The phase diagram and tetragonal superstructures of the rare earth cobaltate phases Ln_1-xSr_xCoO_3-d (Ln = La³⁺ - Yb³⁺)

Michael James (ANSTO), David Cassidy (ANSTO), Darren Goossens (ANU) and Ray Withers (ANU).

Single phase perovskite-based rare earth cobaltates (Ln _1-x Sr_xCoO_3-d) (Ln = La³⁺ - Yb ³⁺ ; 0.67 £ x £ 0.9) have been synthesised at 1100 °C under 1 atmosphere of oxygen. X-ray diffraction of phases containing the larger rare earth ions La³⁺ , 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 ´ 2a_p) superstructure phase (Figure 1(a)).

Orientational twinning is also present for Ln1-xSrxCoO3-d (x > 0.6) compounds containing rare earth ions smaller than Nd3+. These compounds show a modulated intermediate parent with a tetragonal superstructure (I4/mmm; 2ap ´ 2ap ´ 4ap) (Figure 1(b))

Figure (a)	Figure (b)

Figure 1. Typical <001>p zone axis EDP of (a) La0.10Sr0.90CoO3-d and (b) Y0.10Sr0.90CoO3-d.

A combination of powder X-ray diffraction and electron diffraction have been used to map out the perovskite phase diagram of these Ln1-xSrxCoO3-d compounds for Ln = La3+ ?C Yb3+ (Figure 2). Numerous structure types are observed for the larger lanthanide ions (Ln = La3+, Pr3+, Nd3+ and Sm3+) with an extended range of solid solution. For rare earth ions smaller than Sm3+, the range of solid solution decreases with decreasing ionic radii.

Figure 2. The Ln1-xSrxCoO3-d tetragonal superstructure phase diagram. Compositions having the metrically tetragonal (I4/mmm; 2ap ´: 2ap ´ 4ap) superstructure are shown by crosses (´). Compositions having rhombohedral structures are indicated by ¨. Simple cubic structures are indicated by ·. Metrically cubic structures with orientational twinning of a local tetragonal (ap ´ ap ´ 2ap) superstructure are indicated by (?). Orthorhombic structures are indicated by ??.

Thermogravimetric measurements have determined the overall oxygen content for these Ln1-xSrxCoO3-d (x > 0.6) compounds with the tetragonal (I4/mmm; 2ap ´ 2ap ´ 4ap) superstructure, which does not tend to change with increasing Sr content d~0.25). These phases show mixed valence (3+/4+) cobalt oxidation states with increasing amounts of Co(IV) with increasing x (up to 50% Co4+).

X-ray diffraction data and Rietveld techniques (Figure 3) have been used to refine the structures of each of these tetragonal superstructure phases (Ln = Sm3+ ?C Yb3+). Coupled Ln/Sr and oxygen/vacancy ordering and associated structural relaxation are shown to be responsible for the observed superstructure (Figure 4).

Figure 3. The observed (+), calculated (-) and difference (-) X-ray diffraction profiles for Ho0.10Sr0.90CoO2.79.

Figure 4. The refined structure of Ho0.10Sr0.90CoO2.79 in a projection close to the supercell <110> direction. Note the CoO3 layers at z = 0 and 1/2 and the CoO4 layers at z=1/4 and 3/4. Approximately 50% of the O2 sites are vacant. The rare earth layers contains disordered Sr/Ln1 ions (in the ratio 6:4), Sr2 ions and Sr3 ions.

Reference

Journal of Solid State Chemistry, In Press (2004).