Ag3(SO3F)4: A rare example of a mixed-valent AgII/AgI compound showing 1D antiferromagnetism

Article abstract of DOI:10.1002/ejic.201100110

Dark brown Ag^I₂Ag^II(SO₃F) ₄, known for over 30 years, is the first known example of four well-characterized mixed-valent (1+/2+) compounds of silver. It crystallizes in the monoclinic space group, P2₁/c, with a = 5.33670(19) A, b = 12.9486(4) A, c = 19.5976(7) A, β = 100.6407(13)°, V = 1330.95(8) A³, Z = 4 and d_calcd. = 3.59 gcm ^-3. Its chemical formula is best written as [Ag^I ₂(SO₃F)]⁺[Ag^II(SO₃F) ₃]^-. Ag^II centres form 1D chains linked through OSO bridges that result in pronounced antiferromagnetic coupling with T _N = 225 K and a superexchange coupling constant (J) of -7.5 meV per pair of coupled Ag^II cations. Ligand environments around Ag ^I and Ag^II differ substantially, which suggests a genuine mixed- (i.e., localized) and not intermediate-valent (i.e., delocalized) character of the title compound. Indeed, electronic absorption is not observed up to 7500 cm^-1, so the intervalence charge-transfer transition across the electronic band gap must fall above 0.8 eV. The compound is stable up to a mere 75 °C, which marks the onset of its thermal decomposition to Ag^ISO₃F and the SO₃F^′· radical. Ag^I₂Ag^II(SO₃F)₄ is, after AgSO₄, the second-known 1D antiferromagnetic semiconducting oxa derivative of Ag^II. Dark brown monoclinic Ag₃(SO ₃F)₄ shows 1D antiferromagnetism with an anomalously large superexchange coupling constant of -7.5 meV per pair of paramagnetic Ag ^II cations (for the spin density see the figure). One-dimensional [Ag^IISO₃F]⁺ chains, linked through fluorosulfate anions into [Ag^II(SO₃F)₃] ^- sheets, constitute characteristic structural features responsible for magnetic behaviour.

Full text of DOI:10.1002/ejic.201100110

FULL PAPER
DOI: 10.1002/ejic.201100110
Ag3(SO3F)4: A Rare Example of a Mixed-Valent AgII/AgI Compound Showing
1D Antiferromagnetism
Tomasz Michałowski,[a] Przemysław J. Malinowski,[a] Mariana Derzsi,[b] Zoran Mazej,[c]
[d]
[b]
[a,b]
ˇ ´
´
Zvonko Jaglicic, Piotr J. Leszczynski, and Wojciech Grochala*
ˇ
Dedicated to Professor Boris Zemva on the occasion of his 70th birthday
Keywords: Silver / Mixed-valent compounds / Density functional calculations / Fluorosulfate
Dark brown AgI2AgII(SO3F)4, known for over 30 years, is the
first known example of four well-characterized mixed-valent
(1+/2+) compounds of silver. It crystallizes in the monoclinic
space group, P21/c, with a = 5.33670(19) Å, b = 12.9486(4) Å,
c = 19.5976(7) Å, β = 100.6407(13)°, V = 1330.95(8) Å3, Z = 4
and dcalcd. = 3.59 gcm–3. Its chemical formula is best written
as [AgI2(SO3F)]+[AgII(SO3F)3]–. AgII centres form 1D chains
linked through OSO bridges that result in pronounced anti-
substantially, which suggests a genuine mixed- (i.e., local-
ized) and not intermediate-valent (i.e., delocalized) character
of the title compound. Indeed, electronic absorption is not
observed up to 7500 cm–1, so the intervalence charge-trans-
fer transition across the electronic band gap must fall above
0.8 eV. The compound is stable up to a mere 75 °C, which
marks the onset of its thermal decomposition to AgISO3F and
the SO3F· radical. AgI2AgII(SO3F)4 is, after AgSO4, the sec-
ferromagnetic coupling with TN = 225 K and a superex- ond-known 1D antiferromagnetic semiconducting oxa deriv-
change coupling constant (J) of –7.5 meV per pair of coupled
AgII cations. Ligand environments around AgI and AgII differ
ative of AgII.
transfer (IVCT) excitations expected for compounds exhib-
iting mixed valence. Black is often indicative of a narrow
energy gap at the Fermi level and the possibility of achiev-
ing metallization either by means of doping or by applying
external pressure. As compounds containing highly oxidiz-
ing AgII species may exhibit unusual electronic and mag-
netic properties (as in the case of black semiconducting
AgSO4[6]) and possibly even superconductivity,[7] detailed
examination of the title compound was desired. Here we
report the crystal structure of AgI2AgII(SO3F)4 as well as
its 1D antiferromagnetism, similar to that of AgSO4.[6]
Introduction
Very little is known about mixed-valent AgI/AgII com-
pounds. At the time of writing only three examples[1] of
such compounds have been described in the literature:
1. silver(I,II) fluorosulfate,[2] AgI2AgII(SO3F)4 (the title
compound, claimed to be black)
2. two dark red complexes of AgI acetylide and AgII with
tetramethylcyclam,[3,4]
3. green silver(I,II) hexafluoroantimonate(V),[5] AgI2AgII-
(SbF6)4.
