Effect of Y³⁺ Doping on Microstructure and Magnetic Transition of CuCrO₂ Ceramics

Abstract

Ceramic samples of CuCr_1−xY_xO₂ (x = 0–0.02) were synthesized via the high temperature solid-state reaction method, and the influence of Y³⁺ doping on their microstructure and antiferromagnetic phase transitions was systematically investigated. Y³⁺ doping increased the unit cell volume from 130.928 ų for x = 0 to 131.147 ų for x = 0.0200, and the average grain size decreased from 3.38 μm for x = 0 to 4.27 μm for x = 0.0200. The Cr and Y elements maintained +3 valence, while the Cu element had +1 valence. All samples showed obvious paramagnetism when the temperature was higher than 140 K. When the temperature continued to decrease, the lattice expansion changed the bond length and bond angle of the Cr-O-Cr bond, resulting in a change in the superexchange interaction, and the magnetic susceptibility increased significantly, gradually showing antiferromagnetism. The T_N of the undoped sample was about 46 K, the T_N of the doped sample with x = 0.0175 was about 21 K, and the T_N of other doped samples was about 30 K. This result indicates that Y³⁺ doping enhanced the antiferromagnetism of the sample but also weakened its antiferromagnetic stability.

1. Introduction

Multiferroic materials, characterized by coexisting ferromagnetic, ferroelectric, or ferroelastic orders with cross-coupled interactions, have emerged as promising candidates for next-generation magnetoelectric devices and advanced information technologies due to their unique multifunctional responses [1,2,3]. Since Curie’s pioneering proposal of intrinsic magnetoelectric coupling in 1894, extensive research has focused on two principal categories: single-phase and composite multiferroics [4]. The ABO₂-type delafossite materials belong to the Type-II single-phase multiferroic materials, such as CuCrO₂. CuCrO₂ emerges as a particularly promising p-type transparent conductive oxide candidate owing to its inherent hole-dominated conductivity and exceptional visible-light transmittance, positioning it as a frontrunner for thermoelectric sensors, solar energy harvesting devices, and advanced display technologies [5,6,7,8]. A comparative analysis within Type-II single-phase multiferroic materials revealed that Cr-O bonding exhibits the strongest covalent character among B-site cations, establishing chromium as the optimal B-site constituent [9,10].

The delafossite compound CuCrO₂ crystallizes as a triangular antiferromagnetic spin-lattice structure, adopting the R-3m space group and C3v point group symmetry at ambient conditions [11]. Its layered architecture comprises alternating Cu-O-Cu dumbbell configurations along the c-axis and edge-sharing CrO₆ octahedra, collectively forming the characteristic delafossite framework. This geometrically frustrated triangular lattice configuration inherently hosts spin frustration, driving unconventional magnetic phase transitions whose microscopic mechanisms remain subjects of active investigation. The different states, synthesis methods, and microstructures of the materials can significantly affect their physical properties [12,13,14,15,16]. Ionic substitution has emerged as an effective strategy for modulating these magnetic functionalities [17,18,19]. Elkhouni et al. demonstrated that Zn²⁺ substitution induces spin dilution through localized magnetic moment suppression, thereby constraining antiferromagnetic domains. Conversely, Ti²⁺ doping triggers a dimensional crossover from anisotropic 3D antiferromagnetic (3D-AF) magnons to 2D-AF magnetic excitations [20,21]. A study by Gao revealed that growth-oriented lattice stresses during synthesis promote preferential c-axis orientation [22]. The coexistence of Fe³⁺ and Cr³⁺ introduces hole-mediated Fe³⁺-Cr³⁺ superexchange interactions, generating ferromagnetic ordering with progressively diminished saturation magnetization as the Fe content increases. Zhang et al. also successfully synthesized Sn⁴⁺-substituted CuCr_1−xSn_xO₂ (x = 0–0.05) ceramics via solid-state reaction, observing enhanced unit cell volumes without altering Cu⁺/Cr³⁺ oxidation states, while significantly modifying magnetic hysteresis characteristics [23].

