Strontium phosphates with β-Ca3(PO4)2-type structures:: Sr9NiLi(PO4)7, Sr9.04Ni1.02Na0.88(PO4)7, and Sr9.08Ni1.04K0.76(PO4)7

Strontium phosphates, Sr9NiLi(PO4)(7) (Li-SNP), Sr9.04Ni1.02Na0.88 (PO4)(7) (Na-SNP), and Sr9.08Ni1.04K0.76 (PO4)(7) (K-SNP), lying near boundary compositions Sr9+2xNi1+x A(1-6x) (PO4)(7) (A = Li, Na, and K) were synthesized by the solid state method at 1000 degreesC. Second-harmonic generation showed that Li-SNP is centrosymmetric whereas Na- and K-SNP are non-centrosymmetric. Li-SNP crystallized in space group I2/m, and Na- and K-SNP are isotypic with beta-Ca-3 (PO4)(2) (space group: R3c) in contrast to A-free Sr9.333Ni1.167(PO4)(7) (x = 1/ 6) belonging to space group R (3) over barm. Structure and lattice parameters of Na- and K-SNP were refined by the Rietveld method from X-ray powder diffraction data. Na- and K-SNP have six metal sites: M1-M6. The M1-M3 sites are fully occupied by Sr, and the M5 site by Ni. Occupancies, g, of the M4 site are g(Sr) = 0.04, g(Ni) = 0.02, and g(Na) = 0.88 in Na-SNP, and g(Sr) = 0.08, g(Ni) = 0.04, and g(K) = 0.76 in K-SNP. The M6 site is vacant in the same manner as with beta-Ca-3 (PO4)(2).