Phonon anomalies in the two phases of the low-dimensional spin compound (VO)2P2O7 -: art. no. 064420

We studied the temperature and pressure dependence of the phonons of the two different phases of (VO)(2)P2O7, the ambient (AP) and the high-pressure (HP) phase, by polarized Raman spectroscopy. Most modes in the lower-symmetry AP phase are split compared to the higher-symmetry HP phase. The V-O vibrations around 920 cm(-1) of both phases reveal an anomalous mode softening with decreasing temperature as well as under moderate hydrostatic pressure (<3 GPa). We show that this is due to the interlayer interaction of neighboring V and O atoms as supported by temperature-dependent lattice parameter determinations. We identify a spin-phonon mode at 141 cm(-1) in the HP phase, the counterpart of the spin-phonon mode at 123 cm(-1) in the AP phase. Several modes of the AP phase around 200 cm(-1) reveal a strongly anharmonic behavior; their integrated intensity disappears as a function of temperature near 30 K. We discuss the origin of this behavior as possibly related to the spin rather than to the lattice structure.