Structural phase transitions and negative thermal expansion in Sc-2(MoO4)(3)

The thermal expansion and phase transitions of the framework material Sc-2( MoO4)(3) have been investigated from 4 to 300 K by powder neutron diffracti on, and from 300 to 1053 K by dilatometry. Below 178 K Sc-2(MoO4)(3) has a monoclinic structure, which has been determined using Rietveld refinement o f X-ray and neutron powder diffraction data collected at 50 K; space group P2(1)/a with a=16.22715(9), b=9.58051(6), c=18.9208(1) Angstrom, beta=125.3 988(4)degrees. Monoclinic Sc-2(MoO4)(3) has a positive coefficient of therm al expansion, alpha(v)=+2.19x10(-5) K-1 between 4 and 170 K. There is signi ficant anisotropy in thermal expansion with the monoclinic b-axis having a negative expansion coefficient between 4 and 86 K. Above 180 K Sc-2(MoO4)(3 ) has the orthorhombic Sc-2(WO4)(3) structure and has a negative coefficien t of thermal expansion with alpha(v) = -6.3 x 10(-6) K-1 between 180 and 30 0 K. The structure has been determined between 4 and 300 K using a parametr ic approach to Rietveld refinement. Structural changes at the monoclinic to orthorhombic phase transition are shown to be intimately related to the co ntraction of the orthorhombic temperature phase. Dilatometry measurements s how that negative thermal expansion continues up to 1053 K, (C) 2000 Elsevi er Science Ltd. All rights reserved.