Article
Reference
Synthesis, crystal structures and spectroscopic investigations on samarium-doped mixed Ba
1-δSr
δMgF
4crystals
KUBEL, Frank, HAGEMANN, Hans-Rudolf, BILL, Hans
Abstract
Single crystal X-ray diffraction analysis was performed on crystals with composition Ba1 − δSrδMgF4 (δ ≤ 0.55). The complete structure was analyzed for single domain crystals with nominal (refined) values of δ = 0.25 (0.27(2)) and 0.5 (0.55(2)). Interatomic distances vary in a characteristic manner, when smaller strontium ions replace the barium ions. Optical studies of Sm(II) doped samples show significant inhomogeneous line broadening and confirm the disorder On the Ba Site.
KUBEL, Frank, HAGEMANN, Hans-Rudolf, BILL, Hans. Synthesis, crystal structures and spectroscopic investigations on samarium-doped mixed Ba
1-δSr
δMgF
4crystals. Materials Research Bulletin , 1997, vol. 32, no. 3, p. 263-269
DOI : 10.1016/S0025-5408(96)00190-0
Available at:
http://archive-ouverte.unige.ch/unige:2958
Disclaimer: layout of this document may differ from the published version.
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PI1 SOO25-5408(96)80190-O
SYNTHESIS, CRYSTAL STRUCTURES AND SPECTROSCOPIC INVESTIGATIONS ON SAMARIUM-DOPED MIXED Bat+!3r&WgF~ CRYSTALS
F. Kubel, H. Hagemann and H. Bill
Departement de Chimie Physique, Universite de Geneve 30, quai E. Ansermet, CH 1211 Geneva 4, Switzerland
(Refereed)
(Received October 16,1996; Accepted November 14, 1996)
ABSTRACT
Single crystal X-ray diffraction analysis was performed on crystals with composition Bal_$r$igF4 (6 5 0.55). The complete structure was analyzed for single domain crystals with nominal (refined) values of 6 = 0.25 (0.27(2)) and 0.5 (0.55(2)). Interatomic distances vary in a characteristic manner, when smaller strontium ions replace the barium ions. Optical studies of Sm(I1) doped samples show significant inhomogeneous line broadening and confirm the disorder on the Ba site. Copyright 8 1997 EIWW Science Ltd
KEYWORDS: A. fluorides, A. optical materials, C. X-ray diffraction, D.
crystal structure, D. luminescence
INTRODUCTION
Fluorides and related compounds are promising host materials for spectral hole burning studies. For best performance, the preparation of disordered inorganic systems with good optical quality showing large inhomogeneous bands is required.
In the course of our ongoing investigations on crystalline materials as potential hosts we prepared and characterized several mixed inorganic compounds (1,2,3). These mixed crystals allowed us to obtain large inhomogeneous optical linewidths of rare-earth ions doped into these samples.
The compound BaMgF., is a potentially interesting optical material due to its polar structure. Although large and good quality single crystals of BaMgF4 can be prepared, there are only limited experimental results available which have been performed on single crystals (4). To our knowledge, there are no structural studies on solid solutions of BaMgF4 with other alkaline earth ions.
263
264 F. KUBEL et al. Vol. 32, No. 3
TABLE 1
Crystal Data* and Conditions of Data Collection and Refinement for Bal_&sMgF4 Crystals (&minsl= 0.25 and 0.5)
Crystal [S] 0.271(18) 0.553(16)
Space group Cmc2, [No. 361
a (Pm) 409.02(8) 401.95(3)
b (pm) 1452.7(5) 1452.2(3)
:
(pm) 578.86(17) 572.70(5)
(lOhpm’) 343.95(17) 334.29(9)
Z 4
Calc. Density ( in gem”) 4.349 4.168
Crystal size (mm) 0.050 x 0.070 x 0.130 0.014 x 0.08 x 0.094
Diffractometer type CAD4
Wavelength, radiation CuKa, 154.183pm
Method o/29-scan
Temperature 300 K
0 range 6.1-75.1
No. of refl. measured 1462 1995
No. of independent refl. 225 397
R I”, 0.045 0.027
No. of refl. with 1>30(1) 213 216
Program used XTAL 3.2 (7)
Absorption coefficient 74.4 54.8
Abs. COIT. anal. T,tJTm,, 0.021 / 0.144 0.067 / 0.504
R (Rw) 0.05 (0.041) 0.021 (0.020)
Number of parameters 42 38
Goodness of fit 0.040 0.03 1
Extinction coefficient 553 (112) 3690 (310)
Min/max e- density (A-)) -5.8 / 3.6 -0.86 / I .30
x&V 0.0 0.0
‘esd’s are given in parentheses.
Experimental data are available on powders of SrMgF4, EuMgFd, and SmMgF., and on some of their solid solutions (5,6). The lack of good quality single crystals did not yet allow for complete structural characterization. The phase diagram of SrFrMgFz (5) shows a peritectic at 50 mole% and 88O”C, corresponding to the compound SrMgF4. There is a solid-
TABLE 2
Standardized Atomic Coordinates (8) and Displacement Parameters (x100)”
popBa=0.729(18); Sr=0.271(18) Ba = 0.447(16); Sr = 0.553(16)
Atom y 2. UkO Y z Ui.
