Stability and microstructure of tricalcium phosphate polymorphs (TCP) in the Ca3(PO4)2–Zn3(PO4)2·Mg3(PO4)2 phase equilibria diagram from experimental studies between 1200°C and 1400°C have been determined. Solid-state compatibilities in the CaO–P2O5–MgO–ZnO system were defined at 900°C for P2O5≤50% mole %, from a complete review of the previously published binary, pseudo-binary, ternary and pseudo-ternary systems and key phase diagrams experiments on CaO–P2O5–MgO–ZnO system. The phase compatibilities of TCP, without considering solid solutions, except for Mg2P2O7-Zn2P2O7 complete solid solution, were stablished and the role of MgO and ZnO on the formation of phases compatible with Ca3(PO4)2 was discussed.
La estabilidad y la microestructura de los polimorfos del fosfato tricálcico (TCP) en el diagrama de equilibrio de fases Ca3(PO4)2-Zn3(PO4)2 Mg3(PO4)2 han sido establecidas mediante estudios experimentales entre 1.200 y 1.400°C. Las compatibilidades en estado sólido en el sistema CaO-P2O5-MgO-ZnO fueron definidas a 900°C para contenidos de P2O5 inferiores al 50% molar a partir de una completa revisión de los datos publicados en la literatura sobre los sistemas binarios, pseudobinarios, ternarios y pseudoternarios, y mediante estudios experimentales de composiciones seleccionadas del sistema cuaternario CaO-P2O5-MgO-ZnO. Las compatibilidades en estado sólido del TCP, sin considerar soluciones sólidas, a excepción de la solución sólida completa entre el Mg2P2O7-Zn2P2O7, fueron establecidas y se discutió el efecto que el contenido de MgO y ZnO, utilizados como dopantes del TCP, ejercen sobre la formación de las fases compatible con este compuesto.
The ternary system Ca3(PO4)2–Zn3(PO4)2–Mg3(PO4)2 within the CaO–P2O5–MgO–ZnO quaternary system is important for biomaterials research because Mg2+ and Zn2+ dopants in HAp and TCP [1–4] enhance synthesis, microstructure, dissolution rate, mechanical strength, bioactivity, and biological and osteogenic performance of calcium-phosphate bioceramics.
The incorporation of divalent ions into TCP structure as is the case of Mg2+ and Zn2+ derives from the necessity to develop smart materials, in which the achievement of a tailored Ca and P ion release during the bio-resorption process is fundamental to enhance the reconstruction of damage bone, specifically bone formation and tissue regeneration and also to optimize the time dependent mechanical strength of the implanted material.
The higher solubility at 37°C of α-TCP (Ksp=10−25.5), and β-TCP (Ksp=10−29.5), compared to HAp (Ksp=10−58.6), is a relevant factor which must be taken into serious consideration when developing a desired bone-substitute material capable to withstand in the human body for a certain period of time [5]. In addition, high densification of these materials is essential to achieve a higher mechanical performance, especially for load bearing applications [6–8]. However, it is difficult to sinter high dense β-TCP polymorph-based materials due to the low temperature of β→α TCP phase transition. These are the main reasons why the high complexity inherent to calcium phosphate-based biomaterials still require further optimization nowadays.
It is reported that Mg2+ and Zn2+ stimulate bone regeneration, osteoblast cell proliferation and synthesis of DNA, inhibiting at the same time osteoclasts bone resorption [9–11]. The incorporation of these ions in solid solution in the TCP structure, which do not break the biocompatibility, has significant consequences in the relative stability of their polymorphic forms, especially in the stabilization of β-TCP. In HAp based materials, Zn2+ and/or Mg2+ presence improving bioresorbility since HAp dissolution in human body is too low to achieve optimum results [12,13]. In the case of doped-TCP, its bioactivity is enhancing, and its chemical stability, essential in the preparation of TCP biomaterial, is improved since transformation temperature of β→α TCP is increasing in more than 250–300°C as a function of Zn2+ and Mg2+ content respectively [14,15].
Although some studies can be found in the literature on the effect of Zn2+ or Mg2+ and other elements [16–19] as doping agents, on properties and behavior of TCP and HAp based biomaterials, few references [20,21] have been found considering the only one effect of Zn2+ and Mg2+ as a whole. Xue et al. [20] found that Mg and Zn dopants play a relevant role toward improving mechanical properties and cell-materials interactions of TCP, while Melo et al. [21] found in Mg-Zn doped TCP samples obtained by solid-sate sintering, a low formation of apatite on the sample surface, after 28 days, in simulate body fluid (SBF). On the other hand, they mention a slight increase of mechanical resistance compared with commercial TCP samples in compression diametral test. Recently, M.A. Sainz et al. [22] have modulated the microstructure, hardness and cell-viability of 1.0wt.% ZnO-TCP sample through MgO incorporation.
Therefore, a study of the primary crystallization volume of TCP in the Ca3(PO4)2–Zn3(PO4)2 Mg3(PO4)2 phase equilibria diagram and solid-state compatibilities of TCP in the CaO–P2O5–MgO–ZnO system are essentials for knowing the effect of these two elements on the phase assemblage and stability regions of Zn2+/Mg2+ doped TCP, if solid solutions limits of ZnO and MgO in TCP is exceeded. All information obtained in the TCP area in both systems, will be a useful tool to design bioceramics materials with tailored composition, microstructure and properties allowing a better understanding of their biological performance. Additionally, formulation of TCP with Zn and Mg dopants over the solid solution limits give place to the formation of the solid- state compatibility secondary phases, whose effect on the bioactivity, and biomechanical properties, among others, is nowadays on discussion or it is unknown.
Literature reviewThe information supplied by CaO–P2O5, MgO–P2O5 and ZnO–P2O5, binary systems is relevant to define and understanding the CaO–ZnO–P2O5 and CaO–MgO–P2O5 ternary systems. Moreover, Ca3(PO4)2–Zn3(PO4)2 and Ca3(PO4)2–Mg3(PO4)2 pseudobinary systems, belong to CaO–ZnO–P2O5 and CaO–MgO–P2O5 ternary systems respectively, are also essentials to a better understanding of the CaO–P2O5–MgO–ZnO quaternary system.
Binary systemsThe CaO–P2O5 system has been extensively studied in the literature [23–26]. Kreidler and Hummel [27] performed the most complete version of the system as part of their studies on P2O5-based systems. Later, experimental studies on this system about phase transitions in amorphous calcium phosphate and high temperature study of the structural phase transition between α and α′ phases by neutron powder diffraction were made by Maciejewski et al. [28] and Yasmina et al. [29] respectively. Finally, theoretical and experimental studies were performed by Serena et al. [30] and Guo-Hui Ding et al. [31]. Fig. 1 shows the version proposed by Kreidler et al. [27] of the system where all intermediate compounds are represented.
![The CaO–P2O5 system [27].](https://static.elsevier.es/multimedia/03663175/0000006500000001/v1_202602230845/S0366317525000688/v1_202602230845/en/main.assets/gr1.jpeg?xkr=YCr5VOU/poQpfs7/7a7NzBMO2l/5LNt7L884TGG/XsVIqRBDfLEKeLVqbAJn9nc2oeScJniYhVSjefRRCV0K+38Z3uno/cwhcciKpR2ftR9+W/2P+bd2mqPEqTgP+3YnCdOOHtm1m/ZGvE+25u2YN8dQk0+bcJxJ0YhWJ8EygqBs+wVtqnt7oY2q24INUdZEiLcIir9BQ1G4S2dcd4Kghcnf+nV2ptokeXI2VpeB9bS6GDOSXTtlOpXpK3nRGSZz6d4VxYOf9gh+lSs2hN87U20Gb0/YpnmOFb4J3TzrmjGenkXY0SRn9UEt1zrp7Tgm)
The CaO–P2O5 system [27].
