Research Article

Synthesis, Structure and Magnetocaloric Properties of La_0.8K_0.15Li_0.05MnO₃ Perovskite Manganite

Y Regaieg^1,2*, W Cheikhrouhou-Koubaa¹, L Sicard³, M Koubaa^1,4, E K Hlil⁵ and A Cheikhrouhou¹

¹LT2S lab, Digital Research Center of Sfax, Sfax Technoparc, 3021 Sfax, Tunisia

²Faculty of Sciences of Gafsa, University of Gafsa, 2112 Gafsa, Tunisia

³ITODYS, Université Paris Diderot, UMR 7086 CNRS, 75205 Paris Cedex 13, France

⁴Institut Supérieur de Biotechnologie de Sfax, University of Sfax, 3000 Sfax, Tunisia

⁵Institut Néel, CNRS, Université Grenoble Alpes, 38042 Grenoble cedex 9, France

Submission: Febraury 27, 2023; Published: March 17, 2023

*Corresponding author: Yassine Regaieg, Digital Research Center of Sfax, Sfax Technoparc, 3021 Sfax, Tunisia

How to cite this article: Y Regaieg*, W Cheikhrouhou-Koubaa, L Sicard, M Koubaa, E K Hlil and A Cheikhrouhou. Synthesis, Structure and Magnetocaloric Properties of La_0.8K_0.15Li_0.05MnO₃ Perovskite Manganite. Curr Trends Biomedical Eng & Biosci. 2024; 21(3): 556061. DOI:10.19080/CTBEB.2024.21.556061

Abstract

The synthesis, structural characterization, and magnetocaloric properties of La_0.8K_0.15-Li_0.05MnO₃ perovskite manganite are reported. Our compound was synthesized using the sol-gel technique at low temperature. The Rietveld refinement of the X-ray powder diffraction shows that La_0.8K_0.15-Li_0.05MnO₃ sample is single phase and crystallizes in a rhombohedral system with space group. Paramagnetic-ferromagnetic phase transition at T_C = 231.2 K is observed for our studied compound. Magnetic entropy change, −ΔS_M, deduced from isothermal magnetization curves, is about 2.39 J kg^-1 K^-1 in a magnetic field change of 5 T. The relative cooling power, RCP(S), value at 5 T reaches 272.4 J kg^-1. This value corresponds to about 66% of that observed in pure gadolinium.

Keywords: Manganites; Sol-gel Technique; X-ray Diffraction; Isothermal Magnetization; Magnetocaloric Effect; Stoichiometric

Introduction

Magnetic refrigeration (MR) based on the magnetocaloric effect (MCE) is a very promising technology to replace the traditional gas-compression refrigeration technology[1,2]. The MCE is an intrinsic property of magnetic materials and defined as the important change of temperature of an adiabatically isolated system with the application and removal of an external magnetic field [3-5]. The MCE is characterized by the magnetic entropy change, −ΔS_M, and the adiabatic temperature change, adTΔ, under magnetic field changes. Mixed valence perovskite manganites with general formula Ln_1-xX_xMnO₃, where Ln is a trivalent rare-earth ion and X a monovalent alkaline or divalent alkaline-earth ion, are typical MCE materials and have been extensively studied these last years in view of their fascinating properties arising from spin-balance sensitivity, orbital contribution, lattice distortion, charge degrees of freedom [6,7]. Monovalent substituted perovskite manganites which are known to introduce large potential fluctuations are typical materials that have been reported [8-10]. Among the evaluated manganites, a family of Ln_1-xX_xMnO₃ appear as very promising candidates for the desired application. In this context, the main objective of our work is to enhance the MCE of La_0.8K_0.2MnO₃ with Li substitution. Therefore, in this pa per, we investigate the structural, magnetic and magnetocaloric properties of La_0.8K_0.15Li_0.05MnO₃ compound synthesized using the sol-gel technique at low temperature.

