Structural phase transition in K1-xCsxDy(WO4)(2) single crystals

Single crystals of dysprosium potassium (caesium) double tungstates were gr own both by typical flux growth and Top Seeded Solution Growth. K2W2O7 was used as flux. Potassium tungstate provides broad range of growth temperatur es, rather low volatility, and does not introduce additional components int o the melt. K1-xCsxDy(WO4)(2) crystals belong to the monoclinic syngony (al pha -KY(WO4)(2) structure) and has the space group C2/c. Dy3+ ions are situ ated on the two-fold axes (C-2) (which coincide with the crystallographic [ 010] directions) inside distorted octahedra of oxygen ions. KDy(WO4)(2) is known to undergo a structural phase transition of Jahn-Teller type at 6.38K . The influence of caesium ions on structural phase transition was investig ated. Spectroscopic studies of this compound at low temperature have reveal ed some peculiarities of optical absorption spectra. The ground state of th e Dy3+ ions is split into Kramer's doubters with the first excited doublet lying closely to the ground doublet what determines the thermal behaviour o f absorption spectra. The increase of the distance between the Dy3+ lowest doublers observed at low temperature is interpreted as the result of the co operative Jahn-Teller effect.