The widespread use of lithium niobate (LN) in several technological applications, notably in optical and electrooptical systems, is a consequence of its remarkable piezoelectric, electrooptical, photoelastic, acousto-optic, and nonlinear optical coefficients. In this chapter, the structural and electrical characterization of LN nanosized particles synthesized by the Pechini route is discussed. Compared to solid-state reaction processes, wet chemistry processes can be advantageous alternatives for the synthesis of polycrystalline LN, because they require lower processing temperatures, and thus the loss of stoichiometry and formation of secondary phases can be minimized. The powders obtained by drying the gel (base powder) were heat-treated for 4 h at temperatures between 400 and 1000°C, according to the differential thermal analysis (DTA) results. It was found that the powders sintered at 450°C contain only the LN phase, while those heat-treated at 500°C already contain the secondary LiNb3O8 phase. The structural and electrical characterization of the samples sintered at 450°C, for different times, was performed using X-ray diffraction (XRD) in conjunction with Rietveld refinement, Raman spectroscopy, scanning electron microscopy (SEM), and impedance spectroscopy in the temperature range between 200 and 360 K and in the frequency range between 100 Hz and 1 MHz and by measuring the ac and dc conductivities.
- lithium niobate
- structural properties
- X-ray spectroscopy
- Raman spectroscopy
- electrical properties
- lithium triniobate
- sol-gel process
Lithium niobate (LiNbO3, LN) is a well-known artificially synthesized ferroelectric material with considerable technological importance, being in competition with barium titanate (BaTiO3, BTO) in several high-tech applications. In fact, an inspection of the number of publications related with LN and BTO will show that since the mid-1990s, the number of reports on both materials has been following the same increasing trend, with similar number of publications, reinforcing the importance of LN among the scientific communities. Table 1 displays some of the main physical properties of single-crystalline LN .
|Melting temperature (°C)||1260|
|Curie temperature (°C)||1210|
|Density at RT (g/cm3)||4.64|
|Refractive index (ordinary),
|Electrooptical coefficient, r33 (m/V)||30 × 10−12|
|Transparency window (μm)||0.4–5|
|Dielectric constant at RT (
||80 (100 kHz)
>1000 (1 kHz)
|Dielectric loss at RT (tan
||≈0 (100 kHz)|
|Coercive field (at 1210°C) (V/m)||20|
|Spontaneous polarization at RT (
|Piezoelectric coefficient (d33 [pC/N])||6|
|Thermal conductivity at RT (W m−1 K−1)||3.92|
LN single crystals display several excellent properties, such as high piezoelectric, electrooptical, photoelastic, acousto-optic, and nonlinear optical coefficients. They are known to have very low acoustic losses, offering a great versatility as a substrate for integrated optic systems: a considerable number of optical devices have been developed based on LN, such as wave guides, surface acoustic wave (SAW) devices, electrooptical wavelength filters and polarization modulators, nonlinear frequency converters (frequency doubling and second harmonic generation), nonvolatile memories, and ultrafast optical processing systems. Its combination of electrooptical and photogalvanic effects makes it photorefractive without the need of applying an external electrical field, thus being able to be applied in holographic data storage. It offers also the possibility of being easily doped, in a controllable way, with optical-active ionic species, using standard techniques such as ion implantation or thermal diffusion.
The physical properties aforementioned are optimized for the case where LN is grown as a single crystal and with stoichiometric composition. Regarding the stoichiometry, LN has a relatively broad composition range, and therefore, it can be labeled as congruent lithium niobate (cLN, 48.35–48.6 mol% Li2O) and stoichiometric (sLN, ~50 mol% Li2O). Figure 1 displays the phase diagram of the Li2O-Nb2O5 binary system, showing the possibility of growing pure LN crystals by using 50% up to ~52% of Nb2O5. It also shows the transition of ferroelectric phase to paraelectric by increasing the synthesis temperature and Nb2O5 content. The large majority of LN single crystals are grown by the conventional Czochralski method, which yields cLN single crystals. Some competitor methods have been developed for growing stoichiometric crystals, including the vapor transport equilibration (VTE) method, which is a post-grown procedure . The former is more suitable for thin and small samples, because for larger and thicker crystals, very large solid-state diffusion times are required for the Li/Nb ratio equilibration. To attain larger stoichiometric single crystals, more direct growth methods can be applied, such as the double crucible Czochralski method with an automatic power supply [5, 6] or the high-temperature top-seeded solution growth (HTTSSG) method from the K2O-Li2O-Nb2O5 ternary mixture, which is the one capable of yielding compositions closest to 50 mol% Li2O [5, 7].
