Research activity

2005

We report the following results obtained by ab inito methods:

An ab initio calculations of the phonon dispersion curves for the cubic phase of magnetite Fe₃O₄ has been calculated (without U), and compared with the phonon spectrum measured by the nuclear inelastic scattering method.
A stability range of the shape memory alloy NiTi dopped with Fe has been studied for a few stoichiometric concentrations of Fe. It proves that the most stable is the phase with 25% of Fe concentrarion.
Vibrational spectra of deuterated methanol in a glassy state has been calculated and compared with neutron spectra. A Boson-like peak was assigned to OH-O bonding.
Phonon dispersion relationships for the superconductor PuCoGa₅ has been calculated. The treatment requires to account the Coulomb interaction U at the atomic site. For the first time the influence of (U, J), the measure of strong electron coupling, on phonon frequencies has been shown.
The energies of the ground states and the phonon dispersion relations of ceramic ZrO₂ for the three know phases: cubic, tetragonal and monoclinic have been calculated. The thermodynamic functions correctly reproduce the phase transition temperature between the tetragonal and monoclinic phases. This phase transition was established as a first order.
Phonon dispersion curve and density of states for clathrathes of Ba₈Si₄₆ type have been computed. The phonon frequencies agree with the Raman measurements. A particularly stron electron-phonon interaction has been found for the Ba atoms vibrating in small cavities. A relation of these vibrations with superconductivity observed in this crystals was discussed.
A phonon density of states of iron has been calculated for beta-FeSi₂ crystal, and the spectra agree with the measurements of the nuclear inelastic scattering.

2004

The main tool of studies were the ab initio calculations of crystal properties. The main results can be summarized as follows:

All elastic constants of AgGaX₂ and CuInX₂ (X=S, Se) have been ab initio calculated, and supplemented by a critical discussion of measured elastic constants.
Elastic constants of the alpha and gamma structures of MgxFe_(2-x)SiO₄ for the pressure from 0 to 240 GPa have been calculated, since these data are required to interpret the sound velocity in geological measurements.
A phonon density of states have been calculate for alpha and theta corundum Al₂O₃, and hence the free energies established. These data indicate that a destabilization at high temperature of alpha phase is accompanied by the porosity of the theta phase.
Formation energies of substitutional defects of Co, Cr, Fe and vacancies in the intermetallic NiAl have been calculated.
Lattice dynamics of ZnTe semiconductor at high-pressure of 12.3 GPa has been considered.
In the SrBi₂Ta₂O₃ ferroelectric crystal two order parameters have been found, due to studies of the phonon modes.
A method to treat the second order phase transition has been proposed and tested on the ferroelastic phase transition of CaCl₂.
Phonon density of states for the urea molecular crystal with strong hydrogen coupling proved to agree with the spectrum mesasured by incoherent neutron scattering method.

2003

Intense ab initio calculations has been carried on. The following topics were considered:

The chalkopiryte CuInSe₂ is used in production of solar cells. However, this crystal shows another structure which is of CuAu-type and appears as precipitates of the chalkopiryte structure. Usually two different structures exists in different regions of the (T,P) plane forming a phase transition. In the present case the chalkopiryte and CuAu-type structures seem to coexists in a large intervals of (T,P). Our ab initio calculations of ground state energy and phonon dynamics, and hence the thermodynamic, show that the free energies of the two structure have identical values, and also the lattice constants are equal, therefore coexistence of the two structures is thermodynamically justifies.
In the HgSe semiconductor with zero energy gap we show how the LO/TO (longitudinal optic/transvers optic) splitting arises, in spite that LO/TO splitting could be observed only in systems with finite energy gap. We estimated the increase of the LO/TO splitting due to change of the charge carriers. Calculated phonon dispersion relations agree very well with the measured ones.
In the hexagonal MgB₂ the BCS mechanism is responsible for the superconductivity, therefore phonon plays a crucial role. We have calculated the intensities of the phonon peaks, which facilitated the search of phonons signals during the x-ray measurements in ESRF, Grenoble, of the phonon dispersion curves of this material.

2002

Using VASP and PHONON programs we have carried out a number of ab initio calculations to find the crystaline structures, and elastic, phonon, magnetic and electronic properties of crystalline systems.
The most important results are listed below:

