Intercalation of hydrotalcites with hexacyanoferrate(II) and (III) - A thermoRaman spectroscopic study

Article abstract of DOI:10.1016/j.jssc.2005.03.045

Raman spectroscopy using a hot stage indicates that the intercalation of hexacyanoferrate(II) and (III) in the interlayer space of a Mg, Al hydrotalcites leads to layered solids where the intercalated species is both hexacyanoferrate(II) and (III). Raman spectroscopy shows that depending on the oxidation state of the initial hexacyanoferrate partial oxidation and reduction takes place upon intercalation. For the hexacyanoferrate(III) some partial reduction occurs during synthesis. The symmetry of the hexacyanoferrate decreases from O_h existing for the free anions to D_3d in the hexacyanoferrate interlayered hydrotalcite complexes. Hot stage Raman spectroscopy reveals the oxidation of the hexacyanoferrate(II) to hexacyanoferrate(III) in the hydrotalcite interlayer with the removal of the cyanide anions above 250 °C. Thermal treatment causes the loss of CN ions through the observation of a band at 2080 cm^-1. The hexacyanoferrate (III) interlayered Mg, Al hydrotalcites decomposes above 150 °C.

Full text of DOI:10.1016/j.jssc.2005.03.045

ARTICLE IN PRESS
Journal of Solid State Chemistry 178 (2005) 1940–1948
www.elsevier.com/locate/jssc
Intercalation of hydrotalcites with hexacyanoferrate(II) and
(III)—a thermoRaman spectroscopic study
Ã
Ray L. Frost , Anthony W. Musumeci, Jocelyn Bouzaid, Moses O. Adebajo,
Wayde N. Martens, J. Theo Kloprogge
Inorganic Materials Research Program, School of Physical and Chemical Sciences, Queensland University of Technology,
GPO Box 2434, Brisbane Queensland 4001, Australia
Received 15 February 2005; received in revised form 28 March 2005; accepted 29 March 2005
Available online 26 April 2005
Abstract
Raman spectroscopy using a hot stage indicates that the intercalation of hexacyanoferrate(II) and (III) in the interlayer space of a
Mg, Al hydrotalcites leads to layered solids where the intercalated species is both hexacyanoferrate(II) and (III). Raman
spectroscopy shows that depending on the oxidation state of the initial hexacyanoferrate partial oxidation and reduction takes place
upon intercalation. For the hexacyanoferrate(III) some partial reduction occurs during synthesis. The symmetry of the
h
hexacyanoferrate decreases from O existing for the free anions to D3d in the hexacyanoferrate interlayered hydrotalcite complexes.
Hot stage Raman spectroscopy reveals the oxidation of the hexacyanoferrate(II) to hexacyanoferrate(III) in the hydrotalcite
interlayer with the removal of the cyanide anions above 250 1C. Thermal treatment causes the loss of CN ions through the
ꢀ
1
observation of a band at 2080 cm . The hexacyanoferrate (III) interlayered Mg, Al hydrotalcites decomposes above 150 1C.
r 2005 Elsevier Inc. All rights reserved.
Keywords: Hydrotalcite; Brucite; Raman microscopy; Hexacyanoferrate(III); Potassium hexacyanoferrate(II)
1
. Introduction
any appropriate counter charge balancing anion can be
placed in the interlayer. Calcinations of the hydrotalcites
leads to the formation of mixed oxides. In the particular
case of hydrotalcites containing iron the properties of
the final oxides depends on the location of the iron in the
brucite-like layers or in the complex anion in the
interlayer. Typically the counter anion may be any
anion with a suitable negative charge including the
hexacyanoferrate(II) and hexacyanoferrate(III) ions
[12–20]. The incorporation of these ions has implications
in electrochemistry [17,19,21–23]. The hydrotalcite may
be considered as a gigantic cation which is counter-
balanced by anions in the interlayer. In hydrotalcites a
broad range of compositions are possible of the type
Hydrotalcites, or layered double hydroxides (LDHs)
are fundamentally anionic clays [1]. The structure of
hydrotalcite can be derived from a brucite structure
3
+
3+
(
sjogrenite) substitutes a part of the Mg [2–8]. The
Mg(OH) ) in which, e.g., Al
¨
or Fe
(pyroaurite-
2
2
+
nature of the cations in the hydroxide layers and of the
interlayer anions can be varied substantially. This
substitution of the trivalent cation for the divalent
cation creates a positive layer charge on the hydroxide
layers, which is compensated by interlayer anions or
anionic complexes [9,10]. Some ternary systems may
also be synthesized and characterised such as the system
based upon Co and Cu as the divalent cation with Fe as
the trivalent cation [11]. When LDHs are synthesized
2
+
3+
nꢀ
2+
[M
M
are the di- and trivalent cations in the octahedral
(OH) ][A ]x/n ꢁ yH O, where M
and
1
ꢀ x
x
2
2
3
+
M
positions within the hydroxide layers with x normally
nꢀ
Ã
between 0.17 and 0.33. A is an exchangeable interlayer
anion [24]. Normally, the carbonate anion is the major
Corresponding author. Fax: +61 7 3864 1804.
