Bonding in silver-oxygen compounds from Ag L3 XANES spectroscopy

~

Solid State Communications, Vol. 81, No. 3, pp. 235-239, 1992. Printed in Great Britain.

0038-1098/9255.00+.00 Pergamon Press plc

BONDING IN SILVER-OXYGEN COMPOUNDS FROM Ag 1.,3 XANES SPECTROSCOPY P. Behrens Fakult~t fOr Chemic, Universit~t Konstanz, Postfach 5560, D-7750 Konstanz, F.R.G. (Received 6 November 1991 by T. P. Martin)

Compounds containing cations with a valence electron configuration that can formally be described as (n-1)dl°ns° (Cu +, Ag +, Au +, Hg2+) exhibit a variety of unusual structural properties, e.g. low coordination numbers and short cation-cation distances. The incorporation of d electrons from the closed d m sub-shell has been debated for a long time as a reason for this extraordinary behaviour. Ag ~ XANES spectra of oxidie silver(I) compounds exhibit a prominent peak at the rising edge. By comparison with the XANES spectrum of a Ag(HI) periodate complex this peak is assigned to a transition from initial 2p states to 4d final states, proving that in oxidic Ag(I) compounds the closed-shell configuration 4d ~° is broken. The intensity of the peak signalling unoccupied 4d states is related to the degree of covalent bonding in a compound. Low coordination numbers2 and/or the formation of clusters or atom pairs are also common in the chemistry of other ions with a valence-electron distribution that can formally be described by (n-1)dl°ns°: Cu +, Au +, Hg2+ as well as Pt~ in platinum complexes exhibiting (unbridged !) Pt°-Pt° bonds5.6'8qi. For compounds containing these species hybridization schemes involving (n-1)d, ns and np states have been forwarded, too, to explain the unusual structural properties6'S'~l; hybridization incorporating d orbitals from a filled d ]° sub-sbell is most probable for elements at the ends of the transition metal series, where the energy difference between (n-l)d and ns states is small due to the ineffective shielding of the increasing nuclear charge across the series by electrons in d orbitals. In the case of heavy atoms, especially Pt, Au and Hg, relativistic effects may play an important role ]2. Despite the various qualitative approaches towards hybridization involving titled d shells, reliable quantitative calculations of band structures and definite experimental evidence for this fact is scarce. Photoelectron spectroscopy gives indications for interactions between Ag 4d and O 2,o states in silver silicates and germanates ~3. The electron-density distribution around the d m ion Zn 2+ in (lgl-I4)3ZnCl5 is non-spherical, suggesting a configuration (3d: e42~-6)4s~ for the Zn 2+ in a tetrahedral environment of CI- ions 14. Formulating the electron configuration resulting from incorporation of d states in chemical bonding as (n-1)dt°~ns 6 implies the presence of unoccupied d states above the Fermi level. One of the means to study unoccupied electronic states is x-ray absorption spectroscopy (XAS). Especially the x-ray absorption near edge structure (XANES) is suitable to investigate bound (local) empty states and the partial density of states of unoccupied bands projected on the atom under study. Electrons are transferred from an initial core state to empty final

Introduction Silver(I) oxygen compounds exhibit a variety of puzzling properties. While the Ag+ ion may behave like an alkali cation in some of its compounds, e.g. Ag2SO4 or the spinel NaAgMoO4,t in many other eases it causes the appearance of special structures. This is due to two tendencies: 0 the preference for low coordination numbers, especially twofold2 as, e.g., in the oxide Ag20 (anti-cristobalite structure type3); note, for comparison, that Na + is coordinated by eight oxygen atoms in Na20 (anti-fluorite