The nonlinear and photorefractive properties will generally degrade with the loss of stoichiometry and consequent increase of defects and impurity density. As a matter of fact, the congruent composition range is regarded as having an intrinsic defect structure which is dominated by lithium vacancies, known as the lithium vacancy model, which translates in Eq. (1) :
In this model, as depicted in Eq. (1), for every four lithium vacancies created, a niobium ion occupies a lithium network site, assuring electrical charge neutrality. The cLN is not suitable for high-temperature applications, because degradation processes can start to occur for temperatures starting from 300°C [6, 8]. On the other hand, studies show that sLN can be stable up to temperatures of at least 900°C [6, 9], because some properties like the electrical conductivity do not change under thermal cycling up to such temperatures.
As it was aforementioned, the dominant process in the growth of LN single crystals is the Czochralski method. However, this method is known for its technical and economic drawbacks, as well as being time-consuming. Thus, alternative preparation processes have been researched and explored. Solid-state reaction processes generally require high processing temperatures (>1000°C), which lead to the loss of lithium by evaporation . As a consequence, secondary crystalline phases such as Li3NbO4 and LiNb3O8 can be formed, changing the stoichiometry and deteriorating the properties. Wet chemistry methods, such as sol-gel methodologies and hydrothermal methods, can be good alternatives because they require lower processing temperatures, such as calcination and thermal-sintering treatments, and thus the formation of secondary phases can be minimized [10, 11]. Single crystals in the nanometer and micrometer size ranges can be synthesized at low temperatures, such as 240°C, by these methods . When such methods are applied, typically polycrystalline LN samples are produced, i.e., a material composed by small single crystals in the micrometric or nanometric range randomly distributed with no evident preferential orientation. Polycrystalline LN finds a lot of applications, especially as thin films for integrated optic applications, although generally the properties of polycrystalline materials are not as good as their single-crystalline counterpart. For example, the piezoelectric properties of polycrystalline LN are inferior to the single crystal, and in the best case, they might approach them if all of its domains are perfectly orientated. Further, relative to single-crystalline thin films, the grain boundaries in polycrystalline films may lead to increased light scattering and larger optical losses in wave guides, which may reduce their utility and potentiality in some applications . However, their production is cheaper and easier compared with the growth processes for single crystals, and all these cons and drawbacks have to be considered and well balanced for potential applications.
Amorphous LN is also important for some applications. In an amorphous material, there is no long-range order, and the network can be described as distorted unitary cells randomly oriented. LN single crystals and polycrystals have low electrical conductivity (~10−12 S/cm at 500 K for single crystals ), and ionic diffusion or mobility is reduced in these materials. However, the amorphous structure is always a more open structure compared with the crystalline composition or, in other words, has a smaller density, which promotes and facilitates the ionic diffusivity, making them suitable for technical application such as solid-state electrolytes for Li-ion batteries. In fact, the reported activation energies for the ionic diffusivity are considerably smaller (half in the 25–150°C temperature range ) in amorphous LN.
In this chapter, the structural and electrical characterization of LiNbO3 nanosized particles prepared by the Pechini route, also known in the sol-gel methodologies as the polymeric precursor route, is discussed. The powders obtained by drying the gel (base powder) were heat-treated for 4 h at temperatures between 400 and 1000°C, according to the differential thermal analysis (DTA) results. The sintering temperature revealed to be, as expected, an important parameter in controlling the development of secondary crystalline phases, and it was found that the powders sintered at 450°C contain only the LN phase, while those heat-treated at 500°C already contain the secondary LiNb3O8 phase. Their structural characterization was performed using X-ray diffraction (XRD) in conjunction with Rietveld refinement, Raman spectroscopy, and scanning electron microscopy (SEM). The grains observed have sizes lower than 100 nm and an approximately spherical geometry. The electrical characterization of pellets made from the base powder heat-treated at 450°C was made by measuring the dc and ac conductivities and measuring the complex impedance (
2. Structural, morphologic, and electrical properties
2.1. Structural properties
LN belongs to a group of materials whose crystalline structure is a perovskite. This structure has the typical chemical formula ABO3, where the cation A usually is too large for an effective complete packaging, causing a distortion in the unit cell and leading to a displacement of the O2− anions from their expected sites. However, in the case of LN, the distortion is related with the small radius of the lithium ion. For temperatures lower than 1415 K, which is the Curie temperature (
As depicted in Figure 2, in the ferroelectric state, the displacement of the Li+ and Nb5+ cations relatively to the center of the oxygen planes originates a spontaneous polarization along the c-axis with a magnitude of 0.7 C/m2 at 300 K (see Table 1). The displacement can be up or down with respect to the oxygen sublattice, and both cations are displaced in the same direction because of the Coulomb repulsion. Above the Curie temperature, due to the thermal expansion of the crystalline lattice axes, the structure is no longer distorted, because the Li+ and Nb5+ cations move to lattice sites lying in the planes of the oxygen layers, as Figure 2(b) displays. Thus, the transition to the paraelectric state occurs, and LN ceases to exhibit a permanent spontaneous polarization [2, 14].
