London Dispersion Effects on the Structure and Properties of Nonlinear Optical BiB3O6 Crystal.

α-BiB3O6 (BiBO) is an important nonlinear optical (NLO) material with high efficiency for applications in harmonic generations and quantum technology. Owing to its low symmetry and cooperative Bi3+ lone pair arrangement, it has also exceptional large piezoelectric and electro-optic coefficients and strong anisotropies on other material characteristics. Previous theoretical calculations on its physical (mainly optical) properties often gave confusing results. It is found here that London dispersion (LD) tends to stabilize structures with closer pack entities like lone pair heavy ion Bi3+ with large polarizabilities, which is ignored in most previous density functional theory (DFT) calculations. Present study shows that without considering the LD effect, the structure of α-BiB3O6 (BiBO) was predicted with an over-estimated (by over 10 %) unique b-axis while underestimates a and overestimates c in a less amount. Consequently it is not possible to use the calculated structure to obtain meaningful properties of this important material. By applying a modified post-DFT LD correction based on linear combination of atomic orbitals (LCAO) and B3LYP functional, the experimental structure is well reproduced with the theoretical optimized one. Many important material property tensors of BiBO crystal are calculated in unprecedented precisions, including: dielectric constants (static and in THz range), elastic and elasto-optic constants, piezoelectric constants, refractive indices, NLO and electro-optic (EO) coefficients. Among them, theoretical calculation of the refractive indices in the THz range by diagonalizing the clamped-ion dielectric constants was firstly achieved at least for BiBO crystal. The calculation also confirms that BiBO has an exceptional large piezoelectric constant d22=40 pC/N and largest free EO coefficients γ 12 T ${{\gamma }_{12}^{T}}$ , γ 22 T ${{\gamma }_{22}^{T}}$ , γ 32 T ${{\gamma }_{32}^{T}}$ on the order of 10 pm/V among borate crystals. The calculation also reveals that the large free EO coefficients are largely originated from the piezoelectric induced photo-elastic effect and for practical high speed applications only the clamped-ion EO coefficients take effect. The clamped ion EO coefficient of γ 53 S ${{\gamma }_{53}^{S}}$ =-4.17 pm/V, γ b 1 S ${{\gamma }_{{\rm b}1}^{S}}$ =-2.61 pm/V are obtained for the first time and may be consulted if one seeks to design BiBO crystal as a high-speed EO modulator. Furthermore, full tensor matrix of the elasto-optic constants was obtained on the first time. Together with the calculated elastic constants, it can help to design acoustic optic modulating devices with preferable figure of merits 10 times that of traditional quartz crystal.

Na12(NbO)3(PO4)7: A Congruent Melting Nonlinear Optical Crystal with Large NbO6 Distortion and High Laser-Induced Damage Threshold.

A new noncentrosymmetric crystal Na12(NbO)3(PO4)7 was successfully synthesized in the niobium phosphate system. Its structure characterizes isolated and highly distorted NbO6 octahedra joining with isolated PO4 groups to form a unit of a (Nb6P6O12) hexagonal star by sharing O atoms. In (Nb6P6O12) hexagonal stars, all Nb and P atoms are in a hexagonal star-like arrangement and all Na atoms are also in a hexagonal star-like arrangement, except for Na(3) and Na(10) atoms. Notably, it exhibits a strong phase-matched second harmonic response: 3 × KDP, which is rare in known niobium phosphate systems. Meanwhile, it also has a wide optically transparent window (0.29-4.44 µm) and a high laser-induced damage threshold (3.5 GW/cm2). More importantly, Na12(NbO)3(PO4)7 is a congruent melting compound that has the potential to be grown into large-sized single crystals by the Czochralski method. These excellent properties make it a promising candidate as a mid-infrared nonlinear optical crystal.

Gd3 TeBO9 : A Rare-Earth Borate with Significant Magnetocaloric Effect.

Magnetic refrigeration technology based on Gd-based paramagnets is expected to be applied to refrigeration in extremely low temperatures, thereby reducing the consumption of liquid helium. Here, we obtained a compound, Gd3 TeBO9 with high Gd3+ concentration through element substitution. The Gd3+ concentration in this compound is as high as 2.4×1024  ions/kg, which is 33 % higher than the commercial Gd3 Ga5 O12 (GGG), and further magnetic tests show that Gd3 TeBO9 has a large magnetic entropy change (57.44 J/(kg K) and 408 mJ/(cm3 K) at 2.6 K and 7 T), which is 1.5 times that of GGG, implying the possibility of its further development as an potential magnetocaloric material.

