Special Issue "New Trends in Lithium Niobate: From Bulk to Nanocrystals"

A special issue of Crystals (ISSN 2073-4352). This special issue belongs to the section "Crystalline Materials".

Deadline for manuscript submissions: closed (31 March 2021).

Special Issue Editors

Prof. Dr. Gábor Corradi
E-Mail Website
Guest Editor
Institute for Solid State Physics and Optics, Wigner Research Centre for Physics, Budapest, Hungary
Interests: Crystal defects and radiation transients in non-linear oxide crystals and scintillators; paramagnetic resonance
Prof. Dr. László Kovács
E-Mail Website
Guest Editor
Institute for Solid State Physics and Optics, Wigner Research Centre for Physics, Budapest, Hungary
Interests: optical crystals; spectroscopy; point defects; hydroxyl ions; localized vibrations; rare-earth absorption and luminescence
Special Issues and Collections in MDPI journals

Special Issue Information

Dear Colleagues,

This planned Special Issue of Crystals is already the fourth one focusing on LiNbO3, and this for a good reason. Since its first preparation in 1937, and especially after its Czochralski growth in 1964/65 as a homogeneous artificial crystal with outstanding ferroelectric, non-linear optical, and holographic properties, the widespread use of lithium niobate as a workhorse for testing and realizing new ideas and applications seems to continue in a number of fields. Recent examples are THz phenomena, ultrashort transients, nanocrystals, and delayed electronic response spanning a time range from femtoseconds to minutes, to mention just a few. Miniaturized integrated optics and expected quantum optics applications also require further development and deeper understanding of the technology and functioning of this paradigmatic material in its new forms, including bottom-up assembly and top-down techniques and methods for their control, as well as the investigation and optimization of the modified properties. The sometimes controversial earlier results and new experiments call for new interpretations and their confirmation by theoretical modeling using also recent progress in computational tools.

Contributions of all kinds addressing these and similar topics from specialized or multidisciplinary aspects, including also short reviews, are invited to the special issue of /Crystals/ "New Trends in Lithium Niobate: from Bulk to Nanocrystals" which is dedicated to the memory of Prof. Dr. Ortwin F. Schirmer and is planned to appear by the end of 2020.

Prof. Dr. Gábor Corradi
Prof. Dr. László Kovács
Guest Editors

Manuscript Submission Information

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Keywords

  • Crystal growth, defects, modeling
  • Transient phenomena
  • Nanocrystals, nanopowders
  • Integrated optics
  • Quantum photonics
  • Photovoltaics, ferroelectrics
  • THz pulse generation

Published Papers (19 papers)

