Protons in piezoelectric langatate; La3Ga5.5Ta0.5O14

Abstract

This contribution reports the hydration and electrical transport properties of effectively acceptor doped single crystalline and polycrystalline langatate, La3Ga5.5Ta0.5O14. The electrical properties are investigated over wide ranges of pH2O, pD2O and pO2 in the temperature range 400 to 1000°C. Acceptor doped langatate is dominated by oxygen vacancies in dry atmospheres and at high temperatures, and by protonic defects in wet atmospheres and at lower temperatures. The corresponding standard hydration enthalpy and entropy are −90±5kJ/mol and −130±5J/molK, respectively. Further, all compositions display pure proton conductivity in wet atmospheres below 700°C with a proton mobility enthalpy in the range of 70–75kJ/mol, depending on doping level and crystallographic direction. Hence, protons are important for the physiochemical properties of langatate even at 1000°C, and could therefore influence the behavior of langatate-based resonator devices. The proton conductivity is slightly anisotropic, being higher in the X- and Y- than in the Z-direction. At high temperatures and under dry conditions, electron holes and oxide ions dominate the conductivity, and the enthalpy of mobility of vacancies is 140±5kJ/mol.