Materials Chemistry and Physics 50 (1997) 267–270 0254-0584/97/$17.00 q 1997 Elsevier Science S.A. All rights reserved PII S0254-0584 ( 97 ) 01940-8 Materials Science Communication IR spectra of VO 2 and V 2 O 3 I.L. Botto a,U , M.B. Vassallo a , E.J. Baran a , G. Minelli b a Centro de Quimica Inorganica (CEQUINOR), Facultad de Ciencias Exactas, Universidad Nacional de La Plata, C. Correo 962, La Plata 1900, Argentina b Centro del Consiglio Nazionale delle Richerche su ‘Struttura ed Attivita Catalitica di Sistemi di Ossidi’, Universita di Roma, 00185 Rome, Italy Received 17 October 1996; revised 4 March 1997; accepted 4 March 1997 Abstract IR spectra of the tetragonal modification of VO 2 and of the trigonal form of V 2 O 3 are recorded at room temperature and compared with that of V 2 O 5 . The investigated samples of the two lower-valent vanadium oxides, obtained on temperature-programmed reduction treatment, were also characterized with diffuse reflectance and electron-paramagnetic resonance spectra. The effect of atmospheric oxygen on these materials was revealed with XPS measurements and also studied with IR spectra. q 1997 Elsevier Science S.A. Keywords: Vanadium oxides; IR spectra; Diffuse reflectance spectra; Surface oxidation 1. Introduction Although VO 2 and V 2 O 3 oxides have been extensively investigated, information about their spectral (in particular vibrational) behaviour is scarce. Both oxides are of theoret- ical and practical interest in several fields of materials science, specially because of their electrical applications and because of their relation to V 2 O 5 , a widely employed catalyst for oxidation reactions. The V(IV) oxide, VO 2 , presents structures derived from that of rutile, with the metal in an octahedral oxygen co- ordination. The high temperature form shows a tetragonal structure (space group P4 2 /mnm) [1], whereas the low tem- perature modification presents a distorted monoclinic struc- ture (space group P2 1 c) [2], similar to that of MoO 2 . The trivalent oxide, V 2 O 3 , is also dimorphic. The high- temperature form has the trigonal corundum structure (space group R c) [3], whereas below 150 K it exists as a mono- # 3 clinic distorted form [4]. In both cases, the metal possesses an octahedral VO 6 arrangement. Difficulties in obtaining well resolved vibrational spectra are expected as these oxides, like other low-valent transition metal oxides, show metallic or free-carrier behaviour. This originates in small separations between the nearest-neighbour metallic ions [5], and in the possibility of direct t 2g –t 2g inter- actions between the MO 6 groups. In this paper we present room-temperature IR spectra of tetragonal VO 2 and trigonal V 2 O 3 phases. The effect of U Corresponding author. Fax: q54 (21) 259 485. atmospheric oxygen on the stability of these materials has also been analysed. 2. Experimental The oxides investigated were obtained from temperature- programmed reduction (TPR)[6] experiments, under low H 2 pressures. TPR measurements were made with a home- made reactor and hydrogen as the reducing agent (10% H 2 in N 2 ). The heating rate was 58C min y1 and consumed H 2 was detected with a thermal conductivity cell. In these expe- riences the consumption of H 2 is plotted against the temper- ature. The starting oxide was a pure V 2 O 5 sample generated on thermal treatment of NH 4 VO 3 (Carlo Erba) at 5008C. All products were carefully characterized with X-ray powder dif- fractometry (XRPD), diffuse reflectance spectra (DRS) and electron paramagnetic resonance (EPR) spectra. Surface characterization was accomplished with additional measure- ments of X-ray photoelectron spectra (XPS). XRPD measurements were performed with a Philips PW 1714 diffractometer, using Ni-filtered Cu Ka radiation. DRS were recorded on a Cary 2300 spectrometer equipped with an IBM P52 computer for data acquisition. MgO was used as a reference. X-band EPR spectra were recorded at room temperature (RT)(293 K) and at liquid nitrogen temperature (LNT) on a Varian E-9 spectrometer, equipped with an on-line com- puter for data treatment. The magnetic field was calibrated with DPPH as a field marker.