Novel compounds in the Zr–O system, their crystal structures and mechanical properties

Abstract

With the motivation of exploring new high-strength ceramics, ab initio evolutionary simulations are performed to search for all the stable compounds in the Zr-O system. We have found that not only the traditional compound ZrO2, but also the ordered suboxides R3̅-Zr6O, R3̅c-Zr3O, P3̅1m-Zr2O and P6̅2m-ZrO are stable at zero pressure. The crystal structure of semimetallic P6̅2m-ZrO consists of Zr-graphene layers and can be described as an intercalated version of the ω-Zr structure. An interesting massive Dirac cone is found in the three-dimensional (3D) band structure of P6̅2m-ZrO at the Γ-point. The elastic properties, the hardness and the correlation between the mechanical properties of Zr-O compounds and the oxygen content have been systematically investigated. Surprisingly, the hardest zirconium oxide is not ZrO2, but ZrO. Both P6̅2m-ZrO and P3̅1m-Zr2O exhibit relatively high hardness values of 14 GPa and 10 GPa, respectively. The anisotropic Young's modulus E, torsion shear modulus Gt and linear compressibility β have been derived for P6̅2m-ZrO and P3̅1m-Zr2O. Further analyses of the density of states, the band structure and the crystal orbital Hamilton population indicate that the electronic structure of Zr-O compounds is directly related to their mechanical properties. The simultaneous occurrence of the 3D-framework of Zr-O and the strong Zr-Zr bonds in P6̅2m-ZrO explains its high hardness.

First-principles investigation of Zr-O compounds, their crystal structures, and mechanical properties

“…P 62m-ZrO and P 62m-HfO are formed on x-Zr and x-Hf, and exhibit interesting structures and electronic properties. 9,20 Besides, P 62m-TiO has been proven to be the ground state of titanium monoxide and it has been experimentally detected using XRD in Amano's work. 11 There is a phase transition for ZrO at 72 GPa, from the hexagonal P 62m-ZrO with fivefold coordination of oxygen atoms to an orthorhombic Pmma-ZrO structure with sixfold coordination of oxygens ( Fig.…”

“…5 In addition, Furuta and Motohashi 6 found a cubic ZrO (a ¼ 4.62 Å ) on a zircaloy oxidized in steam at 1000 C. Ni 7 observed thin intermediate oxide layers with composition close to ZrO. Theoretical calculations [8][9][10] indicated that hexagonal P 62m-ZrO instead of NaCl-type ZrO is the ground state of zirconium monoxide, and P 62m-ZrO should be stable up to 1000 K. 8,9 Even though P 62m-ZrO had not been experimentally reported, just recently, the experiment 11 successfully synthesized the P 62m-TiO by using a bismuth flux and demonstrated that NaCl-type TiO was actually a hightemperature phase.…”

“…In addition, -Fe 2 N type Zr 2 O was proposed using Xray and neutron diffraction in Hashimoto's work. 17 Firstprinciples calculations 8,9,18 indicated that R 3-Zr 6 O, R 3c-Zr 3 O and P 31m-Zr 2 O were ground-state structures.…”

Pressure-induced novel compounds in the Hf-O system from first-principles calculations

“…It is useful in practice to show the directional variation of Young's modulus by three-dimensional diagrams, where deviations from sphericity directly reflect the degree and direction of anisotropy. We calculated the elastic anisotropy of five special phases: P62m- calculations [29], therefore the band structure of P62m-HfO calculated by HSE06 is supposed to be similar. However the more accurate approach HSE06 result turns out that P62m-HfO has the tiny band gap 0.112 eV between the conduction band and valence band while DFT-GGA band structure shows that very few states at the Fermi level and P62m-HfO should be a semimetal.…”

Effect of oxidation and nitriding on the properties of zirconium alloys

“…In paper [24], the following thermo-dynamically stable compounds were revealed: Zr 6 O, Zr 3 O, Zr 2 O, ZrO and ZrO 2 , as well as the Gibbs powers, at which the formation of the above compounds takes place, were established ( Depending on the temperature-temporal and gas-dynamics parameters of zirconium interaction with oxygen-containing medium, in the near-surface layer of metal, an oxide film and/or only a diffusion layer is formed. Paper [10] also shows that in the "Zr-O" system there is only one stable oxide ZrO 2 , which has several modifications.…”