Structural characterization of titania by X-ray diffraction, photoacoustic, Raman spectroscopy and electron paramagnetic resonance spectroscopy

Abstract

A titania mineral (obtained from East coast, Orissa, India) was investigated by X-ray diffraction (XRD), photoacoustic spectroscopy (PAS), Raman and Electron Paramagnetic Resonance (EPR) studies. XRD studies indicated the presence of rutile (91%) and anatase (9%) phases in the mineral. Raman investigation supported this information. Both rutile and anatase phases have tetragonal structure (rutile: space group P42/mnm, a=4.5946(1) Å, c=2.9597(1) Å, V=62.48(1) (Å)3, Z=2; anatase: space group I41/amd, 3.7848(2) Å, 9.5098(11) Å, V=136.22(2) (Å)3, Z=4). The deconvoluted PAS spectrum showed nine peaks around 335, 370, 415,485, 555, 605, 659, 690,730 and 785nm and according to the ligand field theory, these peaks were attributed to the presence of V4+, Cr3+, Mn4+ and Fe3+ species. EPR studies revealed the presence of transition metal ions V4+(d1), Cr3+(d3), Mn4+(d3) and Fe3+(d5) at Ti4+ sites. The EPR spectra are characterized by very large crystal filed splitting (D term) and orthorhombic distortion term (E term) for multiple electron system (s>1) suggesting that the transition metal ions substitute the Ti4+ in the lattice which is situated in distorted octahedral coordination of oxygen. The possible reasons for observation of unusually large D and E term in the EPR spectra of transition metal ions (S=3/2 and 5/2) are discussed.