In addition, detailed studies carried out by Riboud et al. [32], on the system CaO–P2O5–H2O, established decomposition temperatures and stability ranges of HAp and TCP at different conditions of water-vapor partial pressure (Pp(H2O)) between 0 and 500mmHg (Pt=1at). These authors showed, Fig. 2, that at water vapor of 500mmHg, HAp is a stable phase at temperatures below 1550°C. As a function of temperature, it can be compatible with tetracalcium phosphate, Ca4(PO4)2O, or CaO, by one side of the diagram or with Ca3(PO4)2 by the other side of the diagram. In anhydrous conditions HAp is not a stable phase and Ca3(PO4)2, is compatible with Ca4(PO4)2O and Ca2P2O7.
![The CaO–P2O5–H2O system at PH2O=0mm Hg and PH2O=500mmHg. Pt=1at. [32].](https://static.elsevier.es/multimedia/03663175/0000006500000001/v1_202602230845/S0366317525000688/v1_202602230845/en/main.assets/gr2.jpeg?xkr=CSpi8V/bQaszYIyff4VeyyfKw+o75d9bNzKjd0grPewPsTqNJEJ/qIyrGXs6KtNSghrULBHlej0CZGerTd+fy3iI5rmuMYFfQgpUpdqwQwFuMyX6A+1C5UN6TkqezicrieV0MHMScfAn0gD/SHbSVxEE9ivtVfcsGjYw6W2Lw4sFKkA1pIKQxms3a8yZFkkY6rboMz2WsxKOCLRVI/kbMA2K0xN7sDYR6OM9kCEGQ9/mzwZoIxJVvvsWhcAeB97XFW0lkM9AbbCvii9KmXKlF52UlKE1wdd4OPjIdf/JUMvc3TUMJRS0rNS9rnKR7FVV)
The CaO–P2O5–H2O system at PH2O=0mm Hg and PH2O=500mmHg. Pt=1at. [32].
In relation to MgO–P2O5[33] and ZnO–P2O5[34,35] binary systems, the most relevant aspects in both diagrams was the presence of intermediate compounds, Mg3(PO4)2, Mg2P2O7, Mg(PO3)2 and MgP4O11 in the MgO–P2O5 system and the equivalent compounds Zn3(PO4)2, Zn2P2O7, Zn(PO3)2, and ZnP4O11 in the ZnO–P2O5 diagram. The equilibrium relationships of both systems were totally similar to the CaO–P2O5 system.
Ternary and pseudobinary systemsThe ternary system CaO–MgO–P2O5 was established by McCauley et al. [36] who experimentally established solid-state compatibilities and solid solution range to content of P2O5 lower than 50% mole. Enderle et al. [37] defined solid solution range in the Ca3(PO4)2–Mg3(PO4)2 system and the temperature of β↔α Ca3Mg3(PO4)4 transformation (1104°C). McCauley et al. [36] did not confirm the γ↔β transformation reported by Slawski [38] but confirmed stability of CaMgP2O7. They reported that CaMgP2O7 located on the Ca2P2O7–Mg2P2O7 tie-line, melts congruently at 1120°C and doesn’t show any solid solution. However, Terpstra et al. [39] in their complete study of the diagram at 1000°C, for P2O5≤33% mol, found that solid solution range in the Ca2P2O7–Mg2P2O7 tie-line encloses the Ca4Mg2P6O21 compound but not CaMgP2O7 compound mentioned by Bobrownicki et al. [40] and McCauley et al. [36], Terpstra et al. also established two and three phases region and their solid solution areas and, as a consequence, the solid state compatiblities for P2O5≤33%mole%. Fig. 3 shows the solid-state compatibliities of the system, without solid solutions, according Terpstra el al [39].
![Solid state compatibilities in the CaO–P2O5–MgO ternary system without considering solid solutions stablished in the present work from previous literature results [39].](https://static.elsevier.es/multimedia/03663175/0000006500000001/v1_202602230845/S0366317525000688/v1_202602230845/en/main.assets/gr3.jpeg?xkr=wy/+/181u7K2GCGrsVnJenG5jXjhlcQG/4WM5A+ra3FP4Gyz7DRRYPkzDVLWblaMWePhxOmOzecVz1wdokD5co7hSYmzVREuXleVvOuk7fsZptGHDKSqFN0OF0V5GFILNjPZdC/NdtSrWpjP3vUa9oXuzQjnc+y9//3pEavPJQe8HjXhcGsjq4Mpq7Rtd4xhftfClmIr9hSAuUPJr3JXsiIt9QBOvJuGtcWVruufzkPruTn2TGFMqJLi4zK+idVRBONnN1oN2u7VfuOkHrF/h54xm9IRReZixt4Ll+kwS3+TTrrY5tQBIyNQWtjph/cR)
Solid state compatibilities in the CaO–P2O5–MgO ternary system without considering solid solutions stablished in the present work from previous literature results [39].
The pseudobinary system Ca3(PO4)2–Mg3(PO4)2 was studied by Ando et al. [41], Slawski et al. [38] and also by Enderle et al. [37] as part of their studies on CaO–P2O5–MgO system. The intermediate compound Ca3Mg3(PO4)4 was confirmed, but not all proposed diagrams agreed with the Gibbs Phase Rule. The last version of the Ca3(PO4)2–Mg3(PO4)2 diagram was published on 2008 by Carrodeaguas et al. [15], who's delimiting the stability and phase transformations of Ca3(PO4)2ss and redrawn the diagram according to the Gibbs Rule Phase.
The ternary system ZnO–MgO–P2O5 and the pseudobinary system Zn3(PO4)2–Mg3(PO4)2, were studied as a whole by Sarver et al. [42,43] and Hummel et al. [44]. These authors established phase relations in the pseudobinary system and all solid solutions in the ternary system at P2O5≤50mole%. It should be noted that both works establish a complete solid solution between Mg2P2O7 and Zn2P2O7, a result that is relevant because it allows the solid-state compatibilities in the system to be defined. Fig. 4 shows the compatibility of phases of the ZnO–MgO–P2O5 system at P2O5≤50mole% considering the mentioned solid solution.
![Solid state compatibilities in the P2O5–MgO–ZnO ternary system without considering solid solution except for Mg2P2O7–Zn2P2O7 system stablished in the present work from previous literature studies [42] The ss point represents the value of a hypothetical solid solution within the Mg2P2O7–Zn2P2O7 system. The position of this point determines the tie-lines, which define the solid-state compatibilities of the ternary system.](https://static.elsevier.es/multimedia/03663175/0000006500000001/v1_202602230845/S0366317525000688/v1_202602230845/en/main.assets/gr4.jpeg?xkr=FJUQajI6oQL5XAoC2AqMmG47qlZGIRdV3iFhFUU+7EMLMaLspMk5CSxX0IG0lWYTMO0+wJSb73D3KTI0Hls9XBt0y855UBgLS9FHDGxzn8QCPSqqcj7zdjh9t1bAiPdS46WpCSKrIH1vHYv+TuVNknrNUAGCzl/nodATgkTBGEr11+3jdwbymOOFpM54Tubib74kHy8vxtIu+yY1RvRCzhNDoZzKsPM3xnBn2SsW+dYrfDbqvM78e8V+trXmq0MWzd147M0F89etV6s2cKPrBaTHIMj96q/FXfpBkngiW0xGE8V+HnBuBIH8NiAsCCBC)
Solid state compatibilities in the P2O5–MgO–ZnO ternary system without considering solid solution except for Mg2P2O7–Zn2P2O7 system stablished in the present work from previous literature studies [42] The ss point represents the value of a hypothetical solid solution within the Mg2P2O7–Zn2P2O7 system. The position of this point determines the tie-lines, which define the solid-state compatibilities of the ternary system.
