Structural and electronic properties of the first iridium containing mixed B-site spinel oxide: $\mathrm{Cu}\left[{\mathrm{Ir}}_{1.5}{\mathrm{Cu}}_{0.5}\right]{\mathrm{O}}_4$

Geometrically frustrated systems populated with large spin-orbit coupled ions are an ideal setting for the exploration of novel exotic states of matter. Here we present an example of iridium on a mixed B-site spinel oxide structure: $\mathrm{Cu}\left[{\mathrm{Ir}}_{1.498\left(2\right)}{\mathrm{Cu}}_{0.502\left(2\right)}\right]{\mathrm{O}}_4$. Synchrotron XRD refinements reveal a face-centered-cubic structure with space group $Fd\overline{3}m$ and mixed Cu-Ir site disorder within the ${\mathrm{B}}_2{\mathrm{O}}_4$ rocksalt substructure. Electrical properties reveal a metallic state within the 50–600-K range with a Kondo effect at $T<50\mathrm{K}$. X-ray absorption spectroscopy (XAS) measurements show a mixed ${\mathrm{Cu}}^{1+/2+}$ and ${\mathrm{Ir}}^{3+/4+}$ charge partitioned picture, which suggests a metallic/band description with reduced on-site Coulomb interactions. Spin-glass-like freezing is seen at $T_{\mathrm{g}}=49\mathrm{K}$, and the hysteresis behavior for $T>T_{\mathrm{g}}$ resembles that of a strongly frustrated magnet. DFT calculations show sizable hybridization between the Cu $3d$ and Ir $5d$ states with an effective mixed ${\mathrm{Ir}}^{3+/4+}$ charge partitioned picture, supporting the electronic and XAS results.

Figure 1

Illustration of the $A{B}_2{X}_4$ spinel structure: isolated $A$-cation tetrahedral sites and the $B_2{X}_4$ rocksalt network where 1/2 of the octahedral sites are occupied (top). The $A$ ions form a diamond sublattice and the $B$ ions form a sublattice of corner-shared tetrahedra within the spinel structure (bottom).

Figure 2

Powder XRD pattern of $\mathrm{Cu}\left[{\mathrm{Ir}}_{1.498\left(2\right)}{\mathrm{Cu}}_{0.502\left(2\right)}\right]{\mathrm{O}}_4$ sample. Blue asterisk symbol indicates a small presence of $\mathrm{Ir}{\mathrm{O}}_2$ impurity phase.

Figure 3

Synchrotron Rietveld refinement of $\mathrm{Cu}\left[{\mathrm{Ir}}_{1.498\left(2\right)}{\mathrm{Cu}}_{0.502\left(2\right)}\right]{\mathrm{O}}_4$ sample at room temperature ($\lambda =0.41283\AA$). Observed (black crosses) and calculated (solid red line) profiles, background (green), and difference curve $\left(I_{\mathrm{obs}}\text{−}{I}_{\mathrm{calc}}\right)$ (blue) are shown. The vertical bars indicate the expected reflection positions for spinel ($Fd\overline{3}m$) phase (black) and $\mathrm{Ir}{\mathrm{O}}_2$ impurity phase (purple).

Figure 4

Cu-$K$-edge and $\mathrm{Ir}\text{−}{L}_3$-edge EXAFS data and model. Real and imaginary parts, as well as magnitude, of the (complex) Fourier transform of $k^2$ weighted EXAFS data and model are shown in the real space plots, and back Fourier transformed data and model are shown in photoelectron wave number ($k$) space. Black line: data; red line: constrained model using the Ir/Cu partitioning and distances obtained in refinements of XRD data.

Figure 5

Top: High temperature resistivity (black circles) and Seebeck (blue triangles) data of $\mathrm{Cu}\left[{\mathrm{Ir}}_{1.498\left(2\right)}{\mathrm{Cu}}_{0.502\left(2\right)}\right]{\mathrm{O}}_4$ sample from ∼300 to ∼550 K. Bottom: Resistivity vs temperature for $\mathrm{Cu}\left[{\mathrm{Ir}}_{1.498\left(2\right)}{\mathrm{Cu}}_{0.502\left(2\right)}\right]{\mathrm{O}}_4$ in the range 2–300 K under 0- and 4-T field.

Figure 6

$\mathrm{Cu}\text{−}{L}_{2,3}$-edge x-ray absorption data for $\mathrm{Cu}\left[{\mathrm{Ir}}_{1.498\left(1\right)}{\mathrm{Cu}}_{0.502\left(1\right)}\right]{\mathrm{O}}_4$ spinel system measured in partial fluorescence yield (PFY) together with Cu 1+ and Cu 2+ reference spectra taken from the literature (Ref. [29]). Data were normalized to Cromer-Liberman (C-L) calculations of the single-atom x-ray absorption cross section (Ref. [30]).

Figure 7

Cu-$K$-edge x-ray absorption spectra for $\mathrm{Cu}\left[{\mathrm{Ir}}_{1.498\left(1\right)}{\mathrm{Cu}}_{0.502\left(1\right)}\right]{\mathrm{O}}_4$ spinel system together with Cu 1+ and Cu 2+ reference spectra (right inset). The derivatives of these spectra are shown in the left inset. The main panel shows weighted averages of Cu +1 and +2 reference data to illustrate the expected enhancement of the pre-edge peak with increasing Cu 1+ content.

Figure 8

XAS measurements at the $\mathrm{Ir}\text{−}{L}_{2,3}$ absorption edges on $\mathrm{Cu}\left[{\mathrm{Ir}}_{1.498\left(2\right)}{\mathrm{Cu}}_{0.502\left(2\right)}\right]{\mathrm{O}}_4$ and two reference compounds with known oxidation state. The insets show the first derivatives of the XAS data.

Figure 9

RIXS wide (top) and narrow (bottom) ranges of the energy loss spectra for the $\mathrm{Cu}\left[{\mathrm{Ir}}_{1.498\left(2\right)}{\mathrm{Cu}}_{0.502\left(2\right)}\right]{\mathrm{O}}_4$ spinel and $\mathrm{Ir}{\mathrm{O}}_2$ and ${\mathrm{Ba}}_2\mathrm{YIr}{\mathrm{O}}_6$ systems.

Figure 10

(a) The zero-field-cooled (zfc) and field-cooled (fc) susceptibility vs temperature for $\mathrm{Cu}\left[{\mathrm{Ir}}_{1.498\left(2\right)}{\mathrm{Cu}}_{0.502\left(2\right)}\right]{\mathrm{O}}_4$ as measured at two different magnitudes of applied field. The hysteresis below the peak at 49 K is evidence for spin glass freezing. (b) The inverse susceptibility with a constant $3.65\times {10}^{\text{–}4}$ subtracted from $\chi \left(T\right)$ vs temperature. The straight line is a least squares fit as discussed in the text.

Figure 11

Density of states obtained using DFT + $U$ (top) and DFT + $U$ + SO (bottom) calculations on $\mathrm{Cu}\left[{\mathrm{Ir}}_{1.498\left(2\right)}{\mathrm{Cu}}_{0.502\left(2\right)}\right]{\mathrm{O}}_4$ spinel system. Cu1 means the Cu ion in the tetrahedral site and Cu2 means the Cu ion in the octahedral site. Hole occupancies in $d$ orbitals are obtained by integrating the partial DOS above the Fermi level.