Fl 0.0246(5) O.OOO(3) 2.6(5) 0.0296(3) O.OOOO(12) 2.8(3)
Mg 0.0851(3) 0.3130(12) 1.1(3) 0.08431(15) 0.3213(6) 1.35(15)
F2 0.1644(6) 0.576(3) 2.1(5) 0.1618(4) 0.5987(13) 3.1(4)
F3 0.1967(5) 0.121(2) 1.5(4) 0.1971(3) 0.1275(13) 2.0(3)
Ba 0.35061(5) 0.34955 2.12(8) 0.34782(4) 0.36235 2.30(4)
Sr 0.35061(5)b 0.34955b 2.2(3) 0.34782(4)b 0.36235b 2.30(4)b
F4 0.5810(6) 0.303(3) 2.3(5) 0.5822(4) 0.3107(13) 3.3(4)
’ Wyckoff position for all atoms is 4(a): 0, y, z. b constraint values.
*
1452- w
b
1450:
E :
Q446j .- c
$1446 0
r
A B
FIG. 1
Unit-cell parameters [106pm3 (A) and pm (B)] of Ba#,MgFs as a function of 6. Values for
SrMgF4 are taken from ref. 6,
solid phase transition at -809°C for this compound. EuMgF4 and SmMgFd also do not melt congruently.
In this paper we present the synthesis and crystal structure of Bal_&ir@gF4 crystals and optical measurements on samarium(ll)-doped crystals.
EXPERIMENTAL
Transparent crystals of Ba&$!lgF~ with 6 I 0.5 (nominal composition) were prepared from the melt of the stoichiometric mixture. The growth experiments were performed in a stainless steel vacuum tight Czochralski resistive fwnace under argon atmosphere. The samples were contained in pyrolytic graphite crucibles. The starting materials were ultrapure precrystallized SrFz (prepared in our laboratory), BaF2 (Merck suprapure), and MgFz (Balzers, vacuum deposition grade). Small crystals with dendritic shape can be grown for 0.5 < 6 < 0.6; they are suitable for X-ray crystal structure analysis. We were not able to obtain single crystals with S b 0.6 by this method. Several attempts to prepare stoichiometric SrMgF4 from the melt yielded predominantly a mixture of the starting compounds SrF2 and MgF2.
Orange-yellow Sm(II) doped crystals, with nominal dopant concentrations of ca. 0.2%, were obtained by adding metallic samarium to the growth charge.
Single crystal refinement data are summarized in Table 1. Both crystals studied do not deviate from orthorhombic symmetry; reflections indicating a symmetry change were not detected, Internal R values of 4% (S = 0.27(2)) and 3% (6 = O-55(2)) suggest no symmetry
266 F. KUBEL et al. Vol. 32, No. 3
FIG. 2
Drawing of the structure Ba&r,, 5MgF4. The magnesium fluoride octahedrons are connected by the corners and form sheets. Alkaline earth ions (Ba and Sr) are shown as filled spheres.
Thick bonds (labeled a to d) decrease when Ba is substituted by Sr, thin bonds (labeled f to h) increase. See also Table 3.
Ba-F
Mg-F
Ba, _,Sr,MgF4
FIG. 3
Selected interatomic distances as a function of Sr substitution. Values for B&&F4 are taken from ref. IO. For the Ba-F distances, the size of the symbols is larger than the esd’s, while for the Mg-F distances the esd’s are approximately twice the symbol size.
TABLE 3
Selected Interatomic Metal Fluoride Distances in pm*
6 0.271(18) 0.553( 16) WI
label:
a:F2 x2 b: F3 c: F3 x2 d: F4 c: Fl x2 f: F2 g: F4 h: F4
Ba(S+F 259.5(9) 251.8(5) 259.8(Q) 256.9(6) 266.9(7) 260.2(5) 280.8( 13) 276.2(7) 286.9(7) 279.9(4) 300.6(10) 302.2(6) 331.4(13) 331.9(E) 335.8(9) 341.6(6)
Mg-F
7.7(10) 2.9(11) 6.7(9) 6.4(15) 7.0(8) -1.6(11) -0.5(15) -5.8(11)
F2 190.9(13) 194.5(6) -3.6(14)
Fl 192.8(11) 194.6(7) -1.8(12)
F3 196.5(10) 197.9(6) -1.4(11)
Fl 201.2(15) 200.4(7) 0.8(16)
F4 x2 204.68(7) 201.09(3) 3.6(7)
*Labels a to h correspond to those in Figure 2. The esd’s are given in parentheses.
change. The refined compositions do not differ by more than 30 from the stoichiometry of the melt.
Luminescence and Raman measurements were obtained on a computer controlled Raman spectrometer consisting of an argon ion laser and a SPEX 1403 monochromator equipped with a photomultiplier tube. The luminescence spectra of the Sm(I1) doped crystals were performed using the 488 nm laser line for excitation.
