ABSTRACT
In the past decade, remarkable advances have been made toward establishing terahertz (THz) spectroscopic techniques in terms of experimentation and theory. Terahertz spectroscopy has proven to be a powerful tool in analytical chemistry for identifying molecules, distinguishing polymorphs, and detecting impurities. With the accumulation of large amounts of spectroscopic data, researchers have recognized another potential application of THz spectroscopy, i.e., refining the structures of crystalline matter. Here, we review the applications of THz spectroscopy to shed light on two crucial problems in crystallography: resolving hydrogen atoms and analyzing disorder. We discuss the first problem using the example of poly(lactic acid) stereocomplex (scPLA). We determined that the derivations of hydrogen atoms from their ideal symmetry sites in the poly(l-lactic acid) (PLLA) and poly(d-lactic acid) (PDLA) break the R3c space group symmetry. We discuss the second problem using two prototype case studies, occupational and orientational disorders, where Form I of diflunisal and the low-temperature orthorhombic phase of the MAPbBr3 perovskite are used as examples, respectively. We found that THz spectroscopy reflects the fingerprint information of the local domains induced by short- range order (SRO). Finally, we report that the disordered domains in a highly oriented film of poly(glycolic acid) (PGA) show an abundance of features in THz spectroscopy that can be used to retrieve microscopic structural information.
