JOURNAL OF SOLID STATE CHEMISTRY 135, 159 — 168 (1998) ARTICLE NO. SC977618 Orthorhombic WO 3 Formed via a Ti-Stabilized WO 3 · 1 3 H 2 O Phase B. Pecquenard,* H. Lecacheux,* J. Livage,* and C. Julien† * Laboratoire de Chimie de la Matie ` re Condense ´ e and † Laboratoire des Milieux De ´ sordonne ´ s et He ´ te ´ roge ` nes, Universite ´ Pierre et Marie Curie, 4 place Jussieu, 75252 Paris, France Received May 7, 1997; in revised form September 3, 1997; accepted September 9, 1997 Stable solutions of WO 3 precursors have been prepared via the dissolution of tungstic acid, H 2 WO 4 , in hydrogen peroxide. A crystalline peroxopolytungstic acid WO 3 ·H 2 O 2 · nH 2 O (n+0.1) is obtained upon drying. Peroxo groups decompose at 200°C, giving an amorphous tungsten oxide that crystallizes into the stable monoclinic WO 3 around 400°C. Completely different results are obtained when Ti(OPr i ) 4 is added to the precursor solution. The orthorhombic phase WO 3 · 1 3 H 2 O is first obtained. As is well known, this hydrated oxide leads to h-WO 3 and m-WO 3 upon heating. However, in the presence of Ti IV , a new metastable orthorhombic tungsten oxide is formed around 400°C. It then transforms irreversibly upon further heating into the stable monoclinic WO 3 . The presence of Ti IV seems to stabilize this new orthorhombic phase. 1998 Academic Press INTRODUCTION A large number of tungsten oxides have been described in the literature (1). Crystalline tungsten oxides exhibit ReO - type structures based on corner-sharing [WO ] octahedra (2). Due to the displacement of W ions, these octahedra are more or less distorted. The type and magnitude of these distortions depend on temperature, giving rise to several polymorphic forms (tetragonal, orthorhombic, monoclinic, and triclinic) which transform reversibly into each other as a function of temperature. Not less than eight phase trans- formations have been reported up to 1000°C for stoi- chiometric tungsten trioxide (3). Several hydrated tungsten oxides WO ) nH O have also been described. They are built from corner-sharing [WO ] or [WO (OH ) ] octahedra. Following the pioneering work of Figlarz et al. (4), metastable tungsten oxides have been obtained from these hydrated phases via ‘‘chimie douce.’’ The orthorhombic oxide WO ) H O is obtained via the hydrothermal treatment of tungstic acid H WO ) H O. Upon thermal dehydration around 300°C it leads to the hexagonal oxide h-WO . A supermetastable orthorhombic WO phase, with the same structure as the hydrate, has even To whom correspondence should be addressed. been reported (5). The dehydration of WO ) 2H O leads to WO ) H O and then to a cubic WO with a perfect ReO structure (6). Tungsten oxides with a pyrochlore-type struc- ture have also been obtained via the thermal decomposition of ammonium tungstate (7, 8). Upon further heating around 400°C, all these metastable oxides lead to the stable mon- oclinic WO phase. Tungsten oxide is a well-known electrochromic material. Its coloration switches reversibly from white to blue upon the electrochemical insertion of monovalent cations such as Li or H (9). Several authors have shown that reversibility can be improved by adding TiO to the tungsten oxide. The lifetime of WO —TiO thin films can be five times longer than that of pure WO (10, 11). Therefore several studies have been undertaken to obtain more information on the structure and electrochemical properties of WO —TiO thin films (12). This paper reports on the synthesis of WO —TiO oxide phases from aqueous solutions in the presence of hydrogen peroxide. It shows that adding small amounts of Ti to the precursor solution strongly changes the nature of the cry- stalline tungsten oxide phases. A new metastable ortho- rhombic phase has even been observed during the thermal treatment of hydrated WO —TiO oxides. EXPERIMENTAL TECHNIQUES Precursor solutions and oxide powders were character- ized using the following experimental techniques. W NMR experiments were performed on aqueous solutions with a Bruker MSL 400 spectrometer. Chemical shifts were measured using Na WO (2 M) or [H SiW O ] solutions in D O as a reference. As the sensitivity of the W nucleus is very weak (natural abundance 14.4%, "1.1210 rad ) T ) s), NMR spectra had to be accu- mulated for more than 24 h ('5000 scans). Phase identification of powders was carried out by X-ray diffraction with a Philips diffractometer equipped (CuK , "1.540598 A s ) in reflection geometry. Infrared absorption spectra were recorded on powders dispersed in KBr pellets using a Fourier transform Nicolet 159 0022-4596/98 $25.00 Copyright 1998 by Academic Press All rights of reproduction in any form reserved.