CaClF: Sm2+ crystal growth and spectroscopy

j........ C R Y S T A L

Journal of Crystal Growth 128 (1993) 1031-1035 North-Holland

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CaC1F" S m 2+ crystal growth and spectroscopy J.P. C h a m i n a d e a, A. G a r c i a a, J.C. Vial b a n d R . M . M a c f a r l a n e c a Laboratoire de Chimie du Solide du CNRS, 351 Cours de la Liberation, F-33405 Talence Cedex, France b Laboratoire de Spectromdtrie Physique (associ~ au CNRS), BP 87, F-38402 Saint Martin d'Hbres, France ¢ IBMAlmaden Research Center, 650 Harry Road, San Jose, California 95120, USA

The growth of CaCIF:Sm2+ single crystals has been investigated in connection with spectral hole burning phenomena. CaC1F:Sm2+ has been grown by slow cooling of a high temperature solution using CaCI 2 as solvent. X-ray diffraction, optical studies and chemical etching were carried out on the single crystals obtained. The luminescence properties of CaC1F:Sm2+ single crystals have been recorded.

1. Introduction A variety of interesting p h e n o m e n a have recently been demonstrated in crystals of the alkaline-earth fluorohalides of the form A X F (where A = Ba, Sr and X = Br, C1) containing divalent rare earth ions. These properties depend on the possibility of photoionizing the divalent rare earths, a process which is optically reversible. Photon gated spectral hole burning was demonstrated in BaC1F : Sm 2+ and SrC1F : Sm 2+ [1,2]. In order to investigate the influence of the ionic radius of the A cation in lowering the 4f5d levels of Sm 2+, the work on B a C 1 F : S m 2+ and S r C 1 F : S m 2+ was extended to the case of CaC1F:Sm 2+ [3]. This material was therefore prepared in single crystal form. The compounds BaC1F and SrC1F melt congruently and both melt growth and indirect flux methods have been used to prepare single crystals of these materials [4-8]. CaC1F, on the other hand, does not show congruent melting, so crystals have to be grown using a flux method. The aim of this work is to present details of the CaC1F:Sm 2+ growth conditions. CaCI 2 has been chosen as the most suitable flux because it helps to stabilize the divalent state of the samarium and indeed it reduces impurity contamina-

tion as it contains common ions. The luminescence properties of Sm 2+ in the CaC1F single crystal matrix, particulary the position of the 5Dj and 7Fj levels has been obtained and compared with those of Sm 2+ in the SrC1F and BaC1F matrices.

2. Crystal growth The crystal growth was carried out in a tubular muffle furnace with Kanthal wires. The furnace temperature was regulated and programmed by a PID Eurotherm controller. Cylindrical shaped P t - 1 0 % Rh crucibles of 30 cm 3 were used. A thin tube at the top of the sealed cap crucible makes filling with powder and then sealing possible. The starting materials were CaCI2, CaF2, N a F and NaC1 with a purity better than 99.9% (Alfa Ventron). SmC12 was prepared by heat treatment of SmC13 at 750°C under an atmosphere of flowing hydrogen. All the manipulations were carried out in a dry box containing dried and deoxygenated Ar gas, because of the highly hygroscopic nature of CaCI2 and SmCI2. Several steps were involved in the reaction p r o c e d u r e . A solid state r e a c t i o n of a Ca0.95Sm0.05C1F sample was followed by dilution

0022-0248/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved

1032

J.P. Chaminade et a L / CaClF : Sm 2 + crystal growth and spectroscopy

Table 1 Growth conditions of CaC1F :Sm 2+ Starting materials

Mol%

Thermal cycle

Results

CaC12 CaCIF: Sm 2 +

40 60

Heating up to 750°C at 100°C/h, soaking, then cooling down to 645°C at l ° C / h , final cooling to room temperature at 50°C/h

Colourless, highly hygroscopic plates with (001) faces up to 10 x 10 × 1.5 mm 3

to the required amount of Sm 2+ by adding weighed quantities of CaC12 and CaF2, typically Ca0.9995Sm0.o005CIF, and finally a crystal growth run was made by mixing the nutrient and chosen flux. The thermal program was the following: heating above the liquidus temperature, soaking time, then very slow cooling of the solution, as crystallisation proceeds by spontaneous nucleation, to the solidification flux temperature and finally a rapid decrease to room temperature. The first serie of experiments were done with the eutectic composition of the N a C I - N a F system (T F = 680°C for 67% NaC1) as a flux [9]. In the second series of experiments, growth was accomplished as a result of the following chemical reaction: Cao.9995Smo.oo05Cl 2 + N a F --~ Cao.9995Smo.ooosC1F + N a C 1 .

