Derivation of a Nonstoichiometric 1/1 Quasicrystal Approximant from a Stoichiometric 2/1 Quasicrystal Approximant and Maximization of the 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 compositional domain into the valence electron-per-atom () parameter space. Focusing on approximant crystals (ACs), a "double heterovalent elemental substitution" is employed in a stoichiometric Ga-Pt-Gd 2/1 AC whereby is lowered from approximately 1.98 to 1.60. Through this approach, a new family of Ga-based Tsai-type 1/1 ACs with an exceptionally wide compositional domain within space is derived. Remarkably, the 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 RCo phases. These findings demonstrate the high potential of a double heterovalent 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.