Change in the Morphology of RbTiOPO4 Introduced by the Presence

CRYSTAL GROWTH & DESIGN

Change in the Morphology of RbTiOPO4 Introduced by the Presence of Nb J. J. Carvajal,† C. F. Woensdregt,‡ R. Sole´,† F. Dıáz,† and M. Aguilo´*,† Fı´sica i Cristal‚lografia de Materials (FiCMA), UniVersitat RoVira i Virgili, Campus Sescelades, c/Marcel‚lı´ Domingo, s/n, 43007 Tarragona, Spain, and Faculty of Geosciences, Department of Petrology, Utrecht UniVersity, P.O. Box 80.021, NL-3508 TA Utrecht, The Netherlands

2006 VOL. 6, NO. 12 2667-2673

ReceiVed January 19, 2006; ReVised Manuscript ReceiVed June 20, 2006

ABSTRACT: RbTiOPO4 crystals doped with Nb show an important flattening of the morphology that makes their use difficult in nonlinear optics applications. Here, we identify the possible causes of this morphological change: Nb acts as an impurity that brakes the velocity of growth of the {100} faces and modifies the growth mechanism of {201} faces of the crystals. We also propose a way of obtaining isometric crystals by forcing crystal growth in the a crystallographic direction. This causes a new change in morphology with benefits for later nonlinear optics applications due to the larger useful area obtained in the x-y plane. 1. Introduction

2. Experimental Section

Rubidium titanyl phosphate, RbTiOPO4 (RTP), crystallizes in the orthorhombic system Pna21 space symmetry group, with cell parameters a ) 12.974(2) Å, b ) 6.494(3) Å, and c ) 10.564(6) Å.1 It is an isostructural with the well-known potassium titanyl phosphate, KTiOPO4 (KTP). Like this latter material, the Rb counterpart should have important applications in nonlinear optics (NLO) and electrooptics due to its high NLO and electrooptical coefficients, high optical damage threshold, low dielectric constant, and chemical stability.2,3 These properties, together with the thermally stable phase matching in these crystals, indicate wavelength conversion to be the dominant field of application. Since the photochromic damage threshold of RTP is 1.8 times higher than that of KTP,4 this material could be useful in high-power applications. Achieving self-induced effects by combining the laser properties of lanthanide (Ln3+) ions and the NLO properties of KTP is hampered by the very low distribution coefficients of Ln3+ in KTP,5 which are not high enough to obtain efficient fluorescence from the Ln3+’s. These distribution coefficients were slightly enhanced in RTP,6 but the Ln concentration achieved was still far below that required for practical laser applications, namely about 1020 atoms cm-3. The use of Nb5+ as codopant has been revealed to be one of the most effective methods to increase the distribution coefficients of Ln3+ in these crystals.7 However, the presence of Nb affects strongly some of the properties of the host and also their morphology.8-10 RTP:Nb crystals are interesting because they show noncritical phase matching up to 984 nm11 and are also potential superionic conductors, due to their high ionic conductivity: 2 orders of magnitude greater than that of RTP.12 In the present paper, we discuss the most suitable conditions to grow RTP crystals containing Nb with optical quality by the top-seeded solution growth (TSSG) technique. We propose an explanation for the morphological changes introduced by Nb in these crystals after an accurate observation of the surface of the crystals. Finally, we expose the methodology we have used to cause a new and controlled morphological change in these crystals, which provide a larger useful area for nonlinear optical applications.

