- Email: [email protected]
The effect of electronegativity on solid and liquid miscibility
JOURNAL OF THE LESS-COMMON METALS
390
Letters to the Editors
The effect of electronegativity
EVANS
AND RAYNOR~
mutual
solid
GSCHNEIDNER~, diagram atomic
and the points The
beyond
by DARKEN
this
units,
“elements
solid solutions have
very
the outer
with
limited ellipses
2.40
,
2.20-
are surrounded
within
the solvent, solubilities,
axes
/
/
,
,
/
very
been
of a modified
8%
ellipse
ellipse outside
intermediate
,
,
,
,
axes
for Yb,
,
f
whilst
solvent,
the larger lying
(Fig.
I) the
2 and f
difference
are predicted
elements
by
of the type of
the electronegativities,
15%
that, for a given
to have
,
f
limited
discussed
form
of major
and
the larger
those lying
against
by ellipses f
the inner
and those
are predicted
exhibit
has recently
in terms
for Th. It is claimed
lying
and thorium fact
12 are plotted
and of minor
ellipse
this
AND GURRY 3. In GSCHNEIDNER’S~~~~~~~
for Th lies just outside
the outer
kind,
ytterbium and
number
for Th and Yb
point
that
the results
for co-ordination
electronegativity radii.
shown
miscibilities,
whointerprets
discussed radii
have
and liquid
on solid and liquid miscibility
that
for Yb
in a diagram
to form
ellipse
beyond
4
in atomic
are predicted
the inner
of
extensive to
but within
behaviours”.
,
,
,
,
,
,
,
-------THORIUM
0”
, ’
-YTTERBIUM
ZOOOB
c 5 1.20F 3 y I.WB _ c 0 5 1.40W
I.ZOl.OO0.20-
I
I
I
0.60
I
1.00
I
I.,
1.20
I
1.40
I
I\1
1.60
RADIUS,
Fig. I. DARKEN AND
GURRY
1
1.60
/
2.00
1/l
I
2.20
I
2.40
I
I
2.60
I
II
2.60
CN = 12 (A)
plot for ytterbium
and thorium metals. (Reproduced Common Metals, 4 (1962) 107.)
J. Less-Common
from J. Less-
Metals, 4 (1962) jgo-392
LETTERS TO THE EDfTORS
391
Examination of GSCWNEIDNER’S diagram makes it very doubtful whether this type of prediction has any justification. Since the minor axes of the ellipses are pro~~ional to the differences in atomic radii, they become smaller as the atomic radius decreases, i.e. as the ellipse moves to the left in Fig. I, whilst the major axes remain unaltered. Qualitative examination of Fig. I reveals the following characteristics : Solvent Au The points for Ag, Cu, Ni, Hg, Cd, Zn, In, and Mg lie well outside larger ellipses of the Gschneidner type. Continuous solid solutions are formed in the system Au-Ag, Au-Cu, and Au-Ni, and wide solid solutions in gold are formed by Hg, Cd, Zn, In and Mg. Solvent Ag The points for Au, Mg, and Li lie well outside the larger Gschneidner ellipse, whilst that for In lies slightly outside. All those elements form extensive solid solutions in Ag (Ag-Au continuous solid solutions). The system Ag-Li is of particular interest, since both elements are univalent, and there is no possibility of the electronegati~ty values referring to a different valency state. Solvent Ct4 The points for Au and In lie well outside the larger Gscheidner ellipse, whilst those for Sb and Be lie slightly outside. All these elements form solid solutions in copper (Cu- Au continuous solid solutions). So&vent Mg The point for Tl lies well outside the larger Gschneidner ellipse, whilst that for Cd lies just on this ellipse. Both metals dissolve freely in Mg (Mg-Cd continuous solid solution). Other examples of this kind can be found, and it seems clear that a given solvent may dissolve large amounts of solutes whose points lie well outside the larger ellipse of GSCRNEIDNER’Stype of diagram, and the claim that such diagrams may be used to predict solubility effects is hardly justifiable. In general it may be suggested that the elements divide themselves into two classes as regards the effect on solid solubility produced by what was originally called the “electrochemical factor”. The electronegative elements at the ends of the horizontal rows of the Periodic Table (e.g. As, Sb) tend to complete their atomic octets of electrons by the formation of stable compounds with metals. The resulting reduction in the extent of the primary solid solutions can be understood by the usual method of drawing tangents to free-energy~com~~~on curves. In such cases the compound usually becomes more stable as the constituent elements differ more in electrochemical characteristics, and the narrowing of the primary solid solutions can be readily understood as due to the combined effects of size-factor and electrochemical factor, and interpreted in terms of free-energy/composition diagrams. In alloys of other elements, systematic examination of equilibrium diagrams sometimes shows clear qualitative effects of the difference between the electrochemical J. Less-Com?non
Metals,
4
(I96Z)
390__392
392
LETTERS TO THE EDITORS
characteristics of the two metals. Thus, in the series Cu-Zn, Ag-Zn, Au-Zn, the electrochemical factor increases on passing from Cu -+ Au, and this passage results in an increasing tendency to form ordered (instead of disordered) body-centred cubic B-phases. It is, however, very difficult to express such effects quantitatively so that any one set of numerical values applies to a wide range of alloys. A standard electrode potential refers to a definite valency state which may not be that required for use with a particular alloy system. The Pauling electronegativities are rtot electrochemical constants, but are bond-energy terms expressing the difference between the energy of an A-B bond and the mean of A-A and B-B bonds. They have generally been determined from non-metallic compounds, and it is uncertain whether their exact values should be usedinthediscussionofalloystructures.Thediagramof GSCHNEIDNER makes it quite clear that an extreme difference (e.g. Au-Mg, Au-In, Ag-Li) between the electronegativity values does not preclude the formation of extensive solid solutions. W. HUME-ROTHERY Departme& of Metallurgy, University of Oxford (Great Britain) 1 D. S. EVANS AND G. V. RAYNOR, J. Less-Common Metals, 3 (1961) 179. 2 I<. A. GSCHNEIDNER, JR.,J. Less-Common Metals, 4 (1962) 107. 3 L. S. DARKEN AND R. W. GURRY, Physical Chemistry of Metals, McGraw-Hill New York, 1953, p. 86.
Book Co., Inc.,
J. Less-Common Metals, 4 (1962) 390-392
390
Letters to the Editors
The effect of electronegativity
EVANS
AND RAYNOR~
mutual
solid
GSCHNEIDNER~, diagram atomic
and the points The
beyond
by DARKEN
this
units,
“elements
solid solutions have
very
the outer
with
limited ellipses
2.40
,
2.20-
are surrounded
within
the solvent, solubilities,
axes
/
/
,
,
/
very
been
of a modified
8%
ellipse
ellipse outside
intermediate
,
,
,
,
axes
for Yb,
,
f
whilst
solvent,
the larger lying
(Fig.
I) the
2 and f
difference
are predicted
elements
by
of the type of
the electronegativities,
15%
that, for a given
to have
,
f
limited
discussed
form
of major
and
the larger
those lying
against
by ellipses f
the inner
and those
are predicted
exhibit
has recently
in terms
for Th. It is claimed
lying
and thorium fact
12 are plotted
and of minor
ellipse
this
AND GURRY 3. In GSCHNEIDNER’S~~~~~~~
for Th lies just outside
the outer
kind,
ytterbium and
number
for Th and Yb
point
that
the results
for co-ordination
electronegativity radii.
shown
miscibilities,
whointerprets
discussed radii
have
and liquid
on solid and liquid miscibility
that
for Yb
in a diagram
to form
ellipse
beyond
4
in atomic
are predicted
the inner
of
extensive to
but within
behaviours”.
,
,
,
,
,
,
,
-------THORIUM
0”
, ’
-YTTERBIUM
ZOOOB
c 5 1.20F 3 y I.WB _ c 0 5 1.40W
I.ZOl.OO0.20-
I
I
I
0.60
I
1.00
I
I.,
1.20
I
1.40
I
I\1
1.60
RADIUS,
Fig. I. DARKEN AND
GURRY
1
1.60
/
2.00
1/l
I
2.20
I
2.40
I
I
2.60
I
II
2.60
CN = 12 (A)
plot for ytterbium
and thorium metals. (Reproduced Common Metals, 4 (1962) 107.)