As it was stated in Section 1, polycrystalline LN is also of great technological importance. In this form, the structure can be described as composed by small single crystals in the micrometric or nanometric range randomly distributed with no evident preferential orientation. Relatively to the ferroelectricity, it is composed by several ferroelectric domains, which are regions with different orientations of the spontaneous polarization
This model is based on the second-order phase transition as described by Landau-Ginsburg; with at least a fourth-order polynomial in
The above-considered model fails when predicting quantities such as the coercive field, both for congruent and stoichiometric LN, because the inversion mechanisms of
As it was stated, the terms depolarization energy
As for the structure of amorphous LN, it was said that the network can be described as distorted unitary cells randomly oriented. Figure 5 reveals the effect of heat treatments (HTs) on amorphous LN prepared by complete hydrolysis of the LN double alkoxide . The XRD spectra of amorphous LN have the typical form of the spectrum for 473 K displayed in Figure 5, with two broad bands around 30 and 50–60°. These broad bands are a trademark of amorphous materials, and they are typically visible for diffraction angles where the crystalline phase has the most intense diffraction peaks, revealing at least a short-range-order preservation. The heat treatments promote the reconfiguration of the amorphous phase to a more thermodynamically stable crystalline phase; although for low treatment temperatures and times, the material may be composed by a heterogeneous mixture of an amorphous and a crystalline phase: for the heat treatment at 573 K for 0.5 h, it is still noticeable the coexistence of a broad band with the diffraction peaks of the LN crystalline phase . The structure of amorphous LN was also described by Kitabatake et al. to be constructed from the network of NbO6 octahedra which contains a micronetwork similar to crystalline LN . The high dielectric constant and a relaxation mechanism were attributed to the high mobility of the Li+ ion in the LN structure .
Going further on the structural properties, the Raman spectroscopy is a useful nondestructive technique to access about the structure and composition of materials. The Raman spectrum of LN will generally depend on its stoichiometry, i.e., the shape, width, and position of some Raman shifts may change according to the Li/Nb ratio . Furthermore, the Li/Nb ratio of a given LN sample may be determined by analyzing the width of some Raman lines, for a given temperature [18, 19].
Figure 6 exhibits experimental full width at half maximum (FWHM) values of the Raman lines detected at about 153 and 876 cm−1, for samples with different lithium contents (mol%) . The measurements were carried out at room temperature (note that the Raman lines’ width also depends on the temperature). The FWHM dependency with the Li content is approximately linear, and hence a calibration line can be obtained. The uncertainty related with the Li content determination by this method was calculated to be 0.05 mol%, with an estimated uncertainty of 0.2 cm−1 in the 876 cm−1 line FWHM and 0.1 cm−1 in the 153 cm−1 line . Therefore, this technique can be a simple nondestructive method to estimate the Li/Nb ratio in LN crystals, with an excellent accuracy.
Figure 7 displays the Raman spectra of a nearly stoichiometric LN crystal, with
2.2. Morphological characteristics
Morphology means “the study of form or pattern,” and morphological characterization techniques, such as scanning electron microscopy (SEM) or transmission electron microscopy (TEM), allow to characterize the morphology of a given material. The morphological characteristics of LN will obviously rely upon the synthesis process, stoichiometry, and concentration of intrinsic defects. Depending on the synthesis process and crystal growth conditions, the grains observed for polycrystalline LN can display well-defined symmetries and a different range of sizes.
