Magnetic and thermal induced ordering in V 2 O 5 xerogel films Carlos A. Brunello, Carlos F.O. Graeff * Departamento de F  ısica e Matem atica, FFCLRP-USP, Av. Bandeirantes 3900, 14040-901 Ribeir~ ao Preto (SP), Brazil Abstract In this work we investigated the possibility of using magnetic fields and thermal treatments to induce ordering in V 2 O 5 xerogels in film form. The xerogel was prepared by standard procedures. The films were prepared either by dip- coating or casting. X-ray powder diffraction, cyclic voltammetry as well as conductivity were used to monitor the changes. We found that the freshly made films are unstable, needing at least 48 h of rest at room temperature to have reproducible X-ray diffractograms, as well as conductivities. The changes are attributed to a decrease in film water content. Magnetic field treatments of several hours were observed to increase the ordering of the V 2 O 5 ribbons, as well as the conductivity. For freshly deposited films the effects were larger than those in films left for a few days at room temperature. The electrochemical response of films exposed to magnetic fields did also show an increase in surface homogeneity. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 61.10.)I; 72.20.)I; 76.30.)v; 81.20.Fw; 82.80.Fk 1. Introduction Among its various properties, V 2 O 5 has a lamellar structure, the [V 2 O 5 ] layers being built up from VO 5 square pyramids sharing edges and corners [1,2]. The sheets are held together via weak interactions between the vanadium atoms and the oxygen atoms at the apex of the VO 5 square pyr- amids of neighboring layers. The lamellar struc- ture of V 2 O 5 is adequate for intercalation, and thus inspired activity in the field of hybrid organic/ inorganic materials. These new hybrid compound materials are considered as innovative materials which may have applications in many fields such as optics, electronics, ionics, mechanics and biol- ogy [3,4]. These colloids are comprised of ribbons 1 nm thick, about 25 nm wide, and about 300 nm long, whose structure is related to that of ortho- rhombic V 2 O 5 [5]. Suspensions from these material form a lyotropic nematic phase [6]. These materials are therefore one of the very few examples of mineral liquid crystals [7]. An outstanding feature of the V 2 O 5 nematic sols is that they can be aligned in magnetic fields [6,8]. The experiments involving magnetic field alignment required very fluid, and therefore dilute, nematic suspensions. In this work we will analyze totally different conditions, and show ordering effects induced by exposing vana- dium pentoxide xerogel films to magnetic fields. The effects that we will be showing are comparable to those occurring during the aging of the xerogel films. As a consequence aging effects will also be discussed. Journal of Non-Crystalline Solids 304 (2002) 265–270 www.elsevier.com/locate/jnoncrysol * Corresponding author. Tel.: +55-16 602 3763; fax: +55-16 633 9949. E-mail address: cgraeff@dfm.ffclrp.usp.br (C.F.O. Graeff). 0022-3093/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII:S0022-3093(02)01033-5