Structure and decomposition of the silver formate Ag(HCO2)

Article abstract of DOI:10.1016/j.jssc.2016.11.022

Crystal structure of the silver formate Ag(HCO₂) has been determined (orthorhombic, sp.gr. Pccn, a=7.1199(5), b=10.3737(4), c=6.4701(3)?, V=477.88(4) ?³, Z=8). The structure contains isolated formate ions and the pairs Ag₂²⁺ which form the layers in (001) planes (the shortest Ag–Ag distances is 2.919 in the pair and 3.421 and 3.716?? between the nearest Ag atoms of adjacent pairs). Silver formate is unstable compound which decompose spontaneously vs time. Decomposition was studied using Rietveld analysis of the powder diffraction patterns. It was concluded that the diffusion of Ag atoms leads to the formation of plate-like metal particles as nuclei in the (100) planes which settle parallel to (001) planes of the silver formate matrix.

Full text of DOI:10.1016/j.jssc.2016.11.022

Author’s Accepted Manuscript
Structure and decomposition of the silver formate
Ag(HCO )
2
Anna N. Puzan, Vyacheslav N. Baumer, Pavel V.
Mateychenko
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http://dx.doi.org/10.1016/j.jssc.2016.11.022
DOI:
Reference: YJSSC19603
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Cite this article as: Anna N. Puzan, Vyacheslav N. Baumer and Pavel V.
Mateychenko, Structure and decomposition of the silver formate Ag(HCO )
2
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Structure and decomposition of the silver formate Ag(HCO2)
Anna N. Puzan*, Vyacheslav N. Baumer, Pavel V. Mateychenko
STC ‘‘Institute for Single Crystals’’, National Academy of Science of Ukraine, 60
Nauky ave., Kharkiv 61001, Ukraine
Abstract. Crystal structure of the silver formate Ag(HCO2) has been determined
(orthorhombic, sp.gr. Pccn, a=7.1199(5), b=10.3737(4), c=6.4701(3)Å,
V=477.88(4) Å3, Z=8). The structure contains isolated formate ions and the pairs
2+
Ag2 which form the layers in (001) planes (the shortest Ag–Ag distances is 2.919
in the pair and 3.421 and 3.716Å between the nearest Ag atoms of adjacent pairs).
Silver formate is unstable compound which decompose spontaneously vs time.
Decomposition was studied using Rietveld analysis of the powder diffraction
patterns. It was concluded that the diffusion of Ag atoms leads to the formation of
plate-like metal particles as nuclei in the (100) planes which settle parallel to (001)
planes of the silver formate matrix.
Keywords: silver formate, crystal structure, Ag–Ag pairs, layered structure,
decomposition, formation of metal phase
Introduction. Silver formate Ag(HCO2) (hereafter I) is a remarkable chemical
compound and a precursor of metallic silver. The state of metallic silver obtained
from decomposition of (I) can be varied within wide limits depending on the
method of its production and further processing. Thus, if aqueous solutions of
silver nitrate AgNO3 and ammonium formate NH4COOH are used to obtain the
precipitate (I), the resulting fine crystalline product may provide both porous and
compact silver particles [1] depending on the method of drying. If the synthesis of
(I) is carried out in the polyethyleneimine (PEI) hydrogel, microporous silver of
colloidal dispersion degree [2] can be obtained as a result of decomposition and
*Corresponding author, e-mail: anna_puzan@mail.ru

