STRUCTURAL AND THERMAL PARAMETERS FOR RUTILE AND ANATASE

Neutron powder diffraction patterns from rutile (TiO2, P4(2)/mnm, a = 4.594, c = 2.959 angstrom) and anatase (TiO2, I4(1)/amd, a = 3.785, c = 9.514 angstrom) have been analysed by the Rietveld method. The positional parameters were determined to be x(O) = y(O) = 0.30478 (6) in rutile and z(O) = 0.20806 (5) in anatase. The anisotropic thermal parameters were also determined. The results from this constant-wavelength neutron diffraction study are in remarkably good agreement with the results from a recently published analysis of time-of-flight neutron data from the same phases. A comparison is made with results from earlier X-ray single-crystal investigations of these polymorphs of titania, and again the agreement is good. In particular, there is no evidence for any significant difference between X-ray and neutron determinations of the oxygen position such as might have resulted from polarization effects. The thermal vibrations show marked anisotropy, which appears to be determined by the stereochemistry of the crystal structure. The Debye temperatures estimated from the diffraction data are 600 (10) K for rutile and 520 (10) K for anatase at room temperature.