High-pressure and temperature dependent Raman studies on negative thermal expansion material Zn2P2O7

Zn2P2O7, a representative material in the A2P2O7 family, exhibits negative thermal expansion (NTE) and possesses a remarkable negative linear expansion coefficient. Here we employ variable temperature and high-pressure Raman spectroscopy to investigate the phonon anomalies arising from the α- to β-phase transition in Zn2P2O7. Combing with the refinement results of our X-ray diffraction (XRD) experiment, we have identified the specific variations of the bond lengths, bond angles and the polyhedral distortion indexes that result in the phonon anomalies. Some certain phonons, whose energy decreases with increasing pressure, have been demonstrated to give rise to negative Grüneisen parameters that can characterize phonon anharmonicity and lead to lower thermal expansion in the α-phase of Zn2P2O7 compared to the β-phase. These results provide new insight into the relationship between lattice deformation and phonon anomalies during the structural phase transition in the NTE material Zn2P2O7.