Fast ion conduction in βAg3SI1−x Br x solid solutionsR. B. Beeken and K. L. Menningen Citation: Journal of Applied Physics 66, 5340 (1989); doi: 10.1063/1.343726 View online: http://dx.doi.org/10.1063/1.343726 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/66/11?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Ferroelectricity and electromechanical coupling in (1 − x)AgNbO3-xNaNbO3 solid solutions Appl. Phys. Lett. 99, 012904 (2011); 10.1063/1.3609234 Simulations of the nucleation of AgBr from solution J. Chem. Phys. 113, 6276 (2000); 10.1063/1.1308517 Photochemistry of adsorbed molecules. XII. Photoinduced ion–molecule reactions at a metal surface forCH3X/RCl/Ag(111) (X=Br, I) J. Chem. Phys. 98, 5954 (1993); 10.1063/1.464889 Fast ion transport in silver halide solid solutions and multiphase systems Appl. Phys. Lett. 37, 757 (1980); 10.1063/1.92023 Determination of the Electronic Conductivity and the Transference Numbers of Ions and Electrons in Solid TlBr J. Chem. Phys. 36, 3101 (1962); 10.1063/1.1732435

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fast ion conduction in f3 .. AgaSI1 _ x Brx sand solutions R. 8. Beeken and K. l. Menningen Department of Physics and Astronomy. UniiJersity of Wisconsin-Steuens Point. Stevens Point. Wisconsin 54481

(Received 24 May 1989; accepted for publication 15 August 1989)

Polycrystalline solid solutions of Ag3 SJ I _ x Br x were prepared from the melt and characterized in the superionic f3 phase by x-ray diffraction and electrical conductivity. Alloy lattice parameters decrease linearly from x = 0,0 to x = 1,0 within experimental uncertainty, Room­temperature conductivity of Ag3SI is enhanced with the substitution of bromide for iodide while a corresponding, though smaller, effect is observed in Ag]SBr as iodide is substituted for bromide. The transition temperature separating the superionic f3 phase and the covalent r phase in these alloys attains a minimum at x = 0.80.

I. iNTRODUCTION

Ag3S1 and Ag3SBr are isostructural compounds at room temperature that exhibit high electrical conductivity owing to the rapid diffusion of mobile silver cations through a crystalline array of sulfide and halide anions. 1 These mate­rials, termed superionic conductors or fast ion conductors, belong to a large class of silver-based compounds with simi­lar properties.2 Their structural diversity with changing temperature accompanied by significant changes in ionic conductivity render them interesting from both a theoretical and a practical standpoint.

In the case of Ag3SI, three phases are experimentally observed. High-temperature a-Ag3S1 is stable above 508 K and crystallizes in a body-centered cubic lattice.} Lew-tem­perature r-Ag3S1 appears below 1 S9 K in a cubic phase with rhombohedral symmetry.4 Between these two temperatures, f3-Ag3S1 exists in the simple cubic structure.-'

The electrical transport properties of Ag3S1 have been investigated by conductivity,3·5-7 by infrared reflectivity, I and by x-ray diffraction4

,8,9 measurements. The following picture of ionic diffusion emerges from these studies. In the superionic a phase, sulfide and iodide anions are randomly distributed over the corners and center of each unit cube while highly mobile silver cations are distributed over four equivalent interstitial sites on each cube face. In the f3 phase, the anions are observed to order although the silver cations are still disordered. Thus, this arrangement is also super­ionic. The a-f3 transition is characterized by a discontinuous change in electrical conductivity by a factor of25 and a cor­responding change in lattice parameter of 1.5 %. The transi­tion to the r phase is much less pronounced as electrical conductivity changes by a factor of 6 and the lattice param­eter changes by just 0.1 %, In the r phase, silver cations are effectively pinned to interstitial sites on each cube face and a covalent configuration is realized.

The fast ion conductor Ag3SBr has not been character­ized as extensively as Ag3SI. There is sti.H some question as to the number of phases exhibited by Ag3SBr in the solid state. A first-order transition dividing the simple cubic f3 phase and the orthorhombic r phase clearly occurs at 126 K as established by conductivity,6 diffraction,4.10 and dielectric ' ! measurements. In addition, evidence fer a weak second-or-

der transition inf3-Ag3SBr at 161 K is provided by the con­ductivity and diffraction measurements. This second-order transition seems to be similar in character to the partial or­dering of silver cations observed in a study!2 of the super­ionic conductor RbAg4Is. An early investigation) of Ag3SBr at elevated temperatures concluded that decomposition oc­curs near 700 K before the appearance of an a phase in the material, but a more recent investigation7 suggests that, in fact, the a phase is produced at 703 K.

