Development of a ReaxFF potential for Au-Pd.

The bimetallic alloys often outperform their single-component counterparts due to synergistic effects. Being widely known, the Au-Pd alloy is a promising candidate for the novel heterogeneous nanocatalysts. Rational design of such systems requires theoretical simulations under ambient conditions.Ab initioquantum-mechanical calculations employ the density functional theory (DFT) and are limited to the systems with few tens of atoms and short timescales. The alternative solution implies development of reliable atomistic potentials. Among different approaches ReaxFF combines chemical accuracy and low computational costs. However, the development of a new potential is a problem without unique solution and thus requires accurate validation criteria. In this work we construct ReaxFF potential for the Au-Pd system based onab initioDFT calculations for bulk structures, slabs and nanoparticles with different stoichiometry. The validation was performed with molecular dynamics and Monte-Carlo calculations. We present several optimal parametrizations that describe experimental bulk mechanical and thermal properties, atomic order-disorder phase transition temperatures and the resulting ordered crystal structures.

Hidden improper ferroelectric phases for design of antiferroelectrics.

Strong anomalous increase of the dielectric constant across a structural phase transition between two centrosymmetric phases, commonly observed in various crystals including prominent antiferroelectrics, is shown to originate from the hidden improper ferroelectric phases. In the vicinity of the phase transition double hysteresis loops of electric polarization vs electric field should be observed, which can be used for targeted design of antiferroelectric compounds. The suggested mechanism is illustrated by theoretical explanation of the recently discovered antiferroelectricity in the Ruddlesden-Popper compound ((CH3)2CHCH2NH3)2CsPb2Br7. Implications of the suggested models for the phase transition between the R and P phases in NaNbO3 are discussed.

Theory of order-disorder phase transitions of B-cations in AB'1/2B''1/2O3 perovskites.

Perovskite-like oxides AB'1/2B''1/2O3 with two different cations in the B-sublattice may experience cation order-disorder phase transitions. In many cases the degree of cation ordering can be varied by suitable synthesis conditions or subsequent sample treatment, which has a fundamental impact on the physical properties of such compounds. Therefore, understanding the mechanism of cation order-disorder phase transition and estimation of the phase transition temperature is of paramount importance for tuning of properties of such double perovskites. In this work, based on the earlier proposed cation-anion elastic bonds model, a theory of order-disorder phase transitions of B-cations in AB'1/2B''1/2O3 perovskites is presented, which allows reliable estimation of the phase transition temperatures and of the reduced lattice constants of such double perovskites.

Linear magnetoelectric effect in göthite, α-FeOOH.

By means of symmetry analysis, density functional theory calculations, and Monte Carlo simulations we show that goethite, α-FeOOH, is a linear magnetoelectric below its Néel temperature T N = 400 K. The experimentally observed magnetic field induced spin-flop phase transition results in either change of direction of electric polarization or its suppression. Estimated value of magnetoelectric coefficient is 0.57 µC · m-2 · T-1. The abundance of goethite in nature makes it arguably the most widespread magnetoelectric material.

Electric polarization of magnetic domain walls in magnetoelectrics.

Two prominent magnetoelectrics MnWO4 and CuO possess low-temperature commensurate paraelectric magnetically ordered phase. Here using Monte Carlo simulations we show that the walls between the domains of this phase are ferroelectric with the same electric polarization direction and value as those in the magnetoelectric phases of these compounds. We also suggest that experimental observation of electric polarization of domain walls in MnWO4 should help to determine the macroscopic interactions responsible for its magnetoelectric properties.

Interpretation of magnetoelectric phase states using the praphase concept and exchange symmetry.

The majority of magnetoelectric crystals show complex temperature-magnetic field or temperature-pressure phase diagrams with alternating antiferromagnetic incommensurate, magnetoelectric, and commensurate phases. Such phase diagrams occur as a result of successive magnetic instabilities with respect to different order parameters, which usually transform according to different irreducible representations (IRs) of the space group of the crystal. Therefore, in order to build a phenomenological theory of phase transitions in such magnetoelectrics one has to employ several order parameters and assume the proximity of various instabilities on the thermodynamic path. In this work we analyze the magnetoelectrics MnWO4, CuO, NaFeSi2O6, NaFeGe2O6, Cu3Nb2O8, α-CaCr2O4 and FeTe2O5Br using the praphase concept and the symmetry of the exchange Hamiltonian. We find that in all the considered cases the appearing magnetic structures are described by IRs entering into a single exchange multiplet, whereas in the cases of MnWO4 and CuO by a single IR of the space group of the praphase structure. Therefore, one can interpret the complex phase diagrams of magnetoelectrics as induced by a single IR either of the praphase or of the symmetry group of the exchange Hamiltonian. Detailed temperature-magnetic field phase diagrams of MnWO4 and CuO for certain field directions are obtained and the magnetic structures of the field-induced phases are determined.

The magnetoelectric effect due to local noncentrosymmetry.

Magnetoelectrics often possess ions located in noncentrosymmetric surroundings. Based on this fact we suggest a microscopic model of magnetoelectric interaction and show that the spin-orbit coupling leads to spin-dependent electric dipole moments of the electron orbitals of these ions, which results in non-vanishing polarization for certain spin configurations. The approach accounts for the macroscopic symmetry of the unit cell and is valid for both commensurate and complex incommensurate magnetic structures. The model is illustrated by the examples of MnWO(4), MnPS(3) and LiNiPO(4). Application to other magnetoelectrics is discussed.

Phenomenological theory of phase transitions in multiferroic MnWO4: magnetoelectricity and modulated magnetic order.

The phenomenological theory of phase transitions in multiferroic MnWO4 is suggested. The theoretical model uses the assumption that the magnetic order is driven by the instability in the (1/4;1/2;1/2) point of the Brillouin zone, which is justified by the symmetry of the low-temperature magnetic phase. It is shown that the experimentally observed incommensurate magnetic order is due to the Lifshitz invariants allowed for the corresponding order parameters. Invariants responsible for the magnetoelectric interaction are found and a schematic phase diagram is calculated. The influence of the magnetic field on the phase transition sequence is also analyzed. It is suggested that the description of the phase transitions in MnWO4 starting from the orthorhombic praphase significantly simplifies the approach and allows us to draw important conclusions.