Structural phase transformation and microwave dielectric studies of SmNb sub(1-x)(Si sub(1/2)Mo sub(1/2)) sub(x)O sub(4) compounds with fergusonite structure

Temperature- and composition-induced phase transition in SmNbO sub(4) was studied by differential scanning calorimetry, Raman spectroscopy and high-temperature powder X-ray diffraction measurements. In situ X-ray diffraction studies revealed that SmNbO sub(4) possesses a monoclinic fergusonite crystal structure at ambient temperature and transforms to a tetragonal scheelite structure above the transition temperature (T sub(o) greater than or equal to 800 degree C). The second-order nature of this transition was confirmed by observing a linear relationship between the spontaneous strain (e sub(s)) of SmNbO sub(4) and the Landau order parameter ( eta ) around the phase transition temperature. We stabilized this high-temperature tetragonal scheelite phase at ambient temperature by substituting Si super(4+) and Mo super(6+) into the Nb site of SmNbO sub(4). The SmNb sub(1-x)(Si sub(1/2)Mo sub(1/2)) sub(x)O sub(4) (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 (I4 sub(1)/a space group, Z= 4) at a critical concentration (x sub(c)). Both the monoclinic and tetragonal phases coexisted in the composition range of 0.06 less than or equal to x< x sub(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.