Raman studies of some alkali metal-lanthanide double tungstates at high pressures

Abstract

Pressure-induced phase transitions in five alkali metal-lanthanide double tungstates, ALn(WO₄)₂, were investigated using high-pressure Raman spectroscopy in the range 0-35 GPa. A phase transition in the scheelite-type double tungstates NaSm(WO₄)₂, NaTb(WO₄)₂ and NaHo(WO₄)₂ to a distorted monoclinic type occurs near 10 GPa. In the pseudo-scheelite-type KLa(WO₄)₂ a similar pressure-induced transition occurs. The behavior is compared with the scheelite-type SrWO₄ and it is proposed that the local compressibility difference at the cationic sites drives the transition in the Na and K double tungstates. The results suggest that the scheelite-type lattice becomes unstable near 10 GPa. In LiY(WO₄)₂, which has the β-LiYb(WO₄)₂-type structure (wolframite-like), a sluggish phase transition to a different monoclinic lattice occurs, which upon release of pressure reverts to the wolframite-type structure. A brief account of the crystal chemistry of the alkali metal-lanthanide double tungstates from the extensive Russian literature is presented to obtain an insight into the pressure-induced phase transitions in them.