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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]

• 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]