Under about 25 GPa in FeBO₃ crystal a phase transition has been found. At this transition the space group R-3c does not change, the unit cell volume exhibits a considerable jump. The antiferromagnetic structure of FeBO₃ disappears due to magnetic moment which diminishes about four times. The small energy gap in electronic structure disappears causing the metal-insulator phase transition. Charges on Fe ions increase slightly.
It has been confirmed that the phase transition under pressure from NaCl- type to CsCl-type phase of AgBr goes over an intermediate monoclinic stable phase. Existence of the monoclinic phase explains unusual behavior of the measured equation of states in the vicinity of the phase transition.
Phonon dispersion curves and elastic constants for chalcopyrite AgGaS₂, AgGaSe₂ and CuInS₂ have been calculated. In AgGaSe₂ a soft mode under pressure has been found. Electronic structure and effective charges have been calculated for CuInS₂.
A quasiharmonic approximation has been used to find the thermal expansion of gamma-Mg₂SiO₄ mineral. Our ab initio calculated thermal expansion agrees with the experimental data.
We used the ab initio calculations to find the critical uniaxial stress for the generation of microcracks in cubic BN material. The critical stress proved to be of about 100 GPa. This confirms that ab initio calculations can be usefull to study an influence of defects on the initial stage of creation material's fractures.
In the shape memory alloy NiTi four phases have been analysed from the point of view of phonon properties. In austenite phase a soft mode has been found. It was shown why the martensitic monoclinic phase is more stable then the orthorhombic one. The overall stability of the hexagonal R-phase has been confirmed.
It was shown that the interaction of Cu and Pd atoms with the surface of alpha-Al₂O₃ has a covalent or ionic character, depending on that whether the surface terminates with Al, or O layers, respectively.
Using LiNbO₃ and SnO₂ as examples we have shown how ab initio calculations can provide the information about peak intensities of coherently scattered neutrons, or X- rays, and select a proper Brillouin zone for most succesful measurements.

2001

We were able to calculate a large number of materials properties. At arbitrary pressure, but at zero temperaturewe are able to predict the crystal structure and symmetry, lattice constants and particle's positions. We could obtain the electronic bands and electronic density of states, phonon dispersion curves, phonon density of states, thermodynamic functions at finite temperatures including heat capacity and Debey-Waller factor, and describe the inelastic incoherent and coherent neutron scattering. We have attempted to calculate the effective charge tensor, and the magnetic structure. These abilities have been applied to a number of crystalline materials, to minerals, chalcopyrites, and superconductors.
The main results are related with the phonon calculations. For that we use so called direct method which requires to calculate the Hellmann-Feynman forces generated by atoms displaced from the equilibrium positions. The force constants, being the core quantities in the lattice dynamics theory, are then fitted in a symmetry controlled way to the Hellmann-Feynman forces. The method is suitable to treat also complex crystals with relatively largeunit cell. We consider that we have a world priority in this calculation technique. Within the direct method we have calculated the phonon dispersion curves in CaTiO₃ crystal. This material have been thought to be an important constituent of the waste forms that are being developed for the disposal high level radioactive wastessince it is able to immobilize lanthanides and actinides by forming a solidsolution with them. It is also known as an important material of ferroelectric ceramics. This compund is also used as a thermally-sensitive resistor elementdue to its negative temperature coefficient. The CaTiO₃ crystallizesin the perovskite structure. We have calculated the phonon dispersion relationsin three phases, and found that the orthorhombic phase of Pmnb symmetry is stable.By calculating ground state energy we were able to prove that the experimentally observed intermediate phase has I4mcm symmetry and not Cmcm symmetry as suggested by another diffraction measurements.
The boron nitride BN, which crystallizes in a cubic structure, is one of the hardest material due to strong covalent coupling. It is, however, not easy to syntetize it, therefore not many experimental data are avaliable. We have calculated the phonon dispersion curves of cubic BN. We used this materialalso to demonstrate that the the direct method can give the LO/TO(longitudinal/transversal) splitting of the phonon optic mode. Our suggested method is equivalent to the approach of planar force constants.
We have carried out the structural and lattice dynamics calculations of silver thiogallate AgGaS₂. The calculated lattice constants and the full set of the elastic constants are in good agreement with the experimental data. The calculated phonon dispersion curves deliever also the Raman and infrared phonon frequencies which fit quite well with the experimental measurements.
Another material for which the lattice dynamics has been calculated is the mineral Mg₂SiO₄ in cubic phase. From the phonon data we could calculate the temperature dependence of heat capacity, which relatively well agree with the experimentally measured thermal properties. Other papers are related with phonons in ZnSnP₂, phonon dispersion curves of supercondunting MgB₂,magnetic FeBO₃, rotational barriers in ammonium hexachlorometallates adsorption mechanisms of metalic Cu and Pt adsorbed on the surface(0001) of Al₂O₃. We have also worked on the dynamics of the order-parameter in mineral lawsonite, and hole-hole interaction in thehigh-T_c supercondictor.

