An in situ time-resolved neutron diffraction study of the hydrothermal crystallisation of barium titanate Richard I. Walton, a Ronald I. Smith, b Franck Millange, a Iain J. Clark, c Derek C. Sinclair c and Dermot O’Hare* a a Inorganic Chemistry Laboratory, University of Oxford, South Parks Road, Oxford, UK OX1 3QR. E-mail: dermot.ohare@chem.ox.ac.uk b ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon, UK OX11 0QX c Department of Engineering Materials, University of Sheffield, Mappin Street, Sheffield, UK S1 3JD Received (in Cambridge, UK) 27th April 2000, Accepted 30th May 2000 Published on the Web 22th June 2000 The hydrothermal crystallisation of barium titanate, BaTiO 3 , has been studied for the first time by in situ time- resolved neutron powder diffraction; we deduce that BaTiO 3 is formed from solution by reaction between [Ti- (OH) x ] (42x)+ (aq) and [Ba(OH) x ] (22x)+ (aq) species, rather than by a heterogeneous reaction between solid TiO 2 and Ba 2+ (aq) ions. Tetragonal barium titanate, t-BaTiO 3 , finds widespread applica- tion in the electroceramics industry. The ferroelectric properties and high permittivity of t-BaTiO 3 are exploited in, for example, thermistors, capacitors, and electro-optic devices. Traditional routes to the synthesis of the material employ direct solid-state reaction between barium carbonate and titanium dioxide at elevated temperature ( > 900 °C). Hydrothermal routes to BaTiO 3 have been the focus of much recent research as an efficient low temperature method for its manufacture (see, for example, refs. 1–3). The reaction of a number of barium and titanium sources in water at temperatures as low as 80 °C has enabled the production of t-BaTiO 3 and by varying reaction conditions and choice of starting materials it has proved possible to change the particle size and morphology of the material produced. Particular attention has been paid to the production of ultrafine (submicron) powders 1–4 and to the production of thin films 5–7 by the hydrothermal method. Such control of morphology is extremely desirable and has huge potential value, both in the manufacture of miniaturised devices 8,9 and in the production of dense fine-grained ceramics by sintering of the fine powders initially prepared. 10 An understanding of the formation mechanism will be vital if controlled growth of t-BaTiO 3 is to be performed and the potential applications described above are to be exploited. Although the kinetics and mechanism of hydrothermal barium titanate production have been discussed by several authors, 11–14 no agreed description of its formation mechanism has been reached. Two extreme models have been postulated: one involving a homogenous solution phase reaction between titanium and barium hydroxy anions (the dissolution–precipita- tion mechanism) and another involving the reaction between solid TiO 2 and soluble barium species (the in situ heterogeneous transformation mechanism). 12 The data previously used to probe the kinetics of barium titanate crystallisation were derived entirely from quenching experiments, whereby material was removed from the reaction cell after given period of time and examined. Because of the absorbing nature of the sample, we were unfortunately unable to probe the barium titanate crystallisation using in situ time-resolved synchrotron X-ray diffraction experiments, which are now well established. 15,16 Powder neutron diffraction, however, offers an alternative means of following the crystallisation. In recent months we have developed a gold coated, null-scattering (67.7 atom% Ti, 32.3 atom% Zr) environmental cell that enables us to record in situ high-resolution powder neutron diffraction patterns of highly reactive and/or corrosive materials reacting under hydrothermal conditions. 17 Only one previous report of the use of neutron diffraction to follow reactions under hydrothermal conditions appears in the literature, and this did not overcome the problem of significant scattering from the reaction vessel appearing in the diffraction patterns, which seriously limited the amount of quantitative and structural information that could potentially be extracted. 18 Here, we describe the first use of the Oxford-ISIS hydrothermal cell to study a hydrothermal synthesis using in situ neutron diffraction. Fig. 1(a) shows the time-of-flight powder neutron diffraction patterns of the crystalline starting materials [TiO 2 (2.12 g) Ba(OD) 2 ·8D 2 O (9.81 g), Ba+Ti ratio of 1.1+1] as a suspension in 10 ml D 2 O in the hydrothermal cell before heating.† The cell was then heated to 125 °C and neutron diffraction patterns Fig. 1 Powder neutron diffraction data obtained from within the hydro- thermal cell. for (a) Ba(OD) 2 ·8D 2 O (upper tick marks) and TiO 2 (lower tick marks) in D 2 O and (b) for the barium titanate produced by heating the mixture at 125 °C for 12 h (lower tick marks are unreacted TiO 2 ). The points are the experimental data, the full line the result of the whole pattern fitting, and the lower line the difference curve. This journal is © The Royal Society of Chemistry 2000 DOI: 10.1039/b003386n Chem. Commun., 2000, 1267–1268 1267