Hyperfine structure of LiNbO₃:⁵⁷Fe(III)

Abstract

Lithium niobate monocrystals exhibit many interesting physical phenomena such as ferro-, piezo- and pyroelectricity, low acoustical losses, small optical absorption in the visible region and, especialy after doping with small amounts of transition metal ions, strong photorefraction and photovoltaic effects [1–3]. According to the multitude of the properties there are numerous applications of LiNbO₃ for e.g. light modulation, Q-switch, second harmonic generation, fibre optics, acoustic transducers, pyroelectric detectors and holographic data storage. In order to understand in detail the microscopic mechanisms of the bulk photovoltaic effect and the photorefraction, which are strongly enhanced by doping the crystals with iron, an exact knowledge of the not yet unambigously known lattice site and the charge compensation [4,5] as well as the electronic structure of the iron impurities is indispensable. Here the Mössbauer investigation of the hyperfine interaction of⁵⁷Fe ions probing the crystal field may contribute to clarification.

As can be seen from the isomer shift iron enters the LiNbO₃-lattice as Fe(II) or Fe(III) [5]. By annealing the samples in oxydizing atmospheres (e.g. ≈p24 hours in air ≈p600‡C) all iron is transformed to Fe(III).