Synthesis, structure, and magnetic characterization of La${}_{2-x}{R}_x$RuO${}_5$ ($R$ $=$ Pr, Nd, Sm, Gd, Dy)

Polycrystalline samples of La${}_{2-x}{R}_x$RuO${}_5$ ($R$ $=$ Pr, Nd, Sm, Gd, Dy) have been prepared, applying a soft-chemistry route based on the thermal decomposition of citric acid precursors. By powder x-ray and neutron diffraction the crystal structures have been investigated in detail. For the unsubstituted parent compound La${}_2$RuO${}_5$, synchrotron x-ray diffraction patterns reveal a broad structural phase transition regime around 170 K without any significant hysteresis. This structural transition is linked with a drastic reduction of the magnetic susceptibility. A similar behavior was also observed for the lanthanide-substituted compounds La${}_{2-x}{R}_x$RuO${}_5$. Magnetic measurements reveal the coexistence of two weakly interacting magnetic sublattices. The effect of rare-earth substitution on the magnetic phase transition is resulting from structural modifications caused by the smaller radius of the $R^{3+}$ ions. These ions are predominantly located within the LaO-layers, which are alternating with LaRuO${}_4$ layers. The transition temperatures determined by differential scanning calorimetry (DSC) are compared to data derived from the susceptibility measurements.

Figure 1

Crystal structure of La${}_2$RuO${}_5$ ($2\times 2\times 2$ unit cells) viewed perpendicular to the $\mathit{ab}$ plane and almost along the $c$ axis. La is represented by dark green spheres, oxygen by red spheres, and the RuO${}_6$ octahedra are drawn in light green. The alternating LaO and LaRuO${}_4$ layers along the $a$ axis are indicated at the top of the figure.

Figure 2

Rietveld refinement of the La${}_{1.25}$Pr${}_{0.75}$ RuO${}_5$ x-ray diffraction pattern, measured with Cu-K${}_{\alpha 1,2}$ radiation at room temperature. In the inset, the angular range above ${80}^{°}$ in $2\Theta$ is shown in more detail.

Figure 3

Rietveld refinement of the La${}_{1.25}$Pr${}_{0.75}$ RuO${}_5$ neutron-diffraction pattern, measured at 300 K with $\lambda =1.494$ Å.

Figure 4

Rietveld refinement of the La${}_{1.25}$Pr${}_{0.75}$ RuO${}_5$ neutron-diffraction pattern, measured at 1.5 K with $\lambda =1.494$ Å.

Figure 5

Cell parameters for La${}_{2-x}{R}_x$RuO${}_5$ derived from Rietveld analysis of powder XRD data at room temperature. Left: cell parameter $a$, right: cell parameter $b$. Error bars are smaller than the size of the symbols.

Figure 6

Cell parameters for La${}_{2-x}{R}_x$RuO${}_5$ derived from Rietveld analysis of powder XRD data at room temperature. Left: cell parameter $c$, right: cell parameter $\beta$. Error bars are smaller than the size of the symbols.

Figure 7

Relative change of the cell parameters for the Pr- (left) and the Gd-substituted compounds (right). The cell parameters of La${}_{2-x}{R}_x$RuO${}_5$ were divided by their corresponding values for La${}_2$RuO${}_5$. Error bars are smaller than the size of the symbols.

Figure 8

Comparison of the cell parameters for the high- and low-temperature phases of La${}_{2-x}$Pr${}_x$RuO${}_5$. Comparable room-temperature parameters have been divided by the corresponding lt data. Left: data from XRD, right: data from neutron diffraction.

Figure 9

Comparison of the octahedral coordination of Ru in La${}_2$RuO${}_5$ and La${}_{1.25}$Pr${}_{0.75}$RuO${}_5$ from room-temperature ND data. The oxygen atoms are labeled corresponding to the tables in the supplementary. The bond lengths are given in Å.

Figure 10

Comparison of the Ru-O2-Ru angles along the $c$ direction (a) and of the local oxygen coordination in the $\mathit{ab}$ plane (b) in La${}_2$RuO${}_5$ (transparent) and La${}_{1.25}$Pr${}_{0.75}$RuO${}_5$ (opaque) from room-temperature ND data. For better comparability of changes, the central ions [Ru in (a) and O5 in (b)] were placed on the same position.

Figure 11

Selected Ru-O bond lengths for the lt phase of La${}_{2-x}$Pr${}_x$RuO${}_5$ derived from ND-data (1.5 K) refinements.

Figure 12

Upper graph: magnetic susceptibility $\chi$ of La${}_2$RuO${}_5$ and La${}_{1.9}{R}_{0.1}$RuO${}_5$ at $H=1$ kOe. Lower graph: inverse magnetic susceptibility 1/$\chi$.

Figure 13

Effective magnetic moments in Bohr magnetons from Curie-Weiss fits of 1/$\chi$ in the temperature range 200–300 K. The full lines represent the summation according to Eq. (4) of the Ru${}^{4+}$ and $R^{3+}$ spin moments calculated from the spin-only and free-ion approximations, respectively.

Figure 14

Curie-Weiss temperatures ${\Theta }_{\text{CW}}$ from the fit of 1/$\chi$ in the temperature range 200–300 K. The lines represent ${\Theta }_{\text{CW}}$ values calculated with Eq. (5).

Figure 15

Quantitative phase analysis of La${}_2$RuO${}_5$ depending on temperature. Fractions of ht and lt phase were calculated from synchrotron-diffraction data for a stepwise cooling from 300 to 54 K.

Figure 16

Inverse magnetic susceptibility of the Pr-substituted samples after subtraction of the high-temperature moment deduced from Curie-Weiss fits. See text for details.

Figure 17

DSC curve from the heating of La${}_{1.75}$Nd${}_{0.25}$RuO${}_5$. The onset temperature was taken from the intersection of the two blue dashed tangents.

Figure 18

Comparison of the transition temperatures obtained by DSC and magnetic measurements for the Pr- and Nd-substitution series.

Figure 19

Transition temperatures $T_{\mathrm{N}}$ obtained from magnetic susceptibility data for La${}_{2-x}{R}_x$RuO${}_5$.