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Research

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Article
Investigations of LiNb1−xTaxO3 Nanopowders Obtained with Mechanochemical Method
Crystals 2021, 11(7), 755; https://doi.org/10.3390/cryst11070755 - 28 Jun 2021
Cited by 1 | Viewed by 409
Abstract
Nanocrystalline compounds LiNb1−xTaxO3 of various compositions (x = 0, 0.25, 0.5, 0.75, 1) were synthesized by high-energy ball milling of the initial materials (Li2CO3, Nb2O5, Ta2O [...] Read more.
Nanocrystalline compounds LiNb1−xTaxO3 of various compositions (x = 0, 0.25, 0.5, 0.75, 1) were synthesized by high-energy ball milling of the initial materials (Li2CO3, Nb2O5, Ta2O5) and subsequent high-temperature annealing of the resulting powders. Data on the phase composition of the nanopowders were obtained by X-ray diffraction methods, and the dependence of the structural parameters of LiNb1−xTaxO3 compounds on the value of x was established. As a result of the experiments, the optimal parameters of the milling and annealing runs were determined, which made it possible to obtain single-phase compounds. The Raman scattering spectra of LiNb1−xTaxO3 compounds (x = 0, 0.25, 0.5, 0.75, 1) have been investigated. Preliminary experiments have been carried out to study the temperature dependences of their electrical conductivity. Full article
(This article belongs to the Special Issue New Trends in Lithium Niobate: From Bulk to Nanocrystals)
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Article
Electron Polarons in Lithium Niobate: Charge Localization, Lattice Deformation, and Optical Response
Crystals 2021, 11(5), 542; https://doi.org/10.3390/cryst11050542 - 13 May 2021
Cited by 1 | Viewed by 418
Abstract
Lithium niobate (LiNbO3), a material frequently used in optical applications, hosts different kinds of polarons that significantly affect many of its physical properties. In this study, a variety of electron polarons, namely free, bound, and bipolarons, are analyzed using first-principles calculations. We perform a full structural optimization based on density-functional theory for selected intrinsic defects with special attention to the role of symmetry-breaking distortions that lower the total energy. The cations hosting the various polarons relax to a different degree, with a larger relaxation corresponding to a larger gap between the defect level and the conduction-band edge. The projected density of states reveals that the polaron states are formerly empty Nb 4d states lowered into the band gap. Optical absorption spectra are derived within the independent-particle approximation, corrected by the GW approximation that yields a wider band gap and by including excitonic effects within the Bethe–Salpeter equation. Comparing the calculated spectra with the density of states, we find that the defect peak observed in the optical absorption stems from transitions between the defect level and a continuum of empty Nb 4d states. Signatures of polarons are further analyzed in the reflectivity and other experimentally measurable optical coefficients. Full article
(This article belongs to the Special Issue New Trends in Lithium Niobate: From Bulk to Nanocrystals)
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Article
Design, Simulation, and Analysis of Optical Microring Resonators in Lithium Tantalate on Insulator
Crystals 2021, 11(5), 480; https://doi.org/10.3390/cryst11050480 - 25 Apr 2021
Viewed by 662
Abstract
In this paper we design, simulate, and analyze single-mode microring resonators in thin films of z-cut lithium tantalate. They operate at wavelengths that are approximately equal to 1.55 μm. The single-mode conditions and transmission losses of lithium tantalate waveguides are simulated for different [...] Read more.
In this paper we design, simulate, and analyze single-mode microring resonators in thin films of z-cut lithium tantalate. They operate at wavelengths that are approximately equal to 1.55 μm. The single-mode conditions and transmission losses of lithium tantalate waveguides are simulated for different geometric parameters and silica thicknesses. An analysis is presented on the quality factor and free spectral range of the microring resonators in lithium tantalate at contrasting radii and gap sizes. The electro-optical modulation performance is analyzed for microring resonators with a radius of 20 μm. Since they have important practical applications, the filtering characteristics of the microring resonators that contain two straight waveguides are analyzed. This work enhances the knowledge of lithium tantalate microring structures and offers guidance on the salient parameters for the fabrication of highly efficient multifunctional photonic integrated devices, such as tunable filters and modulators. Full article
(This article belongs to the Special Issue New Trends in Lithium Niobate: From Bulk to Nanocrystals)
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Article
Study of Type II SPDC in Lithium Niobate for High Spectral Purity Photon Pair Generation
Crystals 2021, 11(4), 406; https://doi.org/10.3390/cryst11040406 - 10 Apr 2021
Cited by 1 | Viewed by 633
Abstract
Recent advances of high-quality lithium niobate (LN) on insulator technology have revitalized the progress of novel chip-integrated LN-based photonic devices and accelerated application research. One of the promising technologies of interest is the generation of entangled photon pairs based on spontaneous parametric down-conversion [...] Read more.
