Synthesis, Structure, and Complex Magnetism of MIr<sub>2</sub>In<sub>8</sub> (M = Eu, Sr)

We report the synthesis, crystal structure, and physical properties of two new polar intermetallic compounds, EuIr<sub>2</sub>In<sub>8</sub> and SrIr<sub>2</sub>In<sub>8</sub>. 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 <i>Pbam</i> with the CeFe<sub>2</sub>Al<sub>8</sub> structure type, which is sometimes also referred to as the CaCo<sub>2</sub>Al<sub>8</sub> structure type. The two analogues have unit cell parameters of <i>a</i> = 13.847(3) Å, <i>b</i> = 16.118(3) Å, and <i>c</i> = 4.3885(9) Å for M = Eu and <i>a</i> = 13.847(3) Å, <i>b</i> = 16.113(3) Å, and <i>c</i> = 4.3962(9) Å for M = Sr at room temperature. SrIr<sub>2</sub>In<sub>8</sub> 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 EuIr<sub>2</sub>In<sub>8</sub> 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 EuIr<sub>2</sub>In<sub>8</sub> relative to that in SrIr<sub>2</sub>In<sub>8</sub>, with estimated values of γ = 118(3) and 18.0(2) mJ mol<sup>–1</sup> K<sup>–2</sup>, respectively.