The Crystal Structure of Ba17Sm10Cl64

Previous X-ray crystal structure refinement of GaAsO4 gave spurious As–O (1.72 ?) and Ga–O (1.77 ?) bond distances by comparison with theoretical values and with experimental values from AlAsO4 and GaPO4 ([As–O]~=1.67 ? and [Ga–O]~= 1.82 ?). Therefore, a complete structural reinvestigation of gallium arsenate was carried out between 15 and 1073 K by both neutron and X-ray diffraction. All new analyses led to experimental bond values in agreement with those predicted, i.e., close to 1.67 and 1.82 ?. This new structural investigation of GaAsO4 over a wide range of temperatures emphasizes the high thermal stability of the alpha-quartz packing in this compound, the best amongst quartz-like materials. The experimental relation between structural deformation and piezoelectric properties of these quartz-like materials also allows us to predict that GaAsO4 would have the best piezoelectric characteristics for its AT-cut. Conversely, its higher covalent character should imply a greater pressure sensitivity, with a crystal structure transition around 9 GPa. Thus, GaAsO4 would be the best piezoelectric candidate for high temperature applications but not for high pressure devices.