Metal-ammonia solutions density

Doped semiconductors, expanded metals, metal-ammonia solutions and rare gas-metal films where the transition occurs because of change of donor concentration or density. 

What is the nature of the transition to the metallic state in a liquid containing a high density of excess electrons When the concentration of excess electrons in a liquid is gradually increased, a transition from behavior characteristic of a localized state to that characteristic of a delocalized state is observed—e.g., in concentrated metal-ammonia solutions and in metal-molten salt mixtures. 

The metallic nature of concentrated metal-ammonia solutions is usually called "well known." However, few detailed studies of this system have been aimed at correlating the properties of the solution with theories of the liquid metallic state. The role of the solvated electron in the metallic conduction processes is not yet established. Recent measurements of optical reflectivity and Hall coefficient provide direct determinations of electron density and mobility. Electronic properties of the solution, including electrical and thermal conductivities, Hall effect, thermoelectric power, and magnetic susceptibility, can be compared with recent models of the metallic state. 

The failure is not limited to metal-ammonia solutions nor to the linear Thomas-Fermi theory (19). The metals physicist has known for 30 years that the theory of electron interactions is unsatisfactory. E. Wigner showed in 1934 that a dilute electron gas (in the presence of a uniform positive charge density) would condense into an electron crystal wherein the electrons occupy the fixed positions of a lattice. Weaker correlations doubtless exist in the present case and have not been properly treated as yet. Studies on metal-ammonia solutions may help resolve this problem. But one or another form of this problem—the inadequate understanding of electron correlations—precludes any conclusive theoretical treatment of the conductivity in terms of, say, effective mass at present. The effective mass may be introduced to account for errors in the density of states—not in the electron correlations. 

A large volume expansion for solutions of sodium in ammonia was first reported by Kraus and Lucasse (17). Since this initial report, many investigations have been made of the volume expansion for a number of alkali metal-ammonia solutions. The techniques employed in these investigations have varied from density measurements for concentrated solutions using the Westphal Balance or Pycnometer to dilatometric studies for dilute solutions, which measure the volume expansion directly. 

The precise nature of the electronic interactions between centers must obviously change dramatically at the NM-M transition, e.g., from van der Waals type interaction to metallic cohesion (112). These gross changes in electronic properties at the transition are sufficient to noticeably influence the thermodynamic features of the system (86,87). The conditions therefore appear highly conducive for a thermodynamic phase transition to accompany the electronic transition at the critical density. In fact, the transition to the metallic state in metal-ammonia solutions is accompanied by a decrease in both enthalpy and entropy (146, 149), and it has been argued convincingly (124, 125) that the phase separation in supercritical alkali metals and metal solutions is... 

Since the first preparation of potassium-ammonia solution (Sir Humphrey Davy, in 1808) alkali metal-ammonia solutions have been at the centre of much theoretical and experimental interest. Novel properties include low density, high electrical conductivity, liquid-liquid phase separation, and a concentration driven metal-nonmetal transition [35]. 

It is apparent from Figs. 2.3 and 2.4 that there is a close correlation between the behavior of the density and that of the conductivity. As we discuss in later chapters, the density variation is the single most influential factor governing the MNM transition in mercury and the alkali metals. In this important respect the MNM trcmsition in fluid metals is similar to the continuous MNM transitions observed in other systems where the density or concentration of a metallic component can be varied. These include heavily doped semiconductors, metal-ammonia solutions, metal-nonmetal aggregates like mercury-xenon, molten metal-salt solutions, etc. (see, e.g., Edwards and Rao, 1985). 

Density and Volume Expansion. Kraus, Johnson, and collaborators have performed careful measurements of the densities of alkali-metal—ammonia solutions. All the measurements were carried out near the boiling point of liquid ammonia and for the concentration range from IM to saturation. They found the densities of the solutions for all three metals to be less than that of pure ammonia. It is more meaningful to consider the apparent expansion in volume per gram atom of metal dissolved which is obtained from the equation... 

Silvery-white lustrous metal face-centered cubic crystal structure ductile ferromagnetic density 8.908 g/cm at 20°C hardness 3.8 Mohs melts at 1,455°C vaporizes at 2,730°C electrical resistivity 6.97 microhm-cm at 20°C total emissivity 0.045, 0.060 and 0.190 erg/s.cm2 at 25, 100 and 1,000°C, respectively modulus of elasticity (tension) 206.0x10 MPa, modulus of elasticity (shear) 73.6x10 MPa Poisson s ratio 0.30 thermal neutron cross section (for neutron velocity of 2,200 m/s) absorption 4.5 barns, reaction cross section 17.5 barns insoluble in water dissolves in dilute nitric acid shghtly soluble in dilute HCl and H2SO4 insoluble in ammonia solution. Thermochemical Properties... 

