A new non-centrosymmetric Gd-based borate crystal Rb7SrGd2(B5O10)3: growth, structure, and nonlinear optical and magnetic properties

A bifunctional primitive strategy induces enhancements of large second harmonic generation and wide UV transmittance in rare-earth borates containing [B5O10] groups

Strong second-harmonic generation (SHG) and a short ultraviolet (UV) cutoff edge are two crucial yet often conflicting parameters that must be finely tuned in the exploration of nonlinear optical (NLO) materials. In this study, two new rare earth borate NLO crystals, K7BaSc2B15O30 (KBSBO) and Rb21Sr3.8Sc5.2B45O90 (RSSBO), were rationally designed through a bifunctional primitive strategy to achieve an optimized balance between favorable SHG efficiency and UV transparency. As anticipated, both KBSBO and RSSBO exhibit a wide UV transparency window below 190 nm. Notably, these tailored crystals display strong SHG responses, with RSSBO achieving a remarkable enhancement in SHG efficiency (2 × KDP), surpassing that of most deep-UV rare earth borates containing [B5O10] groups known to date. Theoretical calculations and structural analyses reveal that the impressive SHG activities primarily stem from the [B5O10] groups and [ScO6] polyhedra. These findings suggest promising potential for KBSBO and RSSBO crystals as beryllium-free deep UV NLO materials.

Three new aluminoborates: from 1D tube to 3D framework

Three new aluminoborates (ABOs) KCs[Al{B5O9(OH)}{BO(OH)2} (1), K0.5Cs[Al{B5O10}1/2{BO2(OH)}] (2), and Cs1.5[Al{B5O10}1/2{BO2(OH)}] (3) have been made under solvothermal conditions. 1 features a 1D tube constructed by the alternation of [B5O9(OH)]4- clusters and AlO4 units, onto which the [BO(OH)2]- triangles are grafted. To further construct higher dimensional structures based on the structure 1, solvents were adjusted for high basicity, resulting in the formations of the 3D ABO frameworks 2 and 3. 2 and 3 are isostructural and built from [B5O10]5- clusters, AlO4 tetrahedra and [BO2(OH)]2- triangles, in which [B5O10]5- clusters are bridged by AlO4 tetrahedra to produce a 2D ABO layer with 14-membered ring (MR) windows, and the [BO2(OH)]2- triangles act as the linkers to bridge the adjacent ABO layers to form 3D ABO frameworks containing eight types of channels. UV-vis diffuse reflectance spectra indicate that 1, 2, and 3 have potential applications in deep ultraviolet (DUV) regions.

Two non-centrosymmetric mixed alkali metal and alkaline earth metal scandium borate nonlinear optical materials with short ultraviolet cutoff edges

Rare earth borates, a subset of the essential nonlinear optical (NLO) materials, have sparked a significant amount of attention in recent years. In self-fluxing systems, two non-centrosymmetric scandium borates with classical B₅O₁₀ groups, namely Rb₇SrSc₂B₁₅O₃₀ (I) and Rb₇CaSc₂B₁₅O₃₀ (II), were successfully discovered. Both I and II exhibit a short ultraviolet (UV) cutoff edge (<200 nm) and appropriate second-harmonic generation efficiency (∼0.76 × KH₂PO₄, ∼0.88 × KH₂PO₄ at 1064 nm, respectively). According to theoretical calculations, it is speculated that the band gap and NLO characteristics of these two compounds are mostly derived from the B₅O₁₀ group and the ScO₆ octahedron. Due to the short cutoff edges of I and II, they may be considered as potential NLO materials in the UV and even deep UV spectral ranges. Furthermore, the advent of I and II adds to the diversity of rare earth borates.

Anion-Centered Polyhedron Strategy for Strengthening Photon Emission Induced by Electron-Phonon Coupling

Electron-phonon coupling emerges as a growing frontier in the heart of condensed matter from physical symmetry to the electronic quantum state, but its quantitative strength dependence on the chemical structure has not been assessed. Here, we originally proposed the anion-centered polyhedron (ACP) strategy for elaborating the electron-phonon coupling interaction in rare-earth (RE) materials comprising three chemical factors, RE-O bond length, the effective charge of the coordinated atom, and structural dimensionality. Using Gd³⁺ cation with 4f⁷ configuration as a fluorescence probe, we found that the "free-O"-centered polyhedron is the most crucial motif in strengthening the phonon-assisted energy transfer and photon emission. The temperature-dependent Huang-Rhys S factors were calculated to identify the electron-phonon coupling intensity based on the fluorescence spectrum quantitatively. Finally, beyond conventional wisdom, a series of structural criteria were presented, serving as useful guidelines for discovering strongly coupled rare-earth optical materials. Our study breaks the long-time "blind"-searching diagram and provides reliable principles for many functional materials associated with electron-phonon coupling, such as superconductors, multiferroics, and phosphors.

