Defect chemistry and DFT modelling of La3Ta0.5Ga5.5O14

Abstract

Langatate (LGT) is a piezoelectric oxide used in the high temperature region. As opposed to α-quartz, which is the most used piezoelectric today, langatate preserves its piezoelectric properties at temperatures up to 1470 °C. The defect chemistry of langatate has, however, not been fully understood. In this thesis, this will be addressed through AC impedance spectroscopy and DFT calculations. The defect chemistry and conductivity of nominally undoped and acceptor doped polycrystalline langatate, as well as single crystals, has been studied through impedance spectroscopy in the temperature range from 400 up to 1200 °C in oxygen partial pressure of 1 to 10-5 atm, as well as water vapour partial pressure of approximately 10-2 to 10-5 atm. Measurements have shown that the nominally undoped langatate was effectively donor doped and was compensated by oxygen interstitial and probably some cation vacancy. This sample showed no significant effect of either pO2 or pH2O on the conductivity at temperatures up to 1200 °C, and did not show any sign of degradation in oxidizing or inert conditions, making it ideal for use as a piezoelectric in varying atmospheric conditions. The major charge carrier is interstitial oxygen and its enthalpy of mobility was calculated to 89±4 kJ/mol.

In the nominally 2.5 mol% gallium doped polycrystalline langatate the major charge compensating defect was oxygen vacancies. These defects dominate the conductivity in dry conditions at low pO2 at temperatures up to at least 1000 °C. In wet conditions at temperatures below 700 °C the major charge carrier was protons with enthalpy of mobility of 70±0.5 kJ/mol. From DFT calculations using GGA-PBE exchange-correlation potential a favourable hydration enthalpy of -90 kJ/mol was obtained. DFT results showed that hydration involves vacancies and protons near the gallium ions, which might indicate that increasing the gallium content, might be favourable to the protonation of the material. At higher temperatures and high pO2 there are significant contributions from electron holes. This composition of langatate could be ideal for use as a humidity sensor at temperatures up to 700 °C.

The Z-cut and X-cut single crystalline langatate showed similar behaviour as the acceptor doped polycrystalline langatate, i.e. it must be effectively acceptor doped with gallium excess. These crystals however exhibited a slight anisotropic effect where proton transport along the xy-plane is more favourable than along the z-axis. Langatate is a modest proton conductor compared to other proton conducting oxides.