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Characterization of the room-temperature phase of Tl2MoO4: crystal structure, symmetry mode analysis and second-harmonic generation measurements.

The crystal structure of the glaserite-related compound dithallium(I)-molybdate(VI), which at 293 K crystallizes monoclinic, space group C121 with lattice parameters a = 10.565 (3), b = 6.418 (1), c = 8.039 (2) A, beta = 91.05 (4) degrees, has been determined. The structure was refined as an inversion twin to a final R(F(all)) value of 0.0611 for 1006 unique reflections [R(F(obs)) = 0.0285 for 644 observed reflections]. Second-harmonic generation measurements led to a value of d(eff) = 5.5 +/- 0.5 pm V(-1) as an estimation of the second-harmonic conversion efficiency at phase matching. Symmetry mode analysis shows that, in general, primary modes have the highest amplitudes, yet surprisingly some of the secondary modes assume amplitudes of comparable magnitude. A comparison of the phase at 293 K with that at 350 K (space group P3m1) shows that the main change can be described as a rotation of the molybdate tetrahedra around the trigonal a(b) axis. The molybdate tetrahedra as well as the octahedra around one of the symmetry-independent Tl atoms are more strongly distorted in the monoclinic phase. The coordination number for the other two Tl atoms is decreased from 12 and 10 in the high-symmetry phase to 10 and 9 in the monoclinic phase. Furthermore, the number of common edges between the Tl and Mo coordination polyhedra is reduced and the common face which is observed between them in the high-temperature phase is changed to a common edge in the low-temperature phase. The contribution of the primary symmetry modes leads exactly to this change in the coordination spheres of the atoms.