Formation enthalpy of schottky defects in alkali halide crystals and of anti-Frenkel defects in CaF2-type crystals

Structural Aspects of the Superionic Transition in AX2 Compounds With the Fluorite Structure

Some AX₂ binary compounds with the fluorite structure (space group Fm3¯m) are well-known examples of materials exhibiting transitions to ionic superconducting phases at high temperatures below their melting points. Such superionic states have been described as either highly defective crystals or part-crystal, part-liquid states where the A ions retain their crystalline order whilst the X ions undergo partial melting. However, no detailed description of the structure of these phases exists. We present here the results of our investigation of the structural changes that occur during these transitions and the structural characteristics of the resulting superionic materials. This work is based on atomic-scale molecular dynamics modelling methods as well as computational diffraction techniques. We employed a set of empirical potentials representing several compounds with the fluorite structure to investigate any potential-dependent effect. We show the importance of small-scale structure changes, with some local environments showing a hexagonal symmetry similar to what is seen in the scrutinyite structure that has been documented for example in UO₂.

Defect engineering in perovskite oxide thin films

Perovskite oxide thin films are a category of multifunctional materials that have intriguing electrical, magnetic, and photovoltaic properties that can be harnessed combinatorially in future microelectronic devices. However, the inevitable existence of defects in perovskites, regardless of the materials' processing conditions, plays a significant role in their functional properties, which could be either detrimental or beneficial, depending on the exact chemical nature of these defects. As such, defect engineering is an important research area in perovskite thin films that aims at understanding the chemical nature of the defects, from which the physical properties of materials can be more precisely manipulated. Here, we review the common defects in perovskite oxide thin films, which include point defects, dopants, domains and domain walls. The factors that impact the appearance and existence of defects and the corresponding mechanisms are also discussed. While summarizing our previous work, the state-of-the-art in the field from other groups has also been discussed. Most of the defects exist as defect dipoles that affect the oxidation states of relevant ions and induce anomalous behaviors, such as ferroelectricity in otherwise non-ferroelectric thin films, as well as enhanced electrical conductivity in insulators. Furthermore, the couplings between defect dipoles and other degrees of freedom including epitaxial strains and interfaces also provide new strategies to modulate the functional properties of perovskite thin films. Particularly, the coupling between defects and domain wall motion can be regarded as a universal tool to modulate the electric and magnetic properties of thin films of perovskite oxides. It is our hope that this review could promote defect engineering as a general regulation strategy to embellish the functional properties of perovskite oxide thin films.