E-mail address: r.frost@qut.edu.au (R.L. Frost).
0022-4596/$ - see front matter r 2005 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2005.03.045

ARTICLE IN PRESS
R.L. Frost et al. / Journal of Solid State Chemistry 178 (2005) 1940–1948
1941
interlayer counter anion. Of course when synthesizing
2.2. X-ray diffraction
hydrotalcites any anion may be used [4,25–28]. Reevesite
and pyroaurite are based upon the incorporation of
carbonate into the interlayer with d(003) spacings of
XRD patterns were collected using a Philips X’pert
wide angle X-ray diffractometer, operating in step scan
˚
˚
around 8 A [29,30]. Normally the hydrotalcite structure
based upon takovite (Ni,Al) and hydrotalcite (Mg,Al)
mode, with CuKa radiation (1.54052 A). Patterns were
collected in the range 3–901 2y with a step size of 0.021
and a rate of 30 s per step. Samples were prepared as a
finely pressed powder into aluminium sample holders.
The Profile Fitting option of the software uses a model
that employs twelve intrinsic parameters to describe the
profile, the instrumental aberration and wavelength
dependent contributions to the profile.
˚
has basal spacings of ꢂ8.0 A where the interlayer anion
is carbonate. The concentration of particular anion may
also effect the d(003) spacing of the hydrotalcite [31].
The potential application of hydrotalcites as catalysts
rests with the ability to make mixed metal oxides at the
atomic level, rather than at a particle level. Such mixed
metal oxides are formed through the thermal decom-
position of the hydrotalcite [32,33]. There are many
other important uses of hydrotalcites such as in the
removal of environmental hazards in acid mine drainage
2.3. Infrared spectroscopy
Infrared spectra were obtained using a Nicolet Nexus
870 FTIR spectrometer with a smart endurance single
bounce diamond ATR cell. Spectra over the 4000ꢀ
[
34,35], and a mechanism for the disposal of radioactive
wastes [36]. Their ability to exchange anions presents a
system for heavy metal removal from contaminated
waters [37]. Structural information on different minerals
has successfully been obtained recently by sophisticated
thermal analysis techniques [38–43]. In order to gain
insight into the redox processes taking place in the
interlayer, we report the hot stage Raman spectroscopy
of hydrotalcite with hexacyanoferrate(II) and hexacya-
noferrate(III) in the interlayer as a function of
temperature.
ꢀ
1
525 cm range were obtained by the co-addition of 64
ꢀ
1
scans with a resolution of 4 cm and a mirror velocity
of 0.6329 cm/s. Spectra were co-added to improve the
signal-to-noise ratio.
2.4. Raman spectroscopy
The crystals of hydrotalcite minerals were placed and
orientated on a polished metal surface on the stage of an
Olympus BHSM microscope, which is equipped with
1
0 Â and 50 Â objectives. The microscope is part of a
2
. Experimental
Renishaw1000 Raman microscope system, wh ich also
includes a monochromator, a filter system and a charge
coupled device (CCD). A Spectra-Physics model 127
2
.1. Synthesis of hydrotalcite compounds
ꢀ
1
He-Ne laser (633 nm) at a resolution of 2 cm in the
ꢀ
1
A mixed solution of aluminium and magnesium
3
range between 100 and 4000 cm was used for Raman
excitation. Repeated acquisition, using the highest
magnification, was accumulated to improve the signal-
to-noise ratio in the spectra. Spectra were calibrated
using the 520.5 cm line of a silicon wafer. Powers of
less than 1 mW at the sample were used to avoid laser
induced degradation of the sample. Slight defocusing of
the laser beam also assists in the preservation of
the sample. Spectra at elevated temperatures were
obtained in an air atmosphere using a Linkam thermal
stage (Scientific Instruments Ltd., Waterfield, Surrey,
England).
