Phase equilibria, water dissolution, and peculiarities of charge transfer in Ca-doped La₂Zr₂O_7–α

Abstract

A series of oxides La_{2 - x} Ca _x Zr₂O_7–α (x = 0.00, 0.05, 0.10, 0.15, 0.20) is synthesized. It is found that in samples with the calcium content x = 0.15, 0.20, the second phase Ca_0.9La_0.2Zr_0.9O₃ is present in the fraction increasing with the increase in x. The solubility limit of calcium to form solid solutions based on La₂Zr₂O₇ corresponds to x = 0.1. By high-temperature gravimetry, the proton concentration in La_1.95Са_0.05Zr₂O_7–α is obtained as a function of temperature in the interval of 300–950°С in Н₂О–О₂ atmosphere. According to temperature programmed desorption studies, in the temperature range of 400–900°С at least two types of OH defects with different binding energies are present in the oxide lattice. The temperature dependences of conductivity are obtained for La_1.95Са_0.05Zr₂O_7–α in dry and humid air atmosphere in the temperature range of 350–800°С by the method of impedance spectroscopy. The electrolyte conductivity in humid air is shown to substantially exceed the corresponding values in dry air, which can be associated with manifestation of protonic conductivity in humid atmosphere. The dependences of oxide conductivity on the oxygen content in the gas phase are determined. The conductivity is divided into its ionic and hole components.