Crystal structure and antiferromagnetic spin ordering ofLnFe2/3Mo1/3O3(Ln=Nd,Pr,Ce,La)perovskites

Abstract

Stoichiometric polycrystalline samples of $Ln\mathrm{F}{\mathrm{e}}_{2/3}\mathrm{M}{\mathrm{o}}_{1/3}{\mathrm{O}}_{3}\phantom{\rule{0.16em}{0ex}}(Ln=\mathrm{Nd},\phantom{\rule{0.16em}{0ex}}\mathrm{Pr},\phantom{\rule{0.16em}{0ex}}\mathrm{Ce},\phantom{\rule{0.16em}{0ex}}\mathrm{La})$ have been prepared by solid-state reaction and studied by means of x-ray and neutron powder diffraction as well as M\"ossbauer spectroscopy and magnetic measurements. All samples were found to be of single phase and to have Pnma symmetry with valence state +3 of Fe and Mo. It is demonstrated that the B-site cations of $Ln\mathrm{F}{\mathrm{e}}_{2/3}\mathrm{M}{\mathrm{o}}_{1/3}{\mathrm{O}}_{3}$ in accord with $Ln\mathrm{Fe}{\mathrm{O}}_{3}$ order in a G-type antiferromagnetic structure with the magnetic moments aligned along the $b$ axis. However, with significantly lower N\'eel temperatures than their $Ln\mathrm{Fe}{\mathrm{O}}_{3}$ parent compounds. The Fe-O-Fe bond lengths and bond angles and thus the magnitude of the antiferromagnetic superexchange interaction are found to systematically change with the ionic radius of Ln such that ${T}_{N}$ increases with increasing radius. Only the $\mathrm{CeF}{\mathrm{e}}_{2/3}\mathrm{M}{\mathrm{o}}_{1/3}{\mathrm{O}}_{3}$ compound experiences a low temperature spin reorientation from alignment along the $b$ axis to the $a$ axis.