type); i0 Ag + ions in ternary oxides tend to aggregate to pairs or duster-like agglomerates in spite of their common positive charge. Often, Ag-Ag distances similar to those observed in metallic silver are found. A great amount of structural details confirming this segregation tendency has been collected by Jansen4'5. Clearly, these features as well as the fact that many Ag + compounds are coloured, cannot be explained by the simple picture of a univalent Ag+ ion with a closed 4d m sub-shell and an unoccupied 5s orbital. The preference for low coordination numbers is of course related to a substantial amount of covalent bonding character, requiring highly directional orbitals created by hybridization. For the Ag + ion in linear twofold coordination as in Ag20, Orgel has proposed hybridization between the 4dz2 and the 5s orbitals6, leading to a valence electron distribution that can be described as 4dl°'55s ~. This approach was extended to incorporate also interactions with the 5Pz orbital2'6. Allowing the closed-shell configuration d ~° to interact in chemical bonding (and thus breaking a basic rule of chemistry) could also provide an explanation for the attractive forces between the (formally) positively charged Ag + ions. 235

236

BONDING IN SILVER-OXYGEN COMPOUNDS

states by x-ray photons. The method is element-specific and, due to the fact that the dipole-selection rule Al = -I-1 is obeyed, also sensitive to the symmetry of the final state. To investigate unoccupied Ag 4d states, measurements at the Ag L3 and I.2 edge, which correspond to transitions from initial states 2P3rz and 2pt n to final states with s and d symmetry, are most promising. Recently, a band structure has been computed for Ag20 by Czyzyk et al. 15, using the augmented spherical wave method. Their results show that the valence band of this binary oxide is basically built out of Ag 4d and O 2p states. Correspondingly, the partial DOS of unoccupied states contains d character. The results of the band structure calculation were used to construct theoretical XANES spectra for the L edges of Ag, leading to good agreement in the region of the edge. For the present work, the most important result is that peaks occurring at the/-,3 and the I-,2edge could be attributed to transitions from 2p to unoccupied Ag 4d states, giving direct proof for the incorporation of d electrons from a nominally closed d ~° subshell in chemical bonding. These peaks may be regarded as a fingerprint of incorporation of Ag 4d states in chemical bonding. A band structure calculation of Cu20, which also crystallizes in the anti-eristobalite structure, led to the same result, although no comparisons with experimental data were given tr. The purpose of this paper is to compare Ag L3 XANF,S spectra of AgO) oxide compounds to the Ag L3 XANES of a Ag0"ID complex and to support the assignment of the peak at the I-,3 edge to a 2p3/2-*4d transition; to show that such transitions also occur for other AgO) oxygen compounds, i.e., to show that incorporation of 4d electrons in chemical bonding is a common feature in AgO) oxide compounds, and to show that the intensity of these peaks is correlated to the degree of covalent bonding in a compound. Ag/-,2 and L~ XANES spectra will be analyzed in a forthcoming paper. Experimental Ag oxide compounds were purchased (Ag20: Merck 1503; AgNO3: Fluka 85228; Ag2CO3: Strem 93-4706) or prepared according to published procedures 0qaAgMoO4: ref. 1, Nas[Ag(IOsOI-I)z] ' 11 H20: ref. 17; AgrAIrSirO24: ref. 18). Samples were checked for phase purity by powder x-ray diffraction. For XAS measurements carefully ground powders were pressed to polyethylene pellets (spectroscopic grade polyethylene: Merck 7422). The amount of sample was adjusted to give edge jumps a t ~ of about 0.5 at the Ag L3 edge. Ag L edge x-ray absorption spectra were collected at the EXAFS-II station m at HASYLABIHamburg/F.R.G. using synchrotron radiation from the storage ring DORIS II operated at 5.301 GeV with currents between 40 and 20 mA during parasitic beam-time. The spectrometer is equipped with a focussing mirror covered with a Ni alloy and a Si(111) double crystal monochromator. The reflectivity cut-off of the mirror at about 7 keV and a detuning of the monochromator to about 50 % of the maximum

