Barocaloric and magnetocaloric effects in the A-site layer-ordered double perovskite YBaCo₂O_5.5

Abstract

Searching for novel caloric materials is essential for realizing environmentally friendly, energy-efficient refrigeration systems. The A-site layer-ordered double perovskite oxide YBaCo₂O_5.5, which had a degree of freedom for the Co³⁺-spin state, was found to exhibit caloric effects by applying multiple external fields regarding three observed phase transitions. Large latent heat of 8.8 J g⁻¹ is observed near the first-order metal–insulator–transition temperature (close to room temperature), where a negative-thermal-expansion-like volume change and a change in the Co³⁺-spin state are accompanied. Significantly, the corresponding entropy change of 29.8 J K⁻¹ kg⁻¹ can be utilized through an inverse barocaloric effect. The ferrimagnetic and antiferromagnetic transitions below the metal–insulator transition temperature can be controlled by applying magnetic fields, inducing normal and inverse magnetocaloric effects, respectively. In addition, when a high magnetic field above 10 kOe is applied, the second-order ferrimagnetic transition merges with the first-order metal–insulator transition as a single first-order transition, potentially enhancing the barocaloric effect. These thermal properties provide valuable insight into the development of efficient refrigeration by means of transition-metal oxides.

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Introduction

In recent years, thermal control has become one of the most critical issues, driven by global warming, increased heat generation from precision equipment in the advanced information society, and the freezing and storage of large amounts of food for long periods. Caloric effects of solids are attracting attention as an alternative to the current gas-compression method. The use of solid-caloric-effect materials allows for environmentally friendly, energy-efficient, and compact cooling systems.¹ In the caloric effects, entropy changes in response to external fields lead to practical thermal conversions, and magnetocaloric, electrocaloric, and barocaloric effects are typical responses to magnetic, electric, and pressure fields, respectively.^2–8

Significant entropy changes are often induced near the phase-transition temperature, leading to substantial caloric effects.^4,9,10 In particular, materials that exhibit first-order transitions have latent heat, so more significant caloric effects can be expected.^10–12 Several benchmark materials have demonstrated giant thermal responses near their first-order phase transitions. For example, Gd₅(Si, Ge)₄ and La(Fe, Si)₁₃-based alloys are well-known for their giant magnetocaloric effects, exhibiting magnetic entropy changes ΔS exceeding 15.0 J K⁻¹ kg⁻¹ under moderate magnetic fields.^10,13 Transition-metal oxides, which correspond to chemically stable inorganic solid materials, are good candidates to show large latent heat due to their strong electron-lattice coupling.^14,15 Indeed, we recently reported large latent heat and the corresponding colossal barocaloric effect (ΔS = 65.1 J K⁻¹ kg⁻¹ under 5.1 kbar) in the A-site-quadrupole perovskite NdCu₃Fe₄O₁₂.¹⁶ In NdCu₃Fe₄O₁₂, the instability of usually high valence Fe^3.75+ induces first-order inter-site charge transfer between Cu and Fe ions near room temperature, which accompanies a metal–insulator transition, a negative-thermal-expansion-like cell–volume change, and a paramagnetic-antiferromagnetic transition simultaneously.¹⁶ While NdCu₃Fe₄O₁₂ shows the barocaloric effect, the latent heat cannot be utilized through a magnetocaloric effect because the antiferromagnetic transition cannot be controlled by applying a magnetic field.

After the discovery of NdCu₃Fe₄O₁₂, we expanded the search range for the substances to a perovskite-related Co³⁺ oxide YBaCo₂O_5.5, which has a degree of freedom for the Co³⁺ spin state. YBaCo₂O_5.5 was reported to show exotic successive phase transitions, which include a Co³⁺-spin-state-change induced first-order metal–insulator transition with a cell-volume change near room temperature and two magnetic transitions (from paramagnetic to antiferromagnetic through ferrimagnetic) below the first-order transition temperature.^17,18 In this paper, we report thermal properties regarding the successive phase transitions in the A-site-layer-ordered double perovskite YBaCo₂O_5.5. We discovered caloric effects by applying multiple external fields, which means both barocaloric and magnetocaloric effects. The metal–insulator transition accompanies a large latent heat, which can be utilized via a barocaloric effect. In addition, YBaCo₂O_5.5 shows both normal and inverse magnetocaloric effects in the vicinity of the two magnetic phase transitions.

