Phase separation and crystallization in sodium lanthanum phosphate glasses induced by electrochemical substitution of sodium ions with protons

Abstract

Electrochemical substitution of sodium ions with protons (alkali-proton substitution; APS), and the injection of proton carriers was applied to sodium lanthanum phosphate glasses. A clear and homogeneous material was obtained for a glass of composition 25NaO_1/2–8LaO_3/2–66PO_5/2–1GeO₂ following APS, with a resulting proton conductivity of 4 × 10⁻⁶ S cm⁻¹ at 250 °C. The glass underwent phase separation and crystallization at temperatures >255 °C, forming a highly hygroscopic and proton conducting H₃PO₄ phase in addition to LaP₅O₁₄ and other unidentified phases. A glass of composition 25NaO_1/2–8LaO_3/2–67PO_5/2 underwent phase separation and crystallization during APS, forming both H₃PO₄ and LaP₅O₁₄ phases. Sodium lanthanum phosphate glasses are prone to phase separation and crystallization during APS unlike the previously reported NaO_1/2–WO₃–NbO_5/2–LaO_3/2–PO_5/2 glasses. The phase separation was explained by a reduction in viscosity following APS and the introduction of protons, which exhibit high field strength. Thus, phase separation and crystallization of glasses during APS was difficult to avoid. An approach to suppress phase separation is discussed.