D.M. Nalecz

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.

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.

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.

Electronic and magnetic properties of (Bi0.5Na0.5)(Mn0.5Nb0.5)O3

(Bi0.5Na0.5)(Mn0.5Nb0.5)O3 has been sintered by hot-sintering method at ambient air. Composition has been checked with use of a scanning microscope. Magnetic behavior of (Bi0.5Na0.5)(Mn0.5Nb0.5)O3 has been investigated by DC magnetization and AC susceptibility techniques. (Bi0.5Na0.5)(Mn0.5Nb0.5)O3 exhibits a magnetic phase transition at 6.8 K. Below this temperature, the interaction between magnetic moments is antiferromagnetic. The resultant non-zero magnetic moment emerges because of weak precipitation of manganese oxide with ordering temperature at T ≈ 40 K. From the fit of the Curie–Weiss law to the AC susceptibility, the Weiss temperature θ = –2.3 K has been determined and Curie constant C = 1.15 emu K/molMn, close to the value expected for spin S = 2 of Mn3+, has been estimated. The antiferromagnetic state with spin-glass type features has been deduced. Crystal structures and partial density of states (DOS) have been simulated by ab initio method. DOS simulations showed half-metal electronic structure of (Bi0.5Na0.5)(Mn0.5Nb0.5)O3.

The crystal field effects in hexagonal 4H-SrMnO3

ABSTRACT We have provided an explanation for the EPR signal in 4H-SrMnO3, revealed at the vicinity of TN and in the paramagnetic region, as related with the doublet originating from the 4A2g subterm of the Mn4+ ion. Our calculations, taking into account the spin–orbit coupling and the crystal–field interactions with a trigonal distortion, reproduce the doublet ground state which is characterised by magnetic moment of ±0.975μB in very good agreement with experimental value of ±0.983μB. It confirms fundamental importance of the spin–orbit coupling and the orbital magnetism for description of 3d oxides. We claim that the EPR result provides a strong argument for the existence in the 4H-SrMnO3 crystal of the discrete atomic-like states with the energy separation of part of meV.

Electronic Properties of 6H Hexagonal SrMnO3

SrMnO3 ceramic was prepared by conventional high-temperature sintering method. Dielectric studies were performed within the temperature range 125–300 K at the frequency 10–400 kHz. The electronic properties of stoichiometric and oxygen defected SrMnO3 crystal in hexagonal 6H- P63/mmc symmetry at 0 K were calculated in ab initio procedure based on density functional theory using the Spanish Initiative for Electronic Simulations with Thousands of Atoms (SIESTA) code. The lattice parameters and density of states were obtained. The ideal 6H-SrMnO3 crystal has the band gap energy about 0.2–0.3 eV at 0 K. Contribution of an exchange-correlation energy into the total energy is about 32%, a magnetic moment about 3 µB.

Spin-Orbit Origin of Anomalous Temperature Dependence of the Magnetic Susceptibility of PbVO3

We have shown that the V4+ ion, realized in PbVO3, with one d electron can have almost non-magnetic ground state under the action of the crystal field in the presence of the spin-orbit coupling. This weakly-magnetic ground state, with low lying crystal-electric-field (CEF) states at 0, 24 and 110 meV, causes anomalous temperature dependence of the magnetic susceptibility at low temperatures violating the magnetic Curie law. The formation of the weakly magnetic ground state is caused by the substantial orbital moment that almost cancels the spin moment.

X-ray and dielectric characterization of Co doped tetragonal BaTiO3 ceramics

ABSTRACT The crystal structure modifications of BaTiO3 induced by cobalt doping were studied. The polycrystalline (1 − x)BaTiO3 + xCo2O3 samples, with x ≤ 10 wt.%, were prepared by high temperature sintering conventional method. According to X-ray phase and structural characterization, performed by full-profile Rietveld refinement technique, all synthesized samples showed tetragonal symmetry perovskite structure with minor amount of parasitic phases. Pure single-phase composition has been detected only in the low level of doping BaTiO3. It was indicated that substitution of Co for the Ti sites in the (1 − x)BaTiO3 + xCo2O3 series led to decrease of tetragonality (c/a) of the BaTiO3 perovskite structure. This effect almost vanished in the (1 − x)BaTiO3 + xCo2O3 samples with nominal Co content higher than ∼1 wt.%, in which precipitation of parasitic Co-containing phases CoO and Co2TiO4 has been observed. Based on the results, the solubility limit of Co in Ti sub-lattice in the (1 − x)BaTiO3 + xCo2O3 series is estimated as x = 0.75 wt.%.