Specific heats of actinide metals
M. Mortimer
To cite this version:
M. Mortimer. Specific heats of actinide metals. Journal de Physique Colloques, 1979, 40 (C4),
pp.C4-124-C4-129. <10.1051/jphyscol:1979438>. <jpa-00218834>
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JOURNAL DE PHYSIQUE
Colloque C4, supplément au n° 4, Tome 40, avril 1979, page C4-124
Specific heats of actinide metals
M . J. Mortimer
Chemistry Division, AERE Harwell, Didcot, Oxon, OX11 ORA, G.B.
Résumé. — Nous discutons les méthodes qui permettent, à partir des paramètres de la chaleur spécifique, de
déterminer le coefficient électronique y et la température de Debye 0D. Nous étudions l'évolution de ces
grandeurs ainsi que celle de la chaleur spécifique le long de la série des actinides. Les valeurs inattendues
obtenues pour le protactinium sont comparées à celles de la résistivité électrique. Nous discutons l'importance
du coefficient de dilatation pour la chaleur spécifique et pour les propriétés électroniques. Finalement nous
comparons les anomalies observées pour a-U, Pu et Am.
Abstract. — After a brief discussion of the methods of extraction of the specific heat parameters, the
electronic specific heat and the Debye temperature, an analysis is given of the trends in these, and in the
measured specific heat, across the actinide series. The unexpected values obtained for protactinium are
considered, with reference to the electrical resistivity. The importance of the expansion coefficient, both in the
derivation of the specific heat parameters, and in any explanation of the electronic origins of their behaviour is
discussed. Finally, the anomalies observed in ct-U, Pu and Am are compared.
1. Introduction. — Since the last r e v i e w of the
specific h e a t s of the actinide metals [1] a d v a n c e s
h a v e b e e n m a d e in a n u m b e r of directions. T h e first
m e a s u r e m e n t s h a v e b e e n r e p o r t e d for the rarer
actinides, protactinium and americium, while meas u r e m e n t s on plutonium h a v e b e e n m a d e at lower
t e m p e r a t u r e s , using t h e less active isotope 242 Pu.
F u r t h e r w o r k has b e e n d o n e on the low t e m p e r a t u r e
anomalies in a - u r a n i u m , a n d o n the variation in
electronic specific h e a t with crystallinity.
In this p a p e r I begin with a s u m m a r y of the
m e t h o d s w e h a v e used in analysing the available
specific heat data in various t e m p e r a t u r e regions.
T h e n the specific h e a t of e a c h actinide element is
r e v i e w e d in turn, with a discussion of absolute
v a l u e s , of the n e w m e a s u r e m e n t s derived p a r a m e t e r s , and of any anomalous behaviour. Finally, the
variation in specific h e a t s across the series is discussed a n d related t o other properties.
2. Analysis of specific heat data. — T h e specific
h e a t s of real materials m a y b e described b y a D e b y e
model in t w o t e m p e r a t u r e ranges : below T = 0 D /5O
a n d a b o v e T = 6J2.
High t e m p e r a t u r e Cp data as measured are first
c o r r e c t e d to C„, t h e dilation correction being given
by:
w h e r e a is the coefficient of linear thermal expansion, V is the molar volume and K the compressibility.
T h e lattice specific C, is calculated for a range of
electronic specific heat coefficients y, and t h e n used
to give the
temperature
Debye
Cl=Cv-yT
temperature
= f(6D,
6D
at
each
T) .
T h e best value of y is t a k e n as that for which 6D is
constant as a function of t e m p e r a t u r e .
At low t e m p e r a t u r e s , t h e m e a s u r e d values of Cp
are given b y
Cp = yT +
pT3
h e r e /8 is related to the D e b y e t e m p e r a t u r e 0 D . T h e
equation is valid u p t o some m a x i m u m t e m p e r a t u r e
T
.
max
E x a m p l e s of t h e s e derivations are given in
figures 1 and 2 for 242 Pu [2].
E a c h table given in this p a p e r is divided into t w o
p a r t s . T h e u p p e r part s u m m a r i z e s results u p to r o o m
t e m p e r a t u r e , with analysis, w h e r e given, as detailed
a b o v e . I call this high temperature data. T h e lower
part of e a c h table gives low t e m p e r a t u r e data.
