FERROELECTRIC-FERROELASTIC TB2(MOO4)3 CRYSTAL-STRUCTURE TEMPERATURE-DEPENDENCE FROM 298-K THROUGH THE TRANSITION AT 436-K TO THE ANTIFERROELECTRIC-PARAELASTIC PHASE AT 523-K

Tb2(MoO4)3 is orthorhombic below T c=436 K with space group Pba2 and tetragonal above with space group P421m. The crystal structure, previously studied at 298 K, has been determined at four higher temperatures. The integrated intensities of 2909, 1325, 1340, and 1310 symmetry-independent structure factors were measured, respectively, at 373, 438, 463, and 523 K with a CAD-4 diffractometer using graphite monochromatized MoKα radiation. The lattice constants at 373 K are a=10.3625(13), b=10.3936(18), c=10.6446(29) Å. A face-centered tetragonal cell with space group C4m21, for orientation identical to that of the orthorhombic cell, has lattice constants a=10.4107(4), c=10.6273(1) Å at 438 K; a=10.4126(4), c=10.6267(1) Å at 463 K; and a=10.4167(4), c=10.6257(1) Å at 523 K. The final agreement factors following structural refinement by the method of least squares are R=0.0274(373 K), 0.0145(438 K), 0.0176(463 K) and 0.0322(523 K). Atomic displacements from the positions occupied at 438 K are greater, by as much as 0.1 Å, at 298 K than those at 373 K: above Tc, the largest comparable displacement is 0.02 Å. The atomic displacements required for polarization reversal or strain reorientation at 373 K are less than those at 298 K by a maximum of slightly over 0.1 Å. Differences in the atomic paths followed under the application of an electric or stress field from those in a thermal field are highly significant, ranging in length to more than 0.2 Å for two oxygen atoms. Two of the three independent MoO42- tetrahedra vibrate highly, the third somewhat less, anisotropically both above and below T c. The three independent tetrahedra rotate gradually from the positions occupied at Tc, through angles less than 8°, as the crystal is cooled to 298 K. © 1980 American Institute of Physics.