Theoretical Diffraction Patterns Research Articles

Understanding the phase transition mechanism in the lead halide perovskite CsPbBr3 via theoretical and experimental GIWAXS and Raman spectroscopy

Metal-halide perovskites (MHPs) exhibit excellent properties for application in optoelectronic devices. The bottleneck for their incorporation is the lack of long-term stability such as degradation due to external conditions (heat, light, oxygen, moisture, and mechanical stress), but the occurrence of phase transitions also affects their performance. Structural phase transitions are often influenced by phonon modes. Hence, an insight into both the structure and lattice dynamics is vital to assess the potential of MHPs. In this study, GIWAXS and Raman spectroscopy are applied, supported by density functional theory calculations, to investigate the apparent manifestation of structural phase transitions in the MHP CsPbBr3. Macroscopically, CsPbBr3 undergoes phase transitions between a cubic (α), tetragonal (β), and orthorhombic (γ) phase with decreasing temperature. However, microscopically, it has been argued that only the γ phase exists, while the other phases exist as averages over length and time scales within distinct temperature ranges. Here, direct proof is provided for this conjecture by analyzing both theoretical diffraction patterns and the evolution of the tilting angle of the PbBr6 octahedra from molecular dynamics simulations. Moreover, sound agreement between experimental and theoretical Raman spectra allowed to identify the Raman active phonon modes and to investigate their frequency as a function of temperature. As such, this work increases the understanding of the structure and lattice dynamics of CsPbBr3 and similar MHPs.

Synthesis and characterization of SSM@NiO/TiO2 p-n junction catalyst for bisphenol a degradation

Photocatalyst immobilization on support materials is essential for large-scale applications. Here, we describe growth of a p-n junction catalyst (NiO/TiO2) on a stainless-steel mesh (SSM) support using a facile hydrothermal method. The morphological superiority of the composite over previously reported NiO/TiO2 catalysts was probed using scanning and transmission electron microscopy. Flower petal–like NiO grew uniformly on SSM, which was evenly covered by TiO2 nanoparticles. Theoretical and experimental X-ray diffraction patterns were compared to analyze the development of the composite during various stages of synthesis. The photocatalytic activity of a powdered catalyst and SSM@catalyst was compared by measuring bisphenol A (BPA) degradation. SSM@NiO/TiO2 achieved the highest rate of BPA degradation, removing 96% of the BPA in 120 min. Scavenging experiments were used to investigate the charge separation and degradation mechanism. SSM@NiO/TiO2 showed excellent reusability potential, achieving and sustaining 91% BPA removal after 10 rounds of cyclic degradation. Reusability performance, composite resilience, apparent quantum yields, and figures of merit suggest that SSM@NiO/TiO2 has excellent utility for practical applications.

Control of the phase composition of advanced calcium phosphates using an x-ray diffractometer with a curved position-sensitive detector

The results of studying the phase composition of advanced calcium phosphates Ca10(PO4)6(OH)2, β-Ca3(PO4)2, α-Ca3(PO4)2, CaHPO4 · 2H2O, Ca8(HPO4)2(PO4)4 · 5H2O using an x-ray diffractometer with a curved position-sensitive detector are presented. Optimal experimental conditions (angular positions of the x-ray tube and detector, size of the slits, exposure time) were determined with allowance for possible formation of the impurity phases during synthesis. The construction features of diffractometers with a position-sensitive detector affecting the profile characteristics of x-ray diffraction peaks are considered. The composition for calibration of the diffractometer (a mixture of sodium acetate and yttrium oxide) was determined. Theoretical x-ray diffraction patterns for corresponding calcium phosphates are constructed on the basis of the literature data. These x-ray diffraction patterns were used to determine the phase composition of the advanced calcium phosphates. The features of advanced calcium phosphates, which should be taken into account during the phase analysis, are indicated. The powder of high-temperature form of tricalcium phosphate strongly adsorbs water from the environment. A strong texture is observed on the x-ray diffraction spectra of dicalcium phosphate dihydrate. A rather specific x-ray diffraction pattern of octacalcium phosphate pentahydrate revealed the only one strong peak at small angles. In all cases, significant deviations are observed for the recorded angular positions and relative intensity of the diffraction peaks. The results of the study of experimentally obtained mixtures of calcium phosphate are presented. It is shown that the graphic comparison of experimental x-ray diffraction spectra and pre-recorded spectra of the reference calcium phosphates and possible impurity phases is the most effective method. In this case, there is no need for calibration. When using this method, the total time for analysis of one sample is no more than 10 min.