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intensity effectively served to reduce contributions from higher energy harmonics to the x-ray beam. Absorption spectra were obtained at room temperature in the transmission mode using ionization chambers to determine the intensifies of the x-ray beam before and behind the sample. Samples were placed in vacuo during the measurement in order to avoid absorption of the soft x-rays used in air. The edge region was scanned with a step-width of 0.25 eV, well below the estimated experimental resolution of ca. 0.6 eV. For calibration of the energy E, a Pd metal foil was scanned simultaneously with each sample, using the count rates of the second and a third ionization chamber. The energy scale was fixed by setting the inflection point of the rising Pd L2 edge to E = 3.3303 keV, giving a calibration error < 0.2 eV between different scans. Usual data reduction procedures were followed, applying a Victoreen-lypo fit for the slow decay of the absorption before the edge and normalizing the amplitude of the spectra to an absorption tad -- 1 at an energy about I00 eV above the edge. Results and Discussion Fig. 1 shows Ag L3 edge spectra of several oxidic silver compounds. The XANES of Nas[Ag(IOsOH)2] • 11 1-120, a periodate complex of Ag0TI), exhibits a strong

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E (keV) Figure 1. Full lines: Ag L3 XANES spectra of silver-oxygen compounds. Dotted lines: Lorentzians fitted to peak A, which is due to a 2P3n-~d transition.

BONDING IN SILVER-OXYGEN COMPOUNDS

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absorption maximum at an energy of 3.352 keV (in the following, all peaks occurring at this energy are designated as A, see Fig. 1). In the case of Aga+, this peak can unequivocally be ascribed to a transition from 2psr2 to empty 4d states; the presence of two holes in the 4d subshell (electron configuration 4de for Ag3+) and the fact that the transition is dipole-allowed explain the high intensity of this peak. No peak is expected to occur at that energy for Ag + ions with a closed d I° shell. However, all AgO) compounds - from Ag20 to AgNO3 - exhibit an A peak, though only with small (and varying) intensity. For Ag20, it is this peak that corresponds to unoccupied states with d character just above the Fermi level according to the band structure calculated by Czyzyk et al)s Thus, the assignment of peak A to a 2 ~ transition is justified on a theoretical Is and - by comparison to the XANES of Nas[Ag(IOsOH)2] ' 11 H20 - also on an exporimental basis. The fact, that all AgO) compounds exhibit a peak at this energy shows that incorporation of 4d electrons in chemical bonding is not restricted to Ag2O, but is a common feature of oxidic AgO) compounds. The intensities of the A peaks, i.e., of the 2p-~4d transition, vary for different compounds. Table 1 lists the coordination environment and the colour of the compounds under investigation. Small coordination numbers, short bond-lengths and the appearance of colour may be regarded as qualitative or semi-quantitative measures for the covalent part in bonding. The compounds under study may thus be ordered according to increasing covalency as follows: AgNO3

Ag6A16Si6024 < Ag2CO3 < Ag20.

= NaAgMoO 4 <

237

In AgNOa, the two crystallographically distinct Ag + ions both possess six oxygen neighbours. The irregular coordination polyhedra, large Ag-O distances and their wide range indicate that the structure is mainly determined by electrostatic forces2°. AgNO3 is thus rendered to be the most ionic of these compounds. N a A g M o O 4 possesses a spinel type structurewith Na + and Ag + ions occupying the octahedral sitesI. As no superstructure corresponding to an ordered distribution of these ions is observed I, it must be assumed that the octahedral sites are occupied statistically, i.e., Ag + must accept the same coordination environment as the "typical ion" Na +. Age[A16Si6024]is an inclusion compound with the sodallte framework. A comparison between its structure (as determined by a recent Rietveld refinement of x-ray powder data21) with that of the corresponding sodium sodatit@ Na6[AI6Si~O24] shows the influence of the covalent Ag-O bonding: Ag+ as well as Na + occupy positions in the six-ring windows of the aluminosilicate framework. However, whereas Na + is coordinated to six oxygen atoms (three at a distance of 2.56 A and three more at 2.69 A), Ag + prefers a lower coordination number, attracting three oxygen atoms to a short Ag-O distance of 2.35 ,/~ and leaving the other three oxygens non-bonded (dAs_O: 2.98 A). The difference in the coordination of Ag+ compared to that of Na + shows that a substantial amount of covalent Ag-O bonding is present in silver sodalite. In Ag2CO3, two oxygen atoms are located at a distance of 2.23 ,/~ from the Ag + ion, this short bond distance indicating a high degree of covalency. Two more oxygens, one at 2.44 A and one at 2.74 A, complete the coordination around Ag to a distorted tetrahedron23. Finally, black Ag20 with linear twofold coordination3 is the prototype example for covalent Ag-O compounds.

