Crystal Structure of the Transition-Metal Molybdates and Tungstates. V. Paramagnetic Alpha-Nh2(MoO4)3

Abstract

Paramagnetic α-Nd2(MoO4)3, prepared below the transition temperature of 960°C, crystallizes in a subtle variant of the scheelite structure. The lattice constants of the basic tetragonal unit cell are a = 5.298 ± 0.007 and c = 11.705 ± 0.023 Å, with space group I41/a and 43 of a Nd2(MoO4)3 formula per unit cell. The ordered distribution of two Nd atoms over every three available sites results in additional weak scattering corresponding to a monoclinic unit cell with a = 27.02, b = 11.705, and c = 11.85 Å, with β = 105.3°, space group C2. The integrated intensities of 2490 scheelitelike reflections within a reciprocal hemisphere of radius (sinθ) / λ = 1.02 Å−1 were measured with PEXRAD, of which 198 were independent and above background. Refinement of the atomic positions, assuming a statistical distribution of 23 Nd on each scheelite 4b site, resulted in an agreement factor between measured and calculated structure factors of 0.0375. Each Nd atom is dodecahedrally coordinated to eight oxygen atoms, with four Nd–O distances of 2.492 ± 0.010 Å and four others of 2.509 ± 0.011 Å. Each Mo atom occupies a tetrahedron of oxygen atoms, with four identical Mo−O distances of 1.800 ± 0.011 Å. An arrangement of Nd atoms and vacancies in the monoclinic cell giving a predicted scattering pattern in satisfactory agreement with observation is presented.