Atomic Scale Investigation of Pressure Induced Phase Transitions in the solid State

In this work, atomic scale investigation of pressure-induced transformations in the solid state have been carried out. A series of compounds including GaN, ZnO, CaF2, and AgI, in addition to elemental phosphorus have been studied. The corresponding transition mechanisms have been elucidated with a clear description of atomic displacements and intermediate structures involved therein. In the first group of compounds, the long standing debate on the transition path of the wurtzite(WZ)-to-rocksalt(RS) transition in semiconductors, GaN and ZnO was resolved using geometrical modeling combined with molecular dynamics (MD) simulations conducted in the frame of transition path sampling (TPS) method. In GaN, a two-step mechanism through a metastable intermediate phase with a tetragonal structure iT has been revealed from simulations. In ZnO, the tetragonal intermediate structure was kinetically less stable, although still part of the real transition mechanism. It appeared at the interface between WZ and RS as consequence of a layers shearing. The transition regime in ZnO was characterized by a competition between iT structure and another hexagonal intermediate with hexagonal symmetry iH. Although possible, the latter is not functional for the transition. In both cases, GaN and ZnO, two points of agreement with experiments have been revealed. The tilting of structures after transition, and the phonon mode softening associated with atomic displacements leading to the tetragonal structure iT In the second group of compounds, the investigation of transitions in superionic conductors, CaF2 and AgI, demonstrated a different and particular behavior of atomic motion under pressure. The solid-solid reconstruction of CaF2 structure was shown to be initiated and precedented by high disorder of the anionic sublattice. The percolation of fluoride ions through voids in the fluorite structure created a thin interface of liquid like state. The sparce regions caused by the departure of anions facilitates the cation sublattice reconstruction. In AgI, ion diffusion during the wurtzite/zincnlende(ZB)$rocksalt transition was more pronounced due to the extended stacking disorder WZ/ZB. The Ag+ ions profited not only from the structure of the interface but used the combination of interstitial voids offered by both phases, WZ and ZB, to achieve long diffusion paths and cause the cation sublattice to melt. Clearly, a proper account for such phenomena cannot be provided by geometry-designed mechanisms based on symmetry arguments. In phosphorus, the question of how the stereochemically active lone pairs are reorganized during the orthorhombic (PI) to trigonal (PV) structural transition was answered by means of simulations. Computation was performed at different levels theory. First, the mechanism of the transition was obtained from TPS MD simulations. MD runs were performed within density functional tight binding method (DFTB). The analysis of atomic displacements along the real transformation path indicated a fast bond switching mechanism. In a second step, the nature of the interplay between orbitals of phosphorus during the bond switching was investigated. A simultaneous deformation of lone pair and P−P bond showed a mutual switching of roles during the transformation. This interplay caused a low dimensional polymerization of phosphorus under pressure. The corresponding structure formed as zigzag linear chain of fourfold coordinated phosphorus atoms (· · ·(P(P2))n · · ·) at the interface between PI and PV phases. A further result of this work was the development of a simulation strategy to incorporate defects and chemical doping to structural transformations. On top of the transition path sampling iterations, a Monte Carlo like procedure is added to stepwise substitute atoms in the transforming system. Introducing a chemically different dopant to a pure system represents a perturbation to the energy landscape where the walk between different phases is performed. Therefore, any change in the transition regime reflects the kinetic preference of a given structural motif at times of phase formation. This method was applied to the elucidation of WZ-RS transition mechanism in the series of semiconducting compounds AlN, GaN, and InN. Simulations showed that In atoms adopt the same transformation mechanism as in GaN and favor it, while Al atoms demonstrated a significant reluctance to the path going through tetragonal intermediate iT. The difference between transition regime in mixed systems InxGa1−xN and AlxGa1−xN is in agreement with experiments on high pressure behavior of AlN, GaN, and InN. While transitions in GaN and InN are reversible down to ambient conditions, AlN is stable. The work presented in this thesis constitutes the seed of new perspectives in the understanding of pressure-induced phase transformations in the solid state, where the physics and the chemistry are brought together by means of computer simulations

