Feh., 1963
MOLTEN SALTSYSTEM GALLIUM R ~ O N O I ~ I ) I D F ~ - GTRIIODIDE ’~LLI~~I
307
MOLTEN SALT SYSTEM GALLIUM MONOIODIDE-GALLIUM TRIIODIDE. I. DESSITIES ASD ELECTIlICAL CONDUCTANCES’ BY E. F’. RIEVLING? ~ N C. D E, ERICKSOS Ralph G . W r i g h t Laboratory, School of Chemistry, Rutgers, The State University, New Brunswick, New Jersey Received J u l y 26, 196.9 The density and electrical conductance of molten compositions in the GaI-Gaz14-GapIaphase system were investigated to determine the composit,ion of any complex ions and to examine the transition between an ionic salt (C;a2T,) and one that is molecular (GaJe). Molar volume evidence indicates the packing of gallium(1) ions in the quasi-lattice of GaJe to form GaJ4 to be more efficient than in the chloride and bromide systcnis. Expansion coefficient data indicate GaJ4 to be ionic and GazIe to be molecular. Molar conductance isotherms between GaIi.eand GaJs possess a maximum a t Gas14 ( A = 15.9 ohm-1 cni.2 at 212’) that is associated with an :tctivat,ion energy peak. This suggests the presence of the tetraiodogalhto ion (Gar4-) at the diiodide cornposition. A general increase of electrical conductance activation energies with the gallium content of the melts is indicative of st,rong coulombic forces in melt compositions close to the diiodide. The temperature dependence of activation encrgies probably is associated with constitutional changes within the melts. The low conductance of C:a216 ( A = 0.197 ohm-’ cm.2 a t 214’) suggests a molecular arrangement for the molten state. Large positive conductance deviations (20 to 50%) from linearity occur between Ga214and C;a,Is and are explained in terms of stoichiometry.
Introduction Recent rlectrical conductance, ~ i s c o s i t yand , ~ Raman studies4,sof molten Ga2CL and Ga2Hr4have indicated the probable existenre of tetrahalogallate anions. The evidence does not show, however, the range of composition over which these ions exist, nor is there very much spcrific information regarding the analogous iodides. The GaI-Ga214-Ga21a system was chosen for further study becausc much less was known about the properties of its liquid regions, becaiise gallium(1) apparently possessed its greatest stability in the iodide and becausc. a more extensive liquid us. composition range was available.* The system was also of value because it afforded the opportunity to study the transition from a molecular melt9 to one that was believed to be essentially ionic. Molar voliime deviations, expansion coefficients, molar conductances, and activation energies for electrical conduction can be related to the degree of ionic character and the possible presence of complex ioss.’O This paper reports the results of a density and electrical conductivity examination that has permitted partial description of the constitution of melts with compositions between GaIl.e and Ga216. Thermodynamic evidence for the presence of the tetraiodogallate ion has been obtained and is presented in part 11.” Experimental Density.--The dilatometers were construrted of 1’yrc.x glass with prccision bore (3.180 z!= 0.008 mm. in diameter) expansion chambers. Thry wcrc calibrated t o score marks (applied with a (1) This paper is based on a tlirsis presented by E. F. Riebling in partial fulfillment of the rcquirernonts for the Ph.1). degree, Rutgers, Tlie State University of New .Jrrsciy, June, 1961. (2) Itesearch a n d nerelopinent IXvision, Corning Glass Works, Corning, New York. (3) N. N. Greenwood and 1. .J. Worrall, J . C h e m . Sac., 1680 (1858). (4) L. A. Woodward, C , . Carton, and 11. L. Roberts. ibid., 3723 (1956). ( 5 ) 1,. A. Woodward, N. N. Greenwood, J. R. Hall, a n d I. J. Worrall, ibid., 1505 (11358). (6) J. I). Corbett and S. von Winbnsh, -1. Am. Chem. Sac., 77, 3964 (105.5). (7) J. D. Corbett, S. von R’inbush, and F. C. Albers, ?Did.,79, 3020 (1957). ( 8 ) J. I). Corbett and It. K. Rlchlullan, ibid., 77, 4217 (1955). (9) N. N. Greenwood and I. J. U’orrall, J . Inorg. Nucl. Chem., 3, 357 (1957). (10) €1. Bloom a n d J. O’M. Bockris. “Molten Electrolytes” in “Modrrn Aspects of ~lectrochemistry,” Vol. 2, ed. by J. O’RI.Bockris, Academic Press, New York. N. Y., 19AO. (11) E. F. Riebling and C. E. Eriekson, J . I’hya. Cliem., 67, 509 (1963).
