Paweł Gnutek

Crystal field analysis of the energy level structure of Cs2NaAlF6:Cr3+

An analysis of the energy level structure of Cr3+ ions in Cs2NaAlF6 crystal is performed using the exchange charge model (ECM) together with the crystal field analysis/microscopic spin Hamiltonian (CFA/MSH) computer package. Utilizing the crystal structure data, our approach enables modelling of the crystal field parameters (CFPs) and thus the energy level structure for Cr3+ ions at the two crystallographically inequivalent sites in Cs2NaAlF6. Using the ECM initial adjustment procedure, the CFPs are calculated in the crystallographic axis system centred at the Cr3+ ion at each site. Additionally the CFPs are also calculated using the superposition model (SPM). The ECM and SPM predicted CFP values match very well. Consideration of the symmetry aspects for the so-obtained CFP datasets reveals that the latter axis system matches the symmetry-adapted axis system related directly to the six Cr?F bonds well. Using the ECM predicted CFPs as an input for the CFA/MSH package, the complete energy level schemes are calculated for Cr3+ ions at the two sites. Comparison of the theoretical results with the experimental spectroscopic data yields satisfactory agreement. Our results confirm that the actual symmetry at both impurity sites I and II in the Cs2NaAlF6:Cr3+ system is trigonal D3d. The ECM predicted CFPs may be used as the initial (starting) parameters for simulations and fittings of the energy levels for Cr3+ ions in structurally similar hosts.

Low symmetry aspects inherent in EMR studies of the orthorhombic to monoclinic structural phase transition in the hexagonal form of barium titanate BaTiO3 doped by Fe3+ ions

Electron magnetic resonance (EMR) studies reveal different spectroscopic properties of transition ions doped in the two crystallographically different forms of barium titanate: cubic (normal) c-BaTiO3 and hexagonal polymorph h-BaTiO3. Recent comparative analysis of EMR data helped to solve the controversy concerning the disparate zero-field splitting (ZFS) parameters for Fe3+ ions in c-BaTiO3. This paper deals with the low symmetry aspects inherent in EMR studies of the orthorhombic to monoclinic structural phase transition in h-BaTiO3 doped by Fe3+ ions. Pertinent spin Hamiltonian notations and choices of axis systems are clarified. The second- and fourth-rank ZFS parameters determined by EMR and the second-rank ones computed using a superposition model for the Fe3+ ions in h-BaTiO3 are reanalyzed. The available ZFS parameters are presented in a well-defined axis system and in a unified way to ensure meaningful comparison. Pertinent transformations of ZFS parameters are carried out using the package CST. Simulations of the low symmetry ZFS parameters are carried out to assess the role of monoclinic and triclinic ZFS terms and to investigate the low symmetry aspects arising with lowering of temperature during the orthorhombic to monoclinic structural phase transition in Fe3+:h-BaTiO3. The procedure for analyzing experimental and theoretical ZFS parameters for transition ions at monoclinic and triclinic symmetry sites proposed here enables a better understanding of the low symmetry aspects involved. This study suggests the need to extend superposition model analysis to the fourth-rank ZFS terms for Fe3+ centers in h-BaTiO3.