R. Bujakiewicz-Koronska

Determination of elastic constants of Na0.5Bi0.5TiO3 from ab initio calculations

Elastic constants and bulk modulus for the tetragonal, rhombohedral, and cubic phase of Na0.5Bi0.5TiO3 crystal were calculated from the first principles. From the calculated elastic constants, other structural properties such as bulk modulus, shear modulus, Youngs modulus, and Poissons ratio can be derived using respective relationships from Voight–Reuss–Hill approximation; bulk modulus was calculated as an example in this article. It was shown that elastic constants show different behavior for compression and elongation. The different values of elastic constants have been calculated for the direction parallel to the bismuth layer (crystallographic a(b)-axis) and the perpendicular direction (crystallographic c-axis). It seems to be caused by bismuth layer structure oxides of Na0.5Bi0.5TiO3 crystal.

First principles calculations of ideal and defected BiMnO3

Presented calculations were designed to evaluate the impact of point defects on the macroscopic properties and the electronic structure of BiMnO3. The calculation of the density functional theory was carried out using the generalized-gradient approximation, the Perdew–Burke–Ernzerhof parametrization and the spin polarization using the SIESTA package. We have modeled systems with a total stochiometry and small deformations caused by the oxygen and bismuth vacancies. The results indicate a significant influence of point defects on the mechanical properties (e.g. the bulk modulus) as well as on the electronic structure of the steady state of BiMnO3 crystal and they are in a good agreement with the experimental data.

Low temperature measurements by infrared spectroscopy in CoFe2O4 ceramic

This paper presents results of new far-infrared and middle-infrared measurements (wavenumber range of 4000–100 cm−1) of the CoFe2O4 ceramic in the temperature range from 300 K to 8 K. The band positions and their shapes remain constant across the wide temperature range. The quality of the sample was investigated by X-ray, EDS and EPMA studies. The CoFe2O4 retains the cubic structure (Fd - 3m) across the temperature range from 85 K to 360 K without any traces of distortion. Based on current knowledge the polycrystalline CoFe2O4 does not exhibit any phase transitions across the temperature range from 8 K to 300 K.

Dielectric and Ferroelectric Properties of Lead-Free NKN and NKN-Based Ceramics

Lead-free ceramics of Na0.5K0.5NbO3 (NKN), Na0.5K0.5(Nb0.94Sb0.06)O3 (NKNS6) and Na0.5K0.5(Nb0.94Sb0.06)O3 + 0.5%MnO2 (NKNS6 + 0.5%MnO2) have been prepared by a solid phase hot pressing sintering process. X-ray diffraction results show that the obtained samples possess the perovskite structure. The micrograph of the fractured surface showed a dense structure in a good agreement with that of 91–94% relative density determined by the Archimedes method. An average grain size decreases with Sb and Mn doping (from about 20 μm for NKN to about 5 and 2 μm for NKNS6 and NKNS6 + 0.5%MnO2, respectively). Low frequency (100 Hz–200 kHz) investigations revealed the diffuse phase transitions. It was found that Mn or Sb doping influence dielectric and ferroelectric properties. The pyroelectric and hysteresis loops measurements show that the samples are ferroelectric with a relatively large remanent polarization (18– 55 μC/cm2) and a low coercive field (7–10 kV/cm). The obtained results are discussed in the framework of foreign ions/lattice imperfections, which create local electric and elastic fields. The obtained materials are considered to be promising candidates for lead-free electronic ceramics.

DOS Calculation for Stoichiometric and Oxygen Defected (Bi1/2Na1/2)(Mn1/2Nb1/2)O3

(Bi1/2Na1/2)(Mn1/2Nb1/2)O3 (BNMNO) ceramics were sintered at ambient air. XRD test exhibited coexistence of phases. The permittivity and conductance were measured at radio frequencies. Activation energy values were estimated as 0.38 eV. Electronic structure was simulated for BNMNO and also for BiMnO3 (BMO) as the reference. The ab initio calculations were performed within DFT using LCAO with localized basis set in the form of spherical functions as implemented in the SIESTA 3.0 code together with a spin polarization. An energy gap was estimated for the ideal BMO and BNMNO lattice to be 0.8 eV and 0.4 eV, respectively. An oxygen vacancy and an interstitial oxygen introduced to calculation caused narrowing the gap.