The crystals obtained by these two methods were identified by X-ray diffraction as CaC1F

Fig. 1. Single crystal plates of CaCIF:Sm 2+ stored in sealed glass tubes (1 division is 1 mm).

type. Luminescence tests allowed us to determine the presence of Sm 2+ or Sm 3+. In all cases, only Sm 3+ appeared; a possible charge compensation effect (2Ca 2+= N a + + Sm 3+) can occur, the ionic radii of Ca 2+, Na + and Sm 2+ being equal to 1.32, 1.38 and 1.46 A, respectively [10]. Several crystals grown in the sodium halide flux were analysed by flame emission spectroscopy; sodium was detected, in good agreement with the suggested hypothesis. In order to prevent such compensation mechanism, another flux system was tried. The CaClz-CaF 2 phase diagram determined by Wenz et al. presents a eutectic point of 645°C for a 18.5 tool% CaC12 composition below the CaC1F decomposition temperature [11]. The composition domain of primary crystallization of CaC1F was investigated. First attempts were successful and the optimal growth conditions were determined and are collected in table 1. The choice of CaCI 2 as flux minimizes crystal impurities as it contains no foreign cations or anions. o

Fig. 2. Optically clear single crystal of CaCIF:Sm 2÷ in paraffin oil selected for spectroscopic analysis (1 division is 1 ram).

J.P. Chaminade et al. / CaCIF : Sm 2 + crystal growth and spectroscopy

Fig. 3. Uniaxial interference figure of a (001) plane of CaC1F:Sm 2÷ with convergent and Bertrand lenses using a polarizing microscope.

Fig. 4. Etch pit pattern on the (001) plane of CaCIF:Sm 2+ single crystal.

After sawing off the upper part of the crucible, single crystals were removed ultrasonically in dry acetone and stored in sealed glass tubes containing a few Torr of Ar. Crystals grew as transparent, optically clear platelets with a size up to 10 × 10 × 1.5 mm 3 (figs. 1 and 2). An examination of the platelets immersed in paraffin oil using a polarizing microscope shows a typical interference figure indicating that the principal plane of the as-grown crystals is normal to the optic axis (fig. 3). The crystals have the PbC1F layer structure with well developed (001) planes, as shown in fig. 3. The averaged lattice parameters of the crystals (a = 3.894(2) A, c = 6.809(4) A, tetrago-

T ( a . u .)

7F o ~

1033

nal system, P 4 / n m m , Z = 2) are in good agreement with those published previously [12]. Etching was performed by adding a drop of concentrated HNO 3 to the paraffin oil at room temperature. Well-defined square-shaped pits with sides parallel to the edges of the crystal platelet were produced (see fig. 4). The etch pattern shows the (001) plane of CaC1F structure and emphasizes the low etch pit density of the crystal. A luminescence test using a high pressure mercury vapour lamp at 365 nm as an excitation source showed the typical red S m 2+ fluorescence, but some Sm 3+ was also present in the crystal sample. A sintering of the sample at 725°C for 2 h

5Dz (E)

Sm2+

Sm 3+

6H~ ~

4G~

A 7F o ~

5 6 6 .'2 . . . .

5 6 6 .'9 . . . .

5D~ (A,)

5 6 7 .'6 . . . .

j /

5 6 & .'3 . . . .

569

.....

Fig. 5. Excitation spectra of broad band luminescence (red and yellow).

nm

034

ZP. Chaminade et al. / CaCIF." Sm 2 + crystal growth and spectroscopy T (a . u .)

5D,(A~)

7F, (E)

~

5D,(A2)

~

7F2(A

~)

a i

640

630

. . . .

i

650

T (a.u.)

. . . .

5Do ~

i

660

I (a.u.)

66o

5D, ~

C 730

.

.

.

.

. . . .

i

nm

7F~ (E)

5 ~ D o ~

,

eao

i

670

7F o

50o ~

b

. . . .

760

7F~ (A2)

7 ~0 . . . . . .7 2. 0 . . . .

nm

7F2

i

740

.

.

.

.

i

750

.

.

.

.

r

760

.

.

.

.

i

770

.

.

.

.

i

nm

Fig. 6. Luminescence spectra of the Sm 2+ centre in CaCIF at 2 K. The excitation wavelength is at 566.9 m

J.P. Chaminade et a L /

1035

CaCIF : Sm 2 + crystal growth and spectroscopy

in a pure H 2 stream enhanced the Sm 2+ fluorescence.

