Heat capacity and thermodynamic properties of some Ca silicides

Article abstract of DOI:10.1016/S0925-8388(99)00692-1

The heat capacities of three Ca compounds, namely CaSi₂, Ca₃Si₄ and Ca₁₄Si₁₉ were measured in the 3-300 K temperature range by adiabatic calorimetry. No thermal anomalies were found in the whole temperature range. In the three Ca silicides, from an analysis of the low temperature data (T<40 K), a power law lower than three in the lattice heat capacity behaviour was observed and tentatively ascribed to the layered structure of the compounds, in agreement with structural informations. From heat capacity data the thermodynamic functions entropy, enthalpy and Gibbs energy were calculated at 298 K.

Full text of DOI:10.1016/S0925-8388(99)00692-1

Journal of Alloys and Compounds 299 (2000) 20–23
L
www.elsevier.com/locate/jallcom
Heat capacity and thermodynamic properties of some Ca silicides
*
F. Canepa , M. Napoletano, P. Manfrinetti, A. Palenzona
INFM and Dipartimento di Chimica e Chimica Industriale, Universit a` di Genova, Via Dodecaneso 31, 16145 Genova, Italy
Received 20 October 1999; accepted 28 October 1999
Abstract
The heat capacities of three Ca compounds, namely CaSi , Ca Si and Ca Si were measured in the 3–300 K temperature range by
2
3
4
14 19
adiabatic calorimetry. No thermal anomalies were found in the whole temperature range. In the three Ca silicides, from an analysis of the
low temperature data (T,40 K), a power law lower than three in the lattice heat capacity behaviour was observed and tentatively ascribed
to the layered structure of the compounds, in agreement with structural informations. From heat capacity data the thermodynamic
functions entropy, enthalpy and Gibbs energy were calculated at 298 K.
 2000 Elsevier Science S.A. All rights reserved.
Keywords: Calcium silicides; Adiabatic calorimetry; Heat capacity; Thermodynamic functions
1
. Introduction
2. Experimental
Ca-silicides were widely investigated in the past with
Elemental Ca (99.5 wt.%) and Si (99.999 wt.%) were
used for direct synthesis of the compounds. Weighed
amounts of the elements (total mass of about 2 g) were
closed, due to the high vapor pressure of Ca, in a tantalum
crucible, then heated in a high frequency induction furnace
respect to their electronic properties. These were analysed
through experimental techniques (Auger Electron Spectop-
scopy, Bremsstrahlung Isochromat Spectroscopy, Inverse
Photoemission), as well as from theoretical studies per-
formed by self-consistent calculations of the electron states
carried out with the Andersen linear muffin-tin orbital
method [1–3]. More recently, the discovery of supercon-
ductivity at 14 K under pressure in a new polymorph of the
CaSi2 stoichiometry [4] and the determination of the
crystal structure of the semiconductor Ca Si [5], in-
and shaken to ensure homogenization. The CaSi
pound was then cooled down to room temperature, while
for the Ca Si and the Ca Si phases a quenching
com-
2
3
4
14 19
procedure from the liquid state was applied. The samples
were then sealed in quartz tubes under reduced Ar pressure
and annealed: CaSi at 1273 K for 3 days, Ca Si at 1173
2
3
4
1
4
19
K for 20 days and Ca Si at 1333 K for 25 days.
duced us to re-analyse the Ca–Si phase diagram in the
whole composition range [6].
14 19
Metallographic examination performed on the three
samples with standard techniques, showed that they were
all single phase.
X-ray analysis was carried out by means of a Guinier-
Stoe camera (Cu Ka radiation) with silicon as internal
standard. Lattice parameters agree well with those of Ref.
However, up to now, no thermodynamic measurements
have been performed on the Ca-silicides. For this reason
we present here the results concerning measurements of
the heat capacity of the CaSi , Ca Si and Ca Si phases
2
3
4
14 19
in a wide temperature range (3–300 K) as well as some
[
6].
calculations to obtain thermodynamic informations at
2
000
0
2
A commercial Oxford Maglab System
measurement
standard temperature (298 K) on enthalpy (H
2
98 K
0
0
0
0
platform was used to measure the heat capacity (adiabatic
method) of the compounds (cylinders of about 0.6 grams
each) in the 3–300 K temperature range under a dynamic
H0 K), entropy (S
) and Gibbs energy (G
2 G ).
298 K
298 K
0 K
2
6
vacuum better than 10 mbar. A calibrated Cernox-type
thermometer from Lake Shore was used as temperature
sensor. The estimated error in the heat capacity experimen-
*
Corresponding author.
0
925-8388/00/$ – see front matter
 2000 Elsevier Science S.A. All rights reserved.
PII: S0925-8388(99)00692-1