Zhan et al.  report in their work a low-temperature hydrothermal route to prepare polycrystalline LN. The XRD characterization revealed the formation of a pure hexagonal single phase of stoichiometric LN. The SEM micrograph of the LN powder, presented in Figure 8, shows a regular rhombohedral grain morphology, in agreement with the XRD results, although some imperfections, such as bended surfaces, are visible. The grain size ranges from 300 nm to approximately 1 μm. Kamali et al.  prepared polycrystalline LN using a modification of the molten salt synthesis (MSS): in conventional MSS, the mixed powders are heated above the liquids’ temperature of the salt mixture, and this molten salt acts as the reaction medium, remaining inert during the synthesis. Salt mixtures such as KCl-NaCl are typically used. In the MSS modification approached by , the salt can react with other reagents during the synthesis process, being labeled as reactive molten salt synthesis (RMSS). They heat-treated at 973 K Nb2Cl5 powder in a LiCl molten salt, in a water-containing atmosphere, whereby the molten salt is one of the precursors for LN synthesis . Using this RMSS approach, they produced single-phased LN, i.e., the loss by evaporation of Li2O was avoided. A SEM micrograph of the obtained LN particles is shown in Figure 8, on the right side . The revealed morphology shows grains with dimensions ranging from several hundreds of nanometers to a few micrometers. The rhombohedral symmetry is also visible, although cleavages and bends are visible.
In Figure 9, it is visible a TEM bright-field micrograph of the polycrystalline LN prepared by the RMSS process. The inset on the right top shows a selected area electron diffraction pattern, being the area marked by the black arrow. The diffraction pattern is consistent with single-crystalline rhombohedral LN. The inset on the left bottom shows a high-resolution micrograph of the area marked by the white arrow. The rhombohedral LN (104) atomic plane patterns are visible, with an interplanar spacing of 0.27 nm .
The grain morphology will be determined by the growth conditions in such a way that the final morphology reflects the configuration with the minimum surface energy. In crystalline solid materials, the surface tension will depend on the crystallographic planes and direction, because to create a new surface, it is necessary to break bonds. At a constant pressure and temperature, the work required to create a new portion of surface
The formation of new surfaces leads to a positive Gibbs energy contribution, whereby smaller particles will be unstable when compared with larger particles. The equilibrium morphology of the crystal will be determined by the surfaces with lower Gibbs energy, while surfaces of higher energy are sacrificed. Different crystallographic planes will have a different number of bonds per unit of area, and the bond strengths can also change according to the composition. Actually, very often when a surface energy value of a given crystalline material is indicated, it is in fact an average of the surface energy of the different crystalline faces. Taking as example a face-centered cubic lattice, when increasing the Miller indices, typically the atomic density of the planes decreases. The exception is that in the plane family [1 1 1], which contains six nearest neighbors, three bonds for each surface atom have to be broken when cutting the crystal along such direction, while for [1 0 0] and [1 1 0] planes, with lower atomic density, four and six bonds have to be broken, respectively . Therefore, the surface energy of the former planes is larger relatively to the [1 1 1] plane family. Planes with the highest density have a lower surface energy and rate of growth, and therefore the final morphology of the crystal growth will be defined by the high-density atomic planes [21, 22]. However, several studies have indicated that a spherical morphology, which is the configuration that minimizes the surface area, is energetically more favorable for solids at high temperatures, and the difference of surface energy between different crystallographic planes becomes a less important factor .
2.3. Electrical and dielectric properties
In this section, we will start to address the electrical properties of LN single crystalline and polycrystalline. Afterward, we will address polycrystalline LN. However, for both cases, it is important to analyze the properties for different temperature ranges and different stoichiometries, since both parameters have influence on the mechanisms of electrical conduction.
