Origins of intrinsic p-type conductivity, p–n transition and substoichiometry in SrO

Abstract

To understand the complex electrical behaviour and deviations from ideal stoichiometry in strontium oxide we have investigated its defect chemistry using a hybrid quantum mechanical/molecular mechanical (QM/MM) embedded-cluster approach. Depending on the temperature and oxygen partial pressure, oxygen interstitials or strontium and oxygen vacancies are found to dominate, while strontium interstitials are rare. Notably, charge-neutral oxygen interstitials form a peroxy-like closed-shell configuration, which is the commonest native point defect in SrO under normal conditions explaining the Sr substoichiometry which is not electrically active. Formally charged double acceptors strontium vacancies prove to be the primary source of the hole excess over negative carriers supplied by donor species, contributing to the material's p-type conductivity. Based on our calculations, we predict that at ultralow oxygen partial pressure (P = 1.0 × 10⁻¹⁵ bar) and high temperatures (>1100 K) in SrO, the electron concentration surpasses the hole concentration, which has previously been reported in pure BaO (also at about 1100 K) and the double barium–strontium oxide (at 850 K) by D. W. Wright, (Nature, 1949, 4173, 714) with the oxygen split interstitial acting as a donor. On increasing the oxygen partial pressure, the hole concentration exceeds the electron concentration, resulting in effective p-type conductivity. Only under low oxygen pressures (e.g., 10⁻⁸ bar) and high extrinsic donor concentrations (>10¹⁷ cm⁻³) might SrO switch to n-type conductivity at high temperatures (>1250 K). This study provides essential insights into intrinsic defects and mechanisms of SrO's p-type conductivity, aiding in understanding and predicting other p-type materials.