S. Kapphan

Infrared absorption study of the OH vibrational band in LiNbO3 crystals

Abstract Stoichiometry and temperature dependence of the OH vibrational band was studied in LiNbO 3 crystals. The decomposition of the bands using the product function of a Lorentzian and a Gaussian showed that in Li deficient crystals the absorption bands consist of at least four components in contrast to more or less perfect stoichiometric crystals which reveal only one to three components. The two new peaks are interpreted by the appearance of two energetically different proton sites in the LiNbO 3 structure having ilmenite-like segments. Above 300°C only one broad band dominates at about 3465 cm −1 indicating the increased mobility of the protons and the interaction with the lattice.

Sharp, temperature dependent OH/OD IR-absorption bands in nearly stoichiometric (VTE) LiNbO3

Abstract In comparison with congruent LiNbO 3 the OH/OD IR-absorption bands in the bulk of nearly stoichiometric LiNbO 3 (prepared by the VTE technique) are about an order of magnitude narrower at room temperature and show a further narrowing to low temperatures. The fundamental stretching absorption band of OH, polarized completely perpendicular to the ferroelectric z -axis (RT at 3465.9 cm −1 , FWHM = 3.25cm −1 ), and of OD (RT at 2561.9cm −1 , FWHM = 2.05cm −1 ), as well as the next higher vibrational transition for OH ( RT : v 02 = 6745.1 cm −1 , FWHM = 10.0cm −1 ) and for OD ( RT : v 02 = 5025.5 cm −1 , FWHM = 6.5 cm −1 ) and their position, halfwidth and integrated absorption are measured as a function of temperature in the range from RT to LHeT. Electrical and mechanical anharmonic contributions are discussed on the basis of a Morse-type potential for a diatomic oscillator. In contrast to the absorption of the OH/OD centers in the bulk of nearly stoichiometric (VTE) LiNbO 3 , the absorption of OH/OD in proton/deuteron exchanged surface layers (at 3508/2589 cm −1 , polarized completely perpendicular z) in the VTE-treated crystals is as broad ( FWHM ≈ 30 21 cm −1 ) and nearly temperature independent as in proton/deuteron exchanged congruent LiNbO 3 . Tempering of the proton exchanged nearly stoichiometric (VTE) crystals at 350 °C in dry N 2 -atmosphere shows the diffusion of protons from the PE-layer into the bulk of the VTE-LiNbO 3 crystal. The integrated optical density of the total OH absorption stays approximately constant for the first 8–10 h of the tempering treatment, indicating about equal oscillator strength for both (bulk and surface layer) absorption bands.

Infrared and dielectric spectroscopy of the relaxor ferroelectric Sr0.61Ba0.39Nb2O6

The dielectric response of strontium barium niobate with 61% Sr has been studied from kHz to THz frequencies by means of several techniques (IR and time domain THz spectroscopy, the coaxial technique and dielectric spectroscopy) over a wide temperature interval: 20?600?K. A strong dielectric anisotropy is present in all the results. Relaxor ferroelectric properties were detected in ?c*(T,?). At very high temperatures a strong relaxation appears in the microwave and THz range, which shifts and broadens to lower frequencies on cooling and then splits into two components. The high frequency one is seen in the THz range at high temperatures and the strong low frequency one weakens below the temperature Tm of the smeared permittivity maximum and broadens extremely in the spectra leaving a constant-loss background at very low temperatures. This relaxation is responsible for the dielectric anomaly near the ferroelectric transition. No anomalies in phonon frequencies were observed, which gives evidence relating to the order?disorder mechanism of the phase transition. The response perpendicular to the polar axis shows anomalous features at low temperatures, which could be connected with the shift of the low frequency limit of a broad dispersion (much weaker than along the polar axis) from the GHz range at 200?K to the kHz range at 40?K. There is no clear evidence of a new phase transition below 100?K suggested by some authors.