Historically the first among these, AgI2AgII(SO3F)4,
briefly described by Leung and Aubke in 1978,[2] seems to
be most interesting as its dark colour is indicative of low-
energy electronic transitions, such as intervalence charge-
Results and Discussion
A mixed-valent silver(I)–silver(II) fluorosulfate can be
synthesized by using a number of methods. It can be ob-
tained by treating a mixture of AgF and AgF2 (2:1 molar
ratio) with an excess amount of sulfur trioxide, see Equa-
tion (1).
[a] Faculty of Chemistry University of Warsaw,
Pasteur 1, 02093 Warsaw, Poland
Fax: +48-22-5540801
E-mail: wg22@cornell.edu
[b] Interdisciplinary Center for Mathematical and Computational
Modelling, University of Warsaw,
Pawinskiego 5a, 02106 Warsaw, Poland
[c] Department of Inorganic Chemistry and Technology, Jozef
ˇ
2AgF + AgF2 + 4SO3 Ǟ AgI2AgII(SO3F)4
(1)
Stefan Institute,
Jamova 39, 1000 Ljubljana, Slovenia
[d] University of Ljubljana, Faculty of Civil and Geodetic
Engineering, and Institute of Mathematics, Physics and
Mechanics,
Alternatively, it can be obtained from the reaction between
AgF2 and HSO3F in which silver(II) fluorosulfate is first
formed according to Equation (2).
Jadranska 19, 1000 Ljubljana, Slovenia
2508
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A Mixed-Valent AgII/AgI Compound Showing 1D Antiferromagnetism
AgF2 + 2HSO3F Ǟ 2HFȆ + Ag(SO3F)2 (2)
and (3). The pattern obtained is shown in Figure 1 together
with the differential curve from three-phase refinement of
the structure. The sample was found to contain 1.5 wt.-%
AgF2 and 9.9 wt.-% Ag(SO3F)2. For these two impurities
only the unit-cell parameters were refined. Soft restrictions
on bond lengths and angles of the SO3F– anions were ap-
plied as described before.[8] The structure of AgI2AgII-
(SO3F)4 was solved by using Jana2006[9] combined with
Expo2004,[10] which provided the starting model for the re-
finement.
and then slowly decomposes to give AgI2AgII(SO3F)4 ac-
cording to Equation (3).
3Ag(SO3F)2 Ǟ Ag3(SO3F)4 + (SO3F)2
(3)
This pathway is very time-consuming, but after four
months the solid residue contains approximately 88.6 wt.-
% of the mixed-valent compound, still contaminated with
AgF2 and Ag(SO3F)2. Remnants of acid may be removed
by heat treatment at 60 °C for 0.5 h.
The highest purity samples of AgI2AgII(SO3F)4 of qual-
ity appropriate for magnetic studies were obtained by treat-
ing silver(I) fluorosulfate with bromine(I) fluorosulfate ac-
cording to the published procedure, see Equation (4).[2]
3AgSO3F + BrSO3F Ǟ AgI2AgII(SO3F)4 + 1/2Br2
(4)
All chemical and physical properties (except for crystal
structure refinement) reported below are for the samples
synthesized according to Equation (4). The elemental con-
tent for these samples is shown in Table 1, as compared
with theoretical values for mixed-valent silver(I,II) fluoro-
sulfate as well as fluorosulfates of silver(I) and silver(II).
Figure 1. Experimental X-ray powder diffractogram of AgI2AgII-
(SO3F)4 and the differential curve. For decomposition of the ob-
served pattern into particular components, see the Exp. Sect.
Table 1. Elemental content analysis (Ag, S and F) for Ag3(SO3F)4.
Two projections of the crystal structure are shown in Fig-
ure 2. The refined unit-cell parameters and selected in-
teratomic contacts are listed in Table 2. AgI2AgII(SO3F)4
Exp.
Theor.
Theor.
Theor.
Element
[Ag3(SO3F)4] [Ag3(SO3F)4] (AgSO3F) [Ag(SO3F)2]
Ag
F
S
45.9
10.9
17.5
45.0
10.6
17.8
52.2
9.2
15.4
35.3
12.4
20.9
Table 2. Unit-cell and refinement parameters and selected bond
lengths for AgI2AgII(SO3F)4.
Space group P21/c
T
Z
Rp
Rwp
R(Bragg)
GOF
298 K
4
0.0153
0.0200
0.0228
1.50
All results are within the error margins of the methods
applied. The results suggest that AgI2AgII(SO3F)4 was in-
deed obtained.
V [Å3]
a [Å]
b [Å]
c [Å]
β [°]
1330.95(8)
5.33670(19)
12.9485(4)
19.5976(7)
100.6407(13)
Crystal Structure: 1D AgII(SO3F)+ Chains Linked into 2D
AgII–O
AgIa–O
AgIb–O
2.055(18)
2.07(2)
2.26(2)
2.36(2)
2.34(2)
2.13(2)
2.15(2)
2.460(19)
2.669(18)
2.49(2)
2.424(17)
2.609(17)
2.718(19)
2.82(2)
2.648(18)
2.78(2)
–
AgII(SO3F)3 Sheets and Isolated AgI2(SO3F)+ Cations
2.843(16)
The best-quality powder X-ray diffractogram was ob-
tained for the sample synthesized over four months from
the reaction between AgF2 and HSO3F, see Equations (2)
2.446(16)
2.52(2)
Figure 2. Two projections of the unit cell of AgI2AgII(SO3F)4 emphasizing SO3F tetrahedra (light grey), AgIIO6 octahedra (dark grey)
and two different AgI coordination spheres (octahedron and pentagonal bipyramid: medium grey). Left: projection on the bc plane
+
emphasizing the presence of 1D [AgII(SO3F)2/2
]
chains (printed in red) and of AgI2(SO3F)+ cations. Right: projection on the ac plane
emphasizing the links between successive [AgII(SO3F)2/2
]
chains (generated with VESTA[15]).
