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Unusual fluorides of silver at high oxidation states

Unusual fluorides of silver at high oxidation states. LECTURE VII. Why fluorides of Ag(2+) i Ag(3+)?. High–T C superonductivity in the hole–doped oxides of Cu 2+ ; Our previous theoretical predictions on large vibronic coupling in the systems built of hard acids and bases

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Unusual fluorides of silver at high oxidation states

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  1. Unusual fluorides of silverat high oxidation states LECTURE VII

  2. Why fluorides of Ag(2+) i Ag(3+)? • High–TC superonductivity in the hole–doped oxides of Cu2+; • Our previous theoretical predictions on large vibronic coupling in the systems built of hard acids and bases • Possibility of the „magic electronic state“ in the systems exhibiting avoided crossing between the „neutral“ and „ionic“ states Ag2+ ~ Cu2+ (d9) Ag3+ ~ Cu3+ (d8) F1– ~ O2– (s2p6) • Yellow Au(0) and compounds of Au(1+); • Increased acidity of – supposedly soft – Au(1+); • Existence of Au–1 (CsAu); • The M–H bond length is some 0.2 Å shorter in AuH than in AgH; • The highest oxidation state of Au is most probably (7+); • Au(2+) has significant tendency for disproportionation. Why not Au(2+)?

  3. Enormous oxidizing properties of Ag(2+) i Ag(3+) • Ag2+ is a very strong oxidizer. • Ag2+ solvated in anhydrous HF oxidizes Xe0 to Xe2+, C6F6 to C6F6+, and oxidizes CF3CF=CF2 quantitatively to CF3CF2CF3. • AgF2 oxidizes fullerene to C60F44 (AgF up to C60F18). • Ag2+ is unknown in oxides and chlorides. • 2. Ag3+ is an enormously strong oxidizer. • Compounds of Ag3+ are best obtained by use of F radicals. • AgF2+solvated in anhydrous HF oxidizes Kr0 to Kr2+, PtF6– to PtF6, and together with Ni4+ is the best oxidizer available to chemistry. • Compounds of Ag3+ easily evolve F2 upon heating. AgF3 is thermodynamically unstable. Cu3+ + O2– Cu2+ + O1– 2 O1–  O22– Ag3+ + F1– Ag2+ + F0 2 F0  F2 Ag2+ + F0 Ag3+ + F1–

  4. The Cu/O vs Ag/F analogy • energetic and spatial proximity of the Ag(4d) and F(2p) orbitals • significant covalency of the Ag–F bonds ! • electronegativity of Ag3+ is close to that of F itself!

  5. Binary fluorides of Ag • Ag2F – superconductivity green, inverse Cd(OH)2 structure • AgF1–x – defected structure yellow to yellow–brown • AgF – photography colorless, NaCl structure • AgF2–x – ??? • AgF2 – organic synthesis brown ferromagnetic, monoclinic CuF2 • [AgF+][AgF4–] – brown kinked 1D AgF+ chains • Ag1+[Ag3+F4] – metastable • [Ag2+][AgF4–]2 – red–brown unique ribbon structure • AgF3 – potent oxidizer brown red, unique helical AuF3 structure

  6. Ternary & higher fluorides of Ag • Ag(I): 2Ag2C2x AgF x 9AgNO3x H2O Isolated Ag2+ centers (i) [Ag2+][AuF4–]2; (ii) [Ag2+][MF6–]2, M=Bi, Sb, Ru, Nb, Ta; (iii) [Ag2+][MF6–], M=Ge, Sn, Pb, Ti, Zr, Hf, Rh, Pd, Pt, Mn, Cr; (iv) Ag3M2F14 & K3Ag2M4F23 M=Zr, Hf; (v) NaAgZr2F11. Infinite [AgF+] chains (straight or kinked) (i) [AgF+][MF4–], M=Au, B; (ii) [AgF+][MF6–], M=Bi, Sb, As, Au, Ir, Ru; (iii) [AgF+]2[AgF4–][MF6–], M=Au, Pt, Ru, Sb, As; (iv) [AgF+][M3M’3F19–], M=Cd, Ca, Hg, M’=Zr, Hf; (v) MAgM’F6, M=Cs, Rb, K, M’= Al, Ga, In, Tl, Sc, Fe, Co. • Ag(II): Infinite [AgF2] planes (i) MAgF3, M=Cs, Rb, K; (ii) M2AgF4, M=Cs, Rb, K, Na. Infinite [AgF3–] chains NaAgF3 Isolated [AgF42–] squares (i) MAgF4, M=Ba, Sr, Ca, Cd, Hg; (ii) Ba2AgF6. Isolated LS [AgF4–] squares, or HS octahedron (i) MAgF4, M=Cs, Rb, K, Na, Li, O2+, XeF5+; (ii) Cs2KAgF6. • Ag(III):

  7. Crystal structures Isolated Ag2+ centers: [Ag2+][SbF6–]2 Infinite [AgF+] chains: [AgF+][BF4–] Infinite [AgF2] planes: [KF][AgF2] Puckered [AgF2] planes: AgF2 Unique [AgF3] helix: AgF3 Isolated [AgF4–] squares: KAgF4

