Z. anorg. allg. Chem. 622 (1996) 343-347
Zeitschrift fur anorganische
und allgemeine Chemie
0 Johann Ambrosius Barth 1996
Synthesis and Structure of Ba,,F,,Cl,
Frank Kubel, Hans Hagemann, and Hans Bill"
Geneva/Switzerland, DCpartement de Chimie Physique de 1'UniversitC de Genkve
Received May 25th, 1995.
Abstract. A new member belonging to the binary phase diagram
of BaF, and BaC1, was synthesized. The single domain crystals
of Ba,,F,,Cl, can be prepared from a nonstoichiometric flux
with molar ratio of 1 : 1 between BaFCl and BaF,. The compound crystallizes at room temperature in the non-centrosymmetric hexagonal space group P62m with a = b = 1408.48(14)
and c = 427.33(5) pm.
Three different barium environements with coordination
number of nine are found. The barium fluorine distances vary
between 250.59(6) - a short distance compared to other Ba-F
distances - and 302.7(1) pm and barium chlorine distances between 331.55(3) and 336.19(15) pm. This compound is further
characterized using Raman spectroscopy.
Keywords: Barium halides; X-ray crystallography; optical
studies; Raman spectroscopy.
Synthese und Struktur von Ba,,F,,Cl,
Inhaltsiibersicht. Eine neue Verbindung zur Erganzung des
binaren Phasendiagramms von BaF, und BaC1, wurde synthetisiert. Aus einer Schmelze von BaFCl und BaF, 1: 1
wurden Einkristalle von Ba,,F,,C15 erhalten. Die Verbindung
kristallisiert bei Raumtemperatur in dgr nichtzentrosymmetrischen hexagonalen Raumgruppe P62m mit a = b =
1408,48(14) und c = 427,33(5) pm.
Fur Barium wurden drei unterschiedliche Umgebungen mit
der Koordinationszahl neun gefunden. Die Abstgnde
Barium-Fluor variieren von 250,59(6) (verglichen mit anderen
Ba-F Abstanden ein sehr kurzer Abstand) bis 302,7(1) pm,
wahrend die Abstande Barium-Chlor
von 331,55(3) bis
336,19(15) pm betragen. Die Strukturdaten werden durch
Ramanmessungen erganzt.
1 Introduction
reported 1955 by Fessenden and Lewin [7]. This compound was not reported in the binary phase diagram of
BaC1,-BaF,
[8, 91 which presents two eutectics for
fluoride mole fractions of 0.19 (854°C) and 0.735
(936°C). The crystals, we obtained either by solid state
reaction or by cooling from the melt, revealed a X-ray diffraction pattern different from the one mentioned 1955 in
reference [7]. They correspond to the new compound
Ba,,F,,Cl,. This compound was characterized by single
crystal X-ray diffraction, medium resolution luminescence, optical microscopy inspection and Raman spectroscopy.
Inorganic layer structure fluoride compounds of the
PbFCl structure type have been of interest to us for some
time already, initially to study intrinsic lattice defects [ 11
and as hosts for the investigation of ligand field effects
on S-state impurities introduced into these hosts [2]. The
cations of these compounds can easily be substituted by
many rare earth (RE) dopants. Recent interest into these
materials dwells on their use in optical applications as
phosphors and more recently for photochemical hole
burning. We have studied many mixed compounds of the
system (Ca, Sr, Ba)F(Cl, Br, I) [3 - 61, for the most part
doped with samarium. These compounds were used for
room temperature spectral hole burning [3, 61, opening
interesting possibilities for optical data storage.
In order to extend the scope of local RE ion ligand
cluster structures, new host materials with more pronounced 3 D network character are needed. With the aim
to find such materials in the alkaline-earth halogen
system with barium, an attempt was made to prepare a
compound with nominal stoichimetry of Ba,F,Cl as
2 Results
Both solid state reaction at 950°C and slow cooling
(1 - 2 "C/min) of a melt with a fluoride mole fraction of
0.75 yielded needle shaped crystals with maximum
dimensions of 0.3 x 0.3 x 2 mm3 as well as smaller
amounts of BaFCl and BaF,. Slow cooling of melts with
higher fluoride content including the stoichiometric composition Ba,,F,,Cl, resulted in increased proportions of
344
Z. anorg. allg. Chem. 622 (1996)
BaFCl and BaF, with respect to Bai2FL9CI5,
but no further crystal phase was observed in the X-ray powder diffraction pattern. These results show that the formation of
BaFCl (melting point 1008 “C) is favored above
ca 1000 “C, while around 950 “C the new compound may
form.
