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We present pressure-induced phase transitions and metallization in VO 2 nanoparticles characterized by synchrotron x-ray diffraction, Raman spectroscopy, and infrared reflectivity measurements. The M1-M1 ′ -Mx phase transitions were found in VO 2 nanoparticles upon compression. The results of IR reflectivity shown that pressure-induced metallization occurs in the M1 ′ phase with increasing pressure and the sample becomes fully metallic at the transition of M1 ′ to Mx . The metallic Mx phase transforms to metastable mixed phases displaying insulating properties upon decompression. We attribute the pressure-induced metallization of the M1 ′ phase to the strong electron correlations, while the metal-insulator transition (MIT) from the Mx to the mixed phases is found to be associated with the structural phase transitions. Both the electron-correlation-driven Mott transition and the structure-driven MIT can be achieved in VO 2 by applying pressure. Compared with bulk samples, the VO 2 nanoparticles exhibit larger bulk moduli and increased transition pressure due to their nanosize effects. High pressure provides an effective method to study the MIT in strongly correlated materials and paves the way for modifying electronic properties of VO 2 . [ABSTRACT FROM AUTHOR]

Layered transition-metal dichalcogenides (TMDs) have recently attracted intense scientific and engineering interest because of their unique semiconducting and opto-electronic properties. We investigated the pressure-induced structural phase transition of layered semiconductor molybdenum disulfide (MoS2) using Raman spectroscopy and studied its metallization using infrared (IR) spectroscopy under both non-hydrostatic and quasi-hydrostatic conditions. Under quasi-hydrostatic and non-hydrostatic conditions, we found that the structural phase transition from 2 Hc stacking to 2 Ha stacking starts at approximately 16 and 21 GPa, respectively, and finishes at ∼35 and ∼41 GPa, respectively. Furthermore, the structural phase transition was followed by a semiconductor-to-metal (S-M) electronic transition. The pressure point of metallization under quasi-hydrostatic conditions is ∼5 GPa lower than that under non-hydrostatic conditions. [ABSTRACT FROM AUTHOR]

Single crystalline C60 nanotubes having face-centered-cubic structure with diameters in the nanometer range were synthesized by a solution method. In situ Raman and photoluminescence spectroscopy under high pressure were employed to study the structural stabilities and transitions of the pristine C60 nanotubes. A phase transition, probably because of the orientational ordering of C60 molecules, from face-centered-cubic structure to simple cubic structure occurred at the pressure between 1.46 and 2.26 GPa. At above 20.41 GPa, the Raman spectrum became very diffuse and lost its fine structure in all wavenumber regions, and only two broad and asymmetry peaks initially centered at 1469 and 1570 cm-1 were observed, indicating an occurrence of amorphization. This amorphous phase remained to be reversible until 31.1 GPa, and it became irreversible to the ambient pressure after the pressure cycle of 34.3 GPa was applied. Copyright © 2012 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

• The high pressure phase transitions of HfSe 2 were studied. • Pressure-induced semiconductor-metal transition appeared in the original P-3m1 phase. • The metallization was due to the overlap of valance and conduction bands with the contribution of Hf d and Se p orbitals. • A new metallic I 4 /mmm structure in HfSe 2 was found under high pressure. [Display omitted] In this study, we report the phase transitions and semiconductor-to-metal (S-M) transition of hafnium selenide (HfSe 2) under high pressure through experiments and theoretical calculations. The starting hexagonal structure transforms to an intermediate phase and I 4 /mmm structure above 10 GPa, and then converts into the I 4 /mmm structure completely above 47.2 GPa. Pressure-induced S-M transition at ~10 GPa was revealed by infrared (IR) reflectivity and resistance measurements. Theoretical calculations indicate that the pressure-induced metallization occurs in the initial hexagonal phase, which arises from the overlap of the valance and conduction bands with the contribution of the Hf d and Se p orbitals under high pressure. Upon pressure releases, the I 4 /mmm phase transforms into the initial hexagonal structure. This work provides new insights into the structures and electronic states of HfSe 2. [ABSTRACT FROM AUTHOR]

Magnetic nanoparticles of the La 0. 6 Ag 0. 2 Mn 1. 2 O 3 manganite with superstoichiometric manganese have been obtained using nitrate pyrolysis method. Structure, phase composition, morphology, and size of the nanoparticles have been determined. Magnetic properties of the La 0. 6 Ag 0. 2 Mn 1. 2 O 3 nanoparticles have been studied near the temperature of their magnetic ordering. It has been shown that magnetization near critical temperature is equilibrium and superparamagnetic. It has been found that magnetization and a change in entropy have critical behavior with a large value of the magnetocaloric effect. Interrelation between the critical indices of magnetization and the magnetocaloric effect has been established. In strong magnetic field, these properties of the nanoparticles have behaviour comparable with bulk samples. In weak magnetic field, the effect of superparamagnetism on the entropy has been found. The magnitude of the contribution from superparamagnetism to the magnetocaloric effect has anomalous temperature dependence. The obtained results of the influence of superparamagnetism on the thermomagnetic properties of the nanoparticles near critical temperature are important to understand and can be used for applied applications. • Magnetically homogeneous La 0 · 6 Ag 0 · 2 Mn 1 · 2 O 3 nanopowder has been prepared. • It has been found that the soft magnetic nanoparticles are in SPM state near T C. • Critical indices obtained experimentally are in good agreement with each other. • In small H , an entropy change Δ S M associated with SPM near T C has anomalous maximum. • High values of Δ S M in the nanopowder under small magnetic field have been obtained. [ABSTRACT FROM AUTHOR]