JOURNAL OF RESEARCH of the National Bureau of Standards-A. Physics and Chemistry Vol. 64A, No. 5, September- October 1960 Systems Silver Iodide-Sodium Iodide and Silver Iodide­ Potassium Iodide1 G . Burley and H. E. Kissinger (May 12, 1960) The phase relations for the systems AgI-NaI and AgI-h] have been delermined for the temperature range from room temperature to 685 0 C, using differential t hermal analy is techniques. The AgI-NaI syslem has a eutectic at 50 mole percent Nal and 38'10 C. The AgI-J';] system has euleclics at 20.8 and 28.5 mole percent ICI and 254 0 C and 244 0 C, re­ spectively. A compound of formula ICAgaI4 is formed wilh a congruent melting poinl of 2680 C. 1. Introduction were calibratcd by using the quartz inversion at 573 0 C as an cxternal standard. In addiLion, AgI During the last decade, silver iodide has been used has a reversible tran formation at 146 0 C. Tills extensively in weather modification attempts. Be­ Lransformation produces a well defined thermal arrest cause of the crys tallographic similarity, ilver iodide which was used as an internal standard. can serve as an epitaxiaJ nucleaLion site for tbe Ncar the eutectics an d n ear the end member com­ growth of ice crystals. This greatly decreases Lbe positions, melting was accompanicd by the char­ supercooling tendency of wH,ter vapor in Lhe atlllos­ acteristic endothermic DTA deHection. For inter­ phrre. mcdiaLe compositions, the beginning and end of On e technique of genera Ling a smoke of this sub­ mclting was indicated by a change in the slope of stance consisLs of di solving a mixture of AgI and an t hc basc linc. alkali iodide in acetone and feeding the resulting A lti gh-Lemperatul'e powder X-ray goniometer [4] solution into the nozzle of a burner [1 ).2 Under was used Lo give an approximaLe indieation of normal operaLing co nditions this gives particles rang­ melting points. Since t he pattern decreases in ing from 10- 2 to 10- 3 J.L in diameter, a nd produces intensity as the temperature is increased, the cxact about 1012 nuclei per second. temperature of disappearance is difficult to detcr­ Since both AgI a nd either N aI or KI arc in Lhe mine by this method. For this reason all results original soluLion and the solvent is presumably reported here are those obtained with the DTA evftporated in Lhe burner, the solid state relations of apparatus. The X -ray method was useful, how­ these constituent compounds are of in terest. Mason ever, in checking for possible pbase changes having [2] has reported that the product resulting from the ncgligible heat effects. combinaLion of AgI and KI had little relation to either of the original compounds. This invesLiga­ 3. Materials t ion of thc phase relationships in the systems AgI-NaI and AgI-KI was undertaken in order to The materials used were reagent grade chemicals. obtain information on equilibrium conditions in these systems. No special purification procedurcs were used. Chem­ ical analysis supplied by thc manufacturer showed a 2 . Apparatus ma}"i.mUl11 of 0.01 percent CI- and 0.0003 perccnt lOa for the KI and 0.02 percent CI- and 0.001 A differential thermal analysis (DTA) unit was percent lOa for the N aI. In order to minimize used to obtain most of the data. The apparatus has absorption of moisture from Lhe atmosphere all been described previously [3] . Specimen holders transfers from reagent to weighing bottles were done made of 5 mil pl atinum sheet in the form of tubular in a drybox through which a stream of dry air was cups 4 cm in depth and 1 cm in diameter 'were used. The bottom of each container included a closed re­ passed. The components were thoroughly mixed entr ant central tube, extending one-half the length and then immediately used as starting materials for of the cup, which fitted ovcr the thermocouple junc­ Lhe DTA. In a number of cases, the thoroughly tion . This arrangement served to contain the moltcn mixed reactants were melted, ground, mixed, and sample and to protect the thcrmocouples. Thermo­ rcmcltcd before using them for the DTA. The couples were made of 15 mil Pt-Pt 10 percent Rh result appeared to be identical wiLh thosc obtained wire. The refcrence material was aluminum oxide. The h eatin ~ rate of the furnace wa gen erally 5° Cimino TIle rate was reduced to 2° C/min to I 'rhis research was supported by the National Science Foundation Grant NSF-G52q2. resolve complex peak shapes. The t hermocouples 2 F igures in brackets indicate the literature references at the end of this paper. 