Na11Ta8P7O43: (Ta8O33) bi-capped triangular prisms connected by PO4 groups resulting in a phase-matched second harmonic response.

Much effort has been devoted to synthesizing nonlinear optical crystals with efficient second harmonic responses. Herein, a tantalum phosphate crystal Na11Ta8P7O43 with a moderate second harmonic response was obtained using self-fluxes of NaH2PO4 and Na2HPO4. Na11Ta8P7O43 belongs to the Cc space group of monoclinic systems with cell parameters of a = 8.5710(2) Å, b = 15.1144(4) Å, c = 28.7712(7) Å, ß = 92.304(2)°, Z = 4. Its structural features were unique (Ta8O33) bi-capped triangular prisms uniformly oriented with a PO4 connection in the ab plane, which were further joined through additional PO4 groups in the c direction, resulting in moderate nonlinear effects. In addition, this crystal has the characteristics of phase-matchable second harmonic generation, a shorter cutoff edge than the known materials LiTaO3 and KTiOPO4, and high transmittance from the visible to the near infrared band, and may serve as a potential nonlinear optical crystal.

Large Difference in Nonlinear Optical Activity of Rare Earth Ion Substitution of Bi3+ in A3TeBO9 (A = Bi, La, Pr, Nd, Sm-Dy).

A series of oxides with high rare earth contents, RE3TeBO9 (RE = La, Pr, Nd, Sm-Dy), were synthesized, and they all crystallize in the noncentrosymmetric space group P63 resembling that of a previously reported Bi3TeBO9 (J. Am. Chem. Soc. 2016, 138, 14190-14193), which showed a very strong second harmonic generation (SHG) signal believed to come from all three structural units [BO3], [TeO6], and [BiO6]. Surprisingly, compared with the isostructural compound Bi3TeBO9, both theoretical calculations and powder SHG tests show that RE3TeBO9 has an abnormally small SHG effect (0.2 × KDP). Based on the group theory analysis, the possible reasons are attributed to the misalignments and lack of distortions of the [BO3], [TeO6], and [REO8] groups in the unit cell. The 21 symmetric operation (a subset operation of the 63-screw axis) of the [TeO6], [BO3], and [REO8] groups leads to complete cancellation of their contributions to the xy directions, while the absence of lone pair electrons in RE3+ and Te6+ results in small distortion in the c axis, hence small contributions to the SHG coefficients dijk involving the z axis. Diffuse reflectance spectra of RE3TeBO9 (RE = La, Pr, Nd, Sm-Dy) show typical absorption of different RE3+, and the similarity of their infrared spectra is due to the same crystal structure and same structural units of the [TeO6] and [BO3] groups. The strong fluorescence intensities of Eu3TeBO9 and Tb3TeBO9 show the characteristic emissions of Eu3+ and Tb3+, which may serve as luminescence materials with proper doping.

Synthesis, Crystal Structure, and Characterizations of Two Tantalum Phosphates A3TaP2O9 (A=K, Na).

Two new compounds K3TaP2O9 and Na3TaP2O9 were obtained by spontaneous crystallization with KH2PO4 and K2HPO4 or NaH2PO4 and Na2HPO4 as fluxes, respectively. K3TaP2O9 belongs to the P21/m space group of the monoclinic system, with cell parameters of a = 9.8912(7) Å, b = 7.1861(5) Å, c = 13.6457(9) Å, ß = 94.047(2)°, and Z = 4. In contrast, Na3TaP2O9 pertains to the P212121 space group of the orthorhombic system, with cell parameters of a = 7.1241(7) Å, b = 9.3071(10) Å, c = 12.3752(13) Å, and Z = 4. Both of the structures feature the existence of the zigzag (TaP2O9)∞ chains with the -(O4Ta)-O-(TaO4)- connection. Also, two compounds have high transparency of about 90% in the 400-2500 nm range. Theoretical calculations and powder second-harmonic generation test show that the compound Na3TaP2O9 has a large band gap of 5.5 eV, a moderate powder second-harmonic generation response of 0.6 × KDP, and a large birefringence of 0.11.