2000

During the year 2000 we continued to apply the ab initio methods to solve a number of physical problems. We are trying to deal with problems and calculate parameters not yet solved by experimental techniques.
Our efforts have been concentrated on the calculations of the lattice dynamics of crystals, since we could carry on phonon calculations for large unit cells. Within this approach we could find phonon dispersion curves and phonon density of states for a number of chalcopyrites, semiconductors and minerals. In particular the calculations of a soft mode in LiNbO₃ ferroelectric material throw additional light on this high-temperature phase transition. Studying soft modes under pressure in AgGaSe₂, we have established the right symmetry changes at the phase transition. The calculated phonon dispersion curves for AgGaS₂, AgGaSe₂, AgGaTe₂, CuInSe₂ and ZnSnP₂ are the only available ones in the literature.
Detailed studies of possible structures of perovskite-type MgSiO₃ showed that its orthorhombic modification with space group Pbnm is stable at T = 0 and at any pressure up to 150 GPa. This crystal structure of the MgSiO₃ is proposed for the lower mantle of the Earth. The upper mantle of the Earth is built of Mg₂SiO₄ olivine. We have started research on this crystal, calculating its lattice dynamics and thermodynamics in a simple cubic structure.
A lot of time was devoted to search for possible high-pressure phases of the GeO₂crystal. Testing under pressure number of possible structures we found that GeO₂ has a second order ferroelastic phase transition from rutile to CaCl₂ - structure, and two other first-order phase transitions to alpha -PbO₂-type, andpiryte structures. The two last phase transitions have not yet been measured.
Special attention was paid to studies of ionic crystal AgBr, which has, under pressure, a phase transition from cubic NaCl - type to cubic CsCl - type structures. Our calculations showed that in the intermediate pressures from 8 to 21 GPa the crystal forms a stable monoclinic phase, which is a subgroup of NaCl and CsCl - type structures simultaneously.
Using the phonomenological force field method, we have predicted the two high-pressure structures and space groups of deuterated urea CO(ND₂)₂. The new space groups are P2₁2₁2 (Z = 2) and P2₁2₁2₁ (Z = 4). Both phase transitions are accompanied by the soft modes.

1999

The potential barriers for SCN ion rotation in KSCN disordered crystal has been calculated with DFT method.
The ab initio phonon dispersion relation have been extended to chalcopyrites CuInSe₂, AgGaSe₂, AgGaTe₂, AgGaS₂. The agreement with dispersion curves measured by inelastic coherent neutron scattering has been achieved. We have proposed to calculate the LO/TO splitting from elongated cell. The LO/TO splitting calculated in MgO agrees with the observations. We have calculated for the first time the phonon phonon dispersion curves in hexagonal 2H-GaN. A soft mode under pressure has been discovered in SnO₂ crystal. The phonons of LiNbO₃ of rhombohedral symmetry has been calculated. The paraelectric-ferroelectric phase transition shows a soft mode.
We have studied the geologically important materials under high-pressure conditions. The perovskite-like MgSiO₃ proves to be stable in orthorhombic Pbnm phase at all investigated pressures and low. The elastic constants of Olivine (MgSiO₃) calculated as a function of pressure do not indicate for any phase transitions at T=0 up to 150GPa. In ionic crystal AgBr a pressure induced phase transition from NaCl type to CsCl type has been found.

1998

Similar phonon dispersion curves calculations for molecular crystals CH₂, OC(ND₂)₂and Na₂CO₃ have been carried on using empirical potentials with tabulated universal parameters. The only available neutron scattering data for OC(ND₂)₂ agrees very well with our result. We add that for crystals with Van der Walls potentials, like majority of molecular crystals, the standard ab initio programs do not work, thus the empirical approach is the only possible way at the moment. We have made as well similar phonon calculations of a number of chalcopyrites AgGaS₂, AgGaSe₂, AgGaTe₂, CdGeAs₂ and CdGeP₂as well as transition metal carbides TiC and ZrC.

Another activity has been related with the Monte Carlo simulation of the pseudospin model for the order-disorder phase transition in KSCN crystal. For this model a calculated correlation function was able to describe, at least qualitatively, an unusual temperature dependence of the width of quasi elastic peak measured by neutron scattering.

Our aim is to relate in future the model calculations, such as for KSCN with complete set of parameters derived from first principle calculations. Part of the parameters comes from harmonic phonons, another part is related to crystal elasticity and anharmonicity. Effort to find these parameters using CASTEP has already been made.

1997

The main activity in the Department is related with ab initio calculations of phonon dispersion curves. The standardization of supercell ab initio calculations in DFT approach, and the development of the direct method allow to calculate phonons from first principles. We have made effort to customize such calculations. Our source of ab initio information comes from the usage of MSI software, in particular from CASTEP module. The Hellmann-Feynman forces calculated there, are imported into our programs, which, in turn, are able to find the force constants and calculate phonon dispersion curves, and phonon densities of states. Using this method we have derived the ab initio phonon dispersion curves for GaAs, GaAl and doped GaAs_xAl_1-x. The calculated dispersion curves agree within a few percents with those which have been measured by the inelastic neutron scattering.

1996

In department of Materials Research by Computers the work is concentrated around ferroelastic properties of crystals. These systems have phase transitions which are accompanied by, usually large, lattice deformations. Therefore, the domain patterns, which arise as a result of the phase transitions, are defined to large extend by stress fields. This idea allowed to establish the law of domain pattern formation. We are working on ferroelastic systems like lead phosphate (Pb₃(PO₄)₂) and YBa₂Cu₃O₇ (YBCO).

The primary tool in our research is a Molecular Dynamics method of simulation of a crystal. This method, together with reduction of number of degrees of freedom of the system, allows to effectively simulate systems of 10^4-10^5 atoms.

Back to our home page

Page maintainer: jochym@ifj.edu.pl