Search

We investigate the impact of polarons on Mo around the oxygen vacancy on the electronic structures and optical properties of SrMoO4 crystals. The screened hybrid density function within the Kohn–Sham density functional theory combined with the finite-size correction are employed in our calculations. Results indicate that the trapped electrons in the form of polarons are all localized on the same Mo atom which is nearest to oxygen vacancy V O × or V O ′ , not on the oxygen vacancy. Optical spectra of polarons around the oxygen vacancy are also obtained. The calculated absorption peaks (4.38 eV) and emission peaks (2.85 eV) are in line with the experimental results, thus providing insight into the origin of the blue luminescence spectra of the SrMoO4. [ABSTRACT FROM AUTHOR]

ZnWO4 is easy to color, which will reduce the luminous efficiency of the crystal and limit the application of the crystal. In order to study the origin of the color in the crystal, in this paper, the effects of the oxygen vacancy on the optical properties for the ZnWO4 crystal have been studied based on the density functional theory (DFT). The hybrid functional method (HSE) and the finite-size correction scheme (FNV) are used to correct the band edge problem and eliminate the artificial interaction of the charged defects, respectively. On the basis of the corrected defect formation energy, we obtain the optical spectra of the F and F + centers containing electron-phonon coupling. The calculated absorption and luminescence peaks are at 2.54 eV and 0.79 eV for the F center and at 2.98 eV and 1.09 eV for the F + center, respectively. The calculated absorption band of the F center is close to the experimental value of 2.48 eV (500 nm), so we speculate that the coloring of the ZnWO4 crystal is related to the F center. Meanwhile, the existence of oxygen vacancy makes ZnWO4 crystal to have self-absorption and to increase decay time, which greatly affects the scintillation properties of the crystal. [ABSTRACT FROM AUTHOR]

Cadmium tungstate crystals have attracted much attention because of its excellent scintillation performance. The existence of oxygen vacancies will reduce the luminous efficiency of the crystal, extend its luminous decay time and reduce light resolution of the crystal. However, the luminescence mechanism of the oxygen vacancies is not clear yet. Here, the optical properties of cadmium tungstate crystal with oxygen vacancies have been studied based on the Density Functional Theory (DFT). We calculated the defect formation energies of oxygen vacancies with three different charges (0, +1, +2), and corrected the band gap to make it more accurate. We obtained the absorption and emission peaks of the F center and F+ center for two different types of oxygen vacancies. The calculated absorption peaks of the F center of V OI and F+ center of V OII at 3.04 eV and 3.59 eV respectively, which are close to the experimental results (3.1 eV (400 nm) and 3.5 eV (350 nm)). So we speculate that these two obvious absorption peaks are caused by oxygen vacancies. [ABSTRACT FROM AUTHOR]

In this work, oxygen vacancies in MgAl2O4 crystal were investigated using the first-principles. The Heyd-Scuseria-Ernzerhof (HSE) was adopted to reproduce the experimental band gap. The finite-size correction scheme was applied to eliminate the unreal interaction between charged defects and their periodic images. Finally, taking into account of the electron-phonon coupling, we obtained the optical spectra of the F and F+ centres. The absorption peaks for the F and F+ centres are at 5.04 eV and 4.67 eV, respectively. Furthermore, we calculate the emission peak for the F centre is 3.28 eV. Comparing with the experimental results, the emission peak at 2.15 eV is related to the F+ centre. [ABSTRACT FROM AUTHOR]

In this paper, we calculated the defect formation energy of oxygen vacancies with different charge states (0, +1, +2) in beryllium oxide crystal by using density functional theory (DFT). Based on defect formation energy, the positions of charge transition levels are obtained. However, there is a well-known problem that DFT will underestimate the band gap, which leads to the positions of charge transition levels are arguable. To obtain more accurate charge transition levels, we employ the hybrid functionals (HSE) to relieve the band edge problem, as well as use the finite-size corrections (FNV) to correct the defect formation energy. After obtaining the location of the charge transition level, we obtain a reliable description of the optical line shape of the F/F + center containing electron–phonon coupling. The absorption spectra of the F center and F + center peak at 7.1 eV and 6.3 eV, respectively. The luminescence band of the F center peaks at 4.7 eV. Furthermore, we speculate that the luminescence band near 3.7 eV is assigned to the F + center. [ABSTRACT FROM AUTHOR]

