Photoluminescence and optical absorption in Cr-doped were observed for the first time and studied within the 300-800 nm spectral region and over a 7-300 K temperature range. `Nominally pure' crystals, with Cr impurities detected by means of EPR, and intentionally Cr-doped crystals grown by the top-seeded solution method were studied. Cr doping results in additional absorption over the whole visible region, increasing towards the band-edge area (at ), and wide absorption bands centred at about 470 nm, 575 nm, and 610 nm. At 74 K, exposure to optical excitation with results in luminescence in the near-infra-red region consisting of four sharp lines, (781.65 nm, ), (776.4 nm, ), (768.8 nm, ), (766.8 nm, ), and several additional weak emission lines. The luminescence excitation spectrum for each A line consists of two complex bands centred at 630 nm and at 400 nm for T = 74 K. With decreasing temperature, all of the sharp lines, such as the zero-phonon lines of in , shift to lower energies, which is opposite to the behaviour of such lines for ionic crystals. The thermal shift for the line is the largest, . Taking into consideration EPR data, temperature transformations, and the lifetime of the sharp emission lines ( at 70 K), we argue that the and lines are lines, i.e. originating from zero-phonon transitions of single ions replacing ions. The nature of the emission lines is not quite clear, but can be considered to originate from ions exchange coupled with other, unknown defects (including exchange-coupled pairs) or with -type transitions of centres perturbed by nearest-site hole polarons, or .

To cite this article: V. S. Vikhnin , S. Kapphan & J. Seglins (1999) Temperature induced red shift of the UV fundamental absorption edge in ferroelectric oxide crystals: effect of charge transfer vibronic excitons, Radiation Effects and Defects in A pronounced temperature induced red shift of the fundamental UV-absorption edge is observed in ferroelectric oxides at T > 300 K. The biggest shift value of -1.3 meV/K is found in tetragonal SBN (Sr,Ba,-rNb20& where no peculiarities were detected in the region of the diffused phase transition ( T , = 350 K). A model involving new in-gap states due to charge transfer vibronic excitons (CTVE) with a new correlated CTVE-phase is proposed to explain the experimental observations.

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(SBN : x) is known to be a mixed crystal with an open tungsten bronze structure and diffuse ferroelectric phase transition [l]. SBN possesses some of the greatest photorefractive [2] and thermal [3] optical nonlinearities. The underlying elementary physical processes as well as the nonlinearities are very sensitive to the modification of the SBN matrix, i.e., to the concentration of intrinsic defects and the introduction of additional doping elements [4 to 61.The usual treatment of SBN carried out at elevated temperatures leads to results which are not completely understood yet. So, fast cooling of nominally pure or slightly cerium-doped (<0.05%) SBN from 1000 "C to room temperature in air results in a new broad absorption band in the NIR region of the spectrum and in a permanent blue color of SBN at room temperature because of the oxygen deficiency [4]. Moreover fast cooling often results in cracks and general degrading of ferroelectric and nonlinear optical properties. Under moderate heating of SBN to 400 to 500 "C (i.e. to dry silver paint electrodes) the crystals change color to brownish yellow or green as observed by eye. Quantitative investigations are still missing.Here we report on optical absorption of various SBN crystals studied in the temperature range from RT (20 "C) to 600 "C. Experimental techniquesThe Sr, -,Ba,Nb,O, crystals studied here were Czochralski grown with x = 0.29,0.39, and 0.54, and nominally pure. For comparison some cerium-and neodymium-doped crystals were studied also. The samples for these investigations have not been poled or depoled during the heat treatment. Some of the nonstoichiometric SBN samples (x = 0.29 and 0.54) also showed striations.The transmission in the spectral range 350 to 900 nm was taken by a Beckman Acta M VII spectrophotometer. The sample was in a heated stainless steel holder in air. The heating and cooling rates between fixed temperatures ranged between 200 and 100 K/h. Results and conclusionsThe absorption spectra of SBN: 0.29 are shown in Fig. 1 for fixed temperatures. No additional absorption bands in the visible appear at heating, but a huge shift of the fundamental electronic absorption edge is observed. The factor characterizing the shift of the absorption curve for e-polarized light by 1.2 meV/K (and ultimately the changes of the band gap Eg) is virtually the same also for other compositions (x = 0.39 I)

SBN) with congruent composition (x = 0.61) and Sr,,, 1Ba,,,,Nb,06 crystals doped with various concentrations of cerium are grown with the Czochralski method. A subsequent doping of the crystals with protons and deuterons is performed either by hydrothermal high temperature treatment or by field supported hydrogen diffusion at elevated temperatures in humid atmosphere. The presence of the protons is revealed by the infrared absorption of the OH stretching vibration. This broad OH absorption band is essentially the same for Sr,.,,Ba,.,,Nb,O, and Sr,.,,Ba,,,,Nb,O, : Ce (0.025 wt% CeO,). It shows a polarization dependence with respect to the c-axis of the tetragonal tungsten bronze structure of Sr,Ba, -,Nbz06, the absorption being stronger for light polarized perpendicular to c than for light polarized parallel to c. The maximum