Introduction

Magnetic refrigeration (MR) based on the magnetocaloric effect (MCE) is a very promising technology to replace the traditional gas-compression refrigeration technology[1,2]. The MCE is an intrinsic property of magnetic materials and defined as the important change of temperature of an adiabatically isolated system with the application and removal of an external magnetic field [3-5]. The MCE is characterized by the magnetic entropy change, −ΔS_M, and the adiabatic temperature change, adTΔ, under magnetic field changes. Mixed valence perovskite manganites with general formula Ln_1-xX_xMnO₃, where Ln is a trivalent rare-earth ion and X a monovalent alkaline or divalent alkaline-earth ion, are typical MCE materials and have been extensively studied these last years in view of their fascinating properties arising from spin-balance sensitivity, orbital contribution, lattice distortion, charge degrees of freedom [6,7]. Monovalent substituted perovskite manganites which are known to introduce large potential fluctuations are typical materials that have been reported [8-10]. Among the evaluated manganites, a family of Ln_1-xX_xMnO₃ appear as very promising candidates for the desired application. In this context, the main objective of our work is to enhance the MCE of La_0.8K_0.2MnO₃ with Li substitution. Therefore, in this pa per, we investigate the structural, magnetic and magnetocaloric properties of La_0.8K_0.15Li_0.05MnO₃ compound synthesized using the sol-gel technique at low temperature.

Experimental Techniques

La_0.8K_0.15Li_0.05MnO₃ sample were prepared by the sol-gel technique (Pechini method) at low temperature [11]. The stoichiometric amounts of La₂O₃, K₂CO₃, Li₂CO₃ and MnO₂ with high purity (Sigma Aldrich 99.9%) were dissolved in concentrated nitric acid to transform them into nitrates. This step was carried out at 60°C in order to accelerate the dissolution. After total dissolution, citric acid, a compliant agent, and ethylene glycol, a polymerization agent, were added. This solution is slowly evaporated at 130 °C until the formation of a residue of high viscosity. A transparent gel is developed during the heating process. The temperature was subsequently raised at 10°C min-1 rate up to 300°C to assure the propagation of a combustion, which transforms the gel into a fine powder. The resulting powder is heated at 450°C during 6 hours in order to decompose the organics. After crushing, the sample was calcined at 650°C during 12 hours and then at 800°C for the same duration with intermediate grinding. Finally, the powder was then pressed into pellets (of about 1 mm thickness and 10 mm diameter) and sintered at 900°C during 24 hours. All annealings are performed in air.

The crystallographic structure was determined by powder X-ray diffraction (XRD) at room temperature using a Panalytical diffractometer (Empyrean model). Structural analysis was carried out using the standard Rietveld technique [12]. The microstructure was studied by Scanning Electron Microscopy (SEM) using a Supra40 ZEISS FEG-SEM microscope operating at 10 kV. Magnetization measurements versus temperature in the range of 5-400K and versus magnetic applied field in the 0-5 T range were carried out using a MPMS-XL Quantum Design SQUID magnetometer. The magnetocaloric effect (MCE) was determined from the magnetization measurements versus magnetic applied field at several temperatures.

Results and Discussion

The powder X-ray diffraction pattern of the La_0.8K_0.15Li_0.05MnO₃ compound synthesized, recorded at room temperature, along with Rietveld refinement is shown in (Figure 1). The diffraction peaks can be indexed in the rhombohedral system with R3c space group. The quality factors indicating the agreement between the observed and the calculated profiles are RB = 2.58, RP = 16.1 and χ2 = 3.13. The corresponding lattice parameters and unit cell volume are a = b = 5.511(2) Å, c = 13.378(4) Å and V = 351.90 Å3. We have also calculated the Mn-O bond length and the Mn-O-Mn bond angle from the position of the ions in the unit cell and lattice parameters, which are found to be 1.961(4) Å and 164.14(3)° respectively. A SEM image of the La_0.8K_0.15Li_0.05MnO₃ sample is shown in (Figure 2). The almost polygonal grains have an average size in the submicrometer range.

Magnetization measurements as a function of temperature, M(T), were plotted in the 5-400 K range for La_0.8K_0.15Li_0.05MnO₃ (0≤x≤0.075) compound by applying a magnetic field of 50 mT (Figure 3). Our sample exhibited a paramagnetic to ferromagnetic transition with decreasing temperature. The Curie temperature, T_C (defined as the temperature at which dM/dT shows a minimum), is found to be 231.2 K.

The Curie-Weiss analysis of the data above TC resulted in effective paramagnetic moment of 4.91 μB higher than the theoretical value = 4.52 μB.

