X-ray absorption spectroscopy investigation of structurally modified lithium niobate crystals [Elektronische Ressource] / von Tonya Vitova. Universität Bonn, Physikalisches Institut

..UNIVERSITAT BONNPhysikalisches InstitutX-ray absorption spectroscopy investigation ofstructurally modified lithium niobate crystalsvonTonya VitovaThetypeandconcentrationofimpuritycentersindifferentvalencestatesare crucial for tuning the photorefractive properties of doped LithiumNiobate (LN) crystals. X-ray Absorption Spectroscopy (XAS) is an ap-propriate tool for studying the local structure of impurity centers. XAScombined with absorption in UV/VIS/IR and High Resolution X-rayEmission Spectroscopy (HRXES) provide information about the valencestate of the dopant ions in as-grown, reduced or oxidized doped LN crys-1+ 2+ 2+ 3+tals. Cu (Cu and Cu ) and Fe (Fe and Fe ) atoms are found in twodifferent valence states, whereas there are indications for a third Mn va-2+ 3+lency, in addition to Mn and Mn in manganese-doped LN crystals.One of the charge compensation mechanisms during reduction of copper-doped LN crystals is outgassing of oxygen atoms. Cu ions in the reduced1+crystals have at least two different site symmetries: twofold (Cu ) and2+sixfold(Cu )coordinatedbyOatoms.FeandMnatomsarecoordinatedbysixOatoms.CuandFeionsarefoundtooccupy onlyLisites,whereasMn ions are also incorporated into Li and Nb sites. The refractive index3 2+change in LNcrystals irradiatedwith He ionsiscaused by structurallydisordered centers, where Nb atoms are displaced from normal crystal-lographic sites and Li or/and O vacancies are present.