Experimental study of second harmonic generation by dipolar configurations in pure and Li-doped KTaO3 and its variation under electric field

Abstract In the incipient ferroelectric crystal KTaO 3 , due to the inversion symmetry of the lattice no second harmonic generation (SHG) is expected. However, even in nominally pure crystals a weak intensity S zzz of frequency doubled light is observed at low temperatures. The SHG intensity S zzz increases with applied electric field. Our results are compared with theoretical predictions [Prusseit-Elffroth and Schwabl, Appl. Phys. A51 , 361 (1990)] and previous results [Fujii and Sakudo, Phys. Rev. B13 , 1161 (1976)] under applied electric field E . Doping of KTaO 3 with off-center Li-ions leads, at low temperature, to higher SHG intensity compared with nominally pure KTaO 3 . Various samples K 1−x Li x TaO 3 with Li-concentrations x = 0.008−;0.063 are measured and the spatial distribution and the total integrated SHG intensity S zzz (calibrated with respect to LiNbO 3 ) was determined as a function of temperature, with or without applied electric field. For small Li-concentrations x S zzz with decreasing temperature appears. Heating and cooling cycles follow the same S zzz curve without thermal hysteresis for these small Li-concentrations. The spatial distribution of S zzz shows a narrow forward scattered peak. When cooling under applied electric field E the intensity S zzz (E) increases drastically at low temperatures indicating the alignment of dipolar regions. The maximum intensity saturates at relatively low electric field E sat with ratios S zzz max (E sat ) S zzz max (0) varying from 250 for x = 0.008 to 50 for x = 0.022. The fact th S zzz max (E sat ) S zzz max (0) of the SHG intensity are much larger than the corresponding birefringence rati Δn max (E) Δn max (0) indicates the alignment of antiparallel ordered regions. S zzz max ( E sat ) as a f Li-concentration is maximum at x ≈ 0.03. The ratios S zzz max (E sat ) S zzz max (0) decrease drastically increasing Li-concentration corresponding to a strong increase of the mean size of ordered dipolar regions for E = 0 at low temperature. This is supported by the appearance (for x > 0.02) of a thermal hysteresis of S zzz and of lateral peaks in the spatial distribution. The variation of S zzz with the polarization direction of the Nd: YAG laser light shows, at low temperatures, a periodicity pointing to dipolar regions with tetragonal symmetry. The ratios of the SHG tensor components in all cases turn out to be ( d 33 d 31 ) 2 = 6.3 ± 0.6 and ( d 31 d 15 ) 2 ≈ 1 . The time dependence of S zzz after a electric field ( E 3 0) reveals for the small Li-concentrations both fast (τ ⩽ 0. l s) and slow relaxation processes. The slow relaxation process can be described by an Arrhenius-like behavior with a concentration dependent activation energy ( E A = k ⋯ 170 K for x = 0.008 to E A = k ⋯ 1000 K for x ⩾ 0.022).

O-D and O-H stretching vibrations in monodomain SrTiO3

Abstract The polarization of the three stretching vibration bands of hydrogen or deuterium impurities in the tetragonal low temperature phase of monodomain SrTiO3 and their splitting under applied electric fields allow these modes to be assigned to energetically different positions - one in the (Ti-O)-plane and two with components in c-direction along the oxygen octahedra.

Semi-empirical calculations of the electronic and atomic structure of polarons and excitons in ABO3 perovskite crystals

Abstract Our quantum chemical calculations performed by means of the intermediate neglect of differential overlap confirm the existence of the self-trapped electrons in KNbO3, KTaO3 and BaTiO3 crystals. The relevant lattice relaxation energies are 0.21, 0.27 and 0.24 eV, whereas the optical absorption energies are 0.78, 0.75 and 0.69 eV, respectively. We suggest theoretical interpretation of the so-called green luminescence (2.2–2.3 eV) in ABO3 perovskite crystals as a result of the recombination of electrons and holes forming the charge-transfer-vibronic-excitons. The calculated luminescence energies for SrTiO3, BaTiO3, KNbO3 and KTaO3 perovskite crystals are in a good agreement with the experimentally observed energies.

