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

Abstract

Strontium phosphates, Sr₉NiLi(PO₄)₇ (Li-SNP), Sr_9.04Ni_1.02Na_0.88(PO₄)₇ (Na-SNP), and Sr_9.08Ni_1.04K_0.76(PO₄)₇ (K-SNP), lying near boundary compositions Sr_9 + 2xNi_1 + xA_1 − 6x(PO₄)₇ (A = Li, Na, and K) were synthesized by the solid state method at 1000 °C. 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 β-Ca₃(PO₄)₂ (space group: R3c) in contrast to A-free Sr_9.333Ni_1.167(PO₄)₇ (x = 1/6) belonging to space group Rm. 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 β-Ca₃(PO₄)₂.