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UDC 539.53:546.302

Y. Al-Khatatbeh* (Amman, Jordan; New Haven, Connecticut, USA)
K. K. M. Lee
(New Haven, Connecticut, USA)

From superhard to hard: a review of transition metal dioxides TiO2, ZrO2, and HfO2 hardness (. 22–39)

The high-pressure, high-temperature behavior of transition metal dioxides TiO2, ZrO2, and HfO2 has been reviewed. In particular, early predictions and measurements that suggested superhard behavior for the transition metal dioxides have been considered. In subsequent studies, it has been concluded that superhardness is not intrinsic in the transition metal dioxides at ambient conditions or to phases produced at high-pressure and/or high-temperature conditions.

Keywords: high-pressure transition metal dioxides, hardness, titanium dioxide, zirconium dioxide, hafnium dioxide.


 UDC 546.261:539.218

Q. Zhu* (Stony Brook, NY, US)
O. D. Feya (Dolgoprudny city, Moscow Region,
Russian Federation)
S. E. Boulfelfel ( Stony Brook, NY, US)
A. R. Oganov**
(Stony Brook, NY, US; Dolgoprudny city,
Moscow Region, Russian Federation; Xi’an, P. R. China)
Metastable host–guest structure of carbon (. 40–52)

A family of metastable host–guest structures, the prototype of which is a tetragonal tP9 structure with 9 atoms per cell has been found. It is composed of an 8-atoms tetragonal host, with atoms filling channels oriented along the c-axis. The tP9 structure has a strong analogy with the recently discovered Ba-IV- and Rb-IV-type incommensurate structures. By considering modulations of the structure due to the variations of the host/guest ratio, it has been concluded that the most stable representative of this family of structures has a guest/host ratio of 2/3 and 26 atoms in the unit cell (space group P42/m). This structure is 0.39 eV/atom higher in energy than diamond. We predict it to have band gap 4.1 eV, bulk modulus 384 GPa, and hardness 61–71 GPa. Due to the different local environments of the host and guest atoms, we considered the possibility of replacing carbon atoms in the guest sublattice by Si atoms in the tP9 prototype and study the properties of the resulting compound SiC8, which was found to have similarly remarkably high bulk modulus 361.2 GPa and hardness 46.2 GPa.

Keywords: density functional theory, evolutionary algorithm, incommensurate crystal, silicon carbide.


 UDC 661.657:539.89

M. Hu, J. He, Q. Wang, Q. Huang, D. Yu, Y. Tian*, B. Xu (Qinhuangdao, P. R. China)
Covalent-bonded graphyne polymers with high hardness (. 53–67)

Eight covalent-bonded graphyne polymers have been proposed using the newly developed USPEX and CALYPSO methods based on the first principle calculations. These polymers are energetically more favorable than the corresponding graphyne under ambient pressure, and seven of them are more stable than fullerene C60, indicating their existence possibilities. The mechanical and dynamic stabilities of these crystal structures have been confirmed by calculating their elastic constants and phonon dispersion curves, respectively. The newly developed variable-cell nudged elastic band (VC-NEV) simulations show that the graphyne ® polymer transformation exhibits lower energy barrier than the graphite ® diamond transformation. Two of the graphyne polymers have been found to be superhard, and the others are hard materials. These graphyne polymers possess tunable electronic properties from metallic to semiconductive.

Keywords: graphyne polymers, hard, electronic characteristics.