Interest in the electrical properties of Ag3SI1 x Br x sol­id solutions has been stimulated by recent investigations 13.14

of anion substituted AgI. Pure AgI is a fast ion conductor above 420 K, but its conductivity drops abruptly by four orders of magnitude upon cooling through this temperature into the covalent phase. As bromide is substituted for iodide in AgI, electrical conductivity in the covalent phase exhibits a substantial increase while the phase transition temperature decreases linearly with bromide concentration. Similar ef­fects are observed in pure AgI with the application of hydro­static pressure, 12,)5,16 Arecentstudy7 ofAg3SI 1 xBrx alloys at high temperature concluded that no enhancement of elec­trical conductivity in the /3 phase occurs as bromide is substi­tuted for iodide in Ag3S1 nor as iodide is substituted for bro­mide in Ag~SBr, but no measurements were obtained for these alloys at low temperature. The current investigation seeks to provide information on the electrical properties of these alloys as temperatures approach the fJ-r transition.

Ii. EXPERIMENTAL METHODS

Samples were prepared using high-purity AgI, AgBr, and Ag2S obtained from Johnson Matthey/AESAR. For each alloy, stoichiometric quantities of these compounds were sealed under vacuum in a quartz ampoule and melted together at 800°C to ensure homogeneous mixing before be­ing cooled slowly to ambient temperature. As each ampoule was opened, the sample material was removed, crushed to a fine powder, and pressed at 100 kpsi into two or three cylin­drical pellets 0.476 cm in diameter and approximately 0.30 em in length. These pellets were resealed under vacuum in individual quartz ampoules and annealed at 200 °C for 60 days to produce uniform solid solutions and remove lattice distortions caused by compression.

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X-ray diffraction patterns recorded on samples prior to the annealing period exhibited broad lines and also indicated the presence ofunreacted parent compounds as observed by other investigators.4

,6,9 Patterns recorded on annealed sam­ples were generally sharp and clear, confirming the simple cubic structure expected of each alloy at room temperature. Pure Ag3S1 and an aHoy of composition x = 0.05 were nota­ble exceptions to this trend. Curiously, the extended anneal­ing period rendered these two samples nearly amorphous and thus their data are not reported. Diffraction patterns for alloys of composition x;>O.lO were recorded with a Debye­Scherrer camera using copper Ka radiation. Lattice param­eters were averaged from the five most prominent diffraction lines in each photograph with a typical standard deviation of 0.005 A.

Electrical conductivity measurements were performed with a Hewlett Packard 4276A LCZ meter. Samples were mounted in a stainless steel cryostat under helium atmo­sphere adjacent to a Chromel-Alumel thermocouple that served to control and measure the sample temperature. An Omega Engineering 149 controller maintained temperature within 0.2°C while a Hewlett Packard 177 digital multi­meter determined the stabilized temperature with an abso­lute accuracy of 1.5 ·C throughout the investigated range.

One of the more pressing problems associated with ac conductivity measurements of Ag3S1 and related com­pounds is to ensure proper electrical contact between a sam­ple pellet and the impedance meter leads. Several different solutions to this problem have been reported in the litera­ture/,,7,12,I7,lR but most of these result in a frequency-depen­dent impedance that is analyzed in terms of a Cole-Cole plot 111 the complex impedance plane. However, the technique used in the current investigation to attach leads to the pellets of Ag3Sl j _ x Br x yielded frequency-independent impedances that were directly equivalent to the sample resistances.

First, the flat faces of a sample pellet were lightly sanded with fine garnet before the sample was rinsed in methanoL Then, a 3-cm-long Cll lead was attached to each face with conductive silver paste (thinned with acetone) that had been obtained from Materials for Electronics, Inc. After air dry­ing for 2 h, the pellet was placed in a quartz tube under continuous vacuum and heated above the 74°C ftash point of the silver paste. Finally, upon removal from vacuum, the sample leads were soldered to the LCZ meter leads.

With this procedure, typical room-temperature impe­dances were found to be frequency independent within 3% over the range of 100-20000 Hz. Corresponding phase an­gles were observed to be under 2° throughout this same range. Significant deviation from this pattern occurred in only three instances. First, if microscopic cracks developed in the dried silver contacts, measured impedance values were characterized by fluctuations, drift, and large phase angles. A simple reapplication of silver contacts would stabilize the readings. Second, attempts to measure impedances of the two amorphous samples always resulted in unstable, fre­quency-dependent values. Finally, low-temperature r-phase measurements of some samples occasionally became fre­quency dependent, though stable, as the impedance ap­proached the 20 Mfi reliability limit of the LCZ meter.

