Solid solutions of Sr 9+x Ni 1.5-x (PO 4 ) 7 that are structurally related to β-Ca 3 (PO 4 ) 2 are formed in the compositional range of 0.14 e x e 0.39. We investigated static disorder in Sr 9.3 Ni 1.2 -(PO 4 ) 7 (x ) 0.3, space group: R3 hm) by X-ray and neutron powder diffraction. The electrondensity distribution in Sr 9.3 Ni 1.2 (PO 4 ) 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 Sr 9.3 Ni 1.2 (PO 4 ) 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 wp ) 5.18% and R B ) 1.03%, which demonstrates that the highly disordered structure of Sr 9.3 Ni 1.2 (PO 4 ) 7 can be expressed more accurately with electron densities than with structure parameters. The asymmetric unit of Sr 9.3 Ni 1.2 (PO 4 ) 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. Sr3 atoms are statistically distributed among four positions near the center of symmetry. Sr 2+ and Ni 2+ ions are split to occupy the M4 site that is 75% vacant. P1O 4 tetrahedra are orientationally disordered.

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