Spectroscopic and thermal properties of FeHg(SCN)4

Article abstract of DOI:10.1016/j.tca.2003.11.010

The preparation and characterization of iron mercury thiocyanate, FeHg(SCN)₄ (abbreviated as FMTC) are described. The spectroscopic properties were characterized by X-ray powder diffraction (XRPD), infrared, Raman and UV-Vis-NIR transmission sp

Full text of DOI:10.1016/j.tca.2003.11.010

Thermochimica Acta 414 (2004) 53–58
Spectroscopic and thermal properties of FeHg(SCN)4
X.Q. Wanga,∗, D. Xua, M.J. Liub, X.Q. Houa,c, X.F. Chenga,
M.K. Lua, D.R. Yuana, J. Huangd
a
State Key Laboratory of Crystal Materials, Institute of Crystal Materials, Shandong University, Jinan 250100, PR China
b
Qingdao Institute of Architecture and Engineering, Qingdao 266033, PR China
c
School of Materials Science, Jinan University, Jinan 250022, PR China
d
College of Environmental Science and Engineering, Institute of Modern Analysis, Shandong University, Jinan 250100, PR China
Received 5 May 2003; received in revised form 7 November 2003; accepted 7 November 2003
Abstract
The preparation and characterization of iron mercury thiocyanate, FeHg(SCN)4 (abbreviated as FMTC) are described. The spectroscopic
properties were characterized by X-ray powder diffraction (XRPD), infrared, Raman and UV-Vis-NIR transmission spectra. The thermal sta-
bility and thermal decomposition of FMTC were investigated by means of thermogravimetric analysis (TGA) and differential thermal analysis
(DTA). The intermediates and final products of the thermal decomposition were identified by X-ray powder diffraction at room temperature.
© 2003 Elsevier B.V. All rights reserved.
Keywords: Iron mercury thiocyanate; X-ray powder diffraction; Infrared spectroscopy; Raman spectroscopy; Transmission spectra; Thermal analysis
1. Introduction
Mn, abbreviated as CMTC, ZMTC and MMTC, respec-
tively) crystals were reported [4–7]. The crystal struc-
ture and SONLO properties of iron mercury thiocyanate
(FeHg(SCN)4, abbreviated as FMTC hereafter) crystal were
reported previously [8]. It belongs to the tetragonal crys-
Recently, good second-order nonlinear optical (SONLO)
materials capable of efficient frequency conversion of in-
frared laser radiation to visible and ultraviolet wavelengths
are attracting much attention since they are of great interest
for applications including telecommunications, optical com-
puting, optical information processing, optical disk data stor-
age, laser remote sensing, laser-driven fusion, color displays,
medical diagnostics, etc. Materials with large second-order
optical nonlinearities, short transparency cutoff wavelengths
and stable physicochemical performances are needed in or-
der to realize many of these applications.
The MHg(SCN)4·nH2O (where M = Co, Cu, Zn, Cd, Ni,
Fe, Mn) series of crystalline complexes have been known
for a century in analytical chemistry for their character-
istic shapes and colors [1]. Along with the development
of nonlinear optics, such coordination compounds have
been discovered to be useful as SONLO materials [2,3].
In recent years, the more detailed preparation, structure
and SONLO properties of MHg(SCN)4 (M = Cd, Zn,
¯
tallographic system, space group I4, with cell parameters
a = 11.1244(2) Å, c = 4.2890(10) Å, V = 542.2(2) Å3,
Z = 2, Dc = 2.993 g/cc. FMTC crystal powder shows a
532 nm second harmonic intensity 0.6 times as that of the
urea crystal powder. To continue this work, in this paper,
the spectroscopic properties of FMTC are investigated by
X-ray powder diffraction (XRPD), infrared (IR), Raman and
UV-Vis-NIR transmission spectra. The thermal properties
of the complex are studied by thermogravimetric analysis
and differential thermal analysis (TGA/DTA) in combina-
tion with XRPD phase analysis of the solid residues at room
temperature.
