Hypervalent hydridosilicate in the Na-Si-H system

Hydrogenation reactions at gigapascal pressures can yield hydrogen-rich materials with properties relating to superconductivity, ion conductivity, and hydrogen storage. Here, we investigated the ternary Na-Si-H system by computational structure prediction and in situ synchrotron diffraction studies of reaction mixtures NaH-Si-H2 at 5-10 GPa. Structure prediction indicated the existence of various hypervalent hydridosilicate phases with compositions NamSiH(4+m) (m = 1-3) at comparatively low pressures, 0-20 GPa. These ternary Na-Si-H phases share, as a common structural feature, octahedral SiH6 2- complexes which are condensed into chains for m = 1 and occur as isolated species for m = 2, 3. In situ studies demonstrated the formation of the double salt Na3[SiH6]H (Na3SiH7, m = 3) containing both octahedral SiH6 2- moieties and hydridic H-. Upon formation at elevated temperatures (>500°C), Na3SiH7 attains a tetragonal structure (P4/mbm, Z = 2) which, during cooling, transforms to an orthorhombic polymorph (Pbam, Z = 4). Upon decompression, Pbam-Na3SiH7 was retained to approx. 4.5 GPa, below which a further transition into a yet unknown polymorph occurred. Na3SiH7 is a new representative of yet elusive hydridosilicate compounds. Its double salt nature and polymorphism are strongly reminiscent of fluorosilicates and germanates.

From CsKTaF7 to CsNaTaF7 : Alkali Metal Cations Regulation to Generate SHG Activity

Inorganic metal halides play important roles in wide range of areas including fluorescence, X-ray detection, and nonlinear-optics. Herein, two new mixed alkali metal tantalum fluorides, CsKTaF₇ and CsNaTaF₇ , have been obtained based on the strategy of cations regulation in A₂ MF₇ (A represents monovalent cations and M is d⁰ transition-metal cation) system by a conventional hydrothermal route. CsKTaF₇ crystallizes in the centric Pnma space group, while CsNaTaF₇ crystallizes in the polar Cmc2₁ space group and exhibits moderate and phase-matchable NLO activity. Both halides possess large optical band gaps above 5.0 eV. The crystal structure evolution, optical properties, and detailed theory calculations of these two halides were elucidated in this work.

KNa2ZrF7: A Mixed-Metal Fluoride Exhibits Phase-Matchable Second-Harmonic-Generation Effect and High Laser-Induced Damage Threshold

Halides are one type of important IR nonlinear-optical material candidate. Here, a mixed alkali metal and d⁰ transition-metal fluoride, namely, KNa₂ZrF₇, was obtained by a facile hydrothermal method. It crystallizes in an orthorhombic system with the polar space group Pmn2₁, and its pseudo-1D structure features isolated (ZrF₇)^3- moncocapped trigonal prisms, which are ionically linked together by countercations K⁺ and Na⁺, representing a new type of fluoride. The powder sample of KNa₂ZrF₇ exhibits a moderate second-harmonic-generation response and a high laser-induced damage threshold. Besides, it can realize phase matchability and possesses a wide-IR transparent window. Thermal stability analysis suggests that KNa₂ZrF₇ is a congruent compound. Structural comparisons with related fluorides and theoretical calculations are also presented in this work.