Derivation of a non-stoichiometric 1/1 quasicrystal approximant from a stoichiometric 2/1 quasicrystal approximant and maximization of magnetocaloric effect

The present research introduces a novel strategy for tuning magnetic properties by overcoming the compositional limitation of stoichiometric intermetallic compounds via extension of their stability into a new dimension within valence electron-per-atom (e/a) parameter space. Focusing on approximant crystals (ACs), a "double hetero-valent elemental substitution" is employed in a stoichiometric Ga-Pt-Gd 2/1 AC whereby e/a is lowered from 1.92 to 1.60. Through this approach a new family of stable Ga-based Tsai-type 1/1 ACs with exceptionally wide composition stability within e/a space is derived. Remarkably, magnetic ground state is altered from initially spin-glass to ferromagnetic (FM) with second order phase transition and mean-field-like critical behavior. More importantly, through this strategy, the isothermal magnetic entropy change enhanced significantly and reached a maximum value of -8.7 J/K mol-Gd under a 5 T magnetic field change, even comparable to leading rare-earth magnetocaloric materials including RCo2 phases. These findings demonstrate the high potential of a double hetero-valent elemental substitution for tailoring magnetic properties and magnetocaloric response in stoichiometric compounds, offering a new pathway for designing high-performance magnetic refrigeration materials even beyond the quasicrystals and ACs.