Ohve green hexagonal crystals density 6.05 g/cm decomposes to platinum metal and chlorine on heating at 581°C insoluble in water and alcohol soluble in hydrochloric acid and ammonia solution. 

White crystalline powder sharp metallic taste orthorhombic structure refractive index 1.5452 density 4.20 g/cm very hygroscopic melts at 394°C vaporizes at 650°C highly soluble in water 447g/100 mL at 20°C aqueous solution acidic very soluble in alcohol, ether, and acetone soluble in alkali hydroxides and ammonia solution. 

If a solution containing approximately 4 mole percent sodium in ammonia is cooled below -42°C (231 K) a remarkable liquid-liquid phase separation occurs (33, 155). The solution physically separates into two distinct layers—a low-density, bronze metallic phase that floats out on top of a more dense, less concentrated dark-blue phase. The first experimental observation of this striking phenomenon in sodium-ammonia solutions was made by Kraus (109, 110) in 1907 more recent studies have mapped out the phase coexistence curves for a variety of alkali and alkaline earth metals in liquid ammonia, and these are delineated and discussed elsewhere (164). 

High density of excess electrons. Concentrated metal ammonia, metal-molten salt solutions and liquid metals exhibit a transition from a localized to the metallic state. 

Like the other alkah metals (45), lithium has appreciable solubiUty in Hquid ammonia. A saturated solution at —33.2° C contains 15.7 mol lithium in 1000 g of ammonia, and at 19°C has a density of 0.477, lower than that of any other known Hquid. Lithium reacts readily in Hquid ammonia to form... 

Iron, cobalt, and nickel catalyze this reaction. The rate depends on temperature and sodium concentration. At —33.5°C, 0.251 kg sodium is soluble in 1 kg ammonia. Concentrated solutions of sodium in ammonia separate into two Hquid phases when cooled below the consolute temperature of —41.6°C. The compositions of the phases depend on the temperature. At the peak of the conjugate solutions curve, the composition is 4.15 atom % sodium. The density decreases with increasing concentration of sodium. Thus, in the two-phase region the dilute bottom phase, low in sodium concentration, has a deep-blue color the light top phase, high in sodium concentration, has a metallic bronze appearance (9—13). 

The interpretation of these remarkable properties has excited considerable interest whilst there is still some uncertainty as to detail, it is now generally agreed that in dilute solution the alkali metals ionize to give a cation M+ and a quasi-free electron which is distributed over a cavity in the solvent of radius 300-340 pm formed by displacement of 2-3 NH3 molecules. This species has a broad absorption band extending into the infrared with a maximum at 1500nm and it is the short wavelength tail of this band which gives rise to the deep-blue colour of the solutions. The cavity model also interprets the fact that dissolution occurs with considerable expansion of volume so that the solutions have densities that are appreciably lower than that of liquid ammonia itself. The variation of properties with concentration can best be explained in terms of three equilibria between five solute species M, M2, M+, M and e ... 

Reduction of benzenoid hydrocarbons with solvated electrons generated by the solution of an alkali metal in liquid ammonia, the Birch reaction [34], involves homogeneous electron addition to the lowest unoccupied 7t-molecular orbital. Protonation of the radical-anion leads to a radical intermediate, which accepts a further electron. Protonation of the delocalised carbanion then occurs at the point of highest charge density and a non-conjugated cyclohexadiene 6 is formed by reduction of the benzene ring. An alcohol is usually added to the reaction mixture and acts as a proton source. The non-conjugated cyclohexadiene is stable in the presence of... 

Golden yellow, soft and ductile metal body-centered cubic structure density 1.93 g/cm melts at 28.44°C vaporizes at 671°C vapor pressure 1 torr at 280°C electrical resistivity 36.6 microhm-cm (at 30°C) reacts with water dissolves in liquid ammonia forming a blue solution. 

The M-NM transition has been a topic of interest from the days of Sir Humphry Davy when sodium and potassium were discovered till then only high-density elements such as Au, Ag and Cu with lustre and other related properties were known to be metallic. A variety of materials exhibit a transition from the nonmetallic to the metallic state because of a change in crystal structure, composition, temperature or pressure. While the majority of elements in nature are metallic, some of the elements which are ordinarily nonmetals become metallic on application of pressure or on melting accordingly, silicon is metallic in the liquid state and nonmetallic in the solid state. Metals such as Cs and Hg become nonmetallic when expanded to low densities at high temperatures. Solutions of alkali metals in liquid ammonia become metallic when the concentration of the alkali metal is sufficiently high. Alkali metal tungsten bronzes... 

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