The IR powder absorption spectrum of the sample shocked at 80 GPa was also recorded at low temperatures to investigate the thermal evolution of the phonon modes of the scheelite phase and also to further study the observed IR
features, as vibrational bands generally sharpen at low temperatures. The effect of cooling on the phonon modes of the scheelite phase is weak (Fig. 2). For instance, the band at 868 cm1 (at room temperature), which is considered to be due
to a SiO stretching vibration, shifted to 871 cm1 at 20 K. The other IR bands showed small or undetectable changes in frequency. This indicates that the scheelite phase of ZrSiO4 has a very weak thermal expansion and the structural
compression caused by the cooling is weak. The data from cooling measurements resolved certain spectral features. At 20 K, the sample showed clearly local maxima near 1000, 1080 and 1180 cm1, which are too high in frequency to bedueto isolated SiO4 groups