crystal growth of yca4o(bo3)3 and its orientation

Journal of Crystal Growth 197 (1999) 228—235

Crystal growth of YCa4O(BO

3)3

and its orientation

Qing Ye*, Bruce H.T. ChaiCenter for Research and Education in Optics and Lasers, University of Central Florida, 4000 Central Florida Blvd., Orlando, FL 32816, USA

Received 24 July 1998; accepted 17 September 1998

Abstract

The growth of YCa4O(BO

3)3

(YCOB), one of the rare-earth calcium oxyborate family of crystals that melts nearlycongruently is reported. Typical crystals produced in our laboratory are about 40 mm in diameter and 150 mm long withgood optical quality. The crystal has good potential to be used for self-frequency doubling due to its excellentcombination of nonlinear and laser properties. The single crystal growth of YCOB with different rare-earth substitutionsis presented. We find that the substitution limit is reduced with either an increase or decrease in the ionic size of thesubstituted rare-earth element. The melt also moves further away from congruently melting with these substitutions.YCOB belongs to monoclinic crystal system. The orientation relationship between the crystallographic axes (a, b, c) andoptical indicatrix axes (X, Y, Z) is discussed. ( 1999 Elsevier Science B.V. All rights reserved.

PACS: 81.10.Fq; 42.70.!a; 61.10.Nz

Keywords: YCOB; Crystal growth; Crystal orientation

1. Introduction

Highly efficient, compact, visible CW laser sour-ces are desirable in various applications, such asoptical data storage, undersea communications, in-terferometric measurements and full color displays.The current approach to achieve visible emission isby frequency doubling of diode-pumped solid statelasers using nonlinear optical crystals such as KTP[1] or periodically poled LiNbO

3(PPLN) [2].

A much better approach is to have a crystal that

*Corresponding author. E-mail: [email protected].

can achieve both lasing and frequency doubling, inother words, self-frequency doubling (SFD). Whilethis is a good idea in principle, unfortunately, veryfew crystals possess this unique combination ofproperties.

The first SFD laser was demonstrated in 1969with Tm-doped LiNbO

3[3]. Before the discovery

of rare-earth calcium oxyborate family compounds,Nd : YAl

3(BO

3)4

(NYAB) was considered the mostpromising SFD crystal due to its large nonlinearcoefficients, high emission cross-section and highdamage threshold, etc. [4—6]. But after intensiveinvestigation of more than a decade [7—9],NYAB crystals have failed to reach commercialreality primarily because of its incongruent melting

0022-0248/99/$ — see front matter ( 1999 Elsevier Science B.V. All rights reserved.PII: S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 0 9 4 7 - 6

behavior and high degree of difficulty to grow highquality crystals by the flux growth. Moreover, crys-tal defects severely limit the yield, making NYABcrystal too expensive to be practical.

The discovery of rare-earth calcium oxyboratefamily compounds brought new hope for SFDmaterials. One of the family of compounds,YCa

4O(BO

3)3(YCOB), shows excellent SFD prop-

erties to generate green emission when doped withneodymium [10]. Its broader absorption band,higher laser damage threshold, and comparablenonlinear coefficient with NYAB and LiB

3O

5(LBO) make Nd : YCOB a very attractive candi-date for compact, diode-pumped visible lasersystem. The first compound of this family— SmCa

4O(BO

3)3, was obtained from a PbO

flux serendipitously during the preparation ofCa

3Sm

2(BO

3)4

in 1991 [11]. Later Norrestamet al. [12] synthesized more compounds ofRCa

4O(BO

3)3

with R3`"La3`, Nd3`, Sm3`,Gd3`, Er3`, Y3` by high temperature solid statereactions and the crystal structure was determinedas monoclinic system with space group of Cm.They also checked the melting behaviors by thedifferential thermal analysis (DTA) method andconcluded that all these compounds melted con-gruently. The largest crystal produced at thattime with dimension of 0.18]0.28]0.29 mm3

was grown from a PbO melt with R3`"Lu3`

[13]. In 1996, Aka et al. [14] reported the success-ful growth of large size GdCa

4O(BO

3)3

(GdCOB)crystals, one of the rare-earth calcium oxyboratefamily compound, and demonstrated that GdCOBwas a promising SFD crystal. These crystalsshowed good optical quality, high damagethreshold and fairly large nonlinear coefficients.Iwai et al. [15] found that YCOB has wider UVtransmission and larger birefringence thanGdCOB. In this paper the growth of YCOB crystaland other rare-earth calcium oxyborate crystalsfrom a nearly congruent melt by Czochralski melt-pulling method is discussed. Since the crystallo-graphic axes (a, b, c) of YCOB are not mutuallyorthogonal and do not align with the optical in-dicatrix axes (X, Y, Z), there is some confusion inthe literature regarding to the exact orientation. Wewill describe in detail our convention of crystalorientation.

