A group‐theory analysis of temperature‐induced phase transitions in ZrO2 has been performed in the framework of the group–subgroup relationship tree (Bärnighausen tree) with the computer tools of the ...Bilbao Crystallographic Server. The transition paths including symmetry‐allowed intermediate phases have been established. The active irreducible representations corresponding to soft phonon modes and spontaneous deformation strains responsible for the phase transitions have been determined. The phonon mode frequencies at the symmetry points of the Brillouin zones of cubic, tetragonal and monoclinic phases have been calculated using the ab initio density functional theory method. As a result, the soft modes and their symmetries have been revealed, which are in a complete agreement with the group‐theoretical predictions.
A theoretical study of phase transitions in ZrO2 is presented.
The atomic sublattices occupying different Wyckoff positions in a host crystal are shown to be subdivided in the supercell model owing to the splitting of the occupied Wyckoff positions. The site ...symmetries of the split Wyckoff positions are in general different, which significantly increases the number of possible occupation schemes for impurities and defects in the supercell model. It is demonstrated that the use of the programs and retrieval tools of the Bilbao Crystallographic Server considerably simplifies the group‐theoretical analysis of Wyckoff position splittings in the supercell model of a crystal with a point defect. Ab initio calculations of electronic states of single defects in ZnO (Zn and O neutral vacancies) within the supercell model are performed to demonstrate the influence of Wyckoff position splittings on the results.
The article considers supercell symmetry and Wyckoff position splittings for the description of defective crystals.
A new trigonal (rhombohedral) SiC phase, existence of which was previously theoretically predicted by a symmetry analysis, is studied. It is shown that the phase can be formed during the growth of ...SiC films by the method of substitution of atoms on the surface of a Si substrate. Ab initio calculations of the crystal structure of a new phase and its Raman spectra are performed by the quantum chemistry method. The difference of the selection rules for the Raman active vibrations for this rhombohedral phase from the selection rules for a cubic phase in the coordinate system aligned with the translation vectors of the rhombohedral phase is established. Series of thin SiC/Si films by annealing time are synthesized by the method of the topochemical substitution of atoms, and their Raman spectra are analyzed. The presence of the spectral line (258 cm
–1
), that is close to the line of a new trigonal (rhombohedral) phase calculated by the ab initio method, has been found in the Raman spectra of the samples at the initial stage of the growth of a SiC film, which indirectly confirms its existence.
Using a new approach based on symmetry analysis, we have determined the magnetic symmetry groups (Shubnikov groups) of ferro- and antiferromagnetic wurtzite nanostructures doped with magnetic atoms ...periodically distributed at cation sites as well as the direction of the spontaneous magnetic field. Quantum wells, nanorods (nanowires), nanotubes, and quantum dots have been considered. The direction of the spontaneous magnetic field is determined by magnetic atoms with higher C3v (3m) or Cs (m) site symmetry group (the site symmetry group is defined with respect to the doped nanostructure). When the magnetic-atom distribution becomes more or less disordered (Diluted Magnetic Semiconductors), it seems reasonable to assume that, in most of the cases, the symmetry in regions with a size of some wurtzite unit-cells remains close to that of a periodic one, hence possibly inducing local ferro- or antiferromagnetic properties. The regions can induce overall ferro- or antiferromagnetic states when they have common spontaneous magnetic field direction or, possibly, induce easy-magnetization direction or plane (direction or plane of maximal magnetization intensity for a given external magnetic field strength) when disorder imposes a spread of spontaneous magnetic field direction over the various regions. Of course, such an effect adds itself to magnetic properties eventually induced by grain boundaries, other crystalline phases, and defects such as vacancies, dislocations or interstitial atoms.
A symmetry analysis of the crystal structure and the phonon spectrum during continuous topochemical conversion of silicon into silicon carbide has been carried out. The transformation of the symmetry ...of phonons at high-symmetry points of the Brillouin zone upon the transition from the initial cubic structure of silicon (diamond) through an intermediate cubic structure of silicon carbide to the trigonal structure of SiC has been determined. The selection rules for the infrared and Raman spectra of all the three phases under investigation have been established.
