FLUX GROWTH OF KTiOPO 4 SINGLE CRYSTALS DOPED WITH Me 41 IONS

V. Nikolov1, I. Koseva1, P. Peshev1, X. Solans2, R. Sole3, X. Ruiz3, Jna. Gavalda3,J. Massons3, M. Aguilo 3, and F. Dıaz3*

1Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences,1113 Sofia, Bulgaria

2Departament de Cristalzlografia i Mineralogia, Univ. Barcelona, 08028 Barcelona, Spain3Laboratori de Fı´sica Aplicada i Cristalzlografia, Univ. Rovira i Virgili,

43005 Tarragona, Spain

(Refereed)(Received July 28, 1998; Accepted November 6, 1998)

ABSTRACTIn the present work, potassium titanyl phosphate single crystals doped withdifferent levels of Ge41, Zr41, and Ce41 were successfully grown by fluxusing the top-seeded solution growth (TSSG) method. Their saturation tem-peratures and crystal habits were compared with those of the undoped singlecrystals and some differences were observed. In addition, the distributioncoefficients of the different dopants were also determined and found to be incorrelation with the ionic radii of the M41 ions. Zr41 doping gives the mostsuitable distribution coefficient, does not introduce problematic absorptionbands in the visible region, and significantly reduces the ionic conductivity inthec direction. © 1999 Elsevier Science Ltd

KEYWORDS: A. optical materials, B. crystal growth, D. ionic conductivity

INTRODUCTION

Potassium titanyl phosphate, KTiOPO4 (hereafter KTP), single crystals are good frequency-doubling materials for laser devices emitting at nearly 1064 nm. KTP can also be used as asubstrate in the production of optical waveguides by ion implantation or by ion exchange

*To whom correspondence should be addressed.

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[1,2]. KTP crystals have, however, a relatively higher value of ionic conductivity at roomtemperature than the one desired for optical applications [3].

According to some published results [4,5], the substitution of Ti41 by trivalent ions helpsthe charge compensation and reduces the potassium vacancies and the ionic conductivity. Forall Me31 substitutions that reduce the ionic conductivity, the distribution coefficients arerather low. Only Cr31 shows a relatively high distribution coefficient, but, in this case,undesired absorption bands are detected in the visible region [4].

Single crystals of KTi12xMe41xOPO4 where Me41 is Ge41, Zr41, or Ce41 have already

been grown. Their structure, electrophysical, and some optical properties were investigated[6–9], but their ionic conductivity was not measured.

The main aim of the present work was to grow Me41 (Ge41, Zr41, and Ce41)-doped KTPcrystals in order to investigate the conditions of growth and distribution coefficients of thedopants and to obtain data on the ionic conductivity of the doped crystals with suitabledistribution coefficient of the dopants, which does not introduce problematic absorptionbands. Ge41 was selected for study because its ionic radius (0.53 Å) is smaller than that ofTi41 (0.605 Å), and Zr41 and Ce41 were selected because their ionic radii (0.72 and 0.87 Å,respectively) are larger than that of Ti41 [10].

EXPERIMENTAL DETAILS

A cylindrical resistance vertical furnace heated by AF Kanthal wire was used in all the crystalgrowth experiments carried out in the present work. The temperature was controlled using aPt/Pt–10%Rh thermocouple connected to a Eurotherm 818P controller/programmer.

Doped KTP single crystals were grown from a K2O–P2O5–TiO2–WO3 solution (42/14/14/30 mol%, respectively), which was chosen on the basis of a previous investigation [11].This flux composition had a KTP concentration of 28 mol%, a saturation temperature of940°C, a density of 2.95 g cm23, and a very favorable dynamic viscosity value (21 cP) at thesaturation temperature [11].

To prepare the working solution, K2CO3, NH4H2PO4, WO3, and TiO2 (Merck, .99%)were used as reagents. The dopants were added as oxides, i.e., GeO2 (Fluka, .99%) andZrO2, CeO2 (Aldrich, .99%). Three levels of doping were applied for each of the dopants(1, 3, and 5 mol% of Me41O2 in relation to TiO2), without reducing the quantity of TiO2 inthe solution. The chemicals were mixed and then decomposed and melted in a Pt crucible 50mm in diameter and 50 mm high. The final solutions, about 160 g in weight, were 40 mm indepth. Smooth thermal gradients were realized in the solution to prevent spontaneouscrystallization on the crucible bottom (the bottom was 5–8°C hotter than the solution surface,while the surface periphery was 2–3°C hotter than the surface center). Before the crystalgrowth, the solutions were homogenized for 4 h at1000°C.

