Ca(SCN)2 and Ca(SCN)2·2 H2O: Crystal structure, thermal behavior and vibrational spectroscopy

Article abstract of DOI:10.1515/znb-2002-1213

The dehydration of Ca(SCN)₂·4H₂O yields single crystals of Ca(SCN)₂·2 H₂O as well as of Ca(SCN)₂. Ca(SCN)₂·2 H₂O crystallizes with a hitherto unknown structure (orthorhombic, Pnma, Z = 4, a = 1280.1(2), b = 790.3(1), c = 726.9(1) pm, R_all = 0.0430). The Ca²⁺ ions are surrounded by four SCN^- ions and four water molecules. The polyhedra are connected to chains along [010] via common oxygen atoms. The SCN^- ions connect these chains to a three-dimensional network so that each thiocyanate group is linked to two Ca²⁺ ions. Hydrogen bonding with sulfur atoms as acceptors is observed. The crystal structure of Ca(SCN)₂ (monoclinic, C2/c, Z = 4, a = 961.7(2), b = 642.4(2), c = 787.2(2) pm, R_all = 0.0673) consists of alternating layers of Ca²⁺ and SCN^- ions. The cations are surrounded by four sulfur and four nitrogen atoms in form of a square antiprism. According to _∞³[Ca(SCN)_8/4] each SCN^- ion connects four Ca²⁺ ions with each other. Thermal investigations show a phase transition of Ca(SCN)₂·4 H₂O followed by dehydration to Ca(SCN)₂ which finally decomposes yielding CaS. IR and Raman measurements have been performed and the resulting frequencies assigned and discussed.

Full text of DOI:10.1515/znb-2002-1213

Ca(SCN)2 and Ca(SCN)2 · 2 H2O: Crystal Structure, Thermal Behavior
and Vibrational Spectroscopy
Claudia Wickleder and Patrick Larsen
Institut fu¨r Anorganische Chemie, Universita¨t zu Ko¨ln, Greinstrasse 6, D-50939 Ko¨ln, Germany
Reprint requests to Dr. Claudia Wickleder Fax: +49 (0)221 470 5083.
E-mail: claudia.wickleder@uni-koeln.de
Dedicated to Professor Albrecht Mewis on the occasion of his 60th birthday
Z. Naturforsch. 57 b, 1419–1426 (2002); received September 6, 2002
Thiocyanates, Vibrational Spectroscopy, Hydrogen Bonds
The dehydration of Ca(SCN)2 4H2O yields single crystals of Ca(SCN)2 2 H2O as well as
of Ca(SCN)2. Ca(SCN)2 2 H2O crystallizes with a hitherto unknown structure (orthorhombic,
Pnma, Z = 4, a = 1280.1(2), b = 790.3(1), c = 726.9(1) pm, Rall = 0.0430). The Ca2+ ions are
surroundedby four SCN ions and four water molecules. The polyhedra are connected to chains
along [010] via common oxygen atoms. The SCN ions connect these chains to a three-dimen-
sional network so that each thiocyanate group is linked to two Ca2+ ions. Hydrogen bonding
with sulfur atoms as acceptors is observed. The crystal structure of Ca(SCN)2 (monoclinic,
C2/c, Z = 4, a = 961.7(2), b = 642.4(2), c = 787.2(2) pm, Rall = 0.0673) consists of alternating
layers of Ca2+ and SCN ions. The cations are surrounded by four sulfur and four nitrogen
3
atoms in form of a square antiprism. According to [Ca(SCN)8 4] each SCN ion connects
four Ca2+ ions with each other. Thermal investigations show a phase transition of Ca(SCN)2
4 H2O followed by dehydration to Ca(SCN)2 which finally decomposes yielding CaS. IR
and Raman measurements have been performed and the resulting frequencies assigned and
discussed.