Theoretical studies on nonlinear optical properties of BaGa4Q7 (Q = S, Se) crystals.

The ever-growing power of theoretical calculations has made it possible to accurately predict physical properties of condensed matter systems. In this Letter, using a linear combination of atomic orbitals (LCAO)-type quantum chemistry calculation package and the coupled perturbed Kohn-Sham (CPKS) method, it is shown that the bandgaps, static refractive indices, and full matrix elements of the nonlinear optical coefficients of recently found chalcogenides, which are potential mid-IR nonlinear optical crystals, can be obtained simultaneously with unprecedented accuracy. The results can serve as the foundation for future applications of those materials with optimum conditions.

Proton-Rich POM-Type K12Mo8O20(HPO4)8(PO4)Cl with Ion-Exchange Capabilities.

A proton-rich POM-type molybdenum phosphate K12Mo8O20(HPO4)8(PO4)Cl was successfully obtained. It crystallizes in a noncentrosymmetric tetragonal space group of P-4 (No. 81) with the unit cell parameters of a = 9.6580(4) Å, c = 14.2607(10) Å, and Z = 1. The occurrence and positions of the light element H in the structure are inferred from single-crystal X-ray diffraction and confirmed by DFT calculations. The hydrogen atoms are found to form hydroxyl bonds with O atoms from P(2)O4 and P(3)O4 constituting the [Mo4P4O26H4]4- layers but are only weakly bound to the isolated P(3)O4 group through hydrogen bonds. The title compound presents a POM-type framework of corrugated [Mo4P4O26H4]4- layers with four K+ ions and mixed ions (K4Cl3+ and isolated PO43-) orderly imbedding in the interlayer spaces with distances of 5.0396 (1) and 5.5966 (3) Å, respectively. The proton-rich nature and the structure feature were further verified by a series of experiments including 1H, 7Li, and 31P MAS NMR spectra, IR spectroscopy, and thermal analysis. Moreover, the weak bonding and large interlayer spaces make K+ and H+ ions susceptible to exchange with ions of Cs+, Ba2+, Zn2+, Pb2+, Cu2+, and Ni2+ commonly presented in chemical pollutants or nuclear wastes. In addition, the title compound shows a small second-harmonic generation signal, consistent with its noncentrosymmetric structure.

Introducing a New d0 Sc3+ Asymmetric Ion for Functional Materials: Large Birefringence Enhancement by ScO6 in Ba3 Sc2 (BO3 )4.

Transition metal ions with d0 electronic states (Ti4+ , Zr4+ , Nb5+ and Ta5+ ) are widely investigated as functional materials. This work first illustrates that Sc3+ ion, long-time ignored, displays a second-order Jahn-Teller (SOJT) effect similar to asymmetric oxide-coordinated transition metal ions, thus providing a new ground to seek for asymmetric functional materials with enhanced performances. In Ba3 Sc2 (BO3 )4 , BO3 groups are parallelly arranged, satisfying the ideal arrangement to produce large birefringence. Importantly, distorted octahedral ScO6 with Sc3+ ion in its d0 electronic state enlarges birefringence unexpectedly up to 0.149 @ 550 nm, which is larger than previously reported borates containing solely BO3 , even to B3 O6 units. Subsequently, the SOJT influence of distorted ScO6 octahedra on birefringence is verified by a comparison between experimental data and theoretical calculations. In addition, Ba3 Sc2 (BO3 )4 also displays a high transmittance in the range of 230 nm-3.5 µm with a UV cut-off wavelength at 198 nm and a large laser induced damage threshold (2.7 GW/cm2 ), comparable to α-BaB2 O4 . Above characteristics imply that the title compound may be a promising birefringent material.

Synthesis, Structure Determination, and Characterizations of a Polar Salt-Inclusion Scandium Germanate, Rb10Li3Sc4Ge12O36F.

A noncentrosymmetric salt-inclusion germanate, Rb10Li3Sc4Ge12O36F, was grown through spontaneous crystallization from a LiF-RbF flux. It crystallizes in the polar space group P31c with cell parameters of a = 10.7587(3) Å, c = 21.6691(10) Å, and Z = 2. Its structure features a complex 3D framework composed of helical [Ge4O12] chains from condensed [GeO4] tetrahedra running along the c axis, which are interconnected by the [ScO6] octahedra. Voids of the 3D net are filled with Rb+ ions, Li+ ions, and isolated trigonal-bipyramidal [Rb3Li2F] superalkali clusters. The title compound has a large band gap of 5.6 eV, a moderate powder second-harmonic-generation response of 0.9KDP, and an extremely small birefringence of 0.001, as was further unraveled by theoretical calculations.