Simulations of the 2D self-assembly of tripod-shaped building blocks.

We introduce a molecular dynamics (MD) coarse-grained model for the description of tripod building blocks. This model has been used by us already for linear, V-shape, and tetratopic molecules. We wanted to further extend its possibilities to trifunctional molecules to prove its versatility. For the chosen systems we have also compared the MD results with Monte Carlo results on a triangular lattice. We have shown that the constraints present in the latter method can enforce the formation of completely different structures, not reproducible with off-lattice simulations. In addition to that, we have characterized the obtained structures regarding various parameters such as theoretical diffraction pattern and average association number.

Low-energy/pulse response and high-resolution-CMOS camera for spatiotemporal femtosecond laser pulses characterization @ 1.55 μm.

In this work, we present a commercial CMOS (Complementary Metal Oxide Semiconductor) Raspberry Pi camera implemented as a Near-Infrared detector for both spatial and temporal characterization of femtosecond pulses delivered from a femtosecond Erbium Doped Fiber laser (fs-EDFL) @ 1.55 µm, based on the Two Photon Absorption (TPA) process. The capacity of the device was assessed by measuring the spatial beam profile of the fs-EDFL and comparing the experimental results with the theoretical Fresnel diffraction pattern. We also demonstrate the potential of the CMOS Raspberry Pi camera as a wavefront sensor through its a nonlinear response in a Shack-Hartmann array and for the temporal characterization of the femtosecond pulses delivered from the fs-EDFL through TPA Intensity autocorrelation measurements. The direct pulse detection and measurement, through the nonlinear response with a CMOS, is proposed as a novel and affordable high-resolution and high-sensitivity alternative to costly detectors such as CCDs, wavefront sensors and beam profilers @ 1.55 µm. The measured fluence threshold, down to 17.5 µJ/cm2, and pJ/pulse energy response represents the lowest reported values applied as a beam profiler and a TPA Shack-Hartmann wavefront sensor, to our knowledge.

Search for common minima in joint optimization of multiple cost functions

We present a novel optimization method, named the Combined Optimization Method (COM), for the joint optimization of two or more cost functions. Unlike the conventional joint optimization schemes, which try to find minima in a weighted sum of cost functions, the COM explores search space for common minima shared by all the cost functions. Given a set of multiple cost functions that have qualitatively different distributions of local minima with each other, the proposed method finds the common minima with a high success rate without the help of any metaheuristics. As a demonstration, we apply the COM to the crystal structure prediction in materials science. By introducing the concept of data assimilation, i.e., adopting the theoretical potential energy of the crystal and the crystallinity, which characterizes the agreement with the theoretical and experimental X-ray diffraction patterns, as cost functions, we show that the correct crystal structures of Si diamond, low quartz, and low cristobalite can be predicted with significantly higher success rates than the previous methods.

Analysis of theoretical and experimental X-ray diffraction patterns for distinct mordenite frameworks

Experimental and theoretical XRD patterns of mordenite frameworks were correlated in this work. The experimental XRD analysis showed that the incorporation of Ag and Fe ions in mordenite modified the intensity of peaks in the diffraction patterns. For theoretical studies, two framework models of mordenite (MOR6 and MOR7) were used. Theoretical results conducted through DFT computational simulations were able to predict correctly the angular positions of the experimental peaks observed in XRD patterns. These theoretical results showed that the ion exchange of $${\hbox {Na}^{+}}$$ by $${\hbox {Ag}^{+}}$$ cations in the zeolitic framework leads to a decrease in intensity of XRD peaks {2 0 0}, {0 2 0} and {1 5 0}, similar to observed experimentally. This is caused by local structural rearrangements produced by the ion exchange. For $${\hbox {Fe}}$$ incorporation in zeolite, two options were considered theoretically: ion exchange and isomorphous substitution of $${\hbox {Al}^{3+}}$$ in tetrahedral positions.

Calculation of the Physicochemical Characteristics of a New Orthorhombic Form of Diamond

A geometrically optimized structure and some properties of a new diamond polymorph have been assessed using density-functional theory calculations in the local-density approximation. The new phase has a primitive orthorhombic lattice (Pmma). The calculation results indicate that the density, sublimation energy, bulk modulus, Vickers hardness, and band gap of this phase are lower than those of cubic diamond by 5–18%. The orthorhombic diamond polymorph can be obtained by compressing graphite in the [001] direction at a pressure of 59.4 GPa. For this phase transition, we calculated theoretical X-ray diffraction patterns of the new phase and intermediate states, which can be used for their identification in experiments.