+
2+
nitrates ([Al ] ¼ 0.25 M and [Mg ] ¼ 0.75 M; 1 M ¼
3
mol/dm ) and a mixed solution of sodium hydroxide
1
ꢀ
(
[OH ] ¼ 2 M) and the desired anion, at the appropriate
ꢀ
1
concentration, were placed in two separate vessels and
purged with nitrogen for 20 min (all compounds were
dissolved in freshly decarbonated water). The cationic
solution was added to the anions via a peristaltic pump
at 40 mL/min and the pH maintained above 9 with the
addition of 0.1 M NaOH. The mixture was then aged at
7
5 1C for 18 h under a N atmosphere. The resulting
2
precipitate was then filtered thoroughly with room
temperature decarbonated water to remove nitrates
and left to dry in a vacuum desiccator for several days.
The presence of nitrates (and carbonates) is readily
checked quite quickly by obtaining the Raman spectrum
of the synthesized hydrotalcite. The nitrate and carbo-
nate anions are characterised by intense sharp bands at
Spectroscopic manipulation such as baseline adjust-
ment, smoothing and normalisation were performed
using the Spectracalc software package GRAMS (Ga-
lactic Industries Corporation, NH, USA). Band com-
ponent analysis was undertaken using the Jandel
‘Peakfit’ software package, which enabled the type of
fitting function to be selected and allows specific
parameters to be fixed or varied accordingly. Band
fitting was done using a Gauss–Lorentz cross-product
function with the minimum number of component
bands used for the fitting process. The Gauss–Lorentz
ꢀ
1
1
047 and 1070 cm , respectively. In this way phase-
pure hydrotalcites with different anions in the interlayer
were synthesized. The phase composition was checked
by X-ray diffraction (XRD) and the chemical composi-
tion by EDXA analyses.

ARTICLE IN PRESS
R.L. Frost et al. / Journal of Solid State Chemistry 178 (2005) 1940–1948
1942
ratio was maintained at values greater than 0.7 and
fitting was undertaken until reproducible results were
3.2. Raman spectroscopy
2
obtained with squared correlations of r greater than
0
The structure of the hexacyanoferrate anion is a
regular octahedron with six cyanide ligands at each site
with a symmetry of O . The selection rules showthat the
.995.
h
vibrational modes A , E and T will be Raman active
1
g
g
2g
only. The T1u mode will be infrared active only. The C3-
axis of this octahedron is oriented perpendicular to the
hydrotalcite brucite-like hydroxyl sheets [18,44]. Brater-
man et al. reasoned that the CN vibrations would
correspond to the vibrational modes A1g, Eg and T1u.
The fact that the first two modes are observed in the
infrared spectrum as lowintensity bands provides an
indication of symmetry reduction. The T1u mode is split
into two modes assigned to Eu and A2u. This is
indicative of a reduction in symmetry from O to D .
3
. Results and discussion
3
.1. X-ray diffraction
The XRD patterns for the hexacyanoferrate(II) and
hexacyanoferrate(III) interlayered hydrotalcites are
shown in Fig. 1. For comparison the XRD patterns of
the sulphate and carbonate interlayered hydrotalcite are
shown. The XRD patterns clearly show the formation of
the hydrotalcites with the different anions in the
interlayer. The XRD patterns also showno impurities
in the synthesized hydrotalcites. Hydrotalcite normally
has a d(003) spacing of 7.9 A. The sulphate interlayered
˚
hydrotalcite has a spacing of 8.0 A. The hexacyanofer-
˚
rate(II) complex has a spacing of 10.9 A and the
˚
hexacyanoferrate(III) hydrotalcite 11.1 A. The increased
h
3d
The Raman spectra of the potassium hexacyanofer-
rate (II) and (III) ions are shown in Fig. 2a. The
hexacyano anion is characterised by bands at 2092 and
˚
ꢀ1
2
064 cm for the potassium hexacynaoferrate(II) salt
ꢀ1
and at 2133 and 2126 cm for the potassium hexacy-
naoferrate(III). Bands in these positions are assigned to
the CN stretching vibrations. In addition two sets of low
intensity bands are observed in the Raman spectra of the
potassium hexacyanoferrate(III) at 2079, 2077 and at
interlayer spacing is due to the size of the anion between
the brucite-like layers.