Experimental

Approximately 1.0 g of pellet-shaped polycrystalline sample of YBaCo₂O_5.5 was prepared by a conventional solid-state reaction similar to that reported by Akahoshi et al., using Y₂O₃, BaCO₃, and Co₃O₄.¹⁷ A portion of the obtained pellet sample was ground into a fine powder for synchrotron X-ray diffraction (SXRD) measurements using the BL02B2 beamline at SPring-8 (λ = 0.4958 and 0.4999 Å). The powder sample was packed into a silica capillary rotated during the measurement. The data obtained were analysed with the Rietveld method using the RIETAN-FP program.^19,20 The corresponding crystal structure models were drawn using the VESTA software.²¹

For measurements other than SXRD, pellet samples cut into appropriate sizes were used. Differential scanning calorimetry was carried out at heating and cooling rates of 10 °C min⁻¹. using NETZSCH DSC3500. The heat flow curves were obtained by subtracting the base change. The latent heat Q and the entropy change S associated with the transition were calculated as and , where dQ̇ is the heat flow and Ṫ is the cooling or heating rate.

Differential thermal analysis (DTA) measurements were performed using a pressure cylinder made of Cu–Be. The details of the equipment and the setup are described in the ref. 4. A T-type thermocouple was adhered with varnish to each sample and the CuO reference pellet. The DTA cell with Daphne7373 pressure medium was inserted in the pressure cylinder. A hydraulic cylinder applied hydrostatic pressure via a piston. The sample temperature was controlled using a mantle heater. The heat flow (divided by heating rate) curves were obtained with the DTA signal δT as dq/dT = − AδT/Ṫ. Because the proportional constant A scarcely changed during measurements with the same setup conditions under pressure, the A value was determined from the heat flow Q̇ measured by DSC under an ambient condition. The entropy was evaluated as the following equation.

The isothermal entropy change at each pressure wasalculated as ΔS_P = S(T, P) − S(T, 0).

DC magnetization measurements were performed using a superconducting quantum interference device (SQUID; Quantum Design MPMS-XL) over the temperature range 200–320 K. All the magnetization measurements were conducted under field cooling conditions. The maximum (or the minimum) value of dM/dT is defined as the magnetic transition temperature T_N (T_C).

Electrical resistivity was measured using the standard four-probe method with a Physical Property Measurement System (PPMS). The sample was a rectangular sintered pellet with dimensions of 2.8 mm in width, 3.0 mm in length, and 0.6 mm in thickness. Silver wires were attached to the pellet using silver paste to ensure electrical contact.

Results and discussion

Single-phase sample of A-site layer-ordered double perovskite YBaCo₂O_5.5 was successfully synthesized. As shown in Fig. S1(a), all the observed SXRD peaks at 300 K can be well fitted with an orthorhombic 2a_p × 2a_p × 2a_p cell, adopting the space group Pmma (a_p denotes the lattice constant of the simple perovskite). Notably, the precise structural refinement confirms nearly perfect ordering between Y/Ba at the A site. The obtained compound was confirmed to exhibit successive phase transitions as follows. In the electrical resistivity data, an abrupt increase is observed at T_MI = 295 K (Fig. 1), indicating a metal–insulator (MI) transition. Associated with the MI transition, a discontinuous jump in the lattice constants and a discontinuous volume change with negative thermal expansion (ΔV = −0.12%) are induced at T_MI, confirming a first-order structural transition. The crystal structure changes to the P2/c monoclinic 2a_p × 2a_p × 2a_p cell at T_MI [Fig. S1(b), Fig. S2 and Table S1], which is consistent with the report by Pantoja.²² In the temperature dependence of the magnetization, following a slight anomaly at T_MI = 295 K, a paramagnetic-ferrimagnetic (PM-FiM) transition at T_C = 288 K and a ferrimagnetic-antiferromagnetic (FiM-AFM) transition at T_N = 266 K are observed. While no structural change is observed at T_C and T_N, a slight increase in the electric resistivity is also induced at T_N. Further decreasing the temperature, the crystal structure returns to the Pmma orthorhombic 2a_p × 2a_p × 2a_p cell at T_s = 230 K (Fig. S2). A positive thermal-expansion-like discontinuous volume change is also observed at T_s = 230 K, indicating a first-order structural transition. These four phase-transition temperatures are in good agreement with those reported previously, confirming the target composition YBaCo₂O_5.5 with the appropriate oxygen content.^17,18,22

