A comprehensive X-ray study of the ferroelectric–ferroelastic and paraelectric–paraelastic phases of Gd2(MoO4)3

Abstract

The crystal structure of Gdz(MoO4)a has been investigated both at room temperature and at elevated temperatures below and above the ferroelectric-ferroelastic transition temperature (~ 160°C). Space group, unit-cell dimensions, and formula units per unit cell Pba2 (C8v), a= 103881 +00003, b= 104194+0.0004, c= 107007+0.0006/~,, Z = 4 at 25°C and PS421m (D~), a=7.393 +0.002, c= 10.670 + 0.004/~, Z= 2 at 183 °C. Full-matrix least-squares refinements have been carried out with anisotropic thermal parameters with two single-crystal diffractometer data sets measured at 25 and 183 °C, yielding conventional R values of 0.032 (3000 reflections) and 0-028 (880). The high-temperature structure comes close to the average structure of the two ferroelectric-ferroelastic orientations. The differences in interatomic distances between the two modifications are all less than 0.05 .~. Ferroelectric-ferroelastic switching can be accomplished through two macroscopically equivalent switching mechanisms. Changes in interatomic distances of nearest neighbors upon switching are all less than 0.05 ,~,. The main difference between the two ferroelectric-ferroelastic orientations results through movements of several oxygen atoms by as much as 0.7.~. The spontaneous polarization was calculated assuming point charges Gd 3+ and [MOO4]'from positional parameters: Ps = 0.175/tC.cm -2, in good agreement with experiment. Since most of the dipole moments cancel out within the unit cell, Gd2(MoO4)3 can be described as a canted antiferroelectric. The temperature dependence of intensities and peak widths of superstructure reflections has been monitored through the transition temperature. Below the transition temperature, temperature-dependent physical properties can be accounted for by gradual changes in positional parameters towards the high-temperature structure. The mechanism of the phase transition is discussed in terms of the 'positional order-disorder' and the 'soft mode' model. The results obtained in the refinement of the room-temperature structure are compared with results of an independent study of this structure by Keve, Abrahams & Bernstein.