Kreidler et al. [45] published the first study on the CaO–P2O5–ZnO ternary system, producing the most complete early version of the Ca3(PO4)2–Zn3(PO4)2 pseudobinary diagram. They established phase relations in the rich-TCP region, showing that β-Ca3(PO4)2 can incorporate up to 10Ymol% Zn3(PO4)2 at 1000°C, which significantly increases the β→α polymorphic transition temperature. However, they did not determine the polymorphic transitions or melting behavior at high temperatures (≥1400°C).
Carbajal et al. [46,47] and later Jantzen et al. [48] expanded these studies. Carbajal et al. produced a complete experimental mapping of the rich-TCP region in the Ca3(PO4)2–Zn3(PO4)2 pseudobinary system, redefining the α, β, and α+β polymorph stability regions as a function of temperature and composition. They also established phase and melting relations of β, α and α′-Ca3(PO4)2 above 1400°C, showing that the β→α transition increases from 1160°C to 1400°C with increasing Zn3(PO4)2 content. They delimited the very narrow α+α′ field, identified two new peritectic points, and defined the Zn2+ solid-solution range in β-Ca3(PO4)2, its influence on phase transformations of Ca3(PO4)2, Zn3(PO4)2 and CaZn2(PO4)2, and the extension of the Ca3(PO4)2 solid-solution field in the rich-TCP region.
An isothermal section at 900°C of the CaO–P2O5–ZnO system was established experimentally, identifying mono, di and triphasic compatibilities in the rich-TCP and rich-ZnO regions [47]. Jantzen et al. [48] completed this work through a thermodynamic assessment of the CaO–P2O5–SiO2–ZnO system, incorporating available experimental data and producing an updated calculated version of the liquidus surface of CaO–P2O5–ZnO system. Fig. 5 presents the solid-state compatibilities stablished in this study for the CaO–P2O5–ZnO system at P2O5≤50Ymol%.
![Solid state compatibilities in the CaO–P2O5–ZnO ternary system at P2O5≤50% mole stablished in the present work from experimental results [46,47], and previous literature results [48].](https://static.elsevier.es/multimedia/03663175/0000006500000001/v1_202602230845/S0366317525000688/v1_202602230845/en/main.assets/gr5.jpeg?xkr=I59scmNnk+Kql3ni/ORX5SSa7VimTfeNLGNrFDmZEZEKqqdDtzgr261RR+DzUaTamlDOp48WA9cGbRS344lOhtT4uO21zgIfvRk7k1R3lpYC9TOxI3P0JCo0jRIg2j9QpJ2aGv4SeC8+YT8TR6Dozt8YuwMXDdCCBn+nltcTexBCf6x17soc0Lfu4nu77vRWpc/Dxy791PHF8mGcYCYGR37VNZBynwFcnacIM72KXrdS8xiPCvkce6Cgf9JCaRL+e3l5qBhPLMq9dkpl9SjtdcJJGSYhylrAH8ZvQavXBsAFdWXb2yqOyYt/q5OJI10r)
Finally, the system CaO–MgO–ZnO is a simple eutectic ternary system, according to the information on binary systems available in the literature [49–51]. No studies on this ternary system were found in the literature. Consequently, CaO, MgO and ZnO were considered solid-state compatibilities phases.
No specific works about Ca3(PO4)2–Zn3(PO4)2–Mg3(PO4)2 and CaO–P2O5–ZnO–MgO phase equilibria diagrams were reported in the literature. The relevant information collected on the pseudobinary and ternary systems belonging to the CaO–P2O5–ZnO–MgO [15–19,39–45], is not enough to a properly definition of primary crystallization volume and stability of polymorphs of TCP in the Ca3(PO4)2–Zn3(PO4)2–Mg3(PO4)2 system and to define the solid-state compatibilities in the quaternary system at P2O5≤50% mole. In the present work, experimental studies on the one hand and a literature review on the other have been used for establishing the stability of Ca3(PO4)2 polymorphs and the solid-state compatibilities of CaO–P2O5–MgO–ZnO system for P2O5≤50.0mole%.
Experimental proceduresCompositions studied were synthesized from pure NH4H2PO4 (≥99.0%-Fluka), CaCO3 (99.0%-Panreac), ultra-pure ZnO (99.9%-Agalsa) and MgO (≥99.9%-Merck). Conventional solid-state sintering process with a previous calcination step was used [46,47]. As preliminary step, each reagent powders were previously attrition-milled for 4h, using ZrO2 balls and isopropyl alcohol as suspension media. Then, different amounts of the milled reagent powders were weighted in an analytical balance and compositions were hand milled three times in an agatha mo using isopropyl media for homogenization. After the milling step the powders were oven dried a 60°C for 24h, passed through a 63μm sieve and isostatically pressed. The pellets were first calcined in platinum crucibles at 900°C for 4h to remove H2O, NH3 and CO2 and subsequently reactivated by crushing and milling again in order to improve the reactivity of samples for the subsequent solid-state reaction sintering step.
The powders were pressed again and pellets of 10mm diameter and 2mm high were heat treated for 12h followed by air-quenched at room temperature. To ensure that the equilibrium conditions were achieved the soaking time was held for a longer period of time, 72h, in some of the samples, appreciating no changes by X-ray in the crystalline phases present in the samples.
Mineralogical characterization of samples was performed by X-ray diffraction (XRD) using a Broker D8 Advance diffractometer (Bruker Germany) in Bragg–Brentano geometry with Lynx Eye providing a monochromatic Cu Kα1 (λ=1.5406Å) radiation. Diffraction data of powders were collected at an angular range of 2θ=10–90° step, scan=0.0197, and a step counting time of 1Ys. Phase identification was made using the PDF-2 release 2000 crystallographic database using ICDD's integrated. Crystalline phase quantification was performed by the Rietveld refinement technique using FullProf.
Field Emission Scanning Electron Microscopy (FE-SEM) (Hitachi S-4700 Japan) equipped with an energy dispersive spectroscopy (EDS) was employed for the characterization of the samples. A microstructural study of some polished or chemically etched samples (diluted acetic acid 1:9 during 30Ys) was also carried out.