2. Crystal growth

2.1. Growth of YCOB

YCOB melts nearly congruently, so that it can bepulled directly from a melt reasonably close to thestoichiometric composition. A 30 kW Pillar powergenerator with maximum frequency of 10 kHz wasused as power supply and a Micricon 823 control-ler was used for the two-loop weight feedback con-trol. The charge was placed in either an iridium ora platinum crucible and heated by radio-frequency(RF) induction. The constant diameter of the crys-tal was controlled by the top-weighing method.A neutral atmosphere was provided by a continu-ous flow of nitrogen gas. The starting material wasprepared by mixing Y

2O

3, CaO and B

2O

3powders

(purity '99.99%) in proportions according to thechemical reaction:

Y2O

3#8CaO#3B

2O

3%2YCa

4O(BO

3)3.

The composition was adjusted to compensate boththe evaporation and noncongruency during thegrowth. The mixture of chemicals was loaded in aniridium crucible of 75 mm in diameter and height,and heated to a temperature slightly higher thanthe melting point of YCOB at 1510°C. When all thecharge was melted and homogenized, the seed, heldon an alumina rod was dipped to touch the melt ata rotation rate of 15—20 rpm. The typical pullingrate was 1—1.5 mm/h. It took about six days togrow a 40 mm in diameter and 150 mm long crystalwith good optical quality (Fig. 1). As much as 80%of the melt can be converted into single crystal.Further growth will produce defects and inclusionsinside the crystal. Commercial size crystal with75 mm in diameter and 200 mm long has beengrown out of a larger crucible using the similarprocedure. The colorless crystals showed two natu-ral facets parallel to the b-axis. These facets pro-vided critical information of the crystal orientation.Similar to GdCOB [14], YCOB has two weakcleavage planes along (2 0 11 ) and (0 1 0) that cancause crack during cooling. Therefore we need takespecial care of the cooling procedure in order tominimize the excess stress exerted on the crystal.The GdCOB single crystals of the same size werealso produced in our laboratory using a similar

Q. Ye, B.H.T. Chai / Journal of Crystal Growth 197 (1999) 228–235 229

Fig. 1. Crystal pictures. (a) YCOB, (b) 5% Nd : YCOB.

procedure. The melting temperature of GdCOB is1480°C and the fraction of usable melt is slightlyless than that of YCOB.

2.2. Growth of other rare-earth calcium oxyboratecrystals

As we mentioned earlier, Norrestam et al. [12]reported that all the compounds of RCa

4O(BO

3)3

with R"La, Nd, Sm, Gd, Er, Y melted congruent-ly according to the DTA results. But one need to becautious on these results since small quantities ofchemicals in DTA measurement may not have suf-ficient signal to show small deviation from truecongruency as compared to large quantities ofmelt. We tried to grow YbCa

4O(BO

3)3

(YbCOB),LuCa

4O(BO

3)3

(LuCOB) and LaCa4O(BO

3)3

(LaCOB) out of large quantities of melt (&1000 g)and found they were not congruent at all. Forexample, stoichiometric LuCOB charge was heatedto melt and homogenize, but no single crystal couldbe pulled out of the melt even with a YCOB seed.The charge after cooling was fine grain polycrystal-

line and in opaque rock-like appearance withouta shiny surface. But the X-ray powder diffractionshowed that the charge had essentially the LuCOBstructure with a few minor peaks that were notsufficient to identify the second phase. Moreover,we even suspect that YCOB and GdCOB are notexactly congruent melts. When we grew YCOBcrystal using about 80% of the total charge, whiteinclusions started to appear inside the crystal andbecame more and more serious with furthergrowth. The last part of the crystal was polycrystal-line and totally opaque. Furthermore, the weightloss due to evaporation cannot account for the shiftof the melt composition. The tentative conclusionbased on our observation is that, if there is a con-gruently melting field for YCOB and GdCOB, it isindeed very small. The substitution of rare-earthelement or slightly composition change can easilybreak this congruent field and the crystal is essen-tially grown from a peritectic melt that is very closeto the stoichiometric composition.

