Crystal growth, characterization and structure refinement of neodymium3+ doped gehlenite, a new laser material [Ca2Al2SiO7]

Pergamon

Materials Research Bulletin, Vol. 29, No. 7, pp. 725-734, 1994 Copyright © 1994 Elsevier Science Ltd Printed in the USA. All rights reserved 0025-5408/94 $6.00 + .00

0025-5408(94)00020-4

CRYSTAL GROWTH, CHARACFERIZATIAON AND STRUCTURE REFINEMENT OF NEODYMIUM3+ DOPED GEHLENITE, A NEW LASER MATERIAL [Ca2A12SiO7]

A.M. LEJUS, A. KAHN-HARARI, J.M. BENITEZ, B. VIANA Laboratoire de Chimie Appliqure de l'Etat Solide, CNRS-URA1466, 11 rue P. et M. Curie, 75231 Paris Cedex 05, France (Received April 8, 1994; Communicated by P. Hagenmuller) ABSTRACT In order to find new Nd 3+ laser materials p u m p a b l e by laser diode, gehlenite ( Ca2 A12 Si 07) was chosen as a host matrix for n e o d y m i u m ions. Large single crystals of Ca2-x Ndx A12+x Sil-x 0 7 (0< x< 0.3) were o b t a i n e d by Czochralski and floating zone m e t h o d s . The main characteristics of these crystals (crystal p e r f e c t i o n , t h e r m a l and mechanical b e h a v i o r , optical properties...) were d e t e r m i n e d . The structure refinement revealed a structural disorder, occuring both on position and distribution of ions a r o u n d N d 3+ and resulting in a b r o a d e n i n g of the absorption bands. This means that less effort is r e q u i r e d for d i o d e laser p u m p i n g , and the laser effect has been o b s e r v e d . T h e r e f o r e , N d 3+ d o p e d Ca2A12SiO7 appears as a good candidate as diode p u m p e d laser. MATE1KIAL INDEX : aluminosilicate, neodymium, laser

INTRODUCTION The increasing interest for solid state laser materials is due to their easy use. In this field, diode p u m p e d materials are particularly suitable for producing compact laser systems with powerful and stable emission (1, 2). Such materials activated by N d 3+ are especially sought for. The purpose is to find a Nd 3+ d o p e d c o m p o u n d obtained as large single crystals and having an intense and broad absorption band in diode emission range (800nm) (3, 4). The gehlenite Ca2AI2SiO7 (CAS) is a possible matrix: the structure exhibits some disorder and can welcome active rare earth ions. The unit cell is tetragonal (melilite type) with space groupe P421m (5, 6). Cations are found on 3 types of sites: large eightfold coordinated sites (Thomson cubes) occupied by the large cation Ca 2+ 725

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T2 FIG.1 - Projection of gehlenite structure o n the (001) plane

o:

TI+T2)laye~

Ca

cleavage

a

b FIG.2

- Gehlenite structure description s h o w i n g the lamellar character of the structure and the easy cleavage direction.

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(or N d 3+ active ion) and two types of tetrahedral sites: a regular one, T1, where half of A13+ ions are located and a very distorted one, T2, smaller than T1, w h e r e Si 4+ and half of AI 3+ ions are statistically distributed(7) (fig. 1)This structure can also be described as a stacking, along C, of alternate layers of (T1 + T2) tetrahedra and T h o m s o n cubes. The tetrahedra layers with AI and Si are dense. In the T h o m s o n cubes, the Ca ions e n s u r e the linkage b e t w e e n two successive layers of tetrahedra (fig.2). Nd 3* substitutes Ca 2+ in the T h o m s o n cubes, according to the charge balance Ca 2+ + Si 4+ --> Nd3++ A13+, leading to the formula Ca2-x Ndx Al2+x Sil-x 07. The structural disorder around Nd ions is favorable to the existence of b r o a d absorption bands. This p a p e r deals w i t h crystal g r o w t h and m a i n p r o p e r t i e s of this d o p e d gehlenite, p a r t i c u l a r l y : crystal perfection, t h e r m a l and m e c h a n i c a l b e h a v i o r , structural e n v i r o n m e n t of the active ion in relation with optical properties. CRYSTAL GROWTH To p r e p a r e large Ca2-x Ndx A12+x Sil-x 07 crystals, two m e t h o d s from the melt were used : floating zone method (8) and Czochralski technique (4, 9). Starting materials In b o t h cases, the starting materials consist of CaCO3,A1203,SiO2 and N d 2 0 3 p o w d e r s mechanically mixed, then pressed into pellets and calcinated at 1000°C to ensure the complete decomposition of CaCO3 and a prereaction. Then, the samples are ground and pressed again either in form of a bar (5 cm length, 5 m m side) in the case of floating zone method, or as a large cylinder (20 cm length, 5 cm diameter), for Czochralski technique. These samples are sintered at 1450°C for 2 days to achieve the formation of pure gehlenite phase. Differential thermal analysis shows that the such obtained c o m p o u n d exhibits congruent melting at 1583°C. Floatin~ zone m e t h o d , J

