Na2/7Gd4/7MoO4: a Modulated Scheelite-Type Structure and Conductivity Properties
Scheelite-type compounds with the general formula (A1,A2)(n)[(B1,B2)O-4](m) (2/3 <= n/m <= 3/2) are the subject of large interest owing to their stability, relatively simple preparation, and optical properties. The creation of cation vacancies (square) in the scheelite-type framework and the ordering of A cations and vacancies can be a new factor in controlling the scheelite-type structure and properties. For a long time, cation-deficient Nd3+:M2/7Gd4/7 square 1/7MoO4 (M = Li, Na) compounds were considered as potential lasers with diode pumping. They have a defect scheelite-type 3D structure (space group I4(1)/a) with a random distribution of Li+(Na+), Gd3+, and vacancies in the crystal. A Na2/7Gd4/7MoO4 single crystal with scheelite-type structure has been grown by the Czochralski method. Transmission electron microscopy revealed that Na2/7Gd4/7MoO4 has a (3 + 2)D incommensurately modulated structure. The (3 + 2)D incommensurately modulated scheelite-type cation-deficient structure of Na2/7Gd4/7MoO4 [super space group (I4) over bar (alpha-beta 0,beta alpha 0)00] has been solved from single-crystal diffraction data. The solution of the (3 + 2)D incommensurately modulated structure revealed the partially disordered distribution of vacancies and Na and Gd cations. High-temperature conductivity measurements performed along the  and  orientation of the single crystal revealed that the conductivity of Na2/7Gd4/7MoO4 at T = 973 K equals sigma = 1.13 X 10(-5) Omega(-1) cm(-1).