RESULTS AND DISCUSSION
X-ray Diffraction The lattice parameters for pure BaMgF4 were calculated from a refined powder diffraction diagram (I& = 4.7% and RBngg = 6.7%) and calibrated against a silicon standard with a = 543.083 pm. The values are a = 413.23(5), b = 1452.91(19), c = 582.18(8) pm, and V = 349.54 106pm3, which are similar to those found in the literature (9).
Replacing Ba completely by Sr (for lattice constants see ref. 6) decreases the unit cell volume by -10%. Intermediate replacements can be fitted by a linear function (see Fig. 1A).
Lattice constants a and c decrease linearly, whereas the long lattice parameter b presents a different behavior (see Fig. 1B).
For the crystal with 6 = 0.27, the four anisotropic atomic displacement factors of Ba and Sr were refmed independently; they were found to be identical within experimental error.
Other crystallographic parameters of Ba$r,MgF., crystals show systematic trends with increasing Sr content. Figure 2 shows a projection of the structure on the b,c plane.
Octahedrons of MgF6 are connected by comers and form sheets linked by a BaFll environment. The substitution of Ba by Sr changes the structure in the following way: (1) The Mg environment becomes more symmetrical. The longest Mg-F distance decreases whereas the other interatomic distances remain invariant or increase slightly. (2) The shorter (~300 pm) Ba-F distances (labeled a-e in Fig. 2) decrease, whereas the distances linking the sheets to one another (label f-h) increase significantly. Selected Ba-F and Mg-F distances as
268
l .s . 4.
3.5 . s- 25-
2- 1.5.
,- osc
Vol. 32, No. 3
Emission Frequency [cm-l]
FIG. 4
Room temperature ‘Dw7Fo emission spectra of Sm(I1) in Bal,Sr,MgF4. The nominal 6 values are indicated in the figure.
a function of composition are shown in Figure 3; values of BaMgF., are taken from (10).
Decreasing Ba-F distances are shown in Figure 2 as thick, increasing distances as thin bonds. Corresponding labeled values are given in Table 3.
Luminescence Spectra. The optical spectra of Sm(I1) in BaMgR have been reported previously (11,12). A notable feature of these spectra is the remarkably high energy of the 5Dr7Fo transition observed at 14715 cm-’ (at 77 K) in BaMgF4. In other halide lattices such as SrF2, CaFz and alkali halides or BaFCl, this transition is found typically around 14550 f 100 cm-’ (13). Similar high values have been observed for this transition in SmMgF, (14684 cm-‘) (6) and SmBeR (-14706 cm-‘) (14). This similarity indicates that the Sm(I1) ion replaces indeed Ba ions in BaMgK. Current ESR studies in our laboratory on Eu(I1) doped crystals of BaMgFd confirm the substitution of barium by the divalent rare-earth ion.
Figure 4 compares the ‘Do-‘Fo emission band of Sm(I1) in Bal,SrJvIgF4 crystals with different 6 values. The width of this band increases from 3.4(2)cm-’ (FWHM) for 6 = 0 (lorentzian line shape) to 42(l) cm-’ for 6 = 0.5. In our previous studies on Sm(I1) doped MeFX crystals, the largest inhomogeneous linewidth observed for this transition was less than 40 cm-’ (1,2,15). It is also interesting to note that the shape of this emission band is not a simple gaussian in Bao.&o.sMgF+ but looks rather like a superposition of 2 gaussians centered at different frequencies. This behavior differs from the one observed in Sm(I1) doped crystals Srl_&a$Cl and Sr&3a&Zl where the shape of this transition was practically gaussian for 6 = 0.5 (1,2). However, Sm(I1) doped crystals of the composition SrFCl,,Br, show complex line shapes which have been explained by the model published in ref. 15. The simplest form of this model takes into account the different chemical (Cl-Br) environments in the fast coordination sphere of the Sm(I1) ion in the crystal. This results in the spectral superposition of 12 different species (15). On the basis of these results the present experimental data need to be completed to be able to decide between the effects of chemical cation disorder or those arising due to a hypothetical change of crystal symmetry, though this one was not observed by X-ray diffraction.. Further spectroscopic studies including hole burning experiments are in progress.
Surprisingly, the emission spectra of SmMgF., at 77K show two bands around 680nm corresponding to the ‘Do--‘Fo transition (6). As this transition is nondegenerate, the
observation of two bands suggests the presence of two different sites occupied by Sm(II), implying a structural phase transition. The similarity of the ionic radii of Sm(I1) and Sr(II) in conjunction with the postulated lower symmetry for SmMgF4 raises the possibility of lower symmetry for SrMgF4. A structural change between 6 = 0.6 and 6 = 1 in Ba&-,MgF4 could account for the nonlinear behavior of the lattice parameter b (see Figure IB). Recent structural studies on SrMgR show indeed a change in symmetry (IO).
ACKNOWLEDGMENTS
The authors are indebted to Dr. Franz Gingl for communicating his unpublished results on BaMgFd and SrMgFd. This work was supported by the Swiss National Science Foundation and the Swiss Priority Program ‘Optique I’.
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