Structural properties of charge disproportionation and magnetic order in ${\mathrm{Sr}}_{2/3}{\mathrm{Ln}}_{1/3}{\mathrm{FeO}}_3$ (Ln=La, Pr, and Nd)

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 Mössbauer 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.

Figure 1

(a) X-ray patterns at 295 K for $\mathrm{S}{\mathrm{r}}_{2/3}\mathrm{L}{\mathrm{a}}_{1/3}\mathrm{Fe}{\mathrm{O}}_3$ and $\mathrm{S}{\mathrm{r}}_{2/3}\mathrm{P}{\mathrm{r}}_{1/3}\mathrm{Fe}{\mathrm{O}}_3$. Inset: Detail of different peak splitting, (b) Rietveld analysis of x-ray diffraction pattern for $\mathrm{S}{\mathrm{r}}_{2/3}\mathrm{N}{\mathrm{d}}_{1/3}\mathrm{Fe}{\mathrm{O}}_3$ at 295 K. The arrow indicates the peaks of the inset with two possible refinements.

Figure 2

Selected regions of x-ray patterns collected at 295 and 5 K for $\mathrm{S}{\mathrm{r}}_{2/3}\mathrm{N}{\mathrm{d}}_{1/3}\mathrm{Fe}{\mathrm{O}}_3$ to compare (a) the peak splitting and (b) the occurrence of new superstructure peaks indexed in the frame of the primitive cubic cell.

Figure 3

Possible atomic shifts for Fe2, Sr(Ln)2 and O2 atoms associated to the condensation of modes from the Irrep LD1. The four models give rise to (${h/3,h/3,h/3)}_{\mathrm{c}}$ superstructure peaks but only models 1 and 2 yield a RXS signal at the Fe K-edge.

Figure 4

Rietveld analysis of the high-resolution x-ray diffraction pattern for $\mathrm{S}{\mathrm{r}}_{2/3}\mathrm{L}{\mathrm{a}}_{1/3}\mathrm{Fe}{\mathrm{O}}_3$. The arrow indicates the region emphasized in the inset with the first superstructure (${h/3,h/3,h/3)}_c$ peak.

Figure 5

Temperature dependence of refined lattice parameters from high-resolution x-ray diffraction patterns (indicated in each figure) and of electrical resistance for $\mathrm{S}{\mathrm{r}}_{2/3}\mathrm{L}{\mathrm{a}}_{1/3}\mathrm{Fe}{\mathrm{O}}_3$ (left), $\mathrm{S}{\mathrm{r}}_{2/3}\mathrm{P}{\mathrm{r}}_{1/3}\mathrm{Fe}{\mathrm{O}}_3$ (middle), and $\mathrm{S}{\mathrm{r}}_{2/3}\mathrm{N}{\mathrm{d}}_{1/3}\mathrm{Fe}{\mathrm{O}}_3$ (right).

Figure 6

Rietveld analysis of the high-resolution neutron diffraction pattern for $\mathrm{S}{\mathrm{r}}_{2/3}\mathrm{L}{\mathrm{a}}_{1/3}\mathrm{Fe}{\mathrm{O}}_3$ collected at 295 K (top) and 5 K (bottom).

Figure 7

Crystal and magnetic structure of $\mathrm{S}{\mathrm{r}}_{2/3}\mathrm{L}{\mathrm{a}}_{1/3}\mathrm{Fe}{\mathrm{O}}_3$ at 5 K deduced from the refinement of neutron diffraction patterns and a group-theoretical analysis. The draw was produced by VESTA program [41].

Figure 8

Mössbauer spectra of $\mathrm{S}{\mathrm{r}}_{2/3}\mathrm{L}{\mathrm{a}}_{1/3}\mathrm{Fe}{\mathrm{O}}_3$ measured between 77 and 300 K.

Figure 9

Temperature dependence of the $\mathrm{S}{\mathrm{r}}_{2/3}\mathrm{L}{\mathrm{a}}_{1/3}\mathrm{Fe}{\mathrm{O}}_3$ hyperfine magnetic field and isomer shift. The dotted line indicates the MIT temperature. In the magnetically split region ($T<T_{\mathrm{MI}}$) the weighted averages: $\langle B_{\mathrm{hf}}\rangle =1/3B_{\mathrm{hf}1}+2/3B_{\mathrm{hf}2}$ and $\langle \delta \rangle =1/3\langle {\delta }_1\rangle +2/3\langle {\delta }_2\rangle$ are also displayed. In the top pannel the dashed lines are guides for the eyes. In the bottom pannel the solid lines are the calculated (and extrapolated) thermal dependences of the isomer shift.

Figure 10

Crystal and magnetic structure of $\mathrm{S}{\mathrm{r}}_{2/3}\mathrm{L}{\mathrm{n}}_{1/3}\mathrm{Fe}{\mathrm{O}}_3$ (Ln = Pr, Nd) at 5 K deduced from the refinement of neutron diffraction patterns and a group-theoretical analysis. The draw was produced by VESTA program [41].