Nonstoichiometric Lithium Niobate Research Articles

Structure particularities and first, second order Raman spectra of LiNbO3:Tb near the concentration threshold

Defective structure particularities have been studied in pure non-stoichiometric (Li/Nb≠1) lithium niobate (NSLN) crystals and LiNbO3:Tb (2.24 wt%) crystal. NSLN Raman spectra contain low-intensity extra bands in the area 100-1000 cm−1 at different scattering geometries. The bands are not provided by the selection rules for LN. The bands are caused by static disorder in cation sublattice of crystals. Besides, we have established additional bands in the area 1300-2000 cm−1 of a LiNbO3:Tb (2.24 wt%) crystal Raman spectrum. The bands correspond to overtone states of crystal lattice polar fundamental modes. Second-order Raman spectrum of LiNbO3:Tb contains bands with frequencies higher than that of overtones of the highest-frequency fundamental polar modes with А1(Z) and Е(X,Y) symmetry type. The particularities are manifestation of bonded states of polar modes – bipolaritons; they convert in two entangled photons or in a bonded state – biphoton at an exit from the crystal.

Local Structure Manipulates Nonlinear Optical Properties in Non-stoichiometric Lithium Niobate

As a typical and commercial nonlinear optical (NLO) material, LiNbO3 usually suffers from non-stoichiometric compositions, but the role of non-stoichiometry in second harmonic generation (SHG) remains unclear. In this study, we prepared non-stoichiometric LiNbO3 with a Li mole percentage amount covering critical ranges from 47 to 50. We found that a 47 mol % Li amount can remarkably weaken the SHG response by 50% without an obvious change of the band gap. A detailed structural analysis indicates that reducing the Li content brings anti-occupied Nb (NbLi4·) together with Li vacancies (VLi′), which decreases local distortions and local polarizations of both NbO6 and LiO6 octahedra and ultimately lead to the decline of the SHG response. Such effects are discussed in terms of the looser and less anisotropic structure character of LiNbO3. The above results are supported by joint synchrotron X-ray/neutron diffraction, atomic pair distribution function (PDF) analysis, and theoretical calculations. These insights are helpful to the future design of functional materials in non-stoichiometric compositions.

Curie temperature of nonstoichiometric lithium tantalate and lithium niobate by a mixed vacancy model

In this work, we present a new vacancy model which proves that both lithium and tantalate (niobium) vacancies can coexist in the lattice structure of lithium tantalate and lithium niobate. Using the theoretical approach to the defect structure analysis, which is developed by Safaryan, a comparison between three defect models (tantalate/niobium vacancies, lithium vacancies, and mixed vacancies) has been proposed. The calculated values of the Curie temperature for different compositions of nonstoichiometric lithium tantalate and lithium niobite show that the mixed vacancy model can effectively describe the defect structure in nonstoichiometric lithium tantalate and lithium niobate. Also, it can reveal good correspondence with experimental results.

Parametrization of nonstoichiometric lithium niobate crystals with different states of defectivity

Abstract Quasi-elastic light scattering (QELS) and Raman scattering have been studied at 296 K in congruent lithium niobate (CLN) crystals with a different degree of defectiveness. The defectiveness degree has been detected due to acoustic Q-factor. A quantitative analysis of spectra has been carried out in the frequency range 0–70 cm−1 in samples with different Q-factor. The applied model considers a connection between low-frequency optical mode A1(TO) symmetry type with relaxing intrinsic energy. The model describes stoichiometry defects. Results of model calculations and experiments allows us to conclude that different types of defects play a substantial role in formation of dynamic central peak at a structure phase transition in CLN crystals.