The device is a light-focussing furnace equiped with two elliptic mirrors. The radiation source is a short arc xenon l a m p (5 KW).The lower end of the feeding bar and the top of a seed are melted and the two d r o p s connect to f o r m the melting zone. Crystal g r o w t h is carried out by regular slow shift u p w a r d of the zone along the bar. The feed and the seed shafts are rotated (20-25 rpm) in opposite directions to ensure h o m o g e n e i t y of the melting zone. The g r o w t h rate is in the range 0.5-1 m m / h . This m e t h o d e n a b l e s to p e r f o r m the crystal g r o w t h u n d e r v a r i o u s atmospheres or v a c u u m by using a gas chamber around the g r o w t h region. Czochralski m e t h o d The device is a classical Czochralski apparatus. It consists of a crystal puller and a 25 KW generator with an induction coil coupled to an iridium crucible used as susceptor and container. A set of stabilized-zirconia e l e m e n t s e n s u r e s thermal insulation. The charge (250g) consists of b r o k e n pieces, coming f r o m the initia~ sintered cylinder, which are m o l t e n to fill up the crucible (50 m m long, 50 m m diameter). In the first experiment, the g r o w t h starts from an iridium rod, d i p p e d

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into the melt, leading, after pulling up, to a polycrystalline block from which single crystal seeds are extracted. For the next experiments, these seeds are used to get good quality crystals. First, the seed is heated above the surface of the melt for several hours to achieve a good thermal equilibrium, then it is dipped into the melt and pulled out very slowly (0.5 mm/h). The seed holder is rotated at a rather high rate (35-40 rpm) due to the viscosity of the melt. The whole device is kept in an envelope under neutral atmosphere (Ar). A video system visualizes the growing crystal. About one week of pulling is required for the crystal growth operation. CHARACTERIZATIONOF Ca2-x Ndx Ab+x Sil-x 07 CRYSTALS 1- Aspect and crystalline quality Crystals with various Nd 3÷ amounts (15 Nd content values, from x=0.005 to x= 0.3) were obtained by the floating zone method. They consist in a transparent single crystal (5mm diameter, 20-30 mm long) which sometimes exhibits fractures and clefts due to the thermal shocks. However, the quality is sufficient for optical characterization. Several other compositions (x=0.05 to x= 0.2) were prepared by Czochralski method. Crystals are large transparent rods (20 mm diameter, up to 100ram long) showing zones with high crystalline quality (laser quality) (fig.3). Variations in the crystal diameter are due to temperature instability. Temperature regulation is now being developed.

FIG.3 Czochralski grown single crystal of Nd: gehlenite Ca2-xNdxA12+xSil-xO7 x=0.02

For these two kinds of crystals, the quality decreases with increasing Nd content (8, 9). For high concentrations (x=0.2 to x= 0.3) the crystals present transparent parts but also cloudy zones, with inclusions or microprecipitates. Yet, this phenomenon is not a problem for the laser application since the actual doping ion rate in the matrix is only a few percent. Moreover, the crystal growth must be carried out under argon for the Czochralski process and under vacuum for the floating zone method to avoid the formation of bubbles, inclusions etc., all defects arising during the crystallization because of air solubility (mainly oxygen) in the melt (10, 11). To eliminate the last remaining bubbles, relatively high rotating speeds (40rpm) are applied. 2-Other ~eneral characteristics Crystals prepared by these two methods present the same characteristics: - They all crystallize with a tetragonal gehlenite structure. v