Structural varieties of polytypes

We present the results of theoretical analysis of structural varieties of compounds for which polytypism is important. As a result, the possibility of isomeric polytype varieties existing has been established, as well as regularities in their formation, depending on the period of repeatability of layers in packaging. In addition, the methods of density functional theory are used to calculate the structure and properties of various polytypic modifications of diamond. Dependences of the interlayer distances and the difference total energy on the hexagonality of diamond polytypes and their isomeric varieties have been found. The theoretical powder X-ray diffraction patterns of diamond polytypes are calculated and their comparative analysis is performed.

New structural model of β-TiO: Superposition of M5X5 superstructures

A new structural model of the high-temperature ordered titanium monoxide phase β-TiO is proposed. This phase has monoclinic symmetry and belongs to the space group P1m1. Its unit cell is a three-fold unit cell of the basis crystal structure B1 of titanium monoxide. The proposed structural model is a superposition of M5X5 (mon) and M5X5 (cub) superstructures, the former of which corresponds to the structure of the low-temperature ordered phase α-TiO and the latter is an additional structure increasing the contribution from the configuration entropy to the free energy with a rise in temperature. In comparison with the familiar simple structural models (M5X5 (mon) and M5X5 (cub) superstructures), the new model describes more accurately the behavior of structural vacancies at elevated temperatures and explains the reason of the misfit between the experimental and theoretical X-ray diffraction patterns of the high-temperature ordered phase.

ТЕОРЕТИЧЕСКОЕ ИССЛЕДОВАНИЕ ФАЗОВОГО ПРЕВРАЩЕНИЯ ТЕТРАГОНАЛЬНОГО ГРАФЕНА L4-8 В ПОЛИМОРФНУЮ РАЗНОВИДНОСТЬ АЛМАЗА LA7

In this paper, the study of phase transition of tetragonal L 4-8 graphene into base-centered orthorhombic LA7 diamond polymorph is carried out using the density functional theory method. Analysis of the possible formation methods of LA7 phase showed that its structure can be obtained as a result of strong uniaxial compression of tetragonal graphite with packing of AB at a pressure of 42,5 GPa. The pressure at which LA7 phase can be synthesized is the lowest in comparison with the pressures at which other diamond polymorphs can be obtained. The calculations also showed that the process of this structural transformation should be accompanied by energy release of 0,52 eV/atom. The polymorphic modification of the diamond can stably exist under normal conditions, since the potential barrier separating states corresponding to graphite L 4-8 and LA7 phase is 0,34 eV/atom. The theoretical X-ray diffraction patterns of the "graphite L 4-8 – LA7" phase transition were calculated for the experimental identification of LA7 phase. The calculated powder X-ray diffraction pattern of the orthorhombic polymorph of diamond differs greatly from the X-ray diffraction patterns of graphite or cubic and hexagonal diamond polytypes.

Surface morphology and thermal figure of merit of a new compound thin film

The third nonlinear optical properties of a new compound 4,4′-bis(3-methoxy benzylidene amino) biphenyl doped poly-methyl methacrylate (PMMA) have been studied using Z-scan technique. Experiments are performed using a continuous waveguide (cw) diode laser at 532 nm wavelength and 0.68 kW/cm2 laser intensity. The optical power limiting behavior of sample doped PMMA was also investigated. It also shows a very good optical limiting behavior with a limiting threshold of 4.7 mW. We attribute the nonlinear absorption and optical limiting property of the sample film to two photon absorption effect at 532 nm. The experimental evidences of observing diffraction pattern in compound 4,4′-bis(3-methoxybenzylideneamino) biphenyl doped PMMA has been present. The refractive index change, Δn, and nonlinear refractive index, n 2 determined from the number of observed ring. We obtained good values of Δn = 105.154 × 10−4and n 2 = 154.154 × 10−7 cm2/W. Variation of refractive index with temperature, dn/dT, and figure of merit, H, are found to be 8.858 × 10−6 1/°C and 5.316 × 10−6, respectively. This large nonlinearity is attributed to a thermal effect resulting from linear absorption. Theoretical diffraction pattern that agree well with experimental one are generated using a wave theory.

