Synthesis, Basic Characterization, and Resonant Ultrasound Studies of Novel Materials: Iron-based Superconductors and Mo3Sb7-based Thermoelectrics

The elastic behavior of materials with unusual low-temperature behavior involving structural, magnetic, and superconducting transitions has been studied using Resonant Ultrasound Spectroscopy (RUS). Two classes of materials have been examined, namely, iron-based superconductors and thermoelectric materials. A series of BaFe2As2 materials as well as TlFe1.6Se2 comprise the former class, and a series of Mo3Sb7 materials is included in the latter.

The elastic behavior of BaFe2As2 reveals that a large softening is observed as the material approaches a tetragonal-to-orthorhombic structural transition accompanied by an antiferromagnetic ordering near 132K. This significant shear softening indicates that the structural transition is due to magnetically-driven nematic fluctuations. The elastic behavior of superconducting cobalt-doped BaFe2As2, with a critical temperature of 22K, supports this finding. In BaFe2As2, an unusually large amount of softening is observed as the material approaches the superconducting transition, and the material begins to stiffen immediately following this transition. The effect of chromium doping in the BaFe2As2 material is also examined. It is found that chromium doping does not produce superconductivity at any concentration. It is shown that the structural transition that occurs in the BaFe2As2 parent material is suppressed with chromium doping, but this suppression is slower than that observed in cobalt-doped compounds. The TlFe1.6Se2 material displays a slight softening at 140K, where a canting of the iron spins occurs. When the original magnetic structure is recovered at 100K, stiffening is observed. A magnetic field of 3T does not affect the elastic response despite the magnetic feature.

The elastic behavior of a series of Mo3Sb7 single crystals is also examined with RUS. The temperature dependence of the elastic response of this material reveals a transition at around 53K, evidenced by a dramatic softening in c’. This softening is associated with a cubic-to-tetragonal structural transition. Single crystals that were doped with chromium, ruthenium, and tellurium were also examined. The chromium doping simply decreases the transition temperature by approximately 5K. While the addition of ruthenium and tellurium suppresses the transition, a structural instability persists, evidenced by a remaining c’ softening.