Low temperature phase transitions in RbDy(MoO4)2 and KDy(WO4)2

Spectroscopic measurements have been made on tetragonal crystals with the scheelite structure of LiR(MoO4) (R = Pr+3, Tb+3, Dy+3 Er+3), as well as on orthorhombic layered-structure crystals of CsPr(MoO4)2, CsTb(MoO4)2, KTb(MoO4)2, RbDy(MoO4)2, and monoclinic crystals of KDy(WO4)2 and KEr(WO4)2 over the temperature range from room temperature down to about 1.5°K over a wavelength range of 1−25 μ in a search for low-temperature phase transitions in rare-earth double molybdates and tungstates. Structural phase transitions have been observed for the dysprosium compounds RbDy(MoO4)2 and KDy(WO4)2 at about 20 and about 10°K, respectively. It is suggested that these phase transitions can be interpreted as due to a cooperative Jahn–Teller effect.