Recent advances of high-quality lithium niobate (LN) on insulator technology have revitalized the progress of novel chip-integrated LN-based photonic devices and accelerated application research. One of the promising technologies of interest is the generation of entangled photon pairs based on spontaneous parametric down-conversion (SPDC) in LNs. In this paper, we investigated, theoretically and numerically, Type II SPDC in two kinds of LNs—undoped and 5-mol% MgO doped LNs. In each case, both non-poled and periodically poled crystals were considered. The technique is based on the SPDC under Type II extended phase matching, where the phase matching and the group velocity matching are simultaneously achieved between interacting photons. The proposed approach has not yet been reported for LNs. We discussed all factors required to generate photon pairs in LNs, in terms of the beam propagation direction, the spectral position of photons, and the corresponding effective nonlinearities and walk-offs. We showed that the spectral positions of the generated photon pairs fall into the mid-infrared region with high potential for free-space quantum communication, spectroscopy, and high-sensitivity metrology. The joint spectral analyses showed that photon pairs can be generated with high purities of 0.995–0.999 with proper pump filtering. Full article
(This article belongs to the Special Issue New Trends in Lithium Niobate: From Bulk to Nanocrystals)
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Article
Correlation of Electrical Properties and Acoustic Loss in Single Crystalline Lithium Niobate-Tantalate Solid Solutions at Elevated Temperatures
Crystals 2021, 11(4), 398; https://doi.org/10.3390/cryst11040398 - 09 Apr 2021
Cited by 2 | Viewed by 476
Abstract
Electrical conductivity and acoustic loss Q−1 of single crystalline Li(Nb,Ta)O3 solid solutions (LNT) are studied as a function of temperature by means of impedance spectroscopy and resonant piezoelectric spectroscopy, respectively. For this purpose, bulk acoustic wave resonators with two different Nb/Ta [...] Read more.
Electrical conductivity and acoustic loss Q−1 of single crystalline Li(Nb,Ta)O3 solid solutions (LNT) are studied as a function of temperature by means of impedance spectroscopy and resonant piezoelectric spectroscopy, respectively. For this purpose, bulk acoustic wave resonators with two different Nb/Ta ratios are investigated. The obtained results are compared to those previously reported for congruent LiNbO3. The temperature dependent electrical conductivity of LNT and LiNbO3 show similar behavior in air at high temperatures from 400 to 700 °C. Therefore, it is concluded that the dominant transport mechanism in LNT is the same as in LN, which is the Li transport via Li vacancies. Further, it is shown that losses in LNT strongly increase above about 500 °C, which is interpreted to originate from conductivity-related relaxation mechanism. Finally, it is shown that LNT bulk acoustic resonators exhibit significantly lower loss, comparing to that of LiNbO3. Full article
(This article belongs to the Special Issue New Trends in Lithium Niobate: From Bulk to Nanocrystals)
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Article
Polaron Trapping and Migration in Iron-Doped Lithium Niobate
Crystals 2021, 11(3), 302; https://doi.org/10.3390/cryst11030302 - 17 Mar 2021
Cited by 1 | Viewed by 489
Abstract
Photoinduced charge transport in lithium niobate for standard illumination, composition and temperature conditions occurs by means of small polaron hopping either on regular or defective lattice sites. Starting from Marcus-Holstein’s theory for polaron hopping frequency we draw a quantitative picture illustrating two underlying [...] Read more.
Photoinduced charge transport in lithium niobate for standard illumination, composition and temperature conditions occurs by means of small polaron hopping either on regular or defective lattice sites. Starting from Marcus-Holstein’s theory for polaron hopping frequency we draw a quantitative picture illustrating two underlying microscopic mechanisms besides experimental observations, namely direct trapping and migration-accelerated polaron trapping transport. Our observations will be referred to the typical outcomes of transient light induced absorption measurements, where the kinetics of a polaron population generated by a laser pulse then decaying towards deep trap sites is measured. Our results help to rationalize the observations beyond simple phenomenological models and may serve as a guide to design the material according to the desired specifications. Full article
(This article belongs to the Special Issue New Trends in Lithium Niobate: From Bulk to Nanocrystals)
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Article
Electro-Optic Control of Lithium Niobate Bulk Whispering Gallery Resonators: Analysis of the Distribution of Externally Applied Electric Fields
Crystals 2021, 11(3), 298; https://doi.org/10.3390/cryst11030298 - 17 Mar 2021
Cited by 1 | Viewed by 505
Abstract
