To whom correspondence should be addressed. Journal of Solid State Chemistry 155, 206 } 215 (2000) doi:10.1006/jssc.2000.8935, available online at http://www.idealibrary.com on High-Temperature Incommensurate-to-Commensurate Phase Transition in the Bi 2 MoO 6 Catalyst D. J. Buttrey,* T. Vogt,- and B. D. White* *Center for Catalytic Science and Technology, Department of Chemical Engineering, University of Delaware, Newark, Delaware 19716; and -Physics Department, Brookhaven National Lab, Upton, New York 11973 Received June 8, 2000; in revised form August 9, 2000; accepted August 15, 2000 DEDICATED TO PROFESSOR J. M. HONIG The temperature-dependent behavior of the low- and inter- mediate-temperature phases of -Bi 2 MoO 6 were investigated. As revealed by high-resolution neutron and synchrotron powder di4raction as well as electron di4raction, the low-temperature Aurivillius phase shows a previously undetected incommensurate modulation along the pseudo-tetragonal a (q a  0.5626(2)) and c(q c  0.2844(2)) axes. A reversible incommensurate-to-com- mensurate phase transition to the intermediate-temperature phase occurs near 840 K, before irreversibly transforming near 880 K into the 6uorite-related high-temperature polymorph described in detail by two of us in an earlier study (Buttrey et al., J. Solid State Chem. 111, 118+127 (1994)).  2000 Academic Press INTRODUCTION Despite the importance of complex oxides in catalysis, our understanding of the interplay between crystal structure and catalytic properties is far from well developed. Bismuth molybdates are prototypical bimetallic oxide catalysts con- tained in multicomponent molybdate (MCM) preparations that are widely used for the selective oxidation and ammoxi- dation of ole"ns (1). The oxidation of propylene to acrolein, for instance, is achieved with a selectivity of up to 95%. The two steps involved in the process are hydrogen abstraction at a bismuth site, resulting in an allyl intermediate, followed by an oxygen insertion at an adjacent molybdenum site to generate the product. The conversion of 1-butene to 1,3- butadiene is also e!ectively accomplished with these mater- ials. With the addition of ammonia, the ammoxidation of propylene to produce acrylonitrile provides another very important application of the bismuth molybdates. Beyond the requirement of site isolation (2) in the topological arrangement of the bismuth and molybdenum reaction sites, which can be accomplished with the appropriate structural motifs, factors such as the surface area and crystal habit, surface reconstruction, site potentials, and stability to oxidation and reduction cycling are important. Note that these are redox catalysts, involving simultaneous reduction and reoxidation to avoid net loss of lattice oxygen. The three catalytically active bismuth molybdates have rather similar Bi : Mo ratios. These ratios are 2 : 3 (- Bi  Mo  O  ), 1 : 1 (-Bi  Mo  O  ), and 2 : 1 (-Bi  MoO  ). The  phase is often described as a defective scheelite structure in which 1 of every 3 Bi sites is vacant (3). For our purposes, it is useful to recognize that the scheelite structure is a special case of the #uorite family. The  form is again a #uorite- related superstructure with metal site vacancies; one of every nine metal sites is absent. In both cases, the vacancies are in an ordered arrangement (at room temperature) within in"nite Bi channels (5). The  composition shows polymor- phism. The low-temperature polymorph, a (rare) naturally occurring mineral called koechlinite, is generally thought to be the catalytically active form since this is the stable bulk phase under reaction conditions (i.e., &650}750 K). Inter- estingly, the highest temperature form is the only one of 3  polymorphs that is clearly a #uorite derivative with in"nite Bi channels, though unlike  and , it has no vacant metal sites. The low and intermediate  forms both belong to the Aurivillius family (6), (Bi  O  )  (Bi  MO  ), with x, n"1 and M"Mo (7). These consist of a strongly dis- torted nearly square layered network of Mo polyhedra separated by Bi  O   layers. These Mo polyhedra are usually described as distorted octahedra, in contrast to the isolated tetrahedra evident in the #uorite-type bismuth mo- lybdates. As we will show, they are actually somewhat intermediate between octahedra and tetrahedra. The -Bi  MoO  composition has been the subject of considerable attention and controversy due to its three- phase polymorphism (8). When heated from room temper- ature, it "rst undergoes a subtle and reversible phase transition near 840 K involving a poorly understood change between the closely related n"1 Aurivillius-type structures 206 0022-4596/00 $35.00 Copyright  2000 by Academic Press All rights of reproduction in any form reserved.