It is well known that the electrical conductivity of LN single crystals, as well as some optical properties, is strongly dependent on the surrounding environmental characteristics, in particular the partial oxygen pressure
The electrical properties of LN are conditioned by the oxidation or reduction atmosphere, during thermal annealing: in a low
The presence of a maximum in
and therefore the release of oxygen during reduction originates free electrons that can get trapped in niobium ions, and the conduction mechanism is assigned to polaronic hopping between Nb5+ and Nb4+ cations . The maximum and minimum observed in Figure 11 can be explained according to the ratio of Nb4+/Nb5+ states: for low reduction temperatures, few Nb4+ states will be created, while for high reduction temperatures, Nb4+ will predominate. However, for intermediate reduction temperature, there will be a case where we will get Nb4+/Nb5+ = 0.5. In that case, a maximum in the conductivity and a minimum in
For higher temperature ranges, in reduced LN single crystals, the VRH of polarons continues to be one of the mechanisms for the electrical conduction. However, and especially for congruent samples, lithium diffusion starts to be thermally activated, and when increasing the temperature, the main contribution for the total electrical conductivity may become ionic, assigned to lithium diffusion through lithium vacancies in the network (once again, we recall the lithium vacancy model) . Figure 12 shows the Arrhenius representation of the total electrical conductivity of LN single-crystalline samples with different lithium oxide molar percentages, in the temperature range between 773 and 1173 K . The conductivity increases with the decrease of the Li2O content, indicating the influence of lithium diffusion through lithium vacancies. The ionic conductivity will be larger for congruent LN, because the density of lithium vacancies is larger.
With respect to the frequency dependency of the electrical properties, typically studied by means of impedance spectroscopy (IS), some results of the work done by Mansingh and Dhar will be addressed, namely, those related with the ac electrical conductivity (
Figure 13 shows for a congruent LN single crystal the dependence of
Figure 14 shows the frequency dependence of
This is the well-known relation found by Mott and Davis which describes the frequency dependency of
The temperature dependence of
It can be noted in Figure 16 that the temperature from which
We will end this section by briefly addressing the electrical and dielectric properties of polycrystalline LN. In such case, the behavior of the referred properties can reflect the presence of grain boundaries in the material. This effect of grain boundaries can be more clearly seen in IS measurements, because the characteristic frequencies (or times) at which grain boundary processes occur are different from those that occur in the bulk of the grains, and therefore Nyquist diagrams [plot of the negative of the imaginary part of the impedance, −Im(Z), in the y-axis over the real part of the impedance, Re(Z), in the x-axis] originate successive semicircles, where each point of the semicircle corresponds to a different frequency value which increases counterclockwise. Lanfredi and Rodrigues report in their work IS studies of the electrical conductivity and dielectric constant of polycrystalline LN . Figure 17 presents Nyquist diagrams for different temperatures of two polycrystalline LN samples
Comparing both diagrams, it is visible that for the same temperature, the real part of the complex impedance Re(Z) of sample
The complex impedance semicircle for the bulk response can be well fitted by simple
To conclude this section, it is included in Figure 19 the frequency dependency, between 5 and 107 Hz, of
For lower frequencies, a strong dispersion of
2.4. Case study: preparation and characterization of polycrystalline LN by the Pechini method
2.4.1. Preparation process: the Pechini route
The sol-gel process is a well-known route for the synthesis of different types of materials, and in its basic description, it can be referred as a technique that synthesizes a solid compound through a chemical reaction in solution at low temperatures. There are different sol-gel methodologies according to the type of precursors used and the chemical reactions leading to the formation of the gel: there is the sol-gel methodology based on the hydrolysis-condensation of metal alkoxides, the “chelate-gel” route, involving aqueous solutions containing metal chelates, and the Pechini route. The sol-gel methodologies have general advantages such as the very good control of the stoichiometry and purity of the final material, low processing temperatures, possibility, and good flexibility in developing thin films as well as the possibility to have control over some important characteristics, such as the size and shape of the particles and homogeneity.
The Pechini route takes its name on its developer, Maggio Pechini in 1967 . In particular, it was developed to include metals which are not suitable for traditional sol-gel reactions due to their unfavorable hydrolysis equilibria, and thus this method has the advantage of not requiring that the metallic species involved form stable hydroxo-complexes. This method is known for its use on the synthesis of multicomponent metal oxide materials , and basically this method uses an R-hydroxycarboxylic acid, such as citric acid (CA), to lead the formation of stable metal complex, i.e., the metallic cations of interest form stable complexes known as chelates. After this step, a polyalcohol, such as ethylene glycol (EG), is used to promote the polyesterification of the chelates, leading to the formation of a polymeric resin, where the metallic cations are trapped in the organic polymeric network. In other words, the polyalcohol is able to create links between the chelates by polyesterification reactions. The formation of the polymeric resin results in the formation of the gel. The subsequent drying process leads to the pyrolysis of the organic compounds, resulting in the formation of multicomponent metal oxide .