ꢀ
1
2
095 and 2091 cm (Fig. 2b). These bands correspond
CO32-
to the positions for the potassium hexacyanoferrate (II).
The presence of bands attributable to the CN stretching
vibrational modes of the hexacyanoferrate(II) in the
spectra of hexacyanoferrate(III) are also confirmed by
the infrared spectra. The infrared spectra of the
potassium hexacyanoferrate (II) and (III) are charac-
terised by bands at 2032 (Fe(II)) and 2113 and 2041
d003 = 7.90Å
ꢀ
1
SO42-
(Fe(III)) cm (Fig. 3). A previous study also found an
additional band for the hexacyanoferrate(III) at
d003 = 7.99Å
ꢀ
1
076 cm [25]. The band position of the free CN ion
ꢀ
2
is 2084 cm . The band at 2076 cm seems to corre-
ꢀ
1
ꢀ1
ꢀ
spond to the free CN ion. The two bands at 2091 and
ꢀ
1
076 cm correspond with the band positions for the
2
potassium hexacyanoferrate (II) salt. This simply shows
that there is some Fe(II) impurity in the Fe(III)
compound.
d003 = 10.9Å
A change in oxidation state of the iron in the
hexacyanoferrate complexes is reflected in the shift in
[
Fe(CN)6]4-
ꢀ
1
the CN stretching wavenumber from 2050–2090 cm
ꢀ
1
for Fe(II) to 2115–2125 cm for Fe(III). This shift can
be attributed to a shift in electron density through the
pi-bonds of the cyano groups in going from hexacya-
noferrate (III) to hexacyanoferrate (II). The Raman
spectra of the hydrotalcite synthesized with hexacyano-
ferrate(II) and (III) in the interlayer are shown in Fig. 4.
d003 = 11.1Å
[
Fe(CN)6]3-
ꢀ
1
Three bands are observed at 2101, 2094 and 2059 cm
for the hexacyanoferrate(II) complex with an additional
5
10
15
20
25
30
ꢀ
1
band at 2048 cm , and four bands at 2127, 2118, 2099
Degrees / 2 Theta
ꢀ
1
and 2059 cm for the hexacyanoferrate(III) inter-
layered hydrotalcite. Some earlier studies reported only
Fig. 1. Powder XRD patterns of the samples used in this study.

ARTICLE IN PRESS
R.L. Frost et al. / Journal of Solid State Chemistry 178 (2005) 1940–1948
1943
092cm-1
2032cm-1
4-
[Fe(CN)6]
2
K [Fe(CN) ]
4
6
2
064cm-1
2
057cm-1
133cm-1
K [Fe(CN) ]
3
6
2
2
113cm-1
3-
[Fe(CN)6]
2
126cm-1
2
041cm-1
2
150
2130
2110 2090
2070
2050
2030
Wavenumber /cm-1
(a)
2
140 2120 2100 2080 2060 2040 2020 2000 1980 1960
Wavenumber /cm-1
K [Fe(CN) ]
3
6
2
079cm-1
2
076cm-1
Fig. 3. Infrared spectra of the potassium salts of hexacyanoferrate(II)
and (III).
[16]. Further Idemura et al. observed a reduction of the
¨
Fe in hexacyanoferrate(III) complexes by Mossbauer
spectroscopy [45]. Crespo pointed out that the shift in
position of the CN stretching wavenumber for the
hexacyanoferrate(II) interlayered hydrotalcite was also
influenced by the hydrogen bonding between the cyano
groups and the interlayer water and also the hydroxyl
surfaces of the brucite-like surface [12]. However, this
concept is not supported by both the infrared and
Raman data of the water and hydroxyl OH bending and
stretching modes. In the infrared spectra the bending
2
095cm-1
2
091cm-1
ꢀ
1
mode is at 1612 cm
and the OH stretching bands
ꢀ
1
between 3000 and 3800 cm . Idemura et al. synthesized
hydrotalcites with varying amounts of the hexacyano-
ferrate(II) and (III) in the hydrotalcite through ion
exchange from the nitrate interlayered hydrotalcite with
the potassium and sodium hexacyano salt solutions with
the exclusion of carbonate. Their infrared spectra
2
100
2090
2080
2070
2060
2050
Wavenumber /cm-1
(b)
Fig. 2. (a) Raman spectra of the potassium salts of hexacyanoferra-
te(II) and (III). (b) Raman spectra of hexacyanoferrate (III) expanded
ꢀ1
in the 2050 to 2100 cm region.
