SolidStateIonics66 (1993) 151-158 North-Holland SOLID STATE IOWICS Electrical characterisation of bismuth orthovanadate using ac impedance spectroscopy P. Wood Johnson Matthey Technology Centre, Blount’s Court, Sonning Common, Reading RG4 9NH. UK D.C. Sinclair IRC in Superconductivity, University of Cambridge, Madingley Road, Cambridge CB3 OHE, UK and F.P. Glasser Chemistry Department, University ofAberdeen. Meston Walk, Aberdeen AB9 2UE, UK Received 22 April 1993; accepted for publication 24 June 1993 Polycrystalline phase pure BiV , 025 0 4+X ceramics were prepared by solid state synthesis. These materials were found to exhibit mixed oxide ion/electronic conduction from ac impedance measurements recorded between 150-800°C in various atmospheres. The results indicate predominantly electronic conduction at both low and high temperatures i.e. i 400°C and > 600°C. Oxide ion conduction was found to dominate the temperature region between 400 and 600°C with an activation energy of 0.80 k 0.01 eV. The bulk conductivity below 400°C was found to be metastable and influenced by the presence of water. The possibility of protonic conduction at low temperatures is therefore identified. 1. Introduction Bismuth orthovanadate, BiV04 exists in three po- lymorphic modifications; one monoclinic and two tetragonal forms, of fergusonite, scheelite and zircon structure types respectively. The zircon polymorph is only obtained by precipitation routes [ 1,2] whereas the fergusonite polymorph can be prepared by both precipitation and solid state techniques [ l-41. A re- versible ferroelastic phase transition occurs, to the scheelite polymorph, on heating the fergusonite po- lymorph at ca. 255°C [5,6]. The scheelite poly- morph is often referred to as the high temperature polymorph as it is unstable at room temperature in the stoichiometric form. Research into BiV04 has concentrated mainly on the fergusonite polymorph and, in particular, its fer- roelastic phase transition. Although it has been widely studied for pigment [ 781 and catalytic applications [ 9,101, there are relatively few reports regarding electrical characterisation [ 1 l-141. BiV04 is found to be a complex electrical material, exhibiting both oxide ion and electronic conduction. Wide discrep- ancies however arise over the precise nature and temperature range of the mixed conductivity; Lu and Steele [ 111 report p-type ( > 350°C), whereas Hi- rota et al. [ 121 report only oxide ion conductivity ( 150-550°C). Detailed electrochemical studies, in- cluding Seebeck and ac impedance measurements have been carried out above 600°C by Vinke et al. [ 131 in which n-type mixed conduction was identified. In the present paper we report ac impedance data for the temperature range 150-800°C in atmo- spheres of air, argon and oxygen, with attention paid to equilibration times at low temperatures. Further- more, interference in the intrinsic conductivity due to water adsorption below 400°C is reported for the first time. 0167-2738/93/$06.00 0 1993 Elsevier Science Publishers B.V. All rights reserved.