Structural changes accompanying negative thermal expansion in Zr 2(MoO 4)(PO 4) 2

Zr ₂(MoO ₄)(PO ₄) ₂ is orthorhombic (Sc ₂W ₃O ₁₂ structure) from 9 to at least 400 K, and shows anisotropic volume negative thermal expansion ( α _a=-8.35(4)×10 ^-6 K ^-1; α _b=3.25(3)×10 ^-6 K ^-1; α _c=-8.27(5)×10 ^-6 K ^-1 in the range 122-400 K) similar in magnitude to A₂M₃O ₁₂ ( 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 Sc ₂W ₃O ₁₂. 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 Sc ₂W ₃O ₁₂.