lathe) with degassed mercury a t room temperature. Weighed amounts of gallium (Alcoa 99.99% reformed into sticks for convenience) and iodine crystals (Matheson, Coleman and Bell reagent grade), to produce about 8-10 g. of salt, were placed in :r reaction chamber for subsequent attachment to the upper end of each dilatomcter. Following a seal-off a t about 25 p pressure, the dilatorncters were placed in a cold furnace and gradually heated to 350 to 500°, where they reacted for several days. This procedure eliminated possible explosions caused by excessive iodine pressure or by generated heat. The molten salts were drained into the dilatometer chambers and the chambers mere sealed off at their upper ends. A 2-quart dewar flask, filled with a straight-chained polymwic siliconefluid (General Electric No. SF-1017) provided the uniform temperature environment (up to 270’) for both the dcnsity and conductance measurements. A small nichrome-wound tube furnace was used t o remelt the salt in a given dilatometer capillary so that it drained down over the crystals in the bulb. This prevented possible fracture of the dilatometer bulb because of expansion of the crystals as they were reheated. Molten salt levels were viewed through an illuminated viewing strip in the dewar flask and were measurcd to f 0 . 0 2 mm. with a rathetometer. The concave menisci (all salts wet the glass surfaces) were about 0.1 mm. high in the 3-mm. tubing. Levels were r a d to the top of the menisci because of the dark red color of the melts. Corrections were applied for the expansion of each dilatometer and for the slight composition change caused by the sublimation of iodine during the initial vacuum seal-off prior to reaction. The net amount of salt drained into the dilatometer chamber was determined by weighing. Total experimental errors (temperature, reading, and menisci) in the density determinations were of the order of ~ k O . l 7 , . Electrical Conductance.-The Conductance cells were constructed of Pyrex glass with 0.015-in. diameter tungsten wire electrodes sealed through capillary tubing. The electrode tips were cleaned by electrolysis in dilute sodium hydroxide solutions. 13ccausc of the wide range of conductance values to be dealt with, cell constants were fixed a t about 10 em.-’. This was a compromiscl btAtween the values required for ionic melts and molecular melts. T h e cell constants were determined with dilute standard potassium chloride solutions and conductance data of Kohlrausch. .4given cell constsnt differed between runs by less than 254. Resistances were mcmured with a standard Wheatstone bridge circuit. The 6-v., 1000 cycle signals and null points were detected with a set of earphones. Each measurement was repeated five times, with the mean value used to calculate the resistances. The standard deviation of the mean measured resistances varied from fO.276 for high resistances (of the order of 1000 ohms) to il.ci0h for low resistances (of the order of 100 ohms). Variation of melt resistances with frequency amounted to only a few per cent, thus indicating the absence of significant polarization eff ects . Known amounts of gallium and iodine were sealed into e w h conductance cell under vacuum and were treated as in the procc-
E. F. RIEBLISGAND C. E. EXICKSON
308 I
Vol. 67
TABLE I1
I
250
I
3.00
0
a
4
si I50
2.83 2.69
I
2 .o 2.5 MOLE RATIO V G A .
1.J
volume (cc.) isotherms as a function of melt composition.
Fig. I.-Molar
dure given for the density samples. Above 270", the reaction furnaw was used as an air bath, and conductances were measured with the help of calibrated leads placed through the door of the rcwtion furnace.
2.48
2.33 2.17
Results Thc molar volumes wcre calculated from thc expcrimental density equations givcn in Table I. For the purpose of calculating molar volumes from Jr,n
=
cc. X m.n-. -~ cc . -P g. g./m.m.
2.06 2.00
m.w.
(1)
the diiodidc and triiodidc wcre assumcd to bc dimcrsg hilc thc monoiodidc was assumed to be a monomcr. Figure 1 presents molar volume isotherms as a function of melt composition (expressed as mole ratio I/Ga). Thc molar volume gencrally incrcascs with the iodine content of the mclts while negative deviations from additivity occur for compositions bctwecn the diiodide and triiodidc.
1.88
1.78 1.63
'I' ("C.) range
a
b
185-222 222-284 284-352 352-400 180-223 223-285 285-400 160-172 172-216 216-285 285-400 143-173 173-2 16 216-350 144-160 169-214 214-375 144-172 172-225 225-350 150-227 227-350 150-21 I 21 1-279 279-350 150-1 72 172-215 2 15-266 26G-350 17 1-2 16 2 1 C-267 267-360 I !)9-223 223-272
3.032 2.193 1,340 -0.2112 2.519 2.055 1.152 3.530 3.012 2.531 0.!)272 4.016 3.385 2.923 4.084 3.467 2.922 4.398 3.711 2.746 4.275 2.942 3.887 3.292 2.460 4.854 3.822 3.331 2.442 4.392 3.699 2.891 4.526 3.777
1.041 0.6244 0.1476 -0.8226 1.083 0.8537 0.3513 1.422 1.191 0,9562 0.05936 1.552 1.269 1.044 1.529 1.257 0.9!)23 1.640 1.335 0.8567 1.622 0.9548 1.401 1.114 0,6534 1.827 1.368 1.129 0.6504 1,663 1.334 0.8977 1.783 1.410
A&A* (kcal.)