Table 2 Transition energies for Sm 2+ in MeCIF hosts 7Fo-SD 0 (cm -1)

7Fo-SD 1 (cm -1)

(~)

3. Luminescence properties For optical spectroscopy, crystals were sealed in glass tubes containing a few Torr of He gas. Fluorescence measurements using Ar + laser excitation at 4545 ,~ confirmed that samarium ions were present in the trivalent and divalent state. Selective excitation into Sm 2+ was necessary to obtain a spectrum free of contamination by Sm 3+. The 5D 2 level of Sm 2+ at 5669 .A was chosen as it enables observation of emission from 5D 1 and 5D 0 yet it lies below t h e 465/2 level of Sm 3+ (fig. 5). Fluorescence spectra were measured at 2 K with a resolution of 0.1 A using CW laser pumping with a bandwidth of 0.3 ,~. Fig. 6 (a, b and c) shows fluorescence line groups from 5D 0 ---, 7F0, 7F1, 7F2, 7F 3 a n d 5 D 1 ~ 7F 1 a n d 7 F 2 in the wavelength range of 630 to 780 nm. In CaCIF, as in BaC1F and SrC1F, the Sm 2÷ ions ocuppy a non-centrosymmetic site of Cnv symmetry so the same optical selection rules are expected. These are: A~, A2, B1, B 2 ~ E ~r polarized electric dipole, A 1 ~ A1, A 2 ~ A2, B 1 B1, B 2 <-* B 2 ,n" polarized electric dipole, A 1 ~ A 2 and B~ <-->B 2 magnetic dipole allowed, and A 1 B1, B2, A 2 ~ B1, B 2 are forbidden. The magnetic dipole transition 7FoA 1 ~ 5DaB 2 and two forbidden transition 7FoA 1 ~, 5DzB1, B 2 have not been observed here. The wavelength of the strongest lines to5D0, 5D t and 5D 2 were measured with a wavemeter and single frequency dye laser so their positions are known to _+0.05 cm-1. No fluorescence was observed from 5D 2 because it lies in the vibronic band of the 4f 6 ~ 4f55d absorption. As the ionic radius of the A cation decreases the strength of the crystal field on the rare earth ion increases. This increases the splitting of the 5d orbital and decreases the frequency of the

Mean Me 2+ ligand distance a)

CaCIF SrC1F b) BaCIF c)

14388 14472.4 14532.2

15715.2 15807 15873

2.75 2.88 3.04

a) From ref. [10]. b) From refs. [2,13]. c) From ref. [14].

lowest 4f n ~ 4f n- 1 5d transition. This we observe and in particular we find that in CaC1F:Sm 2÷ the 4f5d band is significantly lower than in the Sr and Ba compounds but it still lies above 5D 0 and 5D14f6 levels. This effect of the increasing crystal field on the f - f transition is shown in table 2.

References [1] A. Winnacker, R.M. Shelby and R.M. Macfarlane, Opt. Letters 10 (1985) 350. [2] R.M. Macfarlane and J.C. Vial, to be published. [3] A. Oppenlander, J.C. Vial and R.M. Macfarlane, J.P. Chaminade, J. Luminescence 42 (1989) 331. [4] F.K. Fong and P.N. Yocom, J. Chem. Phys. 41 (1964) 1383. [5] A.R. Patel and R.P. Singh, J. Crystal Growth 5 (1969) 70. [6] E. Nicklaus and F. Fisher, J. Crystal Growth 12 (1972) 337. [7] H.L. Bhat and P.S. Narayaman, J. Mater. Sci. 10 (1975) 2007. [8] K. Somaiah and V. Babu, Indian J. Pure Appl. Phys. 14 (1976) 702. [9] C.E. Johnson and E.J. Hathaway, J. Electrochem. Soc. 118 (1971) 631. [10] R.D Shannon, Acta Cryst. A, B32 (1976) 751. [11] D.A. Wenz, I. Johnson and R.D. Wolson, J. Chem. Eng. Data 14 (1969) 252. [12] H.P. Beck, J. Solid State Chem. 17 (1976) 275. [13] Y.R. Shen, T. Gregorian, T. Tr6ster, O. Schulte and W.B. Holzapfel, High Pressure Res. 3 (1990) 144. [14] Z.J. Kiss and H.A. Weakliem, Phys. Rev. Letters 15 (1965) 457.