Introduction
carried out. Main feature of the crystal structure is
the coordination of the M2+ ions with four sulfur
Due to its strong parity allowed electronic tran- and just as many nitrogen atoms in form of a square
sitions, the luminescence of Eu2+ ions is used in antiprism. The unusual surrounding causes a quite
several applications, like tricolour lamps and X-ray uncommon and very intense green emission of the
storage phosphors [1]. In this case the energy of the Eu2+ doped thiocyanates M(SCN)2 (M = Sr, Ba) at
luminescence, e. g. the position of the opposite par- low temperatures [4]. In order to shift the 4f55d1
ity 4f55d1 state, depends very strongly on the host 4f6 emission to even lower energy we considered
lattice. Above all, the wavelength of the emission Ca(SCN)2 to be an appropriate host lattice. Un-
is determined by the nature of the ligands. There fortunately, structural data are not available up to
are many investigations on the optical properties of now. On the other hand the tetrahydrate Ca(SCN)2
divalent europium in oxide and halide host lattices
4 H2O has been characterized previously [5] and
where mostly an emission in the blue or ultraviolet was thought to be an appropriate starting material
range is observed [2]. Investigations of other host to prepare the anhydrous compounds by dehydra-
lattices like chalcogenides are very scarce although tion. Indeed, Ca(SCN)2 can be pepared in this way,
a red shifted emission can be expected bearing po- even in single crystalline form. Furthermore, we
tential for several new applications.
were able to obtain a dihydrate as an intermedi-
ate of the dehydration reaction and to determine
its crystal structure. Thus, in the present paper we
present the crystal structures, the thermal behavior
and the vibrational spectroscopy of the two calcium
thiocyanates Ca(SCN)2 2 H2O and Ca(SCN)2.
Recently, we started to investigate solvent free
“binary” earth alkaline thiocyanates, M(SCN)2
(M = Sr, Ba), in view of their properties as host
lattices [3]. Fortunately, Eu(SCN)2 turned out to be
isotypic [3] so that doping experiments can be easily
c
¨
¨
0932–0776/02/1200–1419 $ 06.00
2002 Verlag der Zeitschrift fur Naturforschung, Tubingen www.znaturforsch.com
K
Brought to you by | University of Sussex Library
Authenticated
Download Date | 9/26/18 10:05 AM

1420
C. Wickleder — P. Larsen · Ca(SCN)2 and Ca(SCN)2 · 2 H2O
Table 1. Crystal-
Ca(SCN)2
Compound
Ca(SCN)2 2 H2O
a = 1280.1(2) pm
b = 790.3(1) pm
c = 726.9(1) pm
= 90.82(3)
lographic data of
a = 961.7(2) pm
Lattice parameter
Ca(SCN)2 2 H2O
b = 642.4(2) pm
and Ca(SCN)2 and
c = 787.2(2) pm
their determination.
Molar volume
No. of formula units
Crystal system
Space group
Measuring device
Radiation
Temperature
Theta range
Rotation angle; increment 0 < < 200 ; 2
Index range
110.74 cm3/mol
4
73.22 cm3/mol
4
monoclinic
C2/c (no. 15)
Stoe IPDS I
orthorhombic
Pnma (No. 62)
Stoe IPDS I
Mo-K (graphite monochrom., = 71.07 pm)
20 C
2 < 2 < 50
20 C
5 < 2 < 56
0 < < 200 ; 2
–16
–9
h
16, –10
k
10, –12
–10
h
l
12, –8
10
k
8,
l
9
No. of exposures
Exposure time
Detector distance
Absorption correction
100
4 min
60
100
5 min
60
numerical after crystal shape optimisation [8]
1
1
13.51 cm
5939
653
19.88 cm
2242
598
Measured reflections
Unique reflections
— with Io > 2 (I)
No. of variables
Rint
498
409
60
0.1382
33
0.0644
Structure determinations
Scattering factors
Goodness of fit
R1; wR2 (Io > 2 (I))
R1; wR2 (all data)
CSD no.
