A Novel Diamagnetic Insulating Quadruple Perovskite Oxide YCu3Rh4O12

Ikuya Yamada; Yuta Kato; Hiroshi Nakajima; Hidekazu Ikeno; Shigeo Mori; Shogo Kawaguchi

doi:10.2320/matertrans.MT-MG2022005

Abstract

A novel oxide YCu₃Rh₄O₁₂ has been obtained using high-pressure and high-temperature conditions of 12 GPa and 1573 K. Electron diffraction and synchrotron X-ray powder diffraction data demonstrates that YCu₃Rh₄O₁₂ crystallizes in a cubic AA′₃B₄O₁₂-type quadruple perovskite structure. The valence state is estimated to be Y³⁺Cu³⁺₃Rh³⁺₄O₁₂ by X-ray absorption spectroscopy. The electric resistivity and magnetization data prove that YCu₃Rh₄O₁₂ is a diamagnetic insulator, which is expected from the electron configurations of Cu³⁺ (3d⁸, low spin, S = 0) and Rh³⁺ (4d⁶, low spin, S = 0) ions. The first-principle calculation displays the insulating band structure for YCu₃Rh₄O₁₂. The valence state transition from Ca²⁺Cu^2.8+₃Rh^3.4+₄O₁₂ to Y³⁺Cu³⁺₃Rh³⁺₄O₁₂ indicates that the doped electrons by the substitution of Y³⁺ for Ca²⁺ are not simply injected to Cu and/or Rh ions, realizing unusual charge redistributions consisting of the simultaneous Cu oxidation (Cu^2.8+ → Cu³⁺) and Rh reduction (Rh^3.4+ → Rh³⁺).

1. Introduction

Quadruple perovskite oxides AA′₃B₄O₁₂, a derivative of perovskite-related structures, have been extensively investigated in recent years.¹^–³⁾ The properties of the quadruple perovskite oxides widely range between large magnetoresistance,⁴⁾ ferrimagnetic half-metallicity,⁵⁾ intersite charge-transfer,⁶^,⁷⁾ negative thermal expansion,⁸^–¹⁰⁾ thermal storage,¹¹⁾ multiferroics,¹²^,¹³⁾ metamagnetism,¹⁴⁾ and electrocatalysis.¹⁵^–¹⁸⁾ Most of the reported properties are derived from the multiple transition metal ions at distinct crystallographic sites of pseudosquare-planar A′-site and octahedral B-site. Since the charge distributions in A′- and B-sites are not unambiguously determined,¹⁹^–²¹⁾ various electromagnetic properties are expected even in the nominally isoelectronic compounds: e.g., ferrimagnetism for YCu₃Fe₄O₁₂ but antiferromagnetism for LaCu₃Fe₄O₁₂.⁶^,²²⁾

The Rh-containing quadruple perovskite CaCu₃Rh₄O₁₂ possesses a mixed valence state of Ca²⁺Cu^2.8+₃Rh^3.4+₄O₁₂,²³⁾ which is located at intermediate between Ca²⁺Cu³⁺₃Co^3.25+₄O₁₂ and Ca²⁺Cu²⁺₃Ir⁴⁺₄O₁₂ along the group 9 element column. In the Co-containing quadruple perovskite ACu₃Co₄O₁₂ series consisting of the divalent, trivalent, and tetravalent A-site ions (Ca²⁺, Y³⁺, and Ce⁴⁺), the valence states sequentially change from Ca²⁺Cu³⁺₃Co^3.25+₄O₁₂ to Y³⁺Cu³⁺₃Co³⁺₄O₁₂ to Ce⁴⁺Cu^2.67+₃Co³⁺₄O₁₂.¹⁹⁾ The Ir-containing ACu₃Ir₄O₁₂ series exhibits a simple valence state transition between just two examples of Ca²⁺Cu²⁺₃Ir⁴⁺₄O₁₂ and La³⁺Cu²⁺₃Ir^3.75+₄O₁₂.²⁴^,²⁵⁾ ACu₃Rh₄O₁₂ series has not been well investigated, although Ca²⁺Cu^2.8+₃Rh^3.4+₄O₁₂ is expected to undergo intriguing valence state transition by hole- or electron-doping.

