High-Temperature X-ray Diffraction and Dilatometric Studies on some Oxygen Ion Conducting Compounds

Abstract

The compounds Bi4V2O11 and Bi4V2(1yx)(Me‡n)2x O11 a (BIMEVOX), where Me is a selected alliovalent or isovalent cation, are the members of aurivillius phase. These compounds are shown to be the best oxygen ion conductors [1, 2] reported so far. Since their advent, many studies have been performed on these compounds. The differential thermal analysis=differential scanning calorimetry (DTA= DSC) studies on Bi4V2O11 reveal that it exists in three different modi®cations before melting, namely a (orthorhombic-I), â (orthorhombic-II) and a (tetragonal). The high-temperature a modi®cation is tetragonal and exhibits high electrical conductivity. The transitions a to â and â to a take place at 720 and 840 K, respectively, and both are reversible. The a and â forms are the ordered superstructure of the disordered a form. The parent Bi4V2O11 structure is still highly controversial. Abraham et al. [1] reported it to have an orthorhombic unit cell with lattice parameters a ˆ 0:5533, b ˆ 0:5611 and c ˆ 1:5288 nm, and Varma et al. [3] explained this based on an edgecentered orthorhombic cell. It was later found that the unit cell is centrosymmetrically monoclinic [4]. However, both the authors [3, 4] agreed that the two unit cells are closely related to each other. Recently, it was found that the structure of Bi4V2O11 is highly sensitive to the kind and purity of the reactants used [5]. Bi4V2O11 is known to lose oxygen, but the phase transitions are not due to this fact as long as the loss or gain of oxygen is not very signi®cant [4]. Many reports have established that the tetragonal a form can be stabilized at room-temperature by suitable metal ion doping at the vanadium site [6, 7]. The ionic conductivity of the BIMEVOX compounds depends on the oxygen vacancy concentration and also on the ionic potential of the substituting cation [8]. An optimum doping of about 10% copper results in the highest conductivity in BIMEVOX oxide [2]. The room-temperature structure (a) of BIMEVOX is a partially ordered superstructure which undergoes a transition to a completely disordered structure (a9) at higher temperatures. It is the a9 form that shows the highest conductivity in BIMEVOX compounds. Although the detailed physical properties like electrical conductivity on these compounds are well known, there are no reports on the structural aspects of the high-temperature phases. There is one report on the thermal expansion of Bi4V2O11 [4], while in the case of Cu-doped samples only a densi®cation study [6] has been reported. Because these compounds conduct only at high temperatures, it is worthwhile to know their thermal behavior in detail, and we therefore undertook the bulk expansion study using dilatometry, unit cell expansion and phase transition studies using high-temperature X-ray diffraction (HTXRD) on the Bi4V2O11 and Bi4V1:8Cu0:2O11ya. Stoichiometric amounts of Bi2O3, V2O5 and CuO (all LR grade) were thoroughly ground, pelletized and heated in static air at 923 K for 20 h with one intermittent grinding to get brown-colored products. The pellets were reground, sintered at 1095 K for 15 h and then cooled to room temperature at the rate of 3 K miny1. The XRD patterns were recorded on a Philips X-ray diffractometer (Model PW 1710) with Ni-®ltered CuKa radiation using silicon as an external standard. The HTXRD studies were carried out using a MRC model X-86-N3 high-temperature diffractometric attachment [11]. Each compound was mounted on a Pt-40%Rh stage, which also served as a resistance heater. The temperature measurement was done using a Pt=Pt-13%Rh thermocouple spotwelded to the bottom of the stage=heater. The accuracy of temperature measurement was about 5 K. The lattice parameters at different temperatures were determined by the least-square re®nement program POWDERX [12]. The dilatometric measurement was performed on a Model LKB 3185 fused quartz push rod dilatometer using sintered cylindrical pellets with about 70% theoretical density. The ®rst heat treatment at 923 K for 12 h resulted in the formation of the desired compounds. To ensure the absence of the last trace of reactants beyond the detection limit of XRD, however, a second heating at 923 K for 8 h was also performed. The XRD pattern of Bi4V2O11 (Fig. 1) shows the characteristic doublets at 2e 318, 398, 488, 548 and a very weak re ection at 2e 24:28. We attribute this weak re ection to the superstructure of the aBi4V2O11 [2]. All the observed re ections excluding this weak re ection could be indexed on an orthorhombic unit cell, and this unit cell was considered as the mean or basis cell of this a-Bi4V2O11. Thus, the mean cell parameters for this phase we found as am ˆ 0:5530 0:0003, bm ˆ 0:5606 0:0002 and cm ˆ 1:5282 0:0007 nm (where m is the mean cell). To include this weak superstructure re ection, a larger unit cell with dimensions of 3 3 am, bm and