A small crystal needle from the solid state reaction products was selected for single crystal X-ray diffraction.
The lattice constants found from diffractometer data confirm
the presence of a new phase. Optical studies of [OOOI] sections
together with diffractometer data allow to determine a hexagonal symmetry. As no further extincti_onwas found, the space
groups P6, P6, P6/m, P622, P6mm, P6 m2, P6 2m, P6/mmm
Table 1 Crystal data and experimental details (e.s.d.s in
brackets)
Formula
Formula mass
Crystal system and space group
a = b (pm)
c (Pm)
V (lo6pm’)
Z
Density (calc.)
Crystal size
Diffractometer type
Wavelength, radiation
Method
Temperature
0 range
No. of reflections mesured
No. of independent reflections
Ba12F&15
2 186.36
hexagonal, P62m (No. 189)
1408.48(14)
427.33(5)
734.1 7(17)
1
4.944 g ~ m - ~
0 . 0 7 6 0.1
~ 14x 0.19 mm
STOE
MoKa, 71.073 pm
o/O-scan
300 K
1.7 - 30.0
8811
472
0.021
Rint
No. of reflections with I > 3a(I) 470
XTAL 3.2
Program used
16.453
Absorption coefficient
Absorption correction, Tmin/Tma analytical, 0.143/0.340
0.016 (0.026)
R (Rw)
45
Number of parameters refined
- 0.02(5)
Absolute structure parameter
Min/max e- density values (A3) - 1.63/1.04
are possible. The structure was solved by direct methods in
space group P6 and refined to -R = 0.02 using the program
package XTAL 3.2 [101. Crystallochemical considerations and
the program MISSYM [Ill allowed to conclude, that the real
space group has higher symmetry. The final refinement was
then made in the corrected space group P6 2m with shifted
atom positions. In this noncentrosymmetric group ferroelectric
twinning is possible. The analysis of the Friedel pairs by refining the absolute structure parameter (Flack [ 121) indicated, that
the chosen crystal is a ferroelectrical single domain. A final
quality factor of 0.016 was obtained for 45 parameters and 468
independent reflections (see Tables 1 -3). The chemical composition is a result of this refinement.
The local structure can be described as follows: there are
three independent barium sites (see figure 1A - 1C) with
coordination number nine (<303 pm for the Ba-F
distance and <340pm for the Ba-CI distances). In
BaFCl [13], a Ba2+coordination of nine atoms with five
chlorine and four fluorine atoms is found, similar to a
compressed monocapped anticube. The barium is located
between a square of fluorine atoms forming the base and
a larger square of four chlorine atoms turned by 45”
around C, through the metal ion. A last chlorine atom
completes the arrangement. In Ba,,FI9C1,, the metal sites
have two types of coordination spheres similar to the
BaFCl structure. Barium (1) and (3) are surrounded by
seven fluorine and two chlorine atoms. Ba(1) together
with the base F(l, 2,2,3), two fluorine ions F(4,4) with
relatively long Ba-F distances of 302.7(1) pm and two
adjacent chlorine atoms C1(2,2) form the second twisted
“square”. One fluorine ion, F(l) completes the arrangement (see Fig. 1A). Ba(3) is connected to F(1,1,2,4),
C1( 1 , l ) and F( 1 , l ) capped by F(2). The local surrounding
of barium (2) is similar to the one in BaFCl. It consists
of four fluorine, F(2,2,3,3) and four chlorine, C1(1,1,
1 , l ) ions, and is completed by a fluorine F(5) ion (see
also figure 1A - C). A long distance between the barium
ion and a 10th halogen (chlorine) is found perpendicular
to and through the fluorine square, similar to the situation in the BaFCl host. In the new compound this
distance is 365.0(6) pm from Ba(2) to the fluorine, F(4).
