High-pressure structural and electronic properties of $\mathrm{Cu}M{\mathrm{O}}_2$ $(M=\mathrm{Cr}, \mathrm{Mn})$ delafossite-type oxides

We report high-pressure x-ray diffraction, x-ray absorption spectroscopy, and electrical transport measurements on $\mathrm{Cu}M{\mathrm{O}}_2$ ($M=\mathrm{Cr}$, Mn) delafossitelike oxides in an attempt to study their structural and electronic evolution with pressure. Recent studies of the similar $\mathrm{CuFe}{\mathrm{O}}_2$ delafossite has revealed a pressure-induced breaking of the unusual high axial anisotropy resulting in a structural phase transition coinciding with the metal-metal intervalence charge-transfer phenomenon. The present study revealed other possible scenarios responsible for the collapse of the high axial anisotropy and evolution of the O-Cu-O bonds in delafossitelike materials under pressure. Thus in $\mathrm{CuMn}{\mathrm{O}}_2$, the O-Cu-O dumbbells tilt with respect to the $c$ axis at $P>13$ GPa, but in contrast to $\mathrm{CuFe}{\mathrm{O}}_2$, the tilting is continuous with pressure increase, justifying a second-order phase transition within the C2/m structure. Meanwhile in $\mathrm{CuCr}{\mathrm{O}}_2$ ($R\overline{3}m$) the first-order structural phase transition to the monoclinic structure ($P{2}_1/m$) is observed at about 26 GPa characterized by the discontinuous bending of the O-Cu-O bond in contrast to the tilting in the case of $\mathrm{CuFe}{\mathrm{O}}_2$ and $\mathrm{CuMn}{\mathrm{O}}_2$. In both studied systems, we did not find clear evidence of valence transformations, similar to that observed in $\mathrm{CuFe}{\mathrm{O}}_2$.

Figure 1

The crystal structure ($C2/m$) of $\mathrm{CuMn}{\mathrm{O}}_2$ at 3 GPa (a) and at 40 GPa (b). The crystal structures of $\mathrm{CuCr}{\mathrm{O}}_2$: $R\overline{3}m$ at 3 GPa (c) and $P{2}_1/m$ at 31 GPa (d) (HP1 phase). The large magenta (shiny) and small yellow (shiny) spheres correspond to the $\mathrm{C}{\mathrm{u}}^{1+}$ and ${\mathrm{O}}^{2-}$ ions, large -green spheres to the $\mathrm{M}{\mathrm{n}}^{3+}$/$\mathrm{C}{\mathrm{r}}^{3+}$, respectively.

Figure 2

X-ray powder-diffraction patterns of $\mathrm{CuMn}{\mathrm{O}}_2$ at RT at various pressures.

Figure 3

Typical examples of analyzed integrated patterns of $\mathrm{CuMn}{\mathrm{O}}_2$ collected at 1.8 and 39 GPa, respectively, assuming a $C2/m$ structure. The refinements have been performed on 33 observables and 20 variables, ${\chi }^2$ is about 1.8. In the diffractogram at 39 GPa the Ne peaks are the ones with black tick marks ($\lambda =0.28953\AA$).

Figure 4

Variations of normalized unit-cell parameters (a), the monoclinic angle β (b), crystal anisotropy (c), calculated as the c*sinβ/$a$ ratio, and unit-cell volume (d) of $\mathrm{CuMn}{\mathrm{O}}_2$ as a function of pressure. The solid and dashed lines are fits for two pressure ranges, 0–13.5 GPa and 15.7–45 GPa, using BM EOS.

Figure 5

X-ray powder-diffraction patterns of $\mathrm{CuCr}{\mathrm{O}}_2$ at RT at various pressures.

Figure 6

Typical examples of analyzed integrated patterns of $\mathrm{CuCr}{\mathrm{O}}_2$ collected at 2.8 and 32.5 GPa, respectively, assuming $R\overline{3}m$ symmetry for low-pressure phase and a $P{2}_1/m$ for the high-pressure phase ($\lambda =0.28953\AA$). The refinements for the hexagonal phase have been performed on 50 observables and 24 variables, ${\chi }^2$ is about 3; for the monoclinic phase, on 50 observables and 20 variables, ${\chi }^2$ is about 2.4.

Figure 7

Variations of the normalized lattice parameters ($a$), unit-cell volume (c), and crystal anisotropy (b), calculated as the $c/a$ ratio of $\mathrm{CuCr}{\mathrm{O}}_2$ as a function of pressure. The solid and dashed lines are theoretical fits for the $R\overline{3}m$ and $P{2}_1/m$ phases using BM2 EOS. The volume was normalized to three unit formulas of $\mathrm{CuCr}{\mathrm{O}}_2$, i.e., $Z=3$ for $R\overline{3}m$ phase and $Z=2$ for $P{2}_1/m$ phase.

Figure 8

Variation of the mean Cr-O, Cu-O distances and O-Cu-O bond angle in $\mathrm{CuCr}{\mathrm{O}}_2$ as a function of pressure.

Figure 9

Cu and Mn K-edge XANES spectra of $\mathrm{CuMn}{\mathrm{O}}_2$ at various pressures.

Figure 10

The energy of the Cu and Mn K-edge absorption onset in $\mathrm{CuMn}{\mathrm{O}}_2$ (a), (b), and the energy of the Cu and Cr K-edge absorption onset in $\mathrm{CuCr}{\mathrm{O}}_2$ (c), (d) as a function of pressure.

Figure 11

Cu and Cr K-edge XANES spectra of $\mathrm{CuCr}{\mathrm{O}}_2$ at various pressures.

Figure 12

(a) Pressure dependence of the resistance at 300 K for $\mathrm{CuCr}{\mathrm{O}}_2$. (b) Temperature dependence of the resistance measured at various pressures. (c) Linearized temperature-dependent data at various pressures assuming variable range-hopping mechanism, $\ln R\propto {(T_0/T)}^{1/4}$. The Mott temperature $T_0$ is obtained from the slope of linear fits to these plots for two temperature ranges, above and below ∼200 K. Inset [panel (a)] shows the pressure dependence of $T_0$ for two temperature ranges: above and below ∼200 K (squares and triangles, respectively).

Figure 13

Pressure dependence of the α-angle between the sheets of $M^{3+}\text{−}\mathrm{O}$ octahedrons and linear ($\mathrm{O}\text{−}\mathrm{C}{\mathrm{u}}^{1+}\text{−}\mathrm{O}$) dumbbells for $\mathrm{CuMn}{\mathrm{O}}_2$.