Structural changes accompanying negative thermal expansion in Zr2(MoO4)(PO4)2

Zr2(MoO4)(PO4)2 is orthorhombic (Sc2W3O12 structure) from 9 to at least 400 K, and shows anisotropic volume negative thermal expansion (αa=−8.35(4)×10−6 K−1; αb=3.25(3)×10−6 K−1; αc=−8.27(5)×10−6 K−1 in the range 122–400 K) similar in magnitude to A2M3O12 (M—Mo or W) with large A3+. The contraction on heating is associated with a pattern of Zr–O–Mo/P bond angle changes that is somewhat similar, but not the same as that for Sc2W3O12. On heating, the most pronounced reductions in the separation between the crystallographic positions of neighboring Zr and P are not associated with significant reductions in the corresponding Zr–O–P crystallographic bond angles, in contrast to what was seen for Sc2W3O12.