Fig. 1 Temperature dependence of the magnetization, the electrical resistivity (top), lattice constants, and cell volume (bottom) of YBaCo₂O_5.5. Vertical dashed lines indicate three phase-transition temperatures.

Among the four phase transitions, the first-order transition at T_MI = 295 K was found to have a large latent heat. Fig. 2 shows a DSC curve and the corresponding entropy change. The DSC data includes a prominent peak at approximately 295 K and a tiny peak at approximately 264 K. The former peak corresponds to the first-order transition at T_MI, while the latter one corresponds to the magnetic transition at T_N. On the other hand, no significant peak is observed at T_s = 230 K, despite the presence of the first-order phase transition accompanied by the discontinuous volume change. Notably, near the phase transition at T_MI, the large latent heat of 8.8 J g⁻¹ is observed. The corresponding entropy change is 29.8 J K⁻¹ kg⁻¹, which is obviously smaller than that of NdCu₃Fe₄O₁₂ (84.2 J K⁻¹ kg⁻¹), but still significant in oxide materials.^14,16,23

Fig. 2 DSC curve (red) of YBaCo₂O_5.5 measured during heating. Corresponding entropy change ΔS (black) calculated from the eqn (1) is also plotted (right axis). The entropy values refer to the value at 240 K.

The large latent heat can be utilized through a barocaloric effect (BCE) by applying hydrostatic pressure. Fig. 3(a) shows the heat flow −dq/dT obtained by differential thermal analysis (DTA) measurements at various pressures. As the applied pressure increases, the DTA peak corresponding to the structural transition at T_MI shifts to lower temperatures, suggesting an inverse BCE. The decrease in T_MI is related to the stabilization of the high-temperature (metallic) phase, which has a smaller cell volume. The pressure dependence of T_MI is summarized in Fig. 3(b). Applying pressure results in a linear decrease in T_MI, with a pressure coefficient of dT_MI/dP = −0.91 K kbar⁻¹. The experimental coefficient can be compared with that obtained from the Clausius–Clapeyron equation

dT/dP = ΔV_tr/ΔS_tr, (2)

where ΔS_tr and ΔV_tr are the entropy and volume changes at the first-order phase transition, respectively. Using experimentally obtained ΔV_tr = 18.98 × 10⁻⁸ m³ kg⁻¹ and ΔS_tr = 29.8 J K⁻¹ kg⁻¹, dT/dP was calculated to be 0.64 K kbar⁻¹, which is roughly consistent with the experimental value. It is worth noting that the DTA peak under P = 6 kbar hardly overlaps with that at ambient pressure, allowing the large latent to be utilized almost entirely as the BCE.

Fig. 3 (a) Heat flow curves divided by heating rate during heating in YBaCo₂O_5.5 under several pressures. (b) Pressure dependence of the DTA peak temperature. The dashed line corresponds to the linear fitting of the data.

