Non-Centrosymmetrical Phases in Compounds of a New Group of Li₃MeO₄ and Li₄MeO₄F(Me=Nb^V, Ta^V) Oxides and Oxyfluorides

Abstract

Polymorphism of double oxides and oxyfluorides–Li4NbO4F, Li3NbO4, Li4TaO4F and Li3TaO4 was studied by the SHG method. Non-centrosymmetrical phases of Li4NbO4F and Li3NbO4 pass irreversibly into the centrosymmetrical phase at 800-850 and 1000°C, respectively. For Li4TaO4F and Li3TaO4 reversible transitions were found at 680 and 660°C respectively. In the vicinity of the phase transition optical susceptibility obeys the law χ∼(T c–T)1/2.