+
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W. Grochala et al.
FULL PAPER
crystallizes in the monoclinic space group P21/c with large atoms form narrow channels along the a crystallographic
values of b and c vectors, which lead to a unit-cell volume axis.
exceeding 1300 Å3. There are three nonequivalent Ag atoms
One might consider a hypothetical Lewis acid/Lewis base
in the structure of which two are AgI (hereafter referred to reaction between Ag(SO3F)2 and AgSO3F, see Equation (5).
as AgIa and AgIb) and one is AgII.
2AgSO3F + Ag(SO3F)2 = Ag3(SO3F)4
(5)
The AgII cations are coordinated by six O atoms ar-
ranged into a deformed elongated octahedron with four
short [2.055(18), 2.07(2), 2.13(2), 2.15(2) Å] and two long
bond lengths [2.424(17), 2.609(17) Å]. The average AgII–O
distance is 2.24(2) Å. The Ag–O distances are very similar
The differential volume of reaction [Equation (5)] is
–27.94 Å3 per formula unit, which corresponds to a volume
drop of a –2% compared with the expected value. The simi-
larity of the average AgII–O and AgI–O bond lengths com-
pared to those found for the Ag(SO3F)2 and AgSO3F pre-
cursors testify that Lewis acidities of both pseudobinary
fluorosulfates are not vastly different.
to
2ϫ2.154(19) Å and 2ϫ2.49(2) Å, i.e., 2.24(2) Å on
average; and 2ϫ2.053(18) Å, 2ϫ2.14(3) Å and
those
found
for
Ag(SO3F)2
[2ϫ2.07(3) Å,
2ϫ2.61(2) Å i.e., 2.27(2) Å on average]. The dimensionless
parameter of Jahn–Teller distortion, D,[11] takes the value
of 1.194, which is comparable to those of 1.179–1.245 seen
for AgII(SO3F)2.[8] The AgII cations are linked through O–
S–O bridges into 1D AgII(SO3F)+ chains propagating along
b with an O–Ag–O angle of 176.4(11)° and Ag–O–O angles
of 176.6(5) and 162.4(5)°. This close to linear geometry of
the Ag···OO···Ag bridge has major consequences for mag-
netic properties of the compound (see below). The
AgII(SO3F)+ chains are further polymerized along a into
2D AgII(SO3F)3– sheets using two additional SO3F– anions.
As for the AgI cations, the first coordination sphere is in
the form of a deformed pentagonal bipyramid for AgIa and
of a deformed octahedron for AgIb. In the bipyramid the
apical O atoms are the nearest to the transition-metal atom.
An average Ag–O distance is 2.59 Å for AgIa, and 2.54 Å
for AgIb. Accordingly, the bond-valence sum[12]for AgIa
[1.10(3)] is larger than that for AgIb [0.99(2)]. The AgO dis-
tances are similar to those found for AgI(SO3F)[13] with a
deformed octahedral coordination of AgI [an average of
2.54(2) Å].
Vibrational Spectra as a Fingerprint of Bonding
Comparison of the far- and mid-infrared spectra mea-
sured for AgI2AgII(SO3F)4 and for two reference com-
pounds, Ag(SO3F)2 and AgSO3F, is shown in Figure 3. The
IR absorption and Raman scattering spectra of AgI2AgII-
(SO3F)4 are shown jointly in Figure 4. Bands appearing in
the spectra were assigned to characteristic vibrational
modes based on the data from the literature,[16] as listed in
Table 3 together with the bands typical for the reference
compounds.
There are four independent SO3F– anions in the struc-
ture. The first SO3F unit (S1) bridges AgII cations through
an OSO linker. The other two SO3F units (S2, S3) bridge
AgII with AgII and AgII with AgI cations also utilizing the
OSO links. The last SO3F unit (S4) bridges only pairs of
AgI cations using both –O– and –OSO– linkers, just like
that seen for AgISO3F.[13] The presence of SO3F– anions
that are not attached to AgII suggests the formulation of
the compound as [AgI2(SO3F)]+[AgII(SO3F)4/2+2/2]–. The
AgI2(SO3F)+ cations are located between the [AgII-
(SO3F)4/2+2/2]– sheets. The first type of fluorosulfate anions
has one terminal O and one terminal F atom (by terminal
atom we mean here one that is not bound strongly to AgII);
terminal O is used for bridging between AgIa and AgIb.
The second type of fluorosulfate anion has one terminal O
and one terminal F. Here the terminal O atom is used for
weak (secondary) bonding to AgII, while completing its co-
ordination sphere from square-planar to elongated octahe-
dral. For the last type of fluorosulfate anion in which all O
atoms are involved in bonding to AgI, only F atoms are
Figure 3. Infrared absorption spectra of AgI2AgII(SO3F)4,
Ag(SO3F)2 and AgSO3F.
Figure 4. Infrared absorption and Raman scattering spectra of
AgI2AgII(SO3F)4.
