Experimental and Ab Initio Studies of Intrinsic Defects in “Antizeolite” Borates with a Ba₁₂(BO₃)₆⁶⁺ Framework and Their Influence on Properties

Abstract

The porous Ba 12 (BO 3 ) 6 6+ framework of the so-called "antizeolite" borates with channels along the c axis is capable of accommodating various guest anionic groups, e.g. and [(Li,Na)F 4 ] 3− . Taking as an example the Ba 12 (BO 3 ) 6 [BO 3 ][LiF 4 ] crystal, we put forward the argument that the optical properties of "antizeolite" borates are strongly influenced by the degree of channel packing with anionic groups and, correspondingly, by the conjugated intrinsic defects. With the use of optical, electron-spin resonance, Raman spectroscopy, and ab initio calculations, it was shown that intrinsic defects largely impact the absorption of light in the visible and UV regions (the color of the bulk crystals can change from colorless to dark brown), absorptionedge position, dichroism, and other optical properties. The change in the optical absorption in the visible range is caused by the appearance of new states in the electronic structure inside the band gap, which are associated mainly with the presence of single and double F centersfluorine vacancies that capture electronsin [□F 4 ] 4− , [F 2 ] 2− , and [LiF 4 ] 3− groups. The formation of F centers in the [F 2 ] 2− group is the most energetically favorable. It has been found that Ba 12 (BO 3 ) 6 [BO 3 ][LiF 4 ] crystals are optically active gyrotropic with an isotropic point at 499 nm at 300 K and are of interest for practical application as narrow-band light filters.

γ-BaB2O4: High-Pressure High-Temperature Polymorph of Barium Borate with Edge-Sharing BO4 Tetrahedra

“…These results indicate that the main optical transitions, according to the Laporte rule, should occur from O 2p orbitals to Ba 5d levels with a change in the orbital angular momentum Δ l = 1. Such transitions are typical of barium borates and were observed, for instance, for BaB 8 O 13 , BaB 4 O 7 , Ba 2 B 10 O 17 , Ba 12 ­(BO 3 ) 6 ­[BO 3 ]­[LiF 4 ], and LiBa 3 ­(OH)­(B 9 O 16 )­[B­(OH) 4 ] …”

Photoluminescence in Cerium-Doped Fluoride Borate Crystals

“…The mixing parameter defining the amount of the exact Hartree–Fock exchange was chosen to be 0.25. This choice described well the bandgap of the fluoride borate crystals. , The on-site Coulomb correlation of f-electrons was taken into account employing Hubbard corrections in the Dudarev parametrization with a U – J value of 5.5 eV for cerium. , Calculations were done using the projector augmented-wave (PAW) formalism, which was developed using the concept of pseudopotentials in the plane-wave basis.…”

“…Compounds LiBa 12 (BO 3 ) 7 F 4 were previously described by Zhao and Li and their structures were reinvestigated in ref . The nature of the variously colored LiBa 12 (BO 3 ) 7 F 4 crystals was discussed in ref . Luminescent properties of LiBa 12 (BO 3 ) 7 F 4 doped with Eu 3+ , Tb 3+ , and Ce 3+ in different combinations were recently studied .…”

“…In the present study, we first carry out DFT calculations on electronic and optical properties of Ce-containing LBBF structures where doping atoms are located at different sites of the host lattice. Previous studies have shown that the presence of defects such as F-centers is critical in describing the optical absorption spectra of antizeolite borates . For this reason, in addition to the effect of doping with Ce ions, we study the contribution of F-centers.…”

Mechanisms of Photoluminescence in Copper-Containing Fluoride Borate Crystals

“…The mixing parameter defining the amount of the exact Hartree–Fock exchange was chosen to be 0.25. This choice describes well the bandgap of the fluoride borate crystals. , The on-site Coulomb correlation of d-electrons was taken into account employing Hubbard corrections in the Dudarev parametrization with U–J value of 6.0 eV for copper . Calculations were done using PAW formalism, which was developed using the concept of pseudopotentials in the plane-wave basis supplied by VASP .…”