ꢀ1
showed two bands at 2120 and 2040 cm ; thus
indicating some partial reduction of the Fe(III) to
(Fe(II) during intercalation. Their infrared work also
showed that only a single band at 2040 cm
observed when the sample was dried in vacuum and the
second band appeared when the sample was dried in air.
Similar observations have been reported by other groups
[14–16,19].
ꢀ
1
two bands for each of the hexacyanoferrate(II) and (III)
interlayered hydrotalcites [12].
Holgadao et al. provided evidence based on Fe–K
XANES measurements for the partial oxidation-reduc-
tion of the iron in the hexacyanoferrate hydrotalcite
was

ARTICLE IN PRESS
R.L. Frost et al. / Journal of Solid State Chemistry 178 (2005) 1940–1948
1944
4
-
3681 cm-1
[
Fe(CN)6]
200°C
3626 cm-1
094 cm-1
2
2
059 cm-1
101 cm-1
3650 cm-1
2
697 cm-1
3
150°C
2
048 cm-1
3
579 cm-1
2
127 cm-1
Fe(CN)6]3-
[
700 cm-1
3647 cm-1
3
1
00°C
3
552 cm-1
2
118 cm-1
2
099 cm-1
655 cm-1
3548 cm-1
3
25°C
2
059 cm-1
3400 cm-1
004 cm-1
2
2
120
2070
2020
1970
1920
3
800
3700
3600
3500
3400
3300
3200
3100
Wavenumber / cm-1
Wavenumbers /cm-1
Fig. 4. Raman spectra of the hexacyanoferrate(II) and (III) inter-
layered Mg, Al hydrotalcite.
Fig. 5. Hot stage Raman spectra of the OH stretching region of the
hexacyanoferrate(II) interlayered Mg, Al hydrotalcite.
3
3
.3. Hot stage Raman spectroscopy
hexacyanoferrate(II) interlayered hydrotalcite intro-
duces O into the system and causes the oxidation of
.3.1. Thermal treatment of the hexacyanoferrate(II)
2
interlayered hydrotalcite
The hot stage Raman spectra of the hexacyanoferra-
te(II) in the OH stretching region and in the
the Fe(II) to (Fe(III). This oxidation is observed
through the development of the additional peaks at
around 2124 and 2070 cm which increase in intensity
with thermal treatment.
Thermal analysis shows that HCN is lost at 270 1C.
The spectrum at 300 1C is too noisy to be curve resolved
and at this temperature the structure of the hydrotalcite
is lost. At 300 1C the hexacyanoferrate(II) hydrotalcite
has decomposed according to the following reaction:
ꢀ
1
ꢀ
1
000–2150 cm region are shown in Fig. 5 and Fig. 6,
2
respectively. The hot stage Raman spectra of the
ꢀ
1
hexacyanoferrate(III) in the 2000–2150 cm region is
shown in Fig. 7. The results of the Raman spectral
analysis are reported in Tables 1 and 2. The Raman
spectra of the Fe(II) hydrotalcite shows two bands at
ꢀ
1
2
094 and 2059 cm with additional bands at 2048 and
ꢀ
1
2101 cm . At 100 1C two bands are observed at 2098
and 2059 cm with two additional bands resolved at
2124 and 2071 cm . In the 150 1C spectrum the
intensity of the latter two bands increases. This intensity
Mg6Al2ðOHÞ16ðFeðCNÞ6Þ0:5 þ 4O2
ꢀ
1
!