The IR spectrum of AgI2AgII(SO3F)4 is similar to those
terminal. In summary, none of the fluorine atoms are of AgSO3F and Ag(SO3F)2 (Figure 3), but clearly not their
bound to a metal cation [such binding occasionally occurs simple superposition. The spectrum is complex and consists
[14]
in fluorosulfates, for example for Ti3Cl10(SO3F)2
or for of many bands in all spectral ranges due to the structural
complexity of the compound and the large size of the unit
AgSO3F[13]]. Similarly to what is found in Ag(SO3F)2,[8]
F
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A Mixed-Valent AgII/AgI Compound Showing 1D Antiferromagnetism
Table 3. Assignment of infrared- and Raman-active bands of AgI2AgII(SO3F)4 relative to AgII(SO3F)2 and AgISO3F and to literature
data.[a]
Raman
Ag3(SO3F)4
Raman
Ag(SO3F)2
IR
IR
IR (lit.)
IR (lit.)
Ag(SO3F)2
IR (lit.)
Assignment
AgSO3F[13]
[8]
[2]
[8]
Ag3(SO3F)4
(product AgF2 Ag3(SO3F)4
+ 2SO3)
1347 vs
1309 mw
1340 s
1309 w
1335 w, sh
1311 s, sh
1273 m
1229 m, sh
1188 s
1173 m, sh
1089 w
1076 s
1320 w, sh
1313 s
SO3 str
1311 s
1275 m, sh
1226 m, sh
1325 s, sh
1278 s
1235 s
1310 s, br., sh
1250 s, br.
1239 vw
1194 vw
1134 s
1086 w
1069 s
1194 m
1195 s
1188 s, sh
1176 vs, br.
1171 s, sh
1089 m
1075 m
1095 w
1082 ms
1070 s
1075 s,sh
1062 s
1065 w
1044 s, sh
850 w
1067 s
1044 s, sh
850 vw
1046 w
1055 s
1046 m, sh
SF str
843 w
815 w
838 s
819 vw
843 w
821 s,sh
804 s
837 m, sh
816 s
804 m, sh
838 w
826 w, sh
820 s
830 s
810 s
795 s
748 s
727 vw
620 s
600 m
748 s
620 s,sh
615 w, sh
608 m
595 m
SO3F def
604 w
598 s
587 s
558 s
434 s
428 s
415 vw
410 w
609 w, sh
594 m
581 m
598 s
582 s
595 s,sh
587 s, 577 s
561 s,sh
585 vw
560 w
497 vw
560 s, 552 s
466 w
435 vw
430 vw
412 w
559 s, sh
560 m
SO3F rock
Ag–O str & lattice
434 w
421 w
428 m
402 w
410 vs
394 vw
258 s
144 s
395 vw
260 w
220 s
238 vs
219 vs
141 w
118 w
91 m
127 w,sh
78 w
[a] v: very, s: strong, m: medium, w: weak, br.: broad, sh: shoulder. str = stretching, def = deformation, rock = rocking.
cell. The obtained IR spectrum shows great similarity to ESR Spectra and Magnetic Properties
the one published by Aubke[2] (with a systematic downshift
by 7 cm–1 – occasionally larger – likely due to incorrect zero
The ESR spectra were measured in the 2.6–296 K tem-
shift in the previous data[2]) and it also bears a strong re- perature range. A very broad line, typical of an ordered
semblance to that measured for the product of the highly magnetic phase, with a g factor of approximately 2.119 is
exothermic reaction between AgF2 and 2SO3.[13]
evident in the spectrum recorded at room temperature (Fig-
The S–O stretching region (1000–1350 cm–1) in the IR ure 5, inset). This is consistent antiferromagnetism of
spectrum is extremely broad and consists of many overlap- Ag3(SO3F)4, which persists at room temperature (Figure 6).
ping bands, clearly as a result of a variety of S–O interac- At 2.6 K a complex fine structure develops, reminiscent of
tions and of engagement of O in primary and secondary that for a paramagnetic specimen (Figure 5) but very dif-
bonding to silver cations at different oxidation states. Pres- ferent from the one measured at 2.6 K for Ag(SO3F)2.[8]
ence of IR absorption above 1300 cm–1 testifies that at least Since all AgII atoms are equivalent in the cell, a fine struc-
one oxygen of the fluorosulfate anions is engaged in second- ture is due to either a rhombic g tensor [recollect a (2+2+2)
ary bonding to silver. The anions may be considered to be coordination of Ag] or even due to the coupling between
(2+1)-dentate. The S–F stretching range (750–850 cm–1) is electronic and nuclear spins.
also rich in bands owing to the presence of four nonequiva-
The presence of AgII with its unpaired 4d electron im-
lent SO3F groups in the structure and to coupling between plies magnetic interactions between AgII centres. Indeed,
oscillators of similar frequency.[17] The S–F band split is the plot of magnetic susceptibility versus temperature (Fig-
revealed in the Raman spectra as well (a sharper band at ure 6) indicates the presence of a broad maximum of
842 cm–1 and a broader and weaker band at 819 cm–1). The susceptibility at 225 K (cf. the previously reported value of
regions of IR absorption due to SO3F deformations (550– 240 K[2]). AgI2AgII(SO3F)4 is thus proven to order antifer-
620 cm–1) and rocking (390–470 cm–1) are rich in bands; the romagnetically, similarly to AgSO4.[6] 1D antiferromagnet-
Ag–O stretching and lattice modes appear below 260 cm–1. ism persists till approximately 400 K; measurements at
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W. Grochala et al.