Lone-pair stabilized channels and blocked transport in apatite-related structures.

Two apatite-related silicates compounds, Tb2Pb3Si3O12 (I) and Pb5SiP2O12 (II), were obtained in the crystalline forms. Structure and ELF calculated results reveal that the lone pair cation of Pb2+ preferentially occupies the Cs site, and the repulsion between the mutual lone pair stabilizes channels. Moreover, combining conductivity results this preference has the important implications of exclusion and blocking the movement of the oxide ion in the channel of apatite-related structures. Our findings provide an experimental foundation for further interpreting the conductivity decrease in lone-pair ions as dopants into apatite-related silicate materials.

A Titanium and Tantalum Phosphate LiNaTiTa2 P2 O13 with An Open Framework hosting Li and Na Ions.

Herein, we report a detailed structural, electric, thermal and optical analysis of a titanium and tantalum phosphate LiNaTiTa2 P2 O13 . The title compound is comprised of typical ReO3 -type layers constituted by corner-sharing TiO6 and TaO6 octahedra, bridged by PO4 tetrahedra, and the structure is closely related to monophosphate niobium bronzes. The existence of pentagonal tunnels, hosting the Li+ and Na+ ions, endows LiNaTiTa2 P2 O13 a moderate ion transportation behavior (4.67×10-4  S/cm at 823 K). In addition, the successful substitution of Nb for Ta in LiNaTiTa2 P2 O13 results in the optical absorption red-shift towards visible range with a narrowing band gap, which may provide a route of isomorphic replacement to band engineering for photo-catalysis.

BiMnPO5 with ferromagnetic Mn2+-(µ-O)2-Mn2+ units: a model for magnetic exchange in edge-linked Mn2+O6 octahedra.

Magnetic interactions within Mn-(µ-O)2-Mn pairs are crucial to the function of some essential enzymes and catalysts, but their nature is unclear. Neutron diffraction reveals that similar units in BiMnPO5 show ferromagnetic coupling which has been rationalized by density functional theory modelling and calculations of magnetic exchange energies. The results are important to many solid state and biological systems.

Tunable deep ultraviolet laser based near ambient pressure photoemission electron microscope for surface imaging in the millibar regime.

A newly developed instrument comprising a near ambient pressure (NAP) photoemission electron microscope (PEEM) and a tunable deep ultraviolet (DUV) laser source is described. This NAP-PEEM instrument enables dynamic imaging of solid surfaces in gases at pressures up to 1 mbar. A diode laser (976 nm) can illuminate a sample from the backside for in situ heating in gases up to 1200 K in minutes. The DUV laser with a tunable wavelength between 175 nm and 210 nm is perpendicularly incident onto the sample surface for PEEM imaging of a wide spectrum of solids with different surface work functions. Using this setup, we have first demonstrated spatiotemporal oscillation patterns of CO oxidation reaction on Pt(110) from high vacuum to NAPs and gas-induced restructuring of metal nanostructures in millibar gases. The new facility promises important applications in heterogeneous catalysis, electrochemical devices, and other surface processes under nearly working conditions.

Molten Salt Synthesis of an Open-Frame Aluminum Phosphate Ba3Al2P4O16 with a Rare-Pyramidal AlO5 Group.

A new open-frame aluminophosphate, Ba3Al2P4O16, was synthesized by a high-temperature flux method. Ba3Al2P4O16 features discrete [Al4P8O32]∞12- chains with rare square-pyramidal AlO5 groups adopted. Especially, this pyramidal AlO5 group acts as an ideal "plane builder" or "vacancy inducer" to construct open-frame structures compared with an ordinary trigonal-bipyramidal AlO5 group.

KNiB4 O6 F3 : A Layered Fluorooxoborate with Charge-Oriented Ordering.