Table 1. Formal oxidation state, coordination number (c.n.), Ag-O distances (dAs-o in A), colour and intensiti~ /(/,3) of the XANES peak A ascribed to the 2par:~4d transition at the Ag La edge (arbitrary units) for the compounas under investigation. The reference for the structural details is also given. AgNO 3

NaAgMoO4

Ag~AlsSi~O24

Ag2COs

Ag20

Nas[Ag(IOsOH)z] • 11 H20

formal oxidation state

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III

c.n.

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ref.

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colour

white

white

off-white

yellow

black

orange-red

I(L3)

0.52

0.63

0.68

0.87

1.74

5.55

a) The only structural parameter of this spinel-type compound, the position of the oxygen atom, has not been determined. The bond-length is estimated from the values observed in Na2MoO 4 and Ag2MoO4 and from the unitcell edge of NaAgMoO4. b) No structural analysis is available for Nas[Ag(IOsOH)2] ' 11 H20; the values given have been determined on the similar compound Ks[Ag(IO~OI'I)2] • 8 H20 (ref. 24).

238

BONDING IN SILVER-OXYGEN COMPOUNDS i

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In conclusion, this work shows that incorporation of 4d states from the formally closed 4d I° shell in chemical bonding is a common feature of oxidic Ag + compounds. The intensity of the Ag /-,3 edge feature signalling unoccupied 4d states is proportional to the covalent part of the Ag-O-bonds. Furthea'more, this work demonstrates the use of XAS to investigate the unoccupied electronic states and to reveal important information about chemical bonding.

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E (keV) Figure 2. Fit for the Ag/-'3XANES spectrum of AgzCO3. Broken line: experimental data; full line: fit; dotted lines: single fit functions (a Lorentzian for peak A at 3.352 keV, symmetric and asymmetric Gaussians for the humps at 3.350 and 3.363 keV, rasp., a modified arctan function for the rise of the edge and a constant with a small negative value).

Acknowledgement. I like to thank Ms. S. AlSmann for preparation of some of the Ag compounds investigated, M. Fr6ba and K. Lochte for help during the measurements and W. Niemann and J. Felscbe for valuable discussion. Thanks are also due to HASYLAB, especially R. Frahm, for allocating beam-time.

1.

2. All L3 XANES spectra were fitted by a least-squaresminimization procedure using a Lorentzian line shape for peak A, a modified arctan function for the rise of the edge and Gaussian peaks to model other humps present in the spectra. A typical fit is shown in Fig. 2. The Lorentzinns of the A peaks for the different compounds as resulting from the fits are inscribed in Fig. 1 by dotted lines. Table 1 lists the intensities of these peaks defined as l(/q) = amplitude × FHWM (FHWM: full width at half maximum). It is obvious that this intensity grows with increasing covalency of the silver--oxygen bonds. This can be rationalized by the simple consideration, that for covalent bonds highly directional orbitals are needed which can be created by hybridization. In the case of AgO) compounds, this process obviously involves breaking the closed-shell d l° configuration. The relation between peak intensity and covalency allows the estimation of bond character in other silver compounds, also in amorphous ones. For example, the ~ edge spectra of ion-conducting AgI-Ag20-B203 glasses presented by Dalba et al. ~ indicate a high degree of covalent bonding in their samples. However, the interpretation of peak A as a fingerprint for linear twofold coordination, as provided by these authors, cannot be supported, as also other compounds with threeor fourfold coordination (Ag~AhiS~O24, Ag2CO3) exhibit a prominent peak at the rising edge. Considering these results, in should be mentioned that in the isolating or semi-conducting compounds studied here, core-hole-electron interactions might modify the electron distribution, so that the registrated features would correspond to an excited state of the Ag + ion. In line with Czyzyk et aL we argue that their ground-state band structure calculation of Ag20 is in good agreement with the measured XANES spectra Is, giving convincing evidence that ground-state properties are proved by Ag L XANES spectroscopy26. Furthermore, it is at least not obvious, how the relation between covalent bonding and intensity of the A peaks should prevail, if core-hole-electron interactions were important.

