Sodium Ion Transport and Phase Transition in the Vanadate Na₃ErV₂O₈ with Glaserite Type Structure

Abstract

The vanadate Na₃ErV₂O₈ with a glaserite type structure was obtained by solid-phase synthesis in air. The unit cell parameters (space group P2₁/n) calculated from powder X-ray diffraction data at room temperature were a = 5.488 Å, b = 9.723 Å, c = 7.213 Å, β = 93.06°. The phase transition from the low-temperature α-phase to the high-temperature β-phase at 866°С has an enthalpy of 36.9 J/g. According to dilatometric measurements, the linear thermal expansion coefficient (α_L) increases from 8.0 × 10^–6 to 12.5 × 10^–6 K^–1 in the α-phase. The α → β phase transition is accompanied by sharp expansion of Na₃ErV₂O₈ and increase in α_L to 15.0 × 10^–6 K^–1 in the β-phase. The electrical conductivity (σ) was studied by impedance spectroscopy as a function of temperature and partial oxygen pressure (${{p}_{{{{{\text{O}}}_{{\text{2}}}}}}}$) in the gas phase. The invariability of σ values over a broad ${{p}_{{{{{\text{O}}}_{{\text{2}}}}}}}$ range attests to the ionic character of electrical conductivity. According to the Tubandt method, the charge carriers are sodium cations.