Even though we are unable to grow pureYbCOB, LuCOB, LaCOB, we can grow YCOBwith significant rare-earth substitution. Our experi-ments show that YCOB can dope up to 50% Yb,30% Lu and 10% La and still produce high opticalquality crystals. However the fraction of usablemelt is proportionally reduced with increasing sub-stitution. This can be explained by the small toler-ance of the trivalent ion radius at Y3` site. In theYCOB structure, six oxygen atoms form a distortedoctahedron surrounding the Y3` ions. It seems tous that the Y3` ion, with radius of 0.900 A_ , has thebest fit to this site. With smaller rare-earth ions, likeYb3` (0.868 A_ ) and Lu3` (0.861 A_ ), or larger oneslike Gd3` (0.938 A_ ), Nd3` (0.983A_ ) and La3`(1.032 A_ ), the amount of dopant incorporated intothe crystal is reduced as shown in Fig. 2. This resultwas also consistent with the apparent meltingtemperatures of these compounds. We found thatYCOB had the highest melting temperature, in-dicating the strongest bonding among all theRCa

4O(BO

3)3

family compounds. We tried to testthis model by doping chromium into the melt. Wewere extremely surprised that there was virtuallyno evidence of any Cr going into the crystal eventhough Cr3` has the strongest crystal stability fieldfor octahedral sites. This effect, therefore, is clearly

230 Q. Ye, B.H.T. Chai / Journal of Crystal Growth 197 (1999) 228–235

Fig. 2. The relation between the doping limit in YCOB crystal and the radii of rare-earth ions.

due to the smaller radius of Cr3` ion (0.615 A_ ). Wealso tried to substitute either strontium or magne-sium atoms for calcium atoms. The fraction ofusable melt was greatly reduced in either case. Theimplication is that the stability field of YCOB ismuch more limited compared to the garnets. Thisalso explains why these compounds were dis-covered at such a late date despite the fact thatpeople have investigated the R

2O

3—CaO—B

2O

3system many years earlier [16]. Nd3` substi-tution is useful for SFD. We estimate that themaximum substitution is about 20%. In our labor-atory we have produced 10% Nd : YCOB withexcellent optical quality. The measurement of flu-orescence lifetime showed no evidence of concen-tration quenching, which is a good feature as laserhost.

3. Crystal structure

The YCOB family of compounds crystallizes inthe monoclinic crystal system with space groupCm. These compounds are negative biaxial crystalsand the number of formula in one unit cell is Z"2[12]. The only symmetrical element in the unit cell

is a mirror plane perpendicular to the b-axis.Y3` ions sit on these planes and each ion is sur-rounding by six oxygen atoms that form a distortedoctahedron. Four of them are borate oxygens. Theother two oxygens have the same environment andact as shared corner of yttrium octahedron and twocalcium (I) octahedrons. There are two types ofcalcium sites, Ca (I) and Ca (II). They both aresix-coordinated with oxygen atoms. The borateatoms also have two different sites, B (I), B (II).They are coordinated with three oxygen atomsforming triangle planes. The crystal is linkedtogether by corner-sharing BO

3triangles and

repeated by edge-sharing along the c direction,which makes the shortest distance between thetwo yttrium atoms the same as c lattice parameter[13].

4. Orientation

Although more than 50% of the crystals found innature belong to the low symmetric monoclinicsystem, essentially all the industrially used crystalshave much higher symmetry. Part of the reason isthat orientating the monoclinic biaxial crystal has


never been an easy job. For laser and nonlinearphase-matching work, the optical indicatrix axes X,Y, Z of the crystal are most commonly used asorientation reference. They are mutually ortho-gonal regardless of the crystal symmetry by defini-tion, which is a mathematical construction of anellipsoid in the X½Z frame with semi-axes equal torefractive indices n

x, n

y, n

z, respectively. On the

other hand, the intrinsic crystallographic axes a, b,c, which carry more information in identifying thethermal and mechanical properties of a crystal, arenot always orthogonal depending on the crystalsymmetry. For crystals with high symmetry such ascubic, tetragonal etc., these two sets of axes arecollinear. In the monoclinic system, only the b-axisis collinear with the Y-axis. The nonorthogonal a-and c-axis are co-planar but position at certainangles with respect to X- and Z-axis. To determinethe angular relationship between the two sets ofaxes can be quite tricky as in the case of YCOBcrystal.

