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 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
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.
ABSTRACT
Cs2Pb(MoO4)2 crystals were prepared by crystallization from their own melt, and the crystal structure has been studied in detail. At 296 K, the molybdate crystallizes in the low-temperature α-form and has a monoclinic palmierite-related superstructure (space group C2/m, a = 2.13755(13) nm, b = 1.23123(8) nm, c = 1.68024(10) nm, ß = 115.037(2)°, Z = 16) possessing the largest unit cell volume, 4.0066(4) nm3, among lead-containing palmierites. The compound undergoes a distortive phase transition at 635 K and incongruently melts at 943 K. The electronic structure of α-Cs2Pb(MoO4)2 was explored by using X-ray emission spectroscopy (XES) and X-ray photoelectron spectroscopy methods. For α-Cs2Pb(MoO4)2, the photoelectron core-level and valence-band spectra and the XES band representing the energy distribution of Mo 4d and O 2p states were recorded. Our results allow one to conclude that the Mo 4d and O 2p states contribute mainly to the central part and at the top of the valence band, respectively, with also significant contributions throughout the whole valence-band region of the molybdate under consideration.
ABSTRACT
Large single crystals of LiGaGe2Se6 were grown, and their structure and linear optical properties were studied. According to XRD results there is some disorder because of the Li ion fluctuation and their redistribution along two cationic sites. The shape of the fundamental absorption edge versus temperature was analyzed, and direct band gap values were estimated from the Tauc plots. Raman spectra were recorded and compared with results of ab initio calculations. The high quality of LiGaGe2Se6 crystals is confirmed by signals from free and self-trapped excitons. Photoluminescence in the 696 nm broad band and a set of bands in the 950 to 1100 nm range are related to self-trapped excitons and cation antisite defects, respectively. The luminescence intensity increases two orders as the crystal is cooled to 80 K. Four peaks are observed in the thermoluminescence curves with dominant ones at 218 and 410 K. Pyroelectric luminescence in the 100 to 180 K range confirms the noncentrosymmetric structure of this crystal.
ABSTRACT
The Lu2.98Ce0.01Y0.01Al5O12 and Y2.99Ce0.01Al5O12 phosphors were synthesized by solid state reaction at temperature 1623 K and pressure 1.5 × 10(7) Pa in (95% N2 + 5% H2) atmosphere. Under the conditions, the compounds crystallize in the form of isolated euhedral partly faceted microcrystals â¼19 µm in size. The crystal structures of the Lu2.98Ce0.01Y0.01Al5O12 and Y2.99Ce0.01Al5O12 garnets have been obtained by Rietveld analysis. The photoluminescence (PL) and X-ray excited luminescence (XL) spectra obtained at room temperature indicate broad asymmetric bands with maxima near 519 and 540 nm for Y2.99Ce0.01Al5O12 and Lu2.98Ce0.01Y0.01Al5O12, respectively. The light source was fabricated using the powder Lu2.98Ce0.01Y0.01Al5O12 phosphor and commercial blue-emitting n-UV LED chips (λ(ex) = 450 nm). It is found that the CIE chromaticity coordinates are (x = 0.388, y = 0.563) with the warm white light emission correlated color temperature (CCT) of 6400 K and good luminous efficiency of 110 lm/W.
ABSTRACT
SrMgF4 has an extremely large bandgap Eg of 12.50 eV as obtained from reflection dispersion. The symmetry of this crystal is monoclinic P21 at room temperature and transforms to the orthorhombic Cmc21 phase near 478 K as the temperature increases. The acentric character of the low-temperature (LT) phase is confirmed by pyroelectric luminescence at T < 440 K. The fundamental absorption edge of the LT phase is located at 122 nm (10.15 eV). A considerable difference between the absorption edge and bandgap Eg is due to the strong exciton absorption. The first-principles electronic structure, refractive indices, nonlinear susceptibility and polarizability were calculated for both LT and high-temperature (HT) phases. Band-to-band transitions are direct for the LT phase but indirect for HT. In spite of relatively low birefringence (â¼0.017) and nonlinear susceptibility (â¼0.044 pm V(-1), an order lower than that in KDP), SrMgF4 crystals are considered promising for nonlinear optics thanks to their transparency far in the vacuum ultraviolet spectral region.
