Upload
faith-hahn
View
224
Download
4
Tags:
Embed Size (px)
Citation preview
Unusual fluorides of silverat high oxidation states
LECTURE VII
Frequency occurrence of different oxidation states
in the compounds of the Group 11 elements
–1 0 1 2 3 4 5 6 7 Cu Cu CuCl CuF2 KCuF4 Cs2CuIVF6 Ag Ag AgF AgF2 AgF3 Cs4AgIIIAgVF12 Au CsAu Au AuCl Au(SbF6)2 CsAuCl4 AuF5 (AuF6) (AuF7)
CuCl frequent AgF2 less frequent AuCl rare AuF5 very rare
Why fluorides of Ag(2+) i Ag(3+)?
Relativistic effects for Au(0)
5d nonrelat
6s relat
5d relat10
16s nonrelat
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.
Ag2+ ~ Cu2+ (d9)Ag3+ ~ Cu3+ (d8)
F1– ~ O2– (s2p6)
1. High–TC superonductivity in the hole–doped oxides of Cu2+;
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. Ag2+ is a very strong oxidizer. A) Ag2+ solvated in anhydrous HF oxidizes Xe0 to Xe2+, C6F6 to C6F6
+, and oxidizes CF3CF=CF2 quantitatively to CF3CF2CF3.
B) AgF2 oxidizes fullerene to C60F44 (AgF up to C60F18).C) Ag2+ is unknown in oxides and chlorides.
2. Ag3+ is an enormously strong oxidizer. A) Compounds of Ag3+ are best obtained by use of F radicals.
B) AgF2+ solvated in anhydrous HF oxidizes Kr0 to Kr2+, PtF6
– to PtF6, and together with Ni4+ is the best oxidizer available to chemistry.
C) Compounds of Ag3+ easily evolve F2 upon heating. AgF3 is thermodynamically unstable.
Enormous oxidizing properties of Ag(2+) i Ag(3+)
Cu3+ + O2– Cu2+ + O1– 2 O1– O2
2–
Ag3+ + F1– Ag2+ + F0 2 F0 F2
Ag3+ + F1– Ag2+ + F0
• 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 !
Cu Ag
O2
F
1+
2+
3+
1+
2+
3+
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 181 1
H2
He2 3
Li4
Be5B
6C
7N
8O
9F
10Ne
3 11Na
12Mg
13Al
14Si
15P
16S
17Cl
18Ar
4 19K
20Ca
21Sc
22Ti
23V
24Cr
25Mn
26Fe
27Co
28Ni
29Cu
30Zn
31Ga
32Ge
33As
34Se
35Br
36Kr
5 37Rb
38Sr
39Y
40Zr
41Nb
42Mo
43Tc
44Ru
45Rh
46Pd
47Ag
48Cd
49In
50Sn
51Sb
52Te
53I
54Xe
6 55Cs
56Ba
* 71Lu
72Hf
73Ta
74W
75Re
76Os
77Ir
78Pt
79Au
80Hg
81Tl
82Pb
83Bi
84Po
85At
86Rn
7 87Fr
88Ra
** 103Lr
104Rf
105Db
106Sg
107Bh
108Hs
109Mt
110Uun
111Uuu
112Uub
113Uut
114Uuq
115Uup
116Uuh
117Uus
118Uuo
* 57La
58Ce
59Pr
60Nd
61Pm
62Sm
63Eu
64Gd
65Tb
66Dy
67Ho
68Er
69Tm
70Yb
** 89Ac
90Th
91Pa
92U
93Np
94Pu
95Am
96Cm
97Bk
98Cf
99Es
100Fm
101Md
102Nb
The Cu/O vs Ag/F analogy
Binary fluorides of Ag
• AgF – photography colorless, NaCl
structure
• AgF3 – potent oxidizer brown red, unique helical AuF3
structure
• AgF2 – organic synthesis brown
ferromagnetic, monoclinic CuF2
• Ag2F – superconductivity green, inverse Cd(OH)2
structure
• Ag1+[Ag3+F4] – metastable • [Ag2+][AgF4
–]2 – red–brown unique ribbon
structure
• [AgF+][AgF4–] –
brown kinked 1D AgF+ chains
• AgF2–x – ???
• AgF1–x – defected structure yellow to
yellow–brown
Ternary & higher fluorides of Ag
• Ag(I):
• Ag(II):
• Ag(III)
:
2 Ag2C2 x AgF x 9 AgNO3 x 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.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.
Crystal structures
Isolated Ag2+ centers: [Ag2+][SbF6
–]2
Puckered [AgF2] planes: AgF2
Infinite [AgF+] chains: [AgF+][BF4
–]Infinite [AgF2] planes:
[KF][AgF2]
Unique [AgF3] helix: AgF3
Isolated [AgF4–]
squares: KAgF4
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
EF
–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 Ag3+: 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 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
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]+[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.”