2. Experimental
2.1. Materials and synthesis
∗
Corresponding author. Tel.: +86-531-8362822;
All the starting materials were analytical reagent grade
(purity ≥ 98.0%) and used as purchased. FMTC was syn-
fax: +86-531-8565403.
E-mail address: xqwang@icm.sdu.edu.cn (X.Q. Wang).
0040-6031/$ – see front matter © 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.tca.2003.11.010

54
X.Q. Wang et al. / Thermochimica Acta 414 (2004) 53–58
Table 1
thesized by a coordination reaction and then a metathetical
Relative experimental intensities, d-spacings (observed and calculated),
2θ (observed and calculated) and their indices for FMTC product
reaction, where M = K, Na, NH4:
4MSCN + HgCl2 = M2Hg(SCN)4 + 2MCl
M2Hg(SCN)4 + FeCl2 = FeHg(SCN)4 + 2MCl
h
k
l
dobs (Å)
dcalc (Å)
2θobs (◦)
2θcalc (◦)
I/I0 (%)
1
2
1
2
3
2
3
3
4
4
4
5
1
4
5
5
3
6
6
3
5
5
7
6
5
7
7
5
8
6
1
0
0
2
1
1
0
3
2
1
3
0
1
4
3
2
1
2
1
3
4
5
1
4
1
0
2
3
0
5
0
0
1
0
0
1
1
0
0
1
0
0
2
0
0
1
2
0
1
2
1
0
0
0
2
1
1
2
0
1
7.951
5.622
4.007
3.975
3.556
3.263
2.822
2.647
7.951
2.301
2.249
7.9484
11.128
15.762
22.182
22.363
25.043
27.329
31.703
33.862
35.671
39.156
40.093
11.131
15.767
22.174
22.37
77
63
63
100
20
63
49
51
83
30
6
5.62037
4.00873
3.9742
3.55463
3.26364
2.82231
2.64947
2.5135
2.2. Physical measurements
25.05
27.325
31.703
33.831
35.721
39.146
40.107
The XRPD pattern of FMTC product was registered with
a Rigaku D/Max-␥A diffractometer, operated at 40 kV and
40 mA, using a Cu target (λ = 1.54178 Å) tube and a
graphite monochrometer. Fixed scatter and divergence slits
of 1◦ and a 0.15 mm receiving slit were used. The inten-
sity data were recorded by continuous scan in a 2θ–θ mode
from 10 to 70◦ with a step size of 0.02◦ and a scan speed
of 4◦/min.
The IR spectrometry measurement of FMTC product was
carried out by use of a Nicolet 750 FTIR spectrometer at
room temperature in the 4000–400 cm−1 region. The Raman
spectrum of FMTC product was measured on a LABRAM
microscopic Raman spectrometer with a slit width of
100 nm, using a 100 mW Argon ion laser at 514.53 nm with
a power density of 5 mW/␮m2 at the sample.
FMTC and CMTC (0.04 mmol each) were dissolved in
100 ml of a mixed solvent of ethanol and water (1:1=V:V),
respectively. Using the above-mentioned solvent as ref-
erence, the transmission spectra in the wavelength range
200–1000 nm of the two solutions were obtained on a Hi-
tachi model U-3500 spectrometer using a matched pair of
1 cm quartz cuvettes.
