Lattice dynamics study of scheelite tungstates under high pressure II.PbWO4

Abstract

Room-temperature Raman scattering has been measured in lead tungstate up to 17 GPa. We report the pressure dependence of all the Raman modes of the tetragonal scheelite phase (PbWO 4 -I, space group I4 1 /a), which is stable at ambient conditions. Upon compression the Raman spectrum undergoes significant changes around 6.2 GPa due to the onset of a partial structural phase transition to the monoclinic PbWO 4 -III phase (space group P2 1 /n). Further changes in the spectrum occur at 7.9 GPa, related to a scheelite-to-fergusonite transition. This transition is observed due to the sluggishness and kinetic hindrance of the I→III transition. Consequently, we found the coexistence of the scheelite, PbWO 4 -III, and fergusonite phases from 7.9 to 9 GPa, and of the last two phases up to 14.6 GPa. Further to the experiments, we have performed ab initio lattice dynamics calculations which have greatly helped us in assigning the Raman modes of the three phases and discussing their pressure dependence. Tel.: + 34 96 387 52 87, Fax: + 34 96 387 71 89 was shown that in BaWO 4 the onset of a partial scheelite-to-BaWO 4 -II phase transition occurs at 6.9 GPa; i.e., at lower pressure than the scheelite-to-fergusonite transition, which was observed at 7.5 GPa. These results are in good agreement with previous ab initio total-energy calculations and ADXRD and XANES measurements [12]. Furthermore, previous high-pressure Raman spectra in BaWO 4 [8] were interpreted on the basis of the results of Ref. 10. Similarly, we will show in this work that PbWO 4 suffers the same phase transitions than BaWO 4 and that the frequencies of the Raman modes in the high-pressure phases of PbWO 4 previously reported [9] can be completely understood on the light of the present work. Our results allow us to develop a picture of the structural behavior of PbWO 4 that solves apparent discrepancies among earlier experiments and theory. II. Experimental detailsThe PbWO 4 samples used in this study were obtained from scheelite-type bulk single crystals which were grown with the Czochralski method starting from raw powders having 5N purity [15,16]. Small platelets (100µm x 100µm x 30µm) were cleaved from these crystals along the {101} natural cleavage plane [17] and inserted in a diamond-anvil cell (DAC). Silicone oil was used as pressure-transmitting medium [18] and the pressure was determined by calibration with the ruby photoluminescence [19].Raman measurements at RT were performed in backscattering geometry using the 488 Å line of an Ar + -ion laser with a power of less than 100 mW before the DAC. Dispersed light was analyzed with a Jobin-Yvon T64000 triple spectrometer equipped with a confocal microscope in combination with a liquid nitrogen (LN)-cooled multi-channel CCD detector. Spectral resolution was better than 1 cm -1 and Ar and He plasma lines were used to calibrate the Raman and photoluminescence spectra.

Effects of pressure on PbWO4-III

“…The existence of only a single sharp and intense Raman peak of PbWO 4 -III also shows the difference from a couple of intense peaks at 899 and 914 cm -1 at 13.7 GPa in previously observed Raman spectrum of the phase interpreted as the PbWO 4 -III (Manjon et al 2006).…”

“…For the room pressure Raman spectra, 31 Raman bands are observed in a wavenumber range of 100-1,000 cm -1 , which are more than 13 and 18 Raman-active modes of the tetrahedral stolzite (PbWO 4 -I, I4 1 /a) and the monoclinic raspite (PbWO 4 -II, P2 1 /a) (Bastians et al 2004), respectively, but less than 72 Raman-active modes of the PbWO 4 -III (P2 1 /n) structure expected by the group theory analysis (C Raman = 36A g ? 36B g ) (Manjon et al 2006;Rousseau et al 1981).…”

“…fergusonite (I2/a) ? P2 1 /n, with the different phase-transition pressures being dependent on the sensibility of different techniques (Manjon et al 2006;Lopez-Solano et al 2007;Errandonea and Manjon 2008). For raspite (PbWO 4 -II), a thermal-driven raspite to scheelite-structured transition was identified at 538°C by an in situ transmission electron microscopy observation (Wang et al 2011).…”

“…The structural stability of stolzite (PbWO 4 -I) has been extensively investigated by Raman spectroscopy, X-ray diffraction, X-ray absorption near-edge structure and neutron diffraction (Jayaraman et al 1985;Christofilos et al 2006;Manjon et al 2006;Zhu et al 2006;Errandonea et al 2006a, b;Grzechnik et al 2006). Previous investigations show that PbWO 4 -I has a pressure-induced structural distortion from tetragonal to monoclinic symmetry following the structural sequence: scheelite (I4 1 /a) ?…”

First-order character of the displacive structural transition in BaWO 4

“…15, Z = 4) [9,10] and cut off the negative compression of the lowest Raman modes. [11,12] The detailed description of phase transition showed a continuous change in the axial parameters and volume with pressure, [10] despite the fact that early study showed a discontinuous fact. [9] The mechanism of phase transition was evidenced further by the analysis of spontaneous strain and the soft mode theory, and a second-order ferroelastic nature was concluded.…”