Phase Transitions and Physical Properties of K₃Nb₃B₂O₁₂ and K₃Nb_{3 – x} M _x B₂O₁₂ (M = Ta, Sb) Crystals

Abstract

This work continues our studies into the nature and mechanisms of phase transitions in K₃Nb₃B₂O₁₂ (KNB) crystals, which have a framework structure with spacious continuous channels and combine ferroelectric, ferroelastic, and superionic properties. Single crystals of KNB-based solid solutions in which Nb⁵⁺ is partially substituted with Ta⁵⁺ (KNBT) or Sb⁵⁺ (KNBS) are grown from off-stoichiometric melts. The KNBT solid solutions containing more than 15 at. % Ta are found to decompose at room temperature. The introduction of Ta leads to marked broadening of phase transitions but has little effect on the transition temperatures. KNBS solid solutions exist in a limited composition range. Increasing the Sb content shifts the high-temperature phase transitions to much lower temperatures and raises the electric conductivity of the crystals by 1–1.5 orders of magnitude.