Structural characterization of the yttrium doped camno3 nanopowders and theoretical modelling of the perovskite structure stability

In the first part of dissertation structural, microstructural and phase analysis ofthe nanopowders with the general formula Ca1-xYxMnO3 (0 ≤ x ≤ 1) were performedusing XRPD diffraction technique and Rietveld refinement. The most abundantcrystal phases in these nanopowders are the members of ABO3 solid solutions with theperovskite structure type. Seven nanopowders of nominal composition CaMnO3(Ca100), Ca0.95Y0.05MnO3 (Ca95), Ca0.85Y0.15MnO3 (Ca85), Ca0.75Y0.25MnO3 (Ca75),Ca0.5Y0.5MnO3 (Ca50), Ca0.25Y0.75MnO3 (Ca25) and YMnO3 (Y100) were preparedusing a modified glycine/nitrate process. Yttrium doped CaMnO3 crystallizes in thespace group Pnma, and one of the goals of the research is to investigate the stability ofthe perovskite structure type depending on the dopant concentration. Two phasecomposition of all samples and amount of each phase was revealed using XRPDdiffraction and Rietveld refinement. The most abundant phase in all samples has theperovskite structure type. As a result of doping with Y, the XRPD analysis showedthat all the perovskite phases are deformed, with symmetry reduced from cubic toorthorhombic, and that they crystallize in the Pnma space group. Unit cell parametersanalysis showed that the increase of the unit cell parameters, which is related to thehigher amount of Y in the structure, is a consequence of reduction of Mn4+ to Mn3+.Besides the reduction of the Mn and the effect of doping with Y, presence ofvacancies in the structure also affects the mechanism of this transformation. Thechemical compositions, calculated from the refined occupancy values, are comparedwith nominal compositions. Effect of yttrium on the bond lengths and bond angles,tilting and deformation of octahedra caused by presence of Mn3+ and Jahn-Tellereffect, was analyzed. In order to investigate the coordination of the A and B sites,bond valence analysis was performed. In addition, concentration of yttrium in thedoped perovskite phases was investigated using X-ray photoelectron spectroscopy(XPS).In the second part of dissertation we have performed a crystal structureprediction study of CaMnO3 focusing on structures generated by octahedral tiltingaccording to group-subgroup relations from the ideal perovskite type ( Pm3m), which173is the aristotype of the experimentally known CaMnO3 compound in the Pnma spacegroup. Using software SPuDS we obtained initial structure parameters for most of theperovskite structure candidates. Furthermore, additional structure candidates havebeen obtained using data mining. For each of the structure candidates, a localoptimization on the ab initio level using density functional theory (LDA and hybridB3LYP) and the Hartree-Fock (HF) method was performed, and we find that severalof the modifications may be experimentally accessible. In the high-pressure regime,we have identified a post-perovskite phase in the CaIrO3 type, not previouslyobserved in CaMnO3. Similarly, calculations at negative pressure predicted a phasetransition from the orthorhombic perovskite to an ilmenite-type (FeTiO3) modificationof CaMnO3

Structural properties of full-scope AlN/BN compounds investigated using ab initio calculations

In the last few decades, aluminum nitride (AlN) and boron nitride (BN) have become a point of interest to many researchers and scholars from different disciplines around the world. Due to its attractive properties, AlN has been successfully used in various applications, starting from advanced ceramics materials, additive for grain size control in micro-alloyed steels, through optoelectronics and microelectronics, and finally to semiconductors. On the other hand, BN has broad applications in various fields, such as 2D material, lubricant material, superhard and semiconductor material as well as many others. This study focuses on the mixed AlN/BN compounds, in particular, boron-rich AlN and aluminum-rich BN systems, thus having the entire range of AlN/BN compositions. The special focus was on structural properties investigated using the hybrid B3LYP method. Important structural properties were investigated to offer novel technological and industrial applications of mixed AlN/BN materials.International Conference on Structural Integrity 2023 (ICSI 2023

Theoretical investigations of novel zinc oxide polytypes and in-depth study of their electronic properties

Zinc oxide is one of the most investigated compounds in materials science, both experimentally and theoretically, while in nature it appears only rarely, as the mineral zincite. Yet there are still many open questions: Is it still possible to observe or synthesize new modifications of zinc oxide? And can we improve the properties of a material that has already been investigated in thousands of studies? What is the connection between zincite, zinc sulfide and zinc oxide, and can we finally explain the controversial mineral matraite? In short, Yes: the answer to these questions is polytypism. We identify a multitude of possible stable polytypes for zinc oxide, and we show that by varying the stacking order, we can fine-tune the electronic properties such as the direct primary and secondary band gaps in zinc oxide without adding dopant atoms

DFT study of the Cr2SiN4 under extreme pressure conditions

Recently predicted Cr2SiN4 phases have been further investigated using ab initio methods to explore their behavior under extreme conditions of pressure. Thermodynamic functions for several different modifications have been calculated for the pressure range from 0 to 10 GPa using the GGAPBE functional. Detailed analysis of the mechanical properties under pressure has been performed using the CRYSTAL solid-state quantum-chemical program. The change in volume, energy, and bulk modulus with pressure elevation has been discussed for each of the phases investigated within this study. The highest value of bulk modulus is found in the equilibrium spinel type modification showing the highest capacity of resistance to volume change under pressure. As this material could potentially have a very wide industrial and technological application, these findings could be of great importance as they provide more insight into this novel Cr2SiN4 compound, and especially its behaviour in the extreme environment

Theoretical study of ground state properties of Na+ , Cs+ , Mg2+ AND Ba2+ doped mayenite and its electride forms under extreme conditions