Raman Scattering in Non-Stoichiometric Lithium Niobate Crystals with a Low Photorefractive Effect

Raman spectra of lithium niobate single crystals strongly doped by zinc and magnesium, it has been established, contain low-intense bands with frequencies 209, 230, 298, 694, and 880 cm−1. Ab ignition calculations fail to attribute these bands to fundamental vibrations of A2 symmetry type unambiguously. Such vibrations are prohibited by the selection rules in the space group C3V6 (R3c). Ab initio calculations also proved that low-intense “extra” bands with frequencies 104 and 119 cm−1 definitely do not correspond to vibrations of A2 symmetry type. We have paid special attention to these extra bands that appear in LiNbO3 single crystals Raman spectra despite the fact that they are prohibited by the selection rules. In order to do so, we have studied a number of lithium niobate single crystals, both nominally pure and doped, by Raman spectroscopy. We have assumed that some “extra” bands correspond to two-particle states of acoustic phonons with a total wave vector equal to zero. We have also detected a Zn concentration area (0.05–0.94 mol.% ZnO in a crystal) where doped crystal structure is more ordered: The order of alternation of the main, doping cations, and vacancies along the polar axis is increased, and oxygen octahedra are less distorted.

Development of high temperature ultrasonic guided wave transducer for continuous in service monitoring of steam lines using non-stoichiometric lithium niobate piezoelectric ceramic

The dielectric, elastic, piezoelectric and electromechanical coupling coefficients of shear mode non-stoichiometric Lithium Niobate (LN) piezoelectric ceramics have been determined using the resonance method from room temperature to 600°C. An ultrasonic guided wave (UGW) transducer was manufactured by bonding a LN piezoelectric ceramic to an aluminium oxide backing block using a high temperature joining technique. Temperature dependence of the transducer from room temperature to 600°C has been experimentally determined. As an aid to data interpretation, a simple approximate model of the temperature dependence of the normalised received signal amplitude of a pulsed ultrasonic propagation system based on LN is developed. This model, has the advantage of avoiding the use of complex electromechanical equivalent circuits. It provides a qualitative explanation of the general form of the data from ultrasonic experiments in the range from room temperature to 600°C. The received ultrasonic pulse power, at the highest temperature, was observed to be as good as at room temperatures. It is therefore concluded that non-stoichiometric LN is potentially suitable for use in direct contact with structures at up to 580°C such as arise with fossil fuel plant steam lines, subject to the results of further work on long term ageing trials. However, the model indicates how the overall performance at all temperatures in the range considered might be enhanced by improved impedance matching and acoustic coupling system design. The Curie Weiss temperature for the crystals (T0) has been estimated from dielectric constant data and found to be much lower than the Curie temperature (Tc).

A new analytical description in C2+ doped LiBO3 C (Ni, Zn…); B (Nb, Ta)

Defect structure models for C-doped non-stoichiometric lithium niobate and lithium tantalate were generally based on the defect model proposed for undoped crystals i.e. Li-site vacancy or Nb-site vacancy descriptions. The atomic radius of C is rC = 135 pm. This work is an extension of the Safaryan theory. A new analytical description has been proposed. It is based on the vibrations of crystal planes. The substitution mechanism of the doped compositions C in LiBO3 crystal is discussed. The mechanism of phase transition due to thermal expansion of crystal is also described.

Defect structure analysis of lithium niobate single crystals and lithium tantalate ceramics with the next-nearest-neighbor interactions

In this work, we have proposed a theoretical description of the defect structure in LiTaO3 and LiNbO3 on the basis of vacancy models combined with a ferroelectric phase transition theory. The calculated values by our approach, which takes into account the next-nearest neighbors interactions, are closer to the experimental data and improves the theoretical approach of Safaryan. A comparative study between the calculated and measured values of the Curie temperature shows that the lithium vacancy model is the best model that has been suggested for describing the defect structure in nonstoichiometric lithium niobate single crystals and lithium tantalate ceramics.

Thermal behavior of the nonstoichiometric lithium niobate powders synthesized via a combustion method

Lithium niobate (LixNb1−xO3+δ) powders with various compositions are prepared via combustion synthesis. The thermal properties, crystal structure, and surface morphology of the as-prepared lithium niobate powders are characterized by thermogravimetric and differential thermal analyses (TG/DTA), powder X-ray diffraction (XRD), and scanning electron microscopy (SEM). When the calcination temperature reached 900°C, the secondary phases Li3NbO4 and LiNb3O8 were observed. The lithium concentration before 900°C was 40–43%. The lattice parameters increased slightly with decreasing concentration of lithium ions. When the calcination temperature was higher than 900°C, the major Li0.91NbO3 phase and the minor LiNbO3 phase coexisted in the nonstoichiometric lithium niobate with 43% lithium content.

