14 Nonstoichiometry, Order, and Microphases in CeCd Solid Solutions
~ 4.5
GUY R. B. ELLIOTT and JOE FRED LEMONS
Downloaded by UNIV OF CINCINNATI on November 11, 2014 | http://pubs.acs.org Publication Date: January 1, 1963 | doi: 10.1021/ba-1964-0039.ch014
University
of California,
Los Alamos Scientific Laboratory,
Los Alamos, Ν. M.
Both a very large number of phases (microphases) and a comparatively broad single phase have been observed in the same composition region near
CeCd
using
~4.5
vapor
pressure
measure
ments with a bithermal, isopiestic equilibrator– balance.
The broad single-phase range results
from random defects dissolved in an ordered par ent lattice.
Further ordering by the defects cre
ates the many different microphases.
Family sim
ilarities exist in the activity coefficients of the mi crophases and in the two-phase regions between them.
The two-phase ranges cover as little as
125-p.p.m. addition of cadmium to the alloy.
In
some cases a periodicity in allowed compositions is observed.
Free energy data for
CeCd
~4.5
struc
tures and for two-phase mixtures with C e C d and with CeCd
~3
~ 6
are reported. Sources of bond
ing energy for the ordering reactions are dis cussed.
C a d m i u m vapor pressure measurements on cerium-cadmium alloys have been ex tended to include C e C d ^ . A s first reported, this alloy was called C e C d (8). Designating it as C e C d ^ indicates that it has an experimentally measur able composition range. These measurements have disclosed the existence of many phases i n what h a d appeared to be a single-phase region. T h e complexity of this system is greater than has been reported for any other metal system. 4 5
2
4
f )
5
Experimental General M e t h o d . T h e method has been described i n detail (4). A t a selected alloy temperature the vapor pressure of cadmium is determined as a function of alloy composition; the cerium solvent has a negligible vapor pres sure. T h e alloy, located i n one leg of a sealed inverted U-tube, is subjected to various specific pressures of cadmium from a supply of pure cadmium at selected temperatures i n the second leg of the tube. T h e U-tube is freely suspended at its midpoint and connected to a balance, so that the transfer of cadmium from one leg of the tube to the other can be measured. This gives information as to the change i n alloy composition and phase equilibrium. In Nonstoichiometric Compounds; Ward, R.; 153 Society: Washington, DC, 1963. Advances in Chemistry; American Chemical
ADVANCES IN CHEMISTRY SERIES
154
Downloaded by UNIV OF CINCINNATI on November 11, 2014 | http://pubs.acs.org Publication Date: January 1, 1963 | doi: 10.1021/ba-1964-0039.ch014
0.75
0.15
0.822
0.820
0.818
CADMIUM ATOM Figure 1.
Microphases
0.816
0.814
FRACTION
observed in CeCd^^.-, region at 639° C.
0.6950
0.6650 0.8225
0.8224
0.8223
0.8222
0.8221
0.8220
CADMIUM ATOM FRACTION Figure 2. Microphases observed in run III Bold faced letters correspond to bold italic in text In Nonstoichiometric Compounds; Ward, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1963.
0.8219
14. ELLIOTT AND LEMONS
Microphases in Ce-Cd Solid Solutions
155
Materials. T h e materials were equivalent to those used previously; however, there was an evident amount of intergranular oxide i n the cerium used i n run I I I . Approximately 4 grams of C e C d ^ was formed in situ b y evaporating cadmium from C e C d ^ . Three separate alloy charges i n different equilibrators were used for three series of measurements: run I, points 1 to 128, and run I I , points 129 to 234, i n Figure 1; run III, points 235 to 297, i n Figure 2. E a c h terminal alloy was observed to have been a single, porous, chunk of alloy. Metallography indicated clean crystals w i t h enough pipes a n d cavities so that cadmium solid diffusion disstances were i n the range 0.01 to 0.1 m m . 4 5
6
E q u i l i b r i u m Criteria. T h e experimentally practical criterion of equilibrium used i n runs I and II was that neither composition nor temperature should change observably for / hour. Experience w i t h the equipment and sample observations for much longer periods of time (days i n some cases) indicated that this time was adequate. I n the composition-structure region represented i n Figure 3 the alloy composition d i d not change detectably within limited ranges of cadmium vapor pressure. F o r these measurements the criterion of equilibrium adopted was that a composition should not have shifted detectably for at least / hour and the cadmium condensate temperature should not have shifted detectably for / hour. Once a pattern of behavior had been established, some measurements were accepted i n w h i c h the procedure or conditions varied i n minor detail from that described. Such points are not indicated i n Figure 1, but are indicated b y a bar through the datum circles i n other figures.
Downloaded by UNIV OF CINCINNATI on November 11, 2014 | http://pubs.acs.org Publication Date: January 1, 1963 | doi: 10.1021/ba-1964-0039.ch014
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2
3
4
1
3
0.28 F
UJ on ZD
Ul93 «94
0.26
167 Γ
o 0.241
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