3. Results. — 3.1 THORIUM (Table I). — N o further high t e m p e r a t u r e m e a s u r e m e n t s h a v e b e e n r e p o r t e d since t h o s e of Griff el and S k o c h d o p o l e [3],
reviewed earlier [1]. At low t e m p e r a t u r e s , as previously reviewed [4, 5] G o r d o n et al. [5] found a y
value of 4.31 ± 0 . 0 5 , for v a n Arkel thorium, total
metallic impurity level 20 p p m , a n d non-metallic
level 230 p p m . N e w low t e m p e r a t u r e results r e ported by L u e n g o et al. [6] show a value of y of
4.08 ± 0.03 for 99.95 % p u r e thorium. T h e y s h o w
that alloying with u r a n i u m causes a rapid increase in
y of 4.85 ± 0.5 mJ . m o l " 1 . KT2 (at % U)" 1 . This is
higher than has b e e n o b s e r v e d in other s y s t e m s .
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979438
SPECIFIC HEATS OF ACTINIDE METALS
Fig. 1. - Extraction of high temperature y and OD values for ='Pu.
I
I
I
I
I
I
I
I
I
I
I
I
~
-
PLUTOYIUH-2L2 +ADDENDA
t
-
111 BRASS STRIP
r 2nd BRASS STRlP
-
-
'
T E H P E R ~ T U R E ~I K ~ I
Fig. 2 . -Low
temperature specific heat of
-
(Fig. 3). Because of the small mass available, only
25 % of the sample was protactinium. A careful
analysis of the two sets of data has failed to reveal.
any systematic error to account for the difference in
y in the two temperature regions. It is worth noting,
however, that while early rather imprecise electrical
resistivity measurements [9] gave results between
those of thorium and uranium, recent more precise
measurements [lo] suggest a value for the electrical
resistivity much lower than that for thorium, perhaps supporting a low y value. Specific heat measurements on more massive samples will clarify the
--L-LLJ
situation.
The specific heat measurements show no effect at
w2pU.
3.2 PROTACTINIUM (Table I). - The first results
of specific heat measurements on protactinium metal present a confusing picture. A single sample,
prepared by the van Arkel process (typical impurities < 400 ppm metallic : 900 ppm 0, 40 ppm N,
50 ppm C) was measured from 10 to 300 K [7]
and below 20 K [8]. The room temperature C,,
33 +- 1 J . mol-' . K-' was higher than expected,
being much higher than that of either of its
neighbours. In turn, the derived y value,
30 mJ . mol-' . K-' is the highest of any of the actinide elements for which results are available. To
confuse the picture, the low temperature
measurements [8] indicate a value close to zero
103 K where both sets of resistivity measurements
show a small change of slope.
-
120-
-
% IOOa
Ole
5
. .
80-
60
Pa
0
100
200
,...
o
Hagh k s t r t p
r
Low k s t r l p
300
7, 2 , u 2 ,
Fig. 3. - Low temperature specific heat of protactinium metal.
LOO
M. J. MORTIMER
C4-126
Table I.
Year of
Pub.
a
8
Griffel and Skochdopole
Smith and Wolcott
Gordon, Montgomery et a/.
Luengo et al.
$
Brown, Hall et al.
Refs.
131
Temperature
range of
measurements
Electronic
coefficient y
mJ mol-' . K-2
.
-
Debye
temperature
OD
-
T max for
linear C,,/ T
-
5 .a Hall, Mortimer, Blaise et al.
$,
.z
'
Jones, Gordon and Long
Clusius and Piesbergen
Flotow and Lohr
Lee, Mendelssohn and Sutcliffe
Sandenaw
Smith and Wolcott
Goodman, Hilliard et al.
Dempesey, Gordon et al.
Flotow and Osborne
Gordon, Montgomery et al.
Ho, Phillips and Smith
Crangle and Temporal
Bader, Phillips and Fisher
Hall and Mortimer
Hall
(*) 10 kbar.
(*) Depends
27.48
not given
27.66
on crystallinity = single crystal -+ polycrystal.
3.3 URANIUM
(Table I). - Previously reviewed
high temperature results 111-141 give a coherent
picture with (C,),,, about 27.5 J . mol-' . K-' and
derived y of about 8 to 8.8 mJ . mol-' . K-,. The one
new paper [IS] does not give y or Cp values, but
looks instead at various specific heat irregularities
and consequent entropy changes as a function of
cold work and annealing.