SHELXS-86 und SHELXL-93 [7]
Intern. Tables, Vol. C [9]
0.888
0.933
0.0277; 0.0531
0.0430; 0.0556
412784
0.0323; 0.0540
0.0637; 0.0612
412783
Table 2. Atomic positions and equivalent isotropic dis-
placement parameters (in pm2) for Ca(SCN)2 2 H2O.
1
Atom Site x/a
y/b
z/c
Ueq 10
Ca
S1
C1
N1
S2
C2
N2
O
4c 0.95583(6) 1/4
4c 0.72077(7) 1/4
4c 0.6356(3) 1/4
4c 0.0769(3) 1/4
4c 0.81755(8) 1/4
4c 0.8898(3) 1/4
4c 0.9369(3) 1/4
8d 0.8978(2) 0.9500(2) 0.0663(3) 22.3(4)
8d 0.884(2) 0.933(3) 0.159(5) 21(9)
8d 0.845(3) 0.937(4) 0.019(5) 47(11)
0.04386(9) 17.7(2)
0.9146(1) 25.5(3)
0.0841(5) 27.6(9)
0.2941(5) 46(1)
0.3906(1) 28.1(3)
0.5778(5) 26.8(9)
0.7140(4) 40.7(9)
Fig. 1. [Ca(SCN)4(H2O)4] polyhedron in the crystal struc-
ture of Ca(SCN)2 2 H2O.
H1
H2
Ueq = 1/3[U11 + U22 + U33] [10].
unsoluble CaCO3, attention had to be paid that no CO2
from air can contaminate the solution. Upon slow evapo-
ration at 50 C the tetrahydrate started to precipitate. The
evaporation must not be continued to dryness in order to
avoid formation of lower hydrates. Instead, the precipi-
tate is to be filtered off. If the evaporation is carried out
at 90 C, samples of the dihydrate, Ca(SCN)2 2 H2O,
could be obtained in pure phases according to X-ray pow-
Experimental Section
Ca(SCN)2 4 H2O was obtained by mixing a slurry
of 100 g of Ca(OH)2 (p. a., Merck) in about 500 ml of
H2O and a stock solution of NH4SCN. The mixture was
stirred for 2 - 3 d at 50 C. To avoid the formation of
Brought to you by | University of Sussex Library
Authenticated
Download Date | 9/26/18 10:05 AM

C. Wickleder — P. Larsen · Ca(SCN)2 and Ca(SCN)2 · 2 H2O
1421
Fig. 2. 1 [Ca(SCN)4 1(H2O)4 2] chain
along [010] in the crystal structure of
Ca(SCN)2 2 H2O.
der diffraction measurements (Stoe, Stadi P). The very
For temperature dependent powder diffraction investi-
hygroscopic solid contained colorless prismatic single gations about 100 mg of Ca(SCN)2 2 H2O was placed
crystals.
on a silver plate under flowing argon in a heatable cham-
ber (Bu¨hler, HDK 2.4). Data were collected by a powder
In contrast to statements that it is not possible to dry
/
diffractometer (Stoe, Stadi P) in the range of 2
=
Ca(SCN)2 2H2O without decomposition [6], pure sam-
ples of Ca(SCN)2 could be obtained by heating the dihy-
drate in a vacuum to 300 C for 5 d. During this procedure
some small colorless cube shaped single crystals formed.
The sample is hygroscopic and must be handled in an
argon box.
7 - 60 between 30 and 600 C with a 20 C step width.
IR and Raman spectra were recorded using an FTIR
spectrometer (Bruker, IFS 66v/S) with a Raman equip-
1
ment (Bruker, FRA 166/S) and a resolution of 2 cm
.
For excitation, the 1064 nm line of a Nd:YAG laser was
employed. For IR measurements the samples were mixed
with dry KBr and pressed to a pellet while for Raman
investigations pure samples melted in glass tubes were
used.
