The phase diagram and tetragonal superstructures of the rare earth cobaltate phases Ln1-xSrxCoO3-δ (Ln = La3+, Pr3+, Nd3+, Sm3+, Gd3+, Y3+, Ho3+, Dy3+, Er3+, Tm3+ and Yb3+)

Single phase perovskite-based rare earth cobaltates (Ln(1-x)Sr(x)CoO(3-sigma)) (Ln = La3+, Pr3+, Nd3+, Sm3+, Gd3+, Dy3+, Y3+, Ho3+, Er3+, Tm3+ and Yb3+; 0.67 less than or equal to x less than or equal to 0.9) have been synthesized at 1100degreesC 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) x a(p) x 2a(p)) superstructure phase. Orientational twinning is also present for Ln(1-x)Sr(x)CoO(3-delta) compounds containing rare earth ions smaller than Nd\(3+). These compounds show a modulated intermediate parent with a tetragonal superstructure (14/mmm; 2a(p) x 2a(p) x 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. (C) 2004 Published by Elsevier Inc.