Structural phase transformation and microwave dielectric studies of SmNb1−x(Si1/2Mo1/2)xO4 compounds with fergusonite structure

Abstract

Temperature- and composition-induced phase transition in SmNbO₄ was studied by differential scanning calorimetry, Raman spectroscopy and high-temperature powder X-ray diffraction measurements. In situ X-ray diffraction studies revealed that SmNbO₄ possesses a monoclinic fergusonite crystal structure at ambient temperature and transforms to a tetragonal scheelite structure above the transition temperature (T_o ≥ 800 °C). The second-order nature of this transition was confirmed by observing a linear relationship between the spontaneous strain (e_s) of SmNbO₄ and the Landau order parameter (η) around the phase transition temperature. We stabilized this high-temperature tetragonal scheelite phase at ambient temperature by substituting Si⁴⁺ and Mo⁶⁺ into the Nb site of SmNbO₄. The SmNb_1−x(Si_1/2Mo_1/2)_xO₄ (x = 0.0–0.69) ceramic compositions were prepared by the conventional solid-state reaction method. Rietveld refinement was carried out on all the compositions to examine the phase purity, and the compositions where x < 0.06 all formed a monoclinic fergusonite structure (I2/a space group, Z = 2). Both the X-ray diffraction and Raman spectroscopy measurements revealed that increasing the concentration of x transformed the structure from monoclinic fergusonite to tetragonal scheelite (I4₁/a space group, Z = 4) at a critical concentration (x_c). Both the monoclinic and tetragonal phases coexisted in the composition range of 0.06 ≤ x < x_c. The Hakki–Coleman and reflection cavity techniques were used to measure the dielectric constant and quality factor of these stabilized phases, respectively. The temperature coefficient of the resonant frequency was measured by using an invar cavity attached to a programmable hot plate. The high-density samples possessed good microwave dielectric properties.