Structural, elastic and vibrational properties of fe2p-type materials for magnetic refrigeration; a density functional theory (dft) study

Abstract

Magnetic refrigeration, also known as adiabatic magnetisation is a physical process that makes use of the magnetic properties of some type of materials to attain extremely low temperatures, through a process defined as the magnetocaloric effect (MCE). The advantages of this technology over the traditional gas compressor-based refrigeration are that, it is low in energy consumption, has a higher refrigeration efficacy and it is ecologically friendly. Hexagonal Fe2P-type materials have been established to exhibit good magnetocaloric properties however, their poor structural, mechanical and dynamical stability along the phase transition limits their application in practical devices. Modifying these material characteristics by alloying or doping is crucial for finely-tuning their performance. Here, a study was conducted into the theoretical aspects of the structural, elastic, and vibrational attributes of FeMnP1−x Ax compounds (where A = Si, Se, In, Sn, and x = 0.33). The primary aim was to assess the viability of employing these compounds as solid-state magnetic refrigerants. To achieve this, first principle computations based on Density Functional Theory (DFT) were executed via the Quantum Espresso software. The investigation encompassed diverse dimensions such as structural parameters (including lattice dimensions and bond lengths), which were determined by iteratively adjusting atomic positions along the x, y, and z axes until the system reached its lowest energy configuration following defined convergence criteria. The calculations of elastic characteristics, including elastic constants, was accomplished using the Thermo_pw software coupled with quantum espresso. The assessment of bulk moduli (B) and shear moduli (G) involved the implementation of the Voigt-Reuss-Hill (VRH) approximation method. The Vibrational properties such as phonon frequency, phonon density of states, phonon band structure and phonon thermal properties were calculated using harmonic approximation by the frozen phonon approach using the phonopy code interfaced with quantum espresso. From study results, the calculated elastic constants were found to fulfill the four Born conditions essential for ensuring elastic stability. The small percentage deviations of the values of lattice constants and bond lengths from the experimental data for FeMnP0.66Si0.33 indicated the accuracy of these calculations. There is also occurrence of phonon anomaly (pseudo gap) which is fundamentally a measure of thermal conductivity and it confirmed that the investigated materials were good thermal conductors. The nonappearance of negative frequencies from the phonon dispersion graphs indicate that all the compounds were dynamically stable in their ferromagnetic phases.