Decomposition of the γ phase in as-cast and quenched U–Zr alloys

An investigation of the decomposition of the high temperature γ phase in as-cast and quenched U–Zr alloys was conducted. Differential scanning calorimetry data clearly showed δ⇌γ transformations in alloys with <10 wt% Zr while XRD data did not contain any peaks which uniquely identify it's presence. Since δ phase forms via ω transformation, a comparison of the theoretical diffraction patterns for ω and δ revealed that the intensities of the peaks which uniquely identify the existence of δ when α-U is present, were either very weak, or were zero in ω, suggesting that the ambiguity can be explained if the phase present in these alloys is ω as opposed to δ. Our data are consistent with the presence of δ and ω in as-cast and quenched U–50Zr alloy, respectively, and (α+ω) in rest of the as-cast and quenched alloys. Based on the experimental data, the transformation sequence from γ phase in U–Zr alloys is proposed.

Структура и свойства алмазоподобной фазы, получаемой из тетрагональных графеновых слоев

Using the density functional theory method in the generalized gradient approximation in this article, crystal structure and properties of the carbon diamond-like LA10 phase, in which all the atomic positions are crystallographically equivalent, were calculated. The diamond-like LA10 phase structure can be theoretical derived by polymerization of tetragonal L4-8 graphene layers, consisting of fourmembered and eightmembered structural units. The LA10 crystalline phase lattice belongs to a tetragonal crystal system with I41/amd space group symmetry. The body-centered tetragonal unit cell has the following parameters: a = 3.581 Å, c = 8.611 Å, and Z = 16 atoms. Band structure and electron density of states calculations showed that the diamond-like LA10 phase is a wide-gap semiconductor with a band gap of 5.0 eV to 6.1 eV. Furthermore, the LA10 phase should have high strength characteristics: the Knoop hardness is equal to 72.3 GPa, the bulk modulus is 351 GPa. It is also established that the LA10 phase should be stable under normal conditions, as the sublimation energy of this phase is only 6.9% less than the corresponding energy of cubic diamond. The most probable synthetic way to obtain the LA10 phase is a strong static compression of graphite, consisting of tetragonal polymorphic variety graphene, perpendicular to the graphene layers. This diamond-like phase can be experimentally identified by a theoretical powder X-ray diffraction pattern, calculated in this work. The calculated X-ray pattern is quite different from the X-ray patterns cubic diamond and lonsdaleite, but is very close to the X-ray pattern hexagonal graphite.

Straightforward understanding of the structures of metastable α″ and possible ordered phases in uranium–niobium alloys from crystallographic simulation

Crystallographic simulation is carried out in the present work to understand the respective structure of metastable α″ and possible ordered phases, which could be the prerequisite for revealing the low-temperature aging mechanism underlying the α″ phase in U–Nb alloys. The space group of No. 11 with C-type lattice is found to describe the structure of α″ phase in a straightforward way, avoiding the mistaken or indirect indexation and analysis of structural data. The obtained theoretical diffraction pattern of α″ phase shows clearly the phenomenon of peak splitting, in which the Bragg positions also agree well with the experimental observation. Additional peaks are observed at the low diffraction angles for the possible ordered phases in U–Nb alloys through simulation on the proposed two types of structures, which need to be carefully detected by the uniting of multiprobe diffraction. The correlation between ordered structures and stress-strain response is also deduced and discussed. The present work would provide some enlighten to the integrity of the phase diagram and the understanding of low-temperature aging mechanism in U–Nb alloys.

Short-range order in disordered transition metal oxides, carbides, and nitrides with the B1 structure

A method has been proposed for the simulation of the atomic structure of disordered phases of nonstoichiometric transition metal oxides, carbides, and nitrides with the B1 structure taking into account the shortrange order in the arrangement of structural vacancies. The simplest structural models constructed with allowance for pair correlations between vacancies within the first four coordination spheres have been considered. A computer simulation of the atomic structure of disordered phases with the inclusion of the shortrange order has been performed. For each stru ctural model, the maximum possible concentration of vacancies and the shortrange order parameters at diffe rent concentrations have been calculated. For a fur� ther experimental verification, the influence of the shortrange order on the shape of the Xray scattering spectrum has been investigated by calculating the theoretical Xray diffraction patterns according to the Debye formula. The influence of the shortrange order on the total energy of the disordered phases has been analyzed by ab initio methods. It has been shown that, in some cases, the proposed structural models of the shortrange order are energetically favorable as compar ed to the model of statistical arrangement of vacan�

Magnetism, electronic structure and half-metallic property of transition metal (V, Cr, Mn, Fe, Co) substituted Zn3P2 dilute magnetic semiconductors: An ab-initio study

