JOURNAL OF SOLID STATE CHEMISTRY 141, 317 — 322 (1998) ARTICLE NO. SC987921 New Cesium Hydrogen Selenate Phosphates: Synthesis and Crystal Structures S. I. Troyanov,* I. V. Morozov,* V. B. Rybakov,* A. Stiewe,- and E. Kemnitz- *Department of Chemistry, Moscow State University, Moscow, Russia; and -Institut fu ( r Chemie der Humboldt-Universita ( t zu Berlin, Hessische Strasse 1-2, 10115 Berlin, Germany Received March 16, 1998; in revised form May 28, 1998; accepted June 8, 1998 Three new compounds, Cs 4 (SeO 4 )(HSeO 4 ) 2 (H 3 PO 4 ) (I), Cs 3 (HSeO 4 ) 2 (H 2 PO 4 ) (II), and Cs 5 (HSeO 4 ) 3 (H 2 PO 4 ) 2 (III), were synthesized from water solutions of CsHSeO 4 /CsH 2 PO 4 . Ac- cording to X-ray single-crystal analysis, the compounds have the following crystal data: I, monoclinic, space group P2 1 , a  5.915(1) A s , b  13.797(3) A s , c  11.828(2) A s ,   95.11(3)°, V  961.4 A s 3 , Z  2, R 1  0.0384; II, monoclinic, space group P2 1 / n, a  20.281(8) A s , b  8.039(4) A s , c  9.160(4) A s ,   99.96(4)°, V  1470.9 A s 3 , Z  4, R 1  0.0580; III, monoclinic, space group C2/c, a  33.855(8) A s , b  7.978(2) A s , c  9.217(2) A s ,   101.00(2)°, V  2443.7 A s 3 , Z  4, R 1  0.0519. All three compounds have in common a coordination number of cesium of 10 or 101, with Cs–O distances from 2.9 to 3.8 A s . P and Se atoms form the individual tetrahedra with different H-bonding connectivity in structures I–III. PO 4 –SeO 4 layers with attached SeO 4 groups are present in I. Structure II contains a three-dimensional hydrogen bonding network, whereas struc- ture III is characterized by PO 4 –SeO 4 bands to which additional SeO 4 tetrahedra are attached.  1998 Academic Press 1. INTRODUCTION The structures of many metal hydrogen sulfates, sele- nates, and phosphates have been investigated in detail using X-ray and neutron diffraction methods. Some of these com- pounds are known to undergo phase transitions to super- protonic or ferroelectric phases (1, 2). Recently, crystal structures and superprotonic phase transitions for a number of CsHSO  —CsH  PO  compounds were reported (3—6). Due to the close similarities in chemical compositions and crystal chemistry of sulfates and selenates, one could expect the analogous mixed selenate phosphate compounds to exist and exhibit properties as in the respective metal sulfate phosphates. In fact, our systematic investigation of the systems MHSeO  —MH  PO  (M"alkali metal) resulted in the syn- To whom correspondence should be addressed. E-mail: erhard" kemnitz@chemie.hu-berlin.de thesis of new compounds for M"Na, K, Rb, and Cs with selenate to phosphate ratios of 3:1, 2:1, 3:2, and 1:1. In this paper, we describe the synthesis and crystal structures of three new cesium hydrogen selenate phosphates, Cs  (SeO  )(HSeO  )  (H  PO  )(I), Cs  (HSeO  )  (H  PO  )(II), and Cs  (HSeO  )  (H  PO  )  (III). Compound II is isotypic with the corresponding cesium hydrogen sulfate phosphate (3), whereas the other two form new structure types. EXPERIMENTAL Synthesis The crystalline phases of cesium hydrogen selenate phos- phates were obtained from solutions of Cs  CO  (p.a.) in a mixture of 70% H  SeO  and 65% H  PO  . X-ray powder diffraction (CoK radiation, DRON-4 diffractometer) was used to control the purity of the crystalline phases. To obtain compounds with different ratios Se:P"3:1, 2:1, and 3:2, the molar ratio of H  SeO  :H  PO  was varied. An excess of H  PO  was used to suppress the dissociation of HSeO  . The selenate to phosphate ratios in solution should exceed those in the crystals because of the incongruent type of crystallization. For example, Cs  (HSeO  )  (H  PO  )  (S:P"3:2) was crystallized from solution with a HSeO  : H  PO  ratio of 65:35. The corresponding HSeO  :H  PO  ratios for the synthesis of Cs  (HSeO  )  (H  PO  ) and Cs  (SeO  )(HSeO  )  (H  PO  ) were 70:30 and 77:23, respec- tively. From solutions with higher H  SeO  content ( '80%), pure cesium hydrogen selenate, CsHSeO  , was obtained. A lower HSeO  :H  PO  ratio of 55:45 led to the precipitation of CsH  PO  . Due to incongruent crystalliza- tion, mixtures of two phases were found in some cases. The exact composition of the compounds was determined by X-ray single-crystal analysis. The compounds are stable in air and form plates (I and II) or blocks (III). Crystal Structure Determination Single-crystal X-ray data were collected at room temper- ature using two different diffractometers, Stoe STADI-4 317 0022-4596/98 $25.00 Copyright  1998 by Academic Press All rights of reproduction in any form reserved.