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Potassium Zinc Borohydrides Containing Triangular [Zn(BH4)3] and Tetrahedral [Zn(BH4)x Cl4–x]2- Anions

Ravnsbæk, Dorthe B.
Smrčok, L'ubomír
Jensen, Torben R.
Published in Journal of Physical Chemistry. C. 2012, vol. 116, no. 1, p. 1563-1571
Abstract Three novel potassium–zinc borohydrides/chlorides are described. KZn(BH4)3 and K2Zn(BH4)xCl4–x form in ball-milled KBH4:ZnCl2 mixtures with molar ratios ranging from 1.5:1 up to 3:1. On the other hand, K3Zn(BH4)xCl5–x forms only in the 2:1 mixture after longer milling times. The new compounds have been studied by a combination of in situ synchrotron powder diffraction, thermal analysis, Raman spectroscopy, and the solid state DFT calculations. Rhombohedral KZn(BH4)3 contains an anionic complex [Zn(BH4)3]− with D3 (32) symmetry, located inside a rhombohedron K8. KZn(BH4)3 contains 8.1 wt % of hydrogen and decomposes at 385 K with a release of hydrogen and diborane similar to other Zn-based bimetallic borohydrides like MZn2(BH4)5 (M = Li, Na) and NaZn(BH4)3. The decomposition temperature is much lower than for KBH4. Monoclinic K2Zn(BH4)xCl4–x contains a tetrahedral complex anion [Zn(BH4)xCl4–x]2– located inside an Edshammar polyhedron (pentacapped trigonal prism) K11. The compound is a monoclinically distorted variant of the paraelectric orthorhombic ht-phase of K2ZnCl4 (structure type K2SO4). K2Zn(BH4)xCl4–x releases BH4 starting from 395 K, forming Zn and KBH4. As the reaction proceeds and x decreases, the monoclinic distortion of K2Zn(BH4)xCl4–x diminishes and the structure transforms at 445 K into the orthorhombic ht-phase of K2ZnCl4. Tetragonal K3Zn(BH4)xCl5–x is a substitutional and deformation variant of the tetragonal (I4/mcm) Cs3CoCl5 structure type possibly with the space group P42/ncm. K3Zn(BH4)xCl5–x decomposes nearly at the same temperature as KZn(BH4)3, i.e., at 400 K, with the formation of K2Zn(BH4)xCl4–x and KBH4, indicating that the compound is an adduct of the two latter compounds.
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CERNY, Radovan et al. Potassium Zinc Borohydrides Containing Triangular [Zn(BH4)3] and Tetrahedral [Zn(BH4)x Cl4–x]2- Anions. In: Journal of Physical Chemistry. C, 2012, vol. 116, n° 1, p. 1563-1571. doi: 10.1021/jp209848r https://archive-ouverte.unige.ch/unige:35128

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Deposited on : 2014-03-31

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