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Figure 7. Far left: band structure for AgI2AgII(SO3F)4 as calculated
with LSDA; middle left: band structure as calculated with
LSDA+U; middle right: partial DOS jointly for all AgII cations
and for
O
atoms from OSO bridges between AgII centres
(LSDA+U); far right: partial DOS jointly for all AgI cations and
for remaining O atoms (LSDA+U). Colour code for partial DOS
is shown. The Fermi level was set to zero.
Figure 5. The ESR spectrum of AgI2AgII(SO3F)4 at 2.6 K. Inset:
the spectrum measured at 296 K.
Figure 8. Projection of spin density, ρsp, as calculated at the
LSDA+U level onto the bc plane of the crystallographic unit cell
of AgI2AgII(SO3F)4 reflecting the antiferromagnetic superexchange
Figure 6. Plot of magnetic susceptibility (χ) versus temperature (T)
for AgI2AgII(SO3F)4 (FC: field-cooled regime, ZFC: zero-field-co-
oled regime).
within the [AgII(SO3F)2/2 + chains (shown on the left). Colour scale
]
[eÅ–3] from blue (excess amount of spin down) to red (excess
amount of spin up).
higher temperatures were not performed due to thermal de-
composition of the sample. The Bonner–Fischer model of
a 1D antiferromagnet[18] with g = 2.119 taken from ESR
spectra (cf. above) gives the exchange integral (J) as –179 K
(i.e., –7.5 meV per pair of AgII cations).
Appearance of strong 1D antiferromagnetic (AFM) cou-
pling is explained by analysis of the crystal structure and
well reproduced by DFT calculations (see the next section).
The |J| value for AgSO4 (217 K) is approximately 20%
larger than that for AgI2AgII(SO3F)4, as also reflected by
the positions of the magnetic susceptibility maxima (264
and 225 K, respectively).
Table 4. The magnetic moments on atoms as calculated for the
AFM ground state of Ag3(SO3F)4 at the LSDA+U level.
Atom
μ [μB]
Atom
μ [μB]
AgIa, AgIb Ϯ0.01
O (OSOintrachain
)
Ϯ0.08
Ϯ0.04 to Ϯ0.05
AgII
S, F
Ϯ0.49
0.00
O (OSOinterchain
)
O (not bound to AgII) Ϯ0.00 to Ϯ0.03
Atomic contributions from AgII and AgI to the density
of states are firmly separated in the energy scale, with AgI
states predominating the valence band and AgII states con-
stituting the conduction band (Figure 7). This feature con-
firms the genuine mixed-valent AgI/AgII character of the
title compound, deduced before from the crystal structure.
DFT Picture of Bonding and Magnetism
To understand the nature of the magnetism and elec- The 2p states from O atoms of the OSO bridges are sub-
tronic structure of Ag3(SO3F)4 we have carried out DFT stantially mixed up with the 4d states of Ag over the entire
calculations using local spin density approximation (LSDA) energy range (“covalence”[19]). As expected, Ag3(SO3F)4 is
and LSDA+U methods. The electronic band structure and a Mott–Hubbard insulator, in contrast to AgII(SO3F)2 (a
atomic electronic densities of states (dos) are shown in Fig- charge-transfer insulator).[8] Introduction of the Mott–
ure 7; a projection of the spin density within the crystallo- Hubbard U is not necessary, however, to open the band gap
graphic unit cell is presented in Figure 8. Magnetic mo- at the Fermi level (Figure 7). A small direct electronic band
ments on atoms are listed in Table 4.
gap of approximately 0.35 eV appears at (1/2 0 0) even in
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A Mixed-Valent AgII/AgI Compound Showing 1D Antiferromagnetism
the LSDA calculations but it increases to ca. 0.6 eV upon
inclusion of the U factor. The smallest indirect band gap is
approximately 0.5 eV. A more precise estimate of the band
gaps requires full optimization of the crystallographic unit
cell.
+
Presence of the 1D [AgII(SO3F)2/2
]
chains bridged into
[AgII(SO3F)4/2+2/2
]
sheets in the crystal structure of
–
Ag3(SO3F)4 (see Figures 2 and 8) determines the magnetic
behaviour of this compound. The lowest-energy solution is
–
antiferromagnetic within the [AgII(SO3F)2/2
]
chains but
Figure 9. TGA/DSC plot of the thermal decomposition of
AgI2AgII(SO3F)4 (at 10 Kmin–1).
ferromagnetic between the neighbouring chains.[20] The
pathway of the strongest magnetic superexchange between
two neighbouring AgII centres goes via the OO bridge, as in
the case of AgSO4.[6,21] Antiferromagnetism of Ag3(SO3F)4
thus arises from superexchange of unpaired Ag(4d) elec-
trons residing at x2–y2 orbitals (localized within mutually
inclined [AgO4] squares) (Figure 8). The calculated J value
(–21.4 meV GGA+U, –23.8 meV LSDA+U) is 2 to 3 times
overestimated with respect to the experimental value of
–7.5 meV, partly due to the fact that the experimental crys-
tal structure was used for calculations without any struc-
tural optimization. The values of magnetic moments on
AgII atoms as obtained from the LSDA+U calculations
(Table 4) are Ϯ0.49 μB, and thus are similar to those calcu-
lated for antiferromagnetic AgSO4 (Ϯ0.43–0.45 μB).[6]
Simultaneously, the calculated magnetic moments on the
bridging O atoms vary between Ϯ0.08 μB (for O atoms
within the chain) and Ϯ0.05 μB (for O atoms from fluoro-
sulfate anions that bridge the infinite chains together). The
values of magnetic moments on AgI cations, and on S, O
and F atoms coming from fluorosulfate anions that bridge
AgI centres, are negligibly small, as expected.
