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Encyclopedia of Physical Science and Technology EN008H-970 June 29, 2001 16:46

Liquid Alkali Metals 667

TABLE V Solubility of Carbon in Liquid Lithium and Sodium molecules being present in the liquid state. The sodium–
rubidium and sodium–cesium phase diagrams are sim-
Temperature Liquid lithium Liquid sodium
ilar to that for sodium–potassium, and the binary mix-
◦
C (at. ppm) (wt. ppm) (at. ppm) (wt. ppm)
◦
tures are liquid at temperatures down to −5 and −29 C,
200 2 3 0.38 × 10 −6 0.2 × 10 −6 respectively.
400 66 114 10.7 × 10 −2 5.6 × 10 −2 With mixtures of three of the alkali metals, the remark-
600 460 800 10.5 5.5 able feature is again the very low melting points that can
800 1550 2700 184 96 be achieved. Because of its immiscibility with the heavier
alkali metals, lithium is not suitable as a major compo-
nent of a lowmelting alloy. Of the other four metals, four
ternary systems are possible, and the lowest melting points
V. ALKALI METAL MIXTURES
that can be achieved are as follows:
The alkali metals resemble one another in atomic struc-
System Melting point ( C)
◦
ture and in the physical and chemical properties of the
bulk metals, and it might be expected that the properties Na–K–Rb −25
of mixtures would lie close to the mean of the separate Na–Rb–Cs −37
liquids. Some properties do, in fact, vary linearly with K–Rb–Cs −38
composition; they include density, viscosity, compress- Na–K–Cs −78
ibility, and specific heat. On the other hand, some proper-
ties of mixtures deviate widely from values that would be These are claimed to be the lowest melting points of
calculated additively from the properties of the separate any known metallic systems, and the alkali metals can
components. therefore be obtained as liquids at temperatures as low as
are normally available with organic solvents.
A. Miscibility
The heavier elements sodium to cesium are miscible with C. Other Properties
oneanotherinallproportionsintheliquidstate,andonlyin
Electrical resistivity measurement is the major technique
the case of mixtures with lithium does immiscibility arise.
used to study reactions in solution, so it is relevant to
Lithium–sodium mixtures are miscible in all proportions
note that the resistivity of any liquid alkali metal mixture
above 305 C, but below this temperature the mixture sep-
◦
is greater than that of either of the separate metals. The
◦
arates into two separate phases. At 171 C, the immiscibil-
extent of this excess resistivity varies remarkably with
ity gap extends from 10.1 to 97.0 at.% of lithium. Liquid
difference in atomic size of the components; the excess
lithium is even less miscible with the heavier alkali metals.
forsodium–lithiummixturesisverysmallindeed,whereas
When liquid lithium and liquid potassium are mixed, two
that for sodium–cesium mixtures is 10 times the resistivity
immiscible liquid phases are formed; at 300 C the lithium
◦
of sodium alone.
phase contains 0.43 at.% of potassium, and the potassium
The surface tension of a liquid alkali metal mixture
phase contains only 0.024 at.% of lithium. The miscibility
also shows pronounced deviation from the mean value.
of lithium with rubidium and cesium is negligible. −1 −3
The surface tension of liquid cesium (74 N m × 10 ),
−3
lies much below that for sodium (197 N m −1 × 10 ), and
B. Melting Points
since surface tension reflects the attraction between atoms
The sodium–potassium system has been the most fully in the liquid, the concentration of cesium in the surface
explored because at one time it seemed possible that of the liquid metal mixture should be much greater than
the eutectic mixture (NaK) might become the accepted in the bulk of the liquid. Consistent with this, the sur-
coolant for fast nuclear reactors. At the eutectic composi- face tension of pure sodium falls rapidly upon addition
tion, 67.8 at.% potassium, the mixture is liquid at temper- of cesium, and in the range 40–100% cesium the surface
atures down to −12.5 C. This low temperature relative tension is near to that for pure cesium. A similar behavior
◦
to the melting points of the two pure metals (97.8 and has been observed for sodium–potassium mixtures. Many
63.2 C, respectively) is remarkable and must be related chemical reactions of the liquid alkali metals and their
◦
to their different atomic sizes. An inflection in the phase mixtures involve reaction with gases, and care must then
diagram suggests the presence of a compound Na 2 K; be exercised in relating reaction rates with bulk composi-
presumably the atoms can pack into a solid structure tion when it is the surface composition that is the relevant
of this composition, and there is no evidence of Na 2 K factor.