Results and discussionStability of tricalcium phosphate polymorphs in the Ca3(PO4)2–Zn3(PO4)2, Ca3(PO4)2–Mg3(PO4)2 and Ca3(PO4)2–Zn3(PO4)2–Mg3(PO4)2 systemsExperimental studies of Ca3(PO4)2–Zn3(PO4)2, Ca3(PO4)2–Mg3(PO4)2 and Ca3(PO4)2–Zn3(PO4)2–Mg3(PO4)2 systems were performed through isothermal analysis between 1200° and 1400°C by XRD and SEM. Table 1 shows the formulated compositions, which were divided in three compositional groups, binary compositions belonging to the Ca3(PO4)2–Zn3(PO4)2 and Ca3(PO4)2–Mg3(PO4)2 pseudobinary systems and finally compositions located inside the Ca3(PO4)2–Zn3(PO4)2–Mg3(PO4)2 system. Characteristics diffraction peaks of TCP polymorphs were identified in the XRD according their JCPDS cards (α-Ca3(PO4)2 PDF# 70-0364) and (β-Ca3(PO4)2 PDF# 70-2065). Phase assemblage coexisting in equilibrium after thermal treatments is also shown in Table 1 considering the crystalline phases observed in the XRD pattern.
Compositions studied in the Ca3(PO4)2–Zn3(PO4)2–Mg3(PO4)2 system.
| Composition (mole%) | Temperature (°C) | Crystalline phases |
|---|---|---|
| 100 Ca3(PO4)2 | 1200 | α-Ca3(PO4)2 |
| 1250 | α-Ca3(PO4)2 | |
| 1350 | α-Ca3(PO4)2 | |
| 1400 | α-Ca3(PO4)2 | |
| 99.75 Ca3(PO4)2–0.25 Zn3(PO4)2 | 1200 | α-Ca3(PO4)2+β-Ca3(PO4)2 |
| 1250 | α-Ca3(PO4)2+β-Ca3(PO4)2 | |
| 1350 | α-Ca3(PO4)2 | |
| 1400 | α-Ca3(PO4)2 | |
| 99.50 Ca3(PO4)2–0.50 Zn3(PO4)2 | 1200 | α-Ca3(PO4)2+β-Ca3(PO4)2 |
| 1250 | α-Ca3(PO4)2+β-Ca3(PO4)2 | |
| 1350 | α-Ca3(PO4)2 | |
| 1400 | α-Ca3(PO4)2 | |
| 99.25 Ca3(PO4)2–0.75 Zn3(PO4)2 | 1200 | α-Ca3(PO4)2+β-Ca3(PO4)2 |
| 1250 | α-Ca3(PO4)2+β-Ca3(PO4)2 | |
| 1350 | α-Ca3(PO4)2 | |
| 1400 | α-Ca3(PO4)2 | |
| 99.00 Ca3(PO4)2–1.00 Zn3(PO4)2 | 1200 | α-Ca3(PO4)2+β-Ca3(PO4)2 |
| 1250 | α-Ca3(PO4)2+β-Ca3(PO4)2 | |
| 1350 | α-Ca3(PO4)2+β-Ca3(PO4)2 | |
| 1400 | α-Ca3(PO4)2 | |
| 98.75 Ca3(PO4)2–1.25 Zn3(PO4)2 | 1200 | α-Ca3(PO4)2+β-Ca3(PO4)2 |
| 1250 | α-Ca3(PO4)2+β-Ca3(PO4)2 | |
| 1350 | α-Ca3(PO4)2+β-Ca3(PO4)2 | |
| 1400 | α-Ca3(PO4)2 | |
| 98.50 Ca3(PO4)2–1.50 Zn3(PO4)2 | 1200 | β-Ca3(PO4)2 |
| 1250 | β-Ca3(PO4)2+α-Ca3(PO4)2 | |
| 1350 | β-Ca3(PO4)2+α-Ca3(PO4)2 | |
| 1400 | α-Ca3(PO4)2 | |
| 98.00 Ca3(PO4)2–2.00 Zn3(PO4)2 | 1200 | β-Ca3(PO4)2 |
| 1250 | β-Ca3(PO4)2+α-Ca3(PO4)2 ↓↓ | |
| 1350 | β-Ca3(PO4)2+α-Ca3(PO4)2 | |
| 1400 | α-Ca3(PO4)2+β-Ca3(PO4)2 ↓↓ | |
| 97.50 Ca3(PO4)2–2.50 Zn3(PO4)2 | 1200 | β-Ca3(PO4)2 |
| 1250 | β-Ca3(PO4)2 | |
| 1350 | β-Ca3(PO4)2+α-Ca3(PO4)2 | |
| 1400 | α-Ca3(PO4)2+β-Ca3(PO4)2 | |
| 97.00 Ca3(PO4)2–3.50 Zn3(PO4)2 | 1200 | β-Ca3(PO4)2 |
| 1250 | β-Ca3(PO4)2 | |
| 1350 | α-Ca3(PO4)2+β-Ca3(PO4)2 | |
| 1400 | α-Ca3(PO4)2+β-Ca3(PO4)2 | |
| 96.00 Ca3(PO4)2–4.50 Zn3(PO4)2 | 1200 | β-Ca3(PO4)2 |
| 1250 | β-Ca3(PO4)2 | |
| 1350 | α-Ca3(PO4)2+β-Ca3(PO4)2 | |
| 1400 | α-Ca3(PO4)2+β-Ca3(PO4)2 | |
| 95.00 Ca3(PO4)2–5.00 Zn3(PO4)2 | 1200 | β-Ca3(PO4)2 |
| 1250 | β-Ca3(PO4)2 | |
| 1350 | β-Ca3(PO4)2 | |
| 1400 | β-Ca3(PO4)2 | |
| 99.75 Ca3(PO4)2–0.25 Mg3(PO4)2 | 1200 | β-Ca3(PO4)2+α-Ca3(PO4)2↓ |
| 1250 | β-Ca3(PO4)2+α-Ca3(PO4)2 | |
| 1350 | α-Ca3(PO4)2 | |
| 1400 | α-Ca3(PO4)2 | |
| 99.50 Ca3(PO4)2–0.50 Mg3(PO4)2 | 1200 | β-Ca3(PO4)2+α-Ca3(PO4)2↓↓ |
| 1250 | β-Ca3(PO4)2+α-Ca3(PO4)2↓↓ | |
| 1350 | α-Ca3(PO4)2 | |
| 1400 | α-Ca3(PO4)2 | |
| 99.25 Ca3(PO4)2–0.75 Mg3(PO4)2 | 1200 | β-Ca3(PO4)2+α-Ca3(PO4)2 |
| 1250 | β-Ca3(PO4)2+α-Ca3(PO4)2 | |
| 1350 | α-Ca3(PO4)2 | |
| 1400 | α-Ca3(PO4)2 | |
| 99.00 Ca3(PO4)2–1.00 Mg3(PO4)2 | 1200 | β-Ca3(PO4)2+α-Ca3(PO4)2 |
| 1250 | β-Ca3(PO4)2+α-Ca3(PO4)2 | |
| 1350 | β-Ca3(PO4)2+α-Ca3(PO4)2 | |
| 1400 | α-Ca3(PO4)2 | |
| 98.75 Ca3(PO4)2–1.25 Mg3(PO4)2 | 1200 | β-Ca3(PO4)2 |
| 1250 | β-Ca3(PO4)2+α-Ca3(PO4)2 | |
| 1350 | β-Ca3(PO4)2+α-Ca3(PO4)2 | |
| 1400 | α-Ca3(PO4)2 | |
| 98.50 Ca3(PO4)2–1.50 Mg3(PO4)2 | 1200 | β-Ca3(PO4)2 |
