T. R. Volk

Ferroelectric properties of strontium barium niobate crystals doped with rare-earth metals

The influence of doping of the SrxBa1−xNb2O6 (x=0.61) crystals (SBN-0.61) by rare-earth element (RE) dopants has been investigated. Taking into account the data available in the literature, it is demonstrated that the introduction of all rare-earth metals is accompanied by a substantial decrease in the temperature of the phase transition Tp and an increase in its smearing. The shift in the Tp temperature for certain rare-earth metals ranges up to 20 K per atomic percent of dopant in the crystal. It is experimentally established that, in the SBN-0.61: Yb, SBN-0.61: Ce, SBN-0.61: Tm, SBN-0.61: La, and SBN-0.61: (Ce + La) crystals, a decrease in Tp brings about a considerable increase in the permittivity and the piezoelectric and electrooptical coefficients. The conclusion is drawn that the doping by rare-earth metals provides a means of optimizing the properties of strontium barium niobates. The pulse switching of the SBN-0.61 and SBN-0.61: RE crystals is studied for the first time. It is found that the switching is characterized by a number of features, the most important of which is a decrease in the switched charge after the application of external fields. This effect associated with the specific features of the switching in the relaxor ferroelectric is assumed to be responsible for the instability of the parameters for strontium barium niobate material.

Atomic structure of Sr0.75Ba0.25Nb2O6 single crystal and composition-structure-property relationin (Sr,Ba)Nb2O6 solid solutions

The structure of the Sr0.75Ba0.25Nb2O6 single crystal has been investigated by x-ray diffraction. The occupancies of the Ba and Sr sites in two structural channels are determined. It is found that these sites are split in the large (pentagonal) channel. A qualitative correlation is revealed between the smearing of the phase transition and the displacement of the Sr atom from the m symmetry plane in the pentagonal channel at different [Sr]/[Ba] ratios. The degree of acentricity of the NbO6 octahedra is analyzed as a function of the [Sr]/[Ba] ratio.

LiNbO3 with the damage-resistant impurity indium

We report on a detailed investigation (EPR, SHG, optical absorption, luminescence and Raman scattering) on the new damage-resistant impurity indium in LiNbO3, where the increased photoconductivity strongly reduces the photorefractive effect. EPR and optical absorption measurements point to a complete disappearance of the Nb antisite in LiNbO3: In for all In concentrations. We believe that the very effective driving out of Nb antisites by In is due to the trivalent charge state of In and the possibility of charge self-compensation. Similarities in the properties of Mg-, Zn- or Indoped samples are discussed. Simultaneous doping with In and Zn leads to an addition of both contributions, in particular for optical frequency doubling and luminescence. Raman studies prove that In does not improve the stoichiometry of the crystals. Indium doping provides the possibility to control simultaneously phase-matching conditions and to reduce drastically photorefraction. Therefore, In co-doped LiNbO3 compositions are promising materials for applications after solving contemporary growth problems.

Threshold concentrations in zinc-doped lithium niobate crystals and their structural conditionality

On the basis of precise X-ray diffraction study of lithium niobate single crystals of congruent composition and four zinc-doped (at 2.8, 5.2, 7.6, and 8.2 mol %) crystals, structural conditionality of the threshold concentrations of the dopant has been established. At these concentrations, the mechanism of zinc incorporation into crystal changes. As the zinc concentration increases, this element first substitutes excess niobium, localized in lithium positions, with a simultaneous decrease in the number of vacancies in these positions. Then zinc substitutes lithium with formation of new lithium vacancies. When a certain limit on the number of vacancies is reached, zinc begins to substitute niobium in its main positions. This process is naturally accompanied by a decrease in the number of vacancies to their complete disappearance and formation of a self-compensating crystal. The character of the dependence of the crystal physical properties on the dopant concentration changes specifically when the impurity concentration passes through the threshold values.