2.1. Crystal Growth. RTP and RTP:Nb crystals were grown from self-fluxes by the top-seeded solution growth technique and slow cooling of the solution. The experiments were carried out in a singlezone vertical cylindrical furnace with a Kanthal AF resistance heating wire,9 with a useful thermal area of 30 cm in length and 7.5 cm in diameter. The furnace was controlled by a Eurotherm 903 controller/ programmer. We prepared the growth solutions by mixing the required amounts of Rb2CO3 (99%), NH4H2PO4 (99%), TiO2 (99.9%), and Nb2O5 (99.9%), used as initial reagents. The compositions used and the solution weights are given in Table 1. We filled 125 cm3 Pt cylindrical crucibles that were 6.5 cm in height and 5.0 cm in diameter. The axial temperature gradient in the solution was 1.3 K cm-1 at the first 1 cm and 0.8 K cm-1 at the next 1.5 cm, being hotter on the bottom than at the surface of the solution. The radial gradient was constant at 1.8 K cm-1. RTP and RTP:Nb prismatic crystal seeds were used in the growth experiments, with the c crystallographic direction normal to the surface of the solution, depending on the experiment (see Table 1). It has been reported that a seed with a dominant (001) artificial face leads to small capping regions and a low dislocation density in the crystal.13,14 We used two different ways of supporting the crystal seeds. In the first runs (see experiments 1-3), we laced a crystal seed using a thin platinum wire at the end of an alumina rod placed out of the center of the surface of the solution. In a second series of experiments (experiments 4-9), two crystal seeds were fixed, also using a thin platinum wire, to a growth device including a platinum turbine immersed in the solution of growth. This system is described in detail in ref 7. In all of the experiments the crystal seeds were rotated at a constant angular speed of 65 rpm and the direction of the rotation was changed every 50 s. We determined the saturation temperature (Ts) by observing the growth or dissolution of the crystal seeds in contact with the surface of the solution. The growth process was carried out by decreasing the temperature under the conditions outlined in Table 1. When the growth run was completed, the crystal was lifted slowly 5-10 mm upward from the surface of the solution and cooled to room temperature at a rate of 15 K h-1 to avoid thermal stresses. The experiments lasted between 2 and 4 weeks on average. With these experiments we checked how the concentration of Nb affects the crystal growth of RTP crystals, how the improvement of mass transport in the solution influences the final result, and how the composition and size of the crystal seeds used affect the quality and size of the crystals. Table 2 briefly summarizes the purpose of each experiment. 2.2. Nb Concentration Analysis. The concentration of Nb in the crystals was measured by electron probe microanalysis (EPMA) operating in the wavelength-dispersive mode. A CAMECA SX-50 electron microprobe was used in the analyses. The concentrations of Rb, Ti, P, Nb, and O were analized at 30 nA electron current and 25

* To whom correspondence [email protected]. † Universitat Rovira i Virgili. ‡ Utrecht University.

should

be

addressed.

E-mail:

10.1021/cg060036o CCC: $33.50 © 2006 American Chemical Society Published on Web 11/02/2006

2668 Crystal Growth & Design, Vol. 6, No. 12, 2006

Carvajal et al.

Table 1. Growth Data Associated with RbTiOPO4 and RbTiOPO4:Nb Single Crystalsa A

B

C

D

E

1

42.0-28.0-30.0-0.0

152

1204

RTP

2

42.0-28.0-29.4-0.6

164

1190

RTP

3

42.0-28.0-28.8-1.2

169

1187

RTP

4

42.9-35.1-22.0-0.0

176

1191

RTP

5

42.9-35.1-21.6-0.4

180

1193

RTP

6

42.9-35.1-21.6-0.4

180

1193

RTP:Nb

7

42.9-35.1-20.7-1.3

185

1195

RTP:Nb

8

40.8-27.2-32.0-0.0

151

1201

RTP

9

40.8-27.2-31.0-1.0

158

1192

RTP:Nb

F

G

H

2.0/0.1 2.0/0.05 9.5/0.02 2.0/0.1 2.0/0.05 12.5/0.02 2.0/0.1 5.0/0.05 15.0/0.02 2.3/0.1 12.0/0.05 3.0/0.1 15.4/0.05 3.0/0.1 10.5/0.05 3.0/0.1 10.0/0.05 2.5/0.1 5.0/0.05 7.0/0.02 2.5/0.1 5.0/0.05 7.5/0.02

