Structure, crystal chemistry and magnetism of rare earth calcium-doped cobaltates: Ln2−xCaxCoO4+δ (Ln = Pr, Nd, Sm, Eu and Gd)

Single-phase samples of Ln2−xCaxCoO4+δ (Ln = Pr3+ − Gd3+) around x∼1 have been prepared. For the first time, the phase field for these compounds has been defined using powder X-ray diffraction and a very narrow solid solution was observed. Under our synthetic conditions, the K2NiF4 structure was not formed for the large La3+ ion, or for compositions containing the smaller rare earths ions (<Gd3+) and Y3+. Thermogravimetric analysis indicates average cobalt oxidation states ranging between ∼3.2+ and ∼3.3+. Rietveld refinement of structures using synchrotron powder X-ray diffraction data indicates that the larger rare earth ions LnCaCoO4+δ (Ln = Pr3+ − Sm3+) form tetragonal (I4/mmm) structures at room temperature, while (Ln = Eu3+, Gd3+) crystallised with orthorhombic (Bmab) structures. The evolution of structure between the low temperature orthorhombic form and the high temperature tetragonal form was investigated between 80 K and 575 K using high-resolution synchrotron X-ray powder diffraction. Orthorhombic to tetragonal transitions were observed for SmCaCoO4.17 (T∼230 K), EuCaCoO4.13 (T∼320 K) and GdCaCoO4.11 (T∼465 K). Magnetic measurements for LnCaCoO4+δ (Ln = Pr3+ − Gd3+) indicate paramagnetic behaviour along with weak antiferromagnetic interactions and Co3+ in a 6t2g low-spin state. PrCaCoO4.17 was found to order antiferromagnetically below 15 K.