Interplay of Crystal Structure and Magnetic Properties of the Eu_5.08-xSr_xAl₃Sb₆ Solid Solution

Zintl phases containing rare-earth metals have gained attention due to their magnetic, electronic, and thermoelectric properties. Eu_5.08Al₃Sb₆ is a new structure type (monoclinic space group C2/m) that can be described as a pseudorock-salt EuSb motif with the Eu-centered Sb octahedra at the origin of the unit cell, and on the C-face center, containing either Eu (8%) or an Al₄ tetrahedron modeled as a dual tetrahedron (37.5%). The complete solid solution of Eu_5.08-xSr_xAl₃Sb₆ can be prepared; however, the cation totals vacillate from 5 to 5.24 depending on the Al content. Al K-edge XANES shows a shift to higher energy relative to the Al metal but at slightly lower energy relative to AlSb, indicating an intermediate oxidation state closer to +3 than 0. The lack of an Al K-edge shift with the incorporation of Sr suggests that changes in Sr content do not have a meaningful impact on the electronics of the Al tetrahedra. Investigation of the solid solution structures provides evidence for classifying this structure type as a polar intermetallic phase with variable composition. Magnetization measurements were collected for the solid solution and show complex magnetic ordering with competing ferromagnetic and antiferromagnetic interactions as the Sr content increases.