2.30109
2.24815
2.071
1.988
1.928
1.876
1.837
1.777
1.698
1.667
1.625
1.589
2.07131
1.9871
1.92777
1.87704
1.8368
1.77732
1.69725
1.66733
1.62478
1.58968
43.717
45.634
47.148
48.531
49.614
51.407
54.008
55.086
56.641
58.029
43.7
5
53
18
27
19
19
13
4
45.654
47.142
48.497
49.63
51.41
54.027
55.078
56.648
58.016
9
12
1.559
1.538
1.504
1.453
1.434
1.405
1.364
1.55881
1.5375
59.281
60.159
61.657
64.073
65.044
66.544
68.825
59.279
60.184
61.671
64.093
65.041
66.548
68.798
20
12
2
8
4
1.50395
1.45283
1.43394
1.40509
1.36452
5
12
TGA/DTA were carried out in the temperature range of
50–1500 ◦C by using of a Netzsch simultaneous thermal an-
alyzer (model STA409) under air flux, at a heating rate of
10 ◦C/min. In order to examine the course of thermal decom-
position of the compounds under study, the solid samples
were heated and decomposed to various temperatures: 200,
330, 370, 500, 600 and 900 ◦C in the furnace within an open
Pt crucible under air atmosphere. The cooled residues were
removed from the crucible and subjected to XRPD analysis
as stated above.
3.2. IR and Raman spectra
The main IR spectral data of FMTC and KSCN [11] are
listed in Table 2. It can be seen that the wavenumbers of the
CN and CS stretching vibration peaks in FMTC are much
higher than that in free SCN radical of KSCN. Meanwhile,
the wavenumbers of SCN bending vibration peaks of FMTC
show a clear contrary in tread. This can be explained by the
electron transformation model [12,13]. In FMTC, N and S in
SCN− are good electron-donors, and at the same time, Fe2+
and Hg2+ are both strong electron-acceptors. The coordina-
tion of N and S to Fe2+ and Hg2+, respectively make the
wavenumbers of CN and CS stretching vibration increase
significantly and those of SCN bending vibration decrease.
The Raman spectrum of FMTC consists of four fre-
quency regions (below 100 cm−1, lattice vibration modes;
3. Results and discussion
3.1. X-ray powder diffraction
The original XRPD pattern of FMTC was reported pre-
viously (JCPDS No. 20-523). However, it was not com-
pleted. The tetragonal unit-cell parameters calculated by
DICVOL91 [9,10] program, according to the values of 2θ
in XRPD pattern are a = 11.2407 Å, c = 4.2909 Å, V =
542.17 Å3, which are more consistent with the results de-
termined by an R3m/E four-circle X-ray diffractometer [8]
than the original data. The XRPD data for FMTC are tabu-
lated in Table 1.
Table 2
Comparison of the main IR spectral data (cm−1) of FMTC with KSCN
Assignment
FMTC
KSCN
ν(CN)
ν(CS)
δ(NCS)
2δ(NCS)
2082.37, 2116.20, 2130.80
784.69
427.11, 444.49, 467.98
891.47, 934.75
2063
746
488
940

X.Q. Wang et al. / Thermochimica Acta 414 (2004) 53–58
55
Table 3
its growth solutions which are easily oxidated in air. Till
now, no considerably large crystal suitable for determining
its transmission spectrum has been obtained. The transmis-
sion spectrum can be studied by recording the transmission
spectra of its solution.
The transmission spectra of FMTC and CMTC solutions
are shown in Fig. 1, which shows that the transparency cut-
offs of FMTC and CMTC in such solution states occur at
303 and 291 nm, respectively. From which, one can see that
the UV transparency cutoff of FMTC is shifted to red band
compared with that of CMTC, which must be due to the
low-energy d–d transmissions of Fe2+ whose electronic con-
figuration is 3d6.
Observed Raman vibrational spectra data (cm−1) and their assignments
of FMTC product in the 50–2500 cm−1 range
Raman spectra
Assignments
61.36, 73.74
105.48
123.07
233.08
447.35, 467.64, 479.46
782.64
881.29, 913.37, 929.37, 973.38
2078.76, 2114.82, 2142.45
Lattice mode
δ(Hg–SCN–Fe)
δ(S–Hg–S)
δ(N–Fe–N)
δ(SCN)
ν(CS)
2δ(SCN)
ν(CN)
100–300 cm−1, vibration bands of Hg and Fe cen-
ters; 300−1200 cm−1, SCN internal vibration modes;
2100−2200 cm−1, CN stretching vibration modes). All the
Raman lines at the low wavenumber are very broad, and the
intensity and count background of all scattering configura-
tions at the low wavenumber are very strong. According to
the vibrational spectra [14–17], the observed Raman bands
along with their vibrational assignments are summarized
in Table 3. The Raman peaks are not split but broaden on
the low-wavenumber side owing to the distortion of the
Fe(NCS)4 and Hg(SCN)4 tetrahedra in FMTC crystal.