To evaluate the cooling performance through the BCE, we calculated the temperature dependence of entropy S near T_MI under several pressures from the DTA data (Fig. 4a). The entropy S can be expressed as

where S_w(T_w, P) is the entropy at the reference state with temperature T_w and pressure P. It is noted that S_w definitely depends on the pressure. However, we can still align the same baseline in the low-temperature-insulator phase despite the presence of a small isothermal entropy difference ΔS_w(P). ΔS_w(P) can be derived using Maxwell's relation:The maximum ΔS_w is 2.65 J K⁻¹ kg⁻¹ under 6 kbar, which is 8.9% of the entropy change at T_MI and at 0 kbar and is negligible in the current case. As shown in the S–T curves under several pressures, the observed entropy jump near T_MI decreases slightly with increasing pressure, which can be explained by the Clausius–Clapeyron equation as follows. While dT/dP is constant regardless of P, ΔV generally decreases with increasing P, thereby leading to a decrease in ΔS.

Fig. 4 (a) Temperature dependence of entropy S during heating process. Entropy S is calculated using the eqn (3). (b) Isothermal entropy changes ΔS_P of YBaCo₂O_5.5.

The corresponding isothermal entropy change, ΔS_P (T) = S (P, T) − S (0, T), is shown in Fig. 4(b). The maximum ΔS_P is 13.5 J K⁻¹ kg⁻¹ under 2 kbar, 19.1 J K⁻¹ kg⁻¹ under 4 kbar, and 19.3 J K⁻¹ kg⁻¹ under 6 kbar [Fig. 4(b)]. One might wonder why the maximum ΔS_P is almost the same under 4 and 6 kbar, because the overlap of the DTA peaks under 4 and 0 kbar is clearly larger than that under 6 and 0 kbar. As mentioned before, the observed entropy change near T_MI slightly decreases with increasing P. As a result, the maximum ΔS_P seems to saturate under approximately 4–6 kbar. The results demonstrate that approximately 2/3 of the entropy changes observed in the DSC measurement are utilized through the BCE under P = 4–6 kbar. In addition, the corresponding maximum adiabatic temperature change ΔT =T(S, 0) − T(S, P) is calculated to be 3.8 K under 4 kbar and 5.3 K under 6 kbar, producing the following refrigerant capacity (approximately expressed as the product of ΔS and ΔT.), 72.6 J kg⁻¹ under 4 kbar and 102.3 J kg⁻¹ under 6 kbar.²⁴

Since YBaCo₂O_5.5 shows magnetic transitions, it is expected that some of the phase transitions can also be controlled by applying a magnetic field. Fig. 5(a) shows the temperature dependence of the magnetization under several magnetic fields. With decreasing temperature, the magnetization shows an increase at 297 K (∼T_MI) under 0.1 kOe due to a change in the Co³⁺-spin state, and the spin-state anomaly temperature does not change with increasing magnetic field (Fig. 5b). This means that the large latent heat near T_MI cannot be utilized through an MCE. On the other hand, ferrimagnetic ordering is formed between T_C and T_N, which could enable us to control the two magnetic transitions with a magnetic field. Under 0.1 kOe, a PM-FiM transition at T_C = 288 K and a FiM-AFM transition at T_N = 266 K are observed. In contrast to the anomaly at T_MI, T_C and T_N largely depend on H. The H dependence of T_C shows an unusual behavior. While the PM-FiM transition temperature T_C increases by increasing H up to 10 kOe, T_C does not change above H = 10 kOe (Fig. 5b and c). Accordingly, T_C appears to coincide with T_MI, suggesting that the second-order PM-FiM transition merges with the first-order MI transition and is observed as a single first-order phase transition. Here, it should be noted again that T_MI is independent of H, as confirmed in the electrical resistivity data under 0 and 5 T (Fig. S3). Thus, T_C is constant above H = 10 kOe, and a first-order-like discontinuous increase of the magnetization is especially observed under H = 50 kOe (Fig. 5a). On the other hand, the FiM-AFM transition temperature T_N linearly decreases with increasing H (Fig. 5a and c). The decrease in T_N under H is also confirmed by the resistivity data, as the jump in electrical resistivity at 266 K is suppressed under 50 kOe (Fig. S3). Summarizing the response of the phase transitions to H, T_C (T_N) tends to increase (decrease) with increasing H due to the stabilization of the ferrimagnetic phase, and only T_C saturates above 10 kOe due to the coincidence of T_C and T_MI.