Some crystals of the two compounds were selected us-
ing a polarizing microscope and then sealed in glass cap-
illaries (d = 0.1 mm). To check their quality, several expo-
sures were taken with an image plate X-ray diffractometer
(STOE IPDS I), which was also used to collect diffraction
data of the best specimen. The structure was solved and
refined using the programs SHELXS-86 and SHELXL-93
[7], respectively. For Ca(SCN)2 2 H2O the position of the
hydrogen atoms were taken from the difference Fourier
map and refined isotropically. Absorption corrections
were applied to the data with the help of the programs X-
red and X-shape [8]. Crystallographic details are summa-
rized in the Tables 1 - 3 and are available from the Fachin-
formationszentrum Karlsruhe, D-76344 Eggenstein-Leo-
poldshafen (crysdata@FIZ-Karlsruhe.de) on quoting the
deposition number given in Table 1.
DTA/TG investigations were performed using a STA
409 thermal analyzer (Netzsch). For that purpose about
20 mg of Ca(SCN)2 4 H2O were filled in corundum con-
tainers and heated at a constant rate of 10 K/min under
flowing argon. The thermal behavior was monitored from
30 C up to 800 C. For the DTA data a baseline cor-
rection was applied. Onset and end temperatures of the
thermal effects were taken from the differentiated DTA
curve following common procedures using the software
supplied with the analyzer [11].
Results and Discussion
Crystal structure
Ca(SCN)2 2 H2O crystallizes with a hitherto un-
kown structure with the orthorhombic space group
Pnma and four formula units per unit cell (Table 1).
There is only one crystallographically independent
Ca2+ ion (Table 2) which is surrounded by four wa-
ter molecules and four thiocyanate ions in form of
a distorted square antiprism (Fig. 1). Two oxygen,
one sulfur and one nitrogen atom belong to each
square plane. The two crystallographically differ-
ent SCN ions within each coordination sphere are
either connected via the terminal sulfur or via the
terminal nitrogen atoms to the cations. There are
two different Ca-S (308 and 315 pm) and Ca-N (239
and 241 pm) distances. The latter are comparable
to those in Ca(SCN)2 4 H2O [5] (245 pm) while
there are no Ca-S contacts in the tetrahydrate. The
Brought to you by | University of Sussex Library
Authenticated
Download Date | 9/26/18 10:05 AM

1422
C. Wickleder — P. Larsen · Ca(SCN)2 and Ca(SCN)2 · 2 H2O
Fig. 3. Perspective view of the crystal structure of Ca(SCN)2 2 H2O. The chains shown in Fig. 2 are connected via
the SCN ions.
Table 3. Selected distances (pm) and angles ( ) for Table 5. Selected distances (pm) and angles ( ) for
Ca(SCN)2 2 H2O.
Ca(SCN)2.
Ca-N1
238.9(4)
C1-N1
C1-S1
C2-N2
C2-S2
O-H1
116.1(5)
164.6(4)
115.9(5)
164.6(4)
71(3)
Ca-N (2 )
Ca-N (2 )
Ca-S (2 )
Ca-S (2 )
255.8(2)
257.6(2)
301.7(1)
308.7(1)
C-N
C-S
117.0(4)
163.9(3)
Ca-N2
241.0(3)
308.0(1)
315.2(1)
248.9(2)
257.9(2)
178.8(4)
177.1(4)
Ca-S2
Ca-S1
Ca-O (2 )
Ca-O (2 )
N1-C1-S1
N2-C2-S2
N-C-S
178.5(3)
O-H2
77(4)
H1-O-H2
101(4)
Hydrogen bonds
O-S1
S1-H1
O-H1-S1
334.9(2)
271(4)
151(2)
O-S2
S2-H2
O-H2-S2
342.5(2)
272(4)
155(2)
Table 4. Atomic positions and equivalent isotropic dis-
placement parameters (in pm2) for Ca(SCN)2.
