China
Africa
Explore chapters and articles related to this topic
Defects and Nonstoichiometry
Published in Elaine A. Moore, Lesley E. Smart, Solid State Chemistry, 2020
Elaine A. Moore, Lesley E. Smart
A Frenkel defect usually occurs only on one sublattice of a crystal and consists of an atom or an ion moving into an interstitial position, thereby creating a vacancy. This is illustrated in Figure 5.1c for an alkali halide–type structure such as NaCl, where one cation is shown as having moved out of the lattice and into an interstitial site. This type of behaviour is seen, for instance, in AgCl, where we observe such a cation Frenkel defect when Ag+ ions move from their octahedral coordination sites into tetrahedral coordination, and this is illustrated in Figure 5.2. The tetrahedral coordination of an interstitial Ag+ ion in AgCl.
Crystal Chemistry and Specific Crystal Structures
Published in David W. Richerson, William E. Lee, Modern Ceramic Engineering, 2018
David W. Richerson, William E. Lee
Figure 5.23 illustrates two simplest point defect structures, the Schottky and Frenkel defect clusters. A Frenkel defect is formed when an atom is displaced from its lattice site onto an interstitial site forming a linked defect pair comprising the vacancy and interstitial. This defect can form on both anion and cation sublattices, although the anion Frenkel defect is often called an anti-Frenkel. Frenkel defects can occur in metals as well as ionic and covalent ceramics, whereas Schottky defects are unique to ionic solids. Schottky defects involve the simultaneous generation of cation and anion vacancies (Figure 5.23a). The vacancies must form in the stoichiometric ratio to preserve charge neutrality so that in MgO, a Schottky pair forms, while in TiO2, a Schottky triplet (one Ti vacancy and two oxygen vacancies) forms, and in Al2O3, a Schottky quintuplet (two aluminum vacancies and three oxygen vacancies) forms. Such defects cannot easily be seen directly even in high-resolution electron microscopes, and the determination of their presence requires indirect techniques such as differences in measured and calculated densities or from electrical conductivity measurements.
Crystal Structure
Published in Alan Owens, Semiconductor Radiation Detectors, 2019
If the missing atom is no longer in the vicinity of the hole or has migrated to the surface, it is known as a Schottky defect. Those that migrate to the surface generally become incorporated into the lattice at the surface. If the atom has moved into an adjacent interstitial site and outside the recombination volume, it is known as a Frenkel defect. A Frenkel defect is in reality a pair of defects – an empty lattice site and an extra interstitially positioned atom. In compound semiconductors, the interstitial ion will always be the positively charged one (i.e., a cation,16 because it is generally smaller than a negatively charged ion and thus fits better into interstitial sites. In other words, its formation enthalpy is lower. Schottky and Frenkel defects tend to form during the growth process and are frozen into the lattice as the crystal crystallizes. They can also be created by energetic particle interactions leading to a phenomenon called displacement damage in radiation physics. The energy of formation is closely related to the crystal binding energy and is typical around 10 eV. Although in both cases the crystal remains neutral (since the total number of positive and negative ions is the same), these defects can affect semiconductor properties as they allow ionic conduction. However in tetrahedrally coordinated materials (e.g., IV, or III-V semiconductors), bonding is predominantly covalent rather than ionic. Consequently, activations energies are high and ionic conduction can be neglected, which may not be the case in ionic materials (e.g., Group I-VII semiconductors).
Variations of chemical bonds in silver halides under high pressures
Published in Phase Transitions, 2019
Shinji Ono, Tomonori Nagai, Ryo Tomizawa
Covalency may also arises from the interactions between the mobile Ag ion and the remaining Ag ions shown in the model cluster. Figure 4 shows the calculated results of the BOP between the mobile Ag ion and the remaining Ag ions. The values of the BOP at the initial position are 0 at each pressure. These facts imply that the covalent interactions do not occur at this position. As the mobile Ag ion migrates, the values of the BOP increase up to the tetrahedral site where the maximum values appear. These results confirm that the covalent interactions between the mobile Ag ion and the remaining Ag ions occur when the mobile Ag ion is migrating. It could be also suggested the stability of the Frenkel defect is strengthened by the covalent interactions between the mobile Ag ion and the remaining Ag ions. The shape of Figure 2 is affected largely by the covalent interactions between the mobile Ag ion and the remaining Ag ions. Figure 4 is also symmetrical.
Electrochemical impedance spectroscopy study of AgI–Ag2O–MoO3 glasses: understanding the diffusion, relaxation, fragility and power law behaviour
Published in Philosophical Magazine, 2021
AgI-based alkali molybdate glasses are one of the rich materials from many scientific and technical aspects. Colloquially, this solid material possesses a structure of super cooled liquid [1], exhibit fast ion conduction which is around 10−5–10−2 Ω−1 cm−1 at room temperature (TR) [2]. For last few decades this material, as an affluent member of the class of fast ion conducting (FIC) glasses, has been under thorough research to scientifically understand its structural basics by applying infrared spectroscopy [3], Raman scattering [4], ionic conductivity measurements [5], high pressure techniques [6], acoustic absorption technique with ultrasonic frequencies and with varying temperature [7], FT-IR, FT-Raman and 95Mo Mas-NMR [8], XANES and EXAFS at Mo K-edge technique [9], neutron diffraction study [10], conductivity relaxation study [11] and technologically exploit it for diverse applications [12,13]. But scientific endeavour is a never ending process because each exploration opens up many new layers of insights. Basically, the structure of this type of glasses involves the high temperature (∼150°C), fast ion (Ag+) conducting α phase of AgI and a surrounding glass matrix, Ag2O–MoO3 in the present case, where this phase is dispersed. The role of this matrix is to hinder the phase transition of AgI at TR [14]. Neutron diffraction study along with reverse Monte Carlo simulation study [10] suggests the unique nature of this glass. Mo atoms are random, i.e. its distribution in the glass matrix is inhomogeneous. Regions with higher Mo density are more orientationally ordered than low Mo density regions. This causes the first sharp diffraction peak (FSDP); this cause of origination of FSDP is much different than other network glasses. On the other hand, the ion migration involves Frenkel defect, oxygen vacancies in the form of non-bridging oxygen (NBO) and hopping [15]. Apparently, these aspects of structure and migration process implicate compelling electrochemical behaviours and properties. Thus to understand the correlation between the structure and migration, diffusion and relaxation, in the light of recent theoretical, experimental and analytical developments is one of the few challenges. In a recent study by Ghosh et al. [16], on the mechanism of cation dynamics in the AgI doped mixed glass forming system, Ag2O–SeO2–MoO3; they observed a mixed glass network effect and also established a correlation between the ion dynamics and the basic structural unit, SeO32– which is parameterised by the content of NBO. Moreover, in one of our earlier studies [17], we observed a phase separation due to the reduction of NBO concentration, over a wide range of composition in the AgI–Ag2O–MoO3 glass system. This phenomenon implicates some significant modifications in the glass structure. In the light of these two studies, we ventured into the charge dynamics mechanism and the effect of structure for AgI–Ag2O–MoO3 glass.