Abstract The ab-initio electronic structure calculations are used to investigate the half-metallic property, structural stability, spin dependent density of states and magnetism of transition metal (V, Cr, Mn, Fe, Co) substituted Zn 3 P 2 . Our results show the half-metallic ground state and ferromagnetic (antiferromagnetic) stability for small concentrations (2.5% and 5%) of transition metal atom substitution considered in our study. The structural and magnetic stability of the substituted compounds have been investigated through the calculation of the heat of formation and analyzing the minimum total energies corresponding to the ferromagnetic, antiferromagnetic and nonmagnetic phases. The substituted transition metal atom is found to give rise to magnetism of these compounds and half-metallic property is observed in many cases. We have used the spin, orbital and atom projected density of states to investigate this point and studied the nature of bonding in these systems using the charge density contours. We have observed from our calculations, magnetically stable phases and large values of the magnetic moments in these compounds and it suggests that these compounds may be useful materials for semiconductor spintronic devices. We have further calculated the theoretical X-ray diffraction pattern of these compounds.

Fine-tuning the theoretically predicted structure of MIL-47(V) with the aid of powder X-ray diffraction

The structural characterization of complex crystalline materials can be simplified by closely comparing theoretical and experimental diffraction patterns.

Effect of microfibril twisting on theoretical powder diffraction patterns of cellulose Iβ

Previous studies of calculated diffraction patterns for cellulose crystallites suggest that distortions that arise once models have been subjected to MD simulation are the result of both microfibril twisting and changes in unit cell dimensions induced by the empirical force field; to date, it has not been possible to separate the individual contributions of these effects. To provide a better understanding of how twisting manifests in diffraction data, the present study demonstrates a method for generating twisted and linear cellulose structures that can be compared without the bias of dimensional changes, allowing assessment of the impact of twisting alone. Analysis of unit cell dimensions, microfibril volume, hydrogen bond patterns, glycosidic torsion angles, and hydroxymethyl group orientations confirmed that the twisted and linear structures collected with this method were internally consistent, and theoretical powder diffraction patterns for the two were shown to be effectively indistinguishable. These results indicate that differences between calculated patterns for the crystal coordinates and twisted structures from MD simulation can result entirely from changes in unit cell dimensions, and not from microfibril twisting alone. Although powder diffraction patterns for models in the 81-chain size regime were shown to be unaffected by twisting, suggesting that a modest degree of twist is not inconsistent with experimental data, it may be that other diffraction techniques are capable of detecting this structural difference. Until such time as definitive experimental evidence comes to light, the results of this study suggest that both twisted and linear microfibrils may represent an appropriate model for cellulose Iβ.

A Combined Theoretical and Experimental Study of Solid Octyl and Decylammonium Chlorides and of Their Aqueous Solutions

The analysis of the thermal behavior of octyl- (OAC) and decylammonium chloride (DAC) has provided relevant new information. OAC shows a polymorphic phase transition that starts at 308 K and is completed at 313 K, between a monoclinic and a tetragonal structure, which belongs to a space group different from what was observed for other alkylammonium chlorides. At the melting temperature (463 K), the growing of a strong first sharp diffraction peak (FSDP) occurring at a d-spacing of ca. 21.0 Å was observed. DAC is characterized by two phase transitions at 318 (T(d1)) and 323 K (T(d2)). At T(d1), the LT polymorph coexists with a new-formed phase. At T(d2) the LT polymorph is replaced by a further polymorphic modification of DAC that becomes the only one at 328 K. This second phase transition signals the occurrence of a phase, closely related to the LT polymorph of HeAC, that crystallizes in the tetragonal crystal system a = ca. 5.00 Å and c = ca. 28.5 Å, P4/nmm space group. DAC melting starts at 458 K and is accompanied, as in the case of OAC, by the growing of a strong FSDP at a d-spacing of ca. 24.8 Å. A parallel DSC study confirmed the above-mentioned transitions. In addition, the structural properties of OAC/water and DAC/water mixtures were studied using an integrated approach, which combines X-ray diffraction and molecular dynamics (MD) techniques. A very good agreement between theoretical and experimental diffraction patterns has been obtained for both investigated mixtures. A thorough analysis of the MD trajectories shows that strong anion-water interactions are present in the mixtures, where Cl(-) forms a rather unstructured first hydration shell of water molecules bound to the anion in a linear Cl···H-O configuration. Moreover, cations and anions were found to interact with each other, and to form "solvent-shared ion pairs", in which one or more water molecules are shared between Cl(-) and the alkylammonium cation.