Synchrotron infrared spectroscopy of domain walls and high pressure phases of multiferroics

Synchrotron light sources provide high throughput, broadband infrared light enabling the development of novel techniques, inaccessible using traditional sources. High pressure techniques benefit greatly, as significant signal loss occurs from focusing the light through diamonds into a small area. Intense infrared light offers an avenue to perform spatially-resolved spectroscopy in areas smaller than the diffraction limit by focusing the light within the near-field limit. We take advantage of the synchrotron light source to perform infrared studies at high pressures and on spots smaller than 20×20 nm². We investigate nanoscale heterogeneity with spatially resolved techniques and reveal pressure-induced phase transitions via high pressure spectroscopy. We then unravel these complicated findings by incorporating group theoretical symmetry analysis and lattice dynamics calculations. The utilization of a synchrotron light source offers the broadband, high throughput infrared light that unifies these projects and enables the understanding of how vibrational modes contribute to unexplored phenomena. Because multiferroic materials exhibit heterogeneity in the form of domains and domain walls, Ca₃Ti₂O7 and h-Lu0.6Sc_0.4FeO₃ provide platforms to reveal the infrared response of different domain walls. These nanoscale objects have eluded study due to their size, but near-field infrared spectroscopy provides an opportunity to investigate domain walls, by performing a line scan over a wall of interest. We reveal that the domain wall widths in Ca₃Ti₂O7 and h-Lu0.6Sc_0.4FeO₃ are 60-100 nm wide and remain insulating. We perform high pressure infrared spectroscopy to reveal pressure-induced structural phase transitions. Combined with symmetry analysis and complimentary lattice dynamics calculations, we assign high pressure phases by comparing experimentally observed changes in the vibrational response with predicted mode patterns for a series of candidate space groups. For the case of hybrid improper ferroelectric Sr₃Sn₂O₇, we discover that the set of structural phase transitions as a function of pressure mirror the reported sequence as a function of temperature. A similar analysis is performed on multiferroic h-Lu0.6Sc_0.4FeO₃ . We reveal a structural transition from a polar → antipolar space group at 15 GPa. We relate this distortion to changes in the bipyramidal tilting modes and competing structural trends in this linear magnetoelectric ferrite.