X-ray study on thermal expansion of ferroelectric Gd2(MoO4)3

The lattice parameters of improper ferroelectric Gd2(MoO4)3 have been accurately measured at various temperatures between 230 and 25 °C. Special emphasis was laid on their temperature dependence near the ferroelectric Curie point. The change of all the lattice parameters at the transition temperature has been found definitely discontinuous with thermal hysteresis of about 3.5 deg. The break of the B-parameter at the transition is extremely small, e.g., 1.584 × 10−3 Å at 159.8 °C. Spontaneous lattice strains in the ferroelectric state are normal in the sense that they increase with decreasing temperature. However, piezoelectric and electrostrictive constants b, Q, and Q evaluated from the spontaneous lattice strains reveal the characteristic nature of the improper ferroelectricity of this crystal; these constants depend on the temperature, in particular, remarkably near the transition temperature, and the electrostrictive constants are three or four orders of magnitude larger than those of KH2PO4.