Phonon instability and charge density wave in U2Ti

Kaur, Gurpreet; Jaya, S. Mathi; Panigrahi, B. K.

doi:10.1016/j.jallcom.2017.09.236

Cited by 4 publications

(5 citation statements)

References 37 publications

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“…There are some distorted structures found in actinide compounds such as PuO 2 , U 2 Ti, and Peierls mechanism is found to be responsible for the distortions. [48][49] To further reveal the phase transformation mechanism, we calculated the electron density of states for both CaF 2 and I4 1 /amd structures, which are shown in Figure .6c. It is found that Fermi energy decreases from 9.4587 eV for CaF 2 phase to 9.4183 eV for the distorted I4 1 /amd phase.…”

Section: Structural Dynamical Stabilitymentioning

confidence: 99%

Stable Calcium Nitrides at Ambient and High Pressures

et al. 2016

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The knowledge of stoichiometries of alkaline-earth metal nitrides, where nitrogen can exist in polynitrogen forms, is of significant interest for understanding nitrogen bonding and its applications in energy storage. For calcium nitrides, there were three known crystalline forms, CaN2, Ca2N, and Ca3N2, at ambient conditions. In the present study, we demonstrated that there are more stable forms of calcium nitrides than what is already known to exist at ambient and high pressures. Using a global structure searching method, we theoretically explored the phase diagram of CaNx and discovered a series of new compounds in this family. In particular, we found a new CaN phase that is thermodynamically stable at ambient conditions, which may be synthesized using CaN2 and Ca2N. Four other stoichiometries, namely, Ca2N3, CaN3, CaN4, and CaN5, were shown to be stable under high pressure. The predicted CaNx compounds contain a rich variety of polynitrogen forms ranging from small molecules (N2, N4, N5, and N6) to extended chains (N∞). Because of the large energy difference between the single and triple nitrogen bonds, dissociation of the CaNx crystals with polynitrogens is expected to be highly exothermic, making them as potential high-energy-density materials.

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Section: Structural Dynamical Stabilitymentioning

confidence: 99%

Stable Calcium Nitrides at Ambient and High Pressures

et al. 2016

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“…The similar behavior of α'-phase in several U-X systems (X = Nb, Ti, Zr, Ru, Re) [36] leads to the question of whether a structure with the Pmm2 space group and composition U 15 X could also be found in the GS of those systems. In addition, the existence of CDW recently found in U 2 Ti [33] highlights that this could be fairly frequent phenomenon among structures with very high content of U as U 15 Mo and U 14 Mo 2 phases. These recent findings inspire and encourage to study the GS structures and properties of those systems.…”

Section: Discussionmentioning

confidence: 81%

“…The most relevant motivations for fully characterizing the structures comprising the GS of a system are related to: i) the identification of stable and metastable structures for all compositions finding out novel phases at low temperatures, ii) the improvement of phase diagram through ab initio thermodynamics or CALPHAD methodology, iii) the discovery of unexpected properties like the Peierls distortion induced by deformation in the U 2 Mo compound, the CDW in the U 2 Ti structure [33] , or the superconducting phase transition of pure U [11] , and iv) the understanding of thermophysical properties at T=0 K in order to be extrapolated, using ab initio Molecular Dynamics [34] , at the experimental condition of interest in a nuclear reactor.…”

Section: Resultsmentioning

confidence: 99%

“…Space group: #191 P6/mmm Lattice parameters: a=b=4.8207Å c=2.7642Å Wyckoff possitions: Mo (1a) (0, 0, 0) U (2d) (1/3,2/3,1/2) U 15 Mo Space group: #25 Pmm2 Lattice parameters: a=4.9563Å b=5.6427Å c=11.5214Å Wyckoff possitions: Mo (1a) (0, 0, 0) U1 (1d) (1/2,1/2,z) z=0.394 U2 (1b) (0,1/2,z) z=0.997 U3 (1c) (1/2,0,z) z=0.402 U4 (1d) (1/2,1/2,z) z=0.899 U5 (1b) (0,1/2,z) z=0.497 U6 (1c) (1/2,0,z) z=0.907 U7 (1a) (0,0,z) z=0.499 U8 (2h) (1/2,± y,z) y=0.759 z=0.146 U9 (2m) (0,± y,z) y=0.761 z=0.247 U10 (2h) (1/2,± y,z) y=0.751 z=0.651 U11 (2m) (0,± y,z) y=0.746 z=0.749 per cell considered in this work. At variance with U 2 Ti [33] , the existence of CDW further stabilizing the Ωphase should be disregarded as no internal relaxation of coordinates was found by duplication of the cell along the c-axis, neither imaginary frequencies were observed in phonon dispersion curves [21] , showing its dynamical stability.…”

Section: ω-U 2 Momentioning

confidence: 99%

“….899 U5 (1b) (0,1/2,z) z=0.497 U6 (1c) (1/2,0,z) z=0.907 U7 (1a) (0,0,z) z=0.499 U8 (2h) (1/2,± y,z) y=0.759 z=0.146 U9 (2m) (0,± y,z) y=0.761 z=0.247 U10 (2h) (1/2,± y,z) y=0.751 z=0.651 U11 (2m) (0,± y,z) y=0.746 z=0.749 per cell considered in this work. At variance with U 2 Ti [33] , the existence of CDW further stabilizing the Ωphase should be disregarded as no relaxation of coordinates was found by duplication of the cell along the c-axis, neither imaginary frequencies were observed in phonon dispersion curves [21] , showing its dynamical stability.…”

Section: Omega-phasementioning

confidence: 99%

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On the ground state of the U-Mo system

Losada

Garcés

2019

Journal of Nuclear Materials

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The ground state of the U-Mo system is studied in this work using an evolutionary algorithm coupled with ab initio calculations. This methodology is applied to several compositions, with a more detailed examination at the U-rich side. A new ground state is identified within an approach that considers structures for up to 3 formula units per cell. A supercell scheme based on the pure phase structure of each element is used for dilute compositions. The ground state at a pressure of 1 atm. is characterized by only two stable compounds: Ω-phase and a new phase, named α-U 15 Mo in this work, with a structure determined by adding a substitutional Mo impurity to a 1 × 2 × 2 supercell of α-U. A range of immiscibility of U in bcc-Mo based structures is extended from 0 to nearby 50 at. % of U. The new α-U 15 Mo phase comprising the ground state has similar features to those of the α -phase formed in quenched samples at room temperature. The internal relaxation of U atoms in this phase resembles the movements due to charge density waves observed in orthorhombic α 1 -U. The ground state of the U 15 Mo compound at T=0 K can not be fully identified due to the charge density wave associated with α-U. That structure could be a supercell based on the structurally undefined α 3 -U phase.

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