  8. Coordination environment of Ag(2+): Ag(1+): very long 2.47 Ag(3+): very short 1.89

  9. Electronic structure of several fluorides of Ag

  10. EF Ag(4d) 12% –0.28 eV 34% –0.72 eV 60% –2.02 eV contribution to the “ligand band”

  11. Conclusions from Theory • DFT computations confirm that the Ag(4d) and F(2p) orbitals exhibit significant energetic and spatial proximity, and they strongly mix with each other in higher fluorides of Ag • the Ag–F bonds are indeed significantly covalent in these compounds • highly untypical situation takes place in the fluorides of Ag3+: more 4d states go to the “ligand band” than to the “metal band” (avoided crossing) ! Hypothesis • properly hole– or electron–doped fluorides of Ag2+ may be high– temperature superconductors, similar to their copper–oxide analogues, if quasi–2D structure is provided, and if defect localization is avoided • self–doped fluorides of Ag2+ may exhibit metallic conductivity

  12. Encouragement When we showed him a draft of this paper, Prof. Bartlett described further his experimental search for superconductivity in Ag/F compound in a private communiation to us (August 2000): “You may be surprised to learn that I have been looking for a superconductor in the Ag/F system for the past 8 years because of observations that we made in 1992. Briefly, we noted that whenever we prepared a [AgF]+[MF6] salt and washed it with anhydrous HF, the magnetic susceptibility exhibited a sharp drop at 63 K, suggestive of a superconducting transition caused by an impurity. Since this anomaly (it looks like a Meissner effect) was independent of M = Sb, As, Au, I assumed that the impurity was a mixed oxidation-state AgII/AgIII fluoride. The material that exhibits the 63 K anomally, does not produce identifying lines in the X-ray diffraction pattern (the parent materials give sharp strong patterns). My surmise has therefore been that the quantity present is small (< 5%). This surmise is obviously not valid if the material is non-crystalline. This set in train a set of investigations (...). My first and still favoured guess was that the 63 K diamagnetic phenomenon was caused by an electron-oxidized AgF2 sheet-structure [ i.e. [AgF2]n+, n<1] intercalated (perhaps non-stoichiometrically) by [AgF4] species. I also allowed that [MF6] could be an intercalating species. It is my belief that some disorder in the placement of the anionic charges is necessary, if hole localization is to be avoided. (...) It was this set of thoughts that caused me to look at the oxidation of AgF2 with [O2]+ salts, unfortunately we only obtained the linearly coordinated [AgF]2[MF6][AgF4] salts. The [AgF]2[MF6][AgF4] salts do not show the anomaly until they are washed with anhydrous HF (i.e. solvolysed). We never obtained an intercalated sheet structure, like that of Au[AuF4]2Au[SbF6]2. It could be that an off-stoichiometry silver relative of the latter is the desired material.”

  13. Experiments Synthesis solid state & AHF/F2 Leicester /UK/ Ljubljana /Slovenia/ Birmingham /UK/ Magnetic susceptibility measurements /SQUID/ ESR XPS XRDP 19F NMR Microwave cavity perturbation ICP MASS Feedback for synthesis

  14. Core XPS

  15. Valence region XPS

  16. Theory vs experiment

  17. Microwave cavity perturbation FM insulating AgF2 PM insulating

  18. SQUID “BeAgF4” FM 0 SC • ESR • zero field signal • g2 signal

  19. XRDP

  20. Conclusions • observations of sudden drops in the magnetic susceptibility of a large number of samples in the BeAgF system • possible superconductivity – and the attendant Meissner–Ochsenfeld effect – at temperatures ranging from 8.5 K to 64 K • composition and structure of the phase(s) responsible for magnetic anomalies is unknown • Li[AgF3–] … [BeF2][AgF2] … [AgF+][BF4–] … ? Surface of AgF2 has been modified? • KAgF3 is metallic above 70 K What next? • identification of superconducting phase and synthesis of a pure compound + repeated XRDP and magnetic susceptibility measurements • electrical resistivity contact or non–contact measurements ? • high–pressure attempts to metallize fluorides of Ag2+, and mixed–valence fluorides of Ag2+/Ag3+ • Epitaxial growth of AgF2, molecular spacers, etc. …???

  21. Literature Review & theory: Grochala W, Hoffmann R, Real and Hypothetical Intermediate-Valence Fluoride AgII/AgIII and AgII/AgI Systems as Potential Superconductors, ANGEW CHEM INT ED ENGL 40 (15): 2743-2781 2001 Experiment: Grochala W, Edwards PP, Meissner–Ochsenfeld Superconducting Anomalies in the Be–Ag–F System, submitted to ANGEW CHEM INT ED ENGL

  22. Acknowledgements • Prof. Roald Hoffmann /Cornell, USA/ • Prof. Peter P. Edwards /Birmingham, UK/ • Prof. Neil Bartlett /Berkeley, USA/ • Prof. Evgenii Antipov /Moscow, Russia/ • Prof. Eric G. Hope & Prof. John Holloway /Leicester, UK/ • Prof. Boris Žemva & Dr. Zoran Mazej /Ljubljana, Slovenia/ • Prof. Russ G. Edgell /Oxford, UK/ • Dr. Simon Kitchin /Birmingham, UK/ • Dr. Adrian Porch /Cardiff, UK/ • Dr. Peter Kroll /Cornell, USA/ • Prof. Kevin Smith /Boston, USA/ • Prof. Andrew Harrison & Dr. Konstantin Kamenev /Edinburgh, UK/ • Prof. David Jefferson /Cambridge, UK/ • Prof. Miguel Moreno /Santander, Spain/ • Prof. Berndt G. Mueller /Giessen , Germany/ • My wife • The Cornell Theory Center (USA) /computational grant/ • The Daresbury Lab (UK) /experimental time at SCIENTA/ • The Cornell Center for Materials Research (USA) /DMR-9632275/ • The NSF (USA) /CHE 99-70089/ • The Royal Society (UK) /Postdoctoral and Research Fellowships/ • The Crescendum Est–Polonia Foundation (Poland)/Research Stipend/

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