These studies showed that CuCrO₂ delafossite material has rich and complex physical properties, and the study of its microstructure is helpful to understanding the magnetoelectric coupling effect with great application potential derived from its unique magnetic structure. However, the magnetic transition mechanism behind CuCrO₂ ceramics is still unclear, and further research is needed. Rare earth ion doping is less commonly used in the existing research to study its effect on the magnetic properties of CuCrO₂ ceramic materials. Therefore, Y³⁺ was selected as the doping element in this paper. On the one hand, Y³⁺ and Cr³⁺ have the same valence state, and Y³⁺ has no magnetism, which can avoid the too-complex influence on the magnetism of Y³⁺. On the other hand, the radius of Y³⁺ is larger than that of Cr³⁺, which allows us to study its influence on the microstructure and magnetic transformation of CuCrO₂ ceramics while minimizing the interference of other factors and explore the possible correlation. Therefore, in this paper, a series of Y³⁺-doped ceramic samples were prepared by the high-temperature solid-state reaction method using non-magnetic rare earth ions Y³⁺ to replace part of Cr³⁺. X-ray diffraction (XRD), Raman spectroscopy (Raman), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and the physical property measurement system (PPMS) were used to systematically study the effect of Y³⁺ doping on the lattice structure, element valence state, microstructure, and magnetic properties of CuCrO₂ with as little magnetic influence as possible. The correlation between the microstructure and magnetic transition was obtained, which provided theoretical and experimental evidence for its magnetic transition mechanism.

2. Materials and Methods

A series of CuCr_1−xY_xO₂ (x = 0–0.02) ceramic samples was fabricated via the high-temperature solid-state reaction method using high-purity CuO (99.5%), Cr₂O₃ (99.9%), and Y₂O₃ (99.9%) nanopowders. The three powders were stoichiometrically weighed, and raw material powders with different doping concentrations were obtained by adjusting the ratio of Cr₂O₃ and Y₂O₃ powders. The three raw material powders were mixed and poured into an appropriate amount of alcohol so that the powder could be completely dispersed into the alcohol, then stirred with an electric stirrer for 24 h to mix the powder evenly. The mixed powder was placed in a drying oven at 85 °C for 24 h. After the alcohol was completely evaporated, the powder was ground for 30 min to make the powder fine enough. The powder was then calcined in a tube furnace in an argon atmosphere (20 mL/min) at a temperature of 900 °C for 12 h. After cooling, the powder was ground again to make the powder fine enough, and it was pressed under a pressure of 12 MPa to make a disc with a diameter of 15 mm. The final ceramic sample was obtained by calcining it in a tubular furnace in an argon atmosphere (20 mL/min) and at a temperature of 1300 °C for 12 h. The synthesized CuCr_1−xY_xO₂ (x = 0–0.02) ceramics were systematically characterized for their microstructure and magnetic properties.

The phase composition of the CuCr_1−xY_xO₂ (x = 0–0.02) ceramics was characterized using a DX-2700A X-ray diffractometer (Dandong HaoYuan Instrument, Dandong City, Liaoning Province, China in English) equipped with Cu-Kα radiation (λ = 1.5418 Å). The XRD data were fitted by GSASII software (version: 3fc6dc from 14 May 2024 14:57), and the changes in the unit cell parameters and crystal density were analyzed. The tube voltage and tube current of the diffractometer were 40 kV and 30 mA, respectively. The step angle was 0.02°, the sampling time was 0.6 s, and the test angle range was 20–80°. Vibrational characteristics were probed using an iHR550 Raman spectrometer (Horiba, South District, Kyoto, Japan) with 532 nm laser excitation, mapping phonon mode modifications across the doping concentrations. The fracture surface microstructure and grain boundary evolution were analyzed through a JSM-6510A analytical scanning electron microscope (JEOL, Toshima City, Tokyo, Japan) operated at 30 kV with the fracture surface magnified 1500 times. Surface chemical states were quantified by ESCALAB Xi+ XPS (Thermo Fisher Scientific, Waltham, Massachusetts, USA), which used a monochromatic X-ray source, with energy calibration referencing the adventitious C 1 s peak at 284.8 eV. Magnetic properties were investigated using the VSM module of a comprehensive physical properties measurement system (Quantum Design, San Diego, California, USA). The temperature dependence of the magnetic susceptibility of the samples in the temperature range of 4 K–290 K was measured under an external magnetic field of 500 Oe. The hysteresis loops at different temperatures (4 K, 26 K, 50 K, 135 K, 145 K, 290 K) were also tested.