Calculations of band structure and electronic density distribution near Fermi energy have been performed for complex nanoporous oxide mayenite 12CaO·7Al2O3 (C12A7) on the ab initio level. The electronic structure of mayenite doped with selected cations from the 1st and 2nd group of the periodic table of elements (Na+ , Cs+ , Mg2+ , and Ba2+) have also been calculated in order to estimate the effect of cationic doping on structural, electronic, and optical properties of mayenite. Partial and complete substitution of the interstitial oxygen anions (there are two O2- anions per unit cell) with electrons (e- “doping”) is also considered in ab initio calculations to observe differences in electronic structure (such as band gap and Fermi level) during the transition from insulator (regular mayenite crystal with O 2- anions in interstitial places) to electride

Crystal Structure and Properties of Theoretically Predicted AlB12

Aluminum borides have various industrial applications, used in fuels, explosives, abrasives, and as additives to consolidated materials based on boron carbide. The structure of AlB12 is similar to that of boron carbide, including almost regular icosahedrons of boron atoms. The absence of the structure data of some higher aluminum borides and the presence of a large number of reflexes in their diffraction patterns makes the identification of phase compositions very difficult and limits the possibilities of the computer modeling of the AlB12. The crystal structure of AlB12 is usually considered as tetragonal α-AlB12 (space group P43212) and orthorhombic γ-AlB12 (space group P212121) which can be synthesized from high-temperature Al-B melts. In our work, we have performed ab initio optimization of the experimentally observed Yannoni’s AlB12 using GGA-PBE functional and obtained relaxed unit cell parameters and atomic positions. Furthermore, we have predicted three different AlB12 structure candidates obtained as a result of the ICSD data mining. The most favorable structure according to total energy ranking was found in the UB12 structure type, which crystallizes in the cubic space group Fm-3m. Therefore, for the new cubic AlB12, we have calculated mechanical properties on different pressures and made the comparison with available experimental data in the AlB12 system

Extreme pressure conditions of bas based materials: Detailed study of structural changes, band gap engineering, elastic constants and mechanical properties

A Density Functional Theory (DFT) study has been performed in order to investigate behaviour of barium sulfide (BaS) at high pressures, and relationship between computed properties, in great detail. Novel predicted and previously synthesized BaS modifications have been calculated using Local Density Approximations (LDA) and Generalized Gradient Approximation (GGA) functionals. In particular, a detailed investigation of structural changes and its corresponding volume effect up to 100 GPa, with gradual pressure increase, has been performed from the first principles. Band gap engineering of the experimentally observed BaS phases at high pressures has been simulated and structure-property relationship is investigated. For each of the predicted and experimentally observed BaS structures, elastic constants and mechanical properties under compression have been investigated (e.g. ductility/brittleness, hardness, anisotropy). This study offers a new perspective of barium sulphide as a high pressure material with application in ceramics, optical and electrical technologies

Theoretical modifications of scandium oxychloride in extreme conditions as an example of rare earth compounds

Theoretical modifications of ScOCl at extreme thermodynamic conditions have been identified and are studied as an example of rare earth element (RRE) compounds. Global optimizations with empirical potentials and local optimizations on the ab initio level were performed, leading to the identification of four new structure candidates on the energy landscape that are expected to be relevant under extreme conditions. The structural, as well as electronic properties of these polymorphs, have been investigated and compared with the modifications of the structure under standard conditions. Such theoretical explorations are very important since literature data regarding ScOCl is rather scarce, although ScOCl and the rare-earth elements (REEs) in general have a wide range of applications; for example, scandium is used in solid oxide fuel cells and could be used for potential future applications in photocatalysis or electronic devices, while oxyhalides of transition metals and REEs exhibit interesting magnetic and electronic properties

Structural, Electronic and Mechanical Properties of Superhard B4C from First Principles

Boron carbide (B4C) has attracted great attention as a semiconducting material with excellent properties and has found various technological applications. High hardness value makes it a potentially superhard material as well as a low density, high degree of chemical inertness, high melting temperature, thermal stability, abrasion resistance, and excellent neutron absorption, contributed to the use of boron carbide as an abrasive material for extreme conditions, wear resistance components, body armors and as a nuclear absorber or solid-state neutron detector. However, B4C is known for its unusual structure, bonding, and substitutional disordering whose nature is not yet fully understood, and exhibits brittle impact behavior. In this study we investigated the chain-model structure with an arrangement of 12-boron atom icosahedra and linear 3-carbon atom chains, using available experimental data. We employed the DFT method with LDA and GGA- PBE functional, as implemented in the CRYSTAL17 software package. Electronic properties of boron carbide have been investigated by calculating the density of states (DOS) and band structure. Calculated mechanical properties have been investigated: bulk modulus, shear modulus, Young modulus, Poisson’s ratio, hardness, and elastic tensor constants, and compared with available experimental data

Data-driven discovery and DFT modeling of Fe4H on the atomistic level

Since their discovery, iron and hydrogen have been two of the most interesting elements in scientific research, with a variety of known and postulated compounds and applications. Of special interest in materials engineering is the stability of such materials, where hydrogen embrittlement has gained particular importance in recent years. Here, we present the results for the Fe-H system. In the past, most of the work on iron hydrides has been focused on hydrogen-rich compounds since they have a variety of interesting properties at extreme conditions (e.g. superconductivity). However, we present the first atomistic study of an iron-rich Fe4H compound which has been predicted using a combination of data mining and quantum mechanical calculations. Novel structures have been discovered in the Fe4H chemical system for possible experimental synthesis at the atomistic level.International Conference on Structural Integrity 2023 (ICSI 2023