calcinations of the product. It is also known that decomposition of (I) is an
autocatalytic reaction in which the resulting metallic silver catalyzes formic acid
decomposition [3,4]. When heated up to ~ 368K (I) explodes [5] to form metallic
silver, hydrogen and carbon dioxide. If the explosion is of a detonating type, it is
accompanied by the formation of fractal clusters of silver in the form of sintered
spheric agglomerates [6]. It is known that silver catalysts are most important in
industrial applications such as oxidation of methanol to methyl formate [7], (I) is
used to produce formic acid ether. Salt formation reaction (I) may also be used to
produce photothermographic composition [8] and to increase the speed of
photoelectron emission by doping silver halide with formate ions [9].
In due course, a lot of authors have studied silver particles morphology, kinetics
and methods of its forming. Methods of producing silver powder of the expected
density have been studied [10]. Nanosilver is used in the synthesis of over-
branched polyesters [11]. The process of silver nanoparticles formation with no
reducing agents [12] has been studied. It has been found that the shape and size of
the resulting silver particles are dependent on the concentration of surfactant, the
initial reagents, synthesis temperature and time [13-19], green synthesis of silver
nanoparticles is also of considerable interest [20-24]. The kinetics of silver
nanoparticles growth in a variety of surface-active solutions has been studied [25].
It has been found that silver clusters obtained under vacuum dehydration in the
zeolite channels and networks are photosensitive which allows to use those
clusters as photocatalysts in solar cells [26] as well as catalyst [27,28] and
practical antimicrobial activity [21,29]. The unique properties of silver produced in
this way are proved by the fact that CO2 recovery rate is twice as high on Ag (110)
than on Ag (111) and Ag (100) [30].
Furthermore, (I) is commonly used for silver plating and obtaining of biologically
active materials [21, 31]. All the above-mentioned suggests that the silver formate
is an important precursor for the particulate metallic silver. As its crystalline
structure has not yet been described, one of the aims of the present study is to
determinate and analyze it. As noted above, silver formate is an unstable

compound and the process of its expansion in the solutions and dispersed media
was studied by many authors, but the decomposition of pure bulk silver formate
has not been described, and it is the second aim of the present work.
1. Experimental
Since methods of obtaining of silver formate from solutions lead to formation of
the disperse product that rapidly decays, they cannot be used to obtain crystals
for X-ray study. To obtain crystals suitable for the study we used a technique
similar to the one described in [32]. Silver carbonate precipitate Ag2CO3 was
obtained by pouring together the stoichiometric amounts of AgNO3 (>99%
purity, 0.75mol/1) and NH4HCO3 (analytical grade, 1mol/1) solutions at 320K,
washed with distilled water, filtered and dried. Obtained yellow powder was
developed with concentrated formic acid (the 5% excess of formic acid as
compared with stoichiometric quantity was added to the powder). At the end of
CO2 gas evolution, the formation of colorless crystals which quickly colored in
yellow and further more slowly black was observed in the solution.
The obtained particles of (I) are shown in Figure 1a,b (JEOL JSM-6390LV X-max
raster electron microscope).
b
c
a
Figure 1. REM photographs: a,b – the silver formate crystals; c – plate-like silver
particles after decomposition.
Fresh-precipitated crystals of (I) were used for X-ray diffraction study using the
"Xcalibur3" automated single-crystal diffractometer (Oxford Diffraction Ltd.),

monochromated MoK radiation, "Sapphire3" CCD detector. The structure was
solved by direct methods and refined by the least squares ("SHELX97" program
package [33]) in the anisotropic approximation (hydrogen atom was refined
isotropically). Main crystal data and refinement results are given in Table 1. Full
data for the structure (I) have been deposited with the ICSD Database, CSD–
431932. Copie of this information may be obtained at http://www.fiz-
karlsruhe.de/obtaining_crystal_structure_data.html.
The rest of silver formate obtained was used for the X-ray powder analysis (Bragg-
Brentano diffractometer "Siemens D500", CuK-radiation, curved graphite
monochromator on the diffracted beam, point scintillation detector, instrumental
profile parameters from LaB6 pattern used as external standard, 10o<2<90o,
Δ2=0.02°, dwell time of 5 sec. per point) using the Rietveld method
(WinPlotr&FullProf program [34]). Initially the qualitative separation of the size
and strain broadening effects on powder pattern of (I) were carried out using
Williamson-Hall method [35]. The dependence of integral breath (hkl·cos for
non-overlapped reflections against dhkl–1 showed y-intercept and slope
simultaneously. Quantitative calculations of these effects were performed using
P.W.Stephens’ formalism [36] implemented in Fullprof program.