Impedance spectroscopy studies of SrMnO3, BaMnO3 and Ba0.5Sr0.5MnO3 ceramics

The impedance spectrum of hexagonal SrMnO3, rhombohedral BaMnO3 and orthorhombic Ba0.5Sr0.5MnO3 ceramics, synthesized by conventional high-temperature method, was studied in a wide temperature and frequency range. The complex impedance plots of Z″ versus Z′ pointed to two contributions originating from grains and grain boundaries. The parameters of electric equivalent circuit were calculated. The semicircles related to the grain boundary are located at the lower frequency side due to higher resistivity and capacity of the grain boundaries. The Ba0.5Sr0.5MnO3 ceramics is characterized by the lowest activation energy related to the grains. The conductivities σac of all the investigated samples show plateaus being a characteristic of the dc conductivity. The conductivity of Ba0.5Sr0.5MnO3 takes the values between those of BaMnO3 and SrMnO3.

Physical Properties of Ba0.95Pb0.05TiO3+0.1%Co2O3

The single-phase perovskite structure of the Ba0.95Pb0.05TiO3+0.1%Co2O3 ceramics was confirmed by the X-ray diffraction method. Microstructural studies revealed that the samples were of good quality and chemically homogeneous. The thermal behaviour of ceramics was studied using the in situ high-temperature X-ray synchrotron powder diffraction investigation. The energy gap of about 3.2 eV was estimated using a reflectance spectroscopy. Measurements showed the influence of Pb and Co on the character of phase transition in the BaTiO3 structure. The results were compared to the ones obtained for pure BaTiO3.

Investigations of Low Temperature Phase Transitions in BiFeO3 Ceramic by Infrared Spectroscopy

In this paper, results of new infrared FIR-MIR measurements (wavenumber range of 4000–100 cm−1) in the range of the temperature 300 ÷ 8 K of the BiFeO3 ceramic are presented. Below 200 K only one symmetric band at ca. 800 cm−1 is visible. The full width at half maximum of this band decreases exponentially on cooling. The shape of the band at ca. 545 cm−1connected with A1 (TO) mode also changes between 60–40 K, this is in the vicinity of the phase transition at 50 K. Initially a broad band reveals its components as shoulder bands. Theirs FWHM decrease with cooling.

The piezoelectric effect in Pb[(Fe1/3Sb2/3) x Ti y Zr z ]O3 ceramics

The piezoelectric properties of Pb[(Fe1/3Sb2/3) x Ti y Zr z ]O3 with x + y + z = 1, x = 0.1, y = 0.43–0.48 ceramics have been investigated over a broad temperature range using a resonance technique. The influence of modification of PZT normal ferroelectric synthesized near the morphotropic phase boundary by a relaxor Pb(Fe1/3Sb2/3)O3 compound on its physical properties was studied. The coefficients s 11, k 31, and d 31 were calculated from the parameters characterizing the behavior of damped harmonic oscillator in the vicinity of the piezoelectric resonance. Several anomalies of the piezoelectric coefficients have been found in the temperature range 300–600 K. Two diffuse phase transitions were observed in Pb[(Fe1/3Sb2/3) x Ti y Zr z ]O3. The anomaly near 530 K for y = 0.43 is responsible for the transition from the rhombohedral phase to the tetragonal one. For y > 0.44 this transition is found to be very diffuse and the coexistence of rhombohedral and tetragonal phases occurs. The observation of low piezoelectric activity confirms the existence of polar regions in Pb[(Fe1/3Sb2/3) x Ti y Zr z ]O3 above T m.

Magnetic Phase Transition in Antiferromagnetic SrMnO3 Perovskites

We study thermal properties of SrMnO3 starting from analysis of the low-energy electronic structure of the Mn4+ ion in oxygen surroundings. As a start point of the studies, simulations of a SrMnO3 unit cell were performed with using the density functional theory (DFT). The DFT calculations have revealed ionic character of the compound. In the next stage of calculations the Mn4+ ion was treated as an atomic-like strongly-correlated 3d3 system with the 28-fold degenerated ground term 4F. This degeneracy is lifted under the combined action of the dominant octahedral crystal field and the on-site spin-orbit coupling. This study was concentrated on the explanation of the temperature dependence of the specific heat showing a pronounced λ-type peak at TN. We have calculated the d-electron contribution to the specific heat both in the antiferromagnetic and paramagnetic states. We have found that the entropy of the magnetic contribution is close Rln4 - a value expected from the 4A2g ground subterm of the Mn4+ ions. e.g. 11.5 J/Kmol.