3.4. Thermal properties
The simultaneously recorded TGA/DTA curves of FMTC
are shown in Fig. 2. FMTC starts its decomposition at
219.4 ◦C. The TGA curve shows the process of thermal
dissociation: breakdown of the three-dimensional steric
structure (3DSS) and the formation of the corresponding
metal(II)-sulfides, carbon dioxide (CO2), nitrogen gas (N2),
sulfur dioxide (SO2) and ferric oxide (Fe2O3). Most of the
volatile degradation products were released until 400 ◦C,
but the weight losses were continued further with rising
temperature. FeHgS2 was formed already at relatively low
temperatures, but not exclusively. The behavior of evolved
sulfides was the continuous sublimation and loss of HgS
and the formation of FeHg(1−x)S(2−x). There was a plateau
in the range of 450–600 ◦C.
3.3. Optical transmission study
Since single crystals are mainly used in optical applica-
tions, the optical transmission range and the transparency
cutoff are important. Because FMTC easily decomposes in
its molten state and it is not sublimable, the entire crys-
tallization process is performed in solution. However, the
growth of FMTC is very difficult due to the instability of
DTA curve presents three steps. The first one between 50
and 400 ◦C, which is the breakdown of the 3DSS and the
formation small-molecule compounds; the second and the
Fig. 1. Transmission spectra of FMTC and CMTC in their 0.4 mM solutions.

56
X.Q. Wang et al. / Thermochimica Acta 414 (2004) 53–58
Fig. 2. TGA/DTA curves of FMTC.
Table 4
third one, which extends up to 700 and 1400 ◦C, respectively.
The analysis of the three principal steps shows that the first
implies a loss of 4C(4CO2), 4N(2N2) and 2S(SO2), and the
solid remainder is FeHgS2. Second exhibits a continuous
loss of HgS and the formation of FeHg(1−x)S(2−x), but no
occurrence of FeS was detected till its oxidation. Third is
the oxidation and the formation of the weight constant final
oxide (Fe2O3). DTA presents four exothermic and two en-
dothermic peaks located respectively at 362.5, 392.4, 452.8,
682.0, 908.4 and 1363.6 ◦C accompany with the degrada-
tion. The first two is related to the first TGA step, i.e. the
breakdown of 3DSS and the elimination of the volatile gases.
The second two is related to the second step, i.e. the subli-
mation of HgS and the formation of part Fe2O3. The third
two is related to the third step, i.e. the formation of Fe2O3.
The XRPD patterns of the sinter for each temperature: 200,
330, 370, 500, 600 and 900 ◦C are exhibited in Fig. 3(a)–(f),
respectively. Fig. 3(a) exhibits that the product is still the
tetragonal phase FMTC. Fig. 3(b) and (c) shows that the
product obtained are cubic phases. Judged by other mixed
sulfides in the old literatures (JCPDS Nos. 2-0463, 5-0566,
6-0261, 22-731, 50-1151), one can presume they must be
new phases: FeHgS2 and FeHg(1−x)S(2−x) (0 < x < 1).
