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, [1][2][3] 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-BaB 2 O 4 (BBO), [4] LiB 3 O 5 (LBO), [5] CsB 3 O 5 (CBO), [6] CsLiB 6 O 10 (CLBO), [7,8] and YCa 4 O(BO 3 ) 3 (YCOB), [9] 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-UV applications, has attracted considerable attention. [10][11][12][13][14] 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.[15] 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 [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 deep-UV 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. [21,22] 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...

Potassium-ion batteries are a compelling technology for large scale energy storage due to their low-cost and good rate performance. However, the development of potassium-ion batteries remains in its infancy, mainly hindered by the lack of suitable cathode materials. Here we show that a previously known frustrated magnet, KFeC 2 O 4 F, could serve as a stable cathode for potassium ion storage, delivering a discharge capacity of~112 mAh g −1 at 0.2 A g −1 and 94% capacity retention after 2000 cycles. The unprecedented cycling stability is attributed to the rigid framework and the presence of three channels that allow for minimized volume fluctuation when Fe 2+ /Fe 3+ redox reaction occurs. Further, pairing this KFeC 2 O 4 F cathode with a soft carbon anode yields a potassium-ion full cell with an energy density of~235 Wh kg −1 , impressive rate performance and negligible capacity decay within 200 cycles. This work sheds light on the development of low-cost and high-performance K-based energy storage devices.

A novel series of alkali and alkaline earth metal combined fluorine beryllium borates NaCaBe 2 B 2 O 6 F, KCa-Be 2 B 2 O 6 F, and KSrBe 2 B 2 O 6 F were successfully synthesized through molecular engineering design and grown in crystals by spontaneous nucleation technique from self-flux systems. The idea, introduction of relatively small alkali and alkaline earth metal cations and the fluorine anion, successfully resulted in the novel UV NLO crystal NaCaBe 2 B 2 O 6 F, the following substitution of cations directed to two centrosymmetric compounds KCa-Be 2 B 2 O 6 F and KSrBe 2 B 2 O 6 F. In all of their structures, the a−b plane is the infinite lattice layer (Be 3 B 3 O 6 F 3 ) ∞ made up of BO 3 and BeO 3 F anionic groups, and for the first time, it was found that the adjacent layers are connected with fluorine bridge atoms to form (Be 6 B 6 O 12 F 3 ) ∞ double layers, instead of oxygen bridge atoms usually occurred in other oxides. This structural characteristic is greatly beneficial to improve the layering-growth habit and eliminate polymorphism of a crystal. Optical measurements on the nonlinear optical crystal of NaCaBe 2 B 2 O 6 F reveal that this crystal is phase-matchable and its shortwavelength absorption edge is down to deep UV (below 190 nm). Theoretical calculations on electronic structure were carried out to explain the experimental results. Our preliminary results indicate that NaCaBe 2 B 2 O 6 F has promising applications in the UV spectrum region.