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•- The solubility of N d 3+ in this matrix is very high. H o w e v e r , for x =1, the c o r r e s p o n d i n g c o m p o u n d CaNdA1307 cannot be p r e p a r e d as single crystal by these m e t h o d s , o w i n g to the u n c o n g r u e n t melting. It d e c o m p o s e s into CaA1204 and NdA103. A total solid solution exist between Ca2A12SiO7 and CaNdA1307 at 1450°C. This last c o m p o u n d is stable at this t e m p e r a t u r e and is d e c o m p o s e d only near the melting point. The lattice constants of gehlenite increase with N d 3+ content a=7.683A c = 5 . 0 6 5 A for x = 0 a = 7.710 A c = 5.081A for x = 0.3 The plot of unit cell p a r a m e t e r s versus N d 3+ content shows a w e a k deviation from the Vegard's law. It could be due to the substitution of (Ca 2+) by an ion of different charge (Nd 3+) in a very distorted and too large a site (Thomson cube). Electron m i c r o p r o b e analysis p e r f o r m e d longitudinally and transversally on some floating zone and Czochralski crystals did not reveal any segregation of N d 3÷ doping ions . A few crystals only had a Ca content below (~ 1%) the starting composition. - The natural g r o w t h axes are [100] or [110](9). So, the crystal g r o w t h is p e r f o r m e d along the dense (001) planes and perpendicularly to t h e - ~ a x i s . Crystals cleave easily into platelets perpendicular to the ~ a x i s . This is a consequence of the pseudo-lamellar character of the structure. Thus, the cleavage fracture should occur along the T h o m s o n cube layer, since tetrahedra layers are compact and Ca-O bonds are w e a k and should break relatively easily (see structure refinement). - The microhardness H was determined on a cleavage plane (001) using the Knoop process. According to the N d 3+ content, H varies slightly from 650 to 680 K g / m m 2. These values are close to those of most silicates. - Thermal dilatation coefficients were determined for the a and ~ directions from the variation of unit cell p a r a m e t e r s with t e m p e r a t u r e , in the range 20 -> 1200°C. They are : ct a = 7.6 10 -6 K -1 a n d ct c=12.1 10 -6 K -1 - Thermal conductivity was d e d u c e d from the formula K=D.d.Cp (D is the diffusivity d e t e r m i n e d by the Parker m e t h o d , Cp is m e a s u r e d by differential calorimetry). The m e a s u r e m e n t s were p e r f o r m e d along the c ' d i r e c t i o n on pellets 8 m m d i a m e t e r and 1.5mm thick (fig.4a). The obtained values are low (0.022 W cm -1 K l ) c o m p a r e d to those of N d d o p e d YAG (Y3AI5012), the usual N d 3+ laser ( K= 0.13 W cm -1 K-I). These m e a s u r e m e n t s , h o w e v e r , were carried out in the less favorable direction (perpendicularly to the T1,T2 layers, fig.2). The easy cleavage plane did not allow to get samples with sufficient thickness (1.Smm) in the other direction (fig.4b) to m e a s u r e D within the layers, which w o u l d certainly give m u c h higher 'values. For such d e t e r m i n a t i o n the e m p l o y e d m e t h o d requires large samples. The p r o b a b l e a n i s o t r o p y in thermal conductivity should be taken into account to p e r f o r m the laser p u m p i n g .

FIG.4 Representation of theoretical directions of thermal conductivity m e a s u r e m e n t s (dashed arrows) deduced from fig.2.

{I _, ............

-

I

,

,,

co

layer" J

,

.

(a)

(Is)

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5/2" 2H9/2 4]9/2__~4~

l /

4I9/2~ 4r74, %/2 C •~

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EL[

1.5

°

<

/,i%

/ I/\/ \ 1

zzo

d0

zgo

do

a c.,,,~

FIG.5 -

Polarized absorption spectra of Nd 3+ in gehlenite, in the range 720-840 nm (T=300K)

1030

-

1 0 5 0 10'70 '1090 1100~(nm) FIG.6

Nd 3+ emission spectrum corresponding to the 4F3/2---~4Ill/2 transition a)- in gehlenite b)- in YAG

