Упругие свойства и ангармонизм твердых тел

The squares of the velocities of the longitudinal and transverse acoustic waves separately are practically not associated with anharmonicity, and their ratio (vL2 / vS2) turns out to be a linear function of the Grüneisen parameter γ - the measure of anharmonicity. The obtained dependence of (vL2 / vS2) on γ is in satisfactory agreement with the experimental data. The relationship between the quantity (vL2 / vS2) and anharmonicity is explained through its dependence on the ratio of the tangential and normal stiffness of the interatomic bond λ, which is a single-valued function of the Grüneisen parameter λ (γ). The relationship between Poisson's ratio μ and Grüneisen parameter γ, established by Belomestnykh and Tesleva, can be substantiated within the framework of Pineda's theory. Attention is drawn to the nature of the Leont'ev formula, derived directly from the definition of the Grüneisen parameter by averaging the frequency of normal lattice vibration modes. The connection between Grüneisen, Leontiev and Belomestnykh-Tesleva relations is considered. The possibility of a correlation between the harmonic and anharmonic characteristics of solids is discussed.

PREDICTION OF BONDING ENERGY BY STRUCTURAL DESCRIPTORS OF METAL NANOALLOYS

Обсуждается проблема предсказания энергии связи для тернарных металлических наночастиц и построение моделей обучения на базе структурных дескрипторов. Были построены регрессионные зависимости удельной межатомной энергии связи для тернарной наносистемы Au - Ag - Cu. Использовался ряд из пяти радиальных признаков, зависящих от попарного межатомного расстояния дескрипторов структуры наночастицы. Для более корректной оценки точности была применена кросс-валидация, далее полученные на валидационных частях выборки результаты усреднялись. Полученная модель ограниченно предсказывает значение удельной межатомной энергии связи внутри группы данных для наночастиц одного состава, а для всей выборки средняя по модулю ошибка составляет 14%. При этом модель практически безошибочно определяет состав наночастицы из нескольких вариантов. Наибольшее значение коэффициента детерминации на всей выборке получено с помощью ансамблевого алгоритма случайный лес. Обнаружена отрицательная корреляция между энергией связи наносплава и положением первого пика радиальной функции распределения для атомов меди. The problem of predicting the binding energy for ternary metal nanoparticles and the construction of learning models based on structural descriptors are discussed. Regression dependences of the specific interatomic bond energy were constructed for the ternary Au - Ag - Cu nanosystem. A number of five radial features were used, depending on the pairwise interatomic distance of the nanoparticle structure descriptors. For a more correct assessment of the accuracy, cross-validation was applied, then the results obtained on the validation parts of the sample were averaged. The resulting model limitedly predicts the value of the specific interatomic binding energy within a group of data for nanoparticles of the same composition. For the entire sample the average error in modulus is 14 %. In this case, the model almost accurately determines the composition of a nanoparticle of several variants. The largest value of the coefficient of determination in the entire sample was obtained using an ensemble random forest algorithm. A negative correlation was found between the binding energy of the nanoalloy and the position of the first peak of the radial distribution function for copper atoms.

Atomistic Line Graph Neural Network for improved materials property predictions

Graph neural networks (GNN) have been shown to provide substantial performance improvements for atomistic material representation and modeling compared with descriptor-based machine learning models. While most existing GNN models for atomistic predictions are based on atomic distance information, they do not explicitly incorporate bond angles, which are critical for distinguishing many atomic structures. Furthermore, many material properties are known to be sensitive to slight changes in bond angles. We present an Atomistic Line Graph Neural Network (ALIGNN), a GNN architecture that performs message passing on both the interatomic bond graph and its line graph corresponding to bond angles. We demonstrate that angle information can be explicitly and efficiently included, leading to improved performance on multiple atomistic prediction tasks. We ALIGNN models for predicting 52 solid-state and molecular properties available in the JARVIS-DFT, Materials project, and QM9 databases. ALIGNN can outperform some previously reported GNN models on atomistic prediction tasks by up to 85% in accuracy with better or comparable model training speed.

Ferromagnetic 1H-LaBr2 monolayer: a promising 2D piezoelectric

The discovery of two dimensional (2D) materials that have excellent piezoelectric response along with intrinsic magnetism is promising for nanoscale multifunctional piezoelectric or spintronic devices. Piezoelectricity requires non-centrosymmetric structures with an electric band-gap, whereas magnetism demands broken time-reversal symmetry. Most of the well-known 2D piezoelectric materials – e.g., 1H-MoS2 monolayer – are not magnetic. Being intrinsically magnetic, semiconducting 1H-LaBr2and 1H-VS2 monolayers can combine magnetism and piezoelectricity. We compare piezoelectric properties of 1H-MoS2, 1H-VS2 and 1H-LaBr2 using density functional theory. Our results show that ferromagnetic 1H-LaBr2 2D monolayer displays a larger piezoelectric strain co-efficient (d_{11}= -4.527 pm/V, which is close to d_{11}= 4.104 pm/V of 1H-VS2 monolayer) compared to that of well-known 1H-MoS2 monolayer (d_{11}= 3.706 pm/V), while 1H-MoS2 monolayer has a larger piezoelectric stress co-efficient (e_{11}= 370.675 pC/m) than the 1H-LaBr2 monolayer (e_{11}= -94.175 pC/m, which is also lower than e_{11}= 298.100 pC/m of 1H-VS2 monolayer). These in-plane piezoelectric d_{11} coefficients are quite comparable with piezo-response of bulk wurtzite nitrides – e.g., d_{33} of GaN is about 3.1 pm/V. The large d_{11} for 1H-LaBr2 monolayer originates from the low elastic constants, C_{11}= 30.338 N/m and C_{12} = 9.534 N/m. Interestingly, the sign of the piezoelectric co-coefficients for 1H-LaBr2 monolayer is different to that of the 1H-MoS2 or 1H-VS2 monolayers. The negative sign arises from the negative ionic contribution of e_{11}, which dominates in the 1H-LaBr2 monolayer, whereas the electronic part of e_{11} dominates in 1H-MoS2 and 1H-VS2. Furthermore, we explain the origin of this large ionic contribution of e_{11} for 1H-LaBr2 in terms of the Born effective charges (Z_{11}) and the sensitivity of the atomic positions to the strain (\frac{du}{d\eta}). Surprisingly, we observe a sign reversal in the Z_{11} of Mo and S compared to the nominal oxidation states, which makes both the electronic and ionic parts of e_{11} positive, and results in the high value of e_{11}. Additionally, our interatomic bond analysis using crystal orbital Hamilton populations indicates that the weaker covalent bond in 1H-LaBr2 monolayer is responsible for large \frac{du}{d\eta} and elastic softening (lower elastic constants).