Whispering gallery resonators made out of lithium niobate allow for optical parametric oscillation and frequency comb generation employing the outstanding second-order nonlinear-optical properties of this material. An important knob to tune and control these processes is, e.g., the linear electro-optic effect, the Pockels [...] Read more.
Whispering gallery resonators made out of lithium niobate allow for optical parametric oscillation and frequency comb generation employing the outstanding second-order nonlinear-optical properties of this material. An important knob to tune and control these processes is, e.g., the linear electro-optic effect, the Pockels effect via externally applied electric fields. Due to the shape of the resonators a precise prediction of the electric field strength that affects the optical mode is non-trivial. Here, we study the average strength of the electric field in z-direction in the region of the optical mode for different configurations and geometries of lithium niobate whispering gallery resonators with the help of the finite element method. We find that in some configurations almost 100% is present in the cavity compared to the ideal case of a cylindrical resonator. Even in the case of a few-mode resonator with a very thin rim we find a strength of 90%. Our results give useful design considerations for future arrangements that may benefit from the strong electro-optic effect in bulk whispering gallery resonators made out of lithium niobate. Full article
(This article belongs to the Special Issue New Trends in Lithium Niobate: From Bulk to Nanocrystals)
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Article
“Seeing Is Believing”—In-Depth Analysis by Co-Imaging of Periodically-Poled X-Cut Lithium Niobate Thin Films
Crystals 2021, 11(3), 288; https://doi.org/10.3390/cryst11030288 - 15 Mar 2021
Cited by 1 | Viewed by 637
Abstract
Nonlinear and quantum optical devices based on periodically-poled thin film lithium niobate (PP-TFLN) have gained considerable interest lately, due to their significantly improved performance as compared to their bulk counterparts. Nevertheless, performance parameters such as conversion efficiency, minimum pump power, and spectral bandwidth [...] Read more.
Nonlinear and quantum optical devices based on periodically-poled thin film lithium niobate (PP-TFLN) have gained considerable interest lately, due to their significantly improved performance as compared to their bulk counterparts. Nevertheless, performance parameters such as conversion efficiency, minimum pump power, and spectral bandwidth strongly depend on the quality of the domain structure in these PP-TFLN samples, e.g., their homogeneity and duty cycle, as well as on the overlap and penetration depth of domains with the waveguide mode. Hence, in order to propose improved fabrication protocols, a profound quality control of domain structures is needed that allows quantifying and thoroughly analyzing these parameters. In this paper, we propose to combine a set of nanometer-to-micrometer-scale imaging techniques, i.e., piezoresponse force microscopy (PFM), second-harmonic generation (SHG), and Raman spectroscopy (RS), to access the relevant and crucial sample properties through cross-correlating these methods. Based on our findings, we designate SHG to be the best-suited standard imaging technique for this purpose, in particular when investigating the domain poling process in x-cut TFLNs. While PFM is excellently recommended for near-surface high-resolution imaging, RS provides thorough insights into stress and/or defect distributions, as associated with these domain structures. In this context, our work here indicates unexpectedly large signs for internal fields occurring in x-cut PP-TFLNs that are substantially larger as compared to previous observations in bulk LN. Full article
(This article belongs to the Special Issue New Trends in Lithium Niobate: From Bulk to Nanocrystals)
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Article
Effects of the Domain Wall Conductivity on the Domain Formation under AFM-Tip Voltages in Ion-Sliced LiNbO3 Films
Crystals 2020, 10(12), 1160; https://doi.org/10.3390/cryst10121160 - 19 Dec 2020
Viewed by 897
Abstract
The specified domain patterns were written by AFM-tip voltages in LiNbO3 films composing LNOI (LiNbO3-on-insulator). The domain wall conductivity (DWC) was estimated in the written patterns. This estimate was based on the effects of load resistors RL inserted between [...] Read more.
The specified domain patterns were written by AFM-tip voltages in LiNbO3 films composing LNOI (LiNbO3-on-insulator). The domain wall conductivity (DWC) was estimated in the written patterns. This estimate was based on the effects of load resistors RL inserted between DWs and the ground, on the features of occurring domains. In this case, the domain formation is controlled by the ratio between RL and the DWs’ resistance RDW. Starting from the comparison of patterns appearing at different RL, the value of RDW in a specified pattern was estimated. The corresponding DWC is of σDW ≈ 10−3 (Ohm cm)−1 which exceeds the tabular bulk conductivity of LiNbO3 by no less than twelve orders of magnitude. A small DW inclination angle of (10−4)0 responsible for this DWC is not caused by any external action and characterizes the domain frontal growth under an AFM-tip voltage. Full article
(This article belongs to the Special Issue New Trends in Lithium Niobate: From Bulk to Nanocrystals)
