Structure of Polymorphous Modifications of Double Sodium and Indium Phosphate

We present the results of neutron powder diffraction experiments with the D2B instrument at Institut Laue Langevin on Al2 (WO4)3 and Y2(WO4)3 over the temperature range 20 to 800°C. The coefficients of thermal expansion are determined to be ALPHAa=-1.31×10-6°C-1, ALPHAb=5.94×10-6°C-1, and ALPHAc=-9.94×10-6°C-1 for Al2(WO4)3 and ALPHAa=-10.35×10-6° C-1, ALPHAb=-3.06×10-6°C-1, and ALPHAc=-7.62×10-6°C-1 for Y2(WO4)3. As in the case of Lu2(WO4)3, the expansivity is unusual in the case of Y2(WO4)3, as the negative thermal expansion is present along all three axes.