The magnetization versus magnetic field, M(H), up to 5 T at different temperatures in the 160-330 K range was also measured for La_0.8K_0.15Li_0.05MnO₃ sample (Figure4(a)). Below T_C, the magnetization increases sharply below 0.5 T and tends to saturation for higher magnetic fields, which confirms the ferromagnetic behavior of our synthesized sample at low temperatures.

We plot in (Figure4(b))the Arrott curves, H/M versus M², deduced from M(H) for La_0.8K_0.15Li_0.05MnO₃ sample. The plots show a positive slope, above T_C, which indicates that a second order ferromagnetic to paramagnetic phase transition occurs [13]. Furthermore, the T_C values deduced from these plots are very close to those determined from M(T).

The magnetic entropy change, −ΔS_M , induced by the magnetic field change, ΔH, was determined using the M(H) curves. According to the classical thermodynamic theory based on Maxwell relations, −ΔS_M , can be evaluated through the formula:

where μ0Hmax is the maximal value of the magnetic applied field. In practice the relation is approximated as:

where M_i and M_i+1 are the experimental values of magnetization measured at temperatures T_i and T_i+1 respectively, under magnetic applied field Hi [14, 15]. (Figure 5) shows the temperature dependence of the -ΔSM under different magnetic field change for La_0.8K_0.15Li_0.05MnO₃ sample. increases with increasing magnetic applied field change. The maximum values of the magnetic entropy change observed around T_C are equal to 0.55, 1.08, 1.56, 1.99 and 2.39 J kg^-1 K^-1 for magnetic field changes of 1, 2, 3, 4 and 5 T respectively.

The variation of the maximum of the magnetic entropy change as a function of the magnetic field change, for the La_0.8K_{0. 15}Li_0.05MnO₃ sample, exhibits a monotone increase, as shown in (Figure 6(a)), which corresponds to the magnetic transition from ferromagnetic to paramagnetic states. In accordance to Oesterreicher et al. [16], the field dependence of the magnetic entropy change at T_C of materials with a second order phase transition can be described by a power law of the type [17]:

where a is a constant and the exponent n depends up on the magnetic state of the sample. The obtained value of n is 0.88, which is significantly different than 2/3, as predicted by the mean field model [17]. The deviation from n = 2/3 is due to the presence of local magnetic inhomogeneities in the vicinity of transition temperature [18]. This result is similar to those obtained for other manganites system [19-21].

In magnetic refrigeration technology, it is important that the magnetocaloric effect extends over a large temperature range. The relative cooling power (RCP(S)) is evaluated as where δTFWHM is the full-width at half-maximum of versus temperature curve [22]. We plot in (Figure 6(b)) the RCP values as a function of the magnetic field change, ΔH. It can be observed that the RCP values increase with ΔH indicating that RCP is strong field dependent. Indeed, RCP can be approximately expressed as a power law:

where m is the critical exponent of the magnetic transition. RCP should scale with field as a power law with an exponent m. The fitting value of m is found to be 1.01 for our sample. This m result is comparable with values previous studies [21,23,24]. (Table 1) shows the obtained results in comparison with those of several magnetic materials that could be used as active refrigerants taken from the literature. It is clear from the table that the obtained values are larger than the parent compound La_0.8K_0.15Li_0.05MnO₃ [8]. On the other hand, our RCP(S) represents about 66% of the prototype magnetic refrigerant material Gd RCP value (410 J/kg [28]) for a magnetic field change of 5T.

Conclusion

In summary, the polycrystalline La_0.8K_0.15Li_0.05MnO₃ sample was prepared by the sol-gel technique at low temperature. Our compound crystallizes in the rhombohedral system with R3c space group. Magnetic measurements revealed that our sample manganite undergoes a second order magnetic transition with the paramagnetic-ferromagnetic transition at a TC ∼ 231.2 K. The maximum of the magnetic entropy change, , and the relative cooling power, RCP(S), associated with the transition are found to be 2.39 J kg^-1 K^-1 and 272.4 J kg^-1 respectively, under a magnetic field change of 5T These results suggest that our synthesized sample could be used as an active magnetic refrigerant working below room temperature..

Acknowledgment

This study has been supported by the Tunisian Ministry of Higher Education and Scientific Research.

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