Structural phase transitions and negative thermal expansion in Sc2(MoO4)3

The thermal expansion and phase transitions of the framework material Sc2(MoO4)3 have been investigated from 4 to 300 K by powder neutron diffraction, and from 300 to 1053 K by dilatometry. Below 178 K Sc2(MoO4)3 has a monoclinic structure, which has been determined using Rietveld refinement of X-ray and neutron powder diffraction data collected at 50 K; space group P21/a with a=16.22715(9), b=9.58051(6), c=18.9208(1) Å, β=125.3988(4)°. Monoclinic Sc2(MoO4)3 has a positive coefficient of thermal expansion, αV=+2.19×10−5 K−1 between 4 and 170 K. There is significant anisotropy in thermal expansion with the monoclinic b-axis having a negative expansion coefficient between 4 and 86 K. Above 180 K Sc2(MoO4)3 has the orthorhombic Sc2(WO4)3 structure and has a negative coefficient of thermal expansion with αV=−6.3×10−6 K−1 between 180 and 300 K. The structure has been determined between 4 and 300 K using a parametric approach to Rietveld refinement. Structural changes at the monoclinic to orthorhombic phase transition are shown to be intimately related to the contraction of the orthorhombic temperature phase. Dilatometry measurements show that negative thermal expansion continues up to 1053 K.