6.3 × 12.2 × 7.7

1.054

3.0 × 17.5 × 6.0

0.676

0.65

small cracks

3.5 × 17.0 × 8.0

0.809

0.52

cracks

8.8 × 10.3 × 9.0 8.0 × 7.0 × 7.5 7.7 × 14.2 × 9.0 6.0 × 14.0 × 9.5 2.7 × 7.3 × 4.5 3.3 × 7.4 × 6.1 3.1 × 3.7 × 4.4 2.5 × 2.8 × 2.8 8.2 × 11.1 × 5.5 10.5 × 15.6 × 7.9

1.417

6.2 × 6.9 × 7.8 6.7 × 5.7 × 6.7

I

J very good

very good

3.889

0.56

inclusions and cracks

0.495

0.56

very good

0.138

0.63

very good

3.378 1.977

small cracks 0.56

very good

a Legend for column heads: (A) experiment number; (B) solution composition (Rb O-P O -TiO -Nb O ) (mol %); (C) solution weight (g); (D) T (K); 2 2 5 2 2 5 s (E) crystal seed; (F) cooling program (K/K h-1); (G) crystal dimensions in the a, b, and c crystallographic directions, respectively (mm); (H) crystal weight (g); (I) distribution coefficient of Nb; (J) quality of the crystal.

Table 2. Purpose of the Different Experiments Detailed in Table 1 expt no.

cryst

1

RTP

2, 3

RTP:Nb

4

RTP

5

RTP:Nb

6, 7

RTP:Nb

8

RTP

9

RTP:Nb

purpose of the expt control natural stirring by the growing cryst influence of the concn of Nb natural stirring by the growing cryst control influence of the mass transport conditions, forced stirring influence of the concn of Nb influence of the mass transport conditions, forced stirring influence of the concn of Nb influence of the mass transport conditions, forced stirring influence of the composition of the cryst seed control influence of the mass transport conditions, forced stirring influence of the size of the cryst seed influence of the concn of Nb influence of the mass transport conditions, forced stirring influence of the composition of the cryst seed influence of the size of the cryst seed

kV accelerating voltage. A pure RTP crystal, grown by us, was used as the standard for Rb, Ti, P, and O with the aim of minimizing the matrix effects in the samples because of its similar chemical composition. Nb was analyzed using LiNbO3 as a standard, provided by C. M. Taylor. The analyses were carried out using the lines Rb LR and P KR measured with the TAP crystal, Ti KR and Nb LR measured with the PET crystal, and O KR measured with a W/Si (2d ) 60 Å) multilayer crystal. The measurements were integrated over 10 s for Rb, Ti, P, and O and over 30 s for Nb. The raw intensities were corrected for dead time, background, and matrix effects using the PAP correction procedure.15 2.3. Microscopic Visualization. Scanning electron microscopy (SEM) images of the crystals were obtained with a JEOL JSM 6400 scanning electron microscope. The samples were sputter-coated with a gold layer before SEM observation to minimize any possible surface charging effects. The crystal surfaces were studied by means of a Park CP scanning force microscope (SFM). Most of the SFM images have been recorded in the contact mode at constant force (deflection images) with a scan rate of 1 Hz and an image size of 256 × 256 pixels. The figures for habit faces and orientation of faces were obtained with the SHAPE16 utility.

2.4. Wet Chemical Etching. RTP and RTP:Nb crystals were chemically attacked with a mixture of H2SO4 (96%) and HF (40%) at a volume ratio of 75:25 at room temperature during 5 min to reveal the etch pits on the different faces of the crystals.