| 1250 | β-Ca3(PO4)2+α-Ca3(PO4)2 | |
| 1350 | β-Ca3(PO4)2+α-Ca3(PO4)2 | |
| 1400 | α-Ca3(PO4)2 | |
| 98.00 Ca3(PO4)2–2.00 Mg3(PO4)2 | 1200 | β-Ca3(PO4)2 |
| 1250 | β-Ca3(PO4)2+α-Ca3(PO4)2 | |
| 1350 | β-Ca3(PO4)2+α-Ca3(PO4)2 | |
| 1400 | β-Ca3(PO4)2+α-Ca3(PO4)2 | |
| 97.50 Ca3(PO4)2–2.50 Mg3(PO4)2 | 1200 | β-Ca3(PO4)2 |
| 1250 | β-Ca3(PO4)2 | |
| 1350 | β-Ca3(PO4)2+α-Ca3(PO4)2 | |
| 1400 | β-Ca3(PO4)2+α-Ca3(PO4)2 | |
| 97.00 Ca3(PO4)2–3.50 Mg3(PO4)2 | 1200 | β-Ca3(PO4)2 |
| 1250 | β-Ca3(PO4)2 | |
| 1350 | α-Ca3(PO4)2+β-Ca3(PO4)2 | |
| 1400 | α-Ca3(PO4)2+β-Ca3(PO4)2 | |
| 96.00 Ca3(PO4)2–4.50 Mg3(PO4)2 | 1200 | β-Ca3(PO4)2 |
| 1250 | β-Ca3(PO4)2 | |
| 1350 | β-Ca3(PO4)2+α-Ca3(PO4)2 | |
| 1400 | β-Ca3(PO4)2+α-Ca3(PO4)2 | |
| 95.00 Ca3(PO4)2–5.00Mg3(PO4)2 | 1200 | β-Ca3(PO4)2 |
| 1250 | β-Ca3(PO4)2 | |
| 1350 | β-Ca3(PO4)2 | |
| 1400 | β-Ca3(PO4)2 | |
| 99.25Ca3(PO4)2–0.25Zn3(PO4)2–0.50Mg3(PO4)2 | 1200 | α-Ca3(PO4)2+β-Ca3(PO4)2 |
| 1250 | α-Ca3(PO4)2+β-Ca3(PO4)2 | |
| 1350 | α-Ca3(PO4)2 | |
| 1400 | α-Ca3(PO4)2 | |
| 98.85Ca3(PO4)2–0.40Zn3(PO4)2–0.75Mg3(PO4)2 | 1200 | α-Ca3(PO4)2+β-Ca3(PO4)2 |
| 1250 | α-Ca3(PO4)2+β-Ca3(PO4)2 | |
| 1350 | α-Ca3(PO4)2+β-Ca3(PO4)2↓↓ | |
| 1400 | α-Ca3(PO4)2 | |
| 98.10Ca3(PO4)2–0.40Zn3(PO4)2–1.50Mg3(PO4)2 | 1200 | β-Ca3(PO4)2 |
| 1250 | α-Ca3(PO4)2+β-Ca3(PO4)2 | |
| 1350 | α-Ca3(PO4)2+β-Ca3(PO4)2 | |
| 1400 | α-Ca3(PO4)2)2 | |
| 98.85 Ca3(PO4)2–1.00Zn3(PO4)2–0.15Mg3(PO4)2 | 1200 | α-Ca3(PO4)2+β-Ca3(PO4)2 |
| 1250 | α-Ca3(PO4)2+β-Ca3(PO4)2 | |
| 1350 | α-Ca3(PO4)2+β-Ca3(PO4)2 | |
| 1400 | α-Ca3(PO4)2 | |
| 98.50Ca3(PO4)2–1.00Zn3(PO4)2–0.50Mg3(PO4)2 | 1200 | β-Ca3(PO4)2 |
| 1250 | α-Ca3(PO4)2+β-Ca3(PO4)2 | |
| 1350 | α-Ca3(PO4)2+β-Ca3(PO4)2 | |
| 1400 | α-Ca3(PO4)2 | |
| 98.00Ca3(PO4)2–1.00Zn3(PO4)2–1.00Mg3(PO4)2 | 1200 | β-Ca3(PO4)2 |
| 1250 | α-Ca3(PO4)2+β-Ca3(PO4)2 | |
| 1350 | α-Ca3(PO4)2+β-Ca3(PO4)2 | |
| 1400 | α-Ca3(PO4)2 | |
| 97.85 Ca3(PO4)2–1.00Zn3(PO4)2–1.15Mg3(PO4)2 | 1200 | β-Ca3(PO4)2 |
| 1250 | α-Ca3(PO4)2+β-Ca3(PO4)2 | |
| 1350 | α-Ca3(PO4)2+β-Ca3(PO4)2 | |
| 1400 | α-Ca3(PO4)2+β-Ca3(PO4)2 | |
| 96.00Ca3(PO4)2–1.00Zn3(PO4)2–3.00Mg3(PO4)2 | 1200 | β-Ca3(PO4)2 |
| 1250 | β-Ca3(PO4)2 | |
| 1350 | β-Ca3(PO4)2+α-Ca3(PO4)2 | |
| 1400 | β-Ca3(PO4)2+α-Ca3(PO4)2 | |
| 95.00Ca3(PO4)2–1.00Zn3(PO4)2–4.00Mg3(PO4)2 | 1200 | β-Ca3(PO4)2 |
| 1250 | β-Ca3(PO4)2 | |
| 1350 | β-Ca3(PO4)2+α-Ca3(PO4)2 | |
| 1400 | β-Ca3(PO4)2+α-Ca3(PO4)2 | |
| 97.00Ca3(PO4)2–1.50Zn3(PO4)2–1.50Mg3(PO4)2 | 1200 | β-Ca3(PO4)2 |
| 1250 | β-Ca3(PO4)2 | |
| 1350 | α-Ca3(PO4)2+β-Ca3(PO4)2 | |
| 1400 | α-Ca3(PO4)2+β-Ca3(PO4)2 | |
| 95.50 Ca3(PO4)2–1.50Zn3(PO4)2–3.00Mg3(PO4)2 | 1200 | β-Ca3(PO4)2 |
| 1250 | β-Ca3(PO4)2 | |
| 1350 | β-Ca3(PO4)2+α-Ca3(PO4)2 | |
| 1400 | β-Ca3(PO4)2+αCa3(PO4)2 | |
| 92.50 Ca3(PO4)2–1.50Zn3(PO4)2–6.00Mg3(PO4)2 | 1200 | β-Ca3(PO4)2 |
| 1250 | β-Ca3(PO4)2 | |
| 1350 | β-Ca3(PO4)2 | |
| 1400 | β-Ca3(PO4)2 | |
| 97.85 Ca3(PO4)2–2.00Zn3(PO4)2–0.15Mg3(PO4)2 | 1200 | β-Ca3(PO4)2 |
| 1250 | α-Ca3(PO4)2+β-Ca3(PO4)2 | |
| 1350 | α-Ca3(PO4)2+β-Ca3(PO4) | |
| 1400 | α-Ca3(PO4)2+β-Ca3(PO4)2 | |
| 96.85 Ca3(PO4)2–2.00Zn3(PO4)2–1.15Mg3(PO4)2 | 1200 | β-Ca3(PO4)2 |
| 1250 | β-Ca3(PO4)2 | |
| 1350 | α-Ca3(PO4)2+β-Ca3(PO4) | |
| 1400 | α-Ca3(PO4)2+β-Ca3(PO4)2 | |
| 95.85 Ca3(PO4)2–4.00Zn3(PO4)2–0.15Mg3(PO4)2 | 1200 | β-Ca3(PO4)2 |
| 1250 | β-Ca3(PO4)2 | |
| 1350 | β-Ca3(PO4)2+α-Ca3(PO4)2 | |
| 1400 | β-Ca3(PO4)2+α-Ca3(PO4)2 | |
| 94.85 Ca3(PO4)2–4.00Zn3(PO4)2–1.15Mg3(PO4)2 | 1200 | β-Ca3(PO4)2 |
| 1250 | β-Ca3(PO4)2 | |
| 1350 | β-Ca3(PO4) | |
| 1400 | β-Ca3(PO4)2 | |
Fig. 6 shows the XRD pattern of selected Ca3(PO4)2–Zn3(PO4)2 compositions, (1.0, 3.5, 4.5 and 5.0 of ZnO), treated at 1350° and 1400°C. At both temperatures, only α-Ca3(PO4)2 and/or β-Ca3(PO4)2 polymorphs were identified.
