Phase diagrams have been determined for the systems MI+AgI (M = Na, K, Rb, Cs, TI or NH4, MBrfAgBr (M = K, Rb or Cs) and MISCuI (M = K or Rb). A group of isomorphous compounds MAg& (M = K, Rb, NH4) with exceptionally high ionic conductivity has been identified. At 20°C the conductivity of polycrystalline RbAg& is 0124f0006 ohm-1 cm-1, i.e., higher than the ionic conductivity of any other known solid. At this temperature RbAg& is stable, but KAg415 and NHdg415 slowly disproportionate to AgI and K2M3 or (NH4)2A@3. The ternary system KI+ RbIfAgI contains a complete range of solid solutions (K,Rb) Ag415. Analogous bromides have not been found, but a compound KCu415 is stable between 257 and 332".
The phase diagram of the KIf AgI system has been redetermined. Two intermediate compounds have been identified. K2AgI3 is stable up to 130°C. At higher temperatures it undergoes a solid state disproportionation reaction to KAg& and Kz.IS is stable between its incongruent melting point (253°C) and 38°C. At lower temperatures it disproportionates to P-AgI and K2AgJ.3.A eutectic between K&& and KI occurs at a nominal composition of 29.5 mole % KI and 238°C.The compound KAg& has an exceptionally high ionic conductivity for a solid, reaching 0.31 ohm-1 cm-1 at the incongruent melting point. The current is carried by the Agf ions. Thermodynamic data derived from e.m.f. measurements indicate that the formation of KAg& from K2AgI3 and AgI is accompanied by an entropy gain, implying an unusually high degree of disorder in KAg& During work on the formation of double halides by solid state reactions, it became apparent that the KI + AgI system contained at least two intermediate compounds, and that one had an exceptionally high electrical conductivity, even at room temperature. The most recent phase diagram of the system, by Nakahara and Sakate,l showed continuous solid solution. Burley and Kissinger 2 considered that the system contained one intermediate compound KAg314 with a congruent melting point. Earlier papers by Rosstkowski,3 Palkin,4 and Dombrovskaya and Koloskova 5 reported a single compound KAg& with an incongruent melting point. Other double iodides, KAgI2 and K2AgI3, had been reported 6p79 8 as crystallizing from aqueous solutions and the existence of K2AgI3 had been confirmed by an X-ray structure determination by Brink and Kroese.9 The present paper describes a thorough investigation of the phase diagram, using thermal and differential thermal analysis, electrical conductivity measurements and X-ray powder photography. The properties of the compound with high electrical conductivity are discussed. EXPERIMENTAL MATERIALS B.D.H. A.R. KI (not less than 99.5 % purity) and B.D.H. reagent-grade AgI (not less than 98 % purity) were used.
The rates of reaction of hydroxyl radicals with ethylene, propane, propylene, methylacetylene, and ailene have been measured at room temperature in a discharge-flow system using electron spin resonance detection. The stoichiometries (n = A[OHJ/A[RJ) were obtained by mass spectrometric analysis of the reacted gas under similar, although not completely identical conditions. The primary rate constants for the C3-hydrocarbons obtained by combining the two are given as : OH + propane k6 = (5.0+ 1 . 0 ) ~ 10" cm3 mol-' s-'The values of n as well as the nature of the products provide some information on the mechanisms involved.A value of k5 = (1 0.3) x 1OI2 cm3 mol-I s-' was obtained for the reaction of OH + ethylene.
The crystal structure of MAg415 (M = K, Rb, NH4) is cubic with a = 11-2A, z = 4 and space group of the enantiomorphic pair P4r32 and P4332. The locations of the I-and M+ ions have been deduced from symmetry and packing considerations. The Ag+ ions do not occupy distinct single sites, but are distributed randomly over a number of interstitial positions. This model is consistent with theexpectionally high mobility of the Ag+ions and the highentropy offormation of thecompound.The absence of Na or Cs analogues is explained by the steric requirements of the proposed structure.The systems '* AgI+MI (M = K, Rb, NH4) contain two intermediate compounds, M2Ag13 and MAg41s. The compounds MAg415 are remarkable for an exceptionally high ionic conductivity. The compound RbAg41S has a conductivity of about 0.12 ohm-' cm-l at room temperature, which exceeds that of all other known solid ionic conductors at this temperature. The current is carried almost entirely by the migration of Ag+ ions. Attempts to determine the transport number of the alkali metal ions indicate a value of the order of lo-' at 130" and a lower value at room temperature. The transport number of the I-ion is even smaller.Although the structures of the compounds M2Ag13 have been determined by Brink and K r o e~e ,~ no previous investigation of the structure of the compounds MAg415 has been reported. However, other compounds with expectionally high ionic conductivity have been inve~tigated,~'~ and these compounds have in common an unusual structural feature. The high temperature (a) forms of AgI, Ag3SI and Ag,S have the anions arranged in a fixed b.c.c. lattice, but the cations do not occupy fixed positions. Instead, the cations are randomly distributed over a number of interstitial sites, and can drift relatively freely through the anion lattice, almost as if the cations were in a liquid state. (The conductivity l o of AgI falls when the a-phase melts.) This paper describes a determination of the structure of MAg41S using powder and single crystal X-ray techniques. The powder photographs of MAg,I, are extremely similar for M = K, Rb and NH4, so work was concentrated on the Rb compound, the others being only metastable at room temperature. With relatively few experimental data, it was possible to locate first the I-ions, and then the Rbf ions from a knowledge of the unit cell dimensions, space group and ionic radii, without recourse to more complex methods of structure analysis. EXPERIMENTAL RbAg415 was obtained by slow cooling of a melt containing 80 mole % AgI and 20 mole % RbI. Powder samples were obtained simply by crushing the solidified mass. Single crystals were obtained by searching among the small lumps produced when the solidified mass had been shattered by a sharp blow. Such crystals were about 0.2 mm diam. and of very irregular shape. Microscopic examination of the crystals between crossed polarizers showed extinction in all directions. This provided evidence for a cubic structure, but was of no assistance in aligning the crystal.
Concentration-time profiles have been measured for hydroxyl radicals generated by the shock-tube decomposition of hydrogen peroxide in the presence of a variety of additives.
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