Halide melting points

Organic molten salts such as tris-n-butyl-dodecylphosphonium halides (melting point below 40°C) have been used as reaction media for nucleophilic aromatic substitution of aryl tosylates by halide ions (Fry and Pienta, 1985). 

These molecules exist in the solid halides, explaining the low melting points of these halides, and also in the vapour phase at temperatures not too far above the boiling point. At higher temperatures, however, dissociation into trigonal planar monomers, analogous to the boron halides, occurs. 

The melting and boiling points of a series of similar covalent halides of a given element are found to increase from the fluoride to the iodide, i.e. as the molecular weight of the halide increases. Thus, the trihalides of phosphorus have melting points PF3 = 121.5 K. PCI3 = 161.2 K, PBrj = 233 K, PI3 = 334 K. 

Zincill) chloride. ZnCl2, is the only important halide—it is prepared by standard methods, but cannot be obtained directly by heating the hydrated salt. It has a crystal lattice in which each zinc is surrounded tetrahedrally by four chloride ions, but the low melting point and solubility in organic solvents indicate some covalent... 

Melting points and boiling points for some representa tive aryl halides are listed in Appendix 1... 

Lithium Halides. Lithium haHde stabiHty decreases with increasing atomic weight of the halogen atom. Hence, the solubiHty increases from the sparingly soluble Hthium fluoride to the very soluble bromide and iodide salts. The low melting points of Hthium haHdes are advantageous for fluxes in many appHcations. 

Cesium Halides. Cesium bromide, [7787-69-1], CsBr, mol wt 212.82, theoretical cesium content 62.45%, is a colorless crystalline soUd, having a melting point of 636°C, a specific gravity of 4433 kg/m, and a solubUity of 1.23 kg/L of water at 25°C. It is usuaUy prepared by neutrali2ing the carbonate or hydroxide with HBr, but it is also the primary product of the Dow process (25) for poUucite processing. 

The mobilities of ions in molten salts, as reflected in their electrical conductivities, are an order of magnitude larger than Arose in Are conesponding solids. A typical value for diffusion coefficient of cations in molten salts is about 5 X lO cm s which is about one hundred times higher Aran in the solid near the melting point. The diffusion coefficients of cation and anion appear to be about the same in Are alkali halides, wiAr the cation being about 30% higher tlrair Are anion in the carbonates and nitrates. 

Figure 4.3 Melting point and boiling point of alkali metal halides.

The saline hydrides are white, high-melting-point solids with crystal structures that resemble those of the corresponding halides. The alkali metal hydrides, for instance, have the rock-salt structure (Fig. 5.39). 

An abrupt change in properties in a series of compounds, as in the melting points or boiling points of halides, is often taken as indicating an abrupt change in bond type. Thus of the fluorides... 

For single crystals, the same method is applied, using an excess of metal and of halogen. After reaction, the mixture of the sulfide halide and the halide is heated to a temperature slightly above the melting point of the respective halide. Perfect, small crystals for X-ray determination are formed. The excess of the trihalide is removed by treatment with anhydrous alcohol 92, 93, 96). CeSI may also be prepared from the sulfides 68, 93) CeSCl is formed by reaction 4 92). 

In contrast to Li and Na, K cannot be produced by the electrolysis of molten halide mixtures. While the melting point of KCl (776°C) is lower than that of NaCl,... 

Salt-inclusion solids described herein were synthesized at high temperature (>500°C) in the presence of reactive alkali and alkaline-earth metal halide salt media. For single crystal growth, an extra amount of molten salt is used, typically 3 5 times by weight of oxides. The reaction mixtures were placed in a carbon-coated silica ampoule, which was then sealed under vacuum. The reaction temperature was typically set at 100-150 °C above the melting point of employed salt. As shown in the schematic drawing in Fig. 16.2, the corresponding metal oxides were first dissolved conceivably via decomposition because of cor-... 

Many electrochemical devices and plants (chemical power sources, electrolyzers, and others) contain electrolytes which are melts of various metal halides (particularly chlorides), also nitrates, carbonates, and certain other salts with melting points between 150 and 1500°C. The salt melts can be single- (neat) or multicomponent (i.e., consist of mixtures of several salts, for their lower melting points in the eutectic region). Melts are highly valuable as electrolytes, since processes can be realized in them at high temperatures that would be too slow at ordinary temperatures or which yield products that are unstable in aqueous solutions (e.g., electrolytic production of the alkali metals). 

In general, the electrolysis of a molten salt at inert electrodes produces the metal at the cathode, e.g., calcium from calcium chloride (melting point 774 °C). The anion is often a halide ion which, on discharge, yields the halogen, e.g., chlorine from calcium chloride. 

Megagraft 1000, the calcium phosphate bioceramic, is synthesized by chemical reaction between calcium and phosphate ion sources (6-9). This synthesis is done by taking the mixture of a calcium and a phosphate source and heating it to a temperature below the starting melting point for an extended period of time. The calcium source can be from calcium phosphates, calcium hydroxide, calcium halides,... 

Alternative equations of state may be derived from Eq. (15). For example, one can obtain an equation of state in terms of variables expressed relative to their values at the melting point. At the melting point, the reduced variables 7rm, rm, and 6m are universal constants. This is illustrated in Table I where rm is given for several salts. Except for the lithium halides, which are not expected to follow this development since the small radius of the lithium ion leads to anion-anion contacts, rm is reasonably... 

---