3. Results and Discussion 3.1. Crystal Growth. As RTP and RTP:Nb melt incongruently,17 crystals cannot be grown from the melt. Instead, a hightemperature solution is the only method reported to grow RTP: Nb crystals.10 We grew RTP and RTP:Nb crystals in self-fluxes by the top-seeded solution growth technique and slow cooling of the solution. To prevent the formation of inclusion flaws during the advanced stages of growth, it is important to rotate the seed and to allow sufficient circulation of the flux. In our first experiments (see experiment 1 in Table 1), we used only the normal stirring produced by the rotation of the growing crystal; i.e., we did not use any other element to stir the solution. In these three experiments, we used a crystal seed of RTP, laced using a thin platinum wire at the end of an alumina rod and placed away from the center of the surface of the solution. The crystal seed had typical dimensions of 1.5 × 1.5 × 5.0 mm in the a × b × c directions. This system was chosen because it proved to be successful in the crystal growth of KTP and derivative crystals18 and because it introduces additional flows other than those of thermal convective origin, which improve its homogeneity. The pure RTP single crystals grown by this methodology were very transparent and were large enough for later optical characterizations, as can be seen in Figure 1a. We then grew single crystals of RTP:Nb by the same methodology (see experiments 2 and 3 in Table 1), in which part of the TiO2 of the solution was substituted by Nb2O5. In these experiments the average time of homogenization increased and the average rate of growth of inclusion-free single crystals decreased as the concentration of niobium in the solution increased. Also, some cracks were seen to come from the seed. Furthermore, the presence of Nb significantly changed the habit of the crystals: RTP:Nb grew as thin plates, as can be seen in

Change in Morphology of RbTiOPO4

Crystal Growth & Design, Vol. 6, No. 12, 2006 2669

Figure 1. Morphologies of (a) RTP and (b) RTP:Nb crystals using the same kinds of crystal seeds.

Figure 1b. Our results show that in general the dimension in the b crystallographic direction was larger than the dimensions in the other directions and the dimension in the a crystallographic direction was the smallest. Another important feature was that the {100} face is the most developed face of the crystal. From what we found when determining the crystallization region of RTP in fluxes containing Nb2O5 and Ln2O3,7 this change in morphology can only be attributed to the presence of Nb in the crystals. This agrees with the previous results for KTP:Nb crystals.19 In a first attempt to obtain more isometric crystals, we tried to improve the mass transport conditions in the solution. In highly viscous solutions, as with the growth solution of RTP and RTP:Nb crystals, a drop in temperature leads to a high level of supersaturation in some areas of unstirred growth solutions. In these areas, crystals grow quickly, especially in the direction with the highest growth velocity. With crystals containing Nb, this quick growth can accentuate their morphological changes. For these reasons, we developed an acentric crystal growth system comprising a stirrer immersed in the growth solution and two crystal seeds symmetrically distributed at about 5 mm from the rotation axis and 15-20 mm up the platinum turbine, which acted as a stirrer.7 This system was expected to increase the stirring of the solution and favor the mass transport conditions, thus minimizing problems associated with inhomogeneous supersaturation in these viscous solutions. Stirring the solution was also proved to decrease the frequency of spontaneous nucleation during the growth process. To obtain the most suitable shape for our stirrer, we checked visually the path that the structural units would follow in the growth solution by experimental simulation in a solution containing 90% glycerine and 10% water. The viscosity of this solution is around 200 cP at room temperature, which is the value we assumed for our solution at high temperature by comparing it with those of similar solutions used in the crystal growth of KTP.20 This solution was placed in a transparent glass crucible with the same dimensions as our Pt crucible. To check visually the path of the structural units in the solution, we used