All growth experiments were carried out by slow cooling, using the top-seeded solutiongrowth (TSSG) technique. Before each run, the corresponding saturation temperature wasaccurately determined by observing the dissolution or growth detected on pure KTP seeds incontact with the free surface of the solution. The seed orientation (parallel to thec axis), theconstant crystal rotation rate (45 rpm), and the slow cooling program were the same in allruns. The cooling program was 2°C/day for the first day, 3°C/day for the second day, and4°C/day for the last three days. Finally, the single crystals grown were removed from thesolution surface, located 1 cm above the free surface, and cooled at 40°C/h to room

1404 V. NIKOLOV et al. Vol. 34, No. 9

temperature. As an example, Figure 1 shows a Zr41-doped single crystal grown in the presentwork.

After a preliminary macroscopic characterization involving dimensions, habit, inclusions,cracks, etc., the concentration of dopants in the crystals was determined by inductivelycoupled plasma atomic emission spectrometry (Joblin Yvon-France JY-38) and by atomicabsorption analyses (Pye Unicam-SP-192). In addition, to get an idea of the dopant distri-bution during the growth, electron probe microanalysis (EPMA) was performed with aCameca Camebax SX 50.

Parallelepipedic samples of about 1 mm thick and 30 mm2 in area cut perpendicularly tothe c axis were used for optical transmission measurements at room temperature, using aShimadzu 2101-PC UV-vis spectrophotometer and a Bomem DA-3IR spectrophotometerwith MCT mir detector. These measurements involved depolarized light propagating in thec direction.

RESULTS AND DISCUSSION

The basic results obtained in the present investigation are shown in Table 1. As can be seen,the addition of a dopant oxide only slightly increased the saturation temperature of thesolution. This is because the sum of the solute concentration (TiO2 1 Me41O2) increasedslightly.

It should be pointed out that the addition of the dopants did not noticeably affect theuncolored nature of the pure KTP single crystals. The crystals were free from inclusions andcracks, except in the case of germanium and zirconium, in which some cracks began toappear when the dopant concentration was above 5 mol%.

Atomic absorption analysis and coupled plasma emission spectrometry measurements ofthe different dopant concentrations of Ge41, Zr41, and Ce41 are presented in Table 1. Thevalues calculated for the corresponding distribution coefficients correlate well with the ionicradii of the dopant Me41 ions. It can be seen that as the ionic radius decreased, thedistribution coefficient increased. In all cases, the distribution coefficients decreased slightly

FIG. 1Zr41-doped KTP single crystal grown in the present study.

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with increase in the dopant concentration. Note that the values of the distribution coefficientsof Ge41 and Ce41 are rather far from one. This indicates that chemical inhomogeneitiesappeared in these doped crystals. In the case of Zr41, the distribution coefficient was nearerto unity; thus, it seems reasonable to think that homogeneous Me41:KTP single crystalscould be obtained for technological use. To check this hypothesis, microprobe analyses wereperformed. Figure 2 shows that for the case of 3 mol% ZrO2 (KTP6), the Zr41 concentrationin the crystal decreased slightly during the growth process.

The experiments showed big differences in the final ratio between the dimensions of thecrystals along thea, b, andc directions. The differences depended on the kind and quantityof the dopants (see Table 1). When the concentration of Ge41 increased, the crystaldimensions in thec direction were smaller than in thea andc directions. An increase in theZr41 concentration led to smaller crystal dimensions in thea direction than in theb andcdirections. In the case of Ce41-doped crystals, there was no significant variation in any of thethree crystal directions, but the real concentration of Ce41 in the crystals was rather low.

Optical measurements showed that there were no important changes in the transmissionspectra of the doped and undoped crystals, either in the visible and near-UV region or in theIR region. In other words, the addition of this kind of tetravalent cation to the crystallinematrix did not generate problematic absorption bands, which can alter the transparency of thedoped crystals in the visible region. Only in the case of CeO2 doping (KTP9) was a smallmovement of the optical transparency region to the lower wavelength detected (see Fig. 3).

