Na_2/7Gd_4/7MoO₄: a Modulated Scheelite-Type Structure and Conductivity Properties

Abstract

Scheelite-type compounds with the general formula (A1,A2)_<i>n</i>[(B1,B2)­O₄]_<i>m</i> (²/₃ ≤ <i>n</i>/<i>m</i> ≤ ³/₂) are the subject of large interest owing to their stability, relatively simple preparation, and optical properties. The creation of cation vacancies (□) 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⁺:M_2/7Gd_4/7□_1/7MoO₄ (M = Li, Na) compounds were considered as potential lasers with diode pumping. They have a defect scheelite-type 3D structure (space group <i>I</i>4₁/<i>a</i>) with a random distribution of Li⁺(Na⁺), Gd³⁺, and vacancies in the crystal. A Na_2/7Gd_4/7MoO₄ single crystal with scheelite-type structure has been grown by the Czochralski method. Transmission electron microscopy revealed that Na_2/7Gd_4/7MoO₄ has a (3 + 2)­D incommensurately modulated structure. The (3 + 2)­D incommensurately modulated scheelite-type cation-deficient structure of Na_2/7Gd_4/7MoO₄ [super space group <i>I</i>4̅ (α–β0,βα0)­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 [100] and [001] orientation of the single crystal revealed that the conductivity of Na_2/7Gd_4/7MoO₄ at <i>T</i> = 973 K equals σ = 1.13 × 10^–5 Ω^–1 cm^–1