Resonant Ultrasound Spectroscopy of complex materials: Thirty years of exploration

Resonant Ultrasound Spectroscopy (RUS) has emerged as a uniquely sensitive probe of elastic properties, phase transitions, and lattice dynamics of a wide array of complex materials. Drawing on three decades of experience with RUS, this presentation will highlight key findings and insights accumulated over an extensive period, showcasing the versatility and power of RUS as a characterization tool in materials science. In particular, I will review results obtained for “glassy crystals” such as clathrates and skutterudites, where RUS provided critical insight into vibrational disorder and low-energy excitations. The case of EuTiO3, a magnetic analog to SrTiO3, will be examined, with emphasis on the structural instability in this compound that remained elusive in other characterization techniques. Finally, more recent investigations into the elastic properties of compositionally complex materials will be presented, illustrating how chemical disorder influences elastic properties in ways that challenge conventional understanding of structure–property relationships.