The phase diagram and tetragonal superstructures of the rare earth cobaltate phases Ln1− xSr xCoO 3– δ ( Ln=La 3+, Pr 3+, Nd 3+, Sm 3+, Gd 3+, Y 3+, Ho 3+, Dy 3+, Er 3+, Tm 3+ and Yb 3+)

Abstract

Single phase perovskite-based rare earth cobaltates ( Ln 1− x Sr x CoO 3− δ ) ( Ln=La 3+, Pr 3+, Nd 3+, Sm 3+, Gd 3+, Dy 3+, Y 3+, Ho 3+, Er 3+, Tm 3+ and Yb 3+; 0.67⩽ x⩽0.9) have been synthesized at 1100°C under 1 atmosphere of oxygen. X-ray diffraction of phases containing the larger rare earth ions La 3+, Pr 3+ and Nd 3+ reveals simple cubic structures; however electron diffraction shows orientational twinning of a local, tetragonal ( I4/ mmm; a p× a p×2 a p) superstructure phase. Orientational twinning is also present for Ln 1− x Sr x CoO 3− δ compounds containing rare earth ions smaller than Nd 3+. These compounds show a modulated intermediate parent with a tetragonal superstructure ( I4/ mmm; 2 a p×2 a p×4 a p). Thermogravimetric measurements have determined the overall oxygen content, and these phases show mixed valence (3 +/4 +) cobalt oxidation states with up to 50% Co(IV). X-ray diffraction data and Rietveld techniques have been used to refine the structures of each of these tetragonal superstructure phases ( Ln=Sm 3+–Yb 3+). Coupled Ln/Sr and oxygen/vacancy ordering and associated structural relaxation are shown to be responsible for the observed superstructure.