At 1350°C a mixture of both phases was observed up to 4.5 mole% Zn3(PO4)2 content whereas only β-Ca3(PO4)2 was detected at high proportion of Zn3(PO4)2. At 1400°C, only α-Ca3(PO4)2 was detected in sample with 1.0mole% of Zn3(PO4)2, whereas different proportions of α and β-Ca3PO4)2 was observed in samples 1<Zn3(PO4)2≤4.5 and only β-Ca3(PO4)2 phase was observed at compositions above 4.5mole% Zn3(PO4)2.
The α-Ca3(PO4)2 decreases and β-Ca3(PO4)2 increases when Zn3(PO4)2 is increasing highlights the stabilizing effect of Zinc on the β-Ca3(PO4)2 polymorph. Furthermore, the shift of XRD characteristics reflections of both polymorphs toward higher 2θ values indicates the formation of Zn-substituted solid solutions in tricalcium phosphate, effect which is more pronounced in β-Ca3(PO4)2 polymorph.
The results obtained in this new study of the Ca3(PO4)2–Zn3(PO4)2 pseudobinary system were in good agreement with the previous research available in the literature.
The Ca3(PO4)2–Mg3(PO4)2 pseudobinary systemXRD diffraction pattern of selected Ca3(PO4)2–Mg3(PO4)2 binary composition, (1.0, 3.5, 4.5 and 5.0 of MgO), treated at 1350° and 1400°C are shown in Fig. 7. A similar trend to that detected in Ca3(PO4)2–Zn3(PO4)2 compositions was observed, and as a consequence the incorporation of Mg2+ in the crystalline structure of Ca3(PO4)2 also promotes the stabilization of β-Ca3(PO4)2 phase.
At 1350°C both polymorphs were detected at compositions between 3.5 and 4.5mole% Mg3(PO4)2, whereas only α-Ca3(PO4)2 was observed in 1 mole% Mg3(PO4)2. β-Ca3(PO4)2 is the stable phase at content greater than 4.5% mole Mg3(PO4)2. At 1400°C, α-Ca3(PO4)2 polymorph is, again, the only phase detected at 1 mole % Mg3(PO4)2 whereas mixtures of α and β-Ca3(PO4)2 phases were found in all compositions containing more than 1 mole % Mg3(PO4)2. The results obtained were in good agreement with the Ca3(PO4)2–Mg3(PO4)2 phase equilibria proposed Carrodeaguas et al. [15].
The Ca3(PO4)2–Zn3(PO4)2–Mg3(PO4)2 pseudoternary system and its solid-state compatibilitiesFig. 8a shows the XRD results of selected ternary compositions treated at 1350° with constant proportion of Zn3(PO4)2 (1mole%) up to 4 mole % of Ca3(PO4)2–Mg3(PO4)2. XRD pattern shows a mixture of α-Ca3(PO4)2 and β-Ca3(PO4)2 phases in samples containing 0.0, 0.5, 1.0 and 2.0 mole% of Mg3(PO4)2, while only β-Ca3(PO4)2 was observed in sample with 4.0 mole% and higher content of Mg3(PO4)2. Phase quantification obtained from Rietveld analysis of XRD pattern, Fig. 8b, shows that the higher the Mg3(PO4)2 content, the higher the proportion of β-Mg3(PO4)2.
Fig. 9 shows the microstructural features of these samples obtained by SEM analysis. Sample without addition of Mg3(PO4)2 exhibits a homogeneous matrix of α-Ca3(PO4)2 with grains of the β-Ca3(PO4)2 phase dispersed in it. The microstructure evolves to biphasic form, approximately 50% α-Ca3(PO4)2–50% β-Ca3(PO4)2, as 0.5mole% Mg3(PO4)2 is added and the proportion of β-Ca3(PO4)2 increases as the Mg3(PO4)2 content increases, reaching 100% of β-Ca3(PO4)2 at 4mole% of Mg3(PO4)2. This microstructural evolution is consistent with XRD analysis, confirming the progressive stabilization of β-Ca3(PO4)2 phase as the Mg3(PO4)2 content increases.
From these results, the stability region of α, α+β and β Ca3(PO4)2 polymorphs (Fig. 10a–d) and the crystallization volume of α-Ca(PO3)2 and α+β-Ca3(PO4)2, in the Ca3(PO4)2–Zn3(PO4)2–Mg3(PO4)2 pseudoternary system were established (Fig. 11).
The results obtained put in evidence that a significant reduction of stability volumes of α and α+β polymorphs takes place, especially at the lowest temperature de treatment, effect very similar to that observed in the pseudobinary systems Ca3(PO4)2–Zn3(PO4)2 and Ca3(PO4)2–Mg3(PO4)2. As a consequence, a significant increases of stability volume of β-Ca3(PO4)2 polymorph was observed.
Finally, two specific compositions, 25mole% Ca3(PO4)2–50mole% Zn3(PO4)2–25mole% Mg3(PO4)2, label 25TCP, and 10mole% Ca3(PO4)2–60mole% Zn3(PO4)2–30mole% Mg3(PO4)2, label 10TCP, Table 2, were analyzed to asses phase compatibility within the Ca3(PO4)2–Zn3(PO4)2–Mg3(PO4)2 pseudoternary system.
The reaction 1 that can be established in this pseudo-ternary phase equilibria diagram is the key to define the solid-state compatibilities in the system:
The ternary composition, 25TCP was formulated in the intersection point of lines that connected Zn2Ca(PO4)2 and Mg3(PO4)2 by one side and Ca3Mg3(PO4)4 and Zn3(PO4)2 by the other, and treated at 900°C, until equilibrium conditions was reached. At this temperature, if there is not liquid formation, phases present in equilibrium will indicate the solid-state compatibilities in the system. The composition, 10TCP, formulated above intersection point, was also treated at the aforementioned temperature. Both samples were heat treated during 4+12+12h, with two intermediate milling steps, to ensure equilibrium conditions.