ðMgOÞ5 MgAl2O4ðFe3O4Þ0:5 þ 3HCN þ 8H2O
ꢀ
1
XRD shows that the products of the thermal
decomposition of the hexacyanoferrate(II) interlayered
hydrotalcite are MgO, Mg AlO (spinel) and Fe O .
increases is observed in the 200 1C spectrum. In the
2
50 1C spectrum the two predominant bands are
2
4
3
4
ꢀ
1
observed at 2119 and 2074 cm . In the 300 1C spectrum
a broad band is revealed at around 2084 cm . This
band corresponds to free CN ion and represents the
HCN molecules which are being lost above 250 1C. No
intensity remains in the bands at around 2048 and
The Raman spectra of the hydroxyl stretching region
of the hexacyanoferrate(II) interlayered hydrotalcite are
shown in Fig. 5. At 25 1C three bands are observed at
ꢀ
1
ꢀ
ꢀ
1
3655, 3548 and 3400 cm . The first band is assigned to
the hydroxyl stretching vibration of the brucite-like
hydroxyl surface and the latter two bands are assigned
ꢀ
1
094 cm . It is proposed that the heating of the
2

ARTICLE IN PRESS
R.L. Frost et al. / Journal of Solid State Chemistry 178 (2005) 1940–1948
1945
2
084 cm-1
-1
3
00°C
2110 cm
-1
300°C
2
069 cm
2074 cm-1
2
119 cm-1
2
50°C
-1
099 cm-1
-1
2112 cm
200°C
2
2187 cm
2
100 cm-1
075 cm-1
2059 cm-1
2
200°C
117 cm-1
-1
-1
2
2118 cm
2102 cm
2
050 cm-1
150°C
-1
2
061 cm
-1
2072 cm-1
2012 cm
2
101 cm-1
-1
1
50°C
2127 cm
2
058 cm-1
2
120 cm-1
-1
2
121 cm
1
00°C
-1
2
102 cm
2
098 cm-1
059 cm-1
-1
2
2078 cm
1
00°C
-1
2
005 cm
071 cm-1
-1
2
2128 cm
2
124 cm-1
2
5°C
-1
-1
2
127 cm
2099 cm
2
094 cm-1
101 cm-1
2059 cm-1
048 cm-1
25°C
-
1
2
072 cm
2
-1
2
2004 cm
2
200
2100
2000
1900
2150
2100
2050
2000
1950
Wavenumbers /cm-1
Wavenumbers /cm-1
ꢀ1
ꢀ
1
Fig. 7. Hot stage Raman spectra of the 1950 to 2150 cm region of
the hexacyanoferrate(III) interlayered Mg, Al hydrotalcite.
Fig. 6. Hot stage Raman spectra of the 1950 to 2150 cm region of
the hexacyanoferrate(II) interlayered Mg, Al hydrotalcite.
to water stretching bands. At 100 1C much of the
intensity of the 3400 cm band has diminished. Three
study considered the equilibrium between water or
hydroxyls with the cyanide ion with water replacing
ꢀ
1
ꢀ
1
ꢀ
ꢀ
bands are observed at 3700, 3647 and 3552 cm in the
00 1C spectrum. Further heating results in the decrease
the CN ion [25]. In this model the CN ions would not
be expelled from the interlayer but rather retained in an
equilibrium process. At the elevated temperatures with
1
ꢀ
1
in intensity of the band at 3552 cm . Thermal analysis
shows that dehydration of the hexacyanoferrate(II)
interlayered hydrotalcite is complete by 150 1C. Thus
the two bands in the 200 1C spectrum are attributed to
the brucite-like hydroxyl stretching vibrations. Two
bands are observed at 3681 and 3626 cm . One possible
assignment of these bands is to AlOH and MgOH
stretching vibrations.
ꢀ
oxygen present it is more likely that the CN ions are
expelled and the octahedral coordination of the Fe(III)
maintained by bonding to oxygen. Hansen and Koch
ꢀ
1
reported an infrared band at 2039 cm and a sharp
ꢀ
1
ꢀ1
band at 2027 cm [15]. They attributed these bands to
ꢀ
ꢀ
free CN ions. However the band position for free CN
ꢀ
1
ions is around 2080 cm . This may simply mean that
the CN ions have replaced the OH units on the brucite-
ꢀ
3.3.2. Thermal treatment of the hexacyanoferrate(III)
interlayered hydrotalcite
like hydroxyl sheets. Such a reaction would release OH
ions into the interlayer. The results of this and a
previous study [25] do not support the concept of free
The Raman spectra of the hexacyanoferrate(III)
interlayered hydrotalcite (shown in Fig. 7) reveals four
ꢀ
CN ions in the interlayer as no bands are observed at
ꢀ1
ꢀ
1
bands at 2127, 2119, 2099 and 2072 cm
at 25 1C.