Large Magnetocaloric Effect in Li3K9Gd3(BO3)7 Crystal Featuring Sandwich-Like Three-Dimensional Framework

A new Gd-based borate crystal, Li₃K₉Gd₃(BO₃)₇, has been successfully obtained via the high-temperature solution method using Li₂O-K₂O-B₂O₃ self-flux. It crystallizes in monoclinic space group P2/n (no. 10) with lattice parameters a = 11.3454(6) Å, b = 9.9881(4) Å, c = 11.4467(7) Å, α = γ = 90 ^o, β = 114.782(7) ^o, and Z = 2. Li₃K₉Gd₃(BO₃)₇ exhibits an intriguing sandwich-like three-dimensional (3D) framework constructed from [Gd-B-O]_∞ layers, KO_n (n = 6 and 8) polyhedra, and LiO₄ tetrahedra, in which [Gd-B-O]_∞ layers are built from two types of GdO₈ polyhedra and triangular BO₃ units. Magnetic measurements showed that Li₃K₉Gd₃(BO₃)₇ exhibits a large magnetocaloric effect with -ΔS_m = 39.3 J kg^-1 K^-1 at 2.0 K for ΔH = 7 T, which is slightly higher than that of the commercial gadolinium gallium garnet under the same conditions. The powder X-ray diffraction, infrared spectrum, and UV-vis-NIR diffuse reflectance spectrum were also performed to characterize Li₃K₉Gd₃(BO₃)₇. The electronic band structures, partial density of states, and refractive indices of Li₃K₉Gd₃(BO₃)₇ were investigated via the first-principle calculations.

Efficient Bi3+ to Eu3+ energy transfer and color tunable emissions in K7CaY2(B5O10)3-based phosphors

Rare-earth borates are well known good photoluminescent materials due to the easy manipulation of activator concentration. K₇CaY₂(B₅O₁₀)₃ belongs to a recently discovered borate family with outstanding nonlinear optical performances. A systematic study on Bi³⁺ and Eu³⁺ doped phosphors was performed to explore their potential in photoluminescence. K₇Ca(Y_1-xBi_x)₂(B₅O₁₀)₃ (0.01 ≤x≤ 0.06), K₇Ca(Y_1-yEu_y)₂(B₅O₁₀)₃ (0.10 ≤y≤ 1) and K₇Ca(Y_0.99-zBi_0.01Eu_z)₂(B₅O₁₀)₃ (0.05 ≤z≤ 0.90) were prepared by high temperature solid state reactions. Rietveld refinements reveal an 8% anti-site occupancy of Ca²⁺ and Eu³⁺ in K₇CaEu₂(B₅O₁₀)₃, and two sets of Bi³⁺ emission and excitation spectra are also observed once Bi³⁺ is introduced. For instance, the two strongest ¹S₀→³P₁ excitations at 270 nm and 281 nm correspond to two ³P₁→¹S₀ emissions at 383 and 334 nm, respectively. The Eu³⁺ emission shows a maximal intensity at y = 0.50 under charge transfer excitation, while there is no concentration quenching effect under f-f excitation. The Bi³⁺-to-Eu³⁺ energy transfer is firmly supported by the steady photoluminescence spectra and the decreased lifetime of Bi³⁺ upon increasing the Eu³⁺ content in K₇Ca(Y_0.99-zBi_0.01Eu_z)₂(B₅O₁₀)₃. This energy transfer mechanism occurs through the electric dipole-dipole interaction. Under excitation at 281 nm, the emission is tunable from deep blue (Bi³⁺) to pink and finally to red (Eu³⁺) all with high quantum yields (>80%).

Nonlinear-Optical Crystal Rb3YB6O12 with Condensed B5O10 Blocks That Exhibits an Intriguing Structural Arrangement and a Short Ultraviolet Absorption Edge

Nonlinear-optical (NLO) crystals, which can regulate the laser wavelength through a cascading second-harmonic-generation technique, have been widely utilized in the field of optoelectronics. In this work, we grew the NLO borate crystal Rb₃YB₆O₁₂ (RYBO) using the spontaneous crystallization method. RYBO crystallizes in a chiral trigonal space group of R32 with a new type of structural arrangement built from Y-O short chains and B₅O₁₀ groups. It is significantly different from the known structure of chemical analogues Rb₃REB₆O₁₂ (RE = Nd, Eu) not only in the halved unit cell parameter but also in the Y-O connection manner. The NLO response of RYBO is about 0.8KDP, 8-fold larger than that of KB₅O₈·4H₂O with the same B₅O₁₀ groups because of the coexistence of two NLO-active units of the distorted YO₆ octahedra and B₅O₁₀ anions. Thanks to the short ultraviolet (UV) cutoff, RYBO may have potential NLO applications in the UV and even deep-UV spectral regions.