Table 2 Atomic positional and isotropic ( x 100) and anisotropic displacement parameters ( x 100). Estimated e.s.d.‘s in parenthesis
~~
Atom
x/a
Y/b
z/c
U
u
1
1
0.16765(3)
0.1884(2)
0.3634(4)
0.1825(4)
0.2786(4)
0.17792(4)
0.4370(4)
0.62733(4)
1/3
0
0.47514(3)
X
0.4881(4)
0.3749(4)
0
0
0
0
2/3
0
1/2
1/2
1/2
0
1/2
0
0
0
0
0
1.31(1)
1.94(8)
1.9(2)
1.7(2)
1.6(2)
1.28(2)
2.1(2)
1.29(2)
1.73(7)
2.2(3)
1.3 l(2)
1.99(8)
1.7(2)
1.6(2)
1.6(2)
1.23(2)
132)
1.40(2)
2u12
2 Ul2
u
2
z
1.08(2)
u
1
1
1.4(2)
2.2(2)
2u12
2u12
2 u12
2u12
2u12
2 u12
u
3
3
UlZ
u
1
3
u
Z
3
1.49(2)
2.0(1)
2.6(2)
1.3(2)
1.5(3)
1.34(3)
2.4(4)
1.26(3)
1.5(1)
3.3(7)
0.56(1)
1.12(9)
0.7(2)
0.9(2)
0.9(1)
0.65(1)
132)
0.57(1)
0.91(4)
0.9(2)
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
The anisotropic displacement factor in the structure factor expression is:
Uij = exp - 2 r ~ ~ ( U , ~ +
h ~UUkZb*2
a * ~ + U3312c*2
+ 2U12hka*b*+ 2Uf3hla*c*+ 2Uzsklb*c*).
345
F. Kubel et al., Synthesis and Structure of Ba,2F,9C1,
Table 3 Selected interatomic distances (in pm) and angles (in
degrees). E.s.d.’s in parenthesis)
Distances:
Ba(1)Ba(2)Cl(2) 331.55(3) x2C1(1)
F(2)
262.6(3) x 2 F(2)
F(3)
258.9(4)
F(3)
F(4)
302.7(1) x 2 F(5)
F(l)
267.0(6)
[F(4)
F(l)
262.9(9)
B
Angles below 90”
Ba(l)
c12- A -c12 80.25(1)
C12- A -F1 70.22(7)
C12- A -F2 72.55(8)
C12- A -F4 81.87(5)
C12- A -F1 70.7(1)
F3- A -F4
66.2(1)
66.1(1)
F2- A -F4
F4- A -F4
89.80(4)
F4- A -F1
69.9(2)
FI- A -F2
71.3(1)
F2- A -F3 72.57(9)
Ba(3)335.2(2) x 4 Cl(1)
267.903) x2F(2)
256.4(3) x 2 F(4)
250.59(6) F(l)
365.0(6)]
Ba(2)
c11- A -c11
C11- A -C11
C11- A -F2
C11- A -F3
CII- A -F5
C11- A -F3
F3- A -F2
F3- A -F2
86.56(6)
79.20(5)
68.0(1)
70.33(9)
69.40(5)
70.3(1)
72.1(1)
72.09(8)
336.2(2) x 2
26946) x 2
268.0(6)
272.8(4) x 4
Ba(3)
C11-A -F1
C11- A -F2
FI- A -F2
FI- A -F1
F1- A -F4
73.37(8)
67.72(9)
69.4(1)
67.8(2)
74.1(1)
C
Fig. 2 Projection of the structure of Ba12F,,C1,along the hexagonal axis. Distorted tricapped “propeller type” trigonal
prisms around Ba( 1) form a network. A similar pattern around
Ba(2) and Ba(3) with one “blade” in common are located in the
channels and shifted by one half of the unit cell in the chex
direction. The cross indicates the position of F(5) in the center
of the channels
Figs. 1A - C Structural unit around Ba(1) (A), Ba(2) (B) and
Ba(3) (C). Small numbered atoms are fluorine atoms, larger terminal atoms are chlorine atoms. The connected F4 unit helps to
compare with the F,-Ba-Cl,-Cl
structural unit present in
BaFCl
The global hexagonal structure of Ba,,F,,Cl, may be
compared with the hexagonal modification of BaC1, [I41
(P62m, a = b = 811.3(8) and c = 467.5(5) pm)) crystallizing in the Fe,P type. In this case the local environement is given by an arrangement of nine chlorine atoms
forming a tricapped trigonal prism. In Ba,,F,,Cl, a
similar ordering can be constructed, as can be seen in
Figure2. Around Ba(1) a connected group of “propeller” type columns is built with the chlorine atoms as
346
“propeller” axis. Six connected edge sharing “propellers” of this type form a channel which contains
another arrangement of three “propellers~~
with common
“blades” and again chlorine atoms as axis. Ba(2) and
Ba(3) are in the center of the individual tricapped prism
generated by this latter arrangement, which is shifted by
112 parallel to the Chex axis. One of the common capping
fluorine atoms (F(5)) is present in the center of this unit.
