crystal structure of tricalcium phosphate

Department of Materials Science and Engineering, Tokyo Institute of Technology* National Institute for Materials Science, Japan.**
○Yoichi Kawaike* Masatomo Yashima* Masahiko Tanaka**

Tricalcium phosphate (Ca3(PO4)2, TCP) has at least three polymorphs, i.e., the β -, α -, and α´-phases, depending on temperature. The β-TCP with a rhombohedral structure (space group R3c) is stable up to 1125 degrees C. The monoclinic α-TCP, which has a complicated crystal structure with the space group of P21/a, is stable between 1125 degrees C and 1430 degrees C, and can be maintained to room temperature as a metastable phase. The α´-TCP is stable above 1430 degrees C and unable to survive quenching to room temperature. In the present work, unit-cell parameters of the α-TCP has been investigated using high-resolution synchrotron powder diffraction and the Rietveld method. The diffraction experiment was conducted at 29 degrees C with a transmission mode at the 15XU experimental station of SPring-8, Japan. Precise unit-cell parameters of the α-TCP were obtained, a= 12.8328(1), b= 27.1958(1), c= 15.1656(1), α= γ =90 deg. and β= 126.2070(5) deg. Calculated density of α-TCP (2.8945 g/cm3) is smaller than that of β -TCP, indicating the “looser” structure of α -TCP. This “looser” structure is consistent with the higher reactivity of the α -TCP in water.