Earlier low temperature results [4, 5, 16, 17, 181
were mostly consistent with a y value of about 10.0,
although there was some variation in the derived
0,. More recent measurements have looked at the
effect of pressure [19] and of variations in
crystallinity [20, 211 on y and at the various low
temperature anomalies in a-uranium [20, 22, 231.
The measurements of Ho, Phillips and Smith [I91
at zero pressure gave a
y
value of
10.3 mJ . mol-' . K-', while at a pressure of 10 kbar
this rose to 12.2 mJ . mol-' . K-'. Furthermore, while the zero pressure measurements showed no magnetic field dependence, and no anomaly characteristic of a superconducting transition down to
0.35 K, the high pressure measurements indicated
that bulk superconductivity occurred at about 2 K,
and was destroyed by the application of a field of
2 000 Oe. Later measurements [21] did show the
existence of bulk superconductivity in a -uranium,
but only below 0.3 K in a single crystal or, smeared
out at slightly higher temperatures, in polycrystalline
uranium. Thus both y and T,rise with pressure.
Another property affecting y is the crystallinity.
Crangle and Temporal [20] measured a single crys-
tal, a polycrystalline sample with large grains, and a
polycrystal, and showed that y rose from
9.14 mJ . mol-' . K-2 through 9.46 mJ . mol-' . K-' to
10 mJ . mol-' . K-Z while 0, fell from 210 to 195.
This rise was confirmed by Bader et al. [21] who
found a rise in y from 9.14 to 9.90 mJ . mol-' . K-2 in
a similar series. Since polycrystals also show an
enhanced T, it is apparent that the inhomogeneous
strains in the crystal affect y and T,in the same way
as hydrostatic pressure. The question of whether
this is mainly electronic in origin [24] or is accounted
for by the effect of pressure on the phonon
spectrum [21] is not yet clear.
Following the early results of Steinitz et al. [25]
which showed 2 f i s t order (at 22 K and 37 K) and 1
2nd order transition (at 43 K) in expansion measurements on a-uranium single crystals, specific heat
measurements [20] confirmed presence of the associated latent heats in single crystals. Having confirmed earlier measurements on a single crystal [20],
Hall [23] heat treated the sample to convert it to a
large grained polycrystal, then progressively decreased the grain size, showing that the 2 first order
transitions were progressively suppressed and the
second order smeared out. This effectively disposed
of the idea that these low temperature anomalies
might not be intrinsic to pure uranium, but be
produced by impurities left in during the crystal
growing process [IS, 221.
The specific heat of uranium in this temperature
range is now fairly well defined. The origin of the
anomalies is not clear. If magnetic in origin it must
SPECIFIC HEATS OF ACTINIDE METALS
presumably be some form of spin density wave,
giving a magnetic structure below 43 K which undergoes minor atomic re-arrangement at 22 K and
37 K [26,20]. If this is the case the magnetism must
be below the detection limit in neutron diffraction
measurements [27].
3.4 NEPTUNIUM (Table 11). - Previously discussed measurements [28, 29, 301 at high temperatures
showed neptunium to have a normal specific heattemperature dependence, with no anomalies. The y
value derived from high temperature data was 14.2
and 8, 187 K. These measurements taken down to
7.5 K [29, 301 permitted low temperature parameters to be determined. They proved not very different.
Two further measurements at low temperatures [2, 311 confirmed this y value, although a much higher value of 8, was found, 240 K.
The materials used were of quite different origins.
Since the temperature region of the latter measurements covered more of the linear C, T region, this
value of 8, is preferred.
3.5 PLUTONIUM
(Table 11). -The high temperature data 129-351 (all taken on '"Pu) mostly gave (C, ),
values between 31.97 and 32.82 J . mol-' . K-', y
values between 11.9 and 15.9 mJ . mol-' . K-2 and 8,
of about 160 K. Since this earlier data two reports
giving high temperature data for '"Pu have been
published [36, 21. In the first of these, Sandenaw
reported some dependence on thermal cycling, but
gave (C, ),,, = 32.9 J . mol-' . K - ' , though with a very high y = 44 mJ . mol-' . K-'. This latter was calculated for the region near 15 K, where C, / T is
C4-127
not yet linear ; if a high temperature fit is made,
as outlined in section 2, a y value of
15.5 d . mol-' . KT2is found. Apart therefore from
some thermal cycling effects these are in good
agreement with previous measurements on 2 3 9 P ~ .
