First-order valence transition: Neutron diffraction, inelastic neutron scattering, and x-ray absorption investigations on the double perovskite ${\mathrm{Ba}}_2{\mathrm{PrRu}}_{0.9}{\mathrm{Ir}}_{0.1}{\mathrm{O}}_6$

Bulk studies have revealed a first-order valence phase transition in ${\mathrm{Ba}}_2{\mathrm{PrRu}}_{1-x}{\mathrm{Ir}}_x{\mathrm{O}}_6$ ($0.10\le x\le 0.25$), which is absent in the parent compounds with $x=0$ (${\mathrm{Pr}}^{3+}$) and $x=1$ (${\mathrm{Pr}}^{4+}$), which exhibit antiferromagnetic order with transition temperatures $T_{\mathrm{N}}=120$ and 72 K, respectively. In the present study, we have used magnetization, heat capacity, neutron diffraction, inelastic neutron scattering, and x-ray absorption measurements to investigate the nature of the Pr ion in $x=0.1$. The magnetic susceptibility and heat capacity of $x=0.1$ show a clear sign of the first-order valence phase transition below 175 K, where the Pr valence changes from 3+ to 4+. Neutron diffraction analysis reveals that $x=0.1$ crystallizes in a monoclinic structure with space group $P{2}_1/n$ at 300 K, but below 175 K two phases coexist, the monoclinic having the Pr ion in a 3+ valence state and a cubic one ($Fm\overline{3}m$) having the Pr ion in a 4+ valence state. Clear evidence of an antiferromagnetic ordering of the Pr and Ru moments is found in the monoclinic phase of the $x=0.1$ compound below 110 K in the neutron diffraction measurements. Meanwhile, the cubic phase remains paramagnetic down to 2 K, a temperature below which heat capacity and susceptibility measurements reveal a ferromagnetic ordering. High energy inelastic neutron scattering data reveal well-defined high-energy magnetic excitations near 264 meV at temperatures below the valence transition. Low energy INS data show a broad magnetic excitation centered at 50 meV above the valence transition, but four well-defined magnetic excitations at 7 K. The high energy excitations are assigned to the ${\mathrm{Pr}}^{4+}$ ions in the cubic phase and the low energy excitations to the ${\mathrm{Pr}}^{3+}$ ions in the monoclinic phase. Further direct evidence of the Pr valence transition has been obtained from the x-ray absorption spectroscopy. The results on the $x=0.1$ compound are compared with those for $x=0$ and 1.

Figure 1

(a) Temperature variation of the ZFC and FC susceptibility of ${\mathrm{Ba}}_2{\mathrm{PrRu}}_{0.9}{\mathrm{Ir}}_{0.1}{\mathrm{O}}_6$ in a measuring field of $H=1$ kOe. The top left inset shows the susceptibility behavior at low temperature, indicating ferromagnetic ordering below 3 K, and the top right inset shows the magnetization isotherm at 1.8 K. (b) Heat capacity data, plotted as $C_p/T$ vs $T$, taken during heating and cooling cycles. The inset shows the low temperature behavior of $C_p/T$ vs $T$, which indicates a magnetic anomaly near 2.5 K.

Figure 2

(a) Powder neutron diffraction patterns of ${\mathrm{Ba}}_2{\mathrm{PrRu}}_{0.9}{\mathrm{Ir}}_{0.1}{\mathrm{O}}_6$ obtained from HRPD in the temperature range 5 to 300 K. Panels (b) and (c) show Rietveld refinements of diffraction data measured at 5 K and 300 K, respectively. Black circles indicate the experimental data points and red solid lines are the simulated curves. The Bragg peak positions of the monoclinic and cubic phases are indicated by blue and maroon vertical ticks, respectively, and the light-blue line shows the difference between the data and the simulated curve.

Figure 3

Temperature dependence of structural parameters of ${\mathrm{Ba}}_2{\mathrm{PrRu}}_{0.9}{\mathrm{Ir}}_{0.1}{\mathrm{O}}_6$. (a) Fraction of monoclinic (space group $P{2}_1/n$) and cubic (space group $Fm\overline{3}m$) phases. (b) Monoclinic lattice parameters $a_{\mathrm{mono}}$ and $b_{\mathrm{mono}}$. (c) Monoclinic lattice parameter $c_{\mathrm{mono}}$ and cubic lattice parameter $a_{\mathrm{cub}}$. (d) Temperature variation of the monoclinic angle ${\beta }_{\mathrm{mono}}$. (e) Single peak analysis which shows the variation of the fraction of monoclinic phase (containing ${\mathrm{Pr}}^{3+}$) and cubic phase (containing ${\mathrm{Pr}}^{4+}$) in warming and cooling measurements. (f) The $d$ spacing of the peaks shown in (e).

