Understanding Negative Thermal Expansion of Zn2GeO4 through Local Structure and Vibrational Dynamics.

Zn2GeO4 is a multifunctional material whose intrinsic thermal expansion properties below ambient temperature have not been explored until now. Herein, the thermal expansion of Zn2GeO4 is investigated by synchrotron X-ray diffraction, with the finding that Zn2GeO4 exhibits very low negative (αv = -2.02 × 10-6 K-1, 100-300 K) and positive (αv = +2.54 × 10-6 K-1, 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 Zn2GeO4 originates from the transverse vibrations of O atoms in the four- and six-membered rings with ZnO4-GeO4 tetrahedra. In addition, the results of temperature- and pressure-dependent Raman spectra identify the low-frequency phonon modes (50-150 cm-1) 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 Zn2GeO4 but also provides further insights into the NTE mechanism of novel structures.