Defects in Solids

Ab initio study of intrinsic point defects in germanium sulfide

Defects in graphene governs electrical and optical properties. Although grain boundaries in graphene inevitably formed during large area synthesis process, which act as scattering centers for charge carriers to degrade mobility, have been studied extensively, point defects have been rarely investigated mainly due to the absence of facile observation tools. Here, we report polarized optical microscopy to observe defect distributions in monolayer graphene. This was realized by aligning liquid crystal s (LC) on graphene where the defect population was modulated by irradiating ultraviolet (UV) light directly on graphene surface under moisture condition. Aromatic rings in LC molecules are oriented with hexagonal rings in graphene to have preferred orientation, providing a way to identify relative orientations of graphene domains and point defects. Our studies show that point defects generated by prolonged UV irradiation time give rise to irregular LC alignment with disclination lines on the graphene surface and a large-size LC domain associated with graphene single domain eventually disappeared. This indicates that defects associated with oxygen-containing functional groups cause to reduce the strong stacking interaction between graphene and LC molecules.

Selected Topics in High Temperature Chemistry - Defect Chemistry of Solids

AlGaN nanowires are promising for deep ultraviolet optoelectronic applications, but their performance is significantly affected by defects, and the mechanism of defect regulation remains unclear. In this work, the effect of vacancy and interstitial defects on the stability, structural and electronic properties of AlGaN nanowires are systematically investigated via first-principles. Our results demonstrate that the N vacancy exhibits the lowest formation energy of 3.68 eV, indicating that it is the most thermodynamically favorable defect type in AlGaN nanowires. Besides, as the defect position migrates from the center to the surface layer of nanowires, the stability of N vacancy gradually decreases (formation energy increases from 3.68 eV to 4.56 eV), while that of Ga vacancy progressively enhances (formation energy decreases from 8.06 eV to 7.88 eV). In addition, the introduction of defects induces significant relaxation in the atomic structure, with bond length variations up to 4.35% ∼ 7.50% for Ga-Ga around N vacancy and 9.92% ∼ 13.95% for N-N around Ga vacancy. Despite the incorporation of point defects, AlGaN nanowires preserve the direct bandgap characteristic. Moreover, Al vacancy, Ga vacancy and N interstitial introduce shallow acceptor levels at 0.12 ∼ 0.37 eV above the valence band maximum, while N vacancy, Ga interstitial and Al interstitial induce deep donor levels at 0.42 ∼ 1.58 eV below the conduction band minimum. Furthermore, Mulliken charge calculations reveal that atomic charge alterations induced by defects mainly occur within the neighboring coordination shell, verifying the localized nature of defect effects.

High‐throughput computational screening and machine learning hold significant potential for exploring diverse chemical compositions and discovering novel inorganic solids. However, the complexity of point defects, which occur in all inorganic solids and are often crucial to their functionality and synthesizability, presents significant challenges. Here, this study presents a defect‐chemistry‐trained, interpretable machine learning approach, designed to accelerate the exploration and discovery of unconventional proton‐conducting inorganic solid electrolytes. By considering dopant dissolution and hydration reactions, the machine learning models provide quantitative predictions and physical interpretations for synthesizable host–dopant combinations with hydration capabilities across various structures. Utilizing these insights, two unconventional proton conductors, Pb‐doped Bi12SiO20 sillenite and eulytite‐type Sr‐doped Bi4Ge3O12, are discovered in the first two synthesis trials. Notably, the Pb‐doped Bi12SiO20 represents an unprecedented class of proton‐conducting electrolyte composed solely of groups 14 and 15 cations and featuring a sillenite structure. It exhibits unique and fast 3D proton conduction along a loosely bonded BiO5 network. This study demonstrates an efficient approach for exploring novel inorganic materials.

A theoretical framework that provides a quantitative relationship between point defect formation energies and growth process parameters is presented. It enables systematic point defect reduction by chemical potential control in metalorganic chemical vapor deposition (MOCVD) of III-nitrides. Experimental corroboration is provided by a case study of C incorporation in GaN. The theoretical model is shown to be successful in providing quantitative predictions of CN defect incorporation in GaN as a function of growth parameters and provides valuable insights into boundary phases and other impurity chemical reactions. The metal supersaturation is found to be the primary factor in determining the chemical potential of III/N and consequently incorporation or formation of point defects which involves exchange of III or N atoms with the reservoir. The framework is general and may be extended to other defect systems in (Al)GaN. The utility of equilibrium formalism typically employed in density functional theory in predicting defect incorporation in non-equilibrium and high temperature MOCVD growth is confirmed. Furthermore, the proposed theoretical framework may be used to determine optimal growth conditions to achieve minimum compensation within any given constraints such as growth rate, crystal quality, and other practical system limitations.

Chapter VI - Thermodynamics of Defects

Chapter 13 - Nonstoichiometric Compounds

Point defect thermodynamics and diffusion in Fe 3C: A first-principles study

Point defects control in non-stoichiometric CuInTe2 compounds and its corresponding effects on the microstructure and thermoelectric properties

Complex Inorganic Solids

Complex Inorganic Solids

General schema for [0 0 1] tilt grain boundaries in dense packing cubic crystals

In ionic crystals, jogs on surfaces and dislocations act as sources and sinks for point defects. Establishment of equilibria with the vacancies, solute ions, and interstitials in the interior produces a net charge on the surface or dislocation, surrounded by a compensating space charge region. Within this space charge region, which typically has a thickness of the order of 10nm, there are very large electric fields (up to about 105 V/cm) and very large perturbations of point defect concentrations (by factors of several hundred). Thus, ion diffusivities can be dramatically perturbed. In addition, determination of the electric potential difference across the space charge region allows one to separate the formation enthalpies and entropies of Frenkel and Schottky defects into contributions from each component of the pair. Examples of such studies in the silver and alkali halides will be described.

We have examined a range of point defects, Frenkel pairs, Schottky defects, and hydrogen-related defects in the ${\mathrm{PuO}}_{2}$ system (supercells of 96 and 768 atoms) using the onetep linear-scaling density functional theory code. Vacancy point defects related to oxygen are found to be more stable than those related to plutonium. The oxygen in the octahedral interstitial is higher in the formation energy than the plutonium in the same octahedral site, although the difference is less than 1 eV. We were also able to identify a stable peroxide species (1.57--2.67 eV) with a O-O distance of 1.46 \AA{}. Of the Frenkel defects we studied, we found that the oxygen is more stable than the plutonium, whereas the Schottky stability changes as a function of supercell size. Finally, we examined a number of likely hydrogen sites in the ${\mathrm{PuO}}_{2}$ lattice: octahedral interstitial, oxygen edge, hydroxyl, oxygen vacancy, and plutonium vacancy. We report hydrogen which exists as a hydride at oxygen and plutonium vacancies to be relatively high in energy (2.69--3.81 and 13.71--15.54 eV, respectively). The hydrogen was found to exist as a radical at the octahedral interstitial site (2.43--3.38 eV) and which is somewhat higher formation energy than other studies find. We find that the hydrogen at the oxygen edge (as a ${\mathrm{H}}^{+}$ cation) and at the oxygen cube corner (as a hydroxyl) are both lower in energy (1.14--1.40 and 1.17--1.56 eV, respectively) as opposed to hydrogen in the octahedral interstitial site but again higher than found by other studies. We discuss the data in the context of potential hydrogen transport pathways and how that might be modified by radiation damage.