Binary perovskite solid solutions and trends in their phase transitions temperatures variation

Some of the known binary ABO3perovskites with ferro- (FE) or antiferroelectric (AFE) at [Formula: see text] and different magnetic phase transitions (PT): ferro-and antiferromagnetic at [Formula: see text] and also some of their solid solutions are considered. Some correlations between their FE or AFE and/or magnetic PTs temperatures, on the one hand, and their interatomic bond [Formula: see text]–O strains, on the other hand, have been constructed. It is shown that in the plotted diagrams these temperatures change with a change in [Formula: see text] values as follows: classical FEs are followed by multiferroics, starting with BiFeO3 and ending with YVO3, followed by classical AFEs. At the same time, the temperatures [Formula: see text] and [Formula: see text] experience maxima at the corresponding points for BiFeO3, then quickly decrease, and the difference between them, [Formula: see text]-[Formula: see text], basically also decreases, slightly increasing along the way to the point EuTiO3. which made it possible to systematize these [Formula: see text] and [Formula: see text].

Dependence of the ratio of squares of the velocity of acoustic waves in solids on the Gruneisen parameter

It is found that in the Leont'ev and Belomestnykh-Tesleva formulas for the Grüneisen parameter, the right-hand sides of the equalities depend on anharmonicity through the dependence of the ratio of the squared acoustic wave velocities ( v L2 / v S2) on the Grüneisen parameter γ. The theoretical dependence of ( v L2 / v S2) on γ generally agrees with experimental data for both crystals and glassy solids. The quantity ( v L2 / v S2) turns out to be a single-valued function of the ratio of the tangential and normal stiffness of the interatomic bond.

Effect of pressure on the mechanical, electronic and optical characters of CsSnBr3 and CsSnI3: ab-initio study

A theoretical comprehensive implementing of the structural, elastic, electronic and optical properties of CsSnX3 (X = Br and I) perovskite compounds under pressures 0 and 20 GPa is performed by ab-initio calculations included within the density functional theory (DFT). The structure of crystal perovskite compounds is found to be stable under induced pressure. The compounds have shown a decrease in the structural properties such as lattice constant and interatomic bond length when the pressure was induced. Whereas, there was an increase in the thermodynamic properties such as Debye temperature and average velocities of sound when pressure was induced. Moreover, the values of mechanical parameters, such as the elastic constant, increased under applied pressure. The electronic parameters indicate that the compounds can be classified as semiconductor materials with a direct (M-M) gap. The induced pressure is found to enhance the optical parameters in the different energy regions. Our calculation predicts that the studied compounds can be the relevant candidates in optical, thermoelectric and mechanical applications.

Temporal pattern of the mechanoluminescent lighting from impact-loaded ZnS ceramics

Mechanoluminescence (ML) from deformed ductile materials is caused by sliding of charged dislocations. The main source of the ML in loaded brittle solids is the interatomic bond breakage. In this work, the ML from impact damaged ductile ZnS ceramics was studied. It was revealed that the time series of ML pulses exhibited two well-separated peaks. A two-stage ML excitation of this kind in ZnS ceramics was observed when the applied load exceeded an ultimate plastic deformation. The positions of each peak along the time axis were found to be dependent in different ways on physical and mechanical properties of crystallites that constituted the given ceramics. The investigated ceramics were produced by four different technological methods. The obtained samples differing in the dimensions of grains and separation of grain boundaries were tested. The statistical analysis of a temporal pattern of the ML lighting showed that the mechanisms of excitation of the two peaks were not the same. The first ML peak was assigned to the plastic deformation preceding the ceramics cracking. The second peak originated from the interatomic bond breakage in nucleated and growing cracks.

The complex oxides with octahedral structures: Existence areas and phase transitions

The experimental and calculated data on the existence of complex oxides in solid state with the octahedral structures of four families, namely perovskites, Bi-containing layered perovskite-like ones, tetragonal tungsten bronzes and pyrochlores, and about their phase transitions are systematized and summarized on the basis of the quasi-elastic or geometric models of these structures. It has been established that similar existence areas and similar correlations between the interatomic bond strains in their structures, on the one hand, and the temperatures of their ferroelectric or antiferroelectric phase transitions, on the other hand, are observed for all of them, despite the differences in the compositions and structures of these oxides, but taking into account their similar parameters.