arXiv subjects
Publications and source records attributed to Th. W. Kool.
A specialized stress equipment has been developed for EPR X-band experiments. Uniaxial stress experiments in EPR of different impurity ions in SrTiO3 and BaTiO3 will be reviewed.
The 105 K second-order displacive phase transition of SrTiO3 has been studied with the help of Electron Paramagnetic Resonance. The photochromic non-cubic Fe5+ centre is used as local probe. Critical phenomena, characterized by an exponent β = 1/3, are presented. Line broadening effects are interpreted as stemming from time-dependent fluctuations near the phase transition point. The data are consistent with those reported earlier on the Fe3+-VO pair centre, indicating cooperative effects in the crystal. Also a model for the non-cubic Fe5+ is proposed, i.e., the ion is substitutional for Ti4+ with an empty adjacent expanded octahedron. Other Fe5+ centres in SrTiO3 and BaTiO3 are reviewed and reinterpreted.
Third order perturbation theory is used to calculate the orthorhombic spin-Hamiltonian zero-field E-term for octahedrally surrounded d3 spin systems (S = 3/2) in strong axial fields in the presence of a weak orthorhombic component.
Numerical and analytical methods are used to investigate the calculation of the zero field splitting |2D| and g(perp) parameters in EPR for octahedrally surrounded d3 spin systems (S = 3/2) in strong and moderate axial crystal fields (|D|>=hν). Exact numerical computer calculations are compared with analytical results obtained from third-order perturbation theory. From the analyses we conclude that EPR measurements performed at a single frequency with the magnetic field H at a magic angle α_M, where 62deg < α_M < 63deg, with respect to the axial crystal field of the d3 spin system, yields an almost exact solution in third-order perturbation theory. For dual frequency experiments, i.e. X-K, X-Q and K-Q band experiments, performed with the magnetic field H at an angle of α = 90deg with respect to the axial crystal field, the ratio h\u{psion}/|2D| has to be smaller than 0.25 in order to use third order perturbation calculations within an error limit of 0.020% in the g(perp) values. For values of h\u{psion}/|2D| >= 0.25 one has to proceed with exact numerical computer calculations. Finally, we conclude that measurements performed at a single EPR frequency experiment with the magnetic field H directed along two specific angles with respect to the axial crystal field of the octahedrally surrounded d3 centre, i.e. α = 90deg and α = 35deg16 respectively, third-order perturbation theory gives non-reliable results for the |D| and g(perp)-values.
In single crystals of SrTiO_3, a paramagnetic center, characterized by S = 3/2 and hyperfine interaction with an I = 5/2 nuclear spin has been observed in the temperature range 4.2K-77K by means of EPR. The impurity center is attributed to Mo3+. No additional line splitting in the EPR spectrum due to the 105K phase transition has been observed. At 4.2K the following spin Hamiltonian parameters for this impurity ion were obtained: g = 1.9546\pm0.0010 and A = (32.0\pm0.05)\times10^-4 cm^-1.