In this paper, we present the optical spectra of the ZrO2 crystal containing oxygen vacancy based on the Density Functional Theory (DFT). The finite-size correction scheme (FNV) is employed to eliminate the artificial interactions and correct the defect formation energy of oxygen vacancies with three different charges (0, +1, +2). Besides, we use hybrid density functionals to relieve the band edge problem. Finally, we obtain the optical spectra for the F center and F + center containing the electron–phonon coupling. The absorption peak of F center of threefold coordinate oxygen vacancy (V O 3 ) near 446 nm (2.78 eV) agrees well with the experimental value (2.83 eV), which can enhance the visible light photocatalytic ability of ZrO2. The luminescence peak of the F + center of fourfold coordinate oxygen vacancy (V O 4 ) is 561 nm (2.21 eV), which is close to the experimental value (2.5 eV). [ABSTRACT FROM AUTHOR]

Erbium oxide ( E r 2 O 3) crystal is expected to be a new generation of persistent luminescence devices. The intrinsic point defects have a great influence on the properties of crystals. So, we investigated the formation energy of intrinsic point defect in E r 2 O 3 crystal by density functional theory (DFT) and thermodynamic calculation. The effect of vibration entropy variation with temperature is considered. The results show that the influence of vibrational entropy on the formation energy should not be ignored under high-temperature. The defect formation energy under varied temperatures and oxygen partial pressure has been obtained. The results indicate that the most stable defect change from oxygen vacancies to erbium vacancies when the Fermi level increases from valence band maximum (VBM) to conduction band minimum (CBM). Erbium vacancies are predicted as shallow level acceptors. Fermi pinning effect causes the crystal to have a p-type conductive behavior. Our calculated result can provide the optimization condition for adjusting the main defect types to satisfy the needs of application. • The influence of vibrational entropy is considered for the defect formation energy. • The point defect V O 4 · · and V E r ‴ are relative stable in most situations. • The complex defects lead to the Fermi-level pinning stabilize at about 0.7 eV above the maximum of value band. • Erbium vacancy is a shallow acceptor, it contributes to the p-type conductive behavior. [ABSTRACT FROM AUTHOR]

• Alkauskas' method is employed to relieve the band edge problem. • The defect formation energy is aligned with the finite-size correction (FNV). • The point defect Vo2+ is relative stable in most situations. • The electron-phonon coupling during the optical transition process is calculated. • F center and F+ center play an indispensable role in luminescence and absorption. To obtain more accurate optical properties, we optimize the oxygen vacancy in gamma-LiAlO 2 with three different charges (0, +1, +2) to study the optical spectrum line shapes of the oxygen vacancy, and the defect formation energies are corrected by finite-size correction scheme (FNV). We obtain the optical spectra containing electron-phonon coupling. The absorption spectra of the F center and F+ center peak at 4.6 eV and 5.5 eV, which are close to the experimental result. So we predicate that the absorption band peak at 5.21 eV should be the origin of the F+ center. The luminescence peaks of the F center and F+ center are at 2.98 eV and 2.56 eV, respectively. Comparing with the experimental results, the emission peak (3.02 eV) is caused by the F center. [ABSTRACT FROM AUTHOR]

In this study, in order to obtain more accurate optical spectrum line shapes, the defect formation energies of oxygen vacancies in YPO 4 crystals with three different charges (0, +1, +2) were calculated using the finite-size correction scheme. The optical properties of the F center and F + center were studied based on electron–phonon coupling. The emission peak calculated for the F + center was close to the experimental value. The luminescence and absorption peaks were in the ultraviolet region for the F and F + centers. We predicted that the absorption peak at 140 nm was related to the F center and that at 152 nm to the F + center. [ABSTRACT FROM AUTHOR]

The defect formation energies of oxygen vacancies in YAlO 3 crystal (YAP) with different charge states (0, +1, +2) are calculated using density functional theory. Advanced hybrid density functionals are employed to relieve the band edge problem (with Alkauskas’s method). The finite-size correction scheme (FNV) is adopted to align the defect formation energy containing artificial interactions between charged defects within the supercell approach. We have analyzed the structures and charge densities of defects with three charge states after the structure optimization. The stability of charged vacancies with respect to neutral one has been discussed. It turns out that Vo 2+ is relative stable in most cases. At last, we give the accurate description of optical line shapes for F/F + centers containing electron-phonon coupling. The results show that F + centers play an important role in color changes in YAP. [ABSTRACT FROM AUTHOR]

The electronic structures of the perfect SrO crystal and SrO containing F color center have been studied within both density functional theory in the generalized gradient approximation (GGA) and the many-body perturbation theory in the G 0 W 0 approximation. The electronic structures reveal that the top of valence band moves down while the bottom of conduction band moves up after G 0 W 0 corrections. The calculated band gap presents a good agreement with experimental result. The F color center could introduce a new electronic state within the band gap. In addition, using many-body perturbation in the G 0 W 0 approximation and the Bethe–Salpeter Equation (BSE), the dielectric functions and the absorption coefficient of both the perfect SrO crystal and the SrO containing F color center have been obtained. The results reveal that the F color center has an optical absorption band peaking at 2.68 eV, which is close to the experimental result (2.5 eV). [ABSTRACT FROM AUTHOR]

