Anisotropic thermal expansion and low-frequency vibrational modes in galactitol: Insights from temperature-dependent terahertz spectroscopy and density functional theory calculations

Galactitol, a hydrogen-bonded sugar alcohol, has attracted considerable attention as a potential organic phase-change material (PCM). In this study, we investigate its thermal expansion and low-frequency lattice vibrations using a combination of temperature-dependent terahertz (THz) spectroscopy and density functional theory (DFT) calculations. This approach enables the characterization of both static and dynamic temperature-dependent changes in the hydrogen-bond network, which plays a crucial role in determining the thermal and phase-transition properties of hydrogen-bonded organic PCMs. Among the observed THz absorption spectra, the lowest-frequency mode exhibited an anomalous blueshift upon heating, whereas all other modes demonstrated a redshift behavior. The quasi-harmonic approximation (QHA) simulations and single-crystal X-ray diffraction (SC-XRD) measurements reveal the pronounced anisotropic thermal expansion of galactitol. It is noteworthy that the QHA captures the qualitative trend of the anomalous blueshift of the lowest-frequency mode through structural changes. The blue-shifting mode is characterized by translational motion primarily along the crystallographic a -axis, with a minor intramolecular O H rocking component contributing to its infrared (IR) activity. This behavior suggests that the O···H–O hydrogen-bond network oriented along the crystallographic c -axis plays a crucial role in the anomalous blueshift.