Compared to other Ba-F distances based on data
from ICSD (1994), the Ba(2)-F(5)
distance of
250.59(6) pm is surprisingly short, compared to the ionic
radii after Shannon [15] of 142 pm (C.N. 8) for Ba2’
and 133 pm (C. N. 6 ) for F-. The short distance can be
understood, as F(5) is a capping atom to three Ba(2)
atoms. This type of distance is normally rather short.
Further its position in the center of a channel (see Fig. 2)
may be an other reason for the short distance. For “normal” Ba-F distances values between 260 and 280 pm are
found.
The density of 4.944 gem-, also reflects the compact
structural arrangement if compared with BaF,
(4.889 gcm-’), BaCl, (3.917cuband 3.856”’”” gem-,) and
BaFCl (4.51 gcm-’).
The atomic displacement parameters of Ba2+and C1vary only slightly from spherical behaviour. Thermal displacements for the fluorine atoms are more anisotropic.
For F(4) a maximal ratio of 2.0 for U,,/U,, was found.
As observed in BaFCl, the isotropic atomic displacements of fluorine and chlorine are in the same order of
magnitude, and atomic displacements of the heavier
metal atoms are small compared to those of the halogen
atoms.
It must be noted that the local non-centrosymmetric
symmetry of the bivalent metal ions may be favourable
for hole burning applications. Experiments in this direction are under progress. Another interesting dopant is
europium, as barium fluorohalides doped with europium
are known to be efficient phosphors for radiation screens.
3 Raman spectra
Group theory predicts for the famor group D,, of
Ba,,F,,Cl, the following lattice vibrations:
11A;+4A,”+ 11A,’+ 10A;+25E’+ I I E ” , of which
1A; and 1E ’ correspond to the acoustic modes. The
symmetrical Raman active vibrations transform as A,’,
E ’ and E ” .
Polarized Raman spectra were performed at ambient
and liquid nitrogen temperatures on a small
(ca 0 . 2 0.2
~ x 2 mm3) crystal needle (see figure 3). Due to
the small sample size, an optimum alignment could not
be achieved. This results in imperfectly polarized spectra.
Nevertheless, these measurements allow to distinguish
clearly 3 sets of bands pertaining to the 3 Raman active
irreducible representations, confirming the axial symmetry of the crystal. Table4 collects the room
temperature Raman shifts observed for Bal2Fl9C1,.
Z . anorg. allg. Chem. 622 (1996)
5,
4
4.5
10‘
I
t
1
li
P1.51-
05-
0
0
50
zx
100
150
200
250
300
350
400
450
Raman Shift [cm-11
Fig. 3 Polarized Raman spectra of Bal,F19C1, at liquid
nitrogen temperature. Labels ZZ, YX and ZX refer to the
laboratory frame which corresponds to tensor elements of A,’,
E’ and E ” symmetry, respectively. The lines marked with * are
laser plasma lines and do not belong to the spectra
We observe quite less than the predicted number of
Raman active bands. This stems partially from the fact
that we are not able to distinguish between polarization
leakage and genuine band for weak bands appearing in
one polarization, while at the same Raman shift there is
a strong band in the other polarization.
