 To whom correspondence should be addressed. E-mail: abraham@ ensc-lille.fr. Journal of Solid State Chemistry 155, 342 } 353 (2000) doi:10.1006/jssc.2000.8923, available online at http://www.idealibrary.com on Synthesis, Crystal Structure, and Comparison of Two New Uranyl Vanadate Layered Compounds: M 6 (UO 2 ) 5 (VO 4 ) 2 O 5 with M  Na, K C. Dion, S. Obbade, E. Raekelboom, and F. Abraham Laboratoire de Cristallochimie et Physicochimie du Solide, UPRESA CNRS 8012, ENSCL, Universite & des Sciences et Technologies de Lille, B.P. 108, 59652 Villeneuved+Ascq Cedex, France and M. Saadi Laboratoire de Chimie de Coordination et Analytique, Faculte & des Sciences, Universite & Chouaib Doukkali, B.P. 20, El Jadida, Morocco Received May 30, 2000; in revised form July 21, 2000; accepted August 9, 2000; published online November 29, 2000 Two new alkali uranyl vanadates, M 6 (UO 2 ) 5 (VO 4 ) 2 O 5 with M  Na, K, have been synthesized by solid state reactions and their structures determined from single-crystal X-ray di4raction data. They crystallize in the monoclinic system with space group P 2 1 / c. Unit cell b, c, and  parameters are almost identical in both compounds while the a parameter in the Na compound is doubled: Na0a  12.584(1) A s , b  24.360(2) A s , c  7.050(1) A s ,   100.61(1)3, V  2124.2 A s , Z  4, D mes  5.60(2) g/ cm 3 , D cal  5.63 g/ cm 3 ; K0a  6.856(1) A s , b  24.797(3) A s , c  7.135(1) A s ,   98.79(8)3, V  1198.8 A s , Z  2, D mes  5.25(2) g/ cm 3 , D cal  5.25 g / cm 3 . A full-matrix least-squares re5nement yielded, respectively, R  0.039, R w  0.043 and R  0.037; R w  0.046 for 3293 and 2096 independent re6ections with I > 3 collected on a Philips PW 1100 di4ractometer (MoK radiation). Both structures are characterized by [(UO 2 ) 5 (VO 4 ) 2 O 5 x ] 6n n corrugated layers parallel to the (100) plane. The layers are built up from VO 4 tetrahedra, UO 7 pentagonal bipyramids, and UO 6 distorted octahedra. The UO 7 pentagonal bipyramids are asso- ciated by sharing opposite equatorial edges to form zig-zag in5nite chains (UO 5 ) n parallel to the c axis. These chains are linked together on one side by VO 4 tetrahedra and on other side by UO 6 octahedra and UO 7 corner sharing. Both structures di4er simply by the symmetry of the layers. The unit cell contains one centrosymmetric layer in the K compound, whereas in the two-layer unit cell of the Na compound two noncentrosymmetric consecutive layers are related by an inversion center. The layers appear to be held together by the alkali ions. The mobility of the M  ions within the interlayer space in M 6 (UO 2 ) 5 (VO 4 ) 2 O 5 and carnotite analog compounds is compared.  2000 Academic Press INTRODUCTION Layered compounds are generally favorable for good electrical ionic conductivity. For example, in -alumina, where fast Na ion transport has been initially observed (1), alkali ions are able to move freely within the conduction planes but cannot penetrate the dense spinel blocks. Numer- ous uranyl phosphate, arsenate, or vanadate compounds adopt layered structures. In the P and As compounds, generally the layers result from the association between XO  tetrahedra (X"P, As) and UO  or UO  bipyramids involving the equatorial oxygen atoms in the plane perpen- dicular to the uranyl ion. This is the case of, for example, hydrogen uranyl phosphate tetrahydrate, HUO  PO  ) 4H  O, also simply called HUP, which is among the best ionic electrical conductors actually known. In HUP, proto- ns H jump between interlayer water molecules. HUP has received much attention because of its potential applica- tions in electrochemical systems (2}7). In the V compounds, generally the UO  or UO  polyhedra are linked by VO  square pyramids. We have already studied monovalent metal uranyl vanadates with a molar ratio U/V"1 (8, 9). These materials are characterized by (UO  VO  )  sheets held together by interlayer cations alone or surrounded with water molecules. They belong to the large class of com- pounds of the general formula M  UO  XO  ) xH  O, where X can be P, As, and V (10). In P and As compounds, the UO  ions are connected by PO  or AsO  tetrahedra, whereas in V compounds they are linked by V  O  units formed by two inverse VO  square pyramids sharing an edge. They therefore have the general formula M  (UO  )  V  O  ) xH  O . We have recently reported the structure of the synthetic carnotite K (UO  )VO  , its Na analog, the proper- ties of Na  K  UO  VO  (04x41) solid solution, and 342 0022-4596/00 $35.00 Copyright  2000 by Academic Press All rights of reproduction in any form reserved.