Fig. 5 (a) Temperature dependence of magnetization under several magnetic fields. (b) An enlarged view of the temperature dependence of magnetization near T_MI and T_C. (c) Magnetic field dependence of T_C and T_N. The yellow dotted line indicates T_MI.

To evaluate the cooling performance through the MCE, we evaluated the magnetic entropy change, ΔS_M, from the isothermal magnetization as a function of applied magnetic field. From the Maxwell relation,

(∂S_M/∂H)_T = μ₀(∂M/∂T)_H, (5)

the magnetic entropy is described asBy using the isothermal magnetization data from 200 to 320 K shown in Fig. 6(a), the magnetic entropy change ΔS_M is thus estimated by the following formula;where ΔT is the temperature internal at which the isothermal magnetization curves are measured (from 0.25 to 2 K in the present experiments) (see Fig. 6a). The resultant ΔS_M as a function of temperature is displayed in Fig. 6b. The maximum magnetic entropy change (|ΔS_M|) under H = 50 kOe reaches 4.1 J K⁻¹ kg⁻¹ near T_C (T_MI) and 1.1 J K⁻¹ kg⁻¹ near T_N. Because the spontaneous magnetization of this compound in the ferrimagnetic phase is at most 0.4μ_B per f.u., the obtained maximum |ΔS_M| is not so large. Even so, the maximum |ΔS_M| near T_C under magnetic fields larger than 10 kOe becomes significant, reflecting the fact that the PM-FiM transition becomes of first order with the fixed T_C (=T_MI). Another characteristic of the MCE in YBaCo₂O_5.5 is that while ΔS_M near T_C is negative, that near T_N is positive. The result implies that in addition to the normal MCE near T_C, inverse MCE near T_N can be utilized in the single compound.

Fig. 6 (a) Isothermal magnetization between 200 and 320 K. (b) Temperature dependence of magnetic entropy change.

As presented above, the MI transition is sensitive to changes in pressure, thus showing the BCE. Considering that T_C coincides with T_MI above H = 10 kOe, applying pressure can also be expected to control the PM-FiM magnetic transition. We then performed magnetization measurements under several pressures. When H is small, like 0.1 kOe, T_MI shifts to lower temperatures, while T_C remains almost unchanged with increasing pressure (Fig. 7a, b and e). In contrast, when H is larger than 10 kOe, T_C (= T_MI) decreases with increasing pressure (Fig. 7c–e). We can therefore conclude that the PM-FiM transition can be tuned by applying pressure above H = 10 kOe, as well as the simultaneous metal–insulator transition.

Fig. 7 (a) Temperature dependence of magnetization under several pressures while applying 0.1 kOe. (b) An enlarged view of the temperature dependence under 0.1 kOe near T_MI and T_C. (c) Temperature dependence of magnetization under several pressures while applying 50 kOe. (d) An enlarged view of the temperature dependence under 50 kOe near T_MI and T_C. (e) Magnetic field dependence of T_C and T_N under several pressures.

We also mention the pressure dependence of the FiM-AFM transition temperature T_N, which is more complicated. When H is small like 0.1 kOe, T_N decreases monotonically with increasing pressure (Fig. 7a, b and e). Above H = 30 kOe, the trend is reversed, and T_N increases monotonically with increasing pressure. In particular, when H is 70 kOe, T_N shifts to higher temperatures by approximately 20 K under P = 8 kbar (Fig. 7e). These results suggest that the stability of the ferrimagnetic and antiferromagnetic phases is very sensitive to external pressure although their magnetic structures are still under debate.^18,25 While the decrease in T_N with increasing P under low magnetic fields remains elusive, its increase under high magnetic fields (≧30 kOe) can be interpreted through the coincidence of T_s and T_N. Based on the Clausius–Clapeyron relation, the negligible latent heat and moderately large positive ΔV at T_s imply a large dT_s/dP, leading to a significant upward shift of T_s under P. Considering that T_N is suppressed by magnetic fields, T_s and T_N. are expected to coincide under high pressures and fields. If we assume a constraint where T_s cannot surpass T_N (analogous to the relationship T_C ≦ T_MI), it is natural that T_N increases with increasing P under high magnetic fields, following the shift of T_s.