tion of Ag2SO4 (orthorhombic Ǟ hexagonal, lit. 425 °C).[22]
Exothermicity of the first step of the thermal decomposi-
tion and evolution of the volatile SO3F radical renders
AgI2AgII(SO3F)4 thermodynamically unstable with respect
to products of thermal decomposition by both enthalpic
and entropic factors.
To identify all crystalline products of thermal decompo-
sition, XRD powder diffractograms were measured for
samples of AgI2AgII(SO3F)4 heated to 180 °C (453 K) and
500 °C (773 K), that is, the temperatures marking the end
of the first and the second step of decomposition, respec-
tively. The main products were identified as AgSO3F and
Ag2SO4, respectively (Figure 10). This result suggests that
AgI2AgII(SO3F)4 decomposes thermally according to the
following simplified reaction equations (6) and (7).
I. 1/3 Ag3(SO3F)4 Ǟ AgSO3F + 1/6 (SO3F)2
(6)
75–180 °C (theor. –13.8% mass)
The appearance of substantial magnetic moments on se-
lected oxygen atoms is equivalent to the partial introduction
of holes into O(2p) states (they attain “free radical” charac-
ter) which – as we will see in the next section – contributes
to facile thermal decomposition of Ag3(SO3F)4 by means
of a redox process.
II. AgSO3F Ǟ 1/2 Ag2SO4 + 1/2 SO2F2
(7)
180–500 °C (theor. –21.2% mass)
Thermal Stability
Thermal behaviour of AgI2AgII(SO3F)4 was studied with
thermogravimetric analysis (TGA)/differential scanning cal-
orimetry (DSC)/evolved gas analysis (EGA). The TGA/
DSC plots are shown in Figure 9. Thermal decomposition
is a two-step process (I: 75–180 °C; II: 180–500 °C) with
an onset at temperatures as low as 75 °C. The first step
of decomposition accounts for about 7.2 wt.-% whereas the
second one for about 16.2 wt.-% mass loss.
The DSC curve is complex, with one broad endothermic
peak around 80 °C of uncertain origin [probably evapora-
tion of traces of the (SO3F)2 substrate occluded in the sam-
ple], a broad exothermic peak at 140 °C corresponding to
the first step of thermal decomposition (with an overlap-
ping sharp endothermic peak at 155 °C corresponding to
melting of AgISO3F[13]) and a large distinct endothermic
Figure 10. XRD powder diffractograms of the products of the first
(top, 180 °C) and the second step (bottom, 500 °C) of the thermal
peak at 424 °C corresponding to a structural phase transi- decomposition of Ag(I)2Ag(II)(SO3F)4.
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Lack of the endothermic DSC peak at 435 °C corre- of 19 (F), 48 (SO), 64 (SO2) and 67 (SOF)[24]] while the
sponding to melting of AgF suggests that AgSO3F does not FTIR spectrum confirms the presence of bands assigned to
evolve SO3 – see Equation (8) – as sometimes observed for S–O and S–F bonds (S–O stretching bands at 1050, 1360,
ionic fluorosulfates.[23]
1374 and 1490 cm–1 and S–F stretching at 770 and
860 cm–1).[25] During the second step of decomposition sim-
ilar signals are seen in both MS and FTIR spectra, but the
III. AgSO3F Ǟ/ AgF + SO3
(8)
The observed values of mass loss for both steps of ther- band at 1050 cm–1 (S–O stretching) and the band at
mal decomposition are much smaller than the theoretical 665 cm–1 (deformation mode), are now much stronger than
values given above. The discrepancies indicate that the com- the remaining ones.
plex process of thermal decomposition is not completed at
500 °C (as clearly seen from the TGA profile) and it in-
Conclusion
volves parallel processes that overlap on the temperature
scale. At the time of the TGA/DSC/EGA measurement,
AgI2AgII(SO3F)4 (despite its high initial purity) must have
been contaminated with a large amount of AgSO3F as a
dead mass. Based on experimentally observed mass losses,
the sample studied must have contained only approximately
40 wt.-% of AgI2AgII(SO3F)4 and as much as approximately
60 wt.-% of AgSO3F. Presence of AgSO3F [i.e., product of
a facile exothermic thermal decomposition of inherently
unstable AgI2AgII(SO3F)4, step I], is not unexpected since
the samples undergoing TGA/DSC/EGA measurements are
conditioned for at least one hour before the beginning of
the measurement in the stream of argon gas (precise mass
stabilization is required for correct TGA experiment). Un-
der such conditions the volatile (SO3F)2 product is dynami-
cally removed while accelerating decomposition.
