The phase diagram and tetragonal superstructures of the rare earth cobaltate phases Ln_1-xSr_xCoO_3-δ (Ln = La³⁺, Pr³⁺, Nd³⁺, Sm³⁺, Gd ³⁺, Y³⁺, Ho³⁺, Dy³⁺, Er ³⁺, Tm³⁺ and Yb³⁺)

Single phase perovskite-based rare earth cobaltates (Ln _1-xSr_xCoO_3-δ) (Ln=La³⁺, Pr³⁺, Nd³⁺, Sm³⁺, Gd³⁺, Dy ³⁺, Y³⁺, Ho³⁺, Er³⁺, Tm³⁺ and Yb³⁺; 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 La³⁺, Pr³⁺ and Nd³⁺ reveals simple cubic structures; however electron diffraction shows orientational twinning of a local, tetragonal (I4/mmm; a_p×a _p×2a_p) superstructure phase. Orientational twinning is also present for Ln_1-xSr_xCoO_3-δ compounds containing rare earth ions smaller than Nd³⁺. These compounds show a modulated intermediate parent with a tetragonal superstructure (I4/mmm; 2a_p×2a_p×4a_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=Sm³⁺-Yb³⁺). Coupled Ln/Sr and oxygen/vacancy ordering and associated structural relaxation are shown to be responsible for the observed superstructure.