Structural and magnetic study ofLaBaCoCuO5+δ

Abstract

The structure and magnetic properties of the compound $\mathrm{La}\mathrm{Ba}\mathrm{Cu}\mathrm{Co}{\mathrm{O}}_{5+\ensuremath{\delta}}$ have been studied for the non-stoichiometric oxygen concentration $\ensuremath{\delta}\ensuremath{\approx}0.6$. The structure is pseudo-cubic with a tripled perovskite unit cell. The crystal structure was determined by a combined Rietveld fit to neutron and synchrotron x-ray powder diffraction data in the orthorhombic $Pmmm$ space group, with cell parameters $a=3.9223(3)\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$, $b=3.9360(3)\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$, $c=11.7073(8)\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$, and $V=180.74(2)\phantom{\rule{0.3em}{0ex}}{\mathrm{\AA{}}}^{3}$ (room temperature). Antiferromagnetic ordering of Cu and Co magnetic moments is observed below $205(4)\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The magnetic structure with cell ${a}_{M}=2a$, ${b}_{M}=2b$, and ${c}_{M}=2c$, could be described with the Shubnikov space group $Fmm{m}^{\ensuremath{'}}$. The magnetic moments of both equivalent $\mathrm{Cu}∕\mathrm{Co}$ sites were determined at 50 and $170\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ to be $0.83(3){\ensuremath{\mu}}_{B}$ and $0.58(3){\ensuremath{\mu}}_{B}$, respectively, consistent with one unpaired electron per atom. The fit of the intensities to a simple mean field magnetic model appeared to be insufficient to account for the variation of moments at temperatures close to ${T}_{N}$ while a three dimensional Heisenberg model could improve the fit. Susceptibility measurements between 4 and $350\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ also show irreversibility below $150\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The local environments of Cu and Co were studied by extended x-ray absorption fine structure spectroscopy at both absorption edges. Cu atoms adopt an elongated octahedral or square-based pyramidal oxygen environment which suggests mainly the presence of Cu(II) in the structure. Co adopts different local environments, depending on the electronic and spin states.