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​://​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


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.




In this context, the new proton conduc