Semiempirical Correlation between the Optical Band Gap of Oxides and Hydroxides and the Electronegativity of Their Const
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Semiempirical Correlation between the Optical Band Gap of Oxides and Hydroxides and the Electronegativity of Their Constituents
Francesco Di Quarto, Monica Santamaria, Salvatore Piazza and Carmelo Sunseri Dipartimento di Ingegneria Chimica dei Processi e dei Materiali, Università di Palermo, Viale delle Scienze, 90128 Palermo (Italy) ABSTRACT On the basis of new experimental results a previous proposed correlation between the optical band gap of oxides and the difference of electronegativity of their constituents is extended to mixed crystalline and amorphous TiO2-Fe2O3 (d,d-metal oxides) as well as to amorphous passive films grown on Al-Ta, Al-Ti, Al-Nb and Al-W alloys (sp,d-metal oxides). Moreover in analogy with previous results on anhydrous oxides, a correlation is proposed between the optical band gap of hydroxides and the electronegativities of their constituents after substituting the electronegativity of hydroxilic group to that of the oxygen. Like in the case of anhydrous oxides, two different interpolation lines have been found for sp-metal and d-metal hydroxides, respectively. INTRODUCTION In recent works [1-3] we have shown that it is possible to make a more quantitative use of Photocurrent Spectroscopy (PCS) in order to characterize corrosion films grown on metals. Such a quantitative approach is based on a semiempirical correlation between the value of optical band gap of semiconducting or insulating oxides, MOy and the Pauling electronegativities of metal, χM, and oxygen, χO, [4]. By plotting the experimental band gap values, Eg, vs (χM-χO)2, two different interpolating lines were obtained for s,p-metal and transitional d-metal oxides, with few exceptions [4] for both types of oxides. The best fitting equations for the two cases are: Eg (eV) = 1.35 (χM-χO)2- 1.49
(1a)
Eg (eV) = 2.17 (χM-χO)2- 2.71
(1b)
for d-metal oxides, and:
for sp-metal oxides. In all calculations the Pauling scale of electronegativity, integrated with the Gordy-Thomas values [5], was used with the exception of Tl(III) for which the value given by Allred [6] has been preferred. It was also suggested [4] that equations 1 could be extended to ternary oxides, AaBbOo, containing only d,d-metal or sp,sp-metal by substituting to χM the arithmetic mean for the cationic group, χav, defined as:
AA4.8.1
χ av =
a χA + b χB a +b
(2)
were a and b are the stoichiometric coefficients of the cations in the ternary oxide, and χA and χB their electronegativities. In the case of mixed sp,d-metal oxides it was found that the d-metal correlation was able to fit quantitatively the data reported in the literature for some mixed oxides. Due to the limited number of systems it remained unsolved the limits of applicability of such correlation to the sp,d-metal mixed oxides. We have to mention that in the case of mixed oxides containing both sp and d-metal eq. 1a seems to hold, provided that the difference in the electronegativity values between the metallic cations is less than 0.5 [4]. In this work we present some results on both crystalline an
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