Study of the Stability of Structural Variants and Magnetic Properties of TmGa₂

We present a detailed study of the electronic structure, chemical bonding, and magnetic properties of TmGa₂. The analysis of chemical pressure in a series of hexagonal REGa₂ compounds (RE = La, Pr, Nd, Gd-Tm and Lu) reveals that steric factors contribute to the instability of the AlB₂-type phase of TmGa₂ and its transition to the orthorhombic KHg₂-type crystal structure. Magnetic properties have been investigated in single crystals grown using a Pb flux. The temperature-dependent magnetic susceptibility measurements at different crystallographic orientations indicate an antiferromagnetic phase transition at T_N = 3.5 K. In the low-temperature regime, magnetic susceptibility deviates from the Curie-Weiss law, showcasing a hump attributed to the crystalline electric field (CEF) effect. The CEF effect has been studied thoroughly based on a point charge model of an isostructural compound TmCu₂, indicating the splitting of 13 nondegenerate levels in TmGa₂. Isothermal magnetization along H∥c exhibits a metamagnetic transition at μ₀H_c = 0.15 T. The overall magnetization remains low, with respect to the theoretical value for Tm³⁺, and does not reach saturation even at 9 T and 2 K. Temperature-dependent heat capacity data confirm the magnetic phase transition at T_N and indicate the presence of the Schottky anomaly. The reduced value of magnetization, the jump in heat capacity, and the magnetic entropy at the magnetic ordering temperature, along with substantial magnetic anisotropy, suggest that CEF interaction plays a crucial role in determining the magnetic ground state in TmGa₂. The nonmagnetic analogue LuGa₂ has also been investigated down to 340 mK, but no superconducting transition has been detected. By employing density functional theory calculations, chemical bonding study, and machine learning-based chemical pressure analysis, we were able to explain the instability of the AlB₂-type structure observed for two of the members of the REGa₂ (RE = lanthanides and Y) series (TmGa₂ and LuGa₂) due to pressure-optimizing transition from the hexagonal AlB₂-type structure.