2 that this is a rhombohedral-rhombohedral symmetry- preserving Mott transition, that seems quite unlikely, be- cause Haumont et al. [22] have found several phase tran- sitions at lower pressure. Thus the symmetry of BiFeO$_{3}$ is not rhombohedral on either side of the high-pressure metal-insulator transition. Whether the transition is a Mott transition or a band transition is unproven. Various theoretical models disagree: Vasquez et al. [3] get an ab initio Mott transition; Clark et al. [23] got a band tran- sition to a semimetal from a screened exchange model. buffer layer by pulsed laser deposition (PLD). A Jovin Yvon T64000 micro-Raman microprobe system with Ar ion laser ( λ = 514.5 nm) in backscattering geometry was used for polarized and temperature dependent Raman scattering. Sample deposition and experimental details are given in [15]. The X-ray diffraction (XRD) pattern (see Fig. 2a) of the BFO films taken using CuK$_{α}$ (1.5406 ˚ A) radiation show c -axis (pseudo-cubic < 001 > direction perpendicu- lar to the substrate) orientation with a high degree of crystallinity. The c -axis length was found to be 3.95 ˚ A, which implies epitaxial strain is quite relaxed. This agrees with the reported values ( c = 3.997 ˚ A) [31]. The comparison of the unpolarized (perpendicular to the < 001 > of the substrate) Raman spectrum of BFO thin film with STO and SRO/STO spectra (cf. Fig. 2a in [15]) precludes any Raman contribution from the sub- strate and bottom electrode; to the contrary, we observed a dip, rather than a peak, at the STO strongest peak po- sition. As is evident from the intensity comparison, all of these peaks are due to the BFO normal modes of vi- brations and none of them arose from the substrate. We verified our results using target materials, single crys- tals, and also by growing (001) BFO films on different substrates. Figs. 1a and b show the temperature variation (from RT up to 1000 ◦ C) of unpolarized Raman spectra of a BFO (001) thin film. A closer observation near the phase transitions reveals two noticeable changes in the signa- ture of the Raman spectra: the disappearance of several stronger modes at ∼ 820 ◦ C and the complete disappear- ance of all the modes above 1000 ◦ C. This temperature behavior implies that BFO maintains its room temper- ature structure up to ∼ 820 ◦ C, indicating the a struc- tural (ferroelectric) phase transition, in agreement with the earlier investigations on BFO bulk single crystal and polycrystalline samples [24]. Note that thin films of BFO show first order phase transitions as in bulk, whereas STO and PbTiO$_{3}$ PTO are known to be first order in bulk but second order in thin films [25, 26]. The presence of the four peaks ( ∼ 213, 272, 820 and 918 cm − $^{1}$) above 820 ◦ C up to ∼ 950 ◦ C (Fig. 1b) shows that the intermediate beta-phase is not cubic (P m ¯ 3 m ) as reported by Haumont et al [24]. In fact, the phase dia- gram of BFO [27], and its more recent revised versions [15], show that BFO possesses a non-cubic β -phase be- tween 820 to 933 ◦ C before it goes to the γ -phase, and the β -phase was recently shown to be orthorhombic by using high temperature X-ray diffraction and domain struc- tures [15] and neutron diffraction [1]. The complete dis- appearance of peaks at above 950 ◦ C – not at 930 ◦ C – indicates that the γ high temperature phase also can- not be cubic (P m ¯ 3 m ), for which any first-order Raman scattering is forbidden. Fig .1 shows the temperature dependent Raman spec- tra of (001)BFO film (300nm thick) on SrTiO$_{3}$ substrates with SrRuO$_{3}$ buffer layer (25nm thick). As can be seen from Fig .1b, four phonon features marked with arrows persist into the γ -phase from 930-950 ◦ C. Therefore this confirms an existence of non-cubic γ -phase. No first- order Raman scattering from phonons is allowed for cu- bic Pm3m (because each ion is at an inversion center, all phonons are odd-parity). This is in agreement with the observation of an orthorhombic symmetry for the γ -phase by Arnold et al. [1]. Although the Raman lines are broad and weak at these temperatures, they exhibit no signifi- cant frequency shift at the β - γ transition at 930 ◦ C; nor are there additional lines above or below 930 ◦ C. There- fore it is very likely that the structure is Pbnm in both β and γ phases and that no primitive-to-body-centered phase change occurs. Note that Fig .1 is for a thin film. Therefore we conclude that the films do not differ from single crystals and powders previously studied with re- gard to the γ -phase. Fig. 2a shows the room-temperature XRD patterns of an as-grown film and film after it underwent 1000 ◦ C ther- mal cycle. As can be seen, the as-grown film is highly epitaxial showing only (00l) peaks and became polycrys- talline after thermal cycling. In principle it is possible that the specimen would melt at high temperatures and then recrystallize in the specimen holder (bottom of the Pt crucible). However, we monitored the sample surface continuously with an optical microscope and no thermal decomposition was observed up to 1000 ◦ C. III. RESULTS AND DISCUSSION A. Phonons in the γ -phase The Raman spectra (Fig. 2b) before and after heat- ing show exactly same number of phonon modes, indicat- ing either no decomposition up to 1000 ◦ C or complete recrystallization, contrary to earlier studies [17], which could be due to the reduced surface/volume ratio, mini- mal surface imperfections and defects, and increased sta- bilization from the substrate. Note that the possibili- ties of subtle structural changes (small changes on angles and/or in-plane lattice parameters) cannot be completely There is renewed interest in this β - γ transition because of the earlier discovery of a metal-insulator transition at 931 ◦ C where the orthorhombic-cubic transition takes place in bulk [15] and that the same transition may oc- cur at 47 GPa at RT. Earlier M¨ ossbauer studies estab- lished that at this pressure the magnetization also dis- appears [21]. Although Gavriliuk et al. [21] concluded