GAUSS-LORENTZ SIZE-STRAIN BROADENNING AND CELL PARAMETERS ANALYSIS OF Mn DOPED SnO₂ PREPARED BY ORGANIC ROUTE

Carlos O. Paiva-Santos, Helder Gouveia, Wanda Las, Jose Arana Varela

Dep. Físico-Química, Instituto de Química - UNESP, R. Prof. Francisco Degni s/n., 14800-900 Araraquara SP Brazil
Email: copsanto@iq.unesp.br

Keywords: Crystallite size, microstrain, lattice parameters

SnO₂ samples doped with x-mol% Mn (x = 0, 0.3, 0.5, 0.7, 1.0) were prepared by organic route, calcined at 800^oC for 4h, and characterized by the Rietveld method with X-ray diffraction data. The Thompson-Cox-Hastings pseudo-Voigt profile function was used as it is in the DBWS 9411 Rietveld analysis software. For the FWHM, were refined only the Gauss and Lorentz coefficients that can be related to size and strain, while the others were kept fixed in the values reached for a WC standard. The Gauss-strain, Lorentz-size and Lorentz-strain broadening coefficients present an almost uniform variation in respect to the Mn inclusion while the Gauss-size coefficient vary disorderly. The crystallite size determined with these coefficients varies uniformly for Lorentz broadening and higly non-uniform for Gauss broadening. The strain determined with the Lorentz coefficient are approximately 5 times smaller than when determined with Gauss coefficient.. The Lorentz and Gauss contributions for crystallite size (also for strain) were weighted in the FWHM formulae of the TCHZ pseudo-Voigt profile function, and used in the evaluation of the mean crystallite size. The crystallite size and strain so determined showed a uniform decrease in the crystallite size and increase in the microstrain with the addition of dopants. It was also observed that the unit cell volume decreases slightly as the amount of added dopant increases. For undoped sample the cell parameters are a = 4.73785(5) A and c = 3.18667(4) A and for 1mol% Mn doped SnO₂ the cell parameters are a = 4.73577(7) A and c = 3.18481(6) A. Based on the cell parameters' variation it is suggested that the Mn dopant occupies the same crystallographic site as Sn, in the SnO₂ crystal structure. Considering that the crystallite size decreases with increasing Mn content, this could explain Mn segregation on the grain boundary of sintered samples and the increase in conductivity observed elsewhere.

Acknowledgments: This work was performed with financial support from FAPESP and CNPq.