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3 - Optical properties The optical behavior of crystals was investigated on cleaved plateLets. - The refractive index is 1.67. - The absorption spectrum at 300 K shows broad bands in the visible and nearinfrared range. The broadest one (795~830 nm) is the most intense and corresponds to the 419/2--~4F5/2, 2H9/2 transition(4, 8). C o m p a r e d to the N d 3+ d o p e d YAG, this absorption line, in the studied compound, is broader (gehlenite: 4.6nm, YAG: 1rim) and well a d a p t e d for further laser applications with commercial laser diodes p u m p i n g (GaA1As). Polarized absorption spectra were recorded in ~ (E//~) and c~ (E _1_4) orientations. The highest absorption is f o u n d in this last orientation (fig.5). The p u m p i n g will have to be realized in this • polarization to get the best results. The emission s p e c t r u m presents the main transitions of N d 3+ used in laser devices, at 900, 1060 and 1330 nm. The g r o u p of lines c o r r e s p o n d i n g to the 4F3/2-~4Ill/2 transition (the most used in the n e o d y m i u m lasers ) spreads from 1040 to 1120 nm, leading to a wide range of tunability. This is an a d v a n t a d g e of N d 3+ d o p e d gehlenite c o m p a r e d to YAG which presents only narrow emission bands (fig.6). - The laser effect has been e v i d e n c e d (4) t h r o u g h laser diode p u m p i n g , on a Czochralski-made crystal, with composition Cal.98Nd0.02A12.02Si0.9807. The yield was 40% but could be increased by the i m p r o v e m e n t of crystal quality and by optimizing N d 3+ content. This study is now in progress, yet this first result is quite promising. It shows the potentiality of Nd 3+ d o p e d gehlenite as new laser material, p u m p e d by diodes. -

-

STRUCTURAL DESCRIPTION In order to determine the exact e n v i r o n m e n t of Nd 3÷ in such p r e p a r e d crystals, a refinement of gehlenite structure was performed on a Czochralski g r o w n single crystal, containing x=0.02 N d 3÷. With the experimental conditions given in table I, the structure was refined d o w n to an agreement factor R= 0.04. The refined atomic parameters are given in table II and the c o r r e s p o n d i n g main interatomic distances are gathered in table III. The temperature factors [3 are especially high for Ca and O. Moreover, the final coordinates of 02 show high estimated standard deviations; this point may be related to the e n v i r o n m e n t of this oxygen, consisting in 3 Ca(Nd) and one A1/Si, which implies a great uncertainty on its real position. The T h o m s o n cube around Ca(Nd) has a very low s y m m e t r y (Cs); oxygen nearest neighbors of Ca (or Nd doping ion) are rather far from the central ion. It comes out of these results that the matrix consists in a rather disordered lattice. Bond-strenghs and valences of the cations are calculated using the bondlengths d e d u c e d from the refined atomic coordinates (12).They confirm the pure a l u m i n u m T1 site (charge = 3) and the statistical distribution of A1 and Si on the T2 site (charge = 3.5). For Ca, the total charge found is lower than 2+, due to the fact that the T h o m s o n cube is s o m e w h a t too large for the central Ca a t o m : the corresponding bond-lengths are larger than expected for classical Ca-O distance and to these long Ca-O bonds correspond weak bond-strengths, which result in too low an electronic environment. Actually the disorder, due to the statistical distribution

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TABLE I - Structural data collection and refinement for gehlenite

Ca2Al2SiO7

Space group

P421m

Calculated density Lattice constants

9 = 3.06 a = 7,684 (1)/~ c = 5.065 (1)

Wavelength Crystal size co scan - scan speed: h
Z=2

V=299/~3

~ MoKot 0.2 x 0.3 x 0.6 ram. 1.8 / 6.7 degrees/rnin. 0max =45 ° 940 0.041 with 450 independent reflections 0.048

Rw

TABLE II - Atomic parameters of gehlenite Ca2Al2SiO7 refined structure (R = 0.041) Atom wyckoff position

x

y x 104

z

~ll

1322

1333

[312

0

22 20 54

22 20 54

85 46 54

0

0 -26 10 12 -8

0 -8 11

All 2a AI/Si 4e Ca 4e

3573(2) 1613(1)

8573(2) 6613(1)

0437(2) 4885(1)

O1 02 03

0 3624(6) 872(3)

5000 8624(6) 1668(3)

8209(8)

56

56

55

7158(6)

52

52

8060(4)

49

43

83 94

2c 4e 8f

0

0

[313

623

B equiv

0

0

-1 2

-1 2

0.6 0.5 1.0

0 -8 10

1.1 1.1 1.0

TABLE III - Main interatomic bond-lengths in Czochralski g r o w n gehlenite crystal (in/~)

All(T1) AI/Si(T2)

Al/Si AI/Si Ca

Ca Ca Ca Ca Ca - Ca

03

4 x 1.748 (2)

02 O1 03

1.661 (3) 1.696 (2) 1.704 (3)

O1 03 02 02 03

2.431 2 x 2.441 2.470 2 x 2.526 2 x 2.825 3.506

(3) (2) (4) (4) (2)

All

3.07 (3)

Al/Si 3.46 (3.5)

Ca

1.74 (2)

Valence deduced from bond-strengths calculation (12)

No.