403 Ll OUID III '" ~ 600 "l 600 LlOU ID ILl 500 - 500 a -A g l t L N OI + L w ~ 4 00 ~ __~~r-~r-~r-~~~ ____~ ____~ ___ '" 400 ::J ~ ~ w w '" '"a. a -A g I + L K 1 + L ~ 300 ::" ..~ 300 a-A g 1+ No l 200 - 200 a-Agl+KAg 3 '. 0----0-----0-------0--0----0----- - - .:. -- 100 100 /l- Ag l tNO I °OL---' ~--~2~O --~370 --740~-7,50~-6~0'-~7~0~'8~0~q9~0--~100 o L-__L- __~ ~L-~ __~ __~ __~ __~ __~ __~ MOLE PE RCENT No l a 10 20 30 40 50 60 70 80 90 100 MOL E PERCEN T KI F IGUHE 1. Phase diagram of the system AgT- Nal . F IGUIG 2. Phase diagram oJ the system A gI-[U . using mechanical mixtures, and it must be assumed that diffusion rates are high enough to assure rapId 5. Accuracy mixing and equilibrium. The reproducibility and accuracy of the composi­ tion of the starting materials is estima ted at ± 0.5 4 . Results mole per cent because of the hygroscopicity of the alkali iodides. The temperature of a transform a­ The phase diagram of the system AgI-NaI is tion arpears to be reproducible to ± 10 C in the given in figure 1 and that of the system AgI-KI III DTA apparatus. In many cases solidification in­ figure 2. volved some super cooling, so t.hat the melting Silver iodide has two low-temperature modifica­ temperatures were judged more reliable. The tem­ tions [5]. These are a face-centered cubic sphalerite perature of a eutectic, when far from the eutectic type ('Y) structure and a hexagonal wurtzite type composition, is subject to a somewhat greater error. (fJ) structure. Only the lat ter appears to be stable In general, the cumulative errors of composition, [6]. At 146 0 C these change reversibly to a body­ tempera ture measurement, and possible loss of centered cubic type (0: ) structure which persists to iodine are es timated to amount to no more than ± 1 the melting point at 555 0 C [7] . mole percent for each measured composition and ± 20 C for each measured temper ature. Interpo­ Sodium iodide has the face-centered cubic sodium lated values are subject to the additional errors chloride type structure. No phase changes occur to inherent in this procedure. the melting point at 662 0 C [7] . Potassium iodide has the same structure type as sodium iodide and no phase changes. I ts melting point is 685 0 [7] . 6. References The AgI-NaI system has a eutectic at 50 mole [1] B. Vo nnegut, J . Colloid Sci. 5, 37 (1 950). percen t N aI and a temperature of 384 0 C. No solid [2] B. J . Mason and J . H allett, Nature 177, 681 (1956). solution r egion was observed. No compound for­ [3 ] H . E. K issinger, J . R esearch N BS 57, 217 (1956) RP 2712. ma tion occurs. The AgI transformation at 1460 C [4] F. A. Mauer and L. H . Bolz, Wright Air D evelop. Center apparently persists across the en tire fi eld, although TR 55- 473 (1957). [5] N. H . K olkmeijel' and J . W. A. van H engel, Z. Kristall og. it is difficul t to observe at low AgI concentration. 88, 317 (1934) . The AgI-KI binary phase diagram shows t wo [6] G. Burley and H . F. M cMurdie, Abstract-Am. Cryst. eutectics and formation of a compound. The eu­ Assoc. Meeting, Wash., D. C. (Jan. 24- 27, 1960). ,: tectics are at 20.8 mole percen t KI and 254 0 C and [7] F. D . R ossini, D. D. Wagma n, "V. H . Evans, S. Levine, and 1. Ja ff e, Selected values of chemical t hermody­ at 28.5 mole percent KI and 244 0 C. A 3AgI .KI namic properties, N BS Cir. 500 (1952). compound is formed with a congruent melting poin t at 268 0 C. (Paper 64A5- 64) 404 .