Novel method for the production of copper(II) formates, their thermal, spectral and magnetic properties

Article abstract of DOI:10.1016/j.jallcom.2020.156208

A new method for the synthesis of various types of copper(II) formates with Cu(NO₃)₂?3H₂O and formic acid, orthorhombic (α) and monoclinic (β) modifications of anhydrous copper(II) formate, as well as Cu(HCOO)₂?2H₂O and Cu(HCOO)₂?4H₂O. The identity of the obtained compounds was confirmed by X-ray phase analysis, optical microscopy and thermogravimetry methods. It was shown that Cu(HCOO)₂?4H₂O can be used as a precursor for producing copper powders with particle size of the order of 150 nm. The results of investigation of vibration and absorption spectra of α and β modifications of anhydrous copper(II) formate are presented. Their magnetic behavior in the temperature interval 2–300 K was estimated, and comparative analysis of magnetic properties was carried out. First-principle calculations of electronic band structure for anhydrous α and β modifications were performed. The results obtained correspond to experimentally observed structural instability of β-Cu(HCOO)₂, fundamental absorption edge enhancement during transition from α to β modification, and low-temperature ferromagnetic ordering in α-Cu(HCOO)₂. Simulation for β modification is indicative of antiferromagnetic ordering of magnetic moments inside chains formed by copper-oxygen octahedra.

Full text of DOI:10.1016/j.jallcom.2020.156208

Journal Pre-proof
Novel method for the production of copper(II) formates, their thermal, spectral and
magnetic properties
V.N. Krasil'nikov, V.P. Zhukov, E.V. Chulkov, I.V. Baklanova, D.G. Kellerman, О.I.
Gyrdasova, Т.V. Dyachkova, А.P. Tyutyunnik
PII:
S0925-8388(20)32572-X
DOI:
https://doi.org/10.1016/j.jallcom.2020.156208
JALCOM 156208
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 8 April 2020
Revised Date: 7 June 2020
Accepted Date: 24 June 2020
Please cite this article as: V.N. Krasil'nikov, V.P. Zhukov, E.V. Chulkov, I.V. Baklanova, D.G. Kellerman,
О.I. Gyrdasova, Т.V. Dyachkova, А.P. Tyutyunnik, Novel method for the production of copper(II)
formates, their thermal, spectral and magnetic properties, Journal of Alloys and Compounds (2020), doi:
https://doi.org/10.1016/j.jallcom.2020.156208.
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V.N. Krasil’nikov: Conceptualization, Investigation, Roles/Writing - original draft, Writing - review & editing,
V.P. Zhukov: Investigation, Formal analysis, Writing - review & editing, E.V. Chulkov: Formal analysis, I.V.
Baklanova: Investigation, Visualization, Formal analysis, D.G. Kellerman: Investigation, Visualization,
Formal analysis, О.I. Gyrdasova: Formal analysis, Т.V. Dyachkova: Formal analysis, А.P. Tyutyunnik:
Investigation, Visualization, Formal analysis.

