Negative thermal expansion driven by acoustic phonon modes in rhombohedral Zn2GeO4

Abstract

Negative thermal expansion (NTE) of materials is an intriguing phenomenon challenging the concept of traditional lattice dynamics and of importance for a variety of applications. Herein, we investigate the mechanism of NTE in the phenacite structure Zn2GeO4 using first-principles calculations with quasi-harmonic approximation (QHA). It is shown that in contrast to the NTE driven by the low energy optical-phonon modes in most open-framework oxides, the NTE in Zn2GeO4 is dominated by the lowest-frequency acoustic phonon modes which possess large negative Grüneisen parameters. Due to the three coordination nature of oxygen in the phenacite structure, the lateral vibration of oxygen is largely inhibited. The Raman and infrared spectra are measured to provide further proof for the rationality of the theoretical calculation. It is predicted and experimentally demonstrated that elemental doping has a negligible effect on the thermal expansion property in the structure because it is dictated by the long wavelength acoustic phonons which should be insensitive to local defects. This work provides an understanding on the NTE in the phenacite structure and may prompt the search for new NTE materials with similar structures.

Keywords

• Negative thermal expansion
• Acoustic phonon modes
• Grüneisen parameters
• Phenacite structure
• First-principles calculations