Polar and Centrosymmetric Phases in Solid Solutions Ca3-xSrx(PO4)2 (0 
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- 作者：A. A. Belik ; F. Izumi ; S. Yu. Stefanovich ; A. P. Malakho ; B. I. Lazoryak ; I. A. Leonidov ; O. N. Leonidova ; and S. A. Davydov
- 刊名：Chemistry of Materials
- 出版年：2002
- 出版时间：July 2002
- 年：2002
- 卷：14
- 期：7
- 页码：3197 - 3205
- 全文大小：239K
- 年卷期：v.14,no.7(July 2002)
- ISSN：1520-5002
文摘
Solid solutions Ca3-xSrx(PO4)2 (0 
x 16/7) were studied by X-ray powder diffraction,infrared spectroscopy, differential scanning calorimetry, electrical-conductivity measurements, and second-harmonic generation. Phosphates with 0 x 12/7 (
-phase) crystallizein space group R3c, Z = 21, a 
p.gif"> 10 Å, and c p.gif"> 38 Å. Sr-rich phosphates with 13/7 x 16/7('-phase) have a somewhat different crystal structure: space group R
m, Z = 10.5, a p.gif"> 10Å, and c p.gif"> 19 Å. The solid solutions with 0 x 12/7 showed a reversible high-temperaturephase transition 
', where the and ' phases have noncentrosymmetric andcentosymmetric structures, respectively. The temperature of the phase transition decreasedwith increasing Sr content. In Ca3(PO4)2, a new phase transition, ', was observed at920 
C in the electrical-conductivity measurements. The structure parameters of polar-Ca2Sr(PO4)2 (x = 1) and centrosymmetric '-Ca5/7Sr16/7(PO4)2 (x = 16/7) at room temperaturewere 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 -Ca2Sr(PO4)2 and space group Rm, Z =10.5, a = 10.7015(2) Å, and c = 19.5787(2) Å for '-Ca5/7Sr16/7(PO4)2. -Ca2Sr(PO4)2 is isotypicwith -Ca3(PO4)2. In -Ca2Sr(PO4)2, the M4 site is 50% occupied by Sr2+ ions, and the M6site is vacant. In both structures, Ca2+ and Sr2+ ions enter the M1-M3 sites whereas Ca2+ions selectively occupy the M5 site. '-Ca5/7Sr16/7(PO4)2 contains some disordered atoms: (1)cations at the M3 site are statistically distributed among several positions near the centerof symmetry, (2) P1O4 tetrahedra are orientationally disordered, and (3) the M4 site (M6site in -Ca2Sr(PO4)2) is occupied by 0.165Sr2+ + 0.085Ca2+ + 0.75
. The mechanism of the ' phase transition in Ca3-xSrx(PO4)2 (0 x 12/7) is discussed on the basis of thecrystal data.
x 16/7) were studied by X-ray powder diffraction,infrared spectroscopy, differential scanning calorimetry, electrical-conductivity measurements, and second-harmonic generation. Phosphates with 0 x 12/7 (-phase) crystallizein space group R3c, Z = 21, a p.gif"> 10 Å, and c p.gif"> 38 Å. Sr-rich phosphates with 13/7 x 16/7('-phase) have a somewhat different crystal structure: space group Rm, Z = 10.5, a p.gif"> 10Å, and c p.gif"> 19 Å. The solid solutions with 0 x 12/7 showed a reversible high-temperaturephase transition ', where the and ' phases have noncentrosymmetric andcentosymmetric structures, respectively. The temperature of the phase transition decreasedwith increasing Sr content. In Ca3(PO4)2, a new phase transition, ', was observed at920 C in the electrical-conductivity measurements. The structure parameters of polar-Ca2Sr(PO4)2 (x = 1) and centrosymmetric '-Ca5/7Sr16/7(PO4)2 (x = 16/7) at room temperaturewere 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 -Ca2Sr(PO4)2 and space group Rm, Z =10.5, a = 10.7015(2) Å, and c = 19.5787(2) Å for '-Ca5/7Sr16/7(PO4)2. -Ca2Sr(PO4)2 is isotypicwith -Ca3(PO4)2. In -Ca2Sr(PO4)2, the M4 site is 50% occupied by Sr2+ ions, and the M6site is vacant. In both structures, Ca2+ and Sr2+ ions enter the M1-M3 sites whereas Ca2+ions selectively occupy the M5 site. '-Ca5/7Sr16/7(PO4)2 contains some disordered atoms: (1)cations at the M3 site are statistically distributed among several positions near the centerof symmetry, (2) P1O4 tetrahedra are orientationally disordered, and (3) the M4 site (M6site in -Ca2Sr(PO4)2) is occupied by 0.165Sr2+ + 0.085Ca2+ + 0.75. The mechanism of the ' phase transition in Ca3-xSrx(PO4)2 (0 x 12/7) is discussed on the basis of thecrystal data.