5.3 3.2 1.2
.. 5.3 4.2 2.1 6.7 5.7 4.7 0.68 7 .3 6.0 5 .0 7.2 6.0 4.8 7.7 6.3 4.2 7.6 4.6 6.6 5. 3 3 .3 8.7 6.4 5.4 3.3 7.8 G.3 4.4 8.3 6.7
Interpolated x and p values mere used to calculate the A (ohm-' cm.z) isotherms which are depicted as a I )ES S I T Y E Q L:ATIONS FOR MOLTENMIXTGRESI N THE SYSTEX: function of composition in Fig. 2. A significant feature GaI-CruJG of the isotherms is the presencc of a maximum at the p = a - by' ( O K . ) = g./cc. diiodide composition. Figurc 3 dcpicts the composition dcpendencc of several hEA* isotherms plotted on a rela b X 10% Temp. r3nge ("C.) I/(;& ratio ative energy basis. Thc activation cncrgics are sharply 4.778 2.377 185-258 3 00 4.805 2.179 193-255 dependent upon tempcrature in an invcrsc manncr, cx2.80 1.996 175-225 4.819 2.60 ccpt for molten GazIaabove 350'. TABLE I
2.20 2.00 1.85 1.70 1.62
4.817 4.841 4.886 4.957 4.971
1.766 I ,688 1.675 1.704 1.707
175-255 181-265 181-265 181-265 189-265
The melt expansion coefficients (proportional to b in Table I) were found to decreasc with an increase of the gallium content. The specific conductance values x (ohm-' cm.-l> w r e calculated from the mcasured resistances (ohms) and the cell constants (G) with thc aid of x = G/ohms (2) A least squarcs treatmcnt was used to obtain equations for log x as a function of l /'r' (OK.) for the regions of constant slopc. The constants for these equations, as a fi,nction of composition, arc givcn in Table 11.
Discussion Molar Volumes and Expansion Coefficients.-The decrease of melt expansion coefficients, as gallium(I) is added to Ga21a,is an indication of the ionic character of the molten diiodide. Ionic melts usually possess smaller expansion coefficients than molecular melts brcause of the stronger coulombic-force fields which tend to restrict thermal expansion.12 The magnitudes of the expansion coefficients for thc molten diiodidc and triiodide are similar to those for the respective gallium chlorides and bromide^,^^'^^'^ thc structures of which arc known with a greater degrcc of certainty. (12) Refrrcnce 10, p. 202. (13) N. N. Greenwood and K. Wade, J . Inorp. Nucl. Chern., 8 , 349 (1937). (14) N. N. Greenwood und I. J. Wormll r b d , 6, 34 (19581.
Feb., 1963
nfOLTEN SALT SYSTEM
GALLIUM MONOIODIDE-GALLIUM TRIIODIDE
The negative deviation of molar volume isotherms represents a 3.5% decrease from linear behavior a t 270' for a composition of GaIz.e in the GazIrGazIG system. This could arise from a packing effect in which the small gallium(1) ions enter into interstitial holes in the liquid Gaz16qua+lattice. The molar volume of the diiodide is 4.47, less than two-thirds that of the triiodide a t 60' above their respective melting points, and -2.97, less a t their melting points. The corresponding factors for the bromides and chlorides, as calculated from Greenwood's datal3# 13*14 are -1.7, -0.2, and -0.2, +1.4y0,respectively. Apparently, the packing of gallium(1) ions in the molten trihalide lattice, to produce an ionic dihalide, is most efficient in the iodide system. The fact that the discrepancies all increase in a negative direction with temperature is a reflection of the larger expansion coefficients of the trihalides compared to the dihalides. The observed molar volume hump (Fig. I) a t the diiodide composition appears to be caused by the arbitrary choice of a monomeric monoiodide. Other sta,tements12 imply that a molar volume hump can be associated with a maximum concentration of bulky complex ions such as tetraiodogallate. Calculation of molar volume isotherms for the Ga11.6-Gaz14region on the basis of a, dimeric monoiodide serves to increase the molar volumes by about 15% a t GaI1.62, 12% a t GaII.,O, and 77, at GaIl.ss a,nd effectively eliminates the hump a t the diiodide. However, assumption of a dimeric monoiodide mould imply Ga+ and Garz- ions, for which there seems to be little other evidence. Conductance of GaJs and Ga214.--Molten Caz16 possesses two unusual conductance properties that were not found for other compositions in the GaI-Ga& system. (1) A plot of log x us. l / T (OK.) for GazIs reverses slope at 350". This results in the broad conductance maximum. ( 2 ) The conductance of the freshly melted GazIs depends somewhat on time. The extent of this time dependence is related to temperature. The first of these could be caused by a significant change in the mobility or number of charge carriers or possibly the occurrence of electronic conduction. The time dependence of the conductance of freshly melted Ga& probably is associated with one or more rate-controlled processes during melting. Thus, a t 214', the initial conductance increase, from 0.00010 to 0.00088 ohm-' cm:-l, occurs during the lengthy isothermal melting process (about 1 hr. for 8 g.) and then is partially offset by a decrease during the next 24 hr. to a constant value of 0.00080 ohm-1 cm.-l. This final value also can be achieved in a short time by rapidly heating the freshly melted salt to 235' and quenching to 214'. Greenwoods has reported a conductance of 0.0001 ohm-l cm.-' for the first liquid to form from tlie crystals a t the melting point. The molar conductance for Ga& at its melting point of 212' (15.9 ohm-l cm.z) is indicative of a highly ionic melt when contrasted with GazIe (0.197 ohm-l cm.2
