Unit‐cell parameters of the α‐tricalcium phosphate [TCP; Ca3(PO4)2] were investigated using high‐resolution synchrotron powder diffraction and the Rietveld method. The diffraction experiment was conducted at 29°C at the BL‐15XU experimental station of SPring‐8, Japan. Precise unit‐cell parameters of the α‐TCP were obtained; a=12.87271 (9), b=27.28034(8), c=15.21275(12) Å, α=γ=90°, and β=126.2078(4)°. The calculated density of α‐TCP (2.8677 g/cm3) is smaller than that of β‐TCP, indicating the “looser” structure of α‐TCP.

The single phase of barium-substituted alpha tricalcium phosphate (Ba-α-TCP) (Ca1- x Ba x )3(PO4)2 (x = 0.05, 0.10, and 0.15) has been prepared by solid-state reactions. The crystal structure of the Ba-α-TCP powders has been investigated by high-resolution synchrotron powder diffraction and the Rietveld method. Diffraction data were successfully analyzed by the single phase Ba-α-TCP (space group P21/a). Unit-cell parameters a, b, c, and β increase with increasing Ba content. Most of the Ba atoms are located at three crystallographic sites among the eighteen cation sites in the α-(Ca1- x Ba x )3(PO4)2 (x = 0.05, 0.10, and 0.15). The refined positional parameters and occupancy factors of the preference sites for Ba substitutions were Ca17:  (0.0909(6), 0.4167(2), 0.7823(5)) g(Ba) = 0.72(2); Ca11:  (0.4068(6), 0.2506(2), 0.2199(5)) g(Ba) = 0.67(1); Ca5:  (0.0983(6), 0.0837(2), 0.7857(5)) g(Ba) = 0.65(1) in the α-(Ca0.85Ba0.15)3(PO4)2. Here, g(Ba) is the occupancy factor of Ba atoms. These sites have relatively smaller bond valence sums, indicating the preference sites for the larger-sized Ba substitutions.

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