Journal of Solid State Chemistry 143, 41 — 51 (1999) Article ID jssc.1998.8067, available online at http://www.idealibrary.com on The Phase Transitions between H 0.13 V 0.13 Mo 0.87 O 3 · 0.26H 2 O and MoO 3 : An X-Ray, Thermal Analysis, and TEM Study L. Dupont, D. Larcher, and M. Touboul Laboratoire de Re ´ activite ´ et Chimie des Solides, Universite ´ de Picardie Jules Verne, UPRES A 6007, 33 rue Saint-Leu, F-80039 Amiens Cedex, France Received June 15, 1998; in revised form October 5, 1998; accepted October 19, 1998 Mixed vanadium-molybdenum oxide hydrates (H x V x Mo 1x O 3 · 0.26H 2 O with 0.064x40.18) have been synthesized by a soft chemistry method. The phase transitions from one of these hydrates (x  0.13) to the final product MoO 3 have been studied by thermal analysis, X-ray powder diffraction, and transmission electron microscopy techniques. Both metastable and stable ox- ides have been observed. H 0.13 V 0.13 Mo 0.87 O 3 · 0.26H 2 O possesses a structure related to hexagonal MoO 3 . Dehydration of the precursor leads to a metastable phase H 0.13 V 0.13 Mo 0.87 O 3 , with a structure similar to that of the hydrate. At 500°C this phase transforms into the metastable V 0.13 Mo 0.87 O 2.935 , with a structure related to the orthorhombic MoO 3 structure. The heating of this last phase above 500°C induces a change from the metastable system to the stable binary (V 2 O 5 – MoO 3 ) one. In agreement with this binary, a liquid phase and a solid phase with a composi- tion closest to MoO 3 , are formed during the phase segregation at 600°C. Models for explaining these phase transitions are pro- posed.  1999 Academic Press INTRODUCTION One decade ago, an hydrated precursor H  V  Mo  O  ) 0.26H  O isotypic with hexagonal MoO  (Fig. 1a) was synthesized using two different ‘‘chimie douce’’ methods by Davies et al. (1). The first approach (1—3) con- sists of the synthesis of an intermediary brannerite-type phase LiVMoO  by LiVO  and MoO  solid state reaction (4). This intermediary phase is soaked in hydrochloric acid solution, where it dissolves, and then the above-mentioned hydrated phase precipitates. In a second approach (5), lith- iated mixed vanadium—molybdenum oxide hydrates iso- typic with hexagonal MoO  structure are synthesized by dissolution of V  O  and MoO  in LiOH aqueous solution or by dissolution of LiVO  and Li  Mo  O  in water. The so-obtained solutions are acidified and heated under reflux until the precipitation of phases with the chemical formula (Li  H  )(V  Mo  )O  ) 0.26H  O occurs. By ionic exchange in HCl at room temperature these phases trans- form into the pure protonic H  V  Mo  O  ) 0.26H  O phase. Recently, our research group reported a new way of obtaining not only the pure protonic hexagonal MoO  type oxide hydrate with the ratio V/Mo"0.13/0.87 but also H  V  Mo  O  ) nH  O phases family with 0.064x40.18 (6). The previously reported synthesis method (6) consists of the dissolution of metallic molybdenum and V  O  in hydro- gen peroxide solution. Then, the solution is heated until a yellow precipitate (the precursor) appears. According to Davies et al. (5), the heating at 350°C of the hydrated precursor leads to V  Mo  O  with an hexagonal MoO  type structure (Fig. 1a), and this oxide is stable up to 460°C. Nevertheless, questions remain concerning the behavior of this oxide heated above 460°C; does it transform into an orthorhombic double layer MoO  type structure oxide (Fig. 1b) or into a monoclinic MoO  type structure oxide (Fig. 1c)? Moreover, is the chemical formula V  Mo  O  in good agreement with an hexagonal MoO  type structure? Based on our experience in the comprehension of phase transition mechanisms from hydrated transition metal oxide precursors synthesized by chimie douce, we decided to carry out the study of the phase transitions ensuing H  V  Mo  O  ) 0.26H  O thermal treatment. The present paper will describe how the phase trans- formations, from the H  V  Mo  O  ) 0.26H  O pre- cursor into the ultimate reaction product MoO  , have been elucidated by a combination of thermal analysis methods, X-ray diffraction, and TEM techniques. EXPERIMENTAL A description of the synthesis method has already been given in detail (6). X-ray diffraction studies were done with a Guinier-Lenne´ heating camera (ENRAF diffractis 581, CuK1 radiation isolated by a monochromator, platinum grid, heating rate 0.1°C/min, film rate 1.5 mm/h, under air) and a D5000 Bragg-Brentano diffractometer (radiation CuK1). The data were treated with Dicvol 91 (7), NBS/AIDS83 (8), and Fullprof (9) software in order to 41 0022-4596/99 $30.00 Copyright  1999 by Academic Press All rights of reproduction in any form reserved.