1
Atom Site x/a
y/b
z/c
Ueq 10
12.0(2)
Ca
S1
C1
N1
4e 1/2
0.0923(1) 3/4
8f 0.79745(8) 0.9403(1) 0.6755(1) 18.1(2)
8f 0.6162(3) 0.3602(4) 0.3985(3) 11.8(6)
8f 0.5565(3) 0.2148(4) 0.4496(3) 15.6(6)
Fig. 4. Chains of the [Ca(SCN)8] polyhedra in Ca(SCN)2.
(Fig. 2) in such a way that the respective SCN
ions are arranged opposite to those of the neighbor-
ing polyhedra. The four SCN ions of one poly-
hedron are part of four different chains, resulting
in a three-dimensional network. This structure can
also be described to consist of layers of SCN and
Ca2+ ions which are connected via water molecules
(Fig. 3). The hydrogen atoms of the water molecules
point towards the nitrogen and the sulfur atoms of
the thiocyanate groups so that S H and N H hy-
2
Ueq = 1/3[U22 + sin
(U11 + U33 + 2 U13 cos ] [10].
Ca-O distances are also similar in both compounds
(Ca(SCN)2 2 H2O: 249 and 258 pm, Ca(SCN)2
4 H2O: 240 - 269 pm). The distances C-N and C-S
show typical values for both SCN ions (Table 3).
One angle S-C-N (178.8 ) is comparable to those in
M(SCN)2 (M = Eu, Ca, Sr, Ba) while the other one
(177.1 for S2-C2-N2) is slightly smaller.
The [Ca(SCN)4 (H2O)4] polyhedra are connected drogen bonds can be formed. While the S H dis-
via common oxygen edges to chains along [010] tances (271 and 272 pm) indicate strong hydrogen
Brought to you by | University of Sussex Library
Authenticated
Download Date | 9/26/18 10:05 AM

C. Wickleder — P. Larsen · Ca(SCN)2 and Ca(SCN)2 · 2 H2O
1423
Table 6. Results of the thermal behavior of Ca(SCN)2
4 H2O (DTA/TG data).
Reaction
Tonset Tend Tmax Mass loss (%)
obs. calc.
C
C
C
Phase transition
Melting of Ca(SCN)2 4H2O 62.0 65.4 76.6
27.2 41.1 62.8
0
0
0
0
Ca(SCN)2 4 H2O
173.9 212.8 241.3 37.7 31.6
Ca(SCN)2
Ca(SCN)2
CaS
408.8 418.2 436.5 70.9 71.8
Fig. 5. Perspective view of the crystal struture of
Ca(SCN)2.
bonds [12] the values for the N H distances (286
and 289 pm) are too long for significant hydrogen
bonding.
Ca(SCN)2 crystallizes isotypically with M(SCN)2
(M = Eu, Sr, Ba [3], Pb [13]) with the monoclinic
space group C 2/c with four formula units per cell
(Table 1). Isotypism of Ca2+ and Ba2+ compounds is
rather unusual due to the significantly different ionic
radii of Ca2+ (126 pm [14]) and Ba2+ (156 pm [14])
for an eightfold coordination but may be explained
by the softness of the SCN anions which pro-
vide space for small as well as large cations in the
structure.
All ions occupy only one crystallographic site
(Table 4). The Ca2+ cations are surrounded by four
sulfur and four nitrogen atoms which belong to eight
SCN ions (Fig. 4). The coordination polyhedron
has the shape of a square antiprism. The Ca-N dis-
tances are 256 and 258 pm (Table 5) and thus much
longer than those in Ca(SCN)2 2 H2O (Table 3).
On the other hand the distances Ca-S are shorter
than those of all isotypic compounds as well as in
the dihydrate.