A layered fluorooxoborate, KNiB4 O6 F3 , contains a new (B4 O6 F4 )4- group built of one planar (BO3 )3- triangle and three tetrahedral (BO3 F)4- units. Those units are joined together by sharing the oxygen atoms to form a 2-dimensinal (BO3/2 F)3 BO3/2 )∞ layer. K+ and Ni2+ occupy on the sides with F- and O2- of the (BO3/2 F)3 BO3/2 )∞ layer with a high (positive charge) to high (negative charge) and low to low coordination. Such kind of charge-oriented ordering is found to be governed by the stabilization energy of Coulomb interaction of the cations in certain sites. It is hoped that this mechanism of ordering may provide an additional tool for designing new structures with favourable properties, such as ferroelectrics or nonlinear optical materials.

Rb4 Li2 TiOGe4 O12 : A Titanyl Nonlinear Optical Material with the Widest Transparency Range.

Practical mid-infrared (MIR) coherent light beams generated by frequency conversion in nonlinear optical (NLO) crystals are indispensable in time-resolved infrared vibrational spectroscopy, remote light detection and ranging, and free-space communications. Herein, a new titanyl germanate Rb4 Li2 TiOGe4 O12 (RLTG) MIR NLO crystal was obtained by heavier element substitution. It features a complicated structure network composed of compressed TiO5 square pyramids and distorted GeO4 tetrahedra, separated by Rb+ and Li+ cations. More importantly, RLTG exhibits concurrently short ultraviolet (0.28 µm) and long IR (5.58 µm) transmittance cutoffs, fully covering the atmospheric transparent window of 3-5 µm. Related to the short UV cutoff, it shows a higher laser-induced damage threshold in comparison to commercial MIR NLO crystals, about twice that of KTiOPO4 (KTP) and 50 times that of AgGaS2 (AGS).

High-Performance Magnetic Refrigerant Featuring One-Dimensional Gd-O Chains and O-Gd3 Triangles.

Magnetic cooling at low temperature has attracted intensive interest in cryogenics research, which may become important as cooling medium for long-wave photon detectors to support space exploration. Here, we report a Gd-based quaternary magnetic refrigerant material, Gd5 BSi2 O13 , containing chains of face-shared GdO9 polyhedra and geometrically frustrated OGd3 triangles. Magnetic measurements indicate that Gd5 BSi2 O13 exhibits a large magnetocaloric effect (MCE) about 1.74 times that of the practical magnetic refrigerant GGG (-ΔSm =67.0 J kg-1 K-1 ). We analyzed the origin of the large MCE by comparing GdIII -containing compounds with different structures and concentrations of GdIII .

K3 Li3 Gd7 (BO3 )9 : A New Gadolinium-Rich Orthoborate for Cryogenic Magnetic Cooling.

Magnetic cooling technology based on magnetocaloric effect (MCE) has attracted great interest in obtaining extremely low temperatures, for example, for space exploration. Here, we grew a new gadolinium-rich orthoborate K3 Li3 Gd7 (BO3 )9 (1) as a promising cryogenic magnetic coolant. It exhibits a complicated three dimensional framework constructed from BO3 groups and gadolinium-oxygen chains. The Gd-O chain consists of two types of clusters of Gd3 O20 and Gd3 O19 interconnection by Gd(4)O8 polyhydron. Due to its high gadolinium concentration, a large -ΔSm of 56.6 J kg-1 K-1 for 1 was obtained at 2 K and ΔH=7 T, much larger than that of the commercial benchmark Gd3 Ga5 O12 (GGG) crystal (38.4 J kg-1 K-1 ), suggesting it to be an excellent MCE material.

Noncentrosymmetric Cubic Cyanurate K6Cd3(C3N3O3)4 Containing Isolated Planar π-Conjugated (C3N3O3)3- Groups.

Single crystals of K6Cd3(C3N3O3)4 (1) were successfully grown via a solid-state cyclotrimerization reaction method from CdCl2 and KOCN. To our best knowledge, it is the first inorganic compound containing isolated six-membered-ring (6-MR) anionic groups that crystallizes in the cubic system. In the structure, the basic 6-MR anionic unit is a planar π-conjugated (C3N3O3)3- group that is isoelectronic with the (B3O6)3- group, as observed in the benchmark nonlinear-optical (NLO) crystal ß-BaB2O4 with strong second-harmonic-generation response. In addition, the electronic structure and linear-optical and NLO properties for 1 were also investigated by the first-principles calculation. The NLO coefficient (d14 = 1.17 pm/V) of 1 is about 3 times that of KH2PO4.