3. Results and Discussion

3.1. XRD Spectra

The XRD patterns of the CuCr_1−xY_xO₂ (x = 0–0.02) ceramics are presented in Figure 1a. All diffraction peaks exhibit sharp and well-defined profiles, indicative of highly crystalline sintered samples. No impurity phases associated with Y-containing compounds were detected, confirming the phase purity of the delafossite CuCrO₂ structure (PDF # 04-010-3330), which verifies the successful incorporation of Y³⁺ into the host lattice without altering its crystallographic phase. A magnified view of the (012) diffraction peak is shown in Figure 1b. Minimal peak shifts are observed for low doping levels (x = 0.0100), suggesting negligible lattice distortion at these concentrations. However, for x > 0.0100, systematic peak shifts toward lower angles occur, signifying progressive lattice expansion with increasing Y³⁺ content. Notably, the (012) peak position shifts monotonically to lower angles at x = 0.0125 and 0.0150, consistent with the larger ionic radius of Y³⁺ compared to Cr³⁺. At higher concentrations (x = 0.0175 and 0.0200), however, a non-monotonic shift pattern emerges: initial displacement to higher angles followed by a return to lower angles. This anomaly implies an optimal doping threshold at x = 0.0150, beyond which lattice distortion becomes dominant. Such behavior likely originates from point defect formation (cation vacancies or oxygen vacancies) that disrupts long-range crystallographic ordering.

Rietveld refinement of the diffraction data was performed using GSASII software. The results are shown in Figure 2 and

Table 1. Lattice parameters of CuCr_1−xY_xO₂ (x = 0–0.02) ceramic samples.

Sample	x = 0	x = 0.0100	x = 0.0125	x = 0.0150	x = 0.0175	x = 0.0200
a = b (Å)	2.9734	2.9761	2.9756	2.9751	2.9759	2.9755
c (Å)	17.1000	17.1086	17.1063	17.1043	17.1060	17.1045
c/a	5.7510	5.7487	5.7489	5.7492	5.7482	5.7484
Cell volume (Å3)	130.928	131.236	131.168	131.114	131.198	131.147
Density (g·cm−3)	5.6758	5.6253	5.6257	5.6124	5.6063	5.6087
RP (%)	9.47	7.88	6.67	6.65	7.47	7.08
GOF	1.62	1.33	1.16	1.16	1.28	1.22

. The R_P parameters of all samples are less than 10%, and the GOF parameters are less than 2, which proves that the refinement results are reliable. The refinement results shown in Table 1 show that Y³⁺ doping leads to lattice expansion (from 130.928 ų for x = 0 to 131.147 ų for x = 0.0200) and a decrease in density (from 5.6758 g·cm⁻³ for x = 0 to 5.6087 g·cm⁻³ for x = 0.0200). This is consistent with the expected lattice structure change caused by large-radius Y³⁺ doping, indicating that Y³⁺ successfully entered the lattice and replaced Cr³⁺. In addition, the c/a values of the doped samples are smaller than those of undoped samples, indicating that there is a clear preferential growth orientation in the doped samples.

