Abstract
The structural properties and the magnetic ground state of $\mathrm{S}{\mathrm{r}}_{2/3}\mathrm{L}{\mathrm{n}}_{1/3}\mathrm{Fe}{\mathrm{O}}_{3}$ (Ln = La, Pr, Nd) samples were studied by means of synchrotron x-ray powder diffraction, neutron powder diffraction, and M\"ossbauer spectroscopy. All samples exhibit a metal-insulator-like transition coupled to a magnetic arrangement at a critical temperature, ${T}_{\mathrm{MI}}$. The diffraction techniques reveal strong structural changes at ${T}_{\mathrm{MI}}$ that lead to new cells with reduced symmetry at low temperature. The new symmetry of the low-temperature phase has been determined for all compounds. The space groups are P$\overline{3}$c1 for La-based compound and A2/n for the rest of samples. The high-resolution x-ray patterns detected superstructure peaks that can be accounted for by a small charge disproportionation between two nonequivalent Fe sites in the low-temperature phase explained in terms of a charge density wave that propagates along one of the body diagonals of the primitive cubic cell of these compounds. Our results clearly reveal that a full charge disproportionation of ${\mathrm{Fe}}^{4+}$ into ${\mathrm{Fe}}^{3+}$ and ${\mathrm{Fe}}^{5+}$ is not produced. We have determined the magnetic ordering of these samples exhibiting an antiferromagnetic structure with a sixfold periodicity with respect to the primitive cubic structure. The magnetic group accounting for the magnetic arrangements was obtained by a symmetry analysis and it is C2/c (15.85) for all samples but with different unit cell depending on the type of ${\mathrm{FeO}}_{6}$ tilts. The collinear ordering of Fe moments is established perpendicular to the charge density wave (along the body diagonal of the primitive cubic cell) and also perpendicular to the unique monoclinic axis.
Highlights
The close relationship between the charge order (CO) and the metal-insulator transition (MIT) has been studied in a multitude of metal transition oxides and their properties are still a matter of discussion [1,2,3,4,5]
The high-resolution x-ray patterns detected superstructure peaks that can be accounted for by a small charge disproportionation between two nonequivalent Fe sites in the low-temperature phase explained in terms of a charge density wave that propagates along one of the body diagonals of the primitive cubic cell of these compounds
The magnetic group accounting for the magnetic arrangements was obtained by a symmetry analysis and it is C2/c (15.85) for all samples but with different unit cell depending on the type of FeO6 tilts
Summary
The close relationship between the charge order (CO) and the metal-insulator transition (MIT) has been studied in a multitude of metal transition oxides and their properties are still a matter of discussion [1,2,3,4,5]. Regarding Sr2/3Pr1/3FeO3 and Sr2/3Nd1/3FeO3, the combination of the a−a−c− tilting system and cation ordering at the B-position leads to a distorted structure adopting a monoclinic cell [40], space group A2/n (standard group C2/c), whose lattice parameters are related to the primitive cubic cell with the following vectors: am = (-2, -2, 2), bm = (1, 0, -1), and cm = (1, -2, 1) with an origin shift of (1/2, -1/2, 0).
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