High-accuracy contactless method for determination of chemical composition of lithium niobate crystals by their birefringence

A high-accuracy method for determining the chemical composition of non-stoichiometric lithium niobate crystals is proposed based on precise measurements of birefringence of thin substrates using the laser ellipsometry in transmission. It is shown that, for compensators made of LiNbO3, this method allows one to monitor the ratio Li/(Li + Nb) with an accuracy of up to ∼10−8 at λ = 0.6328 μm. The accuracy is limited by temperature fluctuations of the crystal.

Composition Dependence of the Transition Temperature for Non-Stochiometric Ferroelectric Lithium Niobate

The transition temperature T C (x) for non-stoichiometric lithium niobate, LiNbO 3 , here denoted by (Li 2 O) x (Nb 2 O 5 ) 1 − x where 0.465 < x < 0.500, evolves from T C (0.465) ≅ 1290 K to T C (0.50) ≅ 1485 K. Using an effective field approach, as done in previous works for isomorphous ferroelectrics solid solutions, we investigate the composition dependence of the critical temperature in this narrow range for ferroelectric lithium niobate. Experimental data are fitted by the function T C (x), deduced from the effective field theory when considering the short composition range of LiNbO 3 as a mixed system with increasing stoichiometry.

First-principles study on the formation energies of intrinsic defects in LiNbO 3

First-principles plane-wave ultrasoft pseudopotential method within local density approach (LDA) has been used to study three possible vacancy-defect models for non-stoichiometric lithium niobate (LiNbO 3): (1) the oxygen-vacancy model V O 2 + + 2 V Li - , (2) the niobium-vacancy model 5 Nb Li 4 + + 4 V Nb 5 - , and (3) the lithium-vacancy model 4 V Li - + Nb Li 4 + . The corresponding formation energies are obtained via energy minimization of a supercell. In Nb-rich environment, the calculated defect formation energies, both under oxidation and reduction conditions, show little effect on the intrinsic defect structures. We find that the lithium vacancy model 4 V Li - + Nb Li 4 + has the most stable configuration in the non-stoichiometric lithium niobate crystals. Our calculations also show that the formation of any type of neutral defects and Frenkel pairs in a Nb-rich environment is difficult.

Experimental and theoretical investigation of lattice defect structures in a series of Zn, Fe-doped nonstoichiometric lithium niobate

A series of the double doped lithium niobate (LiNbO 3, LN) single crystals had been grown by Czochralski method. The Curie temperatures of various concentrations doped or [Li]/[Nb] ratio LN crystals measured by differential thermal analysis (DTA) were discussed to investigate their defect structures with Safaryan et al. new approach about LN lattice defect structure using Curie temperatures calculated. Infrared transmission spectra of various concentrations doped were used to compare the investigation above. The results show that the lithium vacancy model is the more probable to describe the lattice defect structure of the doped LN single crystal.

Optical index profile at an antiparallel ferroelectric domain wall in lithium niobate

Unexpected optical contrast at antiparallel domain walls is observed in non-stoichiometric lithium niobate. This is imaged using near-field scanning optical microscopy. A detailed modeling of the imaging process is performed, and a comparison of the experimental and simulation images is used to extract the index profile across a single antiparallel domain wall.

