109Ag chemical shifts in some solid compounds

JOURNAL

OF MAGNETIC

RESONANCE

64,

76-80 (1985)

logAg Chemical Shifts in Some Solid Compounds H. LOOSER*AND D. BRINKMANN Physik-lnstitut,

University of Ziirich. 8001 Zurich, Switzerland

Received March 13. 1985 The ‘09Ag chemical shift 15has been measured in the following solid compounds: (i) powder samples of Ag halides where 6 extends from 7 10 ppm (AgI) to - 110 ppm (AgF); (ii) single-crystal and powder samples of the superionic conductors RbAg& and KAg& where the shift is about 800 ppm at room temperature; anisotropies and temperature dependenciesare discussed,(iii) single crystals of the superionic conductor Ag2,&W40i6 with 580 ppm for the isotropic component at 288 K. o 1985 Academic PUSS. IIIC. INTRODUCTION

The importance of chemical-shift data in chemical structure analysis is well established. It is, however, less recognized that the chemical shift can play an important role in solid-state physics. For instance, in recent studies of superionic conductors in our laboratory we have found that in substances like RbAg& and KAg& (I), Ag2Ji8W40i6 (2), and AgsBrS (3) the d o m inant nuclear relaxation mechanism of the highly m o b ile Ag nuclei is fluctuating chemical shift. O n the other hand, the chemical-shift data found in the literature for the silver isotopes io7Ag and ‘09Ag are sparse (4) because these isotopes are difficult to observe experimentally. Both isotopes have spin i, thus relaxation can occur only by magnetic interactions which are relatively inefficient because of the small magnetic moments. For example, in a 1 m aqueous AgN03 solution without paramagnetic ions the spin-lattice relaxation tim e T, of ‘09Ag is 1000 s (5). In this paper we report lo9Ag chemical-shift measurements in some solid compounds including the silver halides where results for solutions are still lacking because of the insolubility of these compounds. EXPERIMENTAL

The silver halides we have investigated were 0.5 cm3 powder samples of LAB grade manufactured by E. Merck, Darmstadt. The RbAg& and KAg,& samples were either single crystals or powder specimens of about 0.35 cm3 obtained by solution growth technique (6). The Ag261i8W40i6single crystals had been grown by the Bridgeman technique. All measurements were performed in a superconducting magnet of 5.1 T field strength with a pulse spectrometer operating at about 10.2 * Present address: Laboratorium fur Festkiirpcrphysik, Swiss Federal Institute of Technology, 8093 Zurich, Switzerland. 0022-2364185 $3.00 Copyright Q 1985 by Academic Fwss, Inc. All rights of reproduction in any form reserved.

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SHIFTS IN SOLIDS

MHz. The Ag signals were digitized by a transient recorder and accumulated and Fourier transformed by an on-line computer. To keep the number of accumulations low special care was taken to optimally match and tune all electronic components. In the case of the silver halides 24 signals were summed up with a repetition time of 1 h yielding a signal-to-noise ratio of 3: 1. The Tr’s are estimated to be about 3 h. For the superionic conductors the repetition time was about T,. For RbAg& this means a range from 600 s at 300 K down to 3 s at 145 K. A S/N ratio of about 12: 1 was achieved by accumulating 16 signals. All shifts were measured at constant field with respect to the Ag resonance of an aqueous solution of 9.1 m AgN03 with 0.24 m Fe(NO& added to reduce the spinlattice relaxation time. This sample itself exhibits a -47 ppm shift with respect to the Larmor frequency of Ag+ ions in infinite dilution (7) which is the commonly accepted zero point on the 6 scale. All our results are given as frequency shifts with respect to this point. RESULTS AND DISCUSSION

Table 1 shows the room-temperature chemical-shift data of the silver compounds we have investigated. The data span about 900 ppm and fall in the range of values presently known for Ag. Silver Halides

The shifts in the silver halide powders exhibit the same sensitivity to halogen substitution as found in other metal halides (although these were mostly measured in solution). While the shielding for AgCl and AgBr is nearly the same, the Ag shift in AgI is larger than that in AgCl, that means 6(I) > 6(Cl). This type of shielding order has been designated in the literature as inverse halogen dependence (IHD); the normal halogen dependence (NHD) refers to 6(Cl) > 6(I). So far the reversal of the halogen dependence when one goes from one group to the other in the periodic table has not been explained (4). Among the transition metals, IHD seems to be restricted to SC,Ti, V, Nb, Cu, and Ag. TABLE 1 losAg Chemical-Shift Data Solids (this work) Compound

‘W AgCl AgBr Ati K.%ds RbAgJ, &26~18W4016

Solutions (for comparison) 6 (mm)

Compound

710 (15) 370 (15) 350 (IS) - 110 (15) 810 (IO) 790 (IO) 580 (10)

9.1 m AgN03 + 0.24 m FeWO& (7) Ag-oxyanions (7) Solutions in ethyl amine (8) Ais1 AgCl AgBr AgF

8 (pm-4 -47 0 to -50 790 495 585 430

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For the silver halides, IHD has already been discovered in Ag halides in 70% aqueous solution of ethyl amine (8) where practically the only silver complex to occur is [Ag(NH2CzH&]+. The 6 values of these shifts, which are listed in Table 1 for comparison, span about 360 ppm while the range for the Ag halide powders has the high value of 820 ppm which is mostly due to the remarkably low value of - 110 ppm for AgF. These results should be helpful in further attempts to explain the origin of the reversal of the halogen dependence. RbAg415 and kL4g415

