Fluoride Borates with [(BO₃)F]^4– ↔ [F₄]^4– Anionic Isomorphism and X-ray Sensitivity

Abstract

Crystals of Ba 7 (BO 3 ) 4-y F 2+3y and Ba 4-x Sr 3+x (BO 3 ) 4-y F 2+3y solid solutions display a phenomenon that has never been described for borates before -X-ray irradiation induces strong absorption in the visible range and, as the result, coloring of the crystals up to dark purple. The induced absorption can be removed by illumination with a wavelength of 300−400 nm. Owing to this phenomenon these crystals are promising sensor applicable to fixing X-ray images. With the use of electron spin resonance and optical spectroscopy it has been found that the formation of color centers is connected with the [(BO 3 )F] 4-and [F 4 ] 4-tetrahedral groups, which are the distinguishing feature of Ba 7 (BO 3 ) 4-y F 2+3y and Ba 4-x Sr 3+x (BO 3 ) 4-y F 2+3y structure. Partial replacement of Ba 2+ to Sr 2+ in the crystal lattice of Ba 4-x Sr 3+x (BO 3 ) 4-y F 2+3y leads to the formation of the stable electron-hole pairs under X-ray irradiation. The opportunity to vary the cationic and anionic sublattice composition within the same crystal structure becomes a prospective tool to vary physical and chemical properties of the phosphors.

Nature of the Color of Borates with “Anti-Zeolite” Structure

“…As the deviation of g x and g y from the g factor of a free electron is caused by the admixture of p x and p y states in the basic p z state because of the spin–orbital interaction, i.e., Δ g x = −2λε 2 / E x and Δ g y = −2λε 2 / E y , , then using the following values of constants of spin–orbital bonding and the coefficient of a degree of covalent bonding λ O – = −150 cm –1 and ε 2 = 0.86, respectively, we obtain a splitting of p levels (distance of overlying p x and p y states from the p z state) for ion O – : E x = 18170 cm –1 (λ = 550 nm), and E y = 19111 cm –1 (λ = 523 nm). The latter are in good agreement with the optical absorption bands at λ = 520 nm and λ = 580 nm and blue-purple color of irradiated fluoride borates. − …”

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

“…Among borates, optically active crystals with a huntite structure, such as EuAl 3 (BO 3 ) 4 , and YbAl 3 (BO 3 ) 4 , with gyrotropic birefringence are known. The observed optical anomaly at 499 nm at 300 K (Figure b) corresponds to the isotropic point at which the dispersion curves of the ordinary and extraordinary X-ray refractive indices intersect. − At this point, the birefringence is equal to zero and the crystal sign changes from uniaxial positive to uniaxial negative. Thus, the Ba 12 (BO 3 ) 6 [BO 3 ]­[LiF 4 ] crystals are optically active gyrotropic and are of practical importance for the production of narrow-band light filters, optical shutters, and devices for measuring the angle of rotation of the polarization plane.…”

Growth and Optical Properties of LixNa1–xBa12(BO3)7F4 Fluoride Borates with “Antizeolite” Structure

“…A study of the optical transmission spectra of NaBa 12 (BO 3 ) 7 F 4 and LiBa 12 (BO 3 ) 7 F 4 crystals and their comparison with the spectra of irradiated fluoride borates Ba 7 (BO 3 ) 4– y F 2+3 y , Ba 4– x Sr 3+ x (BO 3 ) 4– y F 2+3 y , has revealed their similaritiesthe presence of broad absorption bands in the visible region (Figure ). The crystals of Ba 7 (BO 3 ) 3.51 F 3.47 ( y = 0.49) are transparent from 215 nm to 3.5 μm, and there are some windows of transparency at 5–6.5, 12–12.5, and 13–16 μm.…”

“…The most intensive induced bands are 518 nm (2.39 eV) and 587 nm (2.11 eV); two weak broad bands with maximum at 409 nm (3.03 eV) and 336 nm (3.69 eV nm) are also present . X-ray irradiation of Ba 4– x Sr 3+ x (BO 3 ) 4– y F 2+3 y crystals induces weak absorption bands at ∼520 and 400 nm and pink-purple coloring of the crystals . The induced absorption can be effectively removed by illumination with a wavelength of 300–400 nm.…”

“…First-principles calculations of Ba 7 (BO 3 ) 4– y F 2+3 y optical absorption spectrum demonstrate that F centers absorption bands occur at 400 nm (3.10 eV) and 353 nm (3.51 eV), which is very close to weak broad bands at 409 nm (3.03 eV) and 336 nm (3.69 eV nm) in the experimental spectrum. Partial replacement of Ba 2+ to Sr 2+ in the crystal lattice of Ba 4– x Sr 3+ x (BO 3 ) 4– y F 2+3 y leads to the formation of the stable electron–hole pairs under X-ray irradiation. , …”