Optical and electrical properties of the yavapaiite-like molybdate NaAl(MoO4)2

Abstract

Polycrystalline sample of the sodium aluminum molybdate, NaAl(MoO4)2, was prepared by conventional solid-state reaction method. Ultraviolet-visible-near-infrared (UV-Vis-NIR) diffuse reflectance spectroscopy revealed that the optical band gap values of this material at ambient conditions are 3.77 and 3.35 eV for direct and indirect transition, respectively. We also report the electrical properties of the sample using alternating current (AC) complex impedance spectroscopy (CIS) technique over a frequency range of 40 Hz to 5 MHz at several temperatures in the range 693–883 K. These studies showed that temperature dependence of the direct current (DC) conductivity (σdc) and the relaxation frequency (fr) obey the Arrhenius law. However, the obtained values of the activation energy are different, confirming that ionic transport in the material is not due to a simple hopping mechanism. Based on DC conductivity data, NaAl(MoO4)2 can be classified as low ionic conductor. The differential thermal analysis (DTA) shows the presence of a structural phase transition at 580 °C, which is confirmed by the variation of fr and σdc as a function of temperature. The bond valence sum map (BVSM) analysis indicates that the sodium ions seem to be trapped in their own sites. Consequently, the Na+ ion transport in the interlayer spaces is very difficult.