The second of the measurements [2] on 242Pugives a
lower (C, ),,, of 3 1.19 J . mol-' . K-', hence a lower y
of 10.5 mJ . mol-' . K-2. Following various tests [2]
no systematic error was found, and this result is
believed genuine, even though low. The sample was
also measured at low temperature [2] when a much
higher y value of 22 was found. There was good
agreement between the two sets of measurements in
the region of overlap. Other measurements of
Gordon [37] also support a higher low temperature y
value.
Whether or not the y value derived from the 242pU
high temperature data is accepted, the measurements contrast with those on neptunium results
taken at the same time, where high and low temperature regions gave y values to within about 4 %. In
the case of plutonium the high temperature y is 50 %
or 100 % lower than the low temperature y depending on which high temperature y value is taken.
Differences in anharmonicity are unlikely to be
significant, so one is forced to conclude that the
electronic specific heat is different in the high and
low temperature regions. It is tempting to associate
this with the decrease in electrical resistivity above
100 K, not directly as a change in the density of
states, but perhaps on a spin fluctuation model,
which has been invoked to explain the resistivity
(Ref. [2] and references therein).
The small anomaly at 60 K seen in several sam~
was not seen in '"'Pu. The point
ples of 2 3 9 P[29,30]
Table 11.
.-
3
.-g
Sandenaw
Lee, Mendelssohn and Sutclie
Lee, Mendelssohn and Sutcliife
Gordon, Hall et al.
Blaise, Mortimer et al.
-
Refs.
(1965)
(1968,
1970)
[28]
[29,
301
(1968,
1970)
(1976)
unpub.
[29,
301
C21
[31]
(1976)
(1976)
unpub.
121
E21
[37]
(1978)
this
cod.
-
Sandenaw, Olsen and Gibney
Sandenaw
Lee, Mendelssohn and Sutcliffe
Taylor, Loasby, Dean et a[.
Lee, Mendelssohn and Sutcliffe
Sandenaw, Gibney (Pu242)
Gordon, Hall et al. (Pu242)
Gordon, Hall et al. (Pu242)
Gordon, Hall et al. (Pu239)
Gordon (Pu242)
'G
.-
Year of
pub.
Miiller, Schenkel et al.
Smith, Stewart, Huang and
(*) Recalculated from high temperature data.
Temperature
range of
measurements
-
8-320
7.5-300
Electronic
coefficient y
mJ . mol-' . K-2
Debye
temperature
- 13.8
- 190
-
14.2 C 0.5
e,
-
185 c 5
T max for
linear CdT
-
C4-128
M. J. MORTIMER
is fully discussed in reference [ 2 ] . Its nonobservance is not due to poorer precision in the
latter case. The suggestion that its presence or
absence depends on the purity of the sample
warrants closer investigation ; a difference between
samples is the higher americium
the '"'Pu and z3h
content of the latter. Magnetic susceptibility studies
might be worth
of low concentrations of Am in Z42Pu
examining.
3 . 6 AMERICIUM (Table 11). - The first reported
specific heat measurements on americium [38] show
a break with results for earlier elements. The room
has fallen to
temperature specific heat (C,,,)
28 J . mol-' . K-I, not much above those for uranium
and thorium, and y to 3 mJ . mol-' . K-', well below
earlier values. While the precision of these measurements is necessarily reduced, in view of the high
self-heat of americium, these results show that the
intervention of the 5f electrons in conduction is
much reduced, a conclusion supported also by electrical resistivity measurements [2] (I).
Both specific heat and electrical resistivity show a
pronounced anomaly at 6 0 K (Fig. 4). That in the
specific heat is much more pronounced than in any
other actinide metal. It is evidently not magnetic in
origin, but seems rather spread out to be the latent
heat associated with a structure change. Measurements of other properties, for example, lattice parameters, are required to give further information.
given to measuring such highly active materials. The
specific heat of protactinium is not at all clear. Here
samples of a greater mass are required to give more
precision.
The derived specific heat y and 8, values may be
compared with other selected parameters :
exp. coeff.
a x lo6
11.6
XRT
0.41
10.3
1.16
14.9
1.6
27.7
2.28
53.7
2.15
-
7.07
2.8
-
(*) At time t = 0,normalized for self-heat.
