Rutile (TiO2) belongs to P42mnm space group. There are two possible sites for impurity cations, i.e., substitutional (regular lattice positions with slightly distorted octahedral surrounding) and interstitial (tetrahedral
surrounding), the latter forming channels parallel to crystallografic c-axis.
The channel structure of TiO2 allows for very fast diffusion of impurity cations.
Emission Mössbauer experiments on rutile single crystal doped with 50 p.p.m. of 57Co were performed. Very high purity sample of 100 microns thickness was used for this purpose. Measurements were carried out either at HT (up to ca. 1400 K) or RT (after subsequent quench from HT) and vs. crystal orientation as well. The sample was kept under flowing gaseous mixture (20% O2 / 80% He) at normal pressure.
Iron following Co EC decay was found to occupy interstitials (channels) and substitutionals (lattice sites) both. The charge state of Fe depends on the local impurity surrounding. There are two possible configurations for lattice and channel at least. Lattice/channel can be ideal or disturbed by nearby oxygen vacancies at the vicinity of impurity. The abrupt quench traps Co2+ diffusing along the open channels (via interstitialcy mechanism) into interstitial or substitutional sites. Parent Co trapped into free of intrinsic defects channel decays to exotic Fe1+(S=3/2) in 3d7 configuration, see Fig. 1 below.
|Fig. 1. Mössbauer spectra vs. temperature of the annealed and quenched sample showing disappearance of the channel components due to the very fast diffusivity.|
Isomer shift of monovalent iron equals -2.1 mm/s and indicates very low electronic density at the Fe nucleus. Fe1+ represents a metastable state. It could be observed due to appropriate time window of the emission Mössbauer spectroscopy. The exotic Fe state is present at temperatures not exceeding ca. 400 K due to enormous channel diffusivity (see Fig. 2 for details). Cobalt impurity "frozen" in a channel site disturbed by oxygen vacancy at neighborhood becomes Fe2+(S=0). This component disappears at elevated temperatures as well. EC decay converts the parent Co2+ located at regular lattice positions either to Fe3+(S=5/2) or to Fe2+(S=2) daughter ions. Substitutional Co decays to Fe3+ in undisturbed (at least locally ideal) octahedral surrounding. Defects induced in the crystal to preserve charge neutrality are extrinsic, i.e., oxygen vacancies which accompany daughter Fe3+ are at the next-nearest-neighbor oxygen positions at least. Co residing in lattice sites and being disturbed by nearby oxygen vacancies converts to Fe2+. The non-equivalent iron-vacancy complexes are possible.
Iron following 57Co decay was observed solely in the host lattice. Divalent and trivalent Fe were found. Fe2+ is present in the vicinity of defects. It exists in two spin states, i.e., with S=2 and S=0. The high spin
Fe2+ converts gradually to low spin state with the increasing temperature. Fe3+ with S=5/2 resides in unperturbed lattice site. A host matrix becomes more covalent at very high temperatures and slightly anharmonic as well. No
significant diffusivity of substitutional Fe could be observed.
The total area under the spectrum follows the unusual pattern (see Fig. 2) due to the gradual disappearance of the signal coming from the iron located in a channel, i.e., a transfer of Fe atoms into fast diffusing interstitials with the increasing temperature occurs (see Fig. 1). More details could be found in Ref. .
|Fig. 2. Effective MSD vs. temperature. The highest temperature point (open circle) was excluded from the fitting procedure due to the onset of anharmonicity.|
|||U.D.Wdowik and K.Ruebenbauer, Phys. Rev. B 63, 125101 (2001) - see PRB www page|