Quantum chemical modelling of ‘green’ luminescence in self activated perovskite-type oxides

We discuss the origin of the intrinsic visible band emission of ABO3 perovskite oxides (so-called ‘green luminescence’) which remains a topic of high interest during the last quarter of the century. We present a theoretical calculation modelling of this emission in the framework of a concept of charge transfer vibronic excitons [Phys. Solid State, 40 (1998) 834], i.e. as a result of radiative recombination of correlated (bound) self-trapped electron and hole polarons in the highly polarizable ABO3type matrix. The Intermediate Neglect of Differential Overlap method combined with the Large Unit Cell periodic defect model was used for quantum chemical calculations and theoretical simulation of the green emission for a series of model ABO3 perovskites. The ‘green’ luminescence energies for PbTiO3, SrTiO3, BaTiO3, KNbO3 and KTaO3 perovskite-type crystals agree well with those experimentally observed earlier. q 2003 Elsevier Science Ltd. All rights reserved.

Characteristics and the nature of the low-frequency dielectric response in moderately concentrated KTaO3:Li

The temperature-frequency behaviour of the complex dielectric permittivity of K0.957Li0.043TaO3 (KLT) was studied in detail. Below 250 K a pronounced relaxation-type dielectric dispersion occurs with two wide temperature maxima shifted to lower temperatures increasing in magnitude as the alternating-current monitoring frequency decreases. The results obtained are analysed by using a theory proposed considering the coupling of the TO1 soft mode (TO standing for transverse optical) to two Li relaxation modes known in moderately concentrated KLT as π- and (π/2)-relaxations.

Dielectric properties and phase transitions in K1−xLixTa1−yNbyO3:Cu

Abstract The ε’(T, f) and tan [δ(T, f)] (10–300 K, 100 Hz – 1 MHz) behavior is studied in K1−xLixTa1−yNbyO3:Cu (0·012 %wt. in batch) with estimated x1 = 0·0014, y = 0·024 (KLTN 0·14/2·4) and x2 = 0·0028, y2 = 0·013 (KLTN 0·28/1·3). In KLTN 0·14/2·4 a strong dipole relaxation is found in the 40–80 K region, with ε’(T)max = 16 000 at 47 K for 100 Hz, and a phase transition dominated by the soft mode coupled with a relaxation freezing mode is displayed at 39 K. Below, at 15·5 K, a nondispersive ε’(T) cusp-shaped maximum with ε’(T)max≈25 000 was found. KLTN 0·28/1·3 reveals a similar behavior, namely, Li+-related dielectric dispersion with ε’(T)max = 8300 at 47 K for 100 Hz, and a phase transition at ∼35 K. At lower temperatures, ε’ increases with a weak shoulder at ∼ 27 K. The so-called “277” species of KLTN:Cu (1900 ppm), with x = 0·001, y = 0·025 stated by content analysis, whose unusual properties were marked recently[1,2], reveal a sharp phase transition, accompanied by dielectric dispersion with a very high ε’(T)max ≈86000 and increased conductivity in the transition region.

Direct OH and OD librational absorption bands in LiNbO3

Abstract The presence of hydrogen (deuterium) defects in ABO3 crystals is revealed by a characteristic OH (OD) stretching vibration near 3500 cm−1 (2600 cm−1). Recently we reported about combination bands of the libration with the vibration of the OH (OD) oscillator observed for the first time in several ABO3 crystals. Direct librational modes have not been reported so far for these crystals. In proton exchanged (PE) surface layers of LiNbO3 with a very high OH concentration the direct OH librational mode at 960 cm−1 is observed by Raman scattering. The direct OD librational mode in deuteron exchanged LiNbO3:DE surface layers is hidden by phonon lines of the host lattice, but can be detected at 698 cm−1 in IR reflection measurements. The observed OH and OD librational wavenumbers are in agreement with a description of the libration by a simple harmonic oscillator model.