Metastable host–guest structure of carbon

A family of metastable host–guest structures, the prototype of which is a tetragonal tP9 structure with 9 atoms per cell has been found. It is composed of an 8-atoms tetragonal host, with atoms filling channels oriented along the c-axis. The tP9 structure has a strong analogy with the recently discovered Ba-IV- and Rb-IV-type incommensurate structures. By considering modulations of the structure due to the variations of the host/guest ratio, it has been concluded that the most stable representative of this family of structures has a guest/host ratio of 2/3 and 26 atoms in the unit cell (space group P42/m). This structure is 0.39 eV/atom higher in energy than diamond. We predict it to have band gap 4.1 eV, bulk modulus 384 GPa, and hardness 61–71 GPa. Due to the different local environments of the host and guest atoms, we considered the possibility of replacing carbon atoms in the guest sublattice by Si atoms in the tP9 prototype and study the properties of the resulting compound SiC₈, which was found to have similarly remarkably high bulk modulus 361.2 GPa and hardness 46.2 GPa.Повідомляється про сімейство метастабільних структур господаргість, прототипом якого є тетрагональна структура tP9 з дев’ятьма атомами в комірці. Вона складається з восьми атомів тетрагонального господаря, що заповнюють канали, орієнтовані вздовж осі с. Структура tP9 аналогічна недавно відкритим Ba-IV- й Rb-IV-типам несумірних структур. Враховуючи модуляцію структури через варіацій співвідношень господар/гість, зроблено висновок, що найбільш стабільний представник цього сімейства структур має відношення гість/господар – 2/3 і 26 атомів в елементарній комірці (просторова група P42/m). Енергія цієї структури на 0,39 еВ/атом вище, ніж алмазу. Ця структура, за прогнозами, повинна мати заборонену зону – 4,1 еВ, об’ємний модуль – 384 ГПа, а твердість – 61–71 ГПа. Через різні локальні стани атомів господаря і гостя розглянуто можливість заміни атомів вуглецю гостьової підґратки атомами Si в прототипі tP9 і вивчені властивості отриманої сполуки SiC₈, які, як було виявлено, мають вельми високий об’ємний модуль пружності – 361,2 ГПа і твердість 46,2 ГПа.Сообщается о семействе метастабильных структур хозяин–гость, прототипом которого является тетрагональная структура tP9 с девятью атомами в ячейке. Она состоит из восьми атомов тетрагонального хозяина, заполняющих каналы, ориентированные вдоль оси с. Структура tP9 аналогична недавно открытым Ba-IV- и Rb-IV-типам несоразмерных структур. Учитывая модуляцию структуры из-за вариаций отношения хозяин/гость, сделан вывод, что наиболее стабильный представитель этого семейства структур имеет отношение гость/хозяин – 2/3 и 26 атомов в элементарной ячейке (пространственная группа P42/m). Энергия этой структуры на 0,39 эВ/атом выше, чем алмаза. Эта структура, по прогнозам, должна иметь запрещенную зону – 4,1 эВ, модуль объемного сжатия – 384 ГПа, а твердость – 61–71 ГПа. Из-за различных локальных состояний атомов хозяина и гостя рассмотрена возможность замены атомов углерода гостевой подрешетки атомами Si в прототипе tP9 и изучены свойства полученного соединения SiC₈, которое, как было обнаружено, имеет весьма высокий модуль объемного сжатия – 361,2 ГПа и твердость 46,2 ГПа

Novel sp³ forms of carbon predicted by evolutionary metadynamics and analysis of their synthesizability using transition path sampling

Experiments on cold compression of graphite have indicated the existence of a new superhard and transparent allotrope of carbon. Numerous metastable candidate structures featuring different topologies have been proposed for “superhard graphite”, showing a good agreement with experimental X-ray data. In order to determine the nature of this new allotrope, we use evolutionary metadynamics to systematically search for low-enthalpy sp³ carbon structures easily accessible from graphite and we employ molecular-dynamics transition path sampling to investigate the corresponding kinetic pathways starting from graphite at 15–20 GPa. Real transformation kinetics are computed and physically meaningful transition mechanisms are produced at the atomistic level of detail in order to demonstrate how nucleation mechanism and transformation kinetics lead to M-carbon as final product of cold compression of graphite. This establishes M-carbon as an experimentally synthesized carbon allotrope.Експерименти по холодному стисненню графіту показали наявність нової надтвердої і прозорою аллотропной форми вуглецю. Численні метастабільні структури з різною топологією були запропоновані для “надтвердого графіту” і показували добру відповідність експериментальним даним рентгенографії. Для однозначного визначення природи цієї нової аллотропной форми нами використано еволюційну метадінаміку, метод, що дозволяє систематичний пошук низькоентальпійних sp³-вуглецевих структур, що кінетично легко одержують з графіту, а також застосований молекулярно-динамічний вибір способів дослідження відповідної кінетики перетворення графіту при тисках 15–20 ГПа. Розрахована реальна кінетика перетворення графіту і отримані на атомистическому рівні реалістичні механізми перетворення, що демонструють, як механізм нуклеації та кінетика перетворення приводять до M-вуглецю – кінцевого продукту холодного стиснення графіту. Ці дослідження дозволяють вважати М-вуглець експериментально синтезованої алотропной формою вуглецю.Эксперименты по холодному сжатию графита показали наличие новой сверхтвердой и прозрачной аллотропной формы углерода. Многочисленные метастабильные структуры с различной топологией были предложены для “сверхтвердого графита” и показывали хорошее согласие с экспериментальными данными рентгенографии. Для однозначного определения природы этой новой аллотропной формы нами использована эволюционная метадинамика, метод, позволяющий систематический поиск низкоэнтальпийных sp³-углеродных структур, кинетически легко получаемых из графита, а также применен молекулярно-динамический выбор способов исследования соответствующей кинетики превращения графита при давлениях 15–20 ГПа. Рассчитана реальная кинетика преобразования графита и получены на атомистическом уровне реалистичные механизмы превращения, которые демонстрируют, как механизм нуклеации и кинетика превращения приводят к M-углероду – конечному продукту холодного сжатия графита Эти исследования позволяют считать М-углерод экспериментально синтезированной аллотропной формой углерода