Tabel 1. Crystal data and refinement indices for (I)
Empirical formula
CHAgO2
152.89
293(2)
Formula weight, a.m.u.
Temperature (K)
Wavelength (Е)
0.71073
Crystal system
Space group
Orthorhombic
Pccn
a (Е)
7.1199(5)
b (Е)
10.3737(4)
c (Е)
6.4701(3)
477.88(4)
V (Е3)
Z
8
Calculated density (g∙cm-3)
Absorption coefficient (mm-1)
F(000)
4.250
8.098
560
Crystal size (mm)
θ range for data collection (є)
Index ranges
0.14 Ч 0.12 Ч 0.09
3.47 to 36.41
-6≤h≤11, -16≤k≤16, -10≤l≤5
2224 / 1038 (Rint = 0.0191)
95.1%
0.5294 and 0.3969
Full-matrix least-squares on F2
1063 / 42
Reflections collected / unique
Completeness to 2θ (35є)
Max. and min. transmission
Refinement method
Data / parameters
Goodness-of-fit on F2
Final R indices [I>2σ(I)]
R indices (all data)
Extinction coefficient
Largest diff. peak and hole (e.Е-3)
0.977
R1 = 0.0283, wR2 = 0.0320
R1 = 0.0586, wR2 = 0.0383
0.0028(2)
0.671 and -0.672
2. Discussion
2.1.Crystal Structure
The main feature of the structure (I) is the presence of the bond metal-metal (Ag-
2+
Ag of 2.919Å), indicating the presence of a cluster Ag2 . This interaction is
described in the literature as the “argentophilic interaction” [37]. Each cluster is
environed with the six formate ions, and each formate-ion is enclosed with six Ag2

clusters (Figure 2). Each silver atom in the cluster is co-ordinated by four oxygen
atoms, belonging to four formate ions, two of which are bridged between the
atoms of the same Ag-cluster with Ag-O distances of 2.32, 2.28Å. The remaining
oxygen atoms are those of formate ions being bridged between adjacent clusters,
Ag-O distances of 2.52, 2.47Å (Table 2) being characteristic for them.
Fig.2. Atomic numbering scheme and thermal ellipsoids for (I). Symmetrically
equivalent atoms are not denoted.

Table 2.Bond lengths (Å) and angles (º)
Ag(1)-O(1)
2.2830(17) Ag(1)-Ag(1)#1
2.9176(5) O(2)-Ag(1)#1 2.3171(16)
1.236(3) O(2)-Ag(1)#5 2.4671(18)
Ag(1)-O(2)#1 2.3171(16) O(1)-C(1)
Ag(1)-O(2)#2 2.4671(18) O(1)-Ag(1)#4
Ag(1)-O(1)#3 2.5168(19) O(2)-C(1)
2.5168(19) C(1)-H(1)
1.259(3)
0.95(2)
O(1)-Ag(1)-O(2)#1
O(1)-Ag(1)-O(2)#2
O(2)#1-Ag(1)-O(2)#2
O(1)-Ag(1)-O(1)#3
O(2)#1-Ag(1)-O(1)#3
O(2)#2-Ag(1)-O(1)#3
O(1)-Ag(1)-Ag(1)#1
O(2)#1-Ag(1)-Ag(1)#1
O(2)#2-Ag(1)-Ag(1)#1
O(1)#3-Ag(1)-Ag(1)#1
161.87(6)
100.45(6)
93.04(4)
91.74(6)
81.11(6)
153.54(7)
74.59(5)
87.71(5)
125.93(5)
79.86(5)
C(1)-O(1)-Ag(1)
C(1)-O(1)-Ag(1)#4
Ag(1)-O(1)-Ag(1)#4
C(1)-O(2)-Ag(1)#1
C(1)-O(2)-Ag(1)#5
Ag(1)#1-O(2)-Ag(1)#5
O(1)-C(1)-O(2)
135.02(18)
122.25(16)
101.35(7)
115.75(16)
114.05(15)
130.12(8)
126.3(2)
O(1)-C(1)-H(1)
O(2)-C(1)-H(1)
115.8(13)
117.8(14)
Symmetry transformations used to generate equivalent atoms:
#1 -x+1,-y+1,-z #2 -x+1,y+1/2,-z+1/2 #3 -x+1/2,y,z-1/2 #4 -x+1/2,y,z+1/2
#5 -x+1,y-1/2,-z+1/2
Crystal structure of (I) (3a,b) is a three-dimensional polymer network in which
cluster pairs of silver atoms extend along the
and
directions, the
distance between nearest silver atoms of the adjacent pairs is 3.421Å. Thus chains
form the layer in the plane (001), the shortest distance Ag...Ag 3.716Å is observed
for the chains located in adjacent layers in the (010) plane. The presence of the
chains and layers in the structure indicates possible diffusion directions under
decomposition of (I).
a
b
Figure 3a,b. Packing in the structure (I), visualized with Vesta software [38]

