Ferroelectric Ferroelastic Paramagnetic Beta-Gd2(MoO4)3 Crystal Structure of the Transition-Metal Molybdates and Tungstates. VI

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Abstract
Beta-gadolinium molybdate, β-Gd2(MoO4)3, is ferroelectric and ferroelastic with a Curie temperature of about 160°C and is paramagnetic with a moment of 7.98 μB. β-Gd2(MoO4)3 crystallizes in the orthorhombic system with space group Pba2 and four formulas in the unit cell. The lattice constants at 298°K are a = 10.38582 ± 0.00014, b = 10.41861 ± 0.00010, and c = 10.70039 ± 0.00004 Å. The integrated intensities of 14 490 reflections were measured with PEXRAD, within a hemisphere of reciprocal space of radius (sinθ) / λ = 1.02 Å−1. The crystal structure was solved by interpretation of Patterson and Fourier series and refined by the method of least squares, using a total of 2545 symmetry independent Fmeas. The final agreement factor R is 0.089, based on isotropic thermal vibrations for all atoms. X-radiation exposure introduces appreciable nonrandom error in the Fmeas. The structure consists of three crystallographically independent discrete sets of (MoO4)2− tetrahedra; one set is oriented with apex oxygen atoms pointing along + c, a second has apex oxygens pointing along − c, and a third has a pseudo-4̄ axis parallel to c. The Gd ions occupy GdO7 polyhedra. The average Mo–O distance is 1.753 Å, the average Gd–O distance is 2.332 Å. The spontaneous polarization Ps is antiparallel to the sense of the polar crystallographic c axis: The calculated value, assuming point charges of Gd3+ and (MoO4)2−, is − 0.26 × 10−2 Cm−2. The experimental value is about 0.2 × 10−2 cm−2. The interchange of axes in ferroelastic switching and the polarity reversal is due to a single process involving small displacements of all atoms. The crystal belongs to the three-dimensional class of ferroelectrics. The phase transition is represented by: ΠPP(4̄2m)FcFcP(mm2).

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