Structural, Mechanical, Optoelectronic, Magnetic and Thermodynamic Properties of GdM₂ (M = Fe, Co, Ni) for Optoelectronic Applications

The structural, mechanical, optoelectronic, magnetic, and thermodynamic properties of the intermetallic compounds GdM₂ (M = Fe, Co, Ni) have been investigated using the full-potential linear muffin-tin orbital (FP-LMTO) method within the local spin density approximation (LSDA) and LSDA + U approaches. In the ferromagnetic phase, both functionals were employed to account for the Coulomb repulsion among electrons of the same atom through the Hubbard U term. These methods were specifically applied to describe the Gd–4f electrons in the electronic and magnetic calculations of ferromagnetic Laves-phase GdM₂ (M = Fe, Co, Ni). Our findings reveal that the LSDA + U approach provides the most stable phase for all the studied compounds. The elastic constant C44 C_{44}C44 indicates that resistance to unidirectional compression is greater than resistance to shear deformation. While LSDA accurately reproduces experimental lattice constants, LSDA + U slightly overestimates them. However, LSDA + U delivers a more precise description of the band structure, density of states, and magnetic moments compared to LSDA. Additionally, a stronger hybridization interaction is observed between Gd-d and Ni-d electrons compared to Gd-d and Co-d, with the weakest interaction occurring between Gd-d and Fe-d electrons. The calculated lattice parameters for GdFe₂, GdCo₂, and GdNi₂ deviate from experimental values by only 0.3%, 0.4%, and 0.2%, respectively, demonstrating a high degree of accuracy. A critical pressure of 10 GPa was found for GdNi₂, indicating a relatively low pressure is needed to transition from the ferromagnetic phase to a non-magnetic state. The calculated total magnetic moments range from approximately 6.5 μB to 7.2 μB per formula unit, with the LSDA + U method providing a more accurate description of the magnetic ordering.