Mg-doped LiNbO3 Crystals Research Articles

Defect chemistry analysis of the defect structure in Mg-doped LiNbO3 crystals

On the basis of the Li vacancy model of intrinsic defects in LiNbO3 crystal, the defect structure formed in LiNbO3:Mg varying with the Mg content are proposed. The Li2O and Nb2O5 contents, the Li/Nb ratio in the crystal, and the density of the crystal are calculated by using the defect structure. The calculated values are in good agreement with the reported experimental results.

Crystalline environment of Fe3+ions in highly Mg-DOPED LiNbO3crystal

The substitutions of Fe3+(I) and Fe3+(II) for both Li+ and Nb5+ ions in LiNbO3 crystal have been studied using the superposition model. From the studies, it is found that a displacement of the impurity ion along the c-axis is required to reach a good fit between the calculated andobsrrved zero-field splittings b(2) degrees. By analyzing the displacement direction from the impurity displacement scheme suggested in this paper, we can conclude that Fe3+(I) replaces Li+ ion and Fe3+(II) replaces Nb5+ ion.

Mg-Doped Lithium Niobate: Some Comments on Stoichiometry, Structure and Properties

The change in properties with composition of Mg-doped LiNbO3 crystals at ∼10% MgO, reported by Hu et al. [Jpn. J. Appl. Phys. 30 (1991) 1412] may be reinterpreted in terms of a different change in solid solution mechanism to the one proposed by Hu et al. For Mg contents greater than ∼10%, phase diagram studies indicate that any crystals would be constrained to have compositions close to or on the stoichiometric join of constant overall cation content, with substitution mechanism 3Li++Nb5+\\rightleftharpoons4Mg2+ rather than to have A site vacancies as suggested by Hu et al. The Curie temperature, Tc, varies with composition. In addition to the well-established decrease in Tc associated with cation vacancy creation, a second mechanism must also operate to reduce Tc, probably associated with substitution of Mg onto B(Nb) sites.

Photoconductivity of Nd, Mg-Doped LiNbO3 Crystals

Optical absorption spectra, OH infrared absorption bands, ESR spectra of Fe3+ ions, and photoconductivities of a group of LiNbO3 crystals, including nominaly undoped LiNbO3, LiNbO3:6 mol% MgO and LiNbO3:6 mol% MgO + 0.5 mol% Nd2O3 crystals, are measured at room temperature. OH band and ESR measurements show that Nd doping will not directly influence the Mg doping concentration threshold effect and the site of incorporation of Fe ions in LiNbO3: Mg. This conclusion is demonstrated by experimental photoconduction results. When no external electric field is applied the photovoltaic current density in LiNbO3: Mg, Nd is larger than that in undoped and Mg-doped LiNbO3 at some wavelengths owing to the larger optical absorption coefficient. Besides, the effect of UV irradiation on the photoconduction in doped LiNbO3 crystals is observed experimentally. The photoconduction in LiNbO3:6mol% MgO will increase after the UV irradiation, but no change is observed in LiNbO3: Mg, Nd crystals. By means of defect chemistry calculations of the threshold effect in Mg-doped LiNbO3 crystals according to the nature of substitution of Nd ions in the Mg-doped crystals, the experimental results are discussed. Es werden die optischen Absorptionsspektren, OH-Infrarotabsorptionsbanden, ESR-Spektren von Fe3+-Ionen und die Photoleitung einer Gruppe von LiNbO3-Kristallen, darunter nominell undotierte LiNbO3-, LiNbO3:6 Mol% MgO- und LiNbO3:6Mol% MgO + 0,5 Mol% Nd2O3-Kristalle, bei Zimmertemperatur gemessen. Wie durch OH-Banden und ESR-Messungen gezeigt wird, beeinflußt Nd-Dotierung nicht direkt den Mg-Dotierungskonzentrationsschwelleneffekt und den Einbauplatz von Fe-Ionen in LiNbO3:Mg. Diese Schlußfolgerung wird durch experimentelle Ergebnisse der Photoleitung demonstriert. Infolge des größeren optischen Absorptionskoeffizienten ist bei einigen Wellenlängen die Photo-EMK-Stromdichte in LiNbO3:Mg, Nd größer als in undotiertem und Mg-dotiertem LiNbO3, wenn kein äußeres elektrisches Feld anliegt. Zusätzlich wird experimentell der Einfluß von UV-Bestrahlung auf die Photoleitung in dotierten LiNbO3-Kristallen beobachtet. Die Photoleitung in LiNbO3:6 Mol% MgO steigt nach UV-Bestrahlung, es wird jedoch keine änderung in LiNbO3: Mg, Nd-Kristallen beobachtet. Mit Defektchemieberechnungen des Schwelleneffekts in Mg-dotierten LiNbO3-Kristallen werden entsprechend der Art der Substitution von Nd-Ionen in Mg-dotierten Kristallen die experimentellen Ergebnisse diskutiert.

Non-photorefractive LiNbO3:Mg as the effective material for the nonlinear optics

Abstract The comparative study of optical properties of Mg-doped and Mg-nondoped LiNbO3 and Fe-doped LiNbO3 crystals was carried out. In all the crystals doped with Mg ≥ 1 wt.% the value of photorefraction decreases not less than by 102 in spite of Fe incorporation up to 0.1 wt%. At room temperature the nonlinear optical elements of LiNbO3:2.4 wt% Mg reveal the conversion efficiency of SHG more than 25% with phase matching angle of 90[ddot]. X-ray and UV-induced changes of the optical absorption spectra reveal the alteration of the electron acceptor type in Mg-doped crystals. On the contrary to pure LiNbO3, in LiNbO3 doped by Mg the Fe3+-ion is not more the electron acceptor center, what is followed by the change of the photoelectric and consequently photorefractive properties. Mg incorporation affects the ESR spectrum of Fe3+ in LiNbO3 as well.

THE OH ABSORPTION SPECTRUM As A PROBE FOR DEFECT STRUCTURE OF LiNbO3 CRYSTAL

The OH- infrared absorption band in the LiNbO3 crystal with various Li/Nb ratios and MgO contents have been measured at room temperature. The structure of the absorption band of nearly stoichiometric crystals consists of three sub-bands with slightly different peak position, their relative heights change with Li/Nb ratios. The OH- absorption band in heavily Mg-doped crystal has a structure consisting of two peaks, the shift of peak position to shorter wavelength is about 54 cm-1. Basing on the crystal structure and the model of the defect structure of the LiNbO3 crystal suggested by Abrahams and Smyth (1-2), these observations are discussed for three different structures, i.e., stroichiometric, congruent and Mg-doped crystals, respectively. The shift of the peak position of the OH- absorption band in heavily Mg-doped crystal to shorter wavelength is suggested to be caused by the Mg2+ ions getting into Nb5+ sites when the Mg-doping level was higher than the threshold level in Mg-doped LiNbO3 crystals.