Temperature and Pressure Dependent Phase Transitions of β'-LiZr2(PO4)3 Studied by Raman Spectroscopy.

LiZr2(PO4)3 (LZP) belongs to the NASICON family of compounds whose ionic conductivity can be tuned by substitution of different cations or by increasing the temperature or pressure. Besides its conductivity, thermal and electrochemical stability makes it useful as a cathode material for lithium-ion energy storage devices. Temperature dependent Raman spectroscopic studies were carried out on the monoclinic (β') phase of LZP in the temperature range 298-853 K. A reversible structural phase transition driven by disorder in lithium sites is observed at 603 K. The spectral data enable an understanding of dynamics of the mobile Li ion and PO4 internal modes across the orthorhombic structural phase transition. On the basis of these studies, a reported change in the conductivity around 600 K is explained. High pressure Raman spectroscopic measurements on β'-LiZr2(PO4)3 reveal the onset of a structural phase transformation at 3.8 GPa and amorphization above 10 GPa. On decompression from 26 GPa, the amorphous phase remains unchanged, indicating irreversible nature of pressure-induced amorphization. Three low frequency Raman modes at 100, 124, and 144 cm(-1), which soften with an increase in pressure could be the driving force for the phase transition at 3.8 GPa. Pressure-induced phase transition prior to amorphization in β'-LiZr2(PO4)3 could be due to collapse of Zr-O-P bond angles. Pressure-induced amorphization in this compound might be due to kinetic hindrance of equilibrium decomposition.