 UDC 546.27:536.713

O. O. Kurakevych (Paris, France)
V. L. Solozhenko* (Villetaneuse, France)
Thermoelastic equation of state of boron suboxide B6O up to 6 GPa and 2700 K: simplified anderson-grüneisen model and thermodynamic consistency (. 68–77)

The p-V-T equation of state of superhard boron suboxide B6O has been measured up to 6 GPa and 2700 K using multianvil technique and synchrotron  X-ray diffraction. To fit the experimental data, the theoretical p-V-T equation of state has been derived in approximation of the constant value of the Anderson-Grüneisen parameter δT. Bulk modulus B0 =181 GPa and its first pressure derivative B0¢ = 6 at 300 K; two parameters describing thermal expansion at 0.1 MPa, i.e., a = 1.4×10–5 K–1 and b = 5×10–9 K–2 as well as dT = 6 have been included in the model. The good agreement between fitted and experimental isobars has been achieved to the absolute volume changes up to 5 % as compared to the volume at standard conditions, V0. The fitted thermal expansion at 0.1 MPa well consistent with the experimental data as well as with ambient-pressure heat capacity cp, bulk modulus B0 and dT describing its evolution with volume and temperature have been discussed. The fitted value of Grüneisen parameter g = 0.85 in agreement with previous empiric estimations for B6O and experimental values for other boron-rich solids has been reported.

Keywords: boron suboxide, equation of state, high pressure, high temperature.


 UDC 661.657:539.89

S. Wang (Las Vegas, NV, USA; Los Alamos, NM, USA; Chengdu, P. R. China)
X. Yu, J. Zhang (Los Alamos, NM, USA)
Y. Zhang, L. Wang (Las Vegas, NV, USA)
K. Leinenweber (Tempe, AZ, USA)
H. Xu (Los Alamos, NM, USA)
D. Popov, Ch. Park (Argonne, Illinois, USA)
W. Yang (Argonne, Illinois, USA; Shanghai, P. R. China)
D. He (Chengdu, P. R. China)
Y. Zhao* (Las Vegas, NV, USA; Los Alamos, NM, USA)
Crystal structures, elastic properties, and hardness of high-pressure synthesized CrB2 and CrB4 (. 78–88)

Chromium tetraboride (CrB4), a recently proposed candidate for superhard materials, has been synthesized at high pressure and temperature by a solid-state reaction. As a byproduct, chromium diboride (CrB2) also forms and co-exists with CrB4 in the final product. The comparative studies of crystal structure, elastic property, and hardness of both phases have been conducted at the same sample environment conditions. The crystal structure of CrB4 has been refined with an orthorhombic symmetry of Immm (space group no. 71) or Pnnm (space group no. 58) using X-ray diffraction data. Further simulations indicate that the structural distinction between Immm and Pnnm can be resolved by neutron diffraction, due to the high scattering cross-section of boron (11B) by neutrons. Although CrB2 and CrB4 have close bulk modulus at about 230 GPa, the measured asymptotic Vickers hardness yields 16 GPa for CrB2 but 30 GPa for CrB4, which is nearly two times that of CrB2. The dramatic enhancement in hardness in CrB4 is attributed to the strong three-dimensional Cr–B network, in contrast to the layered lattice structure of hexagonal CrB2.

Keywords: Chromium borides, CrB4, CrB2, high-pressure synthesis, structure, compressibility, superhard material.


 UDC 546.171.1/.718:539.89

Z. L. Zhao, K. Bao, D. F. Duan, X. L. Jin, F. B. Tian, D. Li, B. B. Liu, T. Cui*(Changchun, P. R. China)
Ideal stoichiometric technetium nitrides under pressure: a first-principles study (. 89–98)

Technetium nitrides with various ideal stoichiometries have been investigated with the first-principle method at the pressures between 0–60 GPa. It have been found that there could be many stable technetium nitrides including Tc3N, Tc2N, TcN, Tc2N3, TcN2, TcN3, and TcN4, among which Tc3N and Tc2N sub-nitrides are synthesizable at zero pressure and could be applied to nuclear waste management, such as separate radioactive 99Tc from nuclear fuel cycle. Moreover, N-rich TcN3 and TcN4 exhibit remarkable bulk properties and can be potential ultrastiff and hard materials.

Keywords: technetium nitride, crystal structure prediction, ultra-incompressible and hard materials, thermodynamic stability, dynamic stability.



4, 2018
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