In this paper, we investigate the valence states and physical properties of the novel Rh-containing quadruple perovskite YCu₃Rh₄O₁₂. YCu₃Rh₄O₁₂ was successfully synthesized under high-pressure and high-temperature conditions of 12 GPa and 1573 K, crystallizing in a cubic quadruple perovskite structure. X-ray absorption spectroscopy revealed the valence state of Y³⁺Cu³⁺₃Rh³⁺₄O₁₂, where simultaneous valence changes in Cu (Cu^2.8+ → Cu³⁺) and Rh (Rh^3.4+ → Rh³⁺) from Ca²⁺Cu^2.8+₃Rh^3.4+₄O₁₂ were induced. YCu₃Rh₄O₁₂ was a diamagnetic insulator, which can be interpreted by the electron configurations of nonmagnetic Cu³⁺ (3d⁸, low spin, S = 0) and Rh³⁺ (4d⁶, low spin, S = 0) ions. The insulating band structure was corroborated by the first-principle calculation.

2. Experimental

Y₂O₃ (99.9%), CuO (99.5%), and Rh₂O₃ (99.9%) at a molar ratio of 1:6:4 were mixed using a mortar to obtain the nominal composition of YCu₃Rh₄O_12−δ (δ = 1.5). An oxidizing agent KClO₄ was added to the mixture to compensate for the oxygen deficiency, preparing the starting mixture. The starting mixture was pelletized, put into an aluminum tube, and covered with Pt film to avoid a reaction between the sample and Pt. The capsule was placed in a (Mg, Co)O pressure-transmitting medium with an octahedral shape to compose a high-pressure cell. The high-pressure cell was surrounded by eight WC anvils. The high-pressure cell was compressed up to 12 GPa using a Walker-type high-pressure apparatus in a few hours. After the compression, the sample was heated to 1573 K for 15 min and held at this temperature for 45 min. The pressure was released in several hours after the heat treatment. The sample was separated by grinding the aluminum tube. A KCl byproduct was removed by washing the as-prepared sample with distilled water several times.

Synchrotron X-ray powder diffraction (SXRD) pattern at room temperature was collected at the BL02B2 beamline of SPring-8.²⁶⁾ The powder sample was filled into a Lindemann glass capillary with an inner diameter of 0.2 mm. The wavelength was determined to be 0.42034 Å using the NIST SRM 672b CeO₂ reference. Structural parameters were refined by using the RIETAN-FP program,²⁷⁾ and the crystal structure model was illustrated using the VESTA-3 program.²⁸⁾ Electron diffraction (ED) patterns at room temperature were collected using a transmission electron microscope accelerated at 200 kV (JEM-2100F, JEOL Co. Ltd.). X-ray absorption spectra at K-edges of Cu and Rh were collected in the transmission mode at the BL14B2 beamline of SPring-8. Electric resistivity was measured by the standard four-probe method using a Physical Properties Measurement System (PPMS, Quantum Design Inc.). Magnetic susceptibility data were obtained in a temperature range between 5 and 300 K in an external field of 1 kOe on field-cooling mode with a superconducting quantum interference device (SQUID, MPMS3, Quantum Design Inc.). The isothermal magnetization curves were collected at 5 and 300 K in an external magnetic field between −50 and 50 kOe.

The electronic structure of YCu₃Rh₄O₁₂ was calculated by the plane-wave basis projector augmented wave method as implemented in the VASP code.²⁹^–³¹⁾ The spin-unpolarized calculations were performed with the Perdew-Burke-Ernzerhof (PBE) functional to describe the exchange-correlation interactions.³²⁾ The plane-wave cutoff energy was set to 550 eV. The strong on-site Coulombic interactions on the localized d electrons were treated with the GGA+U approach. The U_eff parameters were 4.0 and 3.3 eV for Cu 3d and Rh 4d, respectively.³³⁾ Geometry optimization was performed using the 4 × 4 × 4 Monkhorst-Pack grid to evaluate the integrals over the Brillouin zone. The lattice constants and internal coordinates were optimized until the total energy difference and residual forces converged to less than 10⁵ eV and 10⁻² eV Å⁻¹, respectively. The density of states (DOS) was calculated using a finer k-point mesh with 12 × 12 × 12.