Novel method for the production of copper(II) formates, their thermal, spectral and magnetic properties
V.N. Krasil’nikov1, V.P. Zhukov1, E.V. Chulkov2,3,4,5, I.V. Baklanova1, D.G. Kellerman1,
О.I. Gyrdasova1, Т.V. Dyachkova1, А.P. Tyutyunnik1
1Institute of Solid State Chemistry Ural Branch Russian Academy Science, Ekaterinburg, Russia
2Departamento de Física de Materiales Universidad del País VascoUPV/EHU, San Sebastián/Donostia, Spain
3Donostia International Physics Center (DIPC), San Sebastián/Donostia, Spain
4Saint Petersburg State University, Saint Petersburg, Russia
5Tomsk State University, Tomsk, Russia
Abstract
A new method for the synthesis of various types of copper(II) formates with Cu(NO3)2∙3H2O and formic acid,
orthorhombic (α) and monoclinic (β) modifications of anhydrous copper(II) formate, as well as
Cu(HCOO)2∙2H2O and Cu(HCOO)2∙4H2O. The identity of the obtained compounds was confirmed by X-ray
phase analysis, optical microscopy and thermogravimetry methods. It was shown that Cu(HCOO)2∙4H2O can be
used as a precursor for producing copper powders with particle size of the order of 150 nm. The results of
investigation of vibration and absorption spectra of α and β modifications of anhydrous copper(II) formate are
presented. Their magnetic behavior in the temperature interval 2-300 K was estimated, and comparative
analysis of magnetic properties was carried out. First-principle calculations of electronic band structure for
anhydrous
structural instability of
modification, and low-temperature ferromagnetic ordering in
α
and
β
modifications were performed. The results obtained correspond to experimentally observed
-Cu(HCOO)2, fundamental absorption edge enhancement during transition from to
-Cu(HCOO)2. Simulation for modification is
β
α
β
α
β
indicative of antiferromagnetic ordering of magnetic moments inside chains formed by copper-oxygen
octahedra.
Keywords: formate, copper(II), UV-Vis-NIR spectra, IR spectra, magnetic properties, first-principle
calculations
Graphical abstract
Introduction
Much attention was focused recently on bivalent copper(II) formates of the general composition
Cu(HCOO)2∙nH2O (n = 0, 2, 4) due to the prospects of their application as precursors in technologies for
producing different functional materials, for example, molecular magnets – perovskite-like organometallic
compounds with framework structure, complex oxides, ultradispersed copper powders, films, profile contacts,
deposited catalysts, metalloorganic inks and polymeric composite materials for various electronic devices [1-
16,17]. The wide range of promising practical applications of copper(II) formates is determined by their
pronounced ability to transform into metal (Cu0) on heating in inert gaseous atmosphere without formation of
impurities of organic polymers and elementary carbon [11,12,18-21] which is typical for compounds of higher
carboxylic acids, for example, succinate, maleate etc. [22-26]. The development of methods for the synthesis of
copper nanoparticles with controllable dimensions, shape and surface properties is of much importance for their
application in various fields of nanotechnology where gold, silver and platinum nanoparticles are already
traditionally used at present [27,28]. Besides, copper(II) formates themselves are known to possess interesting
magnetic and dielectric properties [18, 29-33]. Presently, Cu(HCOO)2 [30, 32], Cu(HCOO)2∙2H2O [34-36],
Cu(HCOO)2∙4H2O [31,37-39] have been fairly well studied and described in the scientific literature. Anhydrous
copper(II) formate exists in three structural modifications, among which α-Cu(HCOO)2 (orthorhombic) and β-
Cu(HCOO)2 (monoclinic) [19, 32, 33] are stable under usual conditions. Considering the importance of
1

copper(II) formates as reagents and functional materials, effective approaches to their synthesis are being
actively developed. Currently, the familiar synthesis methods are based on the interaction of formic acid with
CuO, Cu(OH)2 and Cu2(OH)2CO3 in aqueous medium [11,18-21,31-39]. Most of these methods are
complicated multistage processes aimed at synthesis of a specific compound by selecting a copper-containing
precursor, formic acid concentration, reaction medium, conditions of reagents interaction and crystallization of
products. Poorly crystallized anhydrous copper(II) formate can be also produced in the form of monoclinic β
modification by dehydration of Cu(HCOO)2∙4H2O [32,39]. In our work we have developed and implemented a
universal and technologically simple method for copper(II) formate synthesis allowing α-Cu(HCOO)2, β-
Cu(HCOO)2, Cu(HCOO)2∙2H2O and Cu(HCOO)2∙4H2O to be obtained. For the purpose of chemical and
structural identification and research of physicochemical properties of compounds synthesized by the developed
technology, we employed X-ray diffraction (XPA), optical and electron microscopy, thermogravimetry (DTA
and TG analysis), vibration and absorption spectroscopy, and magnetometry methods. The experimental
results of magnetic properties investigations were compared with the first-principles calculation data.
The primary focus was on the examination of anhydrous modifications of copper(II) formate as the most
stable ones under real conditions.
Experiment technique
1. Synthesis
The process simplicity and universal character of the developed method for the synthesis of copper(II)
formates are provided by the use of Cu(NO3)2∙3H2O as a source of copper(II), which is highly soluble in
formic acid at room temperature irrespective of its concentration. The interaction of reagents is described
by the following reactions:
Cu(NO3)2∙3H2O + 2HCOOH = Cu(HCOO)2 + 2NO2 + 1/2O2 + 4H2O
Cu(NO3)2∙3H2O + 7HCOOH = Cu(HCOO)2 + N2 + 5СO2 + 5H2O
(1)
(2)
Reaction (1) proceeds with the use of concentrated formic acid, it begins spontaneously at room
o
temperature and is accompanied by strong heating (up to ~100 C) and intensive emission of brown
vapors of nitrogen dioxide. The interaction according to reaction (2) takes place with weak heating of
copper(II) nitrate solution in diluted formic acid, the major part of which is used to reduce nitrate ion to
molecular nitrogen by reaction [40]:
2HNO3 + 5HCOOH→N2 + 5СO2 + 6H2O
(3)
Considering the probabilities of reactions (1 and 2) in the system copper nitrate – formic acid, the following
options of copper(II) formates synthesis were proposed.
Synthesis of monoclinic β modification of anhydrous copper(II) formate. A powder of copper nitrate of
the composition Cu(NO3)2∙3H2O in the amount of 2.5 g was dissolved in 3.0 ml of 99.7% formic acid HCOOH
at room temperature. The interaction begins spontaneously after dissolution of nitrate and is accompanied by
strong heating and formation of blue crystals, whose X-ray diffraction pattern (Fig. 1S) corresponds to β-
Cu(HCOO)2 with lattice parameters: a = 8.1998(8), b = 7.9337(7), c = 3.6266(2) Å, β = 122.17(1)o, V =
199.71 Å3. Under the microscope, β-Cu(HCOO)2 crystals are observed mainly as oblique extended plates with
refractive indices: Ng = 1.722, Nm = 1.681, Np = 1.630.
Synthesis of orthorhombic α modification of anhydrous copper(II) formate. 2.5 g of copper nitrate
Cu(NO3)2∙3H2O powder were dissolved in 3.0 ml of 99.7% formic acid HCOOH at room temperature. β-
o
Cu(HCOO)2 crystals were formed whereupon 1 ml of distilled water was added and heated at 80 C until blue
color of the deposit changed to dark blue, which according to XPA data (Fig. 2S) was due to the formation of
orthorhombic α modification of anhydrous copper formate with lattice parameters: a = 14.203(1), b =
8.9441(8), c = 6.2305(7) Å, V = 791.48 Å3. Under the microscope, dark blue crystals in the form of needles and
elongated plates with refractive indices: Ng = 1.689, Nm = 1.643, Np = 1.597 were observed. Synthesis of α-
Cu(HCOO)2 without passing through the stage of β-Cu(HCOO)2 formation is possible, for example, by mixing
Cu(NO3)2∙3H2O with 70% formic acid and heating the mixture at 60-80 oC.
Synthesis of monoclinic modification of copper(II) formate dihydrate. Copper nitrate Cu(NO3)2∙3H2O
powder taken in the amount of 2.5 g was dissolved in 50 ml of 20% formic acid HCOOH at a temperature of 50
oC. The solution was evaporated until a greenish-blue deposit was formed, which was separated by vacuum
filtration, dried at room temperature and placed into an airtight vessel for storage. According to XPA data (Fig.
3S), a monoclinic modification of copper formate of the composition Cu(HCOO)2∙2H2O with lattice
parameters: a = 8.5190(8), b = 7.1346(7), c = 9.439(1) Å, β = 96.91(2)o, V = 569.53 Å3 is formed.
Cu(HCOO)2∙2H2O crystals have a form of hexagonal plates with refractive indices: Ng = 1.591, Nm = 1.540,
2