The structural analysis confirms the successful substitution of Cr³⁺ by Y³⁺ in the CuCrO₂ lattice without secondary phase formation, preserving the delafossite structure (PDF # 04-010-3330). Low Y³⁺ doping (x = 0.0100) minimally perturbs the lattice structure. However, for x > 0.0100, lattice expansion becomes evident. The optimal doping threshold is identified at x = 0.0150, beyond which lattice distortion emerges. Despite these distortions, the overall structural integrity remains preserved, as evidenced by the absence of secondary phases and maintained crystallographic stability.

3.2. Raman Spectroscopy

To further investigate the impact of Y³⁺ doping on the lattice structure of CuCrO₂, Raman spectroscopy was conducted on the samples, as illustrated in Figure 3. Four distinct vibrational bands are observed across all compositions: A_g (208 cm⁻¹), E_g (455 cm⁻¹), P₁ (533 cm⁻¹), and A_1g (704 cm⁻¹), consistent with prior studies [24,25]. The A_1g phonon mode corresponds to atomic vibrations along the Cu-O bond direction, while the E_g mode arises from vibrations perpendicular to the Cu-O bonds. The P₁ mode originates from the native intrinsic acceptor point defects in CuCrO₂ [26]. These spectral features confirm that Y³⁺ doping does not change the delafossite structure of CuCrO₂. However, increasing the Y³⁺ concentration induces progressive attenuation of the E_g and A_1g mode intensities, accompanied by frequency downshifts. In contrast, the P₁ mode exhibits enhanced intensity until x = 0.0200, where its relative intensity normalizes, albeit with a significantly broadened full width at half-maximum. These observations indicate that Y³⁺ doping substantially modulates phonon interactions and molecular polarization in CuCrO₂ ceramics [27,28], weakening (Cr, Y)-O bonding. However, Y³⁺ doping does not change the delafossite structure of CuCrO₂, which is consistent with the results of XRD.

3.3. Scanning Electron Microscopy

SEM images of the fracture surfaces of CuCr_1−xY_xO₂ (x = 0–0.02) ceramics are presented in Figure 4. The undoped sample exhibits a smooth surface with well-defined interfaces, characteristic of a layered hexagonal structure. Pores formed during synthesis due to oxygen release are observed [29,30]. In contrast, Y³⁺-doped samples display a gradual transition from hexagonal-phase to amorphous-phase growth, accompanied by blurred grain boundaries. To further analyze the effect of Y³⁺ doping on the grain size, the SEM images were analyzed by ImageJ software (version: ij154-win-java8) and the average particle size was obtained. The results are shown in Figure 5. The results show that doping significantly changes the grain size of the samples. The average grain size of all doped samples is larger than that of undoped samples, and with an increase in the doping concentration, it shows a linear growth trend (except for the sample with x = 0.0175). The results of the particle size analysis are consistent with the XRD diffraction pattern, which further confirms that Y³⁺ doping leads to lattice expansion and a density reduction, hindering the synthesis of dense CuCrO₂ ceramics. Furthermore, Y³⁺ incorporation impedes intergranular fusion, promoting the formation of small precipitates through liquid-phase recrystallization. The content of precipitates reaches the maximum when x = 0.0150. In summary, the SEM results confirm the significant effect of Y³⁺ on the surface microstructure of CuCrO₂ ceramics.