2027 or 2039 cm . The observation of a broadish band
ꢀ
1
ꢀ
Significant changes are observed in the 100 1C spectrum.
Four bands are observed at 2128, 2121, 2102 and
at ꢂ2080 cm
may mean that the CN ions have
moved out of the interlayer and are adsorbed on the
surface of the metal oxides which are being formed
during the decomposition.
ꢀ
1
072 cm . In addition a broad feature is observed at
2
2
ꢀ
1
080 cm . This latter band is present in all the spectra
and increases in intensity as the temperature is raised.
ꢀ
In the 150 1C spectrum four bands are observed at
2127, 2118, 2102 and 2061 cm . The first two bands are
ꢀ1
The band is attributed to free CN ions. A previous

Table 1
Results of the Raman spectral analysis of the hexacyanoferrate(II) as a function of temperature
Room Temp.
100 1C
150 1C
200 1C
250 1C
300 1C
Wave number FWHM %** Wave number FWHM
%
Wave number FWHM
%
Wave number FWHM
%
Wave number FWHM
%
Wave number FWHM
%
3
699.7
3.5 3646.7
45.0
98.1
2.7 3696.7
4.9 3650.0
42.6
78.4
2.4 3681.2
6.0
58.4
4.6
4.1
3654.9
101.2
3
625.7
102.5
3
579.2
123.6
4.5
3
3
547.6
400.5
183.5
306.6
3.6 3552.2
184.7
4.9
16.3
2
124.4
15.9
21.8
3.1 2119.5
28.6 2101.2
22.7
20.3
6.7 2117.1
16.4 2100.0
26.3
16.2
11.4 2119.2
14.2 2098.7
33.9
7.3
11.5
1.1
2
2
101.3
093.7
18.4
12.0
10.2 2098.3
12.4
2084.3
66.4
27.3
2
070.6
15.5
18.7
60.4
6.1 2071.7
22.8 2058.4
15.7 2045.5
25.8
8.3
11.3 2075.1
1.2 2059.4
14.7 2049.6
6.9 1555.8
12.5 1498.7
22.5
11.6
59.7
95.5
53.3
8.5 2074.3
4.0
45.3
92.2
18.9
2
2
059.3
047.8
18.9
67.3
16.0 2059.4
26.8 2032.2
59.0
76.8
51.4
14.7
15.7 1541.1
8.3
1
1
548.1
493.6
48.0 1544.9
84.7
63.3
52.9
19.8
1276.4
10.9 1122.7
36.1
47.7
2.3
15.5 1121.1
1
1
113.4
016.3
44.8
39.7
11.6 1117.6
1.0
49.5
6
5
5
4
98.7
86.8
07.5
76.7
34.6
31.4
21.2
32.9
0.5
0.9
1.0
1.4
507.0
472.5
17.7
34.5
46.2
82.6
0.5 510.3
1.5 471.8
0.5
17.2
13.4
0.3
0.2
361.7
277.1
288.6
42.3
1.2
2.5
2
57.4
56.8
41.3
1.4 241.8
1.7 224.0
192.7
25.1
12.6
18.5
0.8
0.7 224.1
0.9 202.8
2
1
33.0
96.3
57.0
41.3
1.9
1.9
218.8
195.2
45.1
26.8
2.3 223.7
1.1
12.1
9.7
1.3
1.3
** % Relative intensity.

ARTICLE IN PRESS
R.L. Frost et al. / Journal of Solid State Chemistry 178 (2005) 1940–1948
1947
Table 2
Results of the Raman spectral analysis of the hexacyanoferrate(III) as a function of temperature
Room Temp
100 1C
150 1C
200 1C
300 1C
Wave
number
FWHM %**
Wave
number
FWHM
%
Wave
number
FWHM
%
Wave
number
FWHM
%
Wave
number
FWHM
%
3
3
3
3
715.7
688.0
633.1
531.5
23.0
46.9
0.8
1.4
3.3
4.2
3696.5
3622.1
42.6
107.4
1.8
9.7
3692.3
3633.7
31.0
90.1
0.9
3.1
100.9
200.2
2
186.5
51.6
71.5
5.5
2
2
2
2
2
2
127.2
118.8
099.4
072.1
059.5
004.3
9.5
12.9
17.9
17.3
13.2
83.5
15.4
12.8
10.6
1.7
2128.3
2121.2
2102.3
2072.5
2058.8
2004.7
6.6
12.7
19.8
24.7
9.1
6.4
11.3
11.4
3.4
2126.5
2118.0
2102.0
2077.9
2060.7
2012.6
9.5
13.9
16.5
24.6
9.6
5.1
10.1
6.3
2112.3
10.0
2109.8
2069.3
42.9
29.0
19.2
12.7
4.1
2.3
4.0
15.9
1.4
20.3
82.9
83.8
75.1
16.2
18.4
1
544.9
1545.1
1116.5
84.5
32.3
1
1
516.1
113.5
103.2
49.5
7.9
3.0
1527.4
1124.5
97.6
55.8
54.4
30.0
1496.8
1121.8
57.5
49.1
13.4
17.0
67.4
67.7
16.3