Fig. 6. DTA/TG investigations of the thermal behavior of
Ca(SCN)2 4H2O (above). Guinier-Simon representation
of the temperature dependent measurement of Ca(SCN)2
2 H2O (below).
Thermal behavior
The results of the DTA/TG measurements of
The distances and angles of the SCN ion Ca(SCN)2 4H2O and the temperature dependent
are comparable to those in other thiocyanates. powder diffraction measurements of Ca(SCN)2
The SCN ion is surrounded by four Ca2+ ions, 2 H2O are shown in Fig. 6 and Table 6. The phase
therefore the compound can be described as transition of Ca(SCN)2 4 H2O is detected by an
[M(SCN)4 4(NCS)4 4]. The [Ca(SCN)8] polyhedra endothermic peak with an onset at 27.2 C (Fig. 6a).
are connected via four common thiocyanate ions to The transition to the so called
form was de-
chains along [010]. The other four ligands connect termined in a previous investigation to occur at
these chains with each other via nitrogen edges and 23.7 C [15]. At 62.0 C Ca(SCN)2 4 H2O be-
sulfur vertices. Therefore, a three-dimensional net- gins to melt as indicated by the endothermic peak
work is built which can be described as alternating in Fig. 6a without a mass loss. At 173.9 C the dehy-
layers of Ca2+ and SCN ions along [101] (Fig. 5). dration starts, but no dihydrate could be detected in
Brought to you by | University of Sussex Library
Authenticated
Download Date | 9/26/18 10:05 AM

1424
C. Wickleder — P. Larsen · Ca(SCN)2 and Ca(SCN)2 · 2 H2O
Table 7. Frequencies (in cm 1) and assignements of the
IR and Raman bands of Ca(SCN)2 2 H2O and Ca(SCN)2.
Ca(SCN)2 2H2O
Ca(SCN)2
IR Raman
Vibration Frequences of
the free species
IR
Raman
72
95
137
87
99
129
160
172
183
228
outer
vibra-
tions
172
199
261
469
478
440
451
473
488
469
477
481
477
490
500
(SCN)
470(
[16]
)
515
627
540
608
?
?
Fig. 7. IR spectra of Ca(SCN)2 2 H2O and Ca(SCN)2.
769
768
764
770
777
(SC)
743(Σ+)
[16]
951
933
942
951
957
958
967
974 2 (SCN)
995
1628
2077
2096
2123
1628
2100
2116
2124
2185
3252
(H2O) 1595(A1) [17]
2013 2010
2077 2029
2048
(CN)
2066(Σ+)
[16]
3242
3398
s(H2O) 3657(A1) [17]
as(H2O) 3756(B2) [17]
3360
the course of the DTA measurement. The observed
mass loss is 37.7% while 31.6% are theoretically
expected. The difference can be explained by the as-
sumption that some water was retained in the sample
or partial decomposition occurs. For comparision,
the powder diffraction measurements (Fig. 6b) show
a dehydration of Ca(SCN)2 2H2O starting at 90 C
which is complete at 110 C, when the powder pat-
tern can be assigned to Ca(SCN)2. The difference
in the observed temperatures detected by the two
methods can be explained by the inaccuracy of the
temperature determination of the powder method.
Both measurements have shown that Ca(SCN)2
starts to decompose at about 420 C. In the DTA
curve, an endothermic peak is detected, and the TG
measurements show a slow complicated decompo-
sition. Probably Ca(SCN)2 starts to melt at 408.8 C
and the decomposition starts directly after melting.
Fig. 8. Raman spectra of Ca(SCN)2 2H2O and Ca(SCN)2.
cyanates M(SCN)2 (M = Eu, Sr, Ba) [3]. Moreover,
the starting point of the decomposition decreases
from Ba(SCN)2 (672 C) to Sr(SCN)2 (650 C)
and Eu(SCN)2 (557 C) [3]. The further decrease
of the decomposition temperature when going to
Ca(SCN)2 is in accordance to this. The mass of the
residue is 28.2% of the initial amount of Ca(SCN)2
4 H2O, indicating that CaS has formed (calculated:
31.9 %).
