ABSTRACT
Three NaBa12(BO3)7F4 crystals were grown in the BaO-BaF2-B2O3-Na2O system from three different compositions of high-temperature solution. With the use of optical spectroscopy, energy-dispersive X-ray spectroscopy, and the Schering bridge method it was found that the crystals revealed sharp differences in their optical and dielectric properties. The relative permittivity of the crystals in direction perpendicular to the optical axis reached 319(5). The minimum deviation technique was used for refractive index measurements.
ABSTRACT
The present study is aimed at unveiling the luminescence potential of Ba4-xSr3+x(BO3)4-yF2+3y (BSBF) crystals doped with Eu3+, Tb3+, and Ce3+. Owing to the incongruent melting character of the phase, the NaF compound was used as a solvent for BSBF crystal growth. The structure of BSBF: Eu3+ with Eu2O3 concentration of about 0.7(3) wt% was solved in the non-centrosymmetric point group P63mc. The presence of Eu2O3 in BSBF: Eu3+ leads to a shift of the absorption edge from 225 nm to 320 nm. The photoluminescence properties of the BSBF: Ce3+, BSBF: Tb3+, BSBF: Eu3+, and BSBF: Eu3+, Tb3+, Ce3+ crystals have been studied. The unusual feature of europium emission in BSBF is the intensively manifested 5D0â7F0 transition at about 574 nm, which is the strongest for BSBF: Eu3+ at 370 nm excitation and for BSBF: Eu3+, Tb3+, Ce3+ at 300 nm and 370 nm excitations. No evidence of Tb3+âEu3+ energy transfer was found for BSBF: Eu3+, Tb3+, Ce3+. The PL spectra of BSBF: Eu3+ at 77 and 300 K are similar with CIE chromaticity coordinates of (0.617; 0.378) at 300 nm excitation and (0.634; 0.359) at 395 nm excitation and low correlated color temperature which implies application prospects in the field of lightning. Due to the high intensity of 5D0â7F0 Eu3+ transition at 370 nm excitation, the BSBF: Eu3+ emission is yellow-shifted.
ABSTRACT
The luminescent properties of single crystals and polycrystalline samples of LiBa12(BO3)7F4 (LBBF) doped and co-doped with Eu3+, Tb3+, and Ce3+ have been studied in order to disclose their potential for application in white light-emitting diodes. Deciphering of LBBF:Eu3+ crystal structure (P42/mbc) makes it possible to determine the scheme of heterovalent isomorphic substitution 3Ba2+ â 2Eu3+ + â¡, â¡ - vacancy in barium sites. Luminescent properties of LBBF crystals co-doped with Eu3+, Tb3+, Ce3+ and Eu3+, Tb3+ are compared. Both crystals demonstrate the luminescence close to day light with CIE coordinates and correlated color temperature (0.280; 0.305), 9232 K for LBBF:Eu3+,Tb3+,Ce3+ and (0.353; 0.390), 4849 K for LBBF:Eu3+,Tb3+ at 300 K at 370 nm excitation. The study of polycrystalline samples LBBF:Ce3+, LBBF:Tb3+, and LBBF:Eu3+, using X-ray diffraction, shows that the homogeneity regions of solid solutions differ significantly. In solid solutions LBBF:Tb3+ and LBBF:Eu3+, a gradual change in symmetry in the "tetragonal â orthorhombic â tetragonal" series takes place as the concentration of the dopant increases. When excited at a wavelength 395 nm, LBBF:Eu3+ samples with the content of Eu3+ 0.5 wt% and 2 wt% demonstrate intense red light with a high quantum yield of 63 and 60%, respectively, which allows them to be used as red components of composite phosphors for white LEDs.
ABSTRACT
This contribution shares experience of teaching an interdisciplinary university course in crystal growth with examples ranging from geology to biology. This is an attempt to combine teaching the basics of the classical and non-classical theories of crystallization with impressive examples of crystals growing around us and in the human body, as well as demonstration of the common phenomena in the growth of minerals in nature, crystalline materials in industry and the laboratory, and biomimetic and stimulus-responsive crystals. Lectures are supported by laboratory exercises. Students can also perform an individual research project and present an oral contribution at a mini-conference. Examples of the topics considered in the course are given, and an extensive list of references to papers and web resources is provided, which may be useful to those who want to implement anything from the authors' experience.
ABSTRACT
Based on ab initio molecular dynamic simulations, dynamic disorder of [BO3] groups in the Ba3(BO3)2 compound has been established. This is the first example of dynamic disorder in borates. It has been shown that static disorder of BO3 groups in the Ba3(BO3)2 crystals [Bekker et al., J. Am. Ceram. Soc., 2018, 101, 450] can be the result of quenching of dynamically disordered high-temperature modifications.
