Why fluorides of Ag(2+) i Ag(3+)? 1.Yellow Au(0) and compounds of Au(1+); 2.Increased acidity of – supposedly soft – Au(1+); 3.Existence of Au –1 (CsAu); 4.The M–H bond length is some 0.2 Å shorter in AuH than in AgH; 5.The highest oxidation state of Au is most probably (7+); 6.Au(2+) has significant tendency for disproportionation. Ag 2+ ~ Cu 2+ (d 9 ) Ag 3+ ~ Cu 3+ (d 8 ) F 1– ~ O 2– (s 2 p 6 ) 1.High–T C superonductivity in the hole–doped oxides of Cu 2+ ; 2.Our previous theoretical predictions on large vibronic coupling in the systems built of hard acids and bases 3.Possibility of the magic electronic state in the systems exhibiting avoided crossing between the neutral and ionic states Why not Au(2+)?
1.Ag 2+ is a very strong oxidizer. A)Ag 2+ solvated in anhydrous HF oxidizes Xe 0 to Xe 2+, C 6 F 6 to C 6 F 6 +, and oxidizes CF 3 CF=CF 2 quantitatively to CF 3 CF 2 CF 3. B)AgF 2 oxidizes fullerene to C 60 F 44 (AgF up to C 60 F 18 ). C)Ag 2+ is unknown in oxides and chlorides. 2. Ag 3+ is an enormously strong oxidizer. A)Compounds of Ag 3+ are best obtained by use of F radicals. B)AgF 2 + solvated in anhydrous HF oxidizes Kr 0 to Kr 2+, PtF 6 – to PtF 6, and together with Ni 4+ is the best oxidizer available to chemistry. C)Compounds of Ag 3+ easily evolve F 2 upon heating. AgF 3 is thermodynamically unstable. Enormous oxidizing properties of Ag(2+) i Ag(3+) Cu 3+ + O 2– Cu 2+ + O 1– 2 O 1– O 2 2– Ag 3+ + F 1– Ag 2+ + F 0 2 F 0 F 2 Ag 3+ + F 1– Ag 2+ + F 0
energetic and spatial proximity of the Ag(4d) and F(2p) orbitals significant covalency of the Ag–F bonds ! electronegativity of Ag 3+ is close to that of F itself ! The Cu/O vs Ag/F analogy
Binary fluorides of Ag AgF – photography colorless, NaCl structure AgF 3 – potent oxidizer brown red, unique helical AuF 3 structure AgF 2 – organic synthesis brown ferromagnetic, monoclinic CuF 2 Ag 2 F – superconductivity green, inverse Cd(OH) 2 structure Ag 1+ [Ag 3+ F 4 ] – metastable [Ag 2+ ][AgF 4 – ] 2 – red–brown unique ribbon structure [AgF + ][AgF 4 – ] – brown kinked 1D AgF + chains AgF 2–x – ??? AgF 1–x – defected structure yellow to yellow–brown
Ternary & higher fluorides of Ag Ag(I): Ag(II): Ag(III): 2 Ag 2 C 2 x AgF x 9 AgNO 3 x H 2 O Isolated Ag 2+ centers (i) [Ag 2+ ][AuF 4 – ] 2 ; (ii) [Ag 2+ ][MF 6 – ] 2, M=Bi, Sb, Ru, Nb, Ta; (iii) [Ag 2+ ][MF 6 – ], M=Ge, Sn, Pb, Ti, Zr, Hf, Rh, Pd, Pt, Mn, Cr; (iv) Ag 3 M 2 F 14 & K 3 Ag 2 M 4 F 23 M=Zr, Hf; (v) NaAgZr 2 F 11. Infinite [AgF + ] chains (straight or kinked) (i) [AgF + ][MF 4 – ], M=Au, B; (ii) [AgF + ][MF 6 – ], M=Bi, Sb, As, Au, Ir, Ru; (iii) [AgF + ] 2 [AgF 4 – ][MF 6 – ], M=Au, Pt, Ru, Sb, As; (iv) [AgF + ][M 3 M 3 F 19 – ], M=Cd, Ca, Hg, M=Zr, Hf; (v) MAgMF 6, M=Cs, Rb, K, M= Al, Ga, In, Tl, Sc, Fe, Co. Infinite [AgF 2 ] planes (i) MAgF 3, M=Cs, Rb, K; (ii) M 2 AgF 4, M=Cs, Rb, K, Na. Infinite [AgF 3 – ] chains NaAgF 3 Isolated [AgF 4 2– ] squares (i) MAgF 4, M=Ba, Sr, Ca, Cd, Hg; (ii) Ba 2 AgF 6. Isolated LS [AgF 4 – ] squares, or HS octahedron (i) MAgF 4, M=Cs, Rb, K, Na, Li, O 2 +, XeF 5 + ; (ii) Cs 2 KAgF 6.
Ag(3+): very short 1.89 Coordination environment of Ag(2+): Ag(1+): very long 2.47
Electronic structure of several fluorides of Ag
–0.28 eV EFEF –0.72 eV –2.02 eV 12% Ag(4d) 34% 60% contribution to the ligand band
Hypothesis 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 Ag 3+ : more 4d states go to the ligand band than to the metal band (avoided crossing) ! Conclusions from Theory properly hole– or electron–doped fluorides of Ag 2+ 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 Ag 2+ may exhibit metallic conductivity
Encouragement 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] + [MF 6 ] 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 Ag II /Ag III 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 AgF 2 sheet-structure [ i.e. [AgF 2 ] n+, n<1] intercalated (perhaps non- stoichiometrically) by [AgF 4 ] species. I also allowed that [MF 6 ] 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 AgF 2 with [O 2 ] + salts, unfortunately we only obtained the linearly coordinated [AgF] 2 [MF 6 ][AgF 4 ] salts. The [AgF] 2 [MF 6 ][AgF 4 ] 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[AuF 4 ] 2 Au[SbF 6 ] 2. It could be that an off-stoichiometry silver relative of the latter is the desired material. 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):
Experiments Birmingham /UK/ Synthesis solid state & AHF/F 2 Magnetic susceptibility measurements /SQUID/ Leicester /UK/ Ljubljana /Slovenia/ XRDP 19 F NMRMicrowave cavity perturbationXPS Feedback for synthesis ESR ICP MASS
Conclusions 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 Ag 2+, and mixed–valence fluorides of Ag 2+ /Ag 3+ Epitaxial growth of AgF 2, molecular spacers, etc. …??? 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[AgF 3 – ] … [BeF 2 ][AgF 2 ] … [AgF + ][BF 4 – ] … ? Surface of AgF 2 has been modified? KAgF 3 is metallic above 70 K
Literature Review & theory: Grochala W, Hoffmann R, Real and Hypothetical Intermediate-Valence Fluoride Ag II /Ag III and Ag II /Ag I 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
Acknowledgements T he 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/ 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