7

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of A1 and Si on the T2 site, results in variable positions for their oxygen neighbors, which are connected either to AI or to Si. This induces an even lower site-symmetry for C a / N d that "sees" either AI or Si as cation neighbors. This disorder occuring on positions and distribution of ions in the matrix is responsible for the very large absorption b a n d for N d 3+. No local ordering between AI and Si can been evidenced through X Ray diffuse scattering investigation. The shortest Ca-Ca distance is about 3.51 A, but N d 3. dilution (2% at) leads to a very low probability (about 1%) of finding two N d ions at such a distance. The m e a n Nd-Nd distance with this concentration, in a r a n d o m distribution, is close to In o r d e r to increase the s t r u c t u r a l d i s o r d e r a n d t h e n b r o a d e n the absorption and emission bands, we have p e r f o r m e d some substitutions in the three kinds of cationic sites in Cal.98Ndo.02AI2.02Sio.9807, as following: a) in the T h o m s o n cube, partial or total substitution of Sr 2+ to Ca 2+ b) in T1 and T? tetrahedra, by replacement of A13+ by a pair 1/2(Mg 2+ + Si4+) In both cases, the substitution does not lead to any i m p r o v e m e n t of the properties, the fluorescence intensity decreasing with increasing N d content. c) in T1 and T2 tetrahedra, by partial or total substitution of Ga 3+ to A13+ In the last case, i n t r o d u c t i o n of Ga 3+ leads to an increase of the fluorescence intensity. H o w e v e r this i m p r o v e m e n t is w e a k and does not d e s e r v e further investigation of this substituted material, owing to the high coast of Ga203. Finally these v a r i o u s a t t e m p t s of ionic substitutions to increase the structural disorder in Cal.98Ndo.02AI2.02Si0.9807 have no significant effects on the optical behavior. -

-

-

CONCLUSION In order to obtain new diode p u m p e d lasers, some properties of gehlenite were investigated and this material a p p e a r s as a r e m a r k a b l e host matrix for N d 3 ions. Large single crystals with composition Ca2-xNdxAll+xSil-xO7 (0
Authors are grateful to D. Saber for his participation in this work, to F. Robert (CNRS-URA 419, UPMC Paris) for careful X Ray data collection on single crystal, to C. Borel, R. Romero and C. W y o n for laser tests (LETI-CENG, Grenoble, France) and to the "DRET" for financial support.

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REFERENCES 1. F. Hanson, D. Dick, H.R. Verdun, M. Kokta, J.of Opt. Soc. Am., B_B_.G1668 (1991). 2. R. Burnham, P. Bournes, J. Kasinski, K. Le and D. Dibiase, OSA Adv. Solid State Lasers, Pinto, Fan Eds., 15, 59 (1993). 3. R.Collongues, A.M.Lejus, J. Thery and D. Vivien, Cryst. Growth, 126, 986 (1993). 4. B. Viana, D. Saber, A.M. Lejus, D. Vivien, C. Borel, R. Romero and C. Wyon, OSA Adv. Solid State Lasers, Pinto and Fan Eds.,15, 242 (1993). 5. S.J. Louisnathan, Can. Miner., 10, 822 (1971). 6. M. Kimata and N. Ii, N. Jb. Miner.Abh., 144, 254 (1982). 7. A.A. Kaminskii, E.L. Belokoneja, B.V. Mill, S.E. Sarkisov, Phys. Status Solidii (a), 97, 279 (1986). 8. B. Viana, D. Saber, N. Duxin, A.M. Lejus and D. Vivien, Opt. Mater. 1994 (submitted). 9. D. Saber, Thesis, University P. et M. Curie, Paris, France (1991). 10. N. Ii and I. Shindo, J. Cryst. Growth, 4_G6569 (1979). 11. C.B. Finch, F.L. Ball and J.L. Bates, J. Cryst. Growth, 54,482 (1981). 12. I.D. Brown and D. Altermatt, Acta Cryst., B41,244 (1985).