Figure 4

(a) Rietveld refinement of powder neutron diffraction data recorded on D20 at 2 K. Black points indicate the experimental data points and the red solid line is the simulated curve. The structural Bragg reflections are indicated by blue and maroon vertical ticks for the monoclinic and cubic phases, respectively, and red ticks mark the magnetic Bragg reflections from the monoclinic phase. Inset: the (001) magnetic reflection together with fitted peak. Panel (b) shows the same for ${\mathrm{Ba}}_2{\mathrm{PrRuO}}_6$ with blue and red ticks for the structural (monoclinic) and magnetic Bragg reflections.

Figure 5

(a) Temperature dependence of Pr and Ru moment of the monoclinic phase in ${\mathrm{Ba}}_2{\mathrm{PrRu}}_{0.9}{\mathrm{Ir}}_{0.1}{\mathrm{O}}_6$. Blue stars indicate the volume fraction of the cubic phase (paramagnetic) with temperature. Panels (b) and (c) depict the alignment of Pr and Ru spins in a magnetic unit cell of ${\mathrm{Ba}}_2{\mathrm{PrRu}}_{0.9}{\mathrm{Ir}}_{0.1}{\mathrm{O}}_6$ and ${\mathrm{Ba}}_2{\mathrm{PrRuO}}_6$, respectively.

Figure 6

Neutron scattering spectra of ${\mathrm{Ba}}_2{\mathrm{PrRu}}_{0.9}{\mathrm{Ir}}_{0.1}{\mathrm{O}}_6$. Panels (a) and (b) show color-coded intensity maps as a function of energy transfer ($\hslash \omega$) and momentum transfer ($\left|\mathbf{Q}\right|$) at 5 K and 222 K, respectively, measured with an incident energy $E_{\mathrm{i}}=100$ meV. Panels (c) and (d) display the same plots for $E_{\mathrm{i}}=500$ meV.

Figure 7

$Q$-averaged INS intensity as a function of energy transfer at different $E_i$ (= 60, 100, 200, and 500 meV). Left and right panels show the low-$Q$ and high-$Q$ regimes, respectively.

Figure 8

INS spectrum of ${\mathrm{Ba}}_2{\mathrm{PrRu}}_{0.9}{\mathrm{Ir}}_{0.1}{\mathrm{O}}_6$ measured on HET with $E_{\mathrm{i}}=15$ meV. Panels (a), (b), and (c) show the color-coded intensity maps as a function of energy transfer and $Q$ recorded at 5 K, 100 K, and 222 K. Panel (d) is the $Q$-averaged scattering at the same temperatures as a function of energy transfer.

Figure 9

INS spectra of ${\mathrm{Ba}}_2{\mathrm{PrRu}}_{0.9}{\mathrm{Ir}}_{0.1}{\mathrm{O}}_6$ at fixed scattering angle $\varphi$ as a function of energy transfer at 5 K, for incident energies of (a) $E_{\mathrm{i}}=900$ meV, averaged from $\varphi =3$ to ${10}^{\circ }$ ($\langle \varphi \rangle =6.5^{\circ }$), and (b) $E_{\mathrm{i}}=500$ meV, averaged from $\varphi =3$ to ${12}^{\circ }$ ($\langle \varphi \rangle =7.5^{\circ }$). The solid lines show the simulated INS spectra for ${\mathrm{Pr}}^{4+}$ (blue) and ${\mathrm{Pr}}^{3+}$ (red) calculated from the cubic CEF model described in the text. (c) Energy level scheme for the $J=5/2$ and $J=7/2$ levels of ${\mathrm{Pr}}^{4+}$ calculated from the CEF model.

Figure 10

XANES spectra at various temperatures are depicted for (a) ${\mathrm{Ba}}_2{\mathrm{PrRu}}_{0.9}{\mathrm{Ir}}_{0.1}{\mathrm{O}}_6$, (b) ${\mathrm{Ba}}_2{\mathrm{PrRuO}}_6$, and (c) ${\mathrm{Ba}}_2{\mathrm{PrIrO}}_6$. The inset of (a) shows an enlarged view of the anomalies near 5.966 keV and 5.979 keV. Panel (d) shows the calculated spectra of ${\mathrm{Ba}}_2{\mathrm{PrRu}}_{0.9}{\mathrm{Ir}}_{0.1}{\mathrm{O}}_6$ from the observed spectra of ${\mathrm{Ba}}_2{\mathrm{PrRuO}}_6$ and ${\mathrm{Ba}}_2{\mathrm{PrIrO}}_6$ at 77 K. The inset of (d) shows an enlarged view of the anomalies near 5.95 keV and 5.99 keV.