Vibrational spectroscopy
The space group C2/c of Ca(SCN)2 leads to the
According to X-ray powder measurements at 500 C factor group C2h with Z = 2 in the primitive unit
this decomposition is quantitative, and the pattern cell. A maximum of 39 vibrations are to be expected
of the product can be assigned to CaS. This is in which can be described by four different irreducible
agreement with the findings for the isotypic thio- representations according to
= 10 Ag + 9 Au +
v
Brought to you by | University of Sussex Library
Authenticated
Download Date | 9/26/18 10:05 AM

C. Wickleder — P. Larsen · Ca(SCN)2 and Ca(SCN)2 · 2 H2O
1425
11 Bg + 9 Bu. 21 vibrations of gerade symmetry are to valence vibrations of the hydrogen bonds because
observable in Raman spectra, while 18 of ungerade they are expected to be at lower energy for relatively
symmetry can be detected in IR measurements. In weak bonds. But they may originate from librational
our experiments, 14 and 7 vibrations were observed modes of hydrogen bonded water molecules which
in the Raman and IR spectra, respectively (Table 7, appear because of hindered rotation. Librational vi-
Fig. 7, 8). The difference between the expected and brations are found as broad intensive bands in IR,
observed IR vibrations is mainly due to the onset of but weak ones in Raman spectra and are positioned
1
the measurements at 400 cm 1, excluding the vibra- at 850 cm in solid H2O [18]. In this case they
tions at low frequencies. Besides, some vibrations are expected to be at lower energy because of the
may have too low intensities to be observed, or the weaker hydrogen bonds.
transitions are too close in energy to be resolved.
The stretching vibrations of the water molecule
1
Ca(SCN)2 2 H2O can be described with the factor are shifted by 400 cm to lower energy compared
group D2h and, therefore, the optical phonon modes to free H2O as it is expected for hydrogen bond sys-
belong to the following irreducible representations: tems [18]. On the other hand, the respective bend-
ing vibrations are blue shifted by a few percent,
= 23 Ag + 16 Au + 16 B1g + 22 B1u + 23 B2g
v
as it is observed in the present case. Additionally,
the stretching vibrations are very broad, which is
+ 15 B2u + 16 B3g + 22 B3u.
another phenomenon attributed to hydrogen bonds.
Note that the present case is an illustrative example
of the large intensity of stretching modes of wa-
ter molecules in the IR spectra compared to Raman
spectra.
All modes with gerade symmetry are to be observed
in Raman spectra while B1u, B2u and B3u modes are
IR active. Again, only a small number of these were
detected due to the reasons mentioned above.
Fig. 7 and 8 show the IR and Raman spectra, re-
spectively. The frequencies and their assignments
are listed in Table 7. The transitions at low fre-
quencies, < 300 cm 1, can be assigned to outer
vibrations, e. g. valence vibrations like (MS) and
(MN) or deformation vibrations like (MSC) and
(MNC). The observed vibrations at frequencies
The position of the modes belonging to SCN
ions depends on the bonding conditions. In the
case of N bonding (SCN) and (SC) are posi-
tioned at lower frequencies compared to the free
ion, and at higher frequencies in the case of S bond-
ing [19]. The respective frequencies of Ca(SCN)2
and Ca(SCN)2 2H2O are similar to those of the free
ion due to SCN bridging. On the other hand (NC)
1
above 400 cm can be correlated to inner modes
of the SCN ions or the H2O molecules. The in-
creasing numbers of these transitions compared to
those of the free species can be due to factor group
multiplication and/or to a removal of the degener-
acy of modes because of the reduced symmetry in
solids.
1
is expected to show values above 2100 cm in
SCN bridging compounds [20] which is indeed
fullfiled for Ca(SCN)2 2 H2O. In contrast, the
stretching S-C modes are at lower frequencies for
the water free compound, owing to several different
factors [21].
In contrast to M(SCN)2 (M = Eu, Sr, Ba) where
the fundamentals and the first overtones of the bend-
ing vibrations (SCN) could exclusivly be detected
in the IR spectra [3], they are observed in the IR
as well as in the Raman spectra in the present case.