3. Results and Discussion

A single-phase YCu₃Rh₄O₁₂ sample was obtained. Figure 1(b) shows the SXRD pattern at room temperature and the Rietveld refinement result. The Bragg reflections were indexed by the cubic AA′₃B₄O₁₂-type quadruple perovskite structure with the $Im\bar{3}$ space group (No. 204). All the ED patterns along [001], $[1\bar{1}0]$, and [111] directions were indexed with the $Im\bar{3}$ space group in Fig. 1(c). The d_hkl spacing values were calculated to be d₂₀₀ ∼ 3.73 Å and d₁₁₀ ∼ 5.25 Å, estimating the cubic lattice constant of a ∼ 7.4 Å. We adopted a stoichiometric structure model with Y at 2a site, Cu at 6b site, Rh at 8c site, and O at 24g site for the structure refinement. The structure parameters obtained from the final refinement are listed in Table 1. In addition to the small deviation between observed and calculated patterns in the Rietveld refinement result (Fig. 1(b)), the small reliability factors (R_wp = 3.229% and R_B = 7.067%) demonstrated that the refinement result is reasonable. The refined lattice constant of a = 7.40774(8) Å for YCu₃Rh₄O₁₂ was comparable to that for CaCu₃Rh₄O₁₂ (7.39267 Å).³⁴⁾ The calculated Cu–O bond length (1.937 Å) for YCu₃Rh₄O₁₂ was larger than that for CaCu₃Rh₄O₁₂ (1.878 Å), whereas the Rh–O bond length (1.994 Å) for YCu₃Rh₄O₁₂ was almost identical to that for CaCu₃Rh₄O₁₂ (1.993 Å). Although the elongated Cu–O bond typically indicates the reduction in Cu valence, the X-ray absorption spectroscopy revealed the Cu oxidation, as discussed below. The reason why the refined structure did not reflect the actual valence states of transition metal ions for YCu₃Rh₄O₁₂ is unclear at the present stage.

Fig. 1

(a) Schematic of the crystal structure of YCu₃Rh₄O₁₂. (b) Rietveld refinement result of the SXRD pattern of YCu₃Rh₄O₁₂. The circles (black) and solid lines (red) represent the observed and calculated patterns, respectively. The difference between the observed and calculated patterns is shown at the bottom (blue). The above vertical marks (green) indicate the Bragg reflection positions. (c) Electron diffraction patterns along [001], $[1\bar{1}0]$, and [111] directions for YCu₃Rh₄O₁₂.

Table 1 Structure parameters for YCu₃Rh₄O₁₂ at room temperature refined by the Rietveld refinement of SXRD data.

The valences of Cu and Rh ions were examined by X-ray absorption spectroscopy. Figure 2(a) shows the Rh K-edge X-ray absorption spectrum of YCu₃Rh₄O₁₂. The onset of the absorption of YCu₃Rh₄O₁₂ (∼23210 eV) was almost identical to that of LaMn³⁺₃Rh³⁺₄O₁₂, suggesting that the Rh ions are close to trivalent for YCu₃Rh₄O₁₂. However, the slight difference at the higher energy side (∼23230 eV) disturbs the precise estimation of the Rh valence, thus we later discuss the total valence state of YCu₃Rh₄O₁₂ consistent with the magnetic and electric resistivity data. Figure 2(b) displays the Cu K-edge X-ray absorption spectrum of YCu₃Rh₄O₁₂. The absorption edge for YCu₃Rh₄O₁₂ was shifted about 1.5 eV to higher energy compared to the Cu²⁺ reference of CaCu₃Ru₄O₁₂. The substantial difference in Cu K-edge absorption indicates that the Cu ions in YCu₃Rh₄O₁₂ are close to trivalent, but the precise estimation was difficult, as well as Rh valence. Therefore, we discuss the precise valence state based on the physical properties as shown below.

Fig. 2

X-ray absorption spectra at (a) Cu and (b) Rh K-edges for YCu₃Rh₄O₁₂.

Figure 3(a) illustrates the electric resistivity data for YCu₃Rh₄O₁₂. The measurement was conducted down to 55 K because the raw resistance exceeded the upper limit of the equipment below this temperature. The resistivity gradually increased on cooling, exhibiting the typical behavior of insulators. Figure 3(b) shows the temperature dependence of the magnetic susceptibility for YCu₃Rh₄O₁₂. A diamagnetism was predominant at room temperature, as observed in the M–H curve at 300 K in Fig. 3(c), whereas a paramagnetic component emerges at low temperature. The magnetic susceptibility in the temperature range between 15 and 300 K was fitted by using a formula: χ(T) = C/(T − θ) + χ_dia, where C, θ, and χ_dia represent Curie constant, Weiss temperature, and temperature-independent diamagnetic component, respectively. The parameters obtained from the fitting were C = 3.617(18) × 10⁻³ emu K mol⁻¹, θ = −10.60(16) K, and χ_dia = 1.7902(10) × 10⁻⁴ emu mol⁻¹. The negative χ_dia value reflects diamagnetism. The obtained C value was assigned to the extrinsic paramagnetic component originating from the tiny amount of Cu²⁺ (S = 1/2) localized spins on the grain boundary because the C value corresponds to only 0.3% of the total Cu ions. The above electronic properties of YCu₃Rh₄O₁₂ are reasonably interpreted by the Cu³⁺ and Rh³⁺ valence states, concluding that the valence state is represented as YCu³⁺₃Rh³⁺₄O₁₂.

