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016
L. Rycerz et al.
compounds [6] are formed; Rb CeBr melts incongruently,
2
in details [2, 3]. Samples (300–500 mg) were contained in
vacuum-sealed quartz ampoules (about 6 mm diameter,
15 mm length). Enthalpies of transition measurements
were conducted at heating and cooling rates between 1 and
5
while Rb CeBr and RbCe Br melt congruently.
7
3
6
2
This present work was devoted to the CeBr –CsBr
3
binary system. A detailed DSC investigation highlighted
the features of phase equilibrium. In addition electrical
conductivity determination of liquid mixtures was con-
ducted and is discussed in terms of possible complex
formation.
-
1
5 K min . Some experiments were also performed at
-
1
0.1 K min cooling rate.
Electrical conductivity measurements were carried out
in capillary quartz cells with cylindrical platinum elec-
trodes, described in details elsewhere [7]. These cells were
calibrated at high temperature with pure molten NaCl [8].
The cell, filled with the substance under investigation, was
placed into a furnace in a stainless steel block, used to
achieve a uniform temperature. The conductivity of the
melt was measured by platinum electrodes with the con-
ductivity meter Tacussel CDM 230 during increasing and
decreasing temperature runs. The mean values of these two
runs were used in calculations. Experimental runs were
Experimental
Sample preparation
Cerium(III) bromide was synthesised from the cerium(III)
carbonate hydrate (Aldrich 99.9%). Ce (CO ) ꢀ xH O was
2
3 3
2
dissolved in hot concentrated HBr acid. Solution was
-
1
evaporated and CeBr ꢀ xH O was crystallized. Ammo-
performed at heating and cooling rates 1 K min . The
temperature was measured with a Pt/Pt–Rh(10) thermo-
couple with 1 K accuracy. Temperature and conductivity
data acquisition was made with PC computer, interfaced to
the conductivity meter. All measurements were carried out
under static argon atmosphere. The accuracy of the mea-
surements was about ± 2%.
3
2
nium bromide was then added and this wet mixture of
hydrated CeBr and NH Br was first slowly heated up to
3
4
4
50 and then up to 570 K to remove the water. Resulting
mixture was subsequently heated to 650 K for sublimation
of NH Br. Finally the salt was melted at 1100 K. Crude
4
CeBr was purified by distillation under reduced pressure
3
(
*0.1 Pa) in a quartz ampoule at 1150 K. CeBr prepared
3
in this way was of a high purity—min. 99.9%. Chemical
analysis was performed by mercurimetric (bromine) and
complexometric (cerium) methods. The results were as
follows: Ce, 36.91 ± 0.03% (36.89% theoretical); Br,
Results and discussion
CeBr –CsBr phase diagram
3
6
3.09 ± 0.04% (63.11% theoretical).
Cesium bromide was Merck Suprapur reagent (min.
9.9%). Prior to use, it was progressively heated up to
The CeBr –CsBr phase diagram was established for the
3
9
first time in the course of the present work. The DSC
investigations were performed on samples with different
compositions; the fusion temperature and enthalpy of the
related mixtures were obtained from the corresponding
curves. The enthalpies of thermal effects obtained from
heating and cooling runs were almost the same (difference
less than 2%). However, supercooling was observed on
cooling curves and hence temperature and enthalpy data
were obtained from heating curves only. All results are
presented in Table 1.
fusion under gaseous HBr atmosphere. HBr in excess was
then removed from the melt by argon bubbling.
The mixtures of CeBr and CsBr (in appropriate pro-
3
portions) were melted in vacuum-sealed quartz ampoules
in an electric furnace. Melts were homogenised by shaking
and solidified. These samples were ground in an agate
mortar in a glove box. Homogenous mixtures of different
composition prepared along the same procedure were used
both in phase diagram and electrical conductivity
measurements.
In all DSC curves, the effect at highest temperature
corresponds to liquidus. In the composition range
All chemicals were handled in an argon glove box with
-
a measured volume fraction of water of about 2 9 10
6
0 \ x B 0.250, where x is molar fraction of CeBr , two
3
and continuous gas purification by forced recirculation
through external molecular sieves.
additional endothermic peaks were also present. The first
one, at 842 K (mean value from measurements), is obser-
vable in all DSC curves up to x \ 0.250 and can be
undoubtedly ascribed to the CsBr–Cs CeBr eutectic. Its
Measurements
3
6
disappearance from x = 0.250 suggests the existence of
Cs CeBr compound. The eutectic composition was
The temperatures and enthalpies of phase transitions
3
6
of CeBr –CsBr binary mixtures were measured with a
3
determined accurately from the Tamman plot (Fig. 1a).
The analysis of this experimental enthalpy vs. composition
plot evidences that no solid solutions form in the system
Setaram DSC 121 differential scanning calorimeter. The
apparatus and the measurement procedure were described
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Rycerz, Leszek
