-
[show abstract]
[hide abstract]
ABSTRACT: Compressibility of high-purity nanostructured cBN has been studied under
quasi-hydrostatic conditions at 300 K up to 35 GPa using diamond anvil cell and
angle-dispersive synchrotron X-ray powder diffraction. A data fit to the Vinet
equation of state yields the values of the bulk modulus B0 of 375(4) GPa with
its first pressure derivative B0' of 2.3(3). The nanometer grain size (\sim20
nm) results in decrease of the bulk modulus by ~9%.
08/2012;
-
[show abstract]
[hide abstract]
ABSTRACT: A strong correlation relationship has been established between the structure and specific Gibbs free energy of the substance
atomization on the one hand, and the substance hardness and volume compressibility on the other. In the framework of the model
proposed hardness is directly proportional to the specific Gibbs free energy per bond in isodesmic crystals. An application
of a correction coefficient to the ionic component of chemical bonds allows one to evaluate the hardness of compounds having
both the covalent (polar and nonpolar) and ion bonds. In the framework of the suggested approach we have been the first to
correctly calculate the temperature dependence of the hardness by the example for diamond and cubic boron nitride.
Journal of Superhard Materials 04/2012; 30(6):368-378. · 0.79 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Cubic zinc oxide (rs-ZnO), metastable under normal conditions, was synthesized from the wurtzite modification (w-ZnO) at 7.7
GPa and ∼800 K in the form of nanoparticles isolated in the NaCl matrix. The phase transition rs-ZnO → w-ZnO in nanocrystalline
zinc oxide under ambient pressure was experimentally studied for the first time by using differential scanning calorimetry
and high-temperature X-ray diffraction analysis. It was shown that the transition occurs in the temperature range from 370
to 430 K and its enthalpy at 400 K is −10.2 ± 0.5 kJ mol−1.
Key wordszinc oxide-high-pressure synthesis-phase transitions
Russian Chemical Bulletin 04/2012; 59(2):325-328. · 0.38 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Metastable LiMeO2–ZnO (Me=Fe 3+, Ti3+) solid solutions with a rock salt crystal structure have been synthesized by the solid-state reaction of ZnO with LiMeO2 complex oxides at 7.7 GPa and 1350–1450 K. The structure, phase composition, thermal stability and thermal expansion of the recovered samples have been studied by X-ray diffraction with synchrotron radiation. At ambient pressure, rock salt LiMeO2–ZnO solid solutions are kinetically stable up to 670–800 K, depending on the composition.
High Pressure Research 06/2011; 31(2):304-309. · 0.78 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: In the present work we have proposed the method that allows one to easily estimate hardness and bulk modulus of known or hypothetical solid phases from the data on Gibbs energy of atomization of the elements and corresponding covalent radii. It has been shown that hardness and bulk moduli of compounds strongly correlate with their thermodynamic and structural properties. The proposed method may be used for a large number of compounds with various types of chemical bonding and structures; moreover, the temperature dependence of hardness may be calculated, that has been performed for diamond and cubic boron nitride. The correctness of this approach has been shown for the recently synthesized superhard diamond-like BC 5 . It has been predicted that the hypothetical forms of B 2 O 3 , diamond-like boron, BC x and CO x , which could be synthesized at high pressures and temperatures, should have extreme hardness.
02/2011;
-
[show abstract]
[hide abstract]
ABSTRACT: X-ray diffraction with synchrotron radiation has been used for the first time
to study chemical interaction in the MnO-ZnO system at 4.8 GPa and temperatures
up to 1600 K. Above 750 K, the chemical reaction between MnO and ZnO has been
observed that resulted in the formation of rock salt (rs) Mn1-xZnxO solid
solutions (0.3 \leq x \leq 0.7). The lattice parameters of these solid
solutions have been in situ measured at high pressure as a function of
temperature, and corresponding thermal expansion coefficients have been
calculated.
02/2011;
-
[show abstract]
[hide abstract]
ABSTRACT: X-ray diffraction with synchrotron radiation has been used for the first time
to study chemical interaction in the FeO-ZnO system at 4.8 GPa and temperatures
up to 1300 K. Above 750 K, the chemical reaction between FeO and ZnO has been
observed that resulted in the formation of rock salt (rs) Fe1-xZnxO solid
solutions (0.3 \leq x \leq 0.85). The lattice parameters of these solid
solutions have been in situ measured as a function of temperature under
pressure, and corresponding thermal expansion coefficients have been
calculated.
