Phase transition and thermal decomposition of [Cd(H2O)6](BF4)2

Article abstract of DOI:10.1023/A:1016126102891

Phase transition and thermal decomposition of [Cd(H₂O)₆](BF₄)₂ were studied by differential scanning calorimetry (DSC), differential thermal analysis (DTA) and thermogravimetry (TG) methods. The solid-solid phas

Full text of DOI:10.1023/A:1016126102891

Journal of Thermal Analysis and Calorimetry, Vol. 68 (2002) 861–864
PHASE TRANSITION AND THERMAL
DECOMPOSITION OF [Cd(H2O)6](BF4)2
*
E. Mikuli1, A. Migda³-Mikuli1 and R. Gajerski2
1Jagiellonian University, Faculty of Chemistry, ul. Ingardena 3, 30-060 Cracow, Poland
2University of Mining and Metallurgy, Faculty of Materials Science and Ceramics,
Al. Mickiewicza 30, 30-059 Cracow, Poland
Abstract
Phase transition and thermal decomposition of [Cd(H2O)6](BF4)2 were studied by differential scan-
ning calorimetry (DSC), differential thermal analysis (DTA) and thermogravimetry (TG) methods.
The solid-solid phase transition at TC1=324 K and the melting point at Tmelt.=391 K were registered.
The thermal dehydration process starts just above TC1 and continues up to Tmelt., where
[Cd(H2O)4](BF4)2 in the liquid phase is formed. Then, dehydration and decomposition take place si-
multaneously until CdF2 is obtained. Final products of the thermal decomposition were identified us-
ing quadrupole mass spectrometry (QMS) and X-ray diffraction methods.
Keywords: DSC, DTA, hexaaquacadmium tetrafluoroborate, phase transition, QMS, TG, thermal
decomposition
Introduction
Crystalline compounds of the type [M(H2O)6](BF4)2, where M=Mn, Fe, Co, Ni and
Zn, exhibit several phase transition both below and above room temperature [1]. In
[Cd(H2O)6](BF4)2 two phase transition proceeding in the solid state were discovered:
at TC1=329 K and at TC2=176 K [2,3]. According to West [4, 5], the crystal structure of
1
[Cd(H2O)6](BF4)2 is trigonal at room temperature (space group no. 156=P3m1=C3v )
with one molecule per unit cell, and it is isomorphic with [Cd(H2O)6](ClO4)2. The lat-
tice constants, determined by Moss et al. [6] for hexaaquacadmium tetrafluoroborate
are quite close to those for hexaaquacadmium chlorate(VII). However, Johansson
3
and Sandström [7] proposed the space group no. 164=P3m1=D3d for
[Cd(H2O)6](ClO4)2. Unfortunately, the structures of the high- and low-temperature
phases of both [Cd(H2O)6](ClO4)2 and [Cd(H2O)6](BF4)2 are still unknown. Since, ac-
cording to Georgiev and Maneva [8], the dehydration process of the
[Cd(H2O)6](BF4)2 starts already at ca 320 K, it is interesting to find its connection
with TC1 phase transition and also with the melting point of the crystals. In addition,
*
It was presented at CCTA 8 Zakopane, Poland
1418–2874/2002/ $ 5.00
Akadémiai Kiadó, Budapest
© 2002 Akadémiai Kiadó, Budapest
Kluwer Academic Publishers, Dordrecht

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MIKULI et al.: HEXAAQUACADMIUM TETRAFLUOROBORATE
results of the thermal decomposition of the investigated compound, will also be pre-
sented for different experimental conditions.
Experimental
[Cd(H2O)6](BF4)2 was synthesized by treating cadmium carbonate with dilute tetra-
fluoroborate acid. The solution was concentrated by mild heating, and the colorless
crystals obtained after cooling of the solution were purified several times by repeated
crystallization from four-times-distilled water. Then, the crystals were dried for sev-
eral days in a desiccator over BaO and later stored in a hygrostat. The composition of
the compound was established through chemical analysis. The infrared and Raman
spectra certified the purity of the sample.
Differential scanning calorimetric (DSC) measurements were performed with a
TA Instruments DSC 2010 apparatus.
Thermogravimetry (TG) and differential thermal analytic (DTA) measurements
were performed using a TA Instruments SDT 2960 apparatus. They were made in the
flow of air, helium and argon at a constant heating rate of 2 and 3 K min–1, up to
500 K, under atmospheric pressure, with the on-line analysis of the evolved gases by
a Balzers ThermoStar GDS 300T3 quadrupole mass spectrometer (QMS).
The X-ray analysis of the solid decomposition product was made with a Sefert
XRD-7 apparatus by applying filtrated CuKα radiation.
Results and discussion
Figure 1 presents heat flow vs. temperature (DSC curve) for [Cd(H2O)6](BF4)2 as reg-
istered on heating the sample at a rate of 10 K min–1 in the temperature range of
300–400 K. There are two endothermic peaks on the DSC curve: at TC1=324 K and at
Fig. 1 DSC curve obtained for [Cd(H2O)6](BF4)2 at a constant heating rate of 10 K min–1
J. Therm. Anal. Cal., 68, 2002