Ilmenite-like stacking defect in nonstoichiometric lithium niobate crystals investigated by Raman scattering spectra

The Raman spectra of three pure lithium niobate (LiNbO3) crystals with different content of Li and several doped LiNbO3 (Mg, In and Ti) crystals have been investigated. A weak peak at about 738 cm−1 reported as a very strong Raman peak of ilmenite phase LiNbO3 was observed for nonstoichiometric LiNbO3 crystals. The dependence of this peak on Li content and doping concentration in crystal suggested that there is a new kind of defect structure which has ilmenite-like stacking and direct relationship with antisite NbLi4+ ion. This result experimentally proved the existence of ilmenite-like stacking defect in nonstoichiometric LiNbO3 for the first time. This defect is in good agreement with the Li vacancy model and has a different structure from that proposed by Smyth [Ferroelectrics 50, 93 (1983); Proc. IEEE, p. 115 (1986)].

Point imperfections and clusters of intrinsic and extrinsic defects in non-stoichiometric and stoichiometric lithium niobate: The regularly ordered crystal

The results of our investigations of different kinds of defects in lithium niobate crystals are summarized in this report. Randomly distributed intrinsic point imperfections are dominating in conventional congruent crystals. This leads to a perturbation of the surroundings of optically or acoustically active impurities and to a broadening of their spectral lines. The great narrowing of resonance lines in nearly stoichiometric samples increases the spectral resolution sufficiently to allow the study of even non-controlled trace impurities and satellite centers, consisting of the impurity and intrinsic defects. The disappearance of satellite centers and the appearance of new centers due to the change of charge compensation mechanism was discovered in stoichiometric crystals. The quantitative characterization of the degree of crystal perfection, a definition of regularly ordered crystal and the necessity of the re-investigation of physical properties for perfect samples are discussed.

An approach to the defect structure analysis of lithium niobate single crystals

A formula for calculating the soft mode frequency and Curie temperature (using ion mass, charge, and the distances between them) was applied to the analysis of the defect structure of nonstoichiometric lithium niobate. Four compositions from near stoichiometric to highly Li2O deficient, including congruent, were studied. The Curie temperatures were calculated for three possible types of defects: (1) oxygen vacancies, (2) niobium vacancies, and (3) lithium vacancies. A comparison between the calculated and experimental values showed that the oxygen and niobium vacancy models did not agree with experimental data. Good quantitative agreement between the calculated and experimental data for the lithium vacancy model allowed us to conclude that this model best describes the defect structure of nonstoichiometric lithium niobate LiNbO3.

Investigation of correlated defects in non-stoichiometric lithium niobate by high resolution electron microscopy

An investigation of intrinsic defects in lithium niobate, using High Resolution Electron Microscopy (HREM) was carried out. It was found that crystals, around and at congruent composition, have a large number of clustered defects about 1 nm in diameter. Models of planar and spatially correlated defects were developed and used for the simulation of HREM images, along with other existing models of non-stoichiometric defects. It was found that neither randomly distributed antisite defects, compensated by lithium or niobium vacancies, nor clustered lithium and oxygen vacancies could produce the contrast variation observed in the images. A new model of small Nb 2O 5 clusters is also suggested as a possible candidate for the defects responsible for the non-stoichiometry of the crystals.

Lattice constants of K- and Mg-doped LiNbO 3. Comparison with nonstoichiometric lithium niobate

The dependencies of lithium niobate lattice constants on stoichiometry and concentration of K and Mg dopants were investigated with the help of X-ray powder diffraction. It was found that in contrast to MgO dopant, an addition of K 2O to the congruent LiNbO 3 melt leads to a decrease of the lattice constants. A similar reduction of a and c parameters is observed if the Li content of the crystal is increased, which is accompanied by an improvement of the lithium niobate lattice structure because of diminishing concentration of intrinsic nonstoichiometric defects.

Model for the Ferroelectric Transition in Nonstoichiometric Lithium Niobate and Lithium Tantalate

A model is developed for the ferroelectric‐to‐paraelectric phase transition in LiNbO3 and LiTaO3. The model is consistent with literature reports of nonstoichiometry, point defect formation, and phase transformation behavior. The model uses lithium Frenkel‐defect‐based order‐disorder as the phase transformation mechanism. The phase transformation (Curie) temperature is predicted to decrease linearly with deviation from stoichiometry in agreement with experimental observations.