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.

https://www.mdpi.com/2073-4352/10/11/990/pdf

Lithium Niobate Single Crystals and Powders Reviewed—Part I

Measurements of refractive indices, electro-optic coefficients and photorefractivity are performed for a set of Zirconium-doped congruent lithium niobate (Zr:LN) crystals as functions of the dopant concentration in the range 0.0-3.0 mol%. The photorefractive properties are studied by measuring the green-light induced birefringence change and by direct observation of the transmitted-beam distortion. The index of refraction data show that the threshold concentration, above which there is a change in the Zr incorporation mechanism, is about 2.0 mol%, but photorefractivity results suggest that the concentration of ZrO2 required to strongly reduce the photorefractive effect is somewhat larger than the 2.0 mol% “threshold” concentration derived from index-of-refraction data. The electro-optic coefficients are little influenced by Zr-doping. All the reported results confirm that Zr:LN is a very promising candidate for the realization of efficient electro-optic and all-optical nonlinear devices working at room temperature.

Zirconium-doped lithium niobate: photorefractive and electro-optical properties as a function of dopant concentration

We report the optical and dielectric properties in hafnium (Hf)-doped lithium niobate (LN) crystals. We investigated samples of congruent composition with various doping concentration varying from 0 to 8 mol%. The clamped and unclamped values of the electro-optic coefficient r 22 of Hf-doped LN and the corresponding dielectric permittivity as well, have been experimentally determined and compared with the results obtained in undoped congruent LN crystals. We show that the electro-optic and dielectric properties are only slightly affected by the introduction of hafnium ions, and therefore Hf-doped LN has the advantage of low photorefractive damage compared with the undoped congruent LN.

/pdf/electro-optic-and-dielectric-properties-of-hafnium-doped-2qqmkzf2jz.pdf

Electro-optic and dielectric properties of Hafnium-doped congruent lithium niobate crystals

The defect structures of impurity-doped congruent lithium niobates (c-LN) were determined for impurities with various valences, including divalent, trivalent, and tetravalent impurities, in a concentration range where antisite niobium (NbLi) exists. On the basis of the “Li site vacancy model,” six kinds of defect structures in impurity-doped c-LN are possible. Using thermodynamic constraints, these can be narrowed down to two kinds. The first structure is that in which impurities, vacancies and Nb exist as defects in the Li site and no defects exist in the Nb site (structure A), described as {[LiLi] 1-5x-jy[NbLi]x[MLi]y[VLi]4x+(j-1)y}[NbNb][OO] 3 (V: vacancy, M: impurity, j: valence of impurity, x, y: compositional variable (≠0), Li/Nb = congruent ratio). {[LiLi×] 1-5x-2y[NbLi••••]x[MLi•]y[VLi′]4x+y}[NbNb×][OO×] 3 is an example by the Kroger-Vink notation for divalent M. In the second structure, vacancies and Nb exist as defects in the Li site and impurities exist as defects in the Nb site (structure B), ...

Investigation of defect structure of impurity-doped lithium niobate by combining thermodynamic constraints with lattice constant variations

Measurements of electro-optic and dielectric properties in Zirconium (Zr)-doped lithium niobate (LN:Zr) crystals are performed as functions of the dopant concentration in the range 0.0-2.5 mol%. The clamped and unclamped electro-optic coefficients r 222 of Zr-doped LN and the corresponding dielectric permittivity as well, have been experimentally determined and compared with the results obtained in undoped congruent LN crystals. We show that the electro-optic and dielectric properties present a kink around 2 mol% of zirconium which seems to be the " threshold " concentration required to strongly reduce the photorefractive effect. All reported results confirm that the LN:Zr is a very promising candidate for several non linear devices.

https://hal.archives-ouvertes.fr/hal-01467230/document

Electro-optic and dielectric properties of Zirconium-doped congruent lithium–niobate crystals

Zirconium doped lithium niobate is a promising candidate as a substrate for nonlinear optical applications, since it does not suffer from the so-called “optical damage.” In order to optimize this aspect, the proper Zr concentration has be used, hence the precise determination of the so-called “threshold concentration,” i.e., the concentration above which the photorefractive effect is markedly reduced, is of great importance. In this work, we prepared by Czochralski growth a series of Zr-doped lithium niobate crystals with various Zr content and studied them using structural (high-resolution x-ray diffraction) and optical (birefringence) measurements as a function of the dopant content in the melt. Both the approaches pointed out a marked change in the crystal characteristics for a Zr concentration between 1.5 and 2 mol %, a value which is identified as the threshold concentration.

Structural and optical properties of zirconium doped lithium niobate crystals

We present a careful investigation of the optical properties of Zr-doped lithium-niobate crystals. We also investigate in detail the threshold concentration for Zr-doping, by means of three different optical measurement techniques.

Optical and structural properties of Zirconium doped lithium niobate crystals

Lithium niobate (LN) is an extremely interesting material for the realization of optical devices and circuits, thanks to its good optical and electro-optical properties. At the state of the art, the use of LN crystals for nonlinear optical functions is limited, despite the very large d33 nonlinear coefficient exhibited by this crystal, because LN is severely affected by the so called “optical damage” due to the photorefractive effect [1]. When a high intensity, spatially inhomogeneous, visible light beam impinges on the LN crystal, a significant and non uniform electric field is produced in the crystal, because of the low crystal photoconductivity, and this yields (through the electro-optic effect) a non-uniform modification of the crystal refractive indices. As a consequence, the efficiency of nonlinear interactions in LN can be greatly reduced, and it becomes necessary to operate the device at high temperature (in some cases even > 200°C), in order to get rid of photorefractivity. In order to operate the nonlinear device at room temperature (or close to it), it is possible to use LN crystals doped with a small quantity of appropriate elements able to significantly increase crystal photoconductivity, like Mg, In, or Sc [1], but it is difficult to obtain high-quality crystals when LN is doped with such ions. Recently it was shown that by using tetravalent ions, like Hf or Zr, the dopant concentration required to significantly reduce the crystal photorefractivity is considerably lower than that required for bivalent and trivalent dopants, even if some uncertainty about the exact required concentration is still present due to the discrepancies between the values reported in the literature. The low dopant-concentration, in combination with the fact that the segregation coefficient of the Zr ion is reported to be very close to 1, should allow growing more easily large homogeneous high-optical-quality Zr:LN crystals.

Photorefractivity, electro-optical coefficients and refractive indices of Zr-doped LiNbO 3 crystals

The experimental data show that the minimum dopant concentration required to strongly reduce the photorefractive effect is close to 2% (in accordance with (3)), and that the presence of Zr in the crystal lattice doesn't reduce the electro-optical properties of the crystal. The impact of the dopant concentration on the ordinary and extraordinary refractive indices was also evaluated. The obtained results indicate that Zr:LN can be a very good candidate for the design of efficient nonlinear all-optical devices.

electro-optical coefficients and refractive indices of Zr-doped LiNb03 crystals

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Structural and optical properties of zirconium doped lithium niobate crystals

Optical and structural properties of Zirconium doped lithium niobate crystals

Photorefractivity, electro-optical coefficients and refractive indices of Zr-doped LiNbO 3 crystals

electro-optical coefficients and refractive indices of Zr-doped LiNb03 crystals