02/2011;
-
[show abstract]
[hide abstract]
ABSTRACT: Series of metastable Me1-xZnxO solid solutions (Me = Ni2+, Co2+, Fe2+, Mn2+)
with the rocksalt (rs) crystal structure have been synthesized from the binary
oxides by quenching from 7.7 GPa and 1450-1650 K. Phase composition of the
samples, as well as structural properties and stoichiometry of synthesized
solid solutions have been studied by X-ray powder diffraction, both
conventional and with synchrotron radiation. The widest (0.3 \leq x \leq 0.8)
composition range of the existence of individual rock-salt solid solution has
been established for the NiO-ZnO system. The bulk rs-Co1-xZnxO, rs-Fe1-xZnxO
and rs-Mn1-xZnxO solid solutions may be quenched down to ambient conditions
only with twice lower ZnO content, i.e. x \leq 0.5, 0.5 and 0.4, respectively;
while formation of rock-salt solid solutions in the CdO-ZnO system has not been
observed in the whole concentration range.
02/2011;
-
[show abstract]
[hide abstract]
ABSTRACT: The structure of graphite-like BCx phases (x = 1, 1.5, 3, 4, 32) has been
studied using conventional X-ray diffraction. The results have been obtained,
which unambiguously point to turbostratic (one- dimensionally disordered)
structure of all phases under study. The crystal lattice parameters, sizes of
coherent scattering domains, and microstrain values have been defined, which
have allowed us to find a correlation between the structure and stoichiometry
of the phases synthesized at the same temperature.
01/2011;
-
[show abstract]
[hide abstract]
ABSTRACT: A number of successful theoretical models of hardness have been developed
recently. A thermodynamic model of hardness, which supposes the intrinsic
character of correlation between hardness and thermodynamic properties of
solids, allows one to predict hardness of known or even hypothetical solids
from the data on Gibbs energy of atomization of the elements, which implicitly
determine the energy density per chemical bonding. The only structural data
needed is the coordination number of the atoms in a lattice. Using this
approach, the hardness of known and hypothetical polymorphs of pure boron and a
number of boron-rich solids has been calculated. The thermodynamic
interpretation of the bonding energy allows one to predict the hardness as a
function of thermodynamic parameters. In particular, the excellent agreement
between experimental and calculated values has been observed not only for the
room- temperature values of the Vickers hardness of stoichiometric compounds,
but also for its temperature and concentration dependencies.
01/2011;
-
[show abstract]
[hide abstract]
ABSTRACT: Measurements of the hardness of a new high-pressure boron phase, orthorhombic
{\gamma}-B28, are reported. According to the data obtained, {\gamma}-B28 has
the highest hardness (~50 GPa) of all known crystalline modifications of boron.
01/2011;
-
[show abstract]
[hide abstract]
ABSTRACT: Our findings show that the hardness of glass-like B2O3 is of about 1.5 GPa,
while the hardness of the high-pressure phase is higher by a factor of 10
(16(5) GPa) and comparable with the hardness (16 GPa) of the WC-10% Co hard
alloy.
01/2011;
-
[show abstract]
[hide abstract]
ABSTRACT: Cubic zinc oxide (rs-ZnO), metastable under normal conditions, was
synthesized from the wurtzite modification (w-ZnO) at 7.7 GPa and ~800 K in the
form of nanoparticles isolated in the NaCl matrix. The phase transition rs-ZnO
\rightarrow w-ZnO in nanocrystalline zinc oxide under ambient pressure was
experimentally studied for the first time by differential scanning calorimetry
and high-temperature X-ray diffraction. It was shown that the transition occurs
in the 370-430 K temperature range and its enthalpy at 400 K is -10.2 \pm 0.5
kJ mol-1.
01/2011;
-
[show abstract]
[hide abstract]
ABSTRACT: Metastable LiMeO2-ZnO (Me = Fe3+, Ti3+) solid solutions with rock salt
crystal structure have been synthesized by solid state reaction of ZnO with
LiMeO2 complex oxides at 7.7 GPa and 1350-1450 K. Structure, phase composition,
thermal stability and thermal expansion of the recovered samples have been
studied by X-ray diffraction with synchrotron radiation. At ambient pressure
rock salt LiMeO2-ZnO solid solutions are kinetically stable up to 670-800 K
depending on the composition.
