Phase transition in WO₃ microcrystals obtained by sintering process

Abstract

The vibrational properties of WO 3 ceramics in the 30-300 K temperature range were probed by Raman spectroscopy. Starting with WO 3 ceramics at room temperature presenting monoclinic (V) and triclinic (VI) mixed phases, structural transitions related to decreasing temperature were observed. At low temperatures a monoclinic (VII) and a new mixed phase were observed, the former characteristic of WO 3 bulk and the latter associated to microcrystals. The analysis of the spectra indicated that the new phase develops gradually and it comes mainly from the monoclinic (V) phase.

Electrospinning Synthesis of Porous CoWO4 Nanofibers as an Ultrasensitive, Nonenzymatic, Hydrogen‐Peroxide‐Sensing Interface with Enhanced Electrocatalysis

“…The powder X‐ray diffraction (XRD) patterns of the as‐prepared samples obtained by sintering the precursors at 625 °C are illustrated in Figure . All of the diffraction peaks in Figure A can be indexed to a monoclinic phase of WO 3 , which matches well with the standard XRD pattern (JCPDS 43‐1035) of pure WO 3 . Figure B displays the XRD patterns of the sample formed by sintering the PVP/Co(Ac) 2 precursor at 625 °C; its diffraction peaks are consistent with a face‐centered cubic (fcc) phase of Co 3 O 4 (JCPDS 43‐1003) .…”

“…. All of the diffractionp eaks in Fig-ure 2A can be indexed to am onoclinic phase of WO 3 ,w hich matches well with the standard XRD pattern (JCPDS 43-1035) of pure WO 3 [15]. …”

High electrochemical activity from hybrid materials of electrospun tungsten oxide nanofibers and carbon black

“…According to the literatures [45,46], the peaks below 200 cm -1 can be ascribed to the lattice modes related to the collective motions of the WO 6 octahedrons and that in 200-500 cm -1 and above 500 cm -1 originate from O-W-O bending modes of the bridging oxygens and W-O stretching modes in the WO 6 octahedral units, respectively. The characteristic peaks for the monoclinic and triclinic phases appear at 32 and 42 cm -1 [34,44], respectively, while their high frequency modes ([200 cm -1 ) are almost identical [44]. Unfortunately, due to the instrument limitation the characteristic peaks cannot be detected in the present study.…”

“…The monoclinic and triclinic phases are thermodynamically stable ones at room temperature and atmospheric pressure [36,39,[41][42][43], as revealed by the phenomenon that the high pressure phases of WO 3 recover to the mixture of the triclinic and monoclinic phases when the high pressure is released [38]. In many cases, the monoclinic and triclinic phases coexist in the WO 3 samples [34,37,38,41,44]. This coexistence results from the minor differences in the W position and the packing pattern of the monoclinic and triclinic structures [42].…”

“…With increasing temperature the phase transitions in the sequence of triclinic, monoclinic, orthorhombic, tetragonal, and hexagonal crystalline phases have been observed [34,35]. However, the dependence of the phase transition on synthetic route, material forms, and crystal size has also been reported, as occurring for thin film [36], micro-/nanoscale [35,37,38], and sintered ceramic [34,39] WO 3 materials. Recently, it was reported that the WO 3 films can transform into W 18 O 49 nanowires after annealing at 500°C [40].…”