Polarized Raman, IR, and time-domain THz spectroscopy of orthorhombic lead zirconate single crystals have yielded a comprehensive picture temperature-dependent quasiharmonic frequencies its low-frequency phonon modes. It is argued that these modes primarily involve vibrations Pb ions librations oxygen octahedra. Their relation to the parent cubic phase proposed. Counts observed IR Raman active belonging distinct irreducible representations agree quite well with group-theory predictions....

The Calcium Silicate Hydrate (C-S-H) nucleation is a crucial step during cement hydration and determines to great extent the rheology, microstructure, properties of paste. Recent evidence indicates that C-S-H involves at least two steps, yet underlying atomic scale mechanism, nature primary particles their stability, or how they merge/aggregate form larger structures unknown. In this work, we use atomistic simulation methods, specifically DFT, evolutionary algorithms (EA), Molecular Dynamics...

In this work, we present ænet-PyTorch, a PyTorch-based implementation for training artificial neural network-based machine learning interatomic potentials. Developed as an extension of the atomic energy network (ænet), ænet-PyTorch provides access to all tools included in ænet application and usage The package has been designed alternative internal capabilities ænet, leveraging power graphic processing units facilitate direct on forces addition energies. This leads substantial reduction time...

Abstract Some Aurivillius compounds exhibit features beyond the single soft-mode picture valid for other ferroelectrics. In SrBi2Ta2O9 (SBT) two primary distortions are needed to reach ferroelectric phase, causing presence of an intermediate phase. However, no phase has been detected in SrBi2Nb2O9 (SBN), and avalanche transition seems happen, with order parameters condensing simultaneously. According ab-initio calculations, usually assumed triggering mechanism through biquadratic coupling...

The Aurivillius compound ${\mathrm{Bi}}_{4}{\mathrm{Ti}}_{3}{\mathrm{O}}_{12}$ exhibits a single phase transition from tetragonal high-temperature to ferroelectric state. By means of first-principles calculations guided by complete symmetry analysis the experimental distortion, we confirm $B1a1$ as (monoclinic) room-temperature but show that involves complex interplay six different normal modes belonging four irreducible representations. Three them are necessary for observed break. We do not...

Antiferromagnetic MnPS${}_{3}$ and diamagnetic ZnPS${}_{3}$ belong to a family of two-dimensional compounds that exhibit rich variety magnetic ordering. Low-temperature Raman spectroscopy SQUID magnetometer measurements are performed on both their alloy in order evaluate possible Jahn-Teller distortion as well reveal signatures. With the aid first-principles calculations, description lattice dynamics system is presented, together with phase diagram across entire compositional range.

Numerous materials with the general formula ${\mathit{A}}_{2}$${\mathit{BX}}_{4}$, where A and B are cations X is anion, isomorphous to \ensuremath{\beta}-${\mathrm{K}}_{2}$${\mathrm{SO}}_{4}$ (space group Pnam) at high temperatures. considerable number of them exhibit a structural instability leading an incommensurately modulated phase identical superspace symmetry. ${\mathrm{K}}_{2}$${\mathrm{SeO}}_{4}$ archetypical example. From analysis incommensurate structures, polarization vector...

The range of validity Landau free-energy potentials with the usual approximation constant coefficients for terms higher than quadratic has recently been questioned (J. Phys. R Condens. Matter 1 (1989) 8327). frequent observation in real systems, within large temperature intervals, clearly outside any possible critical region, power laws type mod Tc-T b order parameter also pointed out as an indication that certain simple general features phase transitions, not related at all to phenomena,...

A simple bond-strain parameter, the so-called global-instability index, defined within bond-valence model reproduces with striking accuracy basic features of ab initio total-energy surface around energy minimum $\mathrm{Sr}{\mathrm{Bi}}_{2}{\mathrm{Ta}}_{2}{\mathrm{O}}_{9}$, but only for degrees freedom subspace energetically unstable modes. These results imply that calculations and criteria could be combined in study prediction structural instabilities. The quantitative agreement between...

The lattice dynamics of potassium selenate is analyzed using a rigid-ion model with the groups reduced to rigid bodies. interatomic forces have been adjusted only static structural data. number adjustable parameters varies from two five. Such simple already sufficient reproduce semiquantitatively phonon real system. In particular, exhibits instability leading existence an incommensurate phase. characteristics resulting soft mode agree those observed experimentally. calculated eigenvector, in...

We study the interplay of structural distortions in double perovskites ${\text{Sr}}_{2}M{\text{WO}}_{6}$ ($M=$ Zn, Ca, Mg) by means first-principles calculations and group theoretical analysis. Structure relaxations cubic, tetragonal, monoclinic phases show that ground states three compounds are monoclinic, although energy difference between tetragonal structures is very small case ${\text{Sr}}_{2}\text{Mg}{\text{WO}}_{6}$. The symmetry analysis involved experimental calculated...

