Vanadium phase transition

Phase transitions are involved in critical temperature thermistors. Vanadium, VO2, and vanadium trioxide [1314-34-7] V2O3, have semiconductors—metal transitions in which the conductivity decreases by several orders of magnitude on cooling. Electronic phase transitions are also observed in superconducting ceramics like YBa2Cu30y but here the conductivity increases sharply on cooling through the phase transition. 

The compound forms a continuous solid solution between 4.2 and 3.0 V and shows two reversible phase transitions at lower potential during cycling. The V3O8 layers of LiV308 are comprised of vanadium in square pyramids (VI), distorted octahedra (V2), and more regular octahedral (V3). ° Experimental and theoretical studies performed by Benedek et al., Picciotto et al., and Jouanneau suggested a number of different possible (octahedral and tetrahedral) sites for Li. 

Pilling-Bedworth ratio of 1 96, anatase phase films can show cracks and fissures with, consequendy, a loss of mechanical stability, however a hydrogen reduction treatment above 600°C leads to phase transition from anatase (101) to rutile (110) [43] with XRD detecting TiH2 upon prolonged hydrogen treatment of titania. As shown in Fig. 4.4, introduction of vanadium increases the intensity of the anatase Ti02 peak above 700°C disappearance of the vanadium (001) peak and the simultaneous appearance of the rutile (110) peak are observed, but anatase continues to dominate even after heat treatment at 800° C. A sharp vanadium (001) peak is observed for heat treatments carried out in air, while no vanadium peak has been seen in the case of heat treatment at 600°C in presence of Ar/H2. 

The fabrication process of vanadium oxide (VO2) has also been studied using RBS/C. Since optieal and electrical properties of VO2 are dramatically changed at 68°C due to phase transition, VO2 is regarded as one of the candidates for thermally activated electronic or optical switching devices for optieal fibers or sensors. To obtain the desired properties, the development of the fabrication process for very thin films, without crystalline defects on various substrates, is required. Single-crystalline VO2 thin films on (0001) plane of a sapphire substrate have been synthesized by a laser ablation method. The quality of VO2 was examined by X-ray diffraction and RBS/C method. The eleetrieal resistanee and the optical transmittance of the VO2 film were measured under inereasing and deereasing temperatures. At a temperature of 68 °C, an abrupt transition of resistanee from metal to... 

Bismuth vanadate, BiV04, exhibits a ferroelashc-paraelastic phase transition and had been the subject of considerable investigation. This is reported in some detail in the entry on Vanadium. 

Fukai Y, Kazama S. NMR studies of anomalous diffusion of hydrogen and phase transition in vanadium-hydrogen alloys . Acta Metall., (1977), 25, 59-70. 

Results of investigations of vanadium dioxide thin films obtained by the electron-beam evaporation method are considered. Structural and surface morphology of VO., thin films on silicon substrates are presented. Analysis of electrical characteristics of the studied samples is given. A hysteresis of temperature dependences of capacitance and resistance, as well as the phase transition at 58 °C are obtained. 

One should pay attention to the processes taking place in the film after the phase transition, as well. Metallic type of conductance was not observed in our experiment after the phase transition. The conductance continued to increase. This effect could be explained by the presence of small amounts of vanadium pentoxide in the VO2 layer. Co-existence of different conduction mechanisms is possible in that case. 

The obtained vanadium dioxide films are characterized by a homogeneous finegrained nanostructure with grain size not exceeding 160 nm. The phase transition temperature was about 58 °C. All the samples are characterized by hysteresis of resistance and capacitance. The obtained results give evidence that there are an unoxidized vanadium and its lower oxides in the VO2 films as a separate scattered nanoinclusions. 

Documentation exists in the literature as to the observation of anomalies in the temperature dependence of some physical properties of vanadium in the range 175-325 K. Although the anomaly was attributed by different workers to an antiferromagnetic transition, a small distortion of the body-centered cubic crystal structure, and impurities, Finkel et al. ( ) recently ascribed the anomaly to a second order phase transition at 230 K. Using low temperature x-ray diffraction techniques in the study of a single crystal of vanadium, Finkel et al. observed a decrease in crystal lattice symmetry form body-centered cubic (T > 230... 

For this reason, a perspective alternative may be a photonic crystal based on opal-vanadium dioxide (VO2) composite, with a semiconductor- metal phase transition in VO2. Experiments on pump-probe measurements of the VO2 phase transition, which use pulses from mode-locked lasers, have shown that this transition occurs for a period of several hundreds of femtoseconds [6]. 

Mixed vanadium-chromium oxide compounds present a wide range of interesting properties for instance, they have excellent catalytic properties, and recently they were shown to be potential candidates for anodes in lithium-ion batteries. DTA, TG and powder XRD analyses were used [101] to monitor the dehydration/crystallization and phase transitions upon heat treatment of the hydrated vanadates obtained through the reaction of peroxo-polyacids of vanadium and chromium, and to determine the ranges of coexistence of the phases in equilibrium. 

Tf is the phase transition temperature and is the concentration of vanadium at the lower boundary of the o-phase homogeneity area. 

Turov, V.V, Gorbik, P.P., Ogenko, V.M., Shulga, O.V., and Chuiko, A.A. 2001. Characteristic properties of metal-semiconductor phase transitions of vanadium dioxide in a polyethylene glycol medium containing tetraethylammonium bromide. Colloids Surf. A Physicochem. Eng. Aspects 178 105-112. 

In all the above mentioned experiments, hydrostatic (i.e. isotropic) pressure was applied to materials. In high-pressure experiments, this is usually assumed to be the case, but it is true only when the stress environment is purely hydrostatic. In other cases the stress state of the sample should be described by stress tensors, which are very difficult to determine. The mean pressure could be some average of the normal stress components over the sample, but one cannot neglect the effects of shear stress, differential stress, and stress inhomogeneity on many physical properties [191]. Influence of different conditions of compression on has been studied in detail on the bcc to rhombohedral phase transition in vanadium. Under a non-hydrostatic compression the phase transition occurred at 30 GPa at ambient temperature and at 37 GPa at 425 K. Under quasi-hydrostatic compression in the Ar pressure medium, Ptr increased to 53 GPa. When Ne was used as the medium, Ptr increased to 61.5 GPa, still short of the ideal value of 65 GPa [192]. 

Takemura K (2007) Pressure scales and hydrostaticity. High Pressure Res 27 465 72 Jenei Zs, liermann HP, Cynn H et al (2011) Structural phase transition in vanadium at high pressure and high temperature influence of nonhydrostatic conditions. Phys Rev B 83 054101 Batsanov SS (2004) Determination of ionic radii from metal compiessibUities. J Struct Chem 45 896-899... 

Catalyst deactivation was also studied. It was shown that crystalline vanadium oxide was formed due to phase transition of anatase into rutil [158]. 

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