New Approach to the β→α Polymorphic Transformation in Magnesium‐Substituted Tricalcium Phosphate and its Practical Implications

Abstract

The transformation β→α in Mg‐substituted Ca3(PO4)2 was studied. The results obtained showed that, contrary to common belief, there is, in the system Mg3(PO4)2–Ca3(PO4)2, a binary phase field where β+α‐Ca3(PO4)2 solid solutions coexist. This binary field lies between the single‐phase fields of β‐ and α‐Ca3(PO4)2 solid solution in the Ca3(PO4)2‐rich zone of the mentioned system. In the light of the results and the Palatnik–Landau's Contact Rule of Phase Regions, a corrected phase equilibrium diagram has been proposed. The practical implications of these findings with regard to the synthesis of pure α‐ and β‐ Mg‐substituted Ca3(PO4)2 powders and to the sintering of related bioceramics with improved mechanical properties are pointed out.

Si-TCP synthesized from "Mg-free" reagents employed as calcium phosphate cement

“…(1250 °C). Based on results reported on the literature [22][23][24] depending on the Magnesium content in the precursors employed the temperature at which pure α-TCP can be synthesized could be raised up to 1500 °C, however, if the material is sintered at the right temperature and for enough time to guarantee the total β → α conversion it is possible to obtain pure α-TCP no matter how high is the Mg contamination. Then, if the powders synthesized on this…”

Las principales contribuciones de Salvador de Aza a las Biocerámicas

“…In these reports the boundary between the field of the low temperature β-TCPss and the high temperature α-TCPss is a line (one-dimension), which contradicts Palatnik-Landau's Contact Rule of Phase Regions (CRPR), lately reformulated as the Boundary Theory (BT) (53)(54)(55)(56)(57). As stated by the CRPR and the BT a phase boundary line (one-dimension) cannot separate two mono-phase fields in a binary phase equilibrium diagram at constant pressure (47).…”

“…Due to the differences in solubility of β-TCPss and α-TCPss the coexistence of both phases in equilibrium becomes evident. Besides, neutron and X-ray diffraction experiments confirmed the reconstructive character of the α↔β transition, which requires to surpass a large activation energy barrier to proceed and indicates that quenching is not required to retain the pure α-TCP phase, but it is only needed to reach a temperature inside the primary field of α-TCP ss and to keep it for a period of time enough to complete the β→α transformation (47).…”

“…unfeasible, unless either a very slow cooling rate or a large dwell at temperature slightly below that of transformation is used. He believed that contradictory reports in literature were due to the influence of small amounts of impurities, such as Mg, commonly present in the materials used in the preparation of TCP and frequently disregarded by the investigators (47,48).…”

“…The presence of β-TCP has sometimes been attributed to partial reversion of the high temperature α-phase already formed during cooling. That is why many authors recommend quenching to avoid the polymorphic transformation α→β (47,48).…”