Metashooting: a novel tool for free energy reconstruction from polymorphic phase transition mechanisms

We introduce a novel scheme for the mechanistic investigation of solid-solid phase transitions, which we dub extit{metashooting}. Combining transition path sampling molecular dynamics and metadynamics, this scheme allows for both a complete mechanistic analysis and a detailed mapping of the free energy surface. This is illustrated by performing extit{metashooting} calculations on the pressure-induced B4/B3 $ightarrow$ B1 phase transition in ZnO. The resulting free energy map helps to clarify the role of intermediate configurations along this activated process and the competition between different mechanistic regimes with superior accuracy. We argue that extit{metashooting} can be efficiently applied to a broader class of activated processes

Hierarchical thermoelectrics: crystal grain boundaries as scalable phonon scatterers

Thermoelectric materials are strategically valuable for sustainable development, as they allow for the generation of electrical energy from wasted heat. In recent years several strategies have demonstrated some efficiency in improving thermoelectric properties. Dopants affect carrier concentration, while thermal conductivity can be influenced by alloying and nanostructuring. Features at the nanoscale positively contribute to scattering phonons, however those with long mean free paths remain difficult to alter. Here we use the concept of hierarchical nano-grains to demonstrate thermal conductivity reduction in rocksalt lead chalcogenides. We demonstrate that grains can be obtained by taking advantage of the reconstructions along the phase transition path that connects the rocksalt structure to its high-pressure form. Since grain features naturally change as a function of size, they impact thermal conductivity over different length scales. To understand this effect we use a combination of advanced molecular dynamics techniques to engineer grains and to evaluate thermal conductivity in PbSe. By affecting grain morphologies only, i.e. at constant chemistry, two distinct effects emerge: the lattice thermal conductivity is significantly lowered with respect to the perfect crystal, and its temperature dependence is markedly suppressed. This is due to an increased scattering of low-frequency phonons by grain boundaries over different size scales. Along this line we propose a viable process to produce hierarchical thermoelectric materials by applying pressure via a mechanical load or a shockwave as a novel paradigm for material design

Understanding the nature of "superhard graphite"

Numerous experiments showed that on cold compression graphite transforms into a new superhard and transparent allotrope. Several structures with different topologies have been proposed for this phase. While experimental data are consistent with these models, the only way to solve this puzzle is to find which structure is kinetically easiest to form. Using state-of-the-art molecular-dynamics transition path sampling simulations, we investigate kinetic pathways of the pressure-induced transformation of graphite to various superhard candidate structures. Unlike hitherto applied methods for elucidating nature of superhard graphite, transition path sampling realistically models nucleation events necessary for physically meaningful transformation kinetics. We demonstrate that nucleation mechanism and kinetics lead to $M$-carbon as the final product. $W$-carbon, initially competitor to $M$-carbon, is ruled out by phase growth. Bct-C$_4$ structure is not expected to be produced by cold compression due to less probable nucleation and higher barrier of formation

Organizational Trust and its Impact on the Organizational Commitment of Human Resources

The present study aims to identify the nature of the impact of organizational trust with its dimensions (trust in colleagues, trust in supervisors, trust in Administration) on the organizational commitment of human resources. For this purpose, a questionnaire was developed by the researchers as the main tool for data collection. It was then distributed to a random sample of 43 employees of the Central Library of Mohammed Seddik Ben Yahia University. Eventually, the collected data were statistically processed using the (SPSS) program. Thus, the results of the study showed that there is a positive effect of organizational trust on organizational commitment of the central library employees. As for the three dimensions of organizational trust, it was confirmed that there is an effect of the dimension of trust in Administration on the organizational commitment of the library employees, while it was completely absent in the two dimensions of trust in colleagues and supervisors

Many-particle Li Ion dynamics in LiMPO4 olivine phosphates (M = Mn, Fe)

LiMPO4 (M = Mn, Fe) olivine phosphates are important materials for battery applications due to their stability, safety, and reliable recharge cycle. Despite continuous experimental and computational investigations, several aspects of these materials remain challenging, including conductivity dimensionality and how it maps onto Li pathways. In this work, we use a refined version of our finite temperature molecular dynamics “shooting” approach, originally designed to enhance Li hopping probability. We perform a comparative analysis of ion mobility in both materials, focused on many-particle effects. Therein, we identify main [010] diffusion channels, as well as means of interchannel couplings, in the form of Li lateral [001] hopping, which markedly impact the overall mobility efficiency as measured by self-diffusion coefficients. This clearly supports the need of many-particle approaches for reliable mechanistic investigations and for battery materials benchmarking due to the complex nature of the diffusion and transport mechanisms