F. Canepa et al. / Journal of Alloys and Compounds 299 (2000) 20–23
21
Fig. 1. Heat capacity of CaSi2 in the 3–300 K temperature range.
tal data is about three per cent in the whole temperature
range.
Below 40 K the hypothesis of comparable values of CP
and C seems reasonable, so the experimental data were
V
firstly tentatively analysed in the framework of the Debye
3
theory which claims a T dependence of the lattice heat
3
. Results and discussion
capacity on temperature, but the fitting procedure was
unsatisfactory. The best fit was obtained for the three
phases using the following equations:
In Figs. 1–3 the heat capacities of the three compounds
CaSi , Ca Si and Ca Si respectively, are reported. For
2
3
4
14 19
1
.8760.02
23
all compounds the heat capacity increases monotonically
up to 300 K. No evidence of structural transitions or
thermal anomalies was observed. At 298 K, the heat
capacity values of the three compounds are 73.9 J/K mol
C (T)
5 A? T
CaSi
2
A 5 (8.060.6)? 10
P
2
.0960.01
22
C (T)
5 A? T
A 5 (4.760.3)? 10
P
Ca Si
14 19
(
(
CaSi ), 179 J/K mol (Ca Si ) and 985 J/K mol
2
3
4
2
.6760.02
24
Ca Si ).
C (T)
5 A? T
4
A 5 (8.760.5)? 10
1
4
19
P
Ca Si
3
Fig. 2. Heat capacity of Ca Si in the 3–300 K temperature range.
3
4

2
2
F. Canepa et al. / Journal of Alloys and Compounds 299 (2000) 20–23
Fig. 3. Heat capacity of Ca14Si19 in the 3–300 K temperature range.
Table 1
Thermodynamic data at 298 K obtained from heat capacity measurements
0
2
0
0
0
298
0
0
298
0
Compound
H
2 H
S
2 S0
G
2 G0
K K
98
K
K
K
K
(
KJ/mol)
(J/K mol)
(KJ/mol)
CaSi2
Ca Si
13.3
32.4
84
197
211.7
226.3
3
4
Ca14Si19
181
1140
2158.7
The crystal structure of the three examined compounds is
characterized by a pronounced silicon sublattice: CaSi is
2
trigonal [7,8] with two layered modifications, i.e., the
three- (3R) and six-layer (6R) types, while in Ca Si and
1
4
19
Ca Si the silicon framework shows a two-dimensional
3
4
feature. These crystal structure characteristics could be
related to the anomalous power in the temperature depen-
dence of the specific heat. In fact a power law in
temperature lower than three has been observed in layered
structures as graphite or boron nitride [9].
The numerical integration of the smoothed values of
heat capacity allow information to be obtained on the total
0
0
entropy (S (T) 2 S (0 K)): the thermal dependence of the
entropy for the three compounds is reported in Fig. 4 while
Table 1 shows the values at room temperature (298 K) of
the thermodynamic functions derived from heat capacity.
Heat capacity measurements on the other three phases of
the Ca–Si phase diagram, namely Ca Si, Ca Si and CaSi,
2
5
3
are now in progress in order to better clarify the influence
of the lattice structure on the phononic vibrations and to
complete the room temperature thermodynamic informa-
tion.
0
0
Fig. 4. Calculated entropy behaviour (S (T) 2 S (0 K)) of the three
compounds in the 3–300 K temperature range.

F. Canepa et al. / Journal of Alloys and Compounds 299 (2000) 20–23
23
[
[
[
2] C. Chemelli, M. Sancrotti, L. Braicovich, F. Cicacci, O. Bisi, A.
Iandelli et al., Phys. Rev. B 40 (1989) 10210.
3] S. Abe, H. Nakayama, T. Nishino, S. Iida, J. Mater. Res. 12 (2)
Acknowledgements
The Authors would like to thank Prof. M.L.Fornasini for
helpful discussions.The present work is a part of the
National Research Program ‘Alloys and Intermetallics
Compounds: Thermodynamics, Physical Properties, Reac-
tivity’ of the Italian MURST.
(
1997) 407.
4] S. Sanfilippo, H. Elsinger, M. N u´ nez-Regueiro, O. Laborde, S.
LeFloch, M. Affronte, et al., Phys. Rev. Lett. (1999) submitted.
[5] A. Currao, S. Wengert, R. Nesper, J. Curda, H. Hillebrecht, Z.
Anorg. Allg. Chem. 622 (1996) 501.
[6] P. Manfrinetti, M.L. Fornasini, A. Palenzona, Intermetallics (1999)
in press.
[
[
[
7] J. Evers, J. Solid State Chem. 28 (1979) 369.
References
¨
8] S. Dick, G. Ohlinger, Z. Kristallogr. 213 (1998) 232.
9] E.S.R. Gopal, in: K. Mendelssohn, K.D. Timmerhaus (Eds.), Spe-
cific Heats at Low Temperatures, Heywood Books, London, 1966,
Chapter 2.
[1] D.D. Sarma, W. Speier, L. Kumar, C. Carbone, A. Spinsanti, O. Bisi
et al., Z. Phys. B: Condensed Matter 71 (1988) 69.