Raman scattering investigation of the structural phase transition in single crystal KDy(WO_4)_2

摘要

The Raman scattering spectrum of a KDy(WO_4)_2 single crystal has been investigated in the frequency range 3-950 cm~(-1) at temperatures 2, 5, 25, 80, and 300K. All 36 vibrational modes predicted by the group-theoretical analysis for the high-temperature phase were detected and their symmetries were determined. The analysis of the temperature behavior of the Raman spectra indicates that the monoclinic symmetry of the crystal remains unchanged upon the phase transition. Low-energy electronic transitions are observed between the levels of the ground multiplet ~6H_(15/2) of the Dy~(3+) ion split by the crystal field of symmetry C_2. It is established that in the region of the first excited Kramers doublet of the Dy~(3+) ion in the KDy(WO_4)_2 crystal, the Raman spectrum above the phase transition temperature of 6.38 K contains two lines (11.0 and 13.6cm~(-1) at 25 K), while at lower temperatures it contains four lines (9.0, 16.2, 16.6, and 18.6 cm~(-1) at 2 K). This fact shows the presence of four nonequivalent dysprosium ions in the low-temperature phase. Three additional phonon lines (41.5, 76.7, 182.3 cm~(-1)) observed in the low-temperature phase at 2 K also indicate that the primitive translation cell is doubled upon the phase transition. The measurements in an external magnetic field directed along the a-axis of the monoclinic cell revealed a considerable transformation of the scattering spectrum in the region of the first exited Kramers doublet of the Dy~(3+) ion in KDy(WO_4)_2. This transformation indicates a magnetic-field-induced phase transition.