In this case study, the precursors lithium nitrate (LiNO3) and niobium chloride (NbCl5) (purity > 99%) were chosen. A molar ratio of 1:1 between LiNO3 and NbCl5 was established in order to enhance the formation of the LiNbO3 stoichiometric phase. Firstly, the LiNO3 and NbCl5 were dissolved in deionized water and in a hydrogen peroxide solution (H2O2, 3%, V/V), respectively. For each gram of NbCl5, 3.2 ml of H2O2 was used, originating a yellow transparent and clear solution. Both precursor solutions were mixed with citric acid (CA), fixing a molar ratio of 1:1 between the CA and the metallic cations, in order to form the metal complexes (chelates). The mixing was performed using a magnetic stirrer for 30 min at room temperature. After the mixing process, ethylene glycol (EG) was added to promote the polyesterification of the chelates. A mass ratio of 2:3 was established between the CA and EG to determine the quantity of EG to use. The final solution was mixed again with a magnetic stirrer for about 3 h, and the final gel was yellow and transparent, maintaining its macroscopic appearance for a long time period (>1 month). Figure 20 outlines the entire preparation process.
The base powder was obtained after drying the gel at 300°C for 1 h, with a heating ramp of 5°C/min, yielding a black/grayish powder.
2.4.2. Thermal, structural, and morphological properties
The base powder was subjected to several heat treatments (HTs) at temperatures between 400 and 1000°C. These temperatures were chosen according to the differential thermal analysis (DTA) results, presented in Figure 21. This thermal technique was performed between room temperature and 1200 °C using a Linseis 63A apparatus. The heating rate was 20 °C/min and Al2O3 powder as used as reference. In Figure 21, the thermogram shows the presence of three exothermic thermal processes at the temperatures of 490, 790, and 1050°C, approximately. Consequently, HTs were performed at 450, 500, 800, and 1000°C, for 4 h.
The XRD measurements performed on these powders (not shown here) revealed that for the HT at 500°C, the LN and LiNb3O8 crystalline phases are present and that the increase of the HT temperature promotes the development of the LiNb3O8 phase. However, the powder HT at 450°C only contained the LiNbO3 phase, whereby this powder was used to prepare pellets which were then sintered at 450°C for 4, 12, 24, 48, and 96 h. Thirty milligrams of powder was used for the preparation of the 10-mm diameter pellets, resulting in a thickness of about 1 mm when applying a uniaxial pressure of 1.5 tons. Hereafter, we will refer to these pellets as samples.
Figure 22 depicts the XRD patterns of the samples sintered during the aforementioned time intervals. The patterns show that the sintering process activated the formation of the LiNb3O8 phase. The XRD technique was performed on a Philips X’Pert MPD (CuKα radiation,
The Rietveld fit parameters indicate a good fit of the structural models to the experimental data (Table 2). Although the XRD patterns shown in Figure 22 may suggest that the LiNb3O8 phase is present in small amounts, the mass percentages shown in Table 2 show that the presence of this phase is relevant, reaching the maximum value for the sample 4 h + 96 h. In fact, both LN and LiNb3O8 have reflections lying in close diffraction angles, and some of the observed peaks contain a contribution of the LiNb3O8 phase, besides the LN phase, explaining the relatively high mass percentages. The crystallite size of both phases stands approximately constant for the different samples, especially for the LiNb3O8 phase, which has larger sizes relatively to the LN phase.
||Crystallite size (nm)
|4 h + 4 h||10.40||8.72||1.42||45.52||61.94||72.48||27.52||8.64||0.06|
|4 h + 12 h||6.26||4.77||1.72||48.30||61.32||66.48||33.52||9.37||0.05|
|4 h + 24 h||6.31||4.49||1.97||50.23||61.03||66.94||33.06||16.85||0.09|
|4 h + 48 h||9.72||8.23||1.39||44.84||62.28||75.90||24.10||16.36||0.13|
|4 h + 96 h||9.37||7.70||1.48||46.65||59.50||61.76||38.24||13.06||0.18|
In Figure 24, the Raman spectra of the sintered samples are presented. The spectra show the presence of vibrational bands which are the result of an overlapping of vibrational models of LN and also of the LiNb3O8 crystalline phase, as a consequence of the polycrystalline structure of the prepared samples. For a further analysis of the LN and LiNb3O8 vibrational modes, the authors suggest the reading of Bartasyte et al. report . The room-temperature Raman spectroscopy was performed in backscattering geometry using a T64000 Jobin-Yvon spectrometer. A microscope objective (50×) focused the exciting light (Ar+ laser,
In Figure 25, SEM micrographs of the samples are shown, as well as the variation of the grain size with the sintering time. The grain size increases significantly with the sintering time from 4 to 12 h and then decreases slightly up to 96 h. As for the morphology, the grains show an approximate spherical geometry. The SEM was executed on a HITACHI S4100-1. All samples were sputtered with carbon before the analysis.
