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

Abstract

Single-phase samples of Ln(2-x)Ca(x)CoO(4+delta) (Ln = Fr(3+) - Gd(3+)) around x similar to 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 K(2)NiF(4) structure was not formed for the large La(3+) ion, or for compositions containing the smaller rare earths ions (<Gd(3+)) and Y(3+). Thermogravimetric analysis indicates average cobalt oxidation states ranging between similar to 3.2+ and similar to 3.3+. Rietveld refinement of structures using synchrotron powder X-ray diffraction data indicates that the larger rare earth ions LnCaCoO(4+delta) (Ln = Pr(3+) - Sm(3+)) form tetragonal (I4/mmm) structures at room temperature, while (Ln = Eu(3+), Gd(3+)) 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 SmCaCoO(4.17) (T similar to 230 K), EuCaCoO(4.13) (T similar to 320 K) and GdCaCoO(4.11) (T similar to 465 K). Magnetic measurements for LnCaCoO(4+delta) (Ln = Pr(3+) - Gd(3+)) indicate paramagnetic behaviour along with weak antiferromagnetic interactions and Co(3+) in a (6)t(2g) low-spin state. PrCaCoO(4.17) was found to order antiferromagnetically below 15 K. Crown Copyright (C) 2011 Published by Elsevier Masson SAS. All rights reserved.