3. 4. 5. 6. 7.

8. 9.

10. 11. 12.

13. 14. 15.

16. 17. 18. 19.

References A. Rulmont, P. Tarte, G. Foumakoye, A.M. Fransolet, and J. Choisnet, J. Solid State Chem. 76, 18 (1988). D.M. Adams, Inorganic Solids, p. 142ff, Wiley, New York (1974). W.G. Wyckoff, Crystal Structures, vol. I, p. 331, Wiley, New York (1963). M. Jansen, J. Less-Convnon Metals 76, 285 (1980). M. Jansen, Angew. Chem. Int. Ed. Engl. 26, 1098 (1987). L.E. Orgel, J. Chem. Soc. 4186 (1958). F.A. Cotton and G. Wilkinson: Advanced Inorganic Chemistry, 4th ed., p. 969, Wiley, New York (1980). J.A. Tossell and D.L Vaughan, Inorg. Chem. 20, 3333 (1981). P.K. Mehrotra and R. Hoffmann, lnorg. Chem. 17, 2187 (1978); Y. Jiang, S. Alvarcz, and R. Hoffmann, Inorg. Chem. 24, 749 (1985); A. Dedieu and R. Hoffmann, J. Am. Chem. Soc. 100, 2974 (1978). C.X. Cui and M. Kertesz, Inorg. Chem. 29, 2568 (1990). H. Sehmidbaur, W. Graf, and G. Mfiller, Angew. Chem. Int. Ed. Engl. (1988). P. Pyykk6, Chem. Rev. 88,563 (1988); P. Pyykk6 and Y. Zhao, Angew. Chem. Int. Ed. Engl. 30, 604 (1991). K. Heidebrecht, M. Jansen, S. Krause, and A.M. Bradshaw, J. Solid State Chem. 89, 60 (1990). S. Ohba, K. Shiokawa, and Y. Saito, Acta Cryst. C43, 189 (1987). M.T. Czyzyk, R.A. de Groot, G. Dalba, P. Fornasini, A. Kisiel, F. Rocca, and E. Burattini, Phys. Rev. B39, 9831 (1989). P. Marksteiner, P. Blaha, and K. Schwarz, Z. Phys. B64, 119 (1986). A. Balikungeri, M. Pelletier and D. Monnier, lnorg. Chim. Acta 22, 7 (1977). S. Al~mann, P. Behrens, and J. Felscbe, to be published. W. Malzfeldt, W. Niemann, R. Haensel, and P.

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20. 21. 22. 23.

BONDING IN SILVER-OXYGEN COMPOUNDS

Rabe, Nucl. lnstrum. Meth. 208, 359 (1985) P. Meyer, A. Rimsky, and R. Chevalier, Acta Cryst. B34, 1457 (1978). P. Behrens, P.B. Kempa, S. Al~mann, J. Felsche, and Ch. Baerlocher, to be published. J. Felsehe, S. Luger, and Ch. Baerlocher, Zeolites 6, 367 (1986). R. Masse, J.C. Guitel, and A. Durif, Acta Cryst. !135, 1428 (1979); ibid. 2823 (erratum).

24. 25. 26.

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R. Masse and A. Simon, J. Solid State Chem. 44, 201 (1982). G. Dalba, P.Fomasini, F. Rocea, and E. Burattini, J. Phys. (Paris) 47, C8-749 (1986). It should however be noted that a feature similar to peak A occurs in the Cu L3 XANES of Cu20 and that it was attributed to a core exeiton: S.L. Hulbert, B.A. Bunker, F.C. Brown, and P. Pianetta, Phys. Rev. B30, 2120 (1984); compare, however, the band structure calculation for Cu20 (ref. 16).