5341 J. Appl. Phys., Vol. 66, No. 11,1 December 1989

Since measured impedance values were found to be fre­quency independent throughout the f3 phase for samples with a composition parameter x;>O. ! 0, conductivities were determined at a single frequency of 1 kHz. With reapplica­tion of sample leads on several pellets, conductivity measure­ments were found to be reproducible within 10% on a given pellet. Measured conductivities of different pellets for a sin­gle composition, however, revealed deviations as high as 50%, which reflects the difficulty of preparing identical sol­id solutions even with a uniform sample preparation tech­nique. The measured conductivity of Ag3SBr agreed wen with previous measurements6

,7 throughout the temperature range employed in this investigation.

III. RESULTS AND DISCUSSION

Lattice parameters of Ag3SI l xBrx measured at room temperature are presented in Fig. 1. The broken line con­necting lattice parameters of Ag3S1 (from the diffractome­tric study of Ref. 9) and Ag3SBr represent Vegard's rule­the alloy lattice parameters expected from a simple hard sphere mixing of silver cations with appropriate proportions of sulfide, iodide, and bromide anions. In all alloys except the x = 0.30 sample, measured lattice parameters conform to Vegard's rule within experimental uncertainty. It is noted that solid solutions prepared for other investigations of Ag3SI j _ x Brx exhibit apparent deviations from Vegard's rule. 3,7

Room-temperature conductivity data for the series of alloys are presented in Fig. 2 including a value for pure Ag3S1 obtained from the recent, detailed measurements of Ref. 6. The error limits illustrated forthex = 0.50 solid solu­tion represent the standard deviation of several measure­ments made on three different pellets. This composition pro­duced the largest error limits observed in any of the reported alloy samples.

Although the electrical conductivities of alloys with in­termediate composition lie dose to a line connecting the con­ductivities of pure Ag3S1 and Ag3SBr, a significant enhance­ment in conductivity is observed for small x as bromide is substituted for iodide in Ag3SI. A similar, though less pro-

-< 4.90 .~g3S1 ,_.Br.

0.0 0.2 O. 4 O. 6 O. 8 1.0

COMposition Parameter. x

FIG. I. Room-temperature lattice parameter vs composition in Ag,SI, xBrx solid solutions. The lattice parameter of Ag,SI was taken from Ref. 9.

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.. ~ -Ie 0

o 0

I .::;

01 0 ! 0 L

::J -2.2 0 0 ..., IIJ. ___ ~ __ .... 0

0 L 0

OJ ---~ (l, ---~ E -2.8 QI

f-

E 0.0 0.2 0.4 0.6 0.8 1.0 0 0 Composition Parameter. " oc

FIG. 2. Common logarithm of room-temperature conductivity vs co~lposi­tioD in Ag3SI, xBr x solid solutions. The room-temperature conductivity of Ag3S1 was obtained from Ref. 6.

nounced, effect is observed for large x as iodide is substituted for bromide in Ag3SBr. These results do not agree with a previous investigation7 of Ag3SI I _ x Brx solid solutions. In that study, electrical conductivity of alloys with a composi­tion parameter O.20<;x<;O.80 exhibited a simple linear de­crease between Ag3S1 and Ag3SBr. Since no samples were reported in the ranges x < 0.20 or x> 0.80, any enhance~ent of conductivity for dilute alloys may have escaped detectlOn. A more plausible explanation for the observed discrepancies, however, may originate with the different techniques used to prepare samples. Alloys measured in the previous investiga­tion were formed by solid-state diffusion for 24 h at 450°C followed by a 25-day annealing period at 200°C-substan­tially shorter than the annealing period used in the current investi.gation. Uniform solid solutions may not have been formed in such a short time period. This could explain the broad temperature range reported for the a-fJ transitions as measured by differenti.al thermal analysis of those alloys.

The low-temperature dependence of electrical conduc­tivity for three representative Ag3SI I .. x Bfx samples is illus­trated by the Arrhenius plots in Fig. 3. Each plot is charac­terized by a sudden change in conductivity by approximately one order of magnitude at the f3-y transition and also exhib-

-2 -: E

-3 u I

.::; b

-4

Dl 0 -5

.J

-6

4 5 6 7 8 9

1000/T (K)-'

FIG. 3. Common logarithm of conductivity vs inverse temperature for three representative samples in the Ag,SJ 1 x Br, alloy series.

5342 J. Appl. Phys., Vol. 66, No. 11,1 December 1989

g 170

:II & Ag 3S I ,_, Br.

