Interplay of crystal structure and magnetic properties of the Eu5.08-xSrxAl3Sb6 solid solution

Zintl phases containing rare earth metals have gained attention due to their magnetic, electronic, and thermoelectric properties. Eu5.08Al3Sb6 is a new structure type (monoclinic space group C2/m) that can be described as a pseudo-rock-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 Eu5.08-xSrxAl3Sb6 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 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.

Methods
Al K-edge x-ray Absorption Near-Edge Structure (XANES) spectra were acquired on single crystals using a scanning transmission x-ray microscope (STXM) in a ~ 0.5 atm He-filled chamber. Spectra were recorded in topoff mode at 220 mA at the spectromicroscopy (SM) beamline 10-ID1 at the Canadian Light Source (CLS) and 500 mA at the Molecular Environmental Science (MES) beamline 11.0.2 at the Advanced Light Source (ALS). The photon energy of the medium-energy diffraction grating was calibrated to the Al K-edge for a 1000 Å aluminum filter sample (Luxel, inflection point = 1559.0 eV). The maximum energy resolution E/ΔE was previously determined at better than 7500, consistent with the observed standard deviation for spectral transitions of ±0.3 eV determined from comparing spectral features over multiple particles and beam runs. Edge shifts for Eu5.08Al3Sb6 and AlSb were determined from the half-height of the Al K absorption edge at 1559 eV and could be referenced to the half-height of the 1000 Å aluminum filter sample: Eu5.08Al3Sb6, 1560.4 eV; AlSb, 1560.7 eV.

Temperature and field-dependent measurements of single crystal samples were performed using a Physical Property Measurement System (PPMS) Dynacool manufactured by Quantum Design Inc. Temperature-dependent magnetization was measured from 1.8 to 300 K with applied fields from 100 Oe to 3000 Oe. Field-dependent magnetization was collected at temperatures from 1.8 to 300 K with a magnetic field up to 9 T.