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/
Synthesissolid state & AHF/F2
Magnetic susceptibility measurements /SQUID/
Leicester /UK/Ljubljana /Slovenia/
XRDP 19F NMR Microwave cavity perturbationXPS
Feedback for synthesis
ESR
ICP MASS
Core XPS
Table 1. Position of the Ag(3d5/2) and F(1s) peaks (eV) (referenced to the C(1s) signal) in
the XPS spectra of several fluorides of Ag.
Compound AgF AgF2 c–AgF2 KAgF3 KAgF4
Ag(3d5/2) 368.20 367.85 368.95 368.70 368.35
F(1s) 683.35 683.35 683.49
685.79
683.47 686.25
0
1000
2000
3000
4000
5000
6000
1 2 3 4 5 6 7 8 9 10 11
E /eV
Inte
nsi
ty /c
ou
nts
KAgF3AgF2KAgF4AgF
Valence region XPS
Table 2. Parameters of the component bands in the valence region of the XPS
spectra for several fluorides of Ag: position of the band (referenced to the
C(1s) signal) (eV), its half width (eV), and intensity at maximum (counts).
These are results of the peak–fitting procedure using Gaussian peak profiles.
Band 2
(“ligand band”)
Band 1
(“metal band”)
Compound Position
/eV/
½ width
/eV/
Intensity
/counts/
Position
/eV/
½ width
/eV/
Intensity
/counts/
AgF 7.77 1.58 774 5.26 1.45 3808
c–AgF2 7.29 2.52 1087 5.57 1.98 3315
AgF2 6.97 3.46 1258 4.90 1.82 3287
KAgF3 6.85 3.12 1422 5.13 1.86 2916
KAgF4 7.27 3.42 1665 5.21 1.62 2389
Table 3. Ratio of the contributions of the Ag(4d) states to the „metal
band” and to the „ligand band” for several fluorides of Ag.
Comparison of experimental and theoretical results.
Ratio of contributions of Ag(4d) to Metal band : ligand band
Compound Theory /DFT/ Experiment /XPS/
AgF 88:12 82:18
c–AgF2 66:34 70:30
AgF2 66:34 58:42
KAgF3 53:47 51:49
KAgF4 40:60 40:60
Theory vs experiment
Microwave cavity perturbation
-6.E+06
-4.E+06
-2.E+06
0.E+00
2.E+06
4.E+06
6.E+06
20 30 40 50 60 70 80 90
-2.86E+06
-2.84E+06
-2.82E+06
-2.80E+06
-2.78E+06
-2.76E+06
0 50 100 150 200 250
AgF2
FM insulating
PM insulating
SQUID
0.E+00
2.E-07
4.E-07
6.E-07
8.E-07
1.E-06
050100150200
T /K
ZFC1.34E-06
1.35E-06
1.36E-06
1.37E-06
1030507090
T /K
ZFCFC
2.12E-06
2.16E-06
2.20E-06
2.24E-06
050100
T /K
2.20E-06
2.21E-06
2.22E-06
2.23E-06
2.24E-06
FCZFC
0.0E+00
1.0E-09
2.0E-09
3.0E-09
4.0E-09
5.0E-09
010203040
T /K
ZFC
a b
c d
Magnetic susceptibility / arbitrary units
T /K T /K
T /K T /K
0
FM
SC
ESR- zero field signal
- g2 signal
“BeAgF4”
XRDP
20 40 60 80
5000
10000
15000
20000
*** **
*
Cou
nts
2
†
#
† AgF2 ‡ AgF # - i -BeF2
* ???
†
†
† † †
† †
† † ‡
‡ ‡ ‡ ‡
*
#
#
#
#
#
# #
# * * *
# #
*
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 Ag2+, and mixed–valence fluorides of Ag2+/Ag3+
• Epitaxial growth of AgF2, molecular spacers, etc. …???
• observations of sudden drops in the magnetic susceptibility of a large number of samples in the BeAgF 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
T /K
0
20
40
60
80
100
120
140
160
180
200
220
Low Temp. SC
Medium Temp.SC
High Temp. SC
Ambient Temp.SC
Absolute Zero
Nb3Geclassical SC alloy
23 K
Liquid nitrogen77K
Lowest recordedtemp. on Earth
185 K
Acetone/CO2
cooling mixture195 K
LixHfNCl25.5 K
HgBa2Ca2Cu3O8+x
(under pressure)166 K
HgBa2Ca2Cu3O8+x
134 K
YBa2Cu3O7 x
93 K
Ba1 xKxBiO3
35 K
Rb2CsC60
35 K
h+ doped C60
52 K
YPd2B2C23 K
[Be/Ag/F]64 K
MgB2
39 K
h+ doped C60: CHBr3
117 K
Typical room temperature (25 oC)
Room Temp. SC
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
Acknowledgements
• 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/
• 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