~ 150 ..., 0 C

L (Jj (L E 130 0 (II I- 0

C 0 0 0

110 0 0 ;J

0 .~

'" 90 t c

C L

f-

;" 0.0 0.2 0.4 0.6 O. 8 1.0

~ Composition ParametElr, x

FIG. 4. f3-y transition temperature vs composition for the Ag,SI,. ,BF, alloy series. The transition temperature of Ag,SI was obtained from Ref. 6.

its a slight upward curvature near room temperature as pre­viously observed in measurements6 of Ag3S1 and Ag3SBr. While significant differences in /3-phase conductivities are evident for alloys of Ag3SI] _ x Br x near room temperature, the magnitude of measured conductivities in the y phase were found to be remarkably similar. This is illustrated in the Arrhenius plots by extrapolation of the three curves beyond 1000/T = 9 but was also an observation made generally across the series of alloys. It must be noted, however, that the measured imnedances in this temperature range approached the reliability limit of the LCZ meter and thus should not be accorded high precision.

A comparison of the electrical conductivities of bro­mide-substituted AgI with bromide-substituted Ag3S1 re­veals a significant feature. A 10% anion substitution in AgI noticeably increases the covalent conductivity while leaving the superionic conductivity unaffected. 14 By contrast, simi­lar substitution in Ag3SI seems to enhance the superionic conductivity while leaving the covalent conductivity intact.

The transition temperature separating the superionic f3 phase from the covalent r phase in Ag3SI I _ x Brx alloys is dependent upon composition. This is illustrated in Fig. 4 where the reported transition temperatures are those at which deviations from /3-phase conductivity are initially ob­served for each alloy. The datum for Ag3S1 is taken from Ref. 6, Transition temperatures measured for Ag3SBr and Ag3Sln.50Bro.5o are in excellent agreement with previous in­vestigations. 6, J 9

In the composition range x<O.60, substitution of bro­mide for iodide in Ag3S1 is seen to result in a linear depres­sion of transition temperature by 0.85 K/mole %. In the composition range x>O.90, substitution of iodide for bro­mide in Ag3SBr results in a depression of transition tempera­ture by 1.7 K/mole %. The transition temperature achieves a minimum at x = 0.80. Bromide substitution in AgI had previously been reported to depress the superionic-covalent transition temperature by 1.9 K/mole %.14

IV. CONCLUSION

Variations in the measured properties of Ag3SI j xBrx alloys among different investigators illustrate the difficulty

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of prepari.ng uniform solid solutions with poiycrystal1ine samples. Although Ag3S1 has been prepared in single crystal form by the Bridgman method,I,8 Ag3SBr has not since it decomposes at high temperatures. Preparation of Ag3SI j _ x Br" single crystals may be equally difficult. It is dear that, for polycrystalline samples, extended annealing periods are necessary to achieve proper uniformity. In the case of Ag3SI, the annealing of finely divided powder is known to yield simple cubic polycrystalline material. 6 The annealing of compressed Ag3S1 pellets in this investigation produced an amorphous material. A reduction of the anneal­ing temperature to 150 ·C did not improve the sample quali­ty. However, the substitution of at least 10 mole % bromide for iodide in Ag3SI appeared to stabilize the crystalline structure under these conditions and resulted in uniform sol­id solutions as evidenced by x-ray diffraction and electrical conductivity.

The measured transport properties of chemically substi­tuted AgI led Shahi and Wagnerl4 to conclude that bromide substitution increases the population of interstitial defects in the AgI lattice and results in an enhanced covalent phase conductivity with an accompanying reduction in the tem­perature separating the superionic and covalent phases. This interpretation was based on a previously proposed mode120

of ionic conduction that related first-order superionic phase transitions to interstitial defects and the resulting strain fields that they induce. Recently, TaUon21 argued against this conclusion for AgI and related compounds citing ther­modynamic constraints and detailed conductivity measure­ments on AgI single crystals. 11

The electrical conductivity measurements of Ag3S1 I _ x Br x alloys in the present investigation do not ap­pear to support the defect model for phase transitions in either Ag3S1 or Ag3SBr. While anion substitution in each of these compounds does indeed act to depress the f3-y transi­tion temperature, no evidence was found for any significant

5343 J. Appl. Phys., Vol. 66, No. 11, 1 December 1989

enhancement of the covalent phase conductivity with anion substitution. However, the mere presence of substitute an­ions in each of these compounds seems to be a more impor­tant factor than changing lattice size for the enhancement of electrical conductivity in the superionic {J phase.

ACKNOWLEDGMENTS

This research was supported by a grant from Research Corporation. Financial support from the University ofWis­consin-Stevens Point Personnel Development Committee is also gratefully acknowledged.

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