small particles of graphite. Stirrers with several shapes, immersed at various distances in the solution and rotated at different angular velocities, were studied. Our results show that the higher the velocity of rotation, the greater the efficiency of the mass transport in the solution. However, the same effect can be obtained by decreasing the rotation velocity and using a stirrer with a higher number of blades. We then used a platinum stirrer with 10-12 blades and rotated it at a velocity that did not cause any rise of the solution surface in order to avoid flux inclusions in the crystals. Using this acentric system of crystal growth, we obtained a higher quantity of high-quality inclusion-free single crystals (see experiments 4-9 in Table 1). We observed that if we increase the cooling interval of the experiment, the number of cracks and flux inclusions increases, as is shown in experiments 5 and 6 in Table 1. It is important to note that whenever Nb was present in the solution, the obtained crystals were smaller than in the experiments to grow pure RTP crystals with a similar solution composition and under the same conditions, as can be seen on comparing experiment 4 with experiments 5-7 in Table 1. With regard to the Ts value of the solution, we have seen that it tends to be lower when Nb is present in the solution than with pure RTP (see experiments 8 and 9 in Table 1). In these crystals, containing Nb and grown using the acentric crystal growth system, the differences in dimensions among the a, b, and c directions decrease and the crystals become more isometric. Our results seem to indicate that the development in the b and c crystallographic directions in RTP:Nb crystals became similar, while the development in the a direction was always lower, as we can see by the size of the crystals in experiments 5-7 in Table 1, being in all cases the smallest dimension. Thus, the change in habit of the RTP:Nb crystals remained. Again, some cracks normally appear in RTP:Nb crystals coming from the seed. We think that this problem may be associated with the mismatch between the cell parameters of RTP and those of RTP:Nb when RTP seeds were used to grow crystals containing Nb. To try to avoid these cracks, we made


Carvajal et al.

Figure 2. Macrohillock of growth on the (100) face of an RTP:Nb crystal. Images were obtained by SEM. The schematic view of the morphology of the crystal was drawn with the Shape utility.

comparative growth attempts with RTP and RTP:Nb seeds (see experiments 6 and 7 in Table 1). At this stage, however, cracks could not be fully avoided, in contrast to reports in the literature for KTP doped with Nb.21 We made comparative growth attempts with seeds with the same composition as the crystal we wanted to grow (see experiment 9 in Table 1). We concluded that, though cracks cannot be fully avoided, they appear less often when the compositions of the seed and the crystals we want to grow are the same. The distribution coefficient of Nb in the crystals is also given in Table 1. We can see that, for the solution compositions used in these crystal growth experiments, the concentration of Nb in the crystals did not change significantly, with distribution coefficients ranging from 0.52 to 0.65. We also observed no significant variations of the distribution coefficient of Nb in the crystals depending on whether additional stirring of the solution was carried out (experiments 2 and 3 in Table 1) or not (experiments 5-7 and 9 in Table 1). 3.2. Morphological Change. The morphology of crystals of the KTP family was first studied by Voronkova and Yanovskii22 and Pavlova et al.23 These crystals crystallize in the orthorhombic system, space group of symmetry Pna21.1 The crystal habit is usually built up of {100}, {201}, {201h}, {011}, {011h}, and {110} forms. RTP and RTP:Nb crystals grew with the same forms as KTP crystals. However, in the case of RTP and RTP:Nb crystals, the {201} and {011} forms develop sharp caps along the c axis, whereas the {011} and {110} forms develop less sharp caps along the b axis. As we presented above, the presence of Nb causes RTP:Nb crystals to grow as platelets with more developed faces corresponding to the {100} form. This shape makes it difficult to use these crystals in applications, such as

second-harmonic generation, that require a special cut of the sample in the x-y plane (which coincides with the (110) crystallographic plane). Then, it is crucial to understand which are the causes of this change in morphology, just to find a way to obtain RTP:Nb crystals with a larger useful area in this plane. It is well-known that changes in the morphology of crystals grown in doped solutions can be caused by the growth sites on the crystal being poisoned by the attachment of dopant species. In fact, we found that the presence of Nb2O5 in the solution, even at low concentrations, affects the morphology of the crystals and slows the crystal growth process. Figure 1 shows a comparison between the morphologies of RTP and RTP:Nb crystals. We observed, by SEM, small grown-in crystallites or twins on the {100} face of RTP:Nb crystals (see Figure 2), maintaining the same orientation for a and b axes, but with a slightly different orientation for the c axis with respect to the substrate. This may appear as a result of imperfect growth in the presence of Nb because of the sedimentation of a 3D nucleus on the crystal surface during growth. Figure 2 also shows a schematic view of the crystal to illustrate its faces and orientation. The edges of these crystallites are parallel to the [010], [011], and [011h] directions. The macrosteps that can be also seen in the picture are parallel to the [010] direction. These twins or crystallites were not observed on the {100} form of pure RTP crystals. No additional features were observed on the other faces of the crystal. We tried to observe crystal growth hillocks using atomic force microscopy (AFM), but the high polarizability of the crystal made it difficult to do this. In general, the faces were dirty and it was therefore difficult to see anything. The results were best when the crystals were cleaned in an ultrasonic bath for 5 min



