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Crystals 2018, 8(5), 210; doi:10.3390/cryst8050210
Lattice Parameters of Optical Damage Resistant In-Doped LiNbO3 Crystals
Shubnikov Institute of Crystallography of FSRC “Crystallography and Photonics” RAS, 119333 Moscow, Russia
NRC Kurchatov Institute, Ploshchad’ Akademika Kurchatova, 123098 Moscow, Russia
Author to whom correspondence should be addressed.
Received: 17 April 2018 / Accepted: 3 May 2018 / Published: 13 May 2018
The lattice parameters in optical damage resistant crystal LiNbO3-In were measured for the first time using the X-ray powder method with an internal standard, which provides a high accuracy of the results. The lattice parameters vs. In concentration were obtained in the concentration range from 0.24 to 3.2 at % In in the crystal. The results are discussed in the framework of currently accepted model of the LiNbO3 intrinsic defect structure.
Keywords:lithium niobate; structure measurements; optical-damage resistance
Lithium niobate remains one of the basic materials for various optical applications, in particular for nonlinear–optical frequency conversion. The main problem limiting the exploitation of LiNbO3 in the high-intensity optics is the photoinduced change of refractive indices and birefringence, referred to as “optical damage” or “photorefraction” . Several means have been proposed to reduce this negative effect. The most efficient method is doping with the group of impurities termed “optical-damage resistant” (“ODRI” below); the development of this approach can be found, e.g., in . Presently, this group includes reliably seven ions, namely the divalent Mg  and Zn , trivalent In  and Sc  and tetravalent Hf , Zr  and Sn  (this numbering refers to the order of publications).
The similarity in the effects of these impurities on various LiNbO3 properties, which originates from a common structure reason, allows us to regard them as a “family”. In the framework of the current approach, ODRI doping similarly to an increase of the Li-content serves as a powerful tool for modifying the intrinsic defect structure which controls all practically important LiNbO3 properties.
Recently, an interest in the ODRI family has revived again due to the encouraging results in LiNbO3-Zr crystals, in which a pronounced reduction of the photorefraction occurs at relatively low Zr concentrations (for refs. see the review ). The effect is comparable to doping with 4–5 at % Mg. Thus, low Zr concentrations strongly reducing the optical damage, at the same time affect the optical quality to a less degree than high Mg-doping. The approach proposed for the interpretation of Mg (Zn) doping effects, appeared to be not completely adequate for the trivalent and tetravalent ODRI, thus, a refinement of the current model is required. Certain progress is achieved owing to recent studies in ODRI-doped stoichiometric LiNbO3 (SLN) crystals, e.g., [10,11].
The approach to the intrinsic defect structure discussed below was based on the pioneering X-ray and neutron investigations of the congruently melting LiNbO3 (CLN) [12,13,14]. Structure studies in ODRI-doped compositions are few in number. X-ray studies of Mg-doped LiNbO3 were executed in [15,16,17,18]. X-ray and neutron investigations of LiNbO3-Zn were performed by us [19,20,21]; later on, the lattice parameters of LiNbO3-Zn were measured in . Recently, in Zr-doped SLN crystals the lattice parameters vs. Zr concentration were reported . In more detail, these studies will be discussed below comparing them to the results obtained in In-doped LiNbO3.
One important comment should be made. As various cations and vacancies can enter both (Li- and Nb-) lattice sites in LiNbO3, so the determination of the occupancy and development of a reliable defect structure require a combination of X-ray and neutron studies using powder samples [14,21,24]. This approach provides an advantage of minor extinction problems in powders and better detection of Li with neutrons. Obviously, the reliability of determination of small impurity concentrations demands extremely high accuracy for such experiments. The precise measurements of LiNbO3-In lattice parameters presented here are aimed at planned investigations of In-incorporation into the lattice for further studies in the photorefractive and related properties of LiNbO3-In, started formerly in [25,26].
Structure measurements in LiNbO3-In crystals which are presented here were performed for the first time.