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Article
Effect of Temperature on Luminescence of LiNbO3 Crystals Single-Doped with Sm3+, Tb3+, or Dy3+ Ions
Crystals 2020, 10(11), 1034; https://doi.org/10.3390/cryst10111034 - 13 Nov 2020
Cited by 1 | Viewed by 425
Abstract
Crystals of LiNbO3 single-doped with Sm3+, Tb3+, or Dy3+ and crystal of LiTaO3 single-doped with Tb3+ were grown by the Czochralski method. Luminescence spectra and decay curves for LiNbO3 samples containing Sm3+ or [...] Read more.
Crystals of LiNbO3 single-doped with Sm3+, Tb3+, or Dy3+ and crystal of LiTaO3 single-doped with Tb3+ were grown by the Czochralski method. Luminescence spectra and decay curves for LiNbO3 samples containing Sm3+ or Dy3+ ions were recorded at different temperatures between 295 and 775 K, whereas those for samples containing Tb3+ ions were recorded at different temperatures between 10 and 300 K. Optical absorption spectra at different temperatures were recorded within the UV-blue region relevant to optical pumping of the samples. It was found that the effect of temperature on experimental luminescence lifetimes consists of the initial temperature-independent stage followed by a steep decrease with the onset at about 700, 600, and 150 K for Sm3+, Dy3+, and Tb3+ ions, respectively. Additionally, comparison of temperature impact on luminescence properties of LiNbO3:Tb3+ and LiTaO3:Tb3+ crystals has been adequately described. Experimental results were interpreted in terms of temperature-dependent charge transfer (CT) transitions within the modified Temperature—Dependent Charge Transfer phenomenological model (TDCT). Disparity of the onset temperatures and their sequence were explained based on the location of familiar zigzag curves connecting the ground state levels of rare earth ions with respect to the band-gap of the host. It was concluded also that LiNbO3:Sm3+ is suitable as an optical sensor within the 500–750 K temperature region whereas LiNbO3:Dy3+ offers the highest sensitivity at lower temperatures between 300 and 400 K. Full article
(This article belongs to the Special Issue New Trends in Lithium Niobate: From Bulk to Nanocrystals)
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Article
Wavelength-Tunable Nonlinear Mirror Mode-Locked Laser Based on MgO-Doped Lithium Niobate
Crystals 2020, 10(10), 861; https://doi.org/10.3390/cryst10100861 - 24 Sep 2020
Cited by 1 | Viewed by 624
Abstract
We present a high-power, wavelength-tunable picosecond Yb3+: CaGdAlO4 (Yb:CALGO) laser based on MgO-doped lithium niobate (MgO:LN) nonlinear mirror mode locking. The output wavelength in the continuous wave (CW) regime is tunable over a 45 nm broad range. Mode locking with [...] Read more.
We present a high-power, wavelength-tunable picosecond Yb3+: CaGdAlO4 (Yb:CALGO) laser based on MgO-doped lithium niobate (MgO:LN) nonlinear mirror mode locking. The output wavelength in the continuous wave (CW) regime is tunable over a 45 nm broad range. Mode locking with a MgO:LN nonlinear mirror, the picosecond laser is tunable over 23 nm from 1039 to 1062 nm. The maximum output power of the mode-locked laser reaches 1.46 W, and the slope efficiency is 18.6%. The output pulse duration at 1049 nm is 8 ps. The laser repetition rate and bandwidth are 115.5 MHz and 1.7 nm, respectively. Full article
(This article belongs to the Special Issue New Trends in Lithium Niobate: From Bulk to Nanocrystals)
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Article
Small-Polaron Hopping and Low-Temperature (45–225 K) Photo-Induced Transient Absorption in Magnesium-Doped Lithium Niobate
Crystals 2020, 10(9), 809; https://doi.org/10.3390/cryst10090809 - 14 Sep 2020
Cited by 2 | Viewed by 731
Abstract
A strongly temperature-dependent photo-induced transient absorption is measured in 6.5 mol% magnesium-doped lithium niobate at temperatures ranging from 45 K to 225 K. This phenomenon is interpreted as resulting from the generation and subsequent recombination of oppositely charged small polarons. Initial two-photon absorptions [...] Read more.
A strongly temperature-dependent photo-induced transient absorption is measured in 6.5 mol% magnesium-doped lithium niobate at temperatures ranging from 45 K to 225 K. This phenomenon is interpreted as resulting from the generation and subsequent recombination of oppositely charged small polarons. Initial two-photon absorptions generate separated oppositely charged small polarons. The existence of these small polarons is monitored by the presence of their characteristic absorption. The strongly temperature-dependent decay of this absorption occurs as series of thermally assisted hops of small polarons that facilitate their merger and ultimate recombination. Our measurements span the high-temperature regime, where small-polaron jump rates are Arrhenius and strongly dependent on temperature, and the intermediate-temperature regime, where small-polaron jump rates are non-Arrhenius and weakly dependent on temperature. Distinctively, this model provides a good representation of our data with reasonable values of its two parameters: Arrhenius small-polaron hopping’s activation energy and the material’s characteristic phonon frequency. Full article
(This article belongs to the Special Issue New Trends in Lithium Niobate: From Bulk to Nanocrystals)
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Review