19.5
1
058.7
810.6
104.6
3.3
7
80.5
127.8
7.4
7
6
5
5
4
3
3
2
2
32.6
86.0
87.9
05.1
68.7
69.9
01.2
47.3
05.4
80.0
50.0
22.9
37.6
26.1
58.6
53.9
62.6
31.1
5.3
2.5
1.0
1.6
1.2
2.9
3.5
4.0
0.9
700.3
588.5
505.6
466.8
369.0
309.6
252.9
206.8
57.9
17.9
41.5
22.3
41.6
43.5
71.2
10.7
3.8
0.5
1.5
0.8
1.4
1.6
4.0
0.3
258.9
204.7
41.6
13.9
1.7
1.3
** % Relative intensity.
assigned to the CN stretching vibrations of the
hexacyanoferrate(III) interlayered hydrotalcite and the
second two bands are assigned to the CN stretching
vibrations of the hexacyanoferrate(II) interlayered
hydrotalcite. The potassium hexacyanoferrate(III) salt
used in the experiment showed only very low concentra-
tions of the hexacyanoferrate(II) ion. Thus it is
concluded that during the synthesis of the hexacyano-
ferrate(III) interlayered hydrotalcite some reduction of
Fe(III) to Fe(II) occurred. At temperatures above
tion has occurred by 150 1C. The loss of the CN ions
occurs simultaneously as the dehydroxylation of the
hexacyanoferrate(III) interlayered hydrotalcite occurs.
The cyanide ion is lost as cyanic acid. This shows that
the cyanide ion reacts with the hydroxyl surface of the
hexacyanoferrate(III) interlayered hydrotalcite to pro-
duce HCN with the oxygen bonded to the cations of the
hydrotalcite Such a concept has been previously
suggested [15].
1
50 1C, the spectral features of the hexacyanoferrate(III)
interlayered hydrotalcite are lost. At 200 1C two broad
4. Conclusions
ꢀ
1
bands are observed at 2187 and 2112 cm . The spectra
suffer from a lack of signal-to-noise, as the thermal
background in the spectra increases greatly at the higher
temperatures.
Thermal decomposition as determined by thermo-
gravimetric methods shows that the hexacyanoferra-
te(III) interlayered hydrotalcite is dehydrated by 150 1C.
This accounts for the very significant changes in the
spectra in going from 150 to 200 1C. TGA coupled to a
gas evolution mass spectra also shows that the cyanide
ion is lost above 200 1C. Dehydration as determined by
gas evolution mass spectrometry shows that dehydra-
Raman spectroscopy confirms the intercalation of
hexacyanoferrate(II) and (III) in the interlayer of Mg,
Al hydrotalcite with the partial reduction of the
hexacyanoferrate(III) interlayered hydrotalcite and the
partial oxidation of the hexacyanoferrate(II) inter-
layered hydrotalcite. This results in a mixed valence
combination in the interlayer irrespective of the initial
starting hexacyanoferrate. Hot stage Raman spectro-
scopy shows that the hexacyanoferrate(II) interlayered
hydrotalcite undergoes oxidation to the hexacyanofer-
rate(III) interlayered hydrotalcite before decomposition

ARTICLE IN PRESS
R.L. Frost et al. / Journal of Solid State Chemistry 178 (2005) 1940–1948
1948
at 250 1C. The hexacyanoferrate(III) interlayered hydro-
talcite remains stable up to 150 1C after which decom-
position with the loss of cyanide ions occurs. Free
cyanide is lost during the thermal treatment of the
hexacyanoferrate(III) hydrotalcite.
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