01/2011;
-
[show abstract]
[hide abstract]
ABSTRACT: γ�B28 is a recently established high�pressure phase of boron. Its structure consists of icosa�hedral B12 clusters and B2 dumbbells in a NaCl�type arrangement (B2)δ+(B12)δ– and displays a significant charge transfer δ ~ 0.5–0.6. The discovery of this phase proved to be essential for the understanding and construction of the phase diagram of boron. It was first experimentally obtained as a pure boron allotrope in early 2004 and its structure was discovered in 2006. This paper reviews recent results and contentious issues related to the equation of state, hardness, putative isostructural phase transformation at ~40 GPa, and debates on the nature of chemical bonding in this phase. Our analysis confirms that (a) calculations based on density functional theory give an accurate description of its equation of state, (b) the reported isostructural phase transformation in γ�B28 is an artifact, (c) the best estimate of hardness of this phase is 50 GPa, (d) chemical bonding in this phase has a significant degree of ionicity. Apart from presenting an overview of the previous results within a consistent view grounded in experiment, thermodynamics and quantum mechanics, we present new results on Bader charges in γ�B28 using different levels of quantum�mechanical theory (GGA, exact exchange, and HSE06 hybrid functional), and show that the earlier conclusion about a significant degree of a partial ionicity in this phase is very robust. An additional insight into the nature of the partial ionicity is obtained from a number of boron structures theoretically constructed in this work.
Journal of Superhard Materials 01/2011; 33:363-379. · 0.79 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The response of graphitelike BC <sub>3</sub> phases ( t-BC <sub>3</sub>) to shock-wave loading has been studied using two types of high explosives, in order to investigate the possible routes to synthesize via dynamic compression superhard materials in the form of high-pressure phases such as the B-doped diamond produced recently under high static pressures and temperatures. The loading conditions resulting from wave propagation in the shock recovery setup have been determined from theoretical predictions confirmed by numerical simulations and velocity measurements. Over the explored range of shock pressure (from 10 to 30 GPa), no detectable diamond phase could be quenched, probably because of insufficient temperature, but Raman and x-ray diffraction studies of the recovered samples indicate permanent structural changes that have been compared to those observed after shorter, laser driven shock compression. These changes include local phase segregation of t-BC <sub>3</sub> and the production of highly disordered phases.
Journal of Applied Physics 11/2010; · 2.17 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The formation of the nanocrystalline rocksalt ZnO (rs-ZnO) has been in situ studied by x-ray diffraction with synchrotron radiation at high pressure and high temperature. A number of rs-ZnO/MgO nanocomposites with preset grain size were synthesized at 7 GPa and 800 K starting from wurtzite ZnO nanoparticles or nanorods. The use of MgO matrix allowed us to recover metastable rs-ZnO in the nanocrystalline form at ambient pressure. The cathodoluminescence measurements demonstrated the blue shift in the luminescence of rs-ZnO nanocrystals down to 402–408 nm that can be attributed to the enhanced incorporation of point defects with lower activation energy.
Journal of Applied Physics 05/2010; · 2.17 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The 300 K equations of state of graphite-like B–C phases with boron content from 0 to 50 at% have been studied using powder X-ray diffraction with synchrotron radiation up to 35 GPa. The compression axis moduli show the non-monotone dependence on boron content and strongly correlate with lattice parameters. The values of bulk moduli B 0 vary from 17.5 (0.6) GPa (t-BC4) to 30.7 (0.5) GPa (t-BC1.5), while for turbostratic graphite B 0=29.5 (1.0) GPa.
High Pressure Research 12/2009; 29(4):605-611. · 0.78 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Boron is a unique element, being the only element, all known polymorphs of
which are superhard, and all of its crystal structures are distinct from any
other element. The electron-deficient bonding in boron explains its remarkable
sensitivity to even small concentrations of impurity atoms and allows boron to
form peculiar chemical compounds with very different elements. These
complications made the study of boron a great challenge, creating also a unique
and instructive chapter in the history of science. Strange though it may sound,
the discovery of boron in 1808 was ambiguous, with pure boron polymorphs
established only starting from the 1950s-1970s, and only in 2007 was the stable
phase at ambient conditions determined. The history of boron research from its
discovery to the latest discoveries pertaining to the phase diagram of this
element, the structure and stability of beta-boron, and establishment of a new
high-pressure polymorph, gamma-boron, is reviewed.
11/2009;
-
[show abstract]
[hide abstract]
ABSTRACT: This Comment points out a number of errors in the recent paper by Zarechnaya et al. (Phys. Rev. Lett. 102, 185501 (2009)). Results and conclusions presented by Zarechnaya et al. (2009) are either incorrect or have been presented before. Comment: This Comment points out a number of errors in the recent paper by Zarechnaya et al. (Phys. Rev. Lett. 102, 185501). It was submitted on 1 June 2009. The Comment was not recommended for publication (decision of 14 August 2009), largely because errors of the Zarechnaya paper can be easily detected by the readers
08/2009;
