The caesium phosphates Cs 3 (H 1.5 PO 4 ) 2 (H 2 O) 2 , Cs 3 (H 1.5 PO 4 ) 2 , Cs 4 P 2 O 7 (H 2 O) 4 , and CsPO 3

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The caesium phosphates ­Cs3(H1.5PO4)2(H2O)2, ­Cs3(H1.5PO4)2, ­Cs4P2O7(H2O)4, and ­CsPO3 Matthias Weil1   · Berthold Stöger2 Received: 9 July 2020 / Accepted: 10 August 2020 / Published online: 8 September 2020 © The Author(s) 2020

Abstract  The caesium phosphates ­Cs3(H1.5PO4)2(H2O)2 and ­Cs3(H1.5PO4)2 were obtained from aqueous solutions, and ­Cs4P2O7(H2O)4 and ­CsPO3 from solid state reactions, respectively. ­Cs3(H1.5PO4)2, ­Cs4P2O7(H2O)4, and ­CsPO3 were fully structurally characterized for the first time on basis of single-crystal X-ray diffraction data recorded at − 173 °C. Monoclinic C ­ s3(H1.5PO4)2 (Z = 2, C2/m) represents a new structure type and comprises hydrogen phosphate groups involved in the formation of a strong non-symmetrical hydrogen bond (accompanied by a disordered H atom over a twofold rotation axis) and a very strong symmetric hydrogen bond (with the H atom situated on an inversion centre) with symmetry-related neighbouring anions. Triclinic ­Cs4P2O7(H2O)4 (Z = 2, P1̄ ) crystallizes also in a new structure type and is represented by a diphosphate group with a P–O–P bridging angle of 128.5°. Although H atoms of the water molecules were not modelled, O···O distances point to hydrogen bonds of medium strengths in the crystal structure. C ­ sPO3 is monoclinic (Z = 4, P21/n) and belongs to the family of catena-polyphosphates (MPO3)n with a repetition period of 2. It is isotypic with the room-temperature modification of ­RbPO3. The crystal structure of ­Cs3(H1.5PO4)2(H2O)2 was re-evaluated on the basis of single-crystal X-ray diffraction data at − 173 °C, revealing that two adjacent hydrogen phosphate anions are connected by a very strong and non-symmetrical hydrogen bond, in contrast to the previously described symmetrical bonding situation derived from room temperature X-ray diffraction data. In the four title crystal structures, coordination numbers of the caesium cations range from 7 to 12. Graphic abstract

Keywords  Solid state · Alkali metals · X-ray structure determination · Hydrogen bonds Electronic supplementary material  The online version of this article (https​://doi.org/10.1007/s0070​6-020-02675​-6) contains supplementary material, which is available to authorized users. * Matthias Weil [email protected] 1



Division of Structural Chemistry, Institute for Chemical Technologies and Analytics, TU Wien, Getreidemarkt 9/164‑SC, 1060 Vienna, Austria



X‑ray Centre, TU Wien, Getreidemarkt 9, 1060 Vienna, Austria

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Introduction The recent interest in the family of caesium phosphates is mainly connected with the high proton conductivity of Cs(H2PO4) to be utilized as a potential electrolyte for intermediate temperature fuel cells [1–3] or for water electrolysis [4]. Another motivation to search for new caesium phosphates is related to acidic salts with formulae MxHy(AO4)z (M = Cs, Rb, K, Na, Li, N ­ H4; A = S, Se, As, P) that likewise exhibit proton conductivity or have ferroelectric properties.

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In this context, the new proton conduc