In stoichiometric compounds (compounds with fixed ratios of elements), the elemental ratios are dictated by chemical stability, which constrains how much the composition, and consequently the number of valence electron-per-atom (e/a) ratio, can be adjusted. Tuning e/a has been proved to be a promising strategy to architecture magnetic properties in many intermetallic compounds, especially those with complex structures including quasicrystals (QCs) and their structurally related approximant crystals (ACs). Owing to their structural complexity, their electronic properties are sensitive to the number of valence electron-per-atom (e/a). Stoichiometric compounds are stable only within a narrow e/a range (this is about 2.00 in QCs), which limits efforts to architecture their magnetic properties.
In a study published in the Journal of the American Chemical Society on August 27, 2025, led by Professor Ryuji Tamura and Assistant Professor Farid Labib from Tokyo University of Science, Japan, present a technique called "double hetero-valent elemental substitution" to overcome this limitation. This method involves engineering the structure by partially replacing certain atoms in the material with others with similar atomic size and chemistry. This expands the compositional domain into a new e/a parameter space and transforms them into non-stoichiometric compounds, enabling the tuning of their magnetic properties while maintaining structural stability.
"This approach offers a powerful way to transform magnetically frustrated stoichiometric compounds into non-stoichiometric materials with tunable magnetic properties and strong magnetocaloric response. It is a major step forward in designing new magnetic refrigeration materials," says Prof. Tamura.
The team applied this strategy to stoichiometric Ga52Pt34Gd14, a 2/1 AC with an e/a of 1.98 that exhibits spin-glass-like freezing behavior. By partially substituting gallium (Ga) and platinum (Pt) with gold (Au), they synthesized a new family of quaternary Ga–Pt–Au–Gd 1/1 ACs with expanded e/a values ranging from 1.60 to 1.83.
The substitution transformed the material's magnetic properties, with the resulting non-stoichiometric ACs exhibiting long-range ferromagnetic order with second-order phase transitions and mean-field-like critical behavior. These materials have Curie temperatures between 8.7 K and 14.9 K, depending on the composition, and showcase a strong magnetocaloric response. Notably, the isothermal magnetic entropy change (ΔSm) peaked at −8.7 J/K·mol-Gd, putting it on par with some of the best rare-earth-based magnetocaloric materials. This large ΔSm reflects the material's ability to absorb or release heat in response to a changing magnetic field, making them promising for magnetic refrigeration.
The team points out that this substitution method could be applied to other elemental pairs, such as Cu/Mg, Ca/Pb, or Ag/Pd, provided the atoms have similar sizes, valence electrons, and electronegativities to maintain overall structural stability. This flexibility could also aid in developing magnetocaloric materials beyond quasicrystal families, with tailored transition temperatures. Also, the substitution method can be made more affordable by replacing expensive precious metals with cheaper alternatives such as copper or silver.
The low transition temperatures observed in these materials make them strong candidates for low-temperature cooling applications, particularly adiabatic demagnetization refrigeration (ADR) and active magnetic regenerators. Additionally, these materials can provide enhanced volumetric entropy capacity through magnetic phase transitions. Overall, the materials synthesized via the substitution method in this work offer enhanced magnetocaloric performance in the temperature range of 8–15 K, favoring usage in practical cryogenic systems.
This study also possesses applications in the field of quantum computing that require helium-free cooling solutions and ultra-low temperature technologies for efficient working. The newly synthesized Ga–Pt–Au–Gd 1/1 ACs can be employed as helium-free high-capacity regenerators or active cooling agents in ADRs.
More broadly, the study introduces a generalizable approach for overcoming stoichiometric limitations in intermetallic compounds, opening new possibilities in the design of magnetically tunable materials.
"These findings demonstrate the potential of double hetero-valent elemental substitution for tailoring magnetic properties and magnetocaloric response in stoichiometric compounds, where the compound can be heated and cooled down upon exposure and removal of magnetic field. This study offers a new pathway for designing high-performance magnetic refrigeration materials," concludes Prof. Tamura.
