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JOURNAL OF MATERIALS SCIENCE 33 (1998) 4173–4180Effects of heating in air and chlorine atmosphereon the crystalline structure of pure Ta 2 O 5or mixed with carbonJ. GONZA¤ LEZ, M. DEL C. RUIZ, J. B. RIVAROLAInstituto de Investigaciones en Tecnologia Qu?´mica, (Universidad Nacional de SanLuis-CONICET), Chacabuco y Pedemera, C.C. 290, 5700 San Luis, ArgentinaE-mail: intequi.unsl.edu.avD. PASQUEVICHCentro Ato&mico Bariloche, CNEA, ArgentinaStructure changes undergone by pure amorphous hydrated tantalum oxide mixed withdifferent types of carbon when heated in air or chlorine atmospheres were monitored byX-ray diffraction (XRD) and scanning electronic microscopy (SEM). Heating in air of pureTa 2 O 5 causes the appearance of the hexagonal structure d-Ta 2 O 5 at 973 K and the ortho-rhombic structure b-Ta 2 O 5 at 1173 K. Heating in chlorine atmosphere markedly lowers thetemperature at which transformation to the orthorhombic phase occurs. This effect isattributed to recrystallization of tantalum oxide from tantalum chloride and oxygen, both ingaseous phase, formed in a previous chlorination step of the amorphous oxide. When thethermal treatment is performed in chlorine atmosphere the presence of carbon permits thedetectionof the hexagonalform at753 K; this temperaturevarieswith the typeof carbonandthe oxide:carbon ration. ? 1998 Kluwer Academic Publishers1. IntroductionTantalum oxides can be present in different crystallinestructures. The existence of each structure depends onvaried factors. Terao [1] studied by X-ray diffraction(XRD) the structural changes suffered by tantalumoxides when heated in oxygen and air and identifiedvarious forms of these oxides, among them an hexag-onal structure d-Ta 2 O 5 obtained at low temperatureswhen amorphous Ta 2 O 5 is heated in air. This hexag-onal structure has also been found by Jeon et al., whenfilms of amorphous Ta 2 O 5 were treated at 1073 K [2].Harvey and Wilman [3] investigated by electron dif-fraction (ED) the effect of heat on thin films ofamorphous tantalum oxide and observed that a cry-stalline phase of hexagonal form occurs above 973 Kand that further heating leads to an orthorhombicstructure, b-Ta 2 O 5 .Bansal [4] investigated by means of XRD, differen-tial thermal analysis (DTA), thermogravimetric analy-sis (TGA) and infrared spectroscopy (i.r.) the phasetransformations undergone by an amorphous Ta 2 O 5gel underthe effect of airheating.This author observedthat the orthorhombic structure forms at 673 K with ashort-range order, and that further heating to 1723 Kleads to the tetragonal structure a-Ta 2 O 5 , the latterbeing a slow reversible process.Ta(V) oxide occurs in two different stable forms:an orthorhombic structure b-Ta 2 O 5 and a tetragonalstructure a-Ta 2 O 5 , for low and high temperatures,respectively. The transition between these two formstakes place at 1633 K [5—7].Amorphous hydrated Ta(V) oxide (Ta 2 O 5 .nH 2 O)can be obtainedby different procedures,suchas TaCl 5hydrolysis or fusion of oxides with KHSO 4 followedby hydrolysis. This hydrated oxide is insoluble inwater and after being dried off it keeps approximately8% of water, which is eliminted by heating at hightemperatures [8,9].The purpose of the present work was to examine byXRD and scanning electron microscopy (SEM) the for-mation of different low-temperature crystalline struc-tures from amorphous hydrated Ta 2 O 5 heated in airand chlorine atmospheres and the effect of carbonwhen this oxide is heated in chlorine current.2. Experimental procedure2.1. MaterialsThe starting material was an amorphous hydratedtantalum oxide, of particle size between 5 and 100 lm,obtained by fusion of an orthorhombic Ta 2 O 5(Fluka AG, CH-9470 Buchs) with KHSO 4 . Thefusion was carried out in a Pt capsule and then anadequate volume of hot water was added to the fusedmaterial.The gel was filtered and washed until completeelimination of potassium. The solid obtained wasdried at 423 K to weight constancy and then analysed0022—2461 ( 1998 Kluwer Academic Publishers 4173