Phase transitions in KTP isostructures: correlation between structure and T c in germanium-doped RbTiOPO4

Abstract

Crystals of germanium-doped rubidium titanyl phosphate, Rb2(Ti)(Ge0.121Ti0.879)O2(PO4)2 (GeRTP#1) and Rb2(Ge0.125Ti0.875)(Ge0.225Ti0.775)O2(PO4)2 (GeRTP#2), have been structurally characterized from X-ray diffraction data at room temperature. In addition, a third structure, Rb2(TiO)2(PO4)2 (RTP), has been reinvestigated. The exchange of titanium for germanium results in a less distorted octahedral coordination around the two crystallographically independent titanium sites. Additionally, rubidium split-cation positions have been found in these doped RTP crystals. Dielectric measurements show that the phase-transition temperature, T c , decreases with increasing germanium concentration, and a direct correlation between the room-temperature split of the rubidium cations and T c has been discovered. General trends regarding the relationship between the room-temperature structures of KTP-like compounds and their T c values are discussed.