Figure 3. Composition of several SFM error images showing (a) a macrohillock of growth in the (100) face and (b) the (201) face of an RTP:Nb crystal. Photographs obtained by optical microscopy of the crystal are included to show where the SFM images were taken (A in (a) and B in (b)).

Figure 4. Composition of several SEM pictures showing chemical attack with a mixture of H2SO4 (96%) and HF (40%) at a volume ratio of 75:25 at room temperature on the {100} faces of (a) an RTP crystal and (b) an RTP:Nb crystal. Magnified pictures were taken at the surrounding areas depicted in the low-magnification images.

with a solution of 0.5 g of Na2HPO4 and 0.5 g of NaH2PO4 dissolved in 40 mL of water. In Figure 3a we show a photomosaic of the (100) face of an RTP:Nb crystal with a growth hillock. The spiral edges are parallel to the same

Figure 5. Composition of several SEM pictures showing chemical attack with a mixture of H2SO4 (96%) and HF (40%) at a volume ratio of 75:25 at room temperature on the {201} face of (a) an RTP crystal and (b) an RTP:Nb crystal. Magnified areas are marked with circles in the low-magnification images.

directions of the crystallites observed by SEM. The steps observed in this figure have a height of between 34 and 232 Å.


Carvajal et al.

Figure 6. Location of the Ti(1) and Ti(2) atoms in the structure with respect to the surfaces of faces (100) and (201). The projection of the structure is parallel to [010] for RTP:Nb.

This corresponds approximately to between 6 and 40 times the thickness of the elementary growth layer d200. Figure 3b shows the photocomposition of several AFM images for the (201) face of the same RTP:Nb crystal as in Figure 3a. As we can see, this face is very flat and there are no signs of growth steps. The other faces of the crystal did not provide any relevant information. Clearly, therefore, the {100} form grows in a way different from that of the {201} form in RTP:Nb crystals. No hillocks were seen on the {100} form of pure RTP crystals. However, due to the difficulties in the observation of the surfaces of pure RTP crystals, we could not conclude whether this behavior is common in this family of crystals. The only reference we have is the paper published by Bolt et al.24 about KTP. If we compare their results with ours, we can see that in the {100} form they found large growth hillocks lying approximately at the center of these faces with growth steps orientated parallel to the b axis, but they did not find the other steps parallel to the [011] and [011h] directions, as we saw on RTP:Nb crystals. Nevertheless, the steps they observed were lower than ours by 1 or 2 orders of magnitude. They observed growth hillocks in the {201} form, albeit with difficulty. We obtained a new proof of the different nature of the growth of the {100} and {201} forms in the RTP and RTP:Nb crystals by chemical etching of these crystals with a mixture of H2SO4 (96%) and HF (40%) at a volume ratio of 75:25. Figure 4 show the results of this chemical attack on the {100} form of these crystals. Both crystals show rhombohedral etchpits corresponding to dislocations in the area of the crystal with the most defects. These dislocations are aligned along the [011] direction, which is the same direction toward which the edges of the etchpits are oriented. Hence, on {100} both RTP and RTP:Nb crystals show the same etchpit morphology. However, a significant change was observed in the chemical attack on the {201} form of these crystals. This face is practically not attacked in RTP crystals, but it is considerably attacked in RTP:Nb crystals (see Figure 5), which may be due

