Wenjiao Yao

NaSr3Be3B3O9F4: a promising deep-ultraviolet nonlinear optical material resulting from the cooperative alignment of the [Be3B3O12F](10-) anionic group.

The demand for deep-ultraviolet (deep-UV) coherent light sources (l< 200 nm) has become increasingly urgent because they have important applications in semiconductor photolithography, laser micromachining, modern scientific instruments (super-high-resolution and angle-resolved photoemission spectrometer, for example) and so forth. To date, the most effective method to generate deep-UV coherent light with solid-state lasers is through cascaded frequency conversion, in particular multiharmonics, using deep-UV nonlinear optical (NLO) crystals. Therefore, the discovery of suitable deep-UV NLO crystals is of great importance. In the past decades, the anionic group theory, which reveals that the overall nonlinearity of a crystal is the geometrical superposition of the microscopic second-order susceptibility tensors of the NLO-active anionic groups, has been very successful in developing borate NLO crystals. Several important NLO crystals have been discovered, including b-BaB2O4 (BBO), [4] LiB3O5 (LBO), [5] CsB3O5 (CBO), CsLiB6O10 (CLBO), [7, 8] and YCa4O(BO3)3 (YCOB), which have been widely used in NLO optics. However, they cannot be used to generate deep-UV coherent light (l< 200 nm) by multiharmonic generation owing to some inherent shortcomings. Thus, the search for new NLO materials, particularly for deep-UVapplications, has attracted considerable attention. A deep-UV NLO material must have a very short absorption edge, and in this respect, beryllium borates are attractive as they are supposed to possess very large energy gap. It is also well known that the incorporation of fluorine can effectively cause the UV absorption edge of a crystal to blue-shift, so our group has made great efforts to search for new deep-UV NLO fluorine beryllium borate crystals. After more than ten years of intensive research in our group, the KBe2BO3F2 [16–18] (KBBF) crystal became the first practically usable deep-UV NLO crystal used to generate coherent 177.3 and 193 nm light. The excellent NLO properties of KBBF crystals are mainly determined by the (Be2BO3F2)1 layer made up of trigonal-planar [BO3] units and the tetrahedral [BeO3F] units. This deep-UV coherent light material has been used as a photon source in modern instruments and revealed many novel scientific phenomena which could not be observed by traditional techniques, as shown in the study of superconductor CeRu2 [19] and Bi2Sr2CaCu2O8+d. [20] Unfortunately, the KBBF crystal is very difficult to grow in thickness because of its strong layering tendency, which severely limits the coherent output power. Therefore, there is great demand for new types of fluorine beryllium borates which have deepUV transmission, moderate birefringence, and relatively large second harmonic generation (SHG) coefficients, and at the same time overcome the crystal-growth problems found in the KBBF crystal. Alkali-metal and alkaline-earth-metal cations are favorable for the transmission of UV light because there are no d–d electron or f–f electron transitions in this spectral region. As shown in numerous explorations, the size and charge of cations have great influence on the macroscopic packing of anions, which in turn determines the overall NLO properties in a crystal. 22] Herein, we utilize both alkali-metal and alkaline-earth-metal cations. Different charge/size combinations of mixed cations may have different influences on the packing of anions, so it is more likely to isolate new phases with interesting stoichiometries, structures, and properties. To date, no fluorine beryllium borates with mixed cations have been reported. Guided by this idea, we successfully obtained a new alkali-metal/alkaline-earth-metal fluorine beryllium borate NaSr3Be3B3O9F4, which contains the novel anionic group [Be3B3O12F] 10 as the basic building unit. Furthermore, the arrangement of these [Be3B3O12F] 10 groups is very favorable for generating large a NLO response and moderate birefringence and especially for avoiding the layering tendency during bulk crystal growth. Herein, we report the synthesis, crystal growth, structure, linear and nonlinear optical properties, thermal behavior, and electronic structure of NaSr3Be3B3O9F. These results indicate that the NaSr3Be3B3O9F4 crystal may be a promising NLO material in the deep-UV range. NaSr3Be3B3O9F4 [23] crystallizes in the noncentrosymmetric trigonal space group R3m. The crystal structure is depicted in Figure 1a. In the asymmetric unit, Sr, Na, Be, B each occupy one crystallographically unique position, and there are two unique F and O positions. The B atom is coordinated to three O atoms to form a planar BO3 unit with B O bond lengths [*] H. Huang, J. Yao, Z. Lin, X. Wang, R. He, W. Yao, N. Zhai, Prof. C. Chen Center for Crystal Research and Development Technical Institute of Physics and Chemistry Chinese Academy of Sciences, Beijing 100190 (China) E-mail: cct@mail.ipc.ac.cn