3.4. X-Ray Photoelectron Spectroscopy

To investigate the influence of doping on elemental valence states, X-ray photoelectron spectroscopy (XPS) was conducted on CuCr_1−xY_xO₂ (x = 0, x = 0.01, and x = 0.02) ceramic samples. The survey spectra and high-resolution Y 3d, Cu 2p, and Cr 2p spectra are shown in Figure 6a–d. The undoped sample exhibits XPS signals exclusively from C, O, Cr, and Cu, with no detectable impurities. In contrast, Y signals emerge in the doped compositions (x = 0.01 and 0.02), confirming successful Y incorporation as designed. Carbon detection across all samples originates from ambient surface contamination. The Y 3d spectra (Figure 6b) display two sharp, symmetric peaks at 156.4 eV (Y 3d_5/2) and 158.4 eV (Y 3d_3/2), where Y 3d_5/2 indicates that the Y element is +3 valence [31]. The Cu 2p spectra (Figure 6c) reveal characteristic peaks at 932.45 eV and 952.30 eV, corresponding to Cu⁺ [32,33], with no satellite features near 940 eV that would indicate Cu²⁺. Similarly, the Cr 2p spectra (Figure 6d) exhibit peaks at 576.45 eV (2p_3/2) and 586.05 eV (2p_1/2), confirming Cr³⁺ valence states [34,35]. These findings collectively demonstrate preserved +3 oxidation states for Y and Cr alongside +1 valence for Cu. Combined with prior structural analyses, this confirms single-phase CuCrO₂ formation where Y³⁺ substitutes for Cr³⁺ in the lattice without inducing valence state alterations in the constituent elements, despite the observed lattice expansion effects [36].

3.5. Magnetic Properties

Figure 7a illustrates the temperature-dependent magnetic susceptibility χ of CuCr_1−xY_xO₂ (x = 0–0.02) ceramic samples across 4–290 K. For the undoped specimen, χ demonstrates a linear enhancement with decreasing temperature above 50 K, reaching a maximum value that signifies paramagnetic behavior. Upon further cooling, a progressive reduction in χ emerges, accompanied by a transition from paramagnetic to antiferromagnetic characteristics. The Néel temperature T_N of the undoped sample was determined as ~46 K, which exhibits elevation compared with previous reports [17,37]. This observation suggests that the 1300 °C sintering temperature critically modifies the low-temperature magnetic properties of CuCrO₂, effectively enhancing T_N and stabilizing the antiferromagnetic order. In contrast, Y³⁺-doped specimens manifest distinctly different χ evolution trends, indicating that Y³⁺ substitution substantially alters the magnetic ground state of CuCrO₂.

A distinct linear enhancement of χ with decreasing temperature above 140 K is observed across the doped samples, indicative of a characteristic paramagnetic response. Below this critical temperature, χ increases sharply and no longer increases linearly. After reaching the peak value, the magnetic susceptibility decreases gradually again, showing obvious antiferromagnetism. The χ-T curve of the doped sample shows that near 140 K, due to the introduction of non-magnetic Y³⁺, lattice expansion or even distortion occurs, which destroys the long-range antiferromagnetic order; this results in some magnetic moments that cannot be completely offset, forming short-range order or spin tilt, so the doped sample exhibits higher magnetic susceptibility. The x = 0.0175 sample exhibits T_N ~21 K, while other doped counterparts demonstrate T_N~30 K, with an additional distinctive peak emergence at 11 K for the x = 0.0100 specimen. Notably, the doped systems display marked suppression of T_N when compared to undoped CuCrO₂. These findings indicate that Y³⁺ doping weakens the antiferromagnetic stability of CCO ceramics at low temperatures, which changes the original magnetic configuration and interaction of spin-triangular-lattice CuCrO₂ ceramics, thus forming new magnetic transition points.

The Curie–Weiss temperatures (T_θ) derived from a linear regression analysis of χ⁻¹-T curves (Figure 7b) within the paramagnetic regime are summarized in

Table 2. Curie–Weiss temperatures for CuCr_1−xY_xO₂ (x = 0–0.02) samples.

Samples	x = 0	x = 0.0100	x = 0.0125	x = 0.0150	x = 0.0175	x = 0.0200
Tθ (K)	−210.04	−224.82	−252.73	−251.43	−218.09	−259.49

. All specimens exhibit negative T_θ values, and Y³⁺ doping leads to a decrease in T_θ. Except for the x = 0.0150 and x = 0.0175 compositions, the T_θ value gradually decreases with an increase in the doping concentration. This means that the antiferromagnetic interaction mainly exists in CuCr_1−xY_xO₂ (x = 0–0.02) ceramics, and the antiferromagnetic interaction mainly comes from the superexchange interaction of Cr-O-Cr, so an increase in the doping concentration enhances this effect.

