Ternary intermetallic RTX compounds are composed of R - a rare-earth metal and T, X - different d or р-elements [1]. This family of intermetallics has a wide range of possible applications, such as magnetocaloric cooling, gas liquefaction and others [2-4]. Experimentally and theoretically it was found that T-sublattice doping can significantly improve their magnetic and electronic properties [3], a composition-Induced magnetic transition was revealed in GdMn_1-xTi_xSi for x = 0 - 1 [4]. Therefore, further study of their properties may be useful for various environmentally sustainable applications.

In this work, the series GdFe_1-xNi_xSi compounds was synthesized and investigated for x = 0 - 0.4 due to the solubility limit [5]. The theoretical calculations were carried out for the tetragonal structure for x ranging between 0 and 0.5. The electronic structure, magnetic moments and types of magnetic orderings were investigated using the DFT+U method taking into account strong electron correlations in the 4f Gd shell [5]. In the self-consistent DFT+U calculations, the theoretical total magnetic moment of GdFe_1-xNi_xSi was found to be solely formed by the Gd ion, and Ni, Si are either non-magnetic or have small magnetic moments 0.02 mB at Si and 0.1mB at Ni. In the calculations for x = 0 - 0.25, in GdFe1-xNixSi the ferromagnetic (FM) ordering of the Gd magnetic moments was found as the most stable. For GdFe_0.7Ni_0.3Si and compositions with the larger content of Ni, the antiferromagnetic (AFM) ordering was found to be more preferable in total energy. Several types of AFM orderings were checked. The one with the Gd moments being aligned antiferromagnetically in ‘‘W slabs’’ and ferromagnetically in ‘‘BaAl₄ blocks’’. This type of AFM became more stable than the ferromagnetic one with the differences in total energy equal to 0.09 meV/f.u. (x=0.3) and 0.29 meV/f.u. (x=0.35). From experimental magnetic measurements, the behaviour of the magnetization curves and Curie temperature for GdFe_1-xNi_xSi differs from the one for V, Cr, Ti [4] with the Ni-doping and a decrease in interatomic distances. The magnetocaloric effect (MCE) of the GdFe_1-xNi_xSi systems changes with a change in composition from 3.8 (x=0.1, T_C = 111 K) and 3.3 (x=0.2, T_C = 104 K) J/kgK to 1.4 (x=0.3, T_C = 106 K) J/kgK which can be attributed to the FM-AFM magnetic transition.

Thus, for the GdFe_1-xNi_xSi series, the transition from a ferromagnetic (low Ni content) to an antiferromagnetic ordering (x > 0.25) was identified using our first-principles calculations. The results are supported by the experimental magnetic data. The data obtained may indicate promising prospects of the GdTSi compounds studied in this work and the whole group of ternary intermetalliс compounds with rare earth metals, which will motivate further research.

The study is supported by the Russian Science Foundation (RSF) project No. 18-72-10098.

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[2] Kuchin, A.G.; Platonov, S.P., Mukhachev, R.D.; Lukoyanov, A.V.; Volegov, A.S.; Gaviko, V.S.; Yakovleva, M.Yu. Large magnetic entropy change in GdRuSi optimal for magnetocaloric liquefaction of nitrogen. Metals 2023, 13, 290. https://doi.org/10.3390/met13020290

[3] Kuchin, A.G.; Platonov S.P.; Lukoyanov, A.V.; Volegov, A.S.; Gaviko, V.S.; Mukhachev, R.D.; Yakovleva, M.Yu. Remarkable increase of Curie temperature in doped GdFeSi compound, Intermetallics 2021, 133, 107183. https://doi.org/10.1016/j.intermet.2021.107183

[4] Mukhachev, R.D.; Lukoyanov, A.V. Composition-Induced Magnetic Transition in GdMn1-xTixSi Intermetallic Compounds for x = 0–1. Metals 2021, 11, 1296. https://doi.org/10.3390/met11081296

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