Deep-Ultraviolet Nonlinear Optical Materials: Na2Be4B4O11 and LiNa5Be12B12O33

Deep-UV coherent light generated by nonlinear optical (NLO) materials possesses highly important applications in photonic technologies. Beryllium borates comprising anionic planar layers have been shown to be the most promising deep UV NLO materials. Here, two novel NLO beryllium borates Na2Be4B4O11 and LiNa5Be12B12O33 have been developed through cationic structural engineering. The most closely arranged [Be2BO5]∞ planar layers, connected by the flexible [B2O5] groups, have been found in their structures. This structural regulation strategy successfully resulted in the largest second harmonic generation (SHG) effects in the layered beryllium borates, which is ~1.3 and 1.4 times that of KDP for Na2Be4B4O11 and LiNa5Be12B12O33, respectively. The deep-UV optical transmittance spectra based on single crystals indicated their short-wavelength cut-offs are down to ~170 nm. These results demonstrated that Na2Be4B4O11 and LiNa5Be12B12O33 possess very promising application as deep-UV NLO crystals.

Sr8MgB18O36: a new alkaline-earth borate with a novel zero-dimensional (B18O36)18- anion ring.

A new polyborate, Sr8MgB18O36, has been synthesized. Its crystal structure was determined from single-crystal X-ray diffraction data, and characterizations were made by differential scanning calorimetry, Fourier transform IR, UV-vis-near-IR diffuse-reflectance, and first-principles calculations. The structure of Sr8MgB18O36 contains a novel isolated anionic group-an 18-membered ring (B18O36)(18-), which is the first found in borates.

Bandgaps in the deep ultraviolet borate crystals: Prediction and improvement

We identify the microscopic structural origins determining the bandgaps in the deep-ultraviolet borates, and propose an efficient method for the prediction of their bandgaps. This method considers only the chemical bond lengths around oxygen atoms and achieves the very high precision with the relative error <5% typically. Its validity is verified by the first-principles studies, which reveal the strong dependence of bandgaps on the coordination environment around oxygen atoms. Our studies have great implications on the search and design of optoelectronic functional materials with large bandgap.

Isotropic Negative Area Compressibility over Large Pressure Range in Potassium Beryllium Fluoroborate and its Potential Applications in Deep Ultraviolet Region

Isotropic negative area compressibility, which is very rare, is observed in KBBF and the related mechanism is investigated by combined high-pressure X-ray diffraction (XRD) experiments and first-principles calculations. The strong mechanical anisotropy leads to a large Poissons ratio and high figure of merit for the acoustic-optics effect, giving KBBF potential applications as smart strain converters and deep-ultraviolet (DUV) acoustic-optic devices.

Sr3BeB6O13: A New Borate in the SrO/BeO/B2O3 System with Novel Tri-Six-Membered Ring (BeB6O15)10– Building Block

A new polyborate Sr3BeB6O13 has been synthesized and grown by the traditional solid-state reaction method and spontaneous crystallization flux method. It crystallizes in orthorhombic space group Pnma (No. 62) with the following unit cell dimensions: a = 12.775(3) Å, b = 10.029(2) Å, c = 8.0453(16) Å, and Z = 4. The crystal is characterized by an infinite two-dimensional network with a tri-six-membered ring (BeB5O13)(9-) anionic group, which was first found in beryllium borates. Ultraviolet (UV)-visible-near-infrared diffuse reflectance spectroscopy demonstrates that its UV cutoff edge is below 200 nm, and the first-principles electronic structure calculations reveal its energy band gap of 7.03 eV (∼175 nm). Thermal analysis exposes its incongruent feature at 1043 °C. IR spectroscopy measurements are consistent with the crystallographic study. These data reveal that this crystal would be applied as a deep-ultraviolet optical material.

Weak anti-localization of the two-dimensional electron gas in modulation-doped AlxGa1−xN∕GaN heterostructures with two subbands occupation

Weak anti-localization of the two-dimensional electron gas (2DEG) in a modulation-doped Al0.22Ga0.78N∕GaN single heterostructure has been investigated through magnetoresistance measurements at low temperatures. The elastic scattering time τe, dephasing time τϕ and spin-orbit scattering time τso at various temperatures are obtained. When the second subband in the triangular quantum well at the heterointerface is occupied by the 2DEG, the weak anti-localization is observed clearly, which is thought to be due to the strong spin-orbit effect induced by the intersubband scattering. The spin-orbit effect and the intersubband scattering become stronger with increasing temperature.

Ca3Na4LiBe4B10O24F: A New Beryllium Borate with a Unique Beryl Borate ∞2[Be8B16O40F2] Layer Intrabridged by [B12O24] Groups

A novel beryllium borate, Ca3Na4LiBe4B10O24F, has been discovered. It possesses a unique ∞(2)[Be8B16O40F2] layer composed of two opposite parallel [Be4B4O12F]∞ layers bridged with [B12O24] polyborates. The linkage of [B12O24] to other structural units is first found in anhydrous borates. In the ∞(2)[Be8B16O40F2] layer, multiple tunnels are arranged along different directions resided by the alkali and alkaline-earth cations. The compound remains stable in an ambient atmosphere from room temperature to the melting point at 830 °C and melts incongruently.