Phases and phase transitions of hexagonal cobalt oxide films onIr(100)-(1 × 1)

Abstract

Cobalt oxides on the unreconstructed Ir(100) surface were prepared byreactive deposition of Co established by simultaneous oxygen flux at about50 °C and subsequent annealing. The films were investigated by low-energy electron diffraction(LEED), scanning tunnelling microscopy (STM) and thermal desorption spectroscopy (TDS).We show that in spite of the quadratic unit mesh of the substrate, oxide films of (111)orientation develop. As long as oxygen-rich conditions are maintained they are of spinel-typeCo3O4(111). They are non-pseudomorphic and transform to rocksalt-type CoO(111) when oxygen lossis induced by annealing at elevated temperatures. Thin films of CoO(111) arecommensurate, and so, in order to realize that, they exhibit a slightly distorted unitcell when below a thickness equivalent to about seven cobalt monolayers. Withincreasing film thickness the uniaxial strain accompanied by the commensurability isgradually relieved by the insertion of dislocations so that eventually the film assumesideal hexagonality. All CoO(111)-type surfaces are reconstructed at low sampletemperatures equivalent to a superstructure. They reversibly transform into a(1 × 1) phaseat about 50 °C.