Hologram recording in Zn-doped LiNbO 3 crystals

Holographic recording in LiNbO3 doped with 2–5% ZnO is reported. High photoconductivities of these crystals result in high-speed recording. Holograms are formed by a combined mechanism including photovoltaic and diffusion fields simultaneously. The contribution from the photovoltaic mechanism increases with light intensity because of strong intensity dependence of the photovoltaic field. The observed values of the photovoltaic fields are higher than those calculated from the available data on photoconductivity and absorption coefficient. The enhanced values of the photovoltaic field are obviously related to the intensity dependence of the optical absorption coefficient that results from a multilevel charge-transport scheme in LiNbO3:Zn crystals.

Zn Atoms in Lithium Niobate and Mechanism of Their Insertion into Crystals

Precision X-ray structural studies were carried out for LiNbO3:Znx single crystals with x=0.0, 2.87, 5.20, and 7.60 at. %. It was found that the insertion of the Zn atoms into the Li position was accompanied by a decrease in the concentration of intrinsic NbLi defects. At x=7.6%, the Zn atoms change their locations in the lattice and partially occupy the Nb positions. This clarifies the structural nature of the “threshold” Zn concentration, which manifests itself as singularities in the concentration dependences of various optical properties. The structural origin of the threshold concentration is likely a common feature of all nonphotorefractive impurities (Mg, Zn, In, and Sc) in LiNbO3. A change in the intrinsic defect structure of the LiNbO3 crystals with different Zn concentrations is discussed.

Periodic domain structures formed under electron-beam irradiation in LiNbO3 plates and Ti:LiNbO3 planar waveguides of the Y cut

Regular domain structures on the Y cuts of the LiNbO3 substrates and Ti:LiNbO3 waveguide structures based on these substrates have been fabricated under electron-beam irradiation. It has been revealed that the domains in undoped and titanium-doped LiNbO3 crystals are formed as a result of different processes. It has been demonstrated using chemical etching and waveguide second-harmonic generation that regular domain structures in the Ti:LiNbO3 waveguides are formed at a depth of approximately 8 μm from the surface, where the titanium concentration does not exceed 2 mol %. The quasi-synchronous waveguide optical second harmonic generation with an efficiency of 8.8% has been obtained using the fabricated structures.

Effects of rare-earth impurity doping on the ferroelectric and photorefractive properties of strontium–barium niobate crystals

Abstract Doping strontium–barium niobate (SBN) crystals with rare earth (RE) impurities strongly lowers the ferroelectric phase transition T c which results in a significant enhancement of the dielectric permittivity and electrooptic coefficients and in a simultaneous decrease of the coercive field E c . In photorefractive SBN:Ce crystals co-doped with a non-photorefractive impurity La we studied the hysteresis dependence of the two-beam coupling gain Γ on the external field.

Ferroelectric phenomena in holographic properties of strontium-barium niobate crystals doped with rare-earth elements

Abstract We report on a relation between the two-beam coupling gain factor Γ in Sr0·61Ba0·39Nb2O6 (SBN:0·6) crystals and their ferroelectric properties. In SBN:0·6 doped with rare-earth-metal impurities Ce and Tm we found a hysteresis dependence of Γ versus the external dc-field E applied after recording which is due to the ferroelectric P-E hysteresis. Particularly, Γ-E hysteresis manifests itself in an alteration of the sign of Γ, i.e., of the direction of the energy transfer between the coupling beams, under applying of E reversing Ps. The value of Γ at room temperature essentially increases with impurity concentrations. It is caused by an increase of the dielectric permittivity, which in its turn is due to a significant lowering of the phase-transition temperature Tc produced by a rare-earth doping.

Study of ferroelectric domain switching by domain wall induced light scattering

The 90°-light scattering on domain walls was probed in various strontium barium niobate (SBN) crystals for studies of the ferroelectric switching under pulsed fields. The validity of this optical method is proved by a good agreement of the switching parameters deduced from optical scattering data with those obtained with electric methods. Scanning of the scattering over the crystal bulk revealed local specialities of the switching, particularly, a marked distribution of the domain wall density D along the polar axis with a maximum close to the negative electrode. In compliance with these in situ observations, the electro-optic coefficient rc reveals a position dependence in all SBN crystals poled in the ferroelectric phase, rc decreasing from the positive to negative electrode. This regularity is interpreted in terms of the domain density distribution D(z) and accounted for by an asymmetry of the domain nucleation.