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We have theoretically analysed temperature dependence of the specific heat at temperatures below 50 K of a Kondo-lattice antiferromagnet CeRh 2 Si 2 . We have reproduced experimentally-observed λ -type peak at the magnetic-ordering temperature T N of 36.5 K as originating from the splitting of the Kramers-doublet ground state of the Ce 3 + ion. We have derived a set of crystal-field parameters, which, apart of the reproduction of crystal-field excitations of 30 and 52 meV, of the temperature dependence of the specific heat and of the paramagnetic susceptibility, reproduces a value of the magnetic moment, of 1.60 μ B . Our results for the temperature dependence of the specific heat and of the entropy are physically adequate and substantially improved compared to theoretical descriptions reported in a recent publication in Nature Comm. (7 (2016) 11029). A good reproduction of the physical magnetic and electronic properties indicates that 99 ± 1 % of Ce ions are in the 4 f 1 configuration. A large value of the entropy removed in the magnetic transition, very close to Rln2, indicates that the hybridization effects and a broadening of crystal-field states in CeRh 2 Si 2 are very small. The Quantum Atomistic Solid-State Theory (QUASST) approach can be used for the theoretical description of magnetic and electronic properties of other Ce, rare-earth and actinides compounds.

Ab initio studies of structural and electronic properties of the monoclinic perovskite-type BiMnO3 and BiMn0.5Nb0.5O3 crystals are presented using the pseudo-potential plane wave method within the density functional theory in generalized – gradient approximation. The calculated equilibrium lattice parameters remain in a good agreement with the available experimental data. The gap energy took values in the range E g = 0 . 2 − 0 . 5 eV for both compounds.

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 i...

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.

For Mott insulators, K2CoF4 and SrMnO3, we have calculated, in the purely ionic model, the low-energy electronic structure both in the paramagnetic and magnetic state as well as zero-temperature magnetic moment, its direction and its temperature dependence. We have calculated the octahedral crystal-field strength 10Dq to be 0.98 and 2.25 eV. We claim that for an adequate theoretical description of magnetic properties even small local distortions and the intra-atomic relativistic spin-orbit coupling have to be taken into account. Our studies have revealed a strong interplay of the magnetism, the orbital moment in particular, with the local crystallographic structure. The calculated orbital moment in K2CoF4 is very large, 1.06 μB, giving 30% contribution to the total moment - this result points the necessity to “unquench” the orbital magnetism in 3d compounds. We consistently described magnetic and some optical properties of these compounds, containing atoms with incomplete 3d shell, in agreement with their insulating ground state.

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.

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.