Point Defects and Defect-Induced Optical Response in Ternary LiInSe₂ Crystals: First-Principles Insight

Many experiments on LiInSe₂ (LISe), a technologically important nonlinear optical crystal, suggest that nonstoichiometric defects play an important role in changing the crystal color and the crystal’s optical applications in infrared and/or near-visible regions. The exact defect species and structures remain unverified by either experiment or theory however. Thereby, density functional theory within the (semi)­local and hybrid exchange-correlation functional is employed to determine the dominant intrinsic point defects in LISe under various environments. It is found that the isolated point defects In antisite In_Li²⁺ and Li vacancy V_Li^– are dominant in a Li-deficient environment, while the Li interstitial Li_i⁺ turns out to be energetically preferable in a Li-sufficient condition. Interstitial In_i³⁺ is regarded as an intermediate state to form In_Li²⁺ (In_i³⁺ + V_Li^– → In_Li²⁺) if there is a Li deficiency. In all possible charge-compensated defect complexes as well as Frenkel and Schottky defects, In_Li²⁺ + 2V_Li^– is the only possible complex configuration under Li-deficient conditions according to the defect structures and formation energies. In particular, the clustering effect decreases the formation energies of all considered defects with respect to the dilute limit. The investigation of the optical response gives further evidence that the intrinsic point defects are responsible for the crystal color change and optical absorption cutoff shift, and conversely, these phenomena could be helpful for recognizing the dominant defects in LISe crystals

Microscopic Properties of Mg in Li and Nb Sites of LiNbO₃ by First-Principle Hybrid Functional: Formation and Related Optical Properties

As the traditional and basic doping ion, Mg is found to strongly lower the photorefractivity of LiNbO₃ when it reaches the threshold concentration, and it is always used to codope with other functional ions to improve both the photorefractive and nonphotorefractive properties of LiNbO₃. Thereby we investigate the basic characteristic of Mg doping, such as the local distortion produced by Mg substitution and the related electronic structures, which mainly determine a broad range of optical properties of LiNbO₃ by employing density functional theory (DFT) within hybrid exchange-correlation functional. The effect of Mg concentration and the interaction of Mg with intrinsic point defects according to the Li-vacancy model are also examined. It is found that, when Li is deficient, Mg_Li with +1 charge state (Mg_Li⁺) and Mg_Nb with −3 charge state (Mg_Nb^3–) are energetically preferable with the increase of Fermi energy. Overall, Mg_Nb^3– exhibits much more contributions than Mg_Li⁺ to the electronic structure and optical properties of LiNbO₃; however, their interaction with intrinsic point defects Nb_Li⁴⁺ and V_Li^– is limited. According to our calculation results, we expect to codope Mg with photorefractive ions such as Fe, Cu, etc. to reduce the electron–hole combination and change the photorefractive properties of LiNbO₃ by controlling Mg doping concentration

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