Inelastic neutron scattering experiments in K2SeO4, Cs2SeO4 and K2CrO4 are reported. The lowest Sigma 3- 2 phonon branches along the alpha *-axis their Pnam phase have been characterised for three compounds. results compared with recent lattice dynamical simulations using rigid-ion models. Potassium chromate, that suggested to be incommensurate at low temperatures, evidences no structural transition down 12 K, but its branch exhibits a significative softening decreases more than 20% as...

La structure modulee est caracterisee par le groupe d'espace P(Pnma):(s11) avec affinement jusqu'e 0,046 pour toutes les reflexions

Despite their structural similarities, ${\mathrm{SrBi}}_{2}{\mathrm{Ta}}_{2}{\mathrm{O}}_{9}$ (SBT) and ${\mathrm{SrBi}}_{2}{\mathrm{Nb}}_{2}{\mathrm{O}}_{9}$ (SBN) undergo a different sequence of phase transitions. The diagram SBT as function the temperature includes an intermediate between high-temperature ferroelectric ground state, while in niobium compound is suppressed direct transition high- low-temperature structures observed. We present ab initio calculations that reveal relevance...

Abstract The elastic and electronic properties of brownmillerite (Ca 2 AlFeO 5 ), the fourth component in mass Portland cement, have been determined using Density Functional Theory. Young’s modulus obtained this work ( E = 169 ± GPa) is much closer to experimental value than all previously reported classical calculations. structure has analyzed by calculating band structure, density states crystal orbital overlap population curves. Results show that there are anti-bonding bonds between iron...

Abstract The quantitative assessment of the displacive degree a structural phase transition is discussed both from microscopical and an empirical viewpoint. Using Monte Carlo results for Φ4model as qualitative guide, relation between perspectives shown. Based on general trends observed, parameter describing any proposed. Rhodes-Wolfarth factor extended to ferroelectrics by Tokunaga [J. Phys. Soc. Jpn (1988) 4275] realization this particular case proper ferroelectrics. power limitations such...

We show that the thermal properties of periodic lattice systems can be approximated to a finite cluster with appropriate boundary conditions which include modified Hamiltonian for variables. Imposing invariance on correlation local site variables is sufficient obtain free parameters Hamiltonian. The degree accuracy calculation depends interaction range allowed in and up are made invariant. Bethe approximation considered trivial case this general method clusters one site. reliability...

Experimental magnetic studies performed on the [{CuLX}2] system (HL = pyridine-2-carbaldehyde thiosemicarbazone, X Cl−, Br−, I−) point to larger electronegativity in X, lower magnitude of antiferromagnetic interactions. In order confirm this and other trends observed dip into them, computational (X Cl− (1), I− (2)) compounds are here reported. The chemical structural comparisons have been extended obtained acid medium. regard, chlorido ligands yield [Cu(HL)Cl2]∙H2O (3) complex, whose crystal...

Molecular-dynamics calculations at different temperatures and pressures have been performed to study the sequence of phase transitions: hexagonal (P${6}_{3}$/mmc)-orthorhombic (Pnam)-orthorhombic (Pna${2}_{1}$) in ${\mathrm{K}}_{2}$${\mathrm{SeO}}_{4}$. The potential model consists rigid-ion long-range Coulombic short-range repulsive forces fitted structural data room-temperature phase, tetrahedral ${\mathrm{SeO}}_{4}$ groups being reduced rigid bodies. A method that uses symmetry-adapted...

A mullite single crystal with composition Al 4.84 Si 1.16 O 9.58 (2) exhibiting sharp satellite reflections was investigated by means of X-ray diffraction. For the refinement a superspace model in group Pbam (α0½)0 ss different scale factors for main and were used order to describe an ordered structure embedded disordered polymorph. The fraction sample exhibits completely vacancy distribution can be described as block blocks (VBs) that alternate vacancy-free (VFBs) along c . incommensurate...

Synchrotron single-crystal X-ray diffraction has revealed diffuse scattering alongside sharp satellite reflections for different samples of mullite (Al 4+2 x Si 2−2 O 10− ). Structural models have been developed in (3+1)-dimensional superspace that account vacancy ordering and Al/Si based on harmonic modulation functions. A constraint scheme is presented which explains the crystal-chemical relationships between split sites average structure. The amplitudes refinements differ significantly by...

The sequence of phase transitions and structural instabilities the Sr2MWO6 double perovskites are investigated using a rigid ion model. parametrization short range empirical potential allows control cation sizes by means independent parameters, in particular, effective size M can be tuned to reproduce behaviour whole family. coupling symmetry modes its role stability phases discussed, molecular dynamics simulations carried out determine as function temperature. A satisfactory agreement...

Abstract Despite many previous investigations, the symmetry and structure of thiourea in its modulated configuration is still under discussion. We report here a new determination this by means X-ray diffraction, using rigid-body model for molecular units. The diffraction diagram up to third order satellites was measured commensurate phase with q=1/9b*, refinement being performed however superspace formalism. group P(Pnma):(sT1) describes adequately symmetry, allowing excellent results...