While the y values (apart from that for
protactinium) change regularly across the series, the
8, values do not follow any trend. There is no clear
correlation between the measured magnetic susceptibility values, which peak at americium with a dip at
plutonium, and either the y values or the resistivity
behaviour. This, and the lack of temperature dependence of the susceptibility is the problem which has
occupied many people attempting to reconcile strong
resistivity ,temperature dependence with essentially
temperature independent susceptibilities (see
Ref. [39] and references therein).
+
+-+
f Specif~c Heat
x-.-.-x
PJD0
0---4
a Expansion
1-------------TEMPERATURE I K)
Fig. 4. -Specific
heat anomalies in the actinide elements.
4. Discussion. - For the well-known actinides,
thorium, uranium, neptunium and plutonium, the
specific heats are fairly well defined. More measurements can be expected to refine the data, and to
define further the anomalous behaviour. In the case
of americium, the general picture is clear, with a
drop in C, compared with earlier elements. The
thermodynamic functions are not well-known, and
before they can be defined, further thought must be
'./.
L-- -vP 3 ~ -./'
0
,.'
N~
..-
.o.
o.......
0
/' :
:
., x .; ..:
v. +
i
:
l n ~ t ~ aSelf
l
Damage r a t e
-
- - - - - -+ - - - - - - - - - - - - - - - .
I
I
I
I
I
1
I
Th
Pa
U
Np
Pu
Am
(') Low temperature measurements by Smith et al. (this
conference) indicate a y value of 2 mJ . mol-' . K-'.
Fig. 5. - Selected results normalized for Pu = 1.
II
SPECIFIC HEATS OF ACTINIDE METALS
The expansion coefficients, on the other hand, are
not often discussed. To make the trend more clear,
the high temperature y values, the resistivity data
and the expansion coefficients have been normalized to Pu = l (Fig. 5) and it is clear that each shows
similar variations across the series. The correlation
is even closer with low temperature y values.
A correlation also exists between expansion coefficients of high temperature phases of plutonium and
their electrical resistivity coefficient. Even where
the expansion coefficients are negative, the coefficients are in every phase inversely related in
sign [40].
C4-129
However the reason for a correlation between
measured expansion coefficients and the electronic
properties is not evident. The expansion coefficient
may be expressed as the sum of a lattice component
and an electronic component. The separation of
these terms has been performed at low
temperatures [41,24] using an equation analogous to
that in section 2 for specific heat. The correlation
between a and y suggests a very important electronic component in the expansion coefficient. This
would merit future study and calculation to separate
these components, as well as careful expansion
coefficient measurements.
References
[I] LEE,J. A., SUTCLIFFE,
P. W., MARTIN,
D. J. and MENDELSSOHN, K., Plutonium 1970 and other actinides. Nuclear
Metallurgy, ed. Miner, W. N. (Metallurgical Society,
New York) Vol. 17, part 2, p. 58.
[2] GORDON,
J. E., HALL,R. 0. A., LEE, J. A. and MORTIMER,
M. J., Proc. R. Soc. London A 3 5 1 (1976) 179.
[33 GRIFFEL,M. and SKOCHDOPOLE,
R. E., J. Am. Chem. Soc.
75 (1953) 5250.
[4] SMITH,P. L. and WOLCOTT,
N. M., Conf. de Physique des
Basses Tempkratures (Paris 1955). Suppl. Bull. Inst.
Intern. Froid, Annexe 3 (1955) 283.
[5] GORDON,J. E., MONTGOMERY,
H., NOER, R. J., PICKETT,
G. R. and TOBON,R., Phys. Rev. 152 (1966) 432.
J. M., SERINI,J. G., SWEE[6] LUENGO,C. A., COTIGNOLA,
DLER, A. R., MAPLE,M. B. and HUBER,J. G., Solid
State Commun. 10 (1972) 459.
[7] BROWN,D., HALL,R. 0. A., LEE, J. A., MORTIMER,
M. J.
and WHITTAKER,
B., 7e Journke des Actinides, Paris
(March 1977) p. 43.
[8] HALL,R. 0. A,, MORTIMER,
M. J., and BLAISE,A., Private
communication.
[9] HALL,R. 0 . A., LEE, J. A. and MORTIMER,
M. J., J. Low
Temp. Phys. 27 (1977) 305.
[lo] BETT,R., Private communication.
[I13 JONES,W. M., GORDON,
J. E. and LONG,E. A., J. Chern.
Phys. 20 (1952) 695.
[12] CLUSIUS,K. and PIESBERGEN,
U., Helv. Phys. Acta 31 (1958)
302.