In the early publication of Aka et al. [14] and theDocument Provisoire of CRISMATEC [17], theorientation of GdCOB crystal was suggested asfollows: YEb, (X, c)"15° and (Z, a)"26°. A simplediagram without referring to the crystal morpho-logy was provided [17]. We found that it wasimpossible to use their assignment since the a, b,c coordinate followed the right-hand rule and theX, Y, Z coordinate was left-handed. We decided topropose a new set of coordinates based on right-hand rule only with projection on crystal morpho-logy. Our proposed coordinates were based onthe combination of all the information on crystalmorphology, d-spacing, X-ray diffraction andrefractive indices results. Here is how we deter-mine it.

1. The YCOB b direction was examined byperforming X-ray diffraction on the plane nor-mal to the b-axis direction. It happened to beone of the two cleavage planes found in thiscrystal with extinction under cross-polarizedlight. We observed both (0 2 0) and (0 6 0) dif-fraction peaks to confirm our result. Since inmonoclinic system, b-axis is perpendicular to a-and c-axis, the normal of the (0 2 0) face is theb-axis.

2. The subsequent growth of crystals using a b di-rection seed showed two sets of natural facetsparallel to the b-axis, one set of which was moredominant. The cross-section of the YCOBcrystal boule showed a parallelogram formedby these two sets of natural facets, which isthe typical morphology of a monoclinic crystal.This further confirms the correctness of theb-axis.

3. The position of the two optical indicatrix axesX and Z can easily be determined by the opticalextinction of a slab cut perpendicular to theb-axis, i.e., (0 1 0) slab, through a cross-polarizer.The traditional convention of n

x(n

y(n

zwas

adopted to distinguish the X- and Z-axis basedon the measured refractive indices using a Abbe60 Refractometer. The angular relationships ofthe X- and Z-axis relative to the natural facetswere also marked. Knowing the position of theX-, Y-, Z-axis, we were able to cut all the secondharmonic phase-matching samples and laserrods based on this coordinate.

4. The determination of a- and c-axis positionis somewhat more complicated. For mostmonoclinic crystals, the two sets of naturalfacets usually have very simple indices suchas (1 0 0), (0 0 1) or (1 0 1), (1 0 11 ). Using thelattice parameter a"8.046 A_ , b"15.959 A_ ,c"3.517 A_ , and b"101.19° reported by Iwaiet al. [15], we measured single crystal X-raydiffraction of two slabs cut along the two sets ofnatural facets. To our surprise, the dominantfacet, which is also the other set of cleavageplane of this crystal, is neither of the two mostcommon sets listed above but rather than (2 0 11 )plane. This determination was quite easy sincethe (2 0 11 ) is the strongest peak of the powderX-ray diffraction pattern (Fig. 3). The minorfacet has a very weak peak positioned at 2hof 31.3° and with d-spacing equal to 2.86 A_ .It is difficult to assign it with certainty ofany face. Without additional information, wefinally assign it as the superlattice of (4 0 1)to be self-consistent. The position of a- andc-axis were then drawn on the parallelogramaccording to the d-spacing of (2 0 11 ) plane andthe result was reported at ASSL’98 conference[10].


Fig. 3. The X-ray powder diffraction pattern of YCOB crystal.