J. Less-Common
from J. Less-
Metals, 4 (1962) jgo-392
LETTERS TO THE EDfTORS
391
Examination of GSCWNEIDNER’S diagram makes it very doubtful whether this type of prediction has any justification. Since the minor axes of the ellipses are pro~~ional to the differences in atomic radii, they become smaller as the atomic radius decreases, i.e. as the ellipse moves to the left in Fig. I, whilst the major axes remain unaltered. Qualitative examination of Fig. I reveals the following characteristics : Solvent Au The points for Ag, Cu, Ni, Hg, Cd, Zn, In, and Mg lie well outside larger ellipses of the Gschneidner type. Continuous solid solutions are formed in the system Au-Ag, Au-Cu, and Au-Ni, and wide solid solutions in gold are formed by Hg, Cd, Zn, In and Mg. Solvent Ag The points for Au, Mg, and Li lie well outside the larger Gschneidner ellipse, whilst that for In lies slightly outside. All those elements form extensive solid solutions in Ag (Ag-Au continuous solid solutions). The system Ag-Li is of particular interest, since both elements are univalent, and there is no possibility of the electronegati~ty values referring to a different valency state. Solvent Ct4 The points for Au and In lie well outside the larger Gscheidner ellipse, whilst those for Sb and Be lie slightly outside. All these elements form solid solutions in copper (Cu- Au continuous solid solutions). So&vent Mg The point for Tl lies well outside the larger Gschneidner ellipse, whilst that for Cd lies just on this ellipse. Both metals dissolve freely in Mg (Mg-Cd continuous solid solution). Other examples of this kind can be found, and it seems clear that a given solvent may dissolve large amounts of solutes whose points lie well outside the larger ellipse of GSCRNEIDNER’Stype of diagram, and the claim that such diagrams may be used to predict solubility effects is hardly justifiable. In general it may be suggested that the elements divide themselves into two classes as regards the effect on solid solubility produced by what was originally called the “electrochemical factor”. The electronegative elements at the ends of the horizontal rows of the Periodic Table (e.g. As, Sb) tend to complete their atomic octets of electrons by the formation of stable compounds with metals. The resulting reduction in the extent of the primary solid solutions can be understood by the usual method of drawing tangents to free-energy~com~~~on curves. In such cases the compound usually becomes more stable as the constituent elements differ more in electrochemical characteristics, and the narrowing of the primary solid solutions can be readily understood as due to the combined effects of size-factor and electrochemical factor, and interpreted in terms of free-energy/composition diagrams. In alloys of other elements, systematic examination of equilibrium diagrams sometimes shows clear qualitative effects of the difference between the electrochemical J. Less-Com?non
Metals,
4
(I96Z)
390__392
392
LETTERS TO THE EDITORS
characteristics of the two metals. Thus, in the series Cu-Zn, Ag-Zn, Au-Zn, the electrochemical factor increases on passing from Cu -+ Au, and this passage results in an increasing tendency to form ordered (instead of disordered) body-centred cubic B-phases. It is, however, very difficult to express such effects quantitatively so that any one set of numerical values applies to a wide range of alloys. A standard electrode potential refers to a definite valency state which may not be that required for use with a particular alloy system. The Pauling electronegativities are rtot electrochemical constants, but are bond-energy terms expressing the difference between the energy of an A-B bond and the mean of A-A and B-B bonds. They have generally been determined from non-metallic compounds, and it is uncertain whether their exact values should be usedinthediscussionofalloystructures.Thediagramof GSCHNEIDNER makes it quite clear that an extreme difference (e.g. Au-Mg, Au-In, Ag-Li) between the electronegativity values does not preclude the formation of extensive solid solutions. W. HUME-ROTHERY Departme& of Metallurgy, University of Oxford (Great Britain) 1 D. S. EVANS AND G. V. RAYNOR, J. Less-Common Metals, 3 (1961) 179. 2 I<. A. GSCHNEIDNER, JR.,J. Less-Common Metals, 4 (1962) 107. 3 L. S. DARKEN AND R. W. GURRY, Physical Chemistry of Metals, McGraw-Hill New York, 1953, p. 86.
Book Co., Inc.,
J. Less-Common Metals, 4 (1962) 390-392