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Review
‘Horror Vacui’ in the Oxygen Sublattice of Lithium Niobate Made Affordable by Cationic Flexibility
Crystals 2021, 11(7), 764; https://doi.org/10.3390/cryst11070764 - 29 Jun 2021
Viewed by 369
Abstract
The present review is intended for a broader audience interested in the resolution of the several decades-long controversy on the possible role of oxygen-vacancy defects in LiNbO3. Confronting ideas of a selected series of papers from classical experiments to brand new [...] Read more.
The present review is intended for a broader audience interested in the resolution of the several decades-long controversy on the possible role of oxygen-vacancy defects in LiNbO3. Confronting ideas of a selected series of papers from classical experiments to brand new large-scale calculations, a unified interpretation of the defect generation and annealing mechanisms governing processes during thermo- and mechanochemical treatments and irradiations of various types is presented. The dominant role of as-grown and freshly generated Nb antisite defects as traps for small polarons and bipolarons is demonstrated, while mobile lithium vacancies, also acting as hole traps, are shown to provide flexible charge compensation needed for stability. The close relationship between LiNbO3 and the Li battery materials LiNb3O8 and Li3NbO4 is pointed out. The oxygen sublattice of the bulk plays a much more passive role, whereas oxygen loss and Li2O segregation take place in external or internal surface layers of a few nanometers. Full article
(This article belongs to the Special Issue New Trends in Lithium Niobate: From Bulk to Nanocrystals)
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Review
Unveiling the Defect Structure of Lithium Niobate with Nuclear Methods
Crystals 2021, 11(5), 501; https://doi.org/10.3390/cryst11050501 - 02 May 2021
Cited by 1 | Viewed by 524
Abstract
X-ray and neutron diffraction studies succeeded in the 1960s to determine the principal structural properties of congruent lithium niobate. However, the nature of the intrinsic defects related to the non-stoichiometry of this material remained an object of controversial discussion. In addition, the incorporation [...] Read more.
X-ray and neutron diffraction studies succeeded in the 1960s to determine the principal structural properties of congruent lithium niobate. However, the nature of the intrinsic defects related to the non-stoichiometry of this material remained an object of controversial discussion. In addition, the incorporation mechanism for dopants in the crystal lattice, showing a solubility range from about 0.1 mol% for rare earths to 9 mol% for some elements (e.g., Ti and Mg), stayed unresolved. Various different models for the formation of these defect structures were developed and required experimental verification. In this paper, we review the outstanding role of nuclear physics based methods in the process of unveiling the kind of intrinsic defects formed in congruent lithium niobate and the rules governing the incorporation of dopants. Complementary results in the isostructural compound lithium tantalate are reviewed for the case of the ferroelectric-paraelectric phase transition. We focus especially on the use of ion beam analysis under channeling conditions for the direct determination of dopant lattice sites and intrinsic defects and on Perturbed Angular Correlation measurements probing the local environment of dopants in the host lattice yielding independent and complementary information. Full article
(This article belongs to the Special Issue New Trends in Lithium Niobate: From Bulk to Nanocrystals)
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Review
Epitaxy of LiNbO3: Historical Challenges and Recent Success
Crystals 2021, 11(4), 397; https://doi.org/10.3390/cryst11040397 - 09 Apr 2021
Viewed by 478
Abstract
High-quality epitaxial growth of thin film lithium niobate (LiNbO3) is highly desirable for optical and acoustic device applications. Despite decades of research, current state-of-the-art epitaxial techniques are limited by either the material quality or growth rates needed for practical devices. In [...] Read more.