Finally, we consider the effect of the coincidence of T_C and T_MI above H = 10 kOe on the thermal properties. Without a magnetic field, the entropy changes due to the Co³⁺-spin-state change, the MI transition, and the structural change should contribute to the large latent heat (8.8 J g⁻¹) near T_MI in YBaCo₂O_5.5. Above H = 10 kOe, the first-order PM-FiM magnetic transition is also induced at T_MI, implying that the magnetic entropy change of Co³⁺ magnetic moments by magnetic ordering also contributes to the latent heat. Consequently, the enhanced latent heat could improve the BCE under high magnetic fields. As such, this material not only exhibits both barocaloric and magnetocaloric effects but also has the potential to exhibit more efficient caloric effects by combining multiple external fields. Therefore, a future challenge is to investigate the thermal properties under simultaneous application of pressure and magnetic fields.

Conclusions

We have found that the A-site layer-ordered double perovskite oxide YBaCo₂O_5.5 shows multiple caloric effects driven by the successive phase transitions at T_MI, T_C, and T_N. The Co³⁺-spin state in YBaCo₂O_5.5 induces the first-order MI transition with a negative-thermal-expansion-like volume change near room temperature, which contributes to a large latent heat of 8.8 J g⁻¹. Most of the corresponding entropy change (∼19 J K⁻¹ kg⁻¹) is utilized under 4–6 kbar via an inverse barocaloric effect. Applying magnetic fields can control PM-FiM and FiM-AFM transitions below T_MI, inducing normal and inverse magnetocaloric effects, respectively. Above 10 kOe, the PM-FiM transition merges with the metal–insulator transition as a single first-order transition. This not only increases the maximum |ΔS_M| near T_C (=T_MI) but also could enhance the latent heat originating in the discontinuous magnetic entropy change of Co³⁺ magnetic moments. In addition, the coincidence of T_C and T_MI under high magnetic fields enables us to control the PM-FiM transition and the simultaneous MI transition by applying pressure, potentially producing the more efficient inverse barocaloric effect. We believe that the present results will serve as a catalyst for further exploration of novel thermal properties by applying multiple external fields in transition-metal oxides.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data that support the findings of this study are available upon request from the authors.

Supplementary information (SI): results of the Rietveld refinement of the synchrotron X-ray diffraction data at 300 and 250 K, structural parameters at 300 and 250 K, temperature dependence of the synchrotron X-ray diffraction data and the refined lattice parameters, and temperature dependence of electrical resistivity under 0 and 50 kOe for YBaCo₂O_5.5. See DOI: https://doi.org/10.1039/d6ma00234j.

Acknowledgements

We thank S. Kawaguchi and S. Kobayashi at SPring-8 for their assistance with the SXRD experiments, M. Iihoshi and R. Watanabe at Kyoto University for their assistance with the SXRD experiments, and K. Ji at Kyoto university and P. Woodward at the Ohio State University for fruitful discussions. The synchrotron radiation experiments were performed at the Japan Synchrotron Radiation Research Institute, Japan (proposal No. 2023B1593, 2024A1727, and 2025A2025). This work was partly supported by Grants-in-Aid for Scientific Research (No. 20H00397, 23H05457, and 23K13814) and by a grant to the Integrated Research Consortium on Chemical Sciences from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan. This work was also supported by the Japan Science and Technology Agency (JST) as part of the Adopting Sustainable Partnerships for Innovative Research Ecosystem (ASPIRE) program, Grant Number JPMJAP2314, and by research grants for Nippon Sheet Glass Foundation for Materials Science and Engineering, Toyota Physical and Chemical Research Institute, and Tokuyama Science Foundation.

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