The mixed-valent AgI/AgII compounds remain largely
unexplored. Herein we have described the synthesis and de-
tailed characterization of dark brown AgI2AgII(SO3F)4,
first prepared in the late 1970s.[2] This compound has a gen-
uine mixed (frozen)-valent character[26] and it is a semi-
conductor of the Mott–Hubbard type. The infinite
{AgII(SO3F)2/2+} chains (structurally analogous to the well-
known AgF+ chains[27]) are linked with additional fluoro-
sulfate anions into {AgII(SO3F)4/2+2/2–} sheets present in
the crystal structure of the title compound AgI2AgII-
(SO3F)4, which behaves as an antiferromagnet with appreci-
able antiferromagnetic exchange within the {AgII-
(SO3F)2/2+} chains (J = –7.5 meV per pair of AgII centres);
the DFT calculations reproduce the experimentally ob-
served antiferromagnetism and additionally predict a very
weak ferromagnetic exchange between the neighbouring
chains. The large absolute value of intrasheet J shows that
fluorosulfate monoanions are capable of transmitting anti-
ferromagnetic superexchange between paramagnetic AgII
centres almost as well as sulfate dianions.[6]
The EGA (a combination of MS and FTIR, Figure 11)
helps to get additional insight into chemical reactions oc-
curring during thermal decomposition. Mass spectra indi-
cate that during the first step of decomposition a fragmen-
tation of evolved (SO3F)2 takes place [signals at m/z ratio
With just two examples known of compounds of
MI2AgII(SO3F)4 stoichiometry (MI = K, Ag), synthesis is
now desired of other compounds of the types MI2AgII-
(SO3F)4, MIAgII(SO3F)3, MIIAgII(SO3F)4, MI2AgII-
(SO3F)2(SO4) and MIIAgII(SO3F)2(SO4). It would be inter-
esting if 2D antiferromagnetism, similar to that seen for
precursors of oxocuprate superconductors, could be ob-
served for certain compositions.
Experimental Section
Synthesis: Due to the high reactivity of silver(II) compounds
towards water all reactions were conducted under an argon atmo-
sphere as provided by Labmaster DP glovebox from MBraun (O2
Ͻ 1.0 ppm; H2O Ͻ 1.0 ppm). All reaction vessels and all laboratory
equipment having contact with the samples were made of Teflon
or fluorinated ethylene propylene (FEP). The silver(I) fluorosulfate
was obtained from commercial AgF (Sigma Aldrich) and an excess
amount of HSO3F (Sigma Aldrich). The solid residue was then
rinsed with a mixture of trifluoroacetic acid (Sigma–Aldrich) and
its anhydride (Sigma Aldrich). The reaction between AgSO3F and
BrSO3F typically on a 0.5–1.0 g scale was conducted for 72 h using
a HF line made of fluoropolymers. Volatile products of the reaction
were removed from the HF line using a vacuum pump. The product
was analyzed as obtained. Ag3(SO3F)4 is black in appearance but
grinding in agate mortar reveals its true dark brown colour, much
darker than that of Ag(SO3F)2 or of AgF2.
Figure 11. The mass spectra of gases evolved during thermal de-
composition of AgI2AgII(SO3F)4 at 125 (top) and 400 °C (middle)
and the temperature dependence of the IR spectra of the evolved
gases (bottom).
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A Mixed-Valent AgII/AgI Compound Showing 1D Antiferromagnetism
The dark red BrSO3F substrate was obtained by reacting an equi-
molar mixture of Br2 and (SO3F)2 for 72 h.[28]
were preheated to 200 °C to avoid condensation of residues. The
silver content was determined using a method by Pendse[33] [pre-
cipitation of disilver(I) chromate]. The method shows an error mar-
gin of less than 1%. An alternative method published by
Tradwell[34] (precipitation of metallic silver during reduction with
H2O2) was attempted but found unreliable while using reference
AgNO3 samples. The sulfur and fluorine content was determined
using the Schöniger method[35] at a detectability level of 0.2 wt.-
% and a typical uncertainty of 0.3 wt.-% for each element. Two
independent analyses were performed with differences between the
measurements reaching no more than 0.2 wt.-% for each element.
The numbers shown in this work are the mean of the two values.
Structure Solution and Refinement: The cell indexation was per-
formed by X-Cell[29] implemented in the Materials Studio pack-
age.[30] Multiphase Le Bail decomposition and Rietveld structure
refinement (Figure 12) were performed in Jana2006. The structure
of the title compound was solved by direct methods in Expo2004
using reflex intensities obtained from Jana2006. For a correct de-
scription of the complex background, 30 Legendre polynomials
were used. The angle and distance uncertainties were obtained after
application of Bérar’s correction.[31]
DFT Calculations: Solid-state density functional theory (DFT) cal-
culations were performed using the VASP code[36] with the projec-
tor-augmented wave method (PAW)[37] as implemented in the Me-
deA package. For the exchange-correlation part of the Hamilto-
nian, the local density approximation (LDA) in its LSDA (local
spin density approximation) variant was applied, since we have no-
ticed previously that LDA allows for much better reproduction
than GGA of many important parameters of related AgSO4.[38]
Due to the very large size of the unit cell (92 atoms) and rather
low symmetry of the magnetic models, the geometry optimization
(i.e., the cell and atomic parameters) was not performed; for the
same reason all calculations were performed in real space. The elec-
tronic iterations convergence was set to 10–7 eV per atom by using
the standard blocked Davidson algorithm. The Γ-centred k-point
mesh automatically generated through the Monhorst–Pack grid of
4ϫ2ϫ1 yielded 6 unique k points. The valence electrons were de-
scribed by plane waves with a kinetic energy cutoff of 600 eV, yield-
ing a satisfactory convergence of total energy. The same parameters
were used for the single-point energy, spin density, band structure
and DOS calculations on a 1ϫ1ϫ1 experimental unit cell.