2.4.3. Electrical and dielectric properties
The electrical analysis of these types of samples is not evident. Figure 26 shows the temperature dependence of the dc conductivity (
However, Figure 27 shows that the activation energy, calculated in the high-temperature region (marked by the Arrhenius regression line in Figure 26), decreases substantially from sample sintered at 4 h to the sample sintered at 96 h. This profile indicates that the height of the potential barriers, from which the charge carriers must pass through, decreases, which should promote an increase in their mobility. Therefore, this analysis indicates a decrease in the number of charge carriers and also an increase in their mobility.
As discussed in Section 2.3, the electrical conduction in crystalline LN can be due to oxygen vacancies, in low-pressure and/or high-temperature conditions (polaron-hopping mechanism), and due to lithium ions in high-pressure conditions (ionic conduction). In this case, the conduction will depend on the ratio between Li/Nb both for high and low pressures. Therefore, in this case context, the conduction related with the lithium-ion (Li+) hypothesis seems to be more relevant. The existence of the LiNb3O8 secondary phase, which is a reach niobium LN phase, implies an increase of the number of Li+ ions not connected to a crystalline structure, which allied to the decrease of the activation energy should improve the
Both dc and ac conductivities (Figures 26–29) increase with the rise of the measurement temperature, showing that both mechanisms are thermally activated. However, with the increase in the sintering time, the
The dielectric constant is maximum for the sample sintered at 24 h (Table 2), measured at room temperature and 10 kHz, which is in accordance with the Rietveld refinement results (Table 2), by presenting the highest crystallite size. The sample sintered at 48 h shows a dielectric constant value very near the maximum, which should be related with the amount of LN phase present in the sample and not the size of their grains (Table 2). It must be noted that the values presented in Table 2 were obtained without assuming the existence of the hypothetical amorphous phase. These two samples present a dielectric loss below 0.15 at room temperature and 10 kHz (Table 2), being almost constant for temperatures between 200 and 300 K.
The 3D plots (Figures 30 and 31) show the presence of a dielectric relaxation phenomenon practically independent on the measurement temperature. However, the frequency of the maximum observed in the loss tangent diagram decreases with the increase of the sintering time, which can be related to the suggested decrease of the amorphous phase content and consequent increase of the LN in crystalline form which is a material where the dielectric depolarization is very difficult, i.e., high-relaxation time characteristic.
Abouelleil M.M. and Leonberger F.J., Waveguides in lithium niobate, J. Am. Ceram. Soc., 1989, 72(8):1311-1321
Graça M.P.F. and Valente M.A. Glass ceramics with para, anti or ferroelectric active phases, Advances in Ceramics – Electric and Magnetic Ceramics, Bioceramics, Ceramics and Environment, Prof. Sikalidis C. (Ed.), InTech (2011). Janeza Trdine 9, 51000 Rijeka, Croatia, ISBN: 978-953-307-350-7.
Volk T. and Wohlecke (2008), Lithium Niobate: Defects, Photorefraction and Ferroelectric Switching ,Springer-Verlag, Berlin, Heidelberg, ISBN: 978-3-540-70765-3.
