Synthesis, Structure, and Complex Magnetism of MIr₂In₈ (M = Eu, Sr)

We report the synthesis, crystal structure, and physical properties of two new polar intermetallic compounds, EuIr₂In₈ and SrIr₂In₈. Both were synthesized in good yield using In metal as a reactive flux medium, enabling the growth of large crystals for physical property measurements. They crystallize in the orthorhombic space group Pbam with the CeFe₂Al₈ structure type, which is sometimes also referred to as the CaCo₂Al₈ structure type. The two analogues have unit cell parameters of a = 13.847(3) Å, b = 16.118(3) Å, and c = 4.3885(9) Å for M = Eu and a = 13.847(3) Å, b = 16.113(3) Å, and c = 4.3962(9) Å for M = Sr at room temperature. SrIr₂In₈ is a diamagnetic metal with no local magnetic moments on either the Sr or Ir sites, and the diamagnetic contribution from core electrons overwhelms the expected Pauli paramagnetism normally seen in intermetallic compounds. Magnetism in EuIr₂In₈ is dominated by the local Eu moments, which order antiferromagnetically at 5 K in low applied fields. Increasing the field strength depresses the magnetic ordering temperature and also induces a spin reorientation at lower temperature, indicating complex competing magnetic interactions. Low-temperature heat capacity measurements show a significant enhancement of the Sommerfeld coefficient in EuIr₂In₈ relative to that in SrIr₂In₈, with estimated values of γ = 118(3) and 18.0(2) mJ mol^-1 K^-2, respectively.