High-quality epitaxial growth of thin film lithium niobate (LiNbO3) is highly desirable for optical and acoustic device applications. Despite decades of research, current state-of-the-art epitaxial techniques are limited by either the material quality or growth rates needed for practical devices. In this paper, we provide a short summary of the primary challenges of lithium niobate epitaxy followed by a brief historical review of lithium niobate epitaxy for prevalent epitaxial techniques. Available figures of merit for crystalline quality and optical transmission losses are given for each growth method. The highest crystalline quality lithium niobate thin film was recently grown by halide-based molecular beam epitaxy and is comparable to bulk lithium niobate crystals. However, these high-quality crystals are grown at slow rates that limit many practical applications. Given the many challenges that lithium niobate epitaxy imposes and the wide variety of methods that have unsuccessfully attempted to surmount these barriers, new approaches to lithium niobate epitaxy are required to meet the need for simultaneously high crystalline quality and sufficient thickness for devices not currently practical by existing techniques. Full article
(This article belongs to the Special Issue New Trends in Lithium Niobate: From Bulk to Nanocrystals)
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Review
Structures of Impurity Defects in Lithium Niobate and Tantalate Derived from Electron Paramagnetic and Electron Nuclear Double Resonance Data
Crystals 2021, 11(4), 339; https://doi.org/10.3390/cryst11040339 - 27 Mar 2021
Cited by 1 | Viewed by 360
Abstract
Point intrinsic and extrinsic defects, especially paramagnetic ions of transition metals and rare-earth elements, have essential influence on properties of lithium niobate, LN and tantalate, LT, and often determine their suitability for numerous applications. Discussions about structures of the defects in LN/LT have [...] Read more.
Point intrinsic and extrinsic defects, especially paramagnetic ions of transition metals and rare-earth elements, have essential influence on properties of lithium niobate, LN and tantalate, LT, and often determine their suitability for numerous applications. Discussions about structures of the defects in LN/LT have lasted for decades. Many experimental methods facilitate progress in determining the structures of impurity centers. This paper gives current bird’s eye view on contributions of Electron Paramagnetic Resonance (EPR), and Electron Nuclear Double Resonance (ENDOR) studies to the determination of impurity defect structures in LN and LT crystals for a broad audience of researchers and students. Symmetry and charge compensation considerations restrict a number of possible structures. Comparison of measured angular dependences of ENDOR frequencies with calculated ones for Li and Nb substitution using dipole–dipole approximation allows unambiguously to determine the exact location of paramagnetic impurities. Models with two lithium vacancies explain angular dependencies of EPR spectra for Me3+ ions substituting for Li+ like Cr, Er, Fe, Gd, Nd, and Yb. Self-compensation of excessive charges through equalization of concentrations of Me3+(Li+) and Me3+(Nb5+) and appearance of interstitial Li+ in the structural vacancy near Me3+(Nb5+) take place in stoichiometric LN/LT due to lack of intrinsic defects. Full article
(This article belongs to the Special Issue New Trends in Lithium Niobate: From Bulk to Nanocrystals)
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Review
Real-Time Identification of Oxygen Vacancy Centers in LiNbO3 and SrTiO3 during Irradiation with High Energy Particles
Crystals 2021, 11(3), 315; https://doi.org/10.3390/cryst11030315 - 22 Mar 2021
Cited by 2 | Viewed by 524
Abstract
Oxygen vacancies are known to play a central role in the optoelectronic properties of oxide perovskites. A detailed description of the exact mechanisms by which oxygen vacancies govern such properties, however, is still quite incomplete. The unambiguous identification of oxygen vacancies has been [...] Read more.
