High temperature structure and vibrational properties of ScXO 4 (X = V 5+and P 5+): A comparative study

ScVO 4 and ScPO 4 represent the zircon (xenotime) type structures with smallest trivalent cations, and that enable them to host both transition metal and rare-earth ions for applied optical materials. Thus, their crystal chemistry and thermophysical properties becomes relevance for their application in non -ambient conditions. In this report, high temperature crystal chemistry and vibrational properties of ScVO 4 and ScPO 4 , as observed from in situ high temperature powder XRD and Raman spectroscopic studies, are reported. The comparative analyses of the results indicate that, though both are isostructural, they show drastically different thermal expansion behavior. In case of ScVO 4 , the c - axis shows significantly larger expansion compared to a - axis, while in ScPO 4 the thermal expansion along and a and c - axes are more or less similar. At ambient condition, the thermal expansion anisotropy in ScPO 4 and ScVO 4 are 1.02 and 3.97, respectively. Additionally, ScPO 4 shows relatively lower coefficient of volume thermal expansion compared to ScVO 4 , ( alpha v = 23.64 x 10 -6 K -1 for ScPO 4 and 26.09 x 10 -6 K -1 for ScVO 4 ), and is contributed by the expansion of ScO 8 units in their structures. The thermal expansion coefficients of ScO 8 unit in ScPO 4 and ScVO 4 are 36.3 x 10 -6 K -1 and 39.1 x 10 -6 K -1 , respectively. Temperature evolution of Raman modes indicates weakening of all the modes, except a symmetric stretching mode, with increasing temperature. The anharmonic analyses of the Raman modes indicate that implicit contributions in ScVO 4 and ScPO 4 are appreciably higher than the explicit contributions, and hence the changes in mode wavenumbers with volume play dominating role in governing their thermal expansion behaviors. Further, it is concluded that ScPO 4 is characterized by more or less like rigid unit cell compared to ScVO 4 .