Phase Transition, Conformational Exchange, and Nonlinear Optical Third Harmonic Generation of ACsP2Se8 (A = K, Rb, Cs)

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• 作者：Alyssa S. Haynes ; Abhishek Banerjee ; Felix O. Saouma ; Calford O. Otieno ; Joon I. Jang ; Mercouri G. Kanatzidis
• 刊名：Chemistry of Materials
• 出版年：2016
• 出版时间：April 12, 2016
• 年：2016
• 卷：28
• 期：7
• 页码：2374-2383
• 全文大小：661K
• 年卷期：0
• ISSN：1520-5002

文摘

The soluble molecular selenophosphate salts ACsP<sub>2sub>Se<sub>8sub> (A = K, Rb, Cs) crystallize in the orthorhombic space group Ccce with a = 14.982(3) Å, b = 24.579(5) Å, and c = 13.065(3) Å for the Cs salt and a = 14.782(3) Å, b = 23.954(5) Å, and c = 13.044(3) Å for the K analogue. ACsP<sub>2sub>Se<sub>8sub> is composed of the molecular 6-membered ring, [P<sub>2sub>Se<sub>8sub>]<sup>2−sup>, in the twist conformation charge balanced by alkali metals. The band gaps of these compounds are 2.44 ± 0.2 eV for Cs<sub>2sub>P<sub>2sub>Se<sub>8sub>, 2.41 ± 0.2 eV for RbCsP<sub>2sub>Se<sub>8sub>, and 2.36 ± 0.2 eV for KCsP<sub>2sub>Se<sub>8sub>. The amorphous versions of these materials can be made by water quenching the melt and have band gaps for all ACsP<sub>2sub>Se<sub>8sub> of 2.12 ± 0.2 eV. Raman spectroscopic studies exhibit active modes of PSe<sub>4sub> and Se−Se in the compound. Solution <sup>31sup>P NMR studies shed light into the interesting conformational fluxionality of the [P<sub>2sub>Se<sub>8sub>]<sup>2−sup> anion, including a conformation that has not been previously observed. Thermal analysis reveals ACsP<sub>2sub>Se<sub>8sub> exhibits a phase transition, which we investigate by in situ synchrotron powder X-ray diffraction. Third harmonic generation (THG) nonlinear optical measurements determined the THG coefficient, χ<sup>(3)sup>, for amorphous and crystalline Cs<sub>2sub>P<sub>2sub>Se<sub>8sub> of 1.8 ± 0.2 × 10<sup>5sup> pm<sup>2sup>/V<sup>2sup> and 2.4 ± 0.1 × 10<sup>5sup> pm<sup>2sup>/V<sup>2sup>, respectively.