First-Principles Study on Polymorphs of AgVO₃: Assessing to Structural Stabilities and Pressure-Induced Transitions

In this paper, we present a comprehensive theoretical study, based on density-functional theory calculations, and which focuses on the structural and electronic properties of silver vanadium oxide (AgVO₃) in the monoclinic [<i>Cm</i> (β-AgVO₃), <i>C</i>2<i>/c</i> (α-AgVO₃), and <i>Cc</i>], orthorhombic (<i>Amm</i>2), and cubic (<i>Pm</i>3̅<i>m</i>) phases from 0–30 GPa. The structural and electronic properties, the stability of different phases, and the pressure-induced solid–solid phase transitions of AgVO₃ have been previously studied. The effects of pressure on the band structures, energy–gap values, density of states, and vibrational frequencies are also studied. Numerical and analytical calculations are conducted to obtain the lattice parameters, the bulk modulus <i>K</i> and their pressure derivative <i>K</i>′, and the energy-volume equations of state. The influence of different parametrizations of the exchange-correlation functional (B3LYP, HSE06, and PBE) on the investigated properties is analyzed, and the results are compared to available experimental data. For the first time, a complex and unexpected structural and chemical behavior as a function of pressure is reported. The β-phase is the most stable and the first phase transition between the monoclinic β-AgVO₃ and <i>Cc</i> phase takes place at 5 GPa (B3LYP), 3 GPa (HSE06), and 2 GPa (PBE). There are pressure-induced transitions among the β-, α-monoclinic, and cubic structures, and the corresponding values for the pressure transitions are dependent on the functional used. Two new polymorphs, monoclinic <i>Cc</i> and orthorhombic (<i>Amm</i>2), have been characterized for the first time, and their contrasting structural stabilities as well as their transition mechanisms can be understood from the intrinsic characteristics of the crystal lattices. The Badger’s rule is fulfilled for <i>Cm</i>, <i>Amm</i>2, and <i>Pm</i>3̅<i>m</i> polymorphs, while it is invalid for the <i>C</i>2/<i>c</i> and <i>Cc</i> phases. Theoretical results show that the studied reactive channels from β-AgVO₃ toward binary oxides, Ag₂O and V₂O₅, AgO and VO₂; the elements Ag, V and O₂; silver pyrovanadate, Ag₄V₂O₇ and V₂O₅, as well as Ag₂V₄O₁₁ and Ag₂O are not thermodynamic favorable processes at pressures up to 30 GPa. These results contribute to the understanding of the pressure behavior of AgVO₃-based compounds. In addition, it would be interesting to determine whether further measurements and calculations would confirm the predicted structural and thermodynamic properties as well as the solid-state transformations of AgVO₃ polymorphs, which have not yet been experimentally shown

Structural and Electronic Properties of Lithiated SnO₂. A Periodic DFT Study

The structural and electronic properties of the intercalation compound Li_<i>x</i>SnO₂ (<i>x</i> = 1/16, 1/8, 1/4, 1/2, 1) as well as the inherent diffusion mechanism of Li ion into the rutile SnO₂ were investigated by means of periodic density functional calculations. Optimized structural parameters, cohesive energies, electronic band structure, and density-of-states and Mulliken charges for the Li_<i>x</i>SnO₂ system at different Li ordering for each Li content are reported. The energetic profiles for the Li diffusion process into rutile SnO₂ are also presented. Our calculation indicates substantial host distortion around intercalation sites, predominantly along the <i>ab</i>-planes. These deformations are found to be related to the soft B_1g, E_u, A_2g, and A_1g vibrational modes of very low frequency and therefore easy to be achieved. The corresponding variation in volume monotonically increases with the Li concentration. Cohesive energies are consistent with continuous and reversible intercalation process. In lithiated SnO₂, lithium is significantly ionized; however, the distribution pattern of the charge transferred from the lithium to the host is very dependent upon the ion concentration. By increasing the Li content, the relative amount of charge transferred to the Sn atoms decreases whereas the charge transferred to oxygen atoms increases. Lithium intercalation causes a chemical reduction of SnO₂ and yields metallic properties. Effects induced by Li intercalation on the electronic band structures of SnO₂ were assessed according to their origins, i.e., if they originate from lattice expansion or from chemical reduction. The energy difference between the valence-band maximum and conduction-band minimum of lithiated SnO₂ decreases with increasing Li content. Lithium diffusion along the <i>c</i>-direction demands significantly lower activation energy than the energy required for diffusion along <i>ab</i>-planes. Energetic barriers related to the lithium diffusion into SnO₂ were found to be dependent upon the Li content

Formation of Ag Nanoparticles on β‑Ag₂WO₄ through Electron Beam Irradiation: A Synergetic Computational and Experimental Study

