New insights into medium-range order around titanium in sol–gel derived silica through isotope difference neutron diffraction and reverse Monte Carlo modelling D.M. Pickup a, * , F.E. Sowrey a , K.O. Drake b , M.E. Smith b , R.J. Newport a a School of Physical Sciences, University of Kent, Canterbury, Kent CT2 7NR, UK b Department of Physics, University of Warwick, Coventry, Warwickshire CV4 7AL, UK Received 8 April 2004 Available online 17 June 2004 Abstract Neutron diffraction with isotopic titanium substitution allows the nature of the cation sites to be determined beyond nearest- neighbor distances for the first time in a sol–gel derived titania–silica glass. Reverse Monte Carlo (RMC) modelling of the results shows unequivocally that titanium substitutes for silicon within the homogeneous glass network, existing mostly as TiO 4 , with a minority TiO 6 component. The fundamental scientific interest, and wide-ranging technologically significant properties of titania additions to silica, demand an understanding of the precise nature of the titanium site. Ó 2004 Elsevier B.V. All rights reserved. Understanding why amorphous materials exhibit the properties that they do is a fundamental scientific problem, and is essential to the task of tailoring useful properties and designing better materials for the future. A key component in achieving this understanding is knowledge of the atomic scale structure of such mate- rials. Titania–silica mixed oxide glasses, (TiO 2 ) x - (SiO 2 ) 1  x , have received much attention over recent years because their physical properties lend them to a number of technological uses: as ultra-low thermal ex- pansion glasses [1] and thin-films with tailored refractive indices [2] for example. Much recent work focuses on their catalytic properties [3]. Sol–gel derived mixed ox- ides are active and selective epoxidation catalysts [4,5], offering an efficient alternative to the conventional tita- nia-on-silica and Ti-substituted zeolites and allowing, for example, better control of porosity. In order to un- derstand fully the nature of these properties, one must have a robust atomic-scale structural model. In the case of TS-1 [6,7], a commercially used Ti-substituted zeolite, this task is made easier by the crystalline nature of the material. However, the structural complexity is much greater in sol–gel derived amorphous materials, and there exists no technique that can provide a complete structural determination: thus, only the short-range or- der in titania–silica sol–gel materials has been studied in detail [4,8,9] hitherto and most of the methods used are intrinsically indirect. The understanding achieved through this work will also be relevant to other funda- mental structural questions concerning titanium such as the degree of intermediate character of titanium within glass formation, and its substitutional role in silicates of importance in Earth sciences and areas of microporous chemistry. Neutron diffraction with isotopic substitution (NDIS), is in principle able to provide, by a direct route, more detailed information on the short and medium range order present in such glassy materials [10]. Using stable isotopes of Ti, NDIS and reverse Monte Carlo (RMC) modelling are combined to produce a three-di- mensional model of a titania–silica sol–gel derived glass which is both consistent with the present quantitative neutron diffraction data as well as with previously collected spectroscopic and NMR data [8,9]. It is unequivocally demonstrated that titanium predomi- nantly substitutes for silicon in a 4-fold coordination * Corresponding author. Fax: +44-1227-827-558. E-mail address: dmp@kent.ac.uk (D.M. Pickup). 0009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.06.005 Chemical Physics Letters 392 (2004) 503–507 www.elsevier.com/locate/cplett