Fig. 12 shows X-ray diffraction patterns of compositions 25TCP and 10TCP samples after thermal treatment at 900°C. Both diffractograms display the characteristics diffraction pattern of the Zn2Ca(PO4)2 phase (JCPDS card 20-0250) identified as the δ-CaZn2(PO4)2 polymorph of calcium zinc phosphate as reported by E.R. Kreidler and F. A. Hummel [45] and also was detected the presence of Mg(3−x)Znx(PO4)2 phase (JCPDS card 81-0776), with different relative intensities respect to previous one sample, as a consequence of the composition differences. The diffraction peaks associated with this phase are slightly shifted reflecting the formation of solid solution of Zn2+ and Mg2+ within crystal structure of the Ca3(PO4)2. This fact is consistent with the existence of considerable solid solution of Zn2+ and Mg2+ in the Ca3(PO4)2 and Mg3(PO4)2 structures reported in the literature [42,44]. Maximum solid solution of Zn2+ in Mg3(PO4)2 is 90mole% while the solid solutions of Mg2+ in Zn3(PO4)2 is 5mole% at 900°C.
The results obtained put in evidence the coexistence of Zn2Ca(PO4)2 and Mg(3-x)Znx(PO4)2 as stable phases indicating that theses phases are compatibles in solid state while Ca3Mg3(PO4)4 and Zn3(PO4)2 are not. Considering that the lines defining solid state compatibilities in a phase equilibrium diagram can never cross, three ternary solid state compatibilities regions in the Ca3(PO4)2–Zn3(PO4)2–Mg3(PO4)2 system can be defined: Zn2Ca(PO4)2–Zn3(PO4)4-Mg3(PO4)2, Zn2Ca(PO4)2–Mg3(PO4)4-Ca3Mg3(PO4)2 and Zn2Ca(PO4)2–Ca3(PO4)2–Ca3Mg3(PO4)2. Furthermore, six binaries solid-state compatibilities were also defined. Figs. 13 and 14 show schematically the solid-state compatibilities of the system, without and with solid solutions.
The solid-state compatibilities in the Ca2P2O7–Mg2P2O7–Zn2P2O7 system was established considering that there is only one possible configuration due to the isostructural characteristics between Zn2P2O7 and Mg2P2O7 compounds. This fact determines the existence of a complete solid solution between them. Fig. 15a shows schematically the solid-state compatibilities of the system considering only the complete solid solution between Zn2P2O7 and Mg2P2O7. No other solid solutions have been considered. Fig. 15b all solid solutions are represented and due to complete solid solution between Zn2P2O7 and Mg2P2O7 and the partial solid solution of Ca2P2O7 and Ca4Mg2P6O21.a region of three phases and three regions of two phases were defined. Finally, two monophasic areas corresponding to Ca2P2O7ss and CaMgP2O7ss compounds were also defined.
Solid state compatibilities of the quaternary system CaO–P2O5–MgO–ZnO
Considering that the temperatures of the invariant points of systems that contain Ca3(PO4)2 are in the range of 950°C or higher, solid state compatibilities of the quaternary system were defined at 900°C To a better understanding of the solid-state compatibilities, in Table 3[52–69] are collected all solid phases stable at room temperature.
Crystallographic description of the stable solid phosphate phases at 25°C belonging to the CaO–MgO–ZnO–P2O5 quaternary system.
| Compound | Polymorph | Crystal Structure |
|---|---|---|
| P2O5 | O′-P2O5 | Orthorhombic (Pnma) [54] |
| CaO | CaO | Cubic (Fm3m) [55] |
| MgO | MgO | Cubic (Fm3m) [56] |
| ZnO | ZnO | Hexagonal (P63MC) [57] |
| CaP4O11 | CaP4O11 | Monoclinic (P21/n) [58] |
| Ca2P6O17 | Ca2P6O17 | Monoclinic (P21/c) [59] |
| Ca(PO3)2 | Ca(PO3)2 | Monoclinic (P21/n) [60] |
| Ca2P2O7 | γ-Ca2P2O7 | Not determined [61] |
| Ca3(PO4)2 | β-Ca3(PO4)2 | Rhombohedral (R3CH) [62] |
| Ca4(PO4)2O | Ca4(PO4)2O | Monoclinic (P21) [63] |
| ZnP4O11 | ZnP4O11 | Not determined |
| Zn(PO3)2 | α-Zn(PO3)2 | Monoclinic (C12/C1) [64] |
| Zn2P2O7 | α-Zn2P2O7 | Monoclinic (I2/c1) [65] |
| Zn3(PO4)2 | α-Zn3(PO4)2 | Monoclinic (C2/c) [66] |
| MgP4O11 | MgP4O11 | Monoclinic (P21/c) [67] |
| Mg(PO3)2 | Mg(PO3)2 | Monoclinic (C2/c) [68] |
| Mg2P2O7 | α-Mg2P2O7 | Monoclinic (C2/m) [69] |
| Mg3(PO4)2 | Mg3(PO4)2 | Monoclinic (P21/b) [70] |
| Ca3Mg3(PO4)4 | Ca3Mg3(PO4)4 | Monoclinic (C12/c1) [71] |
| Ca4Mg2P6O21 | Ca4Mg2P6O21 | Not determined |
| Zn2Ca(PO4)2 | Zn2Ca(PO4)2 | Not determined |
It is relevant to point out that in CaO–P2O5 binary system in air Ca4(PO4)2O and HAp are not compatible phases, but both are compatible with CaO and Ca3(PO4)2. Then, HAp and CaO are solid-state compatibility phases, although CaO–Ca4(PO4)2O will be the solid-sate compatibility in anhydrous conditions.
From the results obtained in the present work and information available and described in the literature review, solid state compatibilities of the ternary systems CaO–MgO–P2O5, MgO–ZnO–P2O5 and P2O5–CaO–ZnO, for P2O5≤50.0mole%, are properly established. In the case of CaO–MgO–ZnO system, a simple eutectic ternary system, CaO, MgO and ZnO are obviously the solid-state compatibility phases.
A limited number of quaternary compositions located in the primary phase field of Ca3(PO4)2ss (Table 4) was studied for defining the stability of TCP in the quaternary system at 900°C. It was not necessary to evaluate more compositions since the definition of the solid state compatibilities in a quaternary system must be based on those defined by ternary systems and the geometrical rules that determine the definition of solid-state compatibilities in a phase equilibria diagram which implies that the lines that configure them can never cross.
Solid state compatibilities of Ca3(PO4)2 in the P2O5–CaO–MgO–ZnO system.
| Phase compatibilities |
|---|
| Zn2Ca(PO4)2–ZnO–Ca3Mg3(PO4)4–Ca3(PO4)2 |
| ZnO–MgO–Ca3Mg3(PO4)4–Ca3(PO4)2 |
| ZnO–MgO–Ca4(PO4)2O/HAp-Ca3(PO4)2 |
| Ca3(PO4)2–Ca2P2O7–Ca4Mg2P6O21–Zn2Ca(PO4)2 |
| Ca3(PO4)2–Ca4Mg2P6O21–Ca3Mg3(PO4)4–Zn2Ca(PO4)2 |
| Ca3(PO4)2–Ca4Mg2P6O21–Ca3Mg3(PO4)4–Mg2-xZnxP2O7 |
| Ca3(PO4)2–Ca2P2O7–Ca4Mg2P6O21–Mg2-xZnxP2O7 |
Fig. 16 shows a schematic representation of quaternary system from ternary systems that constituted it. Three ternary intermedium compounds, two in the CaO–P2O5–MgO system (Fig. 3), and one in the CaO–P2O5–ZnO system (Fig. 5) are represented. The solid solutions of the different phases were drawn in this figure, including the complete solid solution between Mg2P2O7–Zn2P2O7. Solid solution of the phases does not change the solid-state compatibilities of the system although generates monophasic regions and diphasic or triphasic volumes of solid state compatibilities. In this work, compatibilities in the CaO–P2O5–MgO–ZnO system were only representing in areas where four phases coexisting at equilibrium.