Bordui P.F., Norwood R.G., Jundt D.H., Fejer M.M., Preparation and characterization of off-congruent lithium niobate crystals, J. Appl. Phys., 1992, 71(2):875-879
Kitamura K., Yamamoto J.K., Iyi N., Kimura S., Hayashi T., Stoichiometric LiNbO 3 single crystal growth by double crucible Czochralski method using automatic powder supply system, J. Cryst. Growth, 1992, 116:327-332
Weidenfelder A., Shi J., Fielitz P., Borchardt G., Becker K.D., Fritze H., Electrical and electromechanical properties of stoichiometric lithium niobate at high-temperatures, Solid State Ionics, 2012, 225:26-29
Szaller Zs., Péter Á., Polgár K., Szabó Gy., High temperature top seeded solution growth of stoichiometric lithium niobate LiNbO 3 (sLN) with planar interface, J. Cryst. Growth, 2012, 360:181-184
Damjanovic D., Materials for high temperature piezoelectric transducers, Curr. Opin. Solid State Mater. Sci., 1998, 3(5):469-473
Ohlendorf G., Richter D., Sauerwald J., Fritze H., High-temperature electrical conductivity and electromechanical properties of stoichiometric lithium niobate, Diff. Fundam., 2008, 8:6.1-6.7
Kamali A.R. and Fray D.J., Preparation of lithium niobate particles via reactive molten salt synthesis method, Ceram. Int., 2014, 40:1835-1841
Graça M.P.F., Prezas P.R., Costa M.M., Valente M.A., Structural and dielectric characterization of LiNbO 3 nano-size powders obtained by Pechini method, J. Sol-Gel Sci. Technol., 2012, 64:78-85
Howlader M.M.R., Suga T., Kim M.J., Room temperature bonding of silicon and lithium niobate, Appl. Phys. Lett., 2006, 89:031914
Rahn J., Hüger E., Dörrer L., Ruprecht B., Heitijans P., Schmidt H., Self-diffusion of lithium in amorphous lithium niobate layers, Z. Phys. Chem., 2012, 226(5-6):439-448
Sanna S. and Schmidt W.G., Ferroelectric phase transition in LiNbO 3 : insights from molecular dynamics, IEEE Ultrason. Ferr., 2012, 59(9):1925-1928
Scrymgeour D.A., Gopalan V., Itagi A., Saxena A., Swart P.J., Phenomenological theory of a single domain wall in uniaxial trigonal ferroelectrics: lithium niobate and lithium tantalite, Phys. Rev. B, 2005, 71(18):184110-1/13
Heitjans P., Masoud M., Feldhoff A., Wilkening M., NMR and impedance studies of nanocrystalline and amorphous ion conductors: lithium niobate as a model system, Faraday Discuss., 2007, 134:67-82
Kitabatake M., Mitsuyu T., Wasa K., Structure and dielectric properties of amorphous LiNbO 3 thin films prepared by a sputtering deposition, J. Appl. Phys., 1984, 56:1780-1784
Ridah A., Bourson P., Fontana M.D., Malovichko G., The composition dependence of the Raman spectrum and new assignment of the phonons in LiNbO 3, J. Phys.: Condens. Matter., 1997, 9(44):9687-9693
Schlarb U., Klauer S., Wesselmann M., Betzler K., Wöhlecke M., Determination of the Li/Nb ratio in lithium niobate by means of birefringence and Raman measurements, Appl. Phys. A, 1993, 56(4):311-315
Damen T.C., Porto S.P.S., Tell B., Raman effect in zinc oxide, Phys. Rev., 1966, 142(2):570-574
Zhan J., Liu D., Du W., Wang Z., Wang P., Cheng H., et al., Synthesis and characterization of high crystallinity, well-defined morphology stoichiometric lithium niobate nanocrystalline, J. Cryst. Growth, 2011, 318(1):1121-1124
Stølen S. and Grande T. (2005), Chemical Thermodynamics of Materials: Macroscopic and Microscopic Aspects, Wiley, Hoboken, NJ, ISBN: 978-0-471-49230-6
Dhar A., Singh N., Singh R.K., Singh R., Low temperature dc electrical conduction in reduced lithium niobate single crystals, J. Phys. Chem. Solids, 2013, 74(1):146-151
Mansingh A. and Dhar A., The ac conductivity and dielectric constant of lithium niobate single crystals, J. Phys. D: Appl. Phys., 1985, 18(10):2059-2071
Lanfredi S. and Rodrigues A.C.M., Impedance spectroscopy study of the electrical conductivity and dielectric constant of polycrystalline LiNbO 3, J. Appl. Phys., 1999, 86(4):2215-2219
Roque-Malherbe R.M.A. (2009), The Physical Chemistry of Materials: Energy and Environmental Applications, CRC Press, 6000 Broken Sound Parkway, NW Suite 300 Boca Raton, FL 33487, United States, ISBN: 9781420082722.
Bartasyte A., Plausinaitiene V., Abrutis A., Stanionyte S., Margueron S., Boulet P., et al. , Identification of LiNbO 3 , LiNb 3 O 8 and Li 3 NbO 4 phases in thin films synthesized with different deposition techniques by means of XRD and Raman spectroscopy, J. Phys.: Condens. Matter., 2013, 25(20):205901.