In the present work, a combined theoretical and experimental study was performed on the structure, optical properties, and growth of Ag nanoparticles in metastable β-Ag₂WO₄ microcrystals. This material was synthesized using the precipitation method without the presence of surfactants. The structural behavior was analyzed using X-ray diffraction and Raman and infrared spectroscopy. Field-emission scanning electron microscopy revealed the presence of irregular spherical-like Ag nanoparticles on the β-Ag₂WO₄ microcrystals, which were induced by electron beam irradiation under high vacuum conditions. A detailed analysis of the optimized β-Ag₂WO₄ geometry and theoretical results enabled interpretation of both the Raman and infrared spectra and provided deeper insight into rationalizing the observed morphology. In addition, first-principles calculations, within the quantum theory of atoms in molecules framework, provided an in-depth understanding of the nucleation and early evolution of Ag nanoparticles. The Ag nucleation and formation is the result of structural and electronic changes of the [AgO₆] and [AgO₅] clusters as a constituent building block of β-Ag₂WO₄, which is consistent with Ag metallic formation

Surfactant-Mediated Morphology and Photocatalytic Activity of α‑Ag₂WO₄ Material

In the present work, the morphology (hexagonal rod-like vs cuboid-like) of an α-Ag₂WO₄ solid-state material is manipulated by a simple controlled-precipitation method, with and without the presence of the anionic surfactant sodium dodecyl sulfate (SDS), respectively, over short reaction times. Characterization techniques, such as X-ray diffraction analysis, Rietveld refinement analysis, Fourier-transform (FT) infrared spectroscopy, FT Raman spectroscopy, UV–vis spectroscopy, transmission electron microscopy (TEM), high-resolution TEM, selected area electron diffraction, energy-dispersive X-ray spectroscopy, field emission-scanning electron microscopy (FE-SEM), and photoluminescence emission, are employed to disclose the structural and electronic properties of the α-Ag₂WO₄ material. First-principles calculations were performed to (i) obtain the relative stability of the six low-index surfaces of α-Ag₂WO₄; (ii) rationalize the crystal morphologies observed in FE-SEM images (using the Wulff construction); and (iii) determine the energy profiles associated with the transformation process between both morphologies induced by the presence of SDS. Finally, we demonstrate a relationship between morphology and photocatalytic activity, evaluated by photodegradation of Rhodamine B dye under UV light, based on the different numbers of unsaturated superficial Ag and W cations (local coordination, i.e., clusters) of each surface

Theoretical and Experimental Insight on Ag₂CrO₄ Microcrystals: Synthesis, Characterization, and Photoluminescence Properties

Ag₂CrO₄ microcrystals were synthesized by means of the coprecipitation method without the use of a surfactant under three different conditions. On the basis of the theoretical and experimental results, we describe the relationship among the structural order/disorder effects, morphology, and photoluminescence of the Ag₂CrO₄ microcrystals. The experimental results were correlated with the theoretical findings for a deeper understanding of the relationship between the electronic structure, morphology, and photoluminescence properties. First-principles computational studies were used to calculate the geometries of bulk Ag₂CrO₄ and its low-index (001), (011), (110), (010), (111), and (100) facets based on a slab model. A good agreement between the experimental and the theoretical morphologies was found by varying the ratio of the superficial energy values

Toward an Understanding of the Growth of Ag Filaments on α‑Ag₂WO₄ and Their Photoluminescent Properties: A Combined Experimental and Theoretical Study

A combined experimental and theoretical study was conducted on the structure and electronic properties of α-Ag₂WO₄ to clarify the nucleation and growth processes of Ag filaments on α-Ag₂WO₄ crystals induced by electron beam irradiation under electron microscopy. X-ray diffraction with Rietveld analysis, micro-Raman and Fourier-transform infrared spectroscopy were used to analyze the structural order/disorder of α-Ag₂WO₄ crystals. These complementary techniques indicated that the microwave-assisted hydrothermal method employed in the synthesis of α-Ag₂WO₄ crystals leads to the freezing of distorted [WO₆] and [AgO_<i>y</i>] (<i>y</i> = 2, 4, 6 and 7) clusters as the constituent polyhedra of α-Ag₂WO₄. On the basis of the theoretical and experimental results, we provide a complete assignment of the structure of α-Ag₂WO₄ and describe the relationship among the disorder, nucleation growth, rate of Ag formation, and photoluminescence behavior before and after the irradiation of the accelerated electron beam. Density functional theory (DFT) studies indicated significant changes in the order–disorder of the initial α-Ag₂WO₄electronic structure, with a decrease in the band gap value from 3.55 to 2.72 eV. The first stages of the electron irradiation on α-Ag₂WO₄ crystal were investigated by DFT calculations, and we have derived a mechanism to describe the formation and growth of Ag filaments during the electronic excitation of the [AgO₂] cluster

Mechanism of Antibacterial Activity via Morphology Change of α‑AgVO₃: Theoretical and Experimental Insights

The electronic configuration, morphology, optical features, and antibacterial activity of metastable α-AgVO₃ crystals have been discussed by a conciliation and association of the results acquired by experimental procedures and first-principles calculations. The α-AgVO₃ powders were synthesized using a coprecipitation method at 10, 20, and 30 °C. By using a Wulff construction for all relevant low-index surfaces [(100), (010), (001), (110), (011), (101), and (111)], the fine-tuning of the desired morphologies can be achieved by controlling the values of the surface energies, thereby lending a microscopic understanding to the experimental results. The as-synthesized α-AgVO₃ crystals display a high antibacterial activity against methicillin-resistant Staphylococcus aureus. The results obtained from the experimental and theoretical techniques allow us to propose a mechanism for understanding the relationship between the morphological changes and antimicrobial performance of α-AgVO₃