Fig. 3

(a) Temperature dependence of the electric resistivity for YCu₃Rh₄O₁₂. (b) Temperature dependence of the magnetic susceptibility for YCu₃Rh₄O₁₂ measured on field cooling in an external field of 1000 Oe. The circles and the curve represent the observed data and the fitting result, respectively. (c) Isothermal magnetization curves at 5 and 300 K for YCu₃Rh₄O₁₂.

The electron configuration of the pseudosquare-coordinatated Cu³⁺ ions is a nonmagnetic low-spin state (total spin quantum number, S = 0), as reported in the Cu³⁺-containing quadruple perovskite oxides such as LaCu³⁺₃Fe³⁺₄O₁₂ and YCu³⁺₃Co³⁺₄O₁₂.³⁵^–³⁷⁾ The Rh³⁺ ions in octahedral coordination possess a low-spin configuration (t_2g⁶e_g⁰) because of the large crystal-field splitting between t_2g and e_g orbitals in 4d electron system. The nonmagnetic states of constituent transition metal ions result in the absence of spin magnetic moments, as illustrated in Fig. 4(a). Substantial splitting energies between fully occupied and unoccupied orbitals are expected for both Cu³⁺ and Rh³⁺, resulting in the insulating band structure with an energy gap. Hence, the diamagnetic insulating properties are expected for YCu₃Rh₄O₁₂.

Fig. 4

(a) Schematic energy diagram of 3d orbitals of Cu³⁺ in square coordination and 4d orbitals of Rh³⁺ in octahedral coordination. (b) Calculated total DOS and partial DOS for YCu₃Rh₄O₁₂.

Figure 4(b) displays the total and partial DOS of YCu₃Rh₄O₁₂ obtained from the first-principle calculation. The valence band down to −7.5 eV consisted of the Cu, Rh, and O bands, whereas the band originating from Y was located at higher energy (>5.5 eV) above Fermi energy. The lowest conduction band was located at 0.3 eV above Fermi energy, exhibiting a substantial band gap.

The above experimental and theoretical study demonstrated that the novel quadruple perovskite oxide YCu₃Rh₄O₁₂ is a diamagnetic insulator, which may be a limited example among several ten kinds of the reported quadruple perovskite oxides. We suggest that the valence-state transformation from CaCu₃Rh₄O₁₂ to YCu₃Rh₄O₁₂ by electron doping is not straightforward because the Cu valence increased from +2.8 to +3, whereas the Rh valence decreased from +3.4 to +3. This can be divided into the following sequence: (1) electron doping to Rh ions (4Rh^3.4+ + e⁻ → 4Rh^3.15+) and (2) charge transfer between Cu and Rh (3Cu^2.8+ + 4Rh^3.15+ → 3Cu³⁺ + 4Rh³⁺). The unusual electronic state transition is not reported in the multielement transition metal oxides, suggesting the complex nature of the quadruple perovskite oxide system.

4. Conclusions

We synthesized the quadruple perovskite oxide YCu₃Rh₄O₁₂ by using high-pressure and high-temperature conditions of 12 GPa and 1573 K. The X-ray absorption spectroscopy revealed the valence state of Y³⁺Cu³⁺₃Rh³⁺₄O₁₂ with Cu³⁺ (3d⁸, low spin, S = 0) and Rh³⁺ (4d⁶, low spin, S = 0) electron configurations, explaining the diamagnetic insulating nature. The first-principle calculation unveiled the insulating band structure. The valence state transition from Ca²⁺Cu^2.8+₃Rh^3.4+₄O₁₂ to Y³⁺Cu³⁺₃Rh³⁺₄O₁₂ is an unusual example in the complex transition metal oxides.

Acknowledgments

The synchrotron X-ray experiment was performed at SPring-8 under the approval of the JASRI (proposal numbers 2022A1493 and 2022B1619). This work was supported by JSPS KAKENHI (grant numbers JP20H02825, JP22H04512), the Asahi Glass Foundation, and the Tanikawa Fund Promotion of Thermal Technology.

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