ABSTRACT
The α- and ß-modifications of barium metaborate are important functional materials used in optoelectronic devices. A new theoretically predicted modification of BaB2O4 has been synthesized under conditions of 3 GPa and 900 °C, using the DIA-type apparatus. The new high-pressure modification, γ-BaB2O4, crystallizes in a centrosymmetrical group of monoclinic syngony (P21/n (#14), a = 4.6392(4) Å, b = 10.2532(14) Å, c = 7.066(1) Å, ß = 91.363(10)°, Z = 4). A distinctive feature of the γ-BaB2O4 structure is the presence of edge-sharing tetrahedra [B2O6] which form infinite double chains ∞[B4O4O8/2] stretching along the a axis. The number of known structural types with the [B2O6] group is limited. Phase γ-BaB2O4 has the shortest distance between boron atoms of shared tetrahedra among all currently known compounds. The [B2O6] group angles are 95.5° and 105.5°. Thermodynamic stability and electronic properties of the γ-BaB2O4 modification were studied. The width of the band gap, calculated using the HSE06 functional, is 7.045 eV which implies transparency in the deep-UV region. Experimental and numerical methods which demonstrate a good match were used to the study the Raman spectra of γ-BaB2O4 and ß-BaB2O4 modifications. In the Raman spectra of γ-BaB2O4, the most intense band at a frequency of 853 cm-1 was found to correspond to the symmetric bending mode of the B-O-B-O ring in edge-sharing tetrahedra.
ABSTRACT
The porous Ba12(BO3)66+ framework of the so-called "antizeolite" borates with channels along the c axis is capable of accommodating various guest anionic groups, e.g. [BO3]3-, [F2]2-, [F4]4-, and [(Li,Na)F4]3-. Taking as an example the Ba12(BO3)6[BO3][LiF4] 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), absorption-edge 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 [â¡F4]4-, [F2]2-, and [LiF4]3- groups. The formation of F centers in the [F2]2- group is the most energetically favorable. It has been found that Ba12(BO3)6[BO3][LiF4] 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.
ABSTRACT
An orthorhombic modification of (Fe,Ni)2P, allabogdanite, found in iron meteorites was considered to be thermodynamically stable at pressures above 8 GPa and temperatures of 1673 K according to the results of recent static high-pressure and high-temperature experiments. A hexagonal polymorphic modification of (Fe,Ni)2P, barringerite, was considered to be stable at ambient conditions. Experimental investigation through the solid-state synthesis supported by ab initio calculations was carried out to clarify the stability fields of (Fe,Ni)2P polymorphs. Both experimental and theoretical studies show that Fe2P-allabogdanite is a low-temperature phase stable at ambient conditions up to a temperature of at least 773 K and, therefore, is not necessarily associated with high pressures. This is consistent with the textural relationships of allabogdanite in iron meteorites.
ABSTRACT
Crystals of the Mn xBa12(BO3)8-2 xF8 x phase were grown from a high-temperature solution. This new fluoride borate is built of positively charged [Ba12(BO3)6]6+ blocks, the so-called "anti-zeolite" pattern. Using X-ray single-crystal diffraction, the bulk atomic arrangement in the centrosymmetric tetragonal unit cell in I4/ mcm could be elucidated. Crystals of the (MnF6)4- group-containing solid solution Mn xBa12(BO3)8-2 xF8 x are dark brown in color in contrast to the differently colored crystals of (LiF4)3- group-containing "anti-zeolite" LiBa12(BO3)7F4 ( P42 bc). According to the electron spin resonance and optical spectroscopic investigation, the absorption spectrum of LiBa12(BO3)7F4 crystals results from the absorption of light by both exciton and free charge carriers and can be tuned by varying the initial composition of the high-temperature solution.
ABSTRACT
Studied LixNa1-xBa12(BO3)7F4 (P42bc) solid solution belongs to the new class of "antizeolite" borates with [Ba12(BO3)6]6+ cation pattern, which contains channels filled by anionic clusters. Optical-quality crystals were grown from the compositions with different sodium-lithium ratio. The results of Rietveld refinement based on powder data demonstrate linear increase of parameter a and unit cell volume with Na/(Na + Li) ratio in cation site. Parameter c is less sensitive to the changes in stoichiometry, which is consistent with channel topology of LixNa1-xBa12(BO3)7F4 structure. Distinctive feature of LixNa1-xBa12(BO3)7F4 crystals is their deep purple color, which is due to both hole-type and electron-type centers. Crystals are characterized by linear dichroism effect.
