High temperature induced phase transitions in Sr_2ZnWO _6 and Sr_2CoWO_6 double perovskite oxides: Raman spectroscopy as a tool

摘要

In this paper we report high-temperature (up to 550 °C) Raman spectroscopy of phase transitions in Sr_2MWO_6 (M = Zn, Co) double perovskite oxides. For Sr_2ZnWO_6 modes centered at 137 cm~(-1), 152 cm~(-1), merge around 80 °C. The same behavior was observed for the 436 cm~(-1) and 447 cm~(-1) modes. The mode centered at 854 cm~(-1) shows a change in the slope at the same temperature, thus indicating a first order phase transition from the monoclinic (P2_1) to tetragonal (I4/m) structure. With increasing temperature, a little change in the slope of the temperature dependence of mode centered at 855 cm~(-1) is observed around 480 °C which might be a sign of the second phase transition in Sr_2ZnWO_6 (tetragonal (I4/m) to cubic (Fm-3m)). The temperature dependence of the full width at half maximum (FWHM) of the most intense modes in Sr _2ZnWO_6 spectra was studied. The width behaves linearly as a function of temperature with a break observed in the slope of the width of the modes at 80 °C; thus, showing the first phase transition occurrence. For Sr_2CoWO_6 the transition from tetragonal (I4/m) structure to cubic (Fm-3m) phase is accompanied by considerable changes in the temperature dependence of modes centered at 438 cm~(-1) and 845 cm~(-1). The transition was observed around 420 °C.