5. Self-consistent result does not necessarily meancorrect. Recently Yoshimura [18] conducted thefour-circle X-ray analysis and obtained the dif-fraction pattern of the reciprocal lattice on acplane, which also fits to our assigned crystallo-graphic coordinate. However, by X-ray the slabsperpendicular to the reciprocal a*- and c*-axisdirections, we realize that our original a- andc-axis has been reversed due to the wrong as-signment of the minor natural facet to (4 0 1).The diffraction peak at 2h"31.3° should be(1 0 1) though it is forbidden base on the crystalsymmetry. Further analysis of our original ap-proach shows that it is possible to have twosolutions of the crystallographic coordinate,both of which can satisfy all the morphologysymmetry and reciprocal lattice requirements.The only way to distinguish them is to do thesingle crystal X-ray diffraction on the slabs cutalong the a face and c face, with the reciprocala*- and c*-axis being their face normal respec-tively. In our X-ray examination of the latestassigned a face, (2 0 0), (3 0 0) and (4 0 0) diffrac-tion peaks were observed, but the peak observedon the c face does not match very well with the(0 0 1) diffraction. Furthermore the forbidden(3 0 0) and (1 0 1) diffraction observed in single

crystal X-ray diffraction experiments, make thiscrystal structure even more interesting. We thinkit is due to the superlattice effect observed inother solids as well [19]. Therefore the crystalstructure and orientation of YCOB has not beenfinally settled yet and further refinement isneeded.

Fig. 4 shows the cross-section of a b-axis grownYCOB crystal. The orientations of both crystallo-graphic axes a, c and the optical indicatrix axes X,Z are indicated superimposing on the traces of theexternal crystal morphology according to our latestresult. There is an angle of 23° between a- andZ-axis, and an angle of 12° between c- and X-axisfor YCOB crystal, which is in reasonable agree-ment with the recent publication of GdCOB crystal[20].

5. Discussion

Since our primary interest of YCOB crystal is itspotential for SFD with Nd doping, we also col-lected the general properties of Nd doped YCOBand made a comparison with published propertiesof NYAB crystal (Table 1). Clearly, NYAB shows


Table 1The comparison of self-frequency doubling crystals NYAB and Nd : YCOB

Crystal Nd : YAl3(BO

3)4

! Nd : YCa4O(BO

3)3

Symmetry (point group) Trigonal (32), uniaxial (!) Monoclinic (m), biaxial (!)1 at% Nd concentration 0.55]1020 ions/cm3 0.45]1020 ions/cm3

Mohs hardness 7.5—8.0 6.0—6.5Density (g/cm3) 3.72 3.31Crystal growth Flux Czochralski growthMelting point (°C) '1400 1510Stability Chemically stable, nonhygroscopic Chemically stable, nonhygroscopicRefractive index at lasing wavelength 1.06 lm n

%"1.6886, n

0"1.7613 n

x"1.6844, n

y"1.7152 n

z"1.7256

Fluorescence lifetime (ls) 60 105Emission cross section (cm2) 2]10~19 0.42]10~19

Absorption peak (nm) 807 796, 812Absorption cross section (cm2) at pump wavelength 2.3]10~20 2.3]10~20

Absorption cross section (cm2) at SFD wavelength 0.55]10~20 0.40]10~20

Transmission range (nm) 1000—2300 1000—2600Damage threshold (GW/cm2) 0.5 at 1060 nm, 10 ns pulse '1 at 532 nm, 6 ns pulsed%&&

of SHG of 1.06 lm (type-I) 1.27 pm/V 1.1 pm/V

!Refs. [4—6].

Fig. 4. The orientation coordinates of the optical indicatrix axes(X, Y, Z) relative to the crystallographic axes (a, b, c) for YCOBcrystal.

some advantages of higher emission cross-section,larger birefringence and larger effective nonlinearcoefficient (d

%&&). However, the ability of producing

large high quality single crystals of YCOB makes itmore viable for compact diode pumped solid statevisible lasers.

6. Summary

We have grown large YCOB and GdCOB singlecrystals from a nearly congruent melt with the meltcomposition a little bit off from the stoichiometriccomposition. The melting behaviors of other rare-earth calcium oxyborate compounds were alsostudied. Our growth results showed that pureYbCa

4O(BO

3)3, LuCa

4O(BO

3)3

and LaCa4O(BO

3)3

cannot be pulled directly from the melt. The rare-earth ions can only partially substitute the yttriumsite depending on their ion sizes. An experimentalmodel is proposed to illustrate this effect. Finally,we describe our method to orient the crystal andimpose both the crystallographic and the opticalindicatrix axes onto a real YCOB crystal.

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crystal growth of yca4o(bo3)3 and its orientation

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