These two isostructural compounds undergo two successive phase transitions at temperatures T,, (around 200 K) and Tc2 (around 130 K). The transition at T,, occurs within the superionic state of conduction separating the cubic high-temperature LYphase from the trigonal medium-temperature /3 phase; the trigonal y phase is low conducting (9). The high silver conductivity arises at least partly from the fact that the number of Ag sites exceeds the number of mobile Ag ions: in the (Yphase there are 16 Ag ions distributed over 56 sites. The high mobility of the silver ions in the LYand p phase has the same consequence as the rapid chemical exchange of nuclei in a liquid: only a single Ag resonance line is observed. In the (Y phase, the measured shift is a weighted average of the different chemical-shift tensors of the 56 Ag sites. Because of the cubic symmetry the shift should be isotropic as has been verified experimentally in single-crystal measurements. The room-temperature shifts (Table 1) are the same within the error limits for both compounds thus reflecting their isostructural property. In the p phase of RbAg& the shift is no longer isotropic. The anisotropy observed is compatible with the trigonal symmetry of this phase and reflects the distortion of the cubic phase: the symmetry axis of the averaged chemical-shift tensor lies parallel to the threefold [ 11 l] axis. The principal value 6,, of the anisotropic part of the chemical-shift tensor varies with temperature as shown in Fig. 1. At Tc2, the anisotropic part of the shift is about 10 ppm compared to 890 ppm for the isotropic

I

IO-

p"

Tc2

0

1' 120

1 150

w 200 T [Kl

FIG. 1. Temperature dependence shiA tensor in &RbAg&.

of the principal

value a,, of the anisotropic

part of the “‘Ag chemical-

SILVER

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SHIFTS

79

IN SOLIDS

part. On approaching T,, the anisotropy decreases and vanishes at the transition into the (Y phase. Although the anisotropy has not been measured in &KAg&, a similar behavior is to be expected because both compounds are isostructural and exhibit very similar physical properties such as ionic conductivity and diffusion. The isotropic contribution to the chemical shift is temperature dependent in both the a! and the /I phases as shown in Fig. 2. For both compounds the shift increases with decreasing temperature. Within experimental errors, the temperature coefficient of the shift depends on the respective phase only but is independent of the composition, it is -0.5 ppm/K in the (Y phase and -0.6 ppm/K in the /I phase. In heavy atoms the major contribution to the shielding constant arises from the paramagnetic term up which is often written in a semiempirical form as up = -B/ AE, where AE is an average electronic excitation energy and B is a factor involving angular momenta. It is tempting to attribute the temperature-dependent shift 6 to an energy gap which varies with temperature. Although a small electronic band gap in RbAg& has been measured by ultraviolet absorption (IO), it varies in the “wrong” direction: the gap decreases with increasing temperature and would, if it were responsible for the temperature dependence of 6, cause the shift to increase with increasing temperature. At the p - y transition in RbAg& and KAg& the Ag signal is lost presumably because of extremely long relaxation times and line broadening due to freezing in of the silver motion.

&dIs

w401c5

This compound, called AgWO for short, exhibits three phase transitions at temperatures 277, 247, and 199 K (II) which separates, similar to RbAg&, different phases denoted (Y, /3, y, and 6 whose ionic conductivities decrease with decreasing temperature (12). Since these phases have either monoclinic (cu,8) or triclinic (y, 6) symmetry the Ag chemical shift is expected to be anisotropic as has been verified experimentally for the (Y,/3, and y phases. 900

I

I

I

I

I 200

I 250

‘X, I 300

‘x

800

I 150

T [Kl FIG. 2. Temperature K&J5 (0).

dependence of the isotropic part of the ‘@‘Ag chemical shift in RbAg&

(X) and

80

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While a complete determination of the shift tensor has not been attempted, at 288 K a mean isotropic shift of 580 ppm with an anisotropy of the order of 30 ppm has been observed. As in RbAg&, this shift must be interpreted as a weighted average of the different chemical-shift tensors “seen” by the Ag ions during their rapid diffusion among the various inequivalent lattice sites. Since part of these sites have iodine neighbors only while the others have a mixed iodine-oxygen coordination the observed mean shift in AgWO lies between the +710 ppm value for Agl (this work) and the value for the AgO bond for which the range 0 to -50 ppm has been quoted (4). REFERENCES 1. H. LOOSER, D. BRINKMANN, M. MALI, AND J. ROOS, Solid State Ionics 5, 485 (1981). 2. H. LOOSER, Dissertation, University of Ziirich, 1983. 3. H. HUBER, M. MALI, J. Roos, AND D. BRINKMANN, Bulletin of the Swiss Physical Society,

March

1985. 4. R. G. K~DD, Nuclear shielding of the transition metals, in “Annual Reports on NMR Spectroscopy,” Vol. IOA, 1980. 5. J. KRONENBITTER, U. SCHWEIZER, AND A. SCHWENK, Z. Nuturfarsch. A 35, 319 (1980). 6. H. AREND, W. HUBER, AND W. FREUDENREICH, J. Cryst. Growth 46, 286 (1979). 7. C.-W. BURGES, R. KOSCHMIEDER, W. SAHM, AND S. SCHWENK, Z. Naturforsch. A 28, 1753 (1973). 8. K. JUCKER, W. SAHM, AND A. SCHWENK, Z. Naturforsch. A 31, 1532 (1976). 9. S. CELLER (Ed.), “Solid Electrolytes,” Springer-Verlag, Berlin, 1977. 10. R. S. BAUER AND B. A. HUBERMAN, Phys. Rev. B 13, 3344 (1976). Il. A. L. GREER, F. HABBBAL, J. F. SCOTT, AND T. TAKAHASHI, J. Chem. Phys. 73, 5833 (1980). 12. S. GELLER, S. A. WILBER, G. F. RUSE, R. J. AKRIDGE, AND A. TURKOVIC, Phys. Rev. B 21, 2506

(1980).