Low temperature Raman spectroscopic study of scandium molybdate

Low temperature Raman spectroscopic measurements are carried out on Sc-2(MoO4)3 to probe the orthorhombic-monoclinic phase transition. Phonon frequencies and their temperature coefficients are found to change discontinuously across the transition. Several modes are found to disappear while some new modes appear across the transition. The evolution of intensities of the modes suggests a two-phase coexistence over a temperature interval of about 20 K both in the cooling and heating cycles. The transition is found to occur at 142 +/- 10 K upon cooling while the reverse transition occurs at 174 +/- 11 K during the heating cycle, i.e., the system exhibits a hysteresis of 32 K. In contrast to the usual behaviour, most of the phonon modes in the monoclinic phase exhibit softening upon further cooling. Separation of the temperature dependence of Raman mode frequencies into quasi-harmonic and purely anharmonic contributions using reported high pressure Raman data reveals that bending modes Of MoO4 ions are the most anharmonic among the modes studied.