Solid state compatibilities in the ZnO–MgO–CaO-orthophosphate region
From our experimental results in CaO–P2O5–ZnO [51] and Ca3(PO4)2–Zn3(PO4)2–Mg3(PO4)2 systems and the information supplied by literature review, solid state compatibilities, in the area delimited by CaO–MgO–ZnO system and orthophosphates phases, Ca3(PO4)2–Zn3(PO4)2–Mg3(PO4)2, were established. Fig. 17 shows seven quaternary subsystems and their solid-state compatibilities. In three of them Ca3(PO4)2 is present
Solid state compatibilities in the orthophosphate-pyrophosphate region
Solid state compatibilities delimited by Ca3(PO4)2–Zn3(PO4)2–Mg3(PO4)2 and Ca2P2O7–Zn2P2O7–Mg2P2O7 systems were determined considering: compatibilities established in the present work for both ternary systems, our experimental results in CaO–P2O5–ZnO diagram, and finally the complete solid solution between Zn2P2O7–Mg2P2O7 phases. Fig. 18 shows eight quaternary subsystems and their corresponding solid-state compatibilities. In four of them Ca3(PO4)2 is present.
Solid state compatibilities in the pyrophosphate-metaphosphate region
Solid-state compatibilities in the area delimited by pyrophosphate Ca2P2O7–Zn2P2O7–Mg2P2O7 and metaphosphate Ca(PO3)2–Zn(PO3)2–Mg(PO3)2 phases were established (Fig. 19). There are five compatibilities volumes where four phases coexist at equilibrium. In this region, as is obvious, Ca3(PO4)2 is not present in any of them.
Practical implications
Table 4 summarizes solid-state compatibilities stablished for P2O5≤50mole% in which Ca3(PO4)2 is a stable and compatible phase. According to the results obtained in the present work, tricalcium phosphate is present in seven tie-tetrahedrons.
Regardless of ZnO and MgO as compatibility phases and considering ZnO and MgO content within limits of solid solution, Ca4(PO4)20 or HAp, as a function of water-vapor partial pressure (Pp(H2O)), and Ca2P2O7 will be the stables phases in excess or defect of CaO respectively.
However, if MgO content, but not ZnO, exceeds tricalcium phosphate solid solution limits, Ca4Mg2P6O21 and/or Ca3Mg3(PO4)4 and/or Ca2P2O7, will appear as stable secondary phases. Similarly, if ZnO content, but not MgO, exceeds tricalcium phosphate solid solution limits, Zn2Ca(PO4)2 will form as secondary stable phase. Finally, in excess of both, presence of Zn2Ca(PO4)2, and/or Ca3Mg3(PO4)4, and/or Ca4Mg2P6O21, and/or Zn2–xMgxP2O7, may take place (see schematic representation of tie-tetrahedroms of Figs. 17 and 18 where Ca3(PO4)2 is a stable phase). As a consequence, when tricalcium phosphate is doped with ZnO and/or MgO, the presence of other phases implies that the material is thermodynamically unstable and therefore reactions to form stable phases can occur.
In this sense, the implantation of a biomaterial in the human body generally produces an inflammatory response that if it is intense can limit or even make its use unfeasible. In this sense, studies carried out by Link et al. [70] who implanted subcutaneously in rats, pure β-Ca3(PO4)2 and β-Ca3(PO4)2 containing Ca2P2O7, (an impurity produced during synthesis of β-Ca3(PO4)2), observed from their histomorphometric analysis, that samples containing Ca2P2O7 can elicit and stimulate the inflammatory responses at the tissue/implant interface. and this effect is enhanced when Ca2P2O7 content increases. On the contrary, pure β-Ca3(PO4)2 samples only presented slight post-implantation inflammatory response. Campillo-Gimenez et al. [71] studied the inflammatory properties of the three crystalline phases and one amorphous calcium pyrophosphates (CPP) and the intracellular pathways involved. They showed that only the amorphous phase did not show an inflammatory property.
In summary, the design of compositions and thermal treatment below temperature of liquid formation should have in consideration solid solutions and the corresponding tie-line, tie triangle or tie-tetrahedron defined by the corresponding solid-state compatibilities in the quaternary systems studied. This fact makes it possible to formulate different based-TCP materials where properties and biological behavior can be tuned as a function of ZnO and MgO content within or without solid solution limits. The presence or absence of second or even third phases, as stable phases or as impurity phases, will be a critical aspect that can determine the biological performance of ZnO/MgO doped based-tricalcium phosphate biomaterials. Compositions treated at temperatures above liquid formation also should have in consideration solid state compatibilities of the system since liquid formation does not break the compatibilities of the corresponding systems and the potential crystallization of one (primary) or various crystalline phases (primary, secondary, etc.) will be determined by the corresponding tie-line, tie-triangle or tie-tetrahedroms of the system.
ConclusionsFrom the results obtained in the present work and literature review, solid state compatibilities of TCP in ternary systems CaO–MgO–P2O5, MgO–ZnO–P2O5 and P2O5–CaO–ZnO, have been established for P2O5≤50.0mole%.
A complete description of the solid-state compatibilities of the CaO–P2O5–MgO–ZnO quaternary system at 900°C has also been established. In particular, solid-state compatibilities of tricalcium phosphate (Ca3(PO4)2), in Ca3(PO4)2–Zn3(PO4)2–Mg3(PO4)2 pseudoternary system was experimentally established while solid-state compatibilities in the Ca2P2O7–Mg2P2O7–Zn2P2O7 system have been deduced considering the isostructural characteristics and the complete solid solution between Zn2P2O7 and Mg2P2O7 compounds.
Seven quaternary subsystems were established in the area delimited by ZnO–CaO–MgO system and orthophosphates phases Ca3(PO4)2–Zn3(PO4)2–Mg3(PO4)2.
In the orthophosphate-pyrophosphate region, delimited by Ca3(PO4)2–Zn3(PO4)2–Mg3(PO4)2 and Ca2P2O7-Zn2P2O7-Mg2P2O7 systems, eight quaternary subsystems were stablished. Solid-state compatibilities in the area delimited by pyrophosphate Ca2P2O7–Zn2P2O7–Mg2P2O7 and metaphosphate Ca(PO3)2–Zn(PO3)2–Mg(PO3)2 phases were established. Five compatibilities volumes where four phases coexist at equilibrium were stablished.
The presence of second and even third phases as stable phase in MgO/ZnO doped TCP based biomaterials depends on the stoichiometry (Ca/P ratio), and solid solution limits.
The studies carry out on CaO–P2O5–MgO–ZnO quaternary system put in evidence that the design of ZnO and MgO codoped TCP compositions may consider not only solid solutions limits but also the corresponding assemblage of phases, given that the presence of second and even third phases could determine the behavior of co-doped Zn/Mg-TCP biomaterials.
Conflict of interestThe authors declare no conflict of interest
This work was supported by the MICINN (Spain), CSIC 201760E028 project and MCIU/AEI/FEDER (UE) project RT12018-095052-B109. The authors wish to thank Leticia Carbajal for her collaboration in the preparation of materials and laboratory tests.