The structure of FeHgS2 were derived from powder X-ray
diffraction data, and the cubic unit-cell parameters calcu-
lated by DICVOL91 program according to the values of 2θ in
Relative experimental intensities, d-spacings (observed and calculated),
2θ (observed and calculated) and their indices for FeHgS2
h
k
l
dobs (Å)
dcalc (Å)
2θobs (◦)
2θcalc (◦)
I/I0 (%)
1
2
2
3
2
1
0
2
1
2
1
0
0
1
2
3.371
2.915
2.063
1.762
1.682
3.37181
2.92008
2.06481
1.76087
1.68591
26.425
30.636
43.836
51.828
54.512
26.433
30.615
43.844
51.926
54.42
100
38
50
42
15
the XRPD patterns are a = 5.8402 Å, V = 199.19 Å3. The
XRPD data for FeHgS2 are listed in Table 4. There is an in-
creasing demand on low-cost compound-semiconductor thin
films consisting of single and complex metal sulfides ob-
tained by spray pyrolysis deposition (SPD) [18]. Therefore,
metallic thiocyanates, for example FMTC, can be used as
the sources to obtained metal sulfides by SPD. Fig. 3(d)–(f)
shows that the main residue is Fe2O3 (JCPDS No. 33-664).
In fact, the mechanism concerning of the thermal decompo-
sition of FMTC would be the formation of both mixed iron
mercury sulfides and Fe2O3 as by product. Fe2O3 was not
observed by XRPD must be due to the very small particle
sizes of Fe2O3 at relatively low temperatures. However, with
the increase of the temperature, these tiny particles would
grow into large size and be observed. The thermoanalytical
data of FMTC are exhibited in Table 5.
Table 5
Thermoanalytical data of FMTC
TGA temperature (◦C)
Stage
DTA peak temperature (◦C)
Weight loss (%)
Evolved moiety
Observed
Calculated
50–400
400–700
700–1400
>1400
1
2
3
362.5, 392.4
452.8, 682.0,
908.4, 1363.6
32.5
48.8
2.1
34.4
47.6
1.6
CO2, N2, SO2, FeHgS2
FeHg(1−x)S(2−x) (0 < x < 1), HgS, FeS, Fe2O3
Fe2O3
Fe2O3
Residue

X.Q. Wang et al. / Thermochimica Acta 414 (2004) 53–58
57
Fig. 3. XRPD patterns at room temperature of the sinters of FMTC at 200 ◦C (a), 330 ◦C (b), 370 ◦C (c), 500 ◦C (d), 600 ◦C (e) and 900 ◦C (f): (᭿)
FeHg(SCN)4; (᭹) FeHgS2; (᭢) FeHg1−xS2−x (0 < x < 1); (᭡) Fe2O3.

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X.Q. Wang et al. / Thermochimica Acta 414 (2004) 53–58
According to above-mentioned analyses, we presume that
Acknowledgements
the following reaction scheme is the most reasonable to de-
scribe the decomposition process (where 0 ≤ x ≤ 1):
This work is supported by an “863” grant (No.
2002AA313070) National Advanced Materials Commit-
tee of China (NAMCC), a grant (No. 50272037) of Na-
tional Natural Science Foundation of China (NNSFC), the
Youth Science Foundation of Shandong University (No.
11370053187029) and the Doctoral Startup Foundation of
Shandong University (No. 10000052182149).
FeHg(SCN)4 + 4O2 → FeHg(1−x)S(2−x) + 2CO2
+ 2SO2 + 2N2 + x HgS
FeHg(1−x)S(2−x) + O2 → Fe2O3 + (1 − x)HgS + SO2
4. Conclusions
References
The preparation of FMTC has been described. Spectro-
scopic properties have been established by using various
ways. The structure of FMTC were derived from powder
X-ray diffraction data, and the tetragonal unit-cell param-
eters calculated by DICVOL91 program according to the
values of 2θ in the XRPD patterns are a = 11.2407 Å,
c = 4.2909 Å, V = 542.17 Å3, which are comparable
with the results determined by an R3m/E four-circle X-ray
diffractometer. Its vibrational spectra were studied by IR
and Raman spectroscopy, which show that the character-
istic vibrational modes of FeHg(SCN)4 crystals consist
of six wavenumber regions: below 100 cm−1, lattice vi-
bration modes; 100–300 cm−1, vibration bands of Hg
and Fe centers (Hg(SCN)4 and Fe(NCS)4 bending vibra-
tion modes); 300–500 cm−1, an SCN bending vibration
mode; 750–800 cm−1, a CS stretching vibration mode;
850–950 cm−1, a doubly degenerate SCN bending vibration
mode; 2100–2200 cm−1, a CN stretching vibration mode.
