Positional and orientational disorder in a solid solution of Sr9+xNi1.5-x(PO4)7 (x=0.3)

Solid solutions of Sr9+xNi1.5-x(PO4)(7) that are structurally related to beta-Ca-3(PO4)(2) are formed in the compositional range of 0.14 less than or equal to x less than or equal to 0.39. We investigated static disorder in Sr9.3Ni1.2-(PO4)(7) (X = 0.3, space group: R (3) over barm) by X-ray and neutron powder diffraction. The electron-density distribution in Sr9.3Ni1.2(PO4)(7) was determined by Rietveld refinement and subsequent whole-pattern fitting based on the maximum entropy method (MEM) from synchrotron X-ray powder diffraction data measured at 100 K. The resulting electron densities served to derive an expedient split-atom model. By adopting this model, we refined the structure parameters of Sr9.3Ni1.2(PO4)(7) by the Rietveld method from the X-ray diffraction data as well as from time-of-flight neutron powder diffraction data measured at 293 K. The Rietveld refinement with the X-ray diffraction data gave R-wp = 5.39% and R-B = 2.80%. Further MEM-based pattern fitting appreciably decreased the R factors to R,p = 5.18% and RB = 1.03%, which demonstrates that the highly disordered structure of Sr9.3Ni1.2(PO4)(7) can be expressed more accurately with electron densities than with structure parameters. The asymmetric unit of Sr9.3Ni1.2(PO4)(7) contains two Sr sites (Sr1 and Sr3), one Ni site (Ni5), one mixed-metal site (M4), two P sites (P1 and P2), and five O sites. Sr2+ atoms are statistically distributed among four positions near the center of symmetry. Sr2+ and Ni2+ ions are split to occupy the M4 site that is 75% vacant. P1O(4) tetrahedra are orientationally disordered.