Acknowledgements
The authors are indebted to Prof. Dr. G. Meyer, Institut
fu¨r Anorganische Chemie, Universita¨t zu Ko¨ln, for gener-
ous support. The preparation of Ca(SCN)2 4H2O by Prof.
Dr. L. Bohaty´ and Dr. P. Held, Institut fu¨r Kristallogra-
phie, Universita¨t zu Ko¨ln, is gratefully acknowledged.
1
The two bands between 500 and 700 cm which
are found in both spectra of Ca(SCN)2 2 H2O could
not be assigned unambiguously. They cannot be due
[3] C. Wickleder, Z. Anorg. Allg. Chem. 627, 1693
(2001).
[1] G. Blasse, B. C. Grabmaier, Luminescent Materials,
Springer Verlag, Berlin (1994).
[4] C. Wickleder, P. Larsen, submitted to J. Lumin.
[2] P. Dorenbos, submitted to Phys. Rev. B.
Brought to you by | University of Sussex Library
Authenticated
Download Date | 9/26/18 10:05 AM

1426
C. Wickleder — P. Larsen · Ca(SCN)2 and Ca(SCN)2 · 2 H2O
[5] P. Held, L. Bohaty, Z. Kristallogr. NCS 216, 337 [13] J. A. A. Mokuolu, J. C. Speakman, Acta Crystallogr.
(2001). B31, 172 (1975).
[6] R. J. Meyer, E. H. E. Pietsch, Gmelins Handbuch [14] R. D. Shannon, C. T. Prettwitt, Acta Crystallogr.
der Anorganischen Chemie, 28 B, Verlag Chemie,
Weinheim (1961).
B25, 925 (1969); R. D. Shannon, Acta Crystallogr.
A32, 751 (1976).
[7] G. M. Sheldrick, SHELXS-86, Programs for X- [15] A. K. McKerrow, V. J. Occleshaw, F. Grabble,
ray structure analysis, Go¨ttingen (1986); G. M. J. Chem. Soc. 5 (1946).
Sheldrick, SHELXL-93, Program for the Refine- [16] L. H. Jones, J. Chem. Phys. 25, 1069 (1956); L. H.
ment of Crystal Structures, Go¨ttingen (1993). Jones, J. Chem. Phys. 28, 1234 (1958).
[8] X-RED 1.07, DataReduction for STADI4 and IPDS, [17] W. S. Benedict, N. Gailar, E. K. Plyler, J. Chem.
Stoe & Cie Darmstadt 1996; X-SHAPE 1.01, Crys- Phys. 24, 1139 (1956).
tal Optimisation for Numerical Absorption Correc- [18] H. Siebert, Anwendungen der Schwingungsspek-
tion, Stoe & Cie Darmstadt (1996).
[9] Th. Hahn (ed.), International Tables for Crystallog-
troskopie in der Anorganischen Chemie, Springer
Verlag, Berlin (1966).
raphy, Vol. C, D. Reidel Publishing Company, Dor- [19] W. Preetz, U. Horns, Z. Anorg. Allg. Chem. 516,
drecht, Boston (1983).
159 (1984); O. Sta¨hler, W. Preetz, Z. Anorg. Allg.
[10] R. X. Fischer, E. Tillmanns, Acta Crystallogr. C44,
775 (1988).
[11] Netzsch GmbH, Thermal Analysis for the Analyzer
STA 409, version3.1, Selb (1996).
Chem. 626, 2077 (2000).
[20] P. C. H. Mitchell, R. J. P. Williams, J. Chem. Soc.
1912 (1960).
[21] R. A. Bailey, S. L. Kozak, T. W. Michelsen, W. N.
Mills, Coord. Chem. Rev. 6, 407 (1971).
[12] T. Steiner, Angew. Chem. 114, 50 (2002).
Brought to you by | University of Sussex Library
Authenticated
Download Date | 9/26/18 10:05 AM