Study of Phase Transition in NbO<sub>2</sub>

Sakata, T.; Sakata, Kimiko; Nishida, Isao

doi:10.1002/pssb.19670200263

Cited by 80 publications

(27 citation statements)

References 6 publications

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“…This loss of correlations, testified by the VO 2 M2 phase having no counterpart in NbO 2 [193], and by the efficacy of ab initio methods to describe NbO 2 [74,[194][195][196], is actually overcompensated by the increase in covalency due to the broader spatial distribution of the 4d orbitals [197], which, in turn, yields a stronger coupling with the zone-boundary lattice modes, and thus a higher transition temperature. The intermediate poorly metallic phase for T MIT < T < T s [188,189,198,199] is thus the counterpart of the monoclinic metal in VO 2 , although the former is more clearly established than the latter.…”

Section: Discussionmentioning

confidence: 92%

“…NbO 2 also undergoes a metal-insulator transition, though at substantially higher temperature of T MIT ∼ 1080 K [184][185][186][187]. This transition occurs slightly below a structural one at T s ∼ 1123 K [188], from a high-temperature rutile structure to a low-temperature body centred tetragonal one that locally resembles the M1 phase of VO 2 [189][190][191][192]. However, the single 4d-electron in Nb 4+ is expected to be less correlated than the 3d-electron in V 4+ .…”

Section: Discussionmentioning

confidence: 97%

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Unraveling the Mott-Peierls intrigue in vanadium dioxide

Grandi

Amaricci

Fabrizio

2020

Phys. Rev. Research

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Vanadium dioxide is one of the most studied strongly correlated materials. Nonetheless, the intertwining between electronic correlation and lattice effects has precluded a comprehensive description of the rutile metal to monoclinic insulator transition, in turn triggering a longstanding "the chicken or the egg" debate about which comes first, the Mott localisation or the Peierls distortion. Here, we show that this problem is in fact ill-posed: the electronic correlations and the lattice vibrations conspire to stabilise the monoclinic insulator, and so they must be treated on equal footing not to miss relevant pieces of the VO2 physics. Specifically, we design a minimal model for VO2 that includes all the important physical ingredients: the electronic correlations, the multi-orbital character, and the two components antiferrodistortive mode that condenses in the monoclinic insulator. We solve this model by dynamical mean-field theory within the adiabatic Born-Oppenheimer approximation. Consistently with the first-order character of the metal-insulator transition, the Born-Oppenheimer potential has a rich landscape, with minima corresponding to the undistorted phase and to the four equivalent distorted ones, and which translates into an equally rich thermodynamics that we uncover by the Monte Carlo method. Remarkably, we find that a distorted metal phase intrudes between the low-temperature distorted insulator and high-temperature undistorted metal, which sheds new light on the debated experimental evidence of a monoclinic metallic phase.

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Section: Discussionmentioning

confidence: 92%

Section: Discussionmentioning

confidence: 97%

Unraveling the Mott-Peierls intrigue in vanadium dioxide

Grandi

Amaricci

Fabrizio

2020

Phys. Rev. Research

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“…19. As c decreases more quickly than a upon cooling in the rutile structure as observed in TiO 2 [92], the c/a ratio becomes small at low temperatures in VO 2 [74] and NbO 2 [29]. Then, the distorted structures are going to be selected at low temperatures.…”

Section: Structural Instability Driving the Mitsmentioning

confidence: 98%

Structural instability of the rutile compounds and its relevance to the metal–insulator transition of VO2

Hiroi

2015

Progress in Solid State Chemistry

100

103

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The metal-insulator transition (MIT) of VO 2 is discussed with particular emphasis on the structural instability of the rutile compounds toward dimerization. Ti substitution experiments reveal that the MIT is robust up to 20% Ti substitutions and occurs even in extremely thin V-rich lamellas in spinodally decomposed TiO 2 -VO 2 composites, indicating that the MIT is insensitive to hole doping and essentially takes on a local character. These observations suggest that either electron correlation in the Mott-Hubbard sense or Peierls (Fermi-surface) instability plays a minor role on the MIT. Through a broad perspective of crystal chemistry on the rutile-related compounds, it is noted that VO 2 and another MIT compound NbO 2 in the family eventually lie just near the borderline between the two structural groups with the regular rutile structure and the distorted structures characterized by the formation of dimers with direct metal-metal bonding. It is also shown that the two compounds of the rutile form do not follow the general trends in structure observed for the other rutile compounds, giving clear evidence of an inherent structural instability present in the two compounds. The MITs of VO 2 and NbO 2 are natural consequences of structural transitions between the two groups, as all the d electrons are trapped in the bonding molecular orbitals of dimers at low temperatures. Such dimer crystals are ubiquitously found in early transition metal compounds having chain-like structures, such as MoBr 3 , NbCl 4 , Ti 4 O 7 , and V 4 O 7 , the latter two of which also exhibit MITs probably of the same origin. In a broader sense, the dimer crystal is a kind of "molecular orbital crystals" in which virtual molecules made of transition metal atoms with partially-filled t 2g shells, such as dimers, trimers or larger ones, are generated by metal-metal bonding and are embedded into edge-or face-sharing octahedron networks of various kinds. The molecular orbital crystallization opens a natural route to stabilization of unpaired t 2g electrons in crystals.2

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“…NbO 2 undergoes a phase transition at around 1073 K, which is, in all respects, akin to that for VO 2 at 341 K (Sakata, Sakata & Nishida, 1967;Sakata, 1969a). The high-temperature (HT) phase has a simple rutile-type structure (space group P42/mnm) and the low-temperature (LT) phase a slightly deformed one, characterized by a series of superlattice reflections on an Xray diffraction pattern.…”

Section: Introductionmentioning

confidence: 96%

Structural features of Nb1−xTixO2

Sakata¹

1979

Acta Crystallogr Sect B

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X-ray structural analysis with Mo Ka radiation was carried out for Nb~_xTixO 2 (0 < x < 0.2) in its lowtemperature deformed rutile phase 14~/a. Crystals were prepared by chemical transport, with TeC1 a as the transport agent. Atomic parameters determined for NbO2 are in good agreement with those obtained by neutron diffraction. For superlattice reflections with h, k = 2n + 1 and l = 2n in the system Nbt_xTixO2, the discrepancy between F o and F c is appreciable. Attempts to clarify this situation are made and discussed. Analysis of interatomic distances for NbO 2 showed that Nb-Nb pair bonds exist along z and strong Nb-O-O-Nb bonds exist along x and y in the form of non-intersecting linear chains. With such a molecular-chain model the structural features and phase transition in the Nbt_xTixO 2 system are discussed.

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Study of Phase Transition in NbO₂

Cited by 80 publications

References 6 publications

Unraveling the Mott-Peierls intrigue in vanadium dioxide

Unraveling the Mott-Peierls intrigue in vanadium dioxide

Structural instability of the rutile compounds and its relevance to the metal–insulator transition of VO2

Structural features of Nb1−xTixO2

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Study of Phase Transition in NbO2

Cited by 80 publications

References 6 publications

Unraveling the Mott-Peierls intrigue in vanadium dioxide

Unraveling the Mott-Peierls intrigue in vanadium dioxide

Structural instability of the rutile compounds and its relevance to the metal–insulator transition of VO2

Structural features of Nb1−xTixO2

Contact Info

Product

Resources

About

Study of Phase Transition in NbO₂