Figure 12. Experimental X-ray powder diffractogram of
AgI2AgII(SO3F)4, its decomposition to contributions from the crys-
talline phases and background, and the differential curve.
Further details on the crystal structure investigation may be
obtained from the Fachinformationszentrum Karlsruhe,
76344 Eggenstein-Leopoldshafen, Germany (Fax: +49-7247-808-
666; e-mail: crysdata@fiz-karlsruhe.de), on quoting the depository
number CSD-422417.
To derive the electronic and magnetic structure of Ag3(SO3F)4,
both the spin-polarized LSDA and LSDA+U single-point calcula-
tions were performed. To mimic the strongly correlated nature of
the 4d electrons of Ag and the interacting p electrons of the bridg-
ing “oxide anions” (within the superexchange AgII–OO–AgII path),
the value of the Coulomb integral U was set to 4 eV and Hund’s
exchange J to 1 eV for both of these ions; the respective values for
S(3p) electrons were 2 and 1 eV, thus allowing direct comparison
with results obtained earlier for AgSO4.[6,38] The Mott–Hubbard U
was not applied to the formally closed-shell (4d10) AgI cations. We
found that inclusion of U for AgI leads to severe overestimation
of magnetic superexchange constant J. Two different schemes of
magnetic ordering were tested in single-point energy calculations:
one ferromagnetic (FM) with FM intrachain and FM interchain
coupling, and one antiferromagnetic (AFM) with AFM intrachain
and FM interchain coupling as well as a nonmagnetic cell. The
AFM ordering corresponds to a global energy minimum at both
LSDA and LSDA+U levels of theory. The sign of interchain super-
exchange was obtained by means of calculations for a modified
2ϫ1ϫ1 supercell of Ag3(SO3F)4 in which the diamagnetic
AgI2(SO3F)+ cations were substituted with Li+ cations to reduce
the size of the model from 184 (overrunning the VASP limit) down
to 136 atoms [structural integrity of the Ag(SO3F)3– layers was pre-
served].[20]
Instrumental: IR spectra were measured using a VERTEX 80V IR
spectrometer from Bruker with AgCl windows used for the mid-IR
range (500–7500 cm–1) and polyethylene windows for the far-IR
(50–650 cm–1) range. Raman spectra were measured using a Horiba
Jobin Yvon LabRam-HR Raman microspectrometer with 632.8 nm
He:Ne laser exciting beam. Si was used as a reference for setting
of the zero shift. X-ray powder diffraction measurements were con-
ducted using a D8 Discover diffractometer from Bruker equipped
with Cu cathode and parallel-beam setting provided by Göbel mir-
rors. Samples were closed under an argon atmosphere in quartz
capillaries (Hilgenberg GmbH,
Ø = 0.3 mm, wall thickness
0.01 mm). The 62 h measurement covered the 2-theta range of 3–
120°. Magnetic measurements were conducted by a superconduct-
ing quantum interference device magnetometer MPMS-XL-5 from
Quantum Design equipped with a 50 kOe magnet. Magnetization
measurements were conducted in both ZFC and FC regimes for a
sample sealed inside a quartz tube in a magnetic field of 1000 Oe
between 2 and 390 K. The data were corrected for temperature-
independent diamagnetism due to the closed atomic shells as esti-
mated from Pascal’s tables.[32] ESR spectra were obtained using a
ESP 300E spectrometer (frequency 9.5 GHz; the X band) from
Bruker for samples sealed under an Ar atmosphere inside a 4 mm-
thick quartz capillary. Thermal analysis was performed using a
STA 409PG TGA/DSC analyzer from Netzsch coupled with an
Aëolos QMS 403C mass spectrometer (Netzsch) and a VERTEX
80v IR spectrometer (Bruker) for EGA. Samples were loaded into
alumina crucibles under an argon (99.9999%) atmosphere. The ex-
periment was conducted in the temperature range of 30–500 °C
(303–773 K) at the heating rate of 10 Kmin–1. Both transfer lines
Acknowledgments
The project “Quest for superconductivity in crystal-engineered
higher fluorides of silver” is operated within the Foundation for
Polish Science “TEAM” Program co-financed by the EU European
Eur. J. Inorg. Chem. 2011, 2508–2516
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2515

W. Grochala et al.
FULL PAPER
rameter smaller from intrachain J by over one order of magni-
tude; this feature renders Ag3(SO3F)4 a quasi-1D antiferromag-
net at room temperature.
Regional Development Fund. This work has been partly supported
by the Slovenian Research Agency (ARRS) within the research
program P1-0045 Inorganic Chemistry and Technology. Calcula-
tions have been performed at ICM supercomputers.
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[24] Other signals are also present: 50 (H2SO2), 66 (H2SO3), 85
(HSO2 ϫHF); presence of H comes from the water content of
the carrier Ar gas.
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[19] AgII forms substantial covalent bonds even to F–1 anion: W.
ˇ
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[20] The antiferromagnetic interchain coupling is weak compared
to the intrachain coupling, with the interchain exchange pa-
Received: February 1, 2011
Published Online: April 26, 2011
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Eur. J. Inorg. Chem. 2011, 2508–2516