[13] FLOTOW,H. E. and LOHR,H. R., J. Phys. Chem. 64 (1960)
904.
1141 LEE,J. A., MENDELSSOHN,
K. and SUTCLIFFE,
P. W., Phys.
Lett. 30A (1969) 106.
[I51 SANDENAW,
T. A., Plutonium 1975 and other Actinides, ed.
Blank H. and Lindner R. (North-Holland Press) p. 487.
[I61 GOODMAN,
B. B., HILLIARET,
J., VEYSSIE,
J. J. and WEIL',L.,
Proc. of Seventh Int. Conf. on Low Temperature Physics, Toronto, 1960. Ed. G. M. Graham and A. G. Hollis
(Univ. of Toronto Press, Toronto, Canada) 1961, p. 3506.
[17] DEMPESEY,
C. W., GORDON,
J. E. and ROMER,
R. H., Phys.
Rev. Lett. 11 (1963) 547.
[IS] FLOTOW,
H. E. and OSBORNE,
D. W., Phys Rev. 151 (1966)
564.
[I91 HO, J. C., PHILLIPS,
N. E. and SMITH,T. F., Phys. Rev. Lett.
17 (1966) 694.
[20] CRANGLE,
J. and TEMPORAL,
J., J. Phys. F 3 (1973) 1097.
[21] BADER,
S. D., PHILLIPS, N. E. and FISHER,E. S., Phys. Rev.
B 12 (1975) 4929.
[22] HALL,R. 0 . A., MORTIMER,
M. J., J. LOW Temp. Phys. 27
(1977) 313.
[23] HALL,R. 0. A., Inst. Phys. Conf. Ser. No. 37 (1978) p. 60.
Int. Conf. on Rare Earths and Actinides, Durham.
[24] ANDRES,K., Phys Rev. 170 (1968) 614.
[25] STENITZ,M. O., BURLESON,
C. E. and MARCUS,J. A., J.
Appl. Phys. 4 1 (1970) 5057.
[26] GARDNER,
W. E. and SMITH,T. F., Phys. Rev. 154 (1967)
309.
1271 LANDER,
G. H. and MUELLER,
M. H., Acta Crystallogr. B 26
(1970) 129.
[28] SANDENAW,
T. A., J. Phys. Chem. Solids 26 (1965) 1075.
[29] SUTCLIFFE,
P. W., Thesis, Univ. of Oxford (1968).
1301 LEE, J. A., MENDELSSOHN,
K. and SUTCLIFFE, P. W., Proc.
R. Soc. London 317A (1970) 303.
1311 BLAISE,A., MORTIMER,
M. J., Private communication.
1321 SANDENAW,
T. A., OLSON,C. E. and GIBNEY,R. S., Plutonium 1960 (Cleaver Hume Press, London) 1961, p. 6679.
[33] SANDENAW,
T. A., J. Phys. Chem. Solids. 23 (1962) 1241.
[34] LEE, J. A., MENDELSSOHN,
K. and SUTCLIFFE,
P. W., Cryogenics 5 (1965) 227.
[35] TAYLOR,
J. C., LOASBY,R. G., DEAN,D. J. and LINFORD,
P. F., Plutonium 1965 (Chapman and Hall, London)
1967, p. 162-175.
[36] SANDENAW,
T. A. and GIBNEY,R. B., J. Chem. Themzodyn.
3 (1971) 85.
[37] GORDON,
J. E., Private communication.
[38] MOLLER,W., SCHENKEL,R., SCHMIDT,H. E., SPIRLET,
J. E., MCELROY,
D. L., HALL,R. 0. A. and MORTIMER,
M. J., J. Low Temp. Phys. 30 (1978) 561.
[39] NELLIS,W. J. and BRODSKY,
M. B., Actinides : Electronic
structure and related properties. Ed. Freeman A. J. and
Darby J. B. (Academic Press) p. 265.
[40] LEE, J. A., HALL,R. 0. A., KING,E. and MEADEN, G. T.,
Plutonium 1960, Ed. Grison, E., Lord, B. H. and Foster, R. D. (Cleaver-Hume, London) p. 39.
[41] WHITE,G. K., Philos. Mag. 6 (1961) 815.
COMMENT
Pr. A. J. FREEMAN.
- The higher y-value for
a - U under pressure appears to be due to the suppression of the low temperature transitions. Therefore, it appears to be correct to assign the higher
y -value (12.2) as the true y -value of a -U.