A large OSL (optically stimulated luminescence) response from copper replacing lithium (Cu Li) in lithium aluminate (LiAlO 2 , LAO) has been reported in earlier studies, and the origin of the OSL response was isolated Cu Li. However, other trapped-hole centers that are not involved in OSL response, such as Cu Li –V Li and STH (self-trapped hole center, a hole localized at O ion adjacent to V Li), also appear in LiAlO 2 :Cu crystal after irradiation. To reach optimum OSL response of LAO crystal, a comprehensive analysis that integrates first-principles and thermodynamic calculations is employed to investigate the relative stability of intrinsic defects and Cu defects. Defect formation energies (DFE) of these defects are obtained from density functional theory (DFT), as well as corrected by finite-size corrections (FNV). To find the region of Fermi level where the crystal maintains electrical neutrality, we combine DFEs as a function of Fermi level for Cu defects and intrinsic defects to discuss the charge compensation mechanism. Then, the definite DFEs for these defects and the energetically preferred sites for substitutional Cu ions are shown after determining the locations of Fermi levels under different conditions. In O-rich conditions, the results show that Cu Al is most stable with the decrease of the chemical potential of Al. Furthermore, Cu Li is energetically preferred under O-poor conditions, and DFE of isolated Cu Li is much lower than other Cu defects, thus, the optimum OSL signal occurs. To validate our conclusions, we also present the deformation charge density to study the chemical bonds of Li–O, Al–O, Cu Li –O and Cu Al –O. The results also suggest that it would cost more energy to destroy Al–O bond than Li–O bond. • DFEs are corrected by finite-size corrections. • Determine the stable range of LAO by thermodynamic calculations. • In O-poor conditions, Cu Li is most stable, and optimum OSL signal occurs. • In O-rich conditions, Cu Al is energetically preferred with the decrease of μ A l . [ABSTRACT FROM AUTHOR]

The density functional theory with the Hubbard U parameter method was used to evaluate the lattice structure, electronic structures, and optical spectra of MgAl 2 O 4 crystals with cation vacancies. We analyzed the electronic structures of oxygen atoms around the cation vacancies. The holes prefers to distribute themselves on all the nearest oxygen atoms around the vacancy rather than to localize on a single oxygen atom. We propose the color center models V Mg 2 − − O 2 3 − 2 + and V Al 3 − − O 2 3 − 3 + for the magnesium vacancy and the aluminum vacancy with localized hole states. Taking into account electron-phonon coupling, we obtained the optical spectra of the cation vacancies with localized hole states. The calculated absorption peaks of the cation vacancies with localized hole states are consistent with the experimental results. The emission peaks of the magnesium vacancy and the aluminum vacancy with localized hole states are at 2.63 eV and 2.62 eV, close to that of the F center at 2.69 eV. We predict that cation vacancies with localized hole states will act as a source of blue luminescence. • The DFT+U method was used to evaluate the electronic structures and optical spectra of MgAl 2 O 4 crystals with cation vacancies. • The color center models V Mg 2 − − O 2 3 − 2 + and V Al 3 − − O 2 3 − 3 + are proposed for a magnesium vacancy and an aluminum vacancy with localized hole states. • Cation vacancies with localized hole states are predicted to act as a source of blue luminescence. [ABSTRACT FROM AUTHOR]

We use the DFT + U approach to investigate the properties of self-trapped holes (STHs) in silicon (Si)-doped potassium dihydrogen phosphate (KH 2 PO 4) crystal. The calculated electronic structure and spin density indicate that the hole is trapped by an oxygen around the Si atom. The STH is stable under room temperature, as the self-trapped energy is as high as 0.50 eV. The optical properties of the localized and delocalized holes of KH 2 PO 4 -Si crystals are also calculated, and the results show that the emission energy of the STH [3.62 eV (343 nm)] is consistent with the experimental photoluminescence of KH 2 PO 4 (342 nm), and the absorption peaks of the STH [3.58 eV (346 nm) and 5.56 eV (223 nm)] are located in the ultraviolet (UV) region, which indicates a close connection between the STH and the absorption band in the UV region. The absorption peak of the delocalized hole [3.44 eV (360 nm)] is consistent with the experimental absorption band (360 nm). Therefore, Si impurities result in UV absorption and a sharp decline of damage threshold in KH 2 PO 4. • The self-trapped hole is localized on a neighboring oxygen atom around Si. • The self-trapped hole is relatively stable at room temperature and the mobility in KH 2 PO 4 is low. • The absorption and luminescence spectra of the self-trapped hole have been calculated. • Self-trapped hole has an indispensable impact on the decline of laser damage threshold in KH 2 PO 4 -Si. [ABSTRACT FROM AUTHOR]