2. Results and Discussion
Table 1 and Figure 1a,b show the lattice parameters vs. In concentration in the crystal. The zero points in the curves correspond to the pure (undoped) CLN. Figure 1a presents a-and c-lattice parameters vs. [In] (in the low-temperature phase LiNbO3 has R3c space group symmetry), depicted by the triangles and squares, respectively. The quality and uniformity of the crystals deteriorates with the In concentration, so relative errors of In content increase. Figure 1b presents the calculated unit cell volume V (the upper curve); for comparison, V vs. Zn-concentration is shown (the low dashed curve) calculated from our former measurements in LiNbO3-Zn .
To discuss these results, in Table 2 we present the ionic radii ri of basic lattice ions and ODRI ions, taken from . As seen, In3+ and Zr4+ are the largest ODRI ions. In Figure 1b, in addition to the graphs V vs. In and Zn concentrations (the lower and upper curves, respectively), we show the unit-cell volume for LiNbO3-3%ZrO2 estimated from the data of . In accordance with Vegard’s law, the unit-cell parameters, in particular the unit-cell volume V smoothly increase with the impurity concentrations, the V value being noticeably larger for the substitutional impurities having the larger ionic radii (In and Zr).
However, at low In concentrations (of 0.25 and 0.5 at %) Vegard’s law seems to be violated, since a drop in the parameter c occurs with respect to this value in the undoped CLN (Figure 1a). At further increase of In concentration, the parameter c steeply grows. Unlike c, the parameter a steeply rises starting from the lowest In concentrations.
A non-monotonous behavior of c-parameter at low In concentrations is an intriguing feature, since this drop in c seems to be a common property of ODRI-doped crystals. It was observed in CLN doped with the divalent Zn at [Zn] ≤3 at %  and in SLN doped with the tetravalent Zr  at ZrO2 concentrations of 1.5 and 2.0 mol %. A drop of c in LiNbO3-In occurs in a narrow concentration range of 0.5 at % whereas in LiNbO3-Zn and LiNbO3-Zr this anomaly is smeared over a range of about 2.5–3% [19,23]. It is reasonable to suggest a common structural origin of these anomalies in the framework of the current model of LiNbO3 intrinsic defect structure (the detailed bibliography can be found, e.g., in ). To start with, the background of this concept should be considered. In [12,13,14] it has been found that a part of Li-sites in CLN (Li-deficient crystals) is occupied by Nb-ions. According to , Nb occupies 4.9% of the Li-sites; the later estimate  gave a value of about 1%. In the framework of the currently accepted “Li-vacancy model” , an excess positive charge of [NbLi]4 is compensated by 4[VLi]′ (here and below the Kröger–Vink notations are used: a heavy point and a prime denote, respectively, the positive and negative charges with respect to the lattice). The existence of Nb-on-Li-sites (“Nb-antisites”) is the cornerstone of the model of LiNbO3 intrinsic defect structure.
In the Li-vacancy model, the CLN chemical formula is presented by the expression:where the group in the braces corresponds to the population of Li sites.
The scenario of ODRI incorporation into the LiNbO3 lattice (refs. in ) is still under debate. Recently, a general consideration of different impurity incorporation into the LiNbO3 lattice was presented in . An a priori confidence about ODRI incorporation into the Li-sites, at least at low doping was confirmed by numerous experiments. Based on the chemical microanalysis measurements in LiNbO3-Mg [17,24], it has been concluded that increasing ODRI concentration leads to a gradual removal of Nb-antisites, which results in a gradual variation of the crystal properties; the effects are qualitatively similar to increasing Li-content. At certain “threshold” concentrations (of 5.5 at % Mg and 7 at % Zn), a drastic decrease of photorefraction and step-wise changes of many properties (a shift of the OH-vibration absorption band, a kink in the concentration dependences of birefringence, etc. ) occur. The structural origin of these thresholds is the most debatable problem. According to the model proposed for the divalent ODRI  and supported by the structure measurements in LiNbO3-Zn , the threshold concentration is related to a partial incorporation of ODRI ions into the Nb-sites.
An anomaly in the concentration dependence of c-parameter, discussed in the present work, is observed in the ranges below the thresholds. The threshold concentrations are of 1.7 at % In and 7 at % Zn, respectively, whereas a decreased c is observed at [In] from 0.25 to 0.5 at % (Figure 1) and [Zn] ≤3 at % . In Zr-doped crystals, the threshold is of 2 mol % ZrO2, while a drop of c appears at ZrO2 of about 1.5 mol % ZrO2 .