Oxygen vacancies are known to play a central role in the optoelectronic properties of oxide perovskites. A detailed description of the exact mechanisms by which oxygen vacancies govern such properties, however, is still quite incomplete. The unambiguous identification of oxygen vacancies has been a subject of intense discussion. Interest in oxygen vacancies is not purely academic. Precise control of oxygen vacancies has potential technological benefits in optoelectronic devices. In this review paper, we focus our attention on the generation of oxygen vacancies by irradiation with high energy particles. Irradiation constitutes an efficient and reliable strategy to introduce, monitor, and characterize oxygen vacancies. Unfortunately, this technique has been underexploited despite its demonstrated advantages. This review revisits the main experimental results that have been obtained for oxygen vacancy centers (a) under high energy electron irradiation (100 keV–1 MeV) in LiNbO3, and (b) during irradiation with high-energy heavy (1–20 MeV) ions in SrTiO3. In both cases, the experiments have used real-time and in situ optical detection. Moreover, the present paper discusses the obtained results in relation to present knowledge from both the experimental and theoretical perspectives. Our view is that a consistent picture is now emerging on the structure and relevant optical features (absorption and emission spectra) of these centers. One key aspect of the topic pertains to the generation of self-trapped electrons as small polarons by irradiation of the crystal lattice and their stabilization by oxygen vacancies. What has been learned by observing the interplay between polarons and vacancies has inspired new models for color centers in dielectric crystals, models which represent an advancement from the early models of color centers in alkali halides and simple oxides. The topic discussed in this review is particularly useful to better understand the complex effects of different types of radiation on the defect structure of those materials, therefore providing relevant clues for nuclear engineering applications. Full article
(This article belongs to the Special Issue New Trends in Lithium Niobate: From Bulk to Nanocrystals)
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Review
Lithium Niobate Single Crystals and Powders Reviewed—Part II
Crystals 2020, 10(11), 990; https://doi.org/10.3390/cryst10110990 - 31 Oct 2020
Cited by 4 | Viewed by 822
Abstract
A review on lithium niobate single crystals and polycrystals has been prepared. Both the classical and recent literature on this topic is revisited. It is composed of two parts with several sections. The current part discusses the available defect models (intrinsic), the trends [...] Read more.
A review on lithium niobate single crystals and polycrystals has been prepared. Both the classical and recent literature on this topic is revisited. It is composed of two parts with several sections. The current part discusses the available defect models (intrinsic), the trends found in ion-doped crystals and polycrystals (extrinsic defects), the fundamentals on dilute magnetic oxides, and their connection to ferromagnetic behavior in lithium niobate. Full article
(This article belongs to the Special Issue New Trends in Lithium Niobate: From Bulk to Nanocrystals)
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Review
Lithium Niobate Single Crystals and Powders Reviewed—Part I
Crystals 2020, 10(11), 973; https://doi.org/10.3390/cryst10110973 - 27 Oct 2020
Cited by 5 | Viewed by 1083
Abstract
A review of lithium niobate single crystals and polycrystals in the form of powders has been prepared. Both the classical and recent literature on this topic are revisited. It is composed of two parts with sections. The current part discusses the earliest developments [...] Read more.
A review of lithium niobate single crystals and polycrystals in the form of powders has been prepared. Both the classical and recent literature on this topic are revisited. It is composed of two parts with sections. The current part discusses the earliest developments in this field. It treats in detail the basic concepts, the crystal structure, some of the established indirect methods to determine the chemical composition, and the main mechanisms that lead to the manifestation of ferroelectricity. Emphasis has been put on the powdered version of this material: methods of synthesis, the accurate determination of its chemical composition, and its role in new and potential applications are discussed. Historical remarks can be found scattered throughout this contribution. Particularly, an old conception of the crystal structure thought as a derivative structure from one of higher symmetry by generalized distortion is here revived. Full article
(This article belongs to the Special Issue New Trends in Lithium Niobate: From Bulk to Nanocrystals)
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