MIKULI et al.: HEXAAQUACADMIUM TETRAFLUOROBORATE
863
Fig. 2 DTA, TG and QMS curves obtained for [Cd(H2O)6](BF4)2 at a constant heating
rate of 2 K min–1 in the flow of air
T
melt.=391 K. The first one is connected with the phase transition proceeding in the
frame of the solid state (∆H=0.6 kJ mol–1) and the second one is sample melting
(∆H=18.9 kJ mol–1).
A thermal analysis of seven [Cd(H2O)6](BF4)2 samples of different masses was
made under different experimental conditions. The results were similar, regardless the
varying sample masses, the kind of the flowing medium and as well as the applied heat-
ing rate. The only difference was that particular steps of the sample decomposition took
place at slightly lower temperatures whereas the heating rate was slower. Figure 2 pres-
ents exemplarily DTA, TG and QMS curves for a [Cd(H2O)6](BF4)2 sample of 9.97 mg
measured in the flow of air at constant heating rate of 2 K min–1. A small (but very well
reproducible) endothermic peak of the DTA curve at ca 325 K, which was not connected
with any mass loss (TG curve) was detected. It was connected with the TC1 phase transi-
tion which took place in the solid state, as we discovered earlier [3] and also registered by
the above mentioned DSC measurements. One can see in Fig. 2 that the thermal decom-
position of [Cd(H2O)6](BF4)2 started just at the TC1 phase transition, and it took effect in
two main stages. The first one was connected with a very broad endothermic peak on the
DTA curve, which was extended from ca 330 to 380 K. DSC measurements as well as
the microscopic observation of the sample proved that this peak was connected with sam-
ple melting. Moreover, the QMS results proved the connection with sample dehydration.
The average mass loss in the first stage of the decomposition, as calculated from the TG
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MIKULI et al.: HEXAAQUACADMIUM TETRAFLUOROBORATE
curve, equaled 9.6±2.4% of the initial sample mass. The calculated value for freeing two
water molecules per one formula of [Cd(H2O)6](BF4)2 equaled 9.1%. Therefore, we sug-
gest that the first stage of the [Cd(H2O)6](BF4)2 decomposition should be connected with
its dehydration to [Cd(H2O)4](BF4)2, which, while being subsequently heated, melted at
ca 391 K. Later, in the second stage, the subsequent decomposition of [Cd(H2O)4](BF4)2
at a liquid phase took place, in fact, in the three steps, as indicated by the QMS curves
presented in Fig. 2. In the first step, at ca 425 K, only H2O molecules were released, and
at ca 445 K, BF3 and HF molecules were released as well as along with water. The aver-
age sample mass loss after the second stage of the decomposition equal to 58.7±2.5% of
the initial sample mass, implies the formation of CdF2 (theoretical value: 61.8%). The
X-ray diffraction patterns for the solid decomposition product confirmed CdF2, as was
identified using JCPDS-ICDD data.
Conclusions
• The solid-solid phase transition from room temperature to the high-temperature
phase of [Cd(H2O)6](BF4)2 proceeds at TC1=324 K.
• The dehydration process of [Cd(H2O)6](BF4)2 starts just at TC1 and, after freeing
two H2O molecules per one formula unit, the crystals of a new complex –
[Cd(H2O)4](BF4)2 – melt at Tmelt.=391 K. Subsequent heating of the melted sample at
first leads only to the subsequent dehydration, and later prompts the simultaneous
freeing of H2O, HF and BF3 molecules, until nothing else but solid CdF2 remains.
• [Cd(H2O)4](BF4)2 obtained in the course of this experiment, as an intermediate
product of the decomposition of [Cd(H2O)6](BF4)2, is not consistent with the results
of some previous studies, [8] and [9], which described the formation of Cd(BF4)2·
• 3H2O and Cd(BF4)2·2H2O, respectively.
References
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2 A. K. Jain and M. Geoffroy, Solid State Commun., 40 (1981) 33.
3 E. Mikuli, A. Migda³-Mikuli, A. Ksi¹¿czak and T. Ksi¹¿czak, to be published.
4 C. D. West, Z. Kristall., 88 A (1934) 198.
5 C. D. West, Z. Kristall., 91 A (1935) 480.
6 K. C. Moss, D. R. Russel and D. W. A. Sharp, Acta Cryst., 14 (1961) 330.
7 G. Johansson and M. Sandström, Acta Chem. Scand., A 41 (1987) 113.
8 M. P. Georgiev and M. Maneva, J. Thermal Anal., 47 (1996) 1729.
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J. Therm. Anal. Cal., 68, 2002