Let us consider the incorporation of ODRI into LiNbO3 at concentrations below the thresholds. For clarity, we present the formulas for the divalent Mg and Zn (denoted Me) (2) and trivalent In (3)where the groups in the brackets correspond to the population of Li sites, x and y represent the number of Li-sites occupied by Nb and ODRI, respectively; and denote the divalent (Mg or Zn) and trivalent In ions, respectively, on the Li-sites.
On the basis of precise chemical data, the authors of [17,24] proposed a two-stage scenario of Mg incorporation into LiNbO3, according to which at concentrations up to 2–3%, Mg ions remove Nb-antisites, and afterwards, they start to substitute for LiLi, forming compensated by Li-vacancies. The discussed anomalies of lattice parameters in LiNbO3-Zn  and LiNbO3-Zr  can be adequately interpreted in the framework of this two-stage scenario. Namely, in LiNbO3-Zn a decrease of c  and a plateau in the lattice-cell volume (Figure 1b) below 3% Zn is accounted for by the removal of Nb-antisites, whereas an increase of c and V at [Zn] >3% is related to increasing number of Li-vacancies. According to the calculations of , Nb-antisites disappear at divalent-impurity concentrations of 3%, which is in excellent agreement with the data for LiNbO3-Zn. For the tetravalent ODRI, the calculations of  predict the removal of Nb-antistes at concentrations of about 1.6 mol %. A drop of c-parameter in LiNbO3-Zr is observed at [ZrO2] of 1.5 and 2.0 mol % , which permits us to attribute this anomaly to the removal of Nb-anisites again. Analogously, it would be reasonable to assign a decrease of c-parameter in LiNbO3-In at [In] ≤0.5 at % (Figure 1a) to the disappearance of NbLi. However, this conclusion disagrees with the calculations of , according to which Nb-antisites disappear at the trivalent ODRI concentration of 2.1%. Moreover, based on the IR measurements in LiNbO3-Sc, the authors of  arrived at the conclusion that Nb-antisites are absent at Sc2O3 > 2 mol %, which agrees with the estimates of .
It is important to emphasize that in the framework of the Li-vacancy model, the only reason of a decreased c-parameter in doped CLN is a decreased content of Nb-antisites. So, the anomaly of c-parameter at In-concentrations ≤0.5 at % is related to the removal of Nb-antisites. We now speculate about the reason of why the concentration range of the structure anomaly in LiNbO3-In does not conform to the consideration of . The calculations of  have shown that for trivalent ions the incorporation into two-cation (Li- and Nb-) sites in LiNbO3 is energetically favorable since it provides the charge self-compensation; in the case of In-doping it would be presented as –. We suggest that because of the large ionic radius (Table 2), the incorporation of In-ions into the Li-sites is hampered, so they enter both cation sites before the total disappearance of NbLi. In other word, a minimum of c at low In concentrations can be related to an incomplete removal of Nb-antisites. A distinction from the trivalent Sc3+ might be caused by the less ionic radius of Sc3+ (Table 2), which facilitates its incorporation into the Li-site. The specificity of In-doping is indirectly supported by the measurements of IR absorption spectra [25,26] which permitted us to suggest the incorporation of In- ions into both cation sites.
Although this reasoning is too speculative, we believe that an anomaly of the lattice parameter c observed in low concentration ranges in several ODRI-doped LiNbO3 originates from a common microscopic reason related to the intrinsic defect structure and is worthy of further investigations.
3. Materials and Methods
The crystals were grown by the Czochralski technique from the congruent melt with the ratio [Li]/[Nb] = 0.942. The concentration of In2O3 in the melt was varied from 0.12 to 2 mol %. In this concentration range the distribution coefficient of In falls down from 1 to 0.8 ; in the graphs shown above we present at % In in the crystals. Emphasize that all compositions under study, both undoped and In-doped ones (as well as Zn-doped LiNbO3 reported by us formerly [3,19,20,21]) were grown from the same starting charge. An identity in the amount of unintended impurity trace substantiates the reliability of comparisons.
