First-principles Study Of Structural Phase Transition And Physical Property Changes In NbS₂

Transition metal dichalcogenides（TMDCs）compounds have attracted much attention because of their unique structure and abundant physical properties.Among them,the charge density wave properties of group-V dichalcogenides,have been a hot topic in recent decades.However,up to now,the theory of charge density wave is still not well developed,and competition or coexistence between charge density waves and superconducting properties has aroused heated discussion in the scientific community.Employing the first-principles calculations,we first explored the high-pressure structural phase transition of transition metal dichalcogenide NbS₂,and then carefully studied the charge density wave of 1T phase NbS₂ and the competition of superconductivity,and finally discussed the stacking effect on the charge density wave of NbS₂.The thesis mainly consists of the following three works:（1）We study the crystal structures,electronic structures,and structural stability of the layered TMDCs NbS₂,and shed more light on the crucial roles of the van der Waals（vd W）interactions.Theoretically calculated results imply that the vd W corrections are important to reproduce the layered crystal structure,which is significant to correctly describe the electronic structure of NbS₂.More interestingly,under hydrostatic pressure or tensile strain in ab plane,an isostructural phase transition from two-dimensional layered structure to three-dimensional bulk in the I4/mmm phase has been uncovered.The abnormal structural transition is closely related to the electronic structure instability and interlayer bonding effects.The interlayer Nb-S distances collapse and the interlayer vd W interactions disappear,concomitant with new covalent bond emerging and increasing coordination number.Present work highlights the significance of the vd W interactions,and provides new insights on the unconventional structural transitions in NbS₂,which will attract wide audience working in the hectic field of TMDCs.（2）Charge-density-wave（CDW）instability and pressure-induced superconductivity in bulk 1T-NbS₂ are predicted theoretically by first-principles calculations.We reveal a CDW instability towards the formation of a stable commensurate CDW order,resulting in a√13×√13 structural reconstruction featured with star-of-David clusters.The CDW phase exhibits one-dimensional metallic behavior with in-plane flat-band characteristics,and coexists with an orbital-density-wave order predominantly contributed by 4(92_-2 orbital from the inner Nb atoms of the star-of-David cluster.By doubling the cell of the CCDW phase along the layer stacking direction,a metal-insulator transition may be realized in the CDW phase in case the interlayer antiferromagnetic ordering and Coulomb correlation effect have been considered simultaneously.Bare electron susceptibility,phonon linewidth and electron-phonon coupling calculations suggest that the CDW instability is driven by softened phonon modes due to the strong electron-phonon coupling interactions.CDW order can be suppressed by pressure,concomitant with the appearance of the superconductivity.Our theoretical predictions call for experimental investigations to further clarify the transport and magnetic properties of 1T-NbS₂.Furthermore,it would also be very interesting to explore the possibility to realize the CDW order coexisting with the superconductivity in bulk 1T-NbS₂.（3）We explored the stacking effects in the charge density wave（CDW）phase of bulk 1T-NbS₂.Metal-insulator transition（MIT）and the origins of the insulating phases are investigated.We identified a band insulator with inherently favorable energy owing to Peierls dimerization.By exploring the electronic structures,we recognized a competitive relationship between Peierls dimerization and electronic correlations,revealing automatic absence of dimerization in traditional stacking.Considering Coulomb correlations in step with spin polarization,we describe a phase diagram to summarize the origin of the band insulator or Mott insulating phases.To understand the interlayer magnetic configuration,interlayer spin-exchange coupling is surveyed along the selected transition pathway.Our results indicate that the interlayer magnetic coupling strength strongly correlates with stacking order,and show that the central Nb-atoms is the key to understand the magnetic behaviors of stacked CCDW phase.Our work emphasizes the important role of interlayer electronic ordering,and shed fresh light on the origin of the insulating behavior of CCDW phase.