Transport and defect mechanisms in cuprous delafossites.: 1.: Comparison of hydrothermal and standard solid-state synthesis in CuAlO2

CuAlO2 exhibits a unique defect structure that is dependent on the synthesis route. High-temperature solid-state and low-temperature hydrothermal techniques are compared to illustrate how cation off-stoichiometry affects the electrical properties of CuAlO2. A defect complex of Al on a Cu-site stabilized by two bound oxygen interstitials (Al(Cu)(..)2O(i)")'' is proposed which serves as an acceptor dopant to set the hole concentration. Trapping of holes (small polarons) by such complexes with decreasing temperature is proposed to account for the decreasing carrier content with temperature. Hydrothermal samples exhibit approximately an order of magnitude increase in hole concentration relative to the solid-state synthesized samples; this is reflected in the electrical conductivity and may explain the variations in electrical properties reported for CuAlO2.