Understanding Negative Thermal Expansion of ZnGeO through Local Structure and Vibrational Dynamics.

ZnGeO is a multifunctional material whose intrinsic thermal expansion properties below ambient temperature have not been explored until now. Herein, the thermal expansion of ZnGeO is investigated by synchrotron X-ray diffraction, with the finding that ZnGeO exhibits very low negative (α = -2.02 × 10 K, 100-300 K) and positive (α = +2.54 × 10 K, 300-475 K) thermal expansion below and above room temperature, respectively. A combined study of neutron powder diffraction and extended X-ray absorption fine structure spectroscopy shows that the negative thermal expansion (NTE) of ZnGeO originates from the transverse vibrations of O atoms in the four- and six-membered rings with ZnO-GeO tetrahedra. In addition, the results of temperature- and pressure-dependent Raman spectra identify the low-frequency phonon modes (50-150 cm) with negative Grüneisen parameters softening upon pressuring and stiffening upon heating during the lattice contraction, thus contributing to the NTE. This study not only reports the interesting thermal expansion behavior of ZnGeO but also provides further insights into the NTE mechanism of novel structures.