TABLE 1Crystal Dimensions, Cell Parameters, Distribution Coefficient, and Ionic Conductivity of

Me41-Doped KTP Crystals

SampleaDopant conc.in solutionb

Saturationtemperature

(°C)

Growthrate

(g/day)

Crystaldimensionsc

Dopant conc.in crystal(wt%)d

Distributioncoefficiente

a(mm)

b(mm)

c(mm)

KTP1 Nondoped 936 0.560 11.5 13.0 12.0 — —KTP2 Ge u 1 u 0.37 939 0.800 14.5 16.5 12.0 1.20 3.24KTP3 Ge u 3 u 1.10 942 0.884 17.0 17.0 12.0 3.51 3.18KTP4 Ge u 5 u 1.84 945 0.575 14.0 14.0 10.0 5.82 3.16KTP5 Zr u 1 u 0.46 939 0.600 12.0 15.0 12.0 0.68 1.48KTP6 Zr u 3 u 1.37 941 0.744 10.0 16.5 15.5 1.85 1.35KTP7 Zr u 5 u 2.28 942 0.522 8.0 16.0 15.0 2.80 1.23KTP8 Ce u 1 u 0.70 948 0.545 10.0 11.0 11.0 0.003 4.33 1023

KTP9 Ce u 3 u 2.10 950 0.585 10.0 11.0 11.0 0.008 3.83 1023

KTP10 Ce u 5 u 3.50 947 0.625 10.0 12.0 12.0 0.012 3.43 1023

aNomenclature used in the text to define the different crystals.bConcentration of the dopants in the initial solution: Meu [100(MeO2/TiO2)] mol% u [100(Me41/KTP)]wt%.cCrystal dimensions ina, b, andc directions, respectively.dDopant concentration in the crystals, according to inductively coupled plasma emission spectrometryanalyses: 100(Me41/KTP).eDistribution coefficient KMe41 defined as KMe41 5 (Me41/KTP)crystal/(Me41/KTP)solution.

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Furthermore, the transmission in the spectral region from 400 to 500 nm slightly increasedand the hydroxyl (OH2) absorption band at 3600 cm21 diminished. All these aspects areconsistent with other published results [9].

FIG. 2Normalized Zr41 distribution in KTP flux grown crystals in theb crystallographic axis. D5distance measured from the center of the crystal.

FIG. 3Transmission spectra obtained for KTP1 and KTP9 crystals. (a) visible and near-UV regions;(b) IR region.

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Zr41-doped crystals were chosen to measure the ionic conductivity inc direction becausethis doping element presents the most suitable distribution coefficient for obtaining chemicalhomogeneous crystals and does not introduce absorption bands in the visible region. Figure4 shows the ac conductivity data as a function of the frequency at room temperature. Theseresults correspond to the samples of pure KTP (KTP1) and Zr41 doped KTP (KTP5 andKTP6). The conductivity values of the undoped KTP ('1026 S/cm) are in agreement withthose of other authors [4,5] and are independent of the frequency, at least in the rangeanalyzed here. Figure 4 also depicts the influence of Zr41 doping over the ionic conductivityin thec direction of the KTP. It is clear that the presence of this cation in the KTP structuresignificantly reduces the ionic conductivity. This reduction was about two and three ordersof magnitude in the case of samples KTP5 and KTP6, respectively.

It is well known that the high level of ionic conductivity of the KTP crystals in thec axis('1026 S/cm) is due to the movement of potassium ions through potassium vacancies. Thus,if the potassium vacancies are reduced, the ionic conductivity must also decrease. One wayto obtain this reduction would be to substitute Ti41 with Me31 [4,5]. Provided that the K1

vacancies are correlated with the O vacancies in the crystal [5], another way to decrease theionic conductivity values might be to reduce the deficiencies of O. In practice, this could bedone by growing the crystals at lower temperature or stabilizing the structure by somesubstitution. From the results of the present work, it seems evident that Zr41 affects thesecond kind of stabilizing, that is to say, it keeps the O deficiencies at a lower level, thusinhibiting the K1 mobility.

ACKNOWLEDGMENTS

This work was supported by CICyT under project TIC96-1039 and the Generalitat deCatalunya under project 1997SGR-00178.

FIG. 4The frequency dependence ionic conductivity at room temperature for KTP1, KTP5, andKTP6 samples.

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