to the different surface energies of the {201} faces in RTP and RTP:Nb crystals. We tried to understand this different behavior by investigating the structure of the RTP:Nb crystal.25 As Nb only substitutes Ti atoms in Ti(1) positions, we centered our attention on the positions of Ti atoms in the structure. We could see that Ti(1) and Ti(2) atoms are placed at different alternating planes parallel to the {100} form, while alternating Ti(1) and Ti(2) atoms are found in the same plane parallel to the {201} form, as can be seen in Figure 6. If we suppose that Nb acts as an impurity that is placed at Ti(1) positions, poisoning the growth sites, this could explain the change in the mechanism and velocity of growth of these faces, braking the velocity of growth of the {100} form. It is also possible that the Nb atoms are preferred 2D nucleation sites for growth and etching of the {201} form, which might explain the kinked appearance of these faces. This may also cause the {201} form to grow faster than the {100} form, and its importance in the final morphology of the crystal will decrease, resulting in a smaller area. 3.3. A Way of Obtaining Isometric Crystals. To solve the morphological problems associated with the crystal habit of crystals containing Nb, we tried to force the growth of these crystals along the a crystallographic axis by using RTP:Nb crystal seeds of 5.0 × 1.5 × 5.0 mm in the a × b × c directions. In this way we obtained isometric RTP:Nb crystals, as can be seen in experiment 9 in Table 1. Figure 7 shows one of the crystals obtained with these thick crystal seeds. With this methodology we obtained high-quality crystals without cracks from solutions containing Nb. In addition we observed that, although the crystals were larger than when crystal seeds of 1.5 × 1.5 × 5.0 mm in the a × b × c directions were used, the size and weight were still lower than in the case of pure RTP crystals grown under the same conditions. The morphology of the crystals grown using these thick crystal seeds was different from that of RTP and RTP:Nb crystals by comparison with the morphology of RTP and RTP: Nb crystals grown using thinner crystal seeds in the a direction,



grow as thin plates. In the {100} form, Nb brakes the velocity of growth of this face by poisoning the Ti(1) growth sites. In the {201} form, Nb atoms seem to be the preferred 2D nucleation sites for growth and etching of the {201} form, which might explain the kinked appearance of these faces, losing its flattened character. We proposed a way of obtaining isometric RTP:Nb crystals by using thick crystal seeds in the a crystallographic direction. This causes a new change in morphology that allows us to obtain a larger useful area in the x-y plane with benefits in later nonlinear optical applications. Acknowledgment. We acknowledge the CICyT for financial support of this work through the Projects MAT2004-20471-E, CIT-020400-2005-14, and MAT2005-06354-C05-02 and the DURSI-Generalitat de Catalunya though 2005SGR00658 and the EU Project NMP3-CT-2003-505580. We are also very grateful to Hans Meeldijk of Utrecht University for providing the AFM images. References

Figure 7. Lateral and frontal views of a single crystal of RTP:Nb grown with the acentric crystal growth system described in ref 7 using thick crystal seeds in the a crystallographic direction.

Figure 8. Morphology of (a) an RTP:Nb crystal grown using a thin crystal seed in the a crystallographic direction and (b) an RTP:Nb crystal grown using a thick crystal seed in the a crystallographic direction. The x-y plane is marked to highlight the larger useful area for nonlinear optical applications in the latter case.

as can be seen in Figure 8. Through the forced crystal growth in the a direction the {110} form disappears. Also, the {011} and the {011h} forms are more important than in RTP crystals containing Nb grown with thin crystal seeds in the a direction. This new change in morphology provides a large useful crystal area in the x-y plane (see Figure 8) with benefits for later applications in second-harmonic generation. 4. Conclusions We identified possible changes in the mechanisms of growth of the {100} and {201} forms that cause RTP:Nb crystals to

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