Table4 Raman shifts (in cm-‘) for BaF, and BaFCl and
BaI2F&l5at 300 K
BaF,
243
BaFCl
25 1
216
165
143
132
82
Bal2Fl9C1,
a;
E‘
E”
222
205
194
122
333
329
263
253
199
142
132
111
285
279
237
189
174
144
123
74
47
Inspection of figure 3 shows that one may divide the
spectra into several regions: below 100 cm-’,
100-150cm-’, 190-300cm-’, above 300cm-’. Comparison with published Raman and IR spectra [16-211
of BaCl,, BaF, and BaFCl suggests to assign the bands
observed above 190cm-’ to modes involving mainly
metal fluoride vibrations. The bands observed at 329 and
333 cm-’ with E’ symmetry appeared to present the
highest observed Raman shift for all crystals known so
far in the BaC1,-BaF, system. This could be related to
the shortest Ba-F bond in the crystal. Group theory
347
F. Kubel et al., Synthesis and Structure of Ba12F19C1,
predicts indeed one mode of E’ symmetry for this individual fluoride ion, corresponding to a motion in the
a,b plane of the crystal. Note that the nearest Ba(2) ions
form an equilateral triangle around this ion in the a,b
plane, and thus one can construct Ba(2)-F(5) valence
modes with E’ symmetry similar to the textbook C0,’vibrations (see Fig. 2). However, one must keep in mind
that the crystal Ba,,F,,Cl, does not present individual
molecular ions, which makes the above description only
qualitative.
The spectra do not change significantly upon cooling,
the Raman shifts increase by 3 to 5 cm-’.
4
Experimental
1) Solid state reaction: Ba12F&15 single crystals can be
prepared by a nonstoichiometric (BaFCVBaF, 1 : 1) solid state
reaction at 950°C for four hours. Guinier films indicate the
presence of small amounts of BaFCl and BaF2 together with
the diffraction lines of the compound under study. Needle
shaped single crystals were present; one crystal was used for
single crystal analysis after optical examination under a high
precision LEITZ microscope.
2) Growth from the liquid state: The compound Ba,,F,,Cl,
was prepared under Argon atmosphere from dry crystallized
BaFCl and ultrapure dry BaF2, in a molar ratio of 1 : 1. The
reactants were kept and ground carefully in in a glove box. A
graphite crucible (previously degassed at 1200 “C) was used for
the reaction and the crystal growth. The reaction chamber and
the products were dried under high vacuum at room
temperature for three days and at 200 “C for 4 h using a liquid
nitrogen trap. The mixture was brought and kept in the liquid
state - controlled by visual inspection - at 1250°C for one
hour. Controlled cooling (with the aid of a PC governing an
EUROTHERM temperature controller) from 1 150 to 800 “ C in
300 minutes was used for crystal growth followed by fast cooling to room temperature. Further synthesis with ratios of BaFCl
to BaF2 of 1 :2 and 1 :4 were performed.
3) Growth from stoichiometric compositions: Experimental
arrangement as for 2) except composition ratio of BaFCl to
BaF2 corresponding to Ba12FI,CI,.
4) Single crystal diffraction: Crystal study conditions are
summarized in Table 1. Standardized [22] atom positions and
isotropic and anisotropic atom displacements are listed in
Table 2. Selected interatomic distances and angles are given in
Table 3.
5 ) Raman spectra were obtained using our laboratory
assembled instrument described in [3]. Low temperature
measurements were done in a home-built liquid nitrogen coldfinger dewar.
The financial support of the Suiss National Science Foundation
and of the “Optique” priority program of the Board of the
Suiss Federal Institutes of Technology is gratefully acknowledged. The authors thank Dr. Karin Cenzual for helpful discussions and Mr. Didier Frauchiger for technical help.
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Authors’ address:
PD Dr. F. Kubel, Dr. H. Hagemann, Prof. Dr. H. Bill
DCpartement de Chimie Physique
Universite de Gen5ve
30 Quai E. Ansermet
CH-1211 Genke 4Awitzerland