X-ray powder diffraction experiment was performed using X’PERT PRO MRD diffractometer (PANalytical, Almelo, the Netherlands) in the reflection Bragg-Brentano parafocusing geometry (CuKα radiation, λ = 1.5418 Å, Ni β-filter). The Kα1 contribution (λ = 1.5405929 Å) was used further for calculations. We used small pieces of single crystals (about 1 mm3) and delicately grinded them between two small sapphire bricks so that not to induce substantial line broadening. We poured a thin layer of the powder of about 10 µm thick onto the standard Si wafer holder which was rotated in its plane during the exposure for better averaging over individual grains. A drop of spirit was used to provide a smoother surface. We used the internal standard method to improve the accuracy of the lattice parameters measurement. Cubic LaB6 NIST №660a standard was used (a = 4.1569162 ± 0.0000097 Å at T = 22.5 °C). As is known, this method enables one to make measurements both for the sample and standard actually in the same condition, thus removing possible deviations during the reloading. Our approach was as follows. The powder patterns of a sample-standard mixture were registered within the range of the diffraction angles of 2θ = 50–120°. Then, we made the mathematical fitting by pseudo-Voigt functions both for the peaks of LiNbO3-In and LaB6. Thus, the obtained values of the diffraction angles for the standard were compared with the reference angles. The difference was interpolated by quadratic parabolas. Finally, the initially measured values were corrected mathematically and, to be sure, the obtained Bragg peak positions of LaB6 were used to calculate the lattice cell parameter by the program DICVOL . Mean absolute angular deviations were within of about Δ2θ = 0.0004°, which enables one to obtain the cubic cell parameter of the standard with a very good accuracy comparable to that designated in the certificate. Thus, typical accuracy for the cell parameter was a = 4.15691(2) Å, which is in a very good coincidence with the reference value. Then we used the same correction curves to calculate the Bragg peak positions for LiNbO3-In and the obtained values were used to calculate the lattice parameters by the program DICVOL. A typical calculation is presented below in Table 3.
As seen, the accuracy is quite good and the standard deviations are small.
For the first time, the lattice parameters were measured with the very high accuracy in optical-damage-resistant LiNbO3-In crystals with In concentrations in the range from 0.12 to 3.2 at % In in the crystal. At [In] ≤0.5% the concentration dependence of the parameter c is non-monotonous and reveals a drop with respect to the c-parameter in undoped LiNbO3. Since similar non-monotonous concentration dependence of the c-parameter were observed in other ODRI-doped LiNbO3 crystals (LiNbO3-Zn, LiNbO3-Zr), a possible common reason is discussed in the framework of the currently accepted model of the intrinsic defect structure of LiNbO3.
S.S. performed the X-ray measurements and analyzed the data, T.V. analyzed the data and wrote the paper.
This work was supported by the Federal Agency of Scientific Organizations (Agreement No. 007-ST/P3363/26). The equipment of the Shared Research Center supported by the Ministry of Education and Science (Project No RFMEFI62114X0005) was used in experiments.
Conflicts of Interest
The authors declare no conflict of interest.
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Figure 1. (a) Lattice parameters a (upper, red triangles) and c (lower, black squares with error bars) vs. In concentration in LiNbO3-In crystal. (b) Unit-cell volume V vs. In (upper) and Zn (lower) concentrations calculated for LiNbO3-In and LiNbO3-Zn crystals. The curve for LiNbO3-Zn is calculated based on the data of . The upper asterisk shows V for LiNbO3-3% ZrO2 calculated from  (A noncoincidence of the reference points for undoped CLN in the curves for LiNbO3-In and LiNbO3-Zn is within the experimental error).
Table 1. Dependence of the lattice parameters on In concentration.
|In at % in the Crystal||c, Å||a, Å||V, Å3|
Table 2. Ionic radii .
Table 3. The DICVOL output for the congruent LiNbO3 after the Bragg peak position correction.
DIRECT PARAMETERS: a = 5.15054 Å, c = 13.86753 Å, V = 318.59 Å3
STANDARD DEVIATIONS: 0.00001 0.00006
FIGURES OF MERIT: 1.- M(17) = 171.8; 2.- F(17) = 67.3(0.0019, 136).
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