Raman scattering study of the structural phase transition in single crystal KDy(MoO4)2

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Raman scattering of light in single-crystal KDy(MoO4)2 is studied at frequencies of 3–1000 cm−1 for temperatures ranging from 2 to 300 K, including that of a structural phase transition of the cooperative Jahn-Teller type (TC ∼ 14.5 K). During the transition to the low-temperature phase, a series of additional phonon lines corresponding to the Ag, B1g, B2g, and B3g modes is observed which indicates a doubling of the unit cell during the phase transition. An analysis of the symmetry of the phonon modes shows that the low-temperature phase has a predominantly monoclinic symmetry with conservation of a second order axis along the crystallographic b direction, i.e., perpendicular to the layers. Excitations are discovered which correspond to low-energy electronic transitions between levels of the ground-state 6H15/2 multiplet of the Dy3+ ion, which is split in the crystal field with a C2 symmetry. In the vicinity of the first excited Kramers doublet of the Dy3+ ion in crystalline KDy(MoO4)2, the scattered spectrum contains four lines [16.5, 21.0, 24.9, and 29.1 cm−1 (2 K)] at low temperatures, instead of a single line [18.3 cm−1 (25 K)] above the phase transition temperature (14.5 K). This indicates the existence of four nonequivalent dysprosium ions in the low-temperature phase.