Polar and Centrosymmetric Phases in Solid Solutions Ca_3-xSr_x(PO₄)₂ (0 ⩽ x ⩽ 16/7)

Abstract

Solid solutions Ca 3-x Sr x (PO 4 ) 2 (0 e x e 16/7) were studied by X-ray powder diffraction, infrared spectroscopy, differential scanning calorimetry, electrical-conductivity measurements, and second-harmonic generation. Phosphates with 0 e x e 12/7 ( -phase) crystallize in space group R3c, Z ) 21, a ≈ 10 Å, and c ≈ 38 Å. Sr-rich phosphates with 13/7 e x e 16/7 ( ′-phase) have a somewhat different crystal structure: space group R3 hm, Z ) 10.5, a ≈ 10 Å, and c ≈ 19 Å. The solid solutions with 0 e x e 12/7 showed a reversible high-temperature phase transition a ′, where the and ′ phases have noncentrosymmetric and centosymmetric structures, respectively. The temperature of the phase transition decreased with increasing Sr content. In Ca 3 (PO 4 ) 2 , a new phase transition, a ′, was observed at 920°C in the electrical-conductivity measurements. The structure parameters of polar -Ca 2 Sr(PO 4 ) 2 (x ) 1) and centrosymmetric ′-Ca 5/7 Sr 16/7 (PO 4 ) 2 (x ) 16/7) at room temperature were refined by Rietveld refinements of the X-ray diffraction data: space group R3c, Z ) 21, a ) 10.5612(2) Å, and c ) 38.0588 (5) Å for -Ca 2 Sr(PO 4 ) 2 and space group R3 hm, Z ) 10.5, a ) 10.7015(2) Å, and c ) 19.5787(2) Å for ′-Ca 5/7 Sr 16/7 (PO 4 ) 2 . -Ca 2 Sr(PO 4 ) 2 is isotypic with -Ca 3 (PO 4 ) 2 . In -Ca 2 Sr(PO 4 ) 2 , the M4 site is 50% occupied by Sr 2+ ions, and the M6 site is vacant. In both structures, Ca 2+ and Sr 2+ ions enter the M1-M3 sites whereas Ca 2+ ions selectively occupy the M5 site. ′-Ca 5/7 Sr 16/7 (PO 4 ) 2 contains some disordered atoms: (1) cations at the M3 site are statistically distributed among several positions near the center of symmetry, (2) P1O 4 tetrahedra are orientationally disordered, and (3) the M4 site (M6 site in -Ca 2 Sr(PO 4 ) 2 ) is occupied by 0.165Sr 2+ + 0.085Ca 2+ + 0.750. The mechanism of the a ′ phase transition in Ca 3-x Sr x (PO 4 ) 2 (0 e x e 12/7) is discussed on the basis of the crystal data.

High-Temperature Phase Transition in the Whitlockite-Type Phosphate Ca9In(PO4)7

“…A similar high-temperature reversible phase transition has been found in solid solutions of Ca 10.5Àx Sr x (PO 4 ) 7 (0 rxr8, Z=6) (25). The temperature of the phase transition as well as the signal of the second harmonic generation (SHG) decreases with increasing strontium content.…”

Determination of Precise Unit‐Cell Parameters of the α‐Tricalcium Phosphate Ca3(PO4)2 Through High‐Resolution Synchrotron Powder Diffraction

“…The unit‐cell parameters obtained from the Rietveld analysis were a =12.87271(9), b =27.28034(8), c =15.21275 (12), α=γ=90°, and β=126.2078 (4)° These values agreed with those determined by the single‐crystal X‐ray diffraction analysis ( a =12.887 (2) Å, b =27.280 (4), c =15.219 (2) Å, α=γ=90°, and β=126.20 (1)° 8 ). It should be noted that the present unit‐cell parameters were more precise and that the estimated standard deviations (±0.0001 Å and ±0.0004°) in this work were much smaller than those in the literature (±0.002–±0.009 Å and ±0.02–±0.1°) 8,11,12,16,17 . Precise unit‐cell parameters in the present work are attributable to the higher angular resolution of the diffractometer.…”

“…reported a modification α″‐TCP. Recently, the β′ phase has also been reported between the low‐temperature β and high‐temperature α phases 11 . The solubility of α‐TCP is about ten times higher than that of hydroxyapatite 4…”

Redistribution of Activator Tuning of Photoluminescence by Isovalent and Aliovalent Cation Substitutions in Whitlockite Phosphors

“…Given that the ion radius of Sr 2+ is larger than that of Ca 2+ (Table 1), 25 an obvious shift to lower 2θ direction can be observed with increasing x from 0 to 1.5 ( Figure 2). Moreover, the Sr 2+ ions could preferentially enter into the site M(4), 23 while trivalent Gd 3+ could randomly distribute into the M(1), M(2), M(3), and even small M(5). 26 3.2.…”