Ferroelectric–ferroelastic Tb2(MoO4)3: Room temperature crystal structure of the transition-metal molybdates. VII

Abstract

Beta-terbium molydbate, Tb2(MoO4)3, is a coupled ferroelectric–ferroelastic crystal with Curie temperature of 436 K. Crystallizing in the orthorhombic system with space group Pba2 and four formulas per unit cell, Tb2(MoO4)3 has lattice constants at 298 K of a=10.344 43±0.00005,b=10.386 23±0.00005, and c= 10.654 85±0.00004 Å.The integrated intensities 19 406 reflections were measured with an Enraf–Nonius CAD-4 diffractometer, using monochromatic MoKα radiation, from a small single-domain crystal. The crystal structure was refined by the method of least squares, using the coordinates of isomorphous β-Gd2(MoO4)3 as the initial model, with 7505 symmetry independent Fmeas. The final agreement factor R is 0.0485, based on anisotropic thermal vibrations for all atoms. The average value of the Mo–O distance in the three independent (MoO4)2− tetrahedra is 1.759±0.005 Å. The Tb3+ ion is seven-coordinated with Tb-O distances ranging from 2.256±0.006 to 2.429±0.004 Å: The average distance is 2.336±0.016 Å. The measured spontaneous polarization in this three-dimensional ferroelectric crystal is 0.19×10−2 C m−2: a point-charge model gives 0.09×10−2 C m−2. A negative charge is generated on (001) with increasing temperature. The largest atomic displacement involved in ferroelastic transformation at room temperature is 0.67 Å for two oxygen atoms: all other displacements are less than 0.35 Å.