2.2.Decomposition
Silver formate is unstable in the air. It decomposes spontaneously at room
temperature to form metallic silver phase. This process was experimentally studied
using powder X-ray analysis. Silver formate freshly produced was ground in a
mortar and placed in a glass sample holder for X-ray pattern registration, which
was repeated after a certain time. The resulting serie of patterns is shown in Figure
4.
Fig. 4. X-ray powder patterns for the decomposing silver formate
Each pattern obtained was refined by the Rietveld method using anisotropic line
broadening approximation, which allows to evaluate the average size of the
forming silver particles in different directions. As an example, Figure 5 shows the
results of the refinement for the pattern obtained after 14 hours of decomposition.

Fig.5. The Rietveld refinement results for the (I) after 14h decomposition.
Silver lines in the first pattern are very weak, whole X-ray pattern corresponds to
the phase of silver formate. Further on, the increase in the intensity of metallic
silver phase lines and the extinction of silver formate phase lines at each
successive stage can be observed. And after about 90 hours of exposure the silver
formate lines disappear completely, the last X-ray pattern corresponds completely
to the formed metallic silver phase.
Results of the Rietveld refinement were used to make the diagrams which reveal
dependence of the obtained silver particle size on the time and of the anisotropic
particle size on the silver phase percentage (Fig.6a,b).
a
b

Fig.6. Dependence of the silver particle size on the time of decomposition (a) and
anisotropic size of the Ag particles vs Ag content (b).
It can be concluded that nuclei of the metal phase are formed at the initial stage of
decomposition. The average size of these nuclei decreases on time from 130Å at
the beginning to ~ 50Å after 42 hours of decomposition. This decreasing can be
explained by the diminution of formate content in the sample and the fact that
decaying particles of the salt matrix eventually became spaced by particles of
product for which the coalescence and re-crystallization processes are impeded at
the room temperature. Further, after about half of the original salt is decomposed,
the average size of the silver particles gradually increases because the silver
resulting from the formate decomposition crystallizes on the existing nuclei.
Defining particle size according to Rietveld method is performed by broadening
the lines profile in comparison with the profile of the standard sample, and in the
case of silver formate the broadening is not the same for all lines, i.e. is
anisotropic, which, in principle, corresponds to the anisotropic size effect. This
means that the resulting silver particles have different sizes in different
crystallographic planes. For the coordinate plane (100) the average size is
essentially smaller than in other crystallographic planes (50 and ~120Å,
respectively). This suggests that formed metal particles have a plate-like habitus.
On considering the microcrystalline effects in the original matrix of silver formate,
it can be noted that the minimum size of the particles during the decomposition is
observed for the (100) planes and is of 360-200Å depending on the decomposition
time (Fig.7a,b), whereas in the other planes it is noticeably greater. Moreover, just
in the (100) planes of the formate matrix are characterized by largest microstrain
values (Fig.7). It was mentioned above that the shortest distances 3.715Å of
between the cluster silver pairs in the initial formate are observed in the (010)
plane. The analysis of microstructural characteristics for the matrix and for the
forming silver allows to suggest that during the initial stage of decomposition
preferable pair diffusion occurs in the (100) plane of the matrix, originating the

formation of silver (100) planes, despite they are not closely packed.
a
b
Fig. 7. Microstructural effects in the silver formate matrix (a – microstains, b –
anisotropic size).
Acknowledgement.
Authors appreciate the International Centre for Diffraction Data for the financial
support (Grants No. 03-02 and 14-01).

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 Silver formate Ag(HCO2) was synthesized and characterized
 Layered packing of Ag-Ag pairs in the structure was found
 Decomposition of Ag(HCO2) and formation of metal phase were studied
 Rietveld-refined micro-structural characteristics during decomposition
reveal the space relationship between the matrix structure and forming Ag
phase

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