An Ab Initio Perspective on the Key Vacancy Defects of KMgF₃

Abstract

Perovskite-type KMgF3, which represents a distinct class of multifunctional materials, has drawn immense interest in the advanced technological fields due to its various attractive properties. However, these properties are strongly influenced by the presence of various vacancy point defects in the host crystal structure. This drives us to gain a detailed knowledge of the defect chemistry in KMgF3 by investigating geometry, electronic structure, defect formation energies, and charge transition levels. To achieve reliable results, we have adopted hybrid density functional calculations. To find out the stable growth condition for all possible vacancy defects and vacancy cluster defects in KMgF3, calculations of defect formation energies under different chemical potential range have been carried out. Among all the vacancy defects considered in this study, the calculated formation energy for VMg in a −2 charge state is found to be a more negative value as the Fermi energy level approached toward the conduction band minimum irrespective of the chemical potential range. The present study explored the role of different vacancy defects in creating impurity states in the band gap region by detailed analysis of electronic structure in each case. It has been observed that the impact of anion vacancy and their aggregation on the electronic structure of KMgF3 is much more prominent than that of cation vacancy. The electrical and optical properties of KMgF3 in the presence of various defects have been described on the basis of calculated thermodynamic and optical charge transition levels, respectively. It has been revealed that the cation vacancy defects are not likely to form deep defect states, while anion vacancy and their aggregation act as deep donor defects. The mixed cluster vacancy of K and F pair behaves differently with respect to Mg and F pair in the optical property. The cation vacancies have hardly any contribution to the observed optical spectrum, which is consistent with experimental observation. Thus, an unambiguous and ultimate clarification of the fundamental absorption and emission processes in KMgF3 has been accomplished in the present study.

Revealing Impacts of Anion Defect Species on Fluoride-Ion Conduction of Ruddlesden-Popper Oxyfluoride Ba₂ScO₃F

“…For example, unlike in perovskite oxides, such as LaGaO3, in which oxide-ion vacancy 5 can be easily introduced, 18,19 in perovskite fluorides, it is indicated by DFT calculation 6 that fluoride-ion vacancy is difficult to be introduced because of its high formation 7 energy. 20 This indication was experimentally confirmed in some typical perovskites an 8 perovskite-related fluorides by our preliminary investigations (detailed in SI). To 9 overcome the difficulty of defect control in simple fluorides, the use of mixed-anion 10 compounds as a host material is a promising approach for controlling defect species and 11 concentration.…”

Probing site-selective doping and charge compensating defects in KMgF3: insights from a hybrid DFT study

“…The formation energy of a point defect system is an important quantity to analyse the thermodynamic stability of all defect systems under various synthetic conditions. The defect formation energy (Δ H f ) of defect D X can be expressed as 66,67 where and stand for the energy of KMgF 3 in the presence of defects (D X ) and in the absence of defects with an identical supercell size (3 × 3 × 3) using the HSE hybrid density functional. μ x is the chemical potential of element X and n X is the number of atoms of element X removed ( n x > 0) or added ( n x < 0) during the formation of the defect system, E F is the Fermi level, ε VBM is the energy of the VBM, and q indicates the defect charge state.…”

Enhancement of Photoactivity of TiO2 for Potential Applications in CO2 Conversion and Water Splitting: A Hybrid Density Functional Theory Study

“…Brillouin zone sampling has been performed by a Γ-centered k -point mesh value of 6 × 6 × 6 for the supercell using the Monkhorst and Pack scheme . To overcome the limitation of standard DFT during band gap calculations, we employed the hybrid functional of Heyd, Scuseria, and Ernzerhof (HSE), which is shown to reproduce the experimental band gap for a wide range of materials. − According to this functional, the ion-core interaction is separated into two parts: short range (SR) and long range (LR). The interaction in the SR is described by both exact Hartree–Fock (HF) exchange and PBE exchange, while the LR is solely described by PBE.…”