The optical transmission spectrum exhibits that the UV
transparency of FMTC is inferior to that of CMTC. The
TGA/DTA analyses reveal that the thermal decomposition
of FMTC in air is breakdown of the 3DSS and the formation
of the corresponding metal(II)-sulfides (FeHg(1−x)S(2−x)
(0 ≤ x ≤ 1)), carbon dioxide (CO2), nitrogen gas (N2),
sulfur dioxide (SO2) and ferric oxide (Fe2O3). From the
thermal decomposition scheme, one can see that metal-
lic thiocyanates can be used as the sources to obtain the
low-cost compound–semiconductor thin films consisting of
metal sulfides.
[1] A. Rosenheim, R. Cohn, Z. Anorg. Allgem. Chem. 27 (1901)
280.
[2] J.G. Bergman, J.J. Mcfee, G.R. Crane, Mater. Res. Bull. 5 (1970)
913.
[3] W. Sturmer, U. Deserno, Phys. Lett. A 32 (1970) 539.
[4] D.R. Yuan, M.G. Liu, D. Xu, Q. Fang, W.T. Yu, W.B. Hou,
Y.H. Bing, S.Y. Sun, M.H. Jiang, Appl. Phys. Lett. 70 (1997)
544.
[5] D. Xu, W.T. Yu, X.Q. Wang, D.R. Yuan, M.K. Lu, P. Yang, S.Y.
Guo, F.Q. Meng, M.H. Jiang, Acta Cryst. C 55 (1999) 1203.
[6] Y.X. Yan, Q. Fang, D.R. Yuan, Y.P. Tian, Z. Liu, X.M. Wang, M.H.
Jiang, D. Williams, A. Siu, Z.G. Cai, Chin. Chem. Lett. 10 (1999)
257.
[7] X.Q. Wang, D. Xu, M.K. Lu, D.R. Yuan, G.H. Zhang, F.Q. Meng,
S.Y. Guo, M. Zhou, J.R. Liu, X.R. Li, Cryst. Res. Technol. 36 (2001)
73.
[8] Y.X. Yan, Q. Fang, W.T. Yu, Y.P. Tian, D.R. Yuan, X.Q. Wang,
F.Q. Meng, Z.G. Cai, L.Z. Zhang, Chin. Chem. Lett. 9 (1998)
1125.
[9] D. Louer, M. Louer, J. Appl. Cryst. 5 (1972) 271.
[10] A. Boultif, D. Louer, J. Appl. Cryst. 24 (1991) 987.
[11] P.O. Kinell, B. Strandberc, Acta Chem. Scand. 13 (1959) 1607.
[12] C. Balarew, R.J. Duhlew, Solid State Chem. 55 (1984) 1.
[13] K. Nakamoto, translated by D.R. Huang, R.Q. Wang, Infrared and
Raman Spectra of Inorganic and Coordination Compounds, 4th ed.,
Chemical Industry Press, Beijing, 1991, p. 304 (in Chinese).
[14] J. Lewis, R.S. Nyholm, P.W. Smith, J. Chem. Soc. 83 (1960) 1992.
[15] A. Sabatini, I. Bertini, Inorg. Chem. 4 (1965) 959.
[16] A. Muller, K.H. Schmidt, K.H. Tytko, J. Bouwma, F. Jellinek, Spec-
trochim. Acta 28A (1972) 381.
[17] G.J. Kubas, L.H. Jones, Inorg. Chem. 13 (1974) 2816.
[18] J. Madarasz, P. Bornbicz, M. Okuya, S. Kaneko, Solid State Ionics
141–142 (2001) 439.