To elucidate the influence of Y³⁺ doping on the magnetic characteristics of CuCrO₂ ceramics, Figure 8 presents magnetic hysteresis loops measured under a ±5000 Oe applied field at different temperatures (4 K, 26 K, 50 K, 135 K, 145 K, and 290 K). All specimens exhibit linear M-H relationships without magnetic saturation, differing solely in their slope magnitudes. Compared with that for the undoped sample, the change in the hysteresis loop slope increases obviously when the doped sample decreases from 145 K to 50 K. The curve slopes of 26 K and 50 K are the largest in all samples, corresponding to T_N, which is consistent with the trend of the χ-T curve. It can be seen that all samples are paramagnetic at 290 K and exhibit obvious antiferromagnetism at other temperatures. As the temperature drops to 26–50 K, the magnetic susceptibility reaches its maximum, and no ferromagnetic phase appears across the entire temperature range.

The collective evidence demonstrates that Y³⁺ doping profoundly modifies the magnetic transitions in CuCr_1−xY_xO₂ (x = 0–0.02) ceramics. While the lattice expansion and structural distortion induced by dopant incorporation preserve the delafossite framework and oxidation states of Cu/Cr, they induce geometric reorganization of Cr-O-Cr bonding configurations through bond angle modification. Concurrently, the non-magnetic Y³⁺ ions spatially decouple adjacent Cr³⁺ moments, creating magnetic isolation effects. The intrinsic antiferromagnetic order of spin-frustrated CuCrO₂ multiferroic materials comes from the superexchange interaction of Cr-O-Cr. Therefore, Y³⁺ doping leads to a change in superexchange interactions, and some magnetic moments cannot be completely offset, forming a short-range order or spin tilt; thus, the doped samples show stronger antiferromagnetism but also weakened antiferromagnetic stability at low temperature.

4. Conclusions

A series of CuCr_1−xY_xO₂ (x = 0–0.02) ceramic samples was synthesized via the solid-state reaction method. All specimens exhibited phase-pure delafossite structures, with no secondary phases detected. Y³⁺ doping led to lattice expansion. The cell volume increased from 130.928 Å3 for x = 0 to 131.147 Å3 for x = 0.0200, the density decreased from 5.6758 g·cm⁻³ for x = 0 to 5.6087 g·cm⁻³ for x = 0.0200, and the average grain size decreased from 3.38 μm for x = 0 to 4.27 μm for x = 0.0200. Y³⁺ doping did not change the lattice structure of CuCrO₂, but with an increase in the doping concentration, the mutual fusion between grains was also affected. Liquid-phase recrystallization made some small precipitates appear in the sample and gradually increased with an increase in the doping concentration. The content of precipitates reached the maximum in the sample with x = 0.0150. An XPS analysis confirmed monovalent Cu⁺ and trivalent Y³⁺/Cr³⁺ states, excluding divalent copper species. All samples showed obvious paramagnetism when the temperature was higher than 140 K. However, when the temperature was lower than 140 K, the lattice expansion and lattice distortion caused by Y³⁺ doping changed the bond angle of the Cr-O-Cr bond and enhanced the superexchange interaction of Cr-O-Cr, resulting in some magnetic moments not being completely offset. As a result, a short-range order or spin tilt formed, enhancing the antiferromagnetism of the sample but also weakening its antiferromagnetic stability at low temperature. The T_N of the undoped sample was about 46 K. The T_N of the sample with x = 0.0175 was about 21 K, while the T_N of the other doped samples was about 30 K. The sample with x = 0.0100 also had an obvious peak at 11 K. In addition, it was found that sintering at 1300 °C significantly enhanced the antiferromagnetic stability of CuCrO₂ and increased the Néel temperature T_N. This study confirmed that low-concentration Y³⁺ doping can fine-tune the lattice structure of CuCrO₂ ceramics without destroying the main phase and regulate its magnetic transition, which provides a new idea for regulating and improving the magnetic properties of CuCrO₂ by rare earth element doping.