We report the results of systematic experimental and theoretical studies of structural and dynamical properties of short-period GaN/AlN superlattices. The multilayer structures with the thicknesses ...of the constituent layers varying from two to several monolayers are grown using the submonolayer digital molecular beam epitaxy technique. In the framework of density functional theory, the lattice dynamics properties of the superlattices are studied. Good agreement between the experiment and theory is found, which made it possible to establish unambiguously a relationship between the features observed in the Raman spectra and the microscopic nature of the acoustic and optical phonon modes. The results obtained enhance the capabilities of Raman spectroscopy as a fast and non-destructive characterization method of short-period GaN/AlN superlattices and can be used to optimize growth process parameters for the fabrication of structurally perfect low-dimensional heterostructures.
Three different transition paths of the phase transition from the high-symmetry trigonal P
m1 to the low-symmetry monoclinic phase C2/c in K
3
Na(SeO
4
)
2
crystals have been analyzed using ab initio ...calculations based on the density functional theory (DFT) as well as programs and retrieval tools of the Bilbao Crystallographic server (BCS). We conclude that the intermediate state observed in experiments within 346-329 K temperature range consists of a mixture of two monoclinic phases C2/m and C2/c, both with the primitive cell doubled along the z-axis. Both monoclinic phases result from the softening of the double-degenerated zone-edge soft phonon
.
Celotno besedilo
Dostopno za:
BFBNIB, DOBA, GIS, IJS, IZUM, KILJ, KISLJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
Abstract
The results of joint theoretical and experimental studies aimed at revealing features in the Raman spectra, which can be used for evaluation of the interface quality between GaN and AlN ...layers in short-period GaN/AlN superlattices (SLs) are presented. The Raman spectra for SLs with sharp interface and with different degree of interface diffusion are simulated by
ab initio
calculations and within the framework of random-element isodisplacement model, respectively. The comparison of the results of theoretical calculations and experimental data obtained on PA MBE and MOVPE grown SLs, leads to conclusion that the spectral region of the A
1
(LO) confined phonons is very sensitive to the degree of interface sharpness. As a result of comprehensive studies, the correlations between the parameters of the A
1
(LO) confined phonons and the structure of SLs are obtained. The results of the complex studies can be used to optimize the parameters of the growth process in order to form structurally perfect short-period GaN/AlN SLs.
We report the results of systematic Raman spectroscopy studies of Al
x
Ga
1 –
x
N (
x
~ 0.75) layers grown using plasma-assisted molecular beam epitaxy at various stoichiometric conditions and growth ...fluxes. The high-intensity asymmetric low-frequency peak obeying Bose statistics is discovered in Raman spectra of the layers grown by temperature-modulated epitaxy at strongly Ga-enriched conditions. Theoretical model is developed to explain the origin and the high intensity of the observed low-frequency peak, which is attributed to the presence of excessive metallic gallium in AlGaN layers and can be explained by vibrations of gallium clusters with a diameter of ~1 nm. The nature of the low-frequency peak is similar to that of the boson peak in glasses, which occupies the same frequency range in Raman spectra. We demonstrate the capabilities of Raman spectroscopy as an express and non-destructive technique for optimization of growth conditions of AlGaN layers to achieve simultaneously the atomically-smooth droplet-free surface morphology and the high structural quality.
Structural and dynamic properties of short-period GaN/AlN superlattices with the thicknesses of the constituent layers varying from two to several monolayers, grown using the method of submonolayer ...digital molecular beam epitaxy, are experimentally and theoretically studied. It is established that in the grown samples there are two types of periodicity. One type is formed by periodic sequence of GaN and AlN layers in the superlattice, and the second one is related with periodic interruptions in the growth of superlattice for the evaporation of excess Ga metal. The dependences of the positions and intensities of the lines in the Raman spectra on the period of the superlattice are determined, and microscopic nature of optical phonon modes is established. The doublet structure of the E(TO) lines localized in the GaN and AlN layers of superlattice, genetically related to the E2(high) and E1 phonon branches of the bulk crystal, is first discovered and explained. A strong dependence of the polar modes localized in the AlN layer on the thickness of layer forming the superlattice is revealed. The results of complex studies will improve the accuracy of quantitative estimation of important parameters of superlattice structures and can be used to optimize growth parameters for the fabrication of structurally perfect short-period GaN/AlN superlattices.
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