Np = 1.518. The color of β-Cu(HCOO)2, α-Cu(HCOO)2 and Cu(HCOO)2∙2H2O powders is demonstrated in the
photographic images in Fig. 4S.
Synthesis of monoclinic modification of copper(II) formate tetrahydrate. 2.5 g of copper nitrate
Cu(NO3)2∙3H2O were dissolved in 50 ml of 20% formic acid HCOOH at 50 oC. The produced solution was kept
open at room temperature until large light blue crystals were deposited (Fig. 5S). According to XPA data, in
this case a monoclinic modification of copper(II) formate of the composition Cu(HCOO)2∙4H2O with lattice
parameters: a = 8.1480(9), b = 8.1318(9), c = 6.3053(7) Å, β = 100.79(2)o, V = 410.39 Å3 is formed.
2. Instrumentation
The phase analysis of the precursors and the products of their thermolysis was carried out with the use of a
POLAM S-112 polarizing microscope in transmitted light (the refractive indices were determined by the
immersion method at room temperature) and a STADI-P automatic X-ray powder diffractometer (STOE,
Germany) in CuKα1-radiation using the powder diffraction file PDF-2 (ICDD Release 2016). The thermal
analysis was performed on a SETARAM Setsys Evolution thermoanalyzer during heating in air with a rate of
10 °C⋅min-1. The morphological features were examined using the scanning electron microscopy (SEM)
method on a JEOLJSM 6390 LA microscope (magnification factor 5-300000, resolving power 3.0 nm at 20
kV). The IR spectra were collected on a Vertex 80 IR Fourier spectrometer (Bruker) in the interval of 4000-400
cm-1 from powder-like samples palletized with CsI. IR spectra in the 4000-360 сm-1 range were recorded on a
Vertex 80 (Bruker) spectrometer using MVP-Pro ATR Accessory with diamond crystal (Harrick). The UV-Vis-
NIR spectra were recorded in the interval 190-1300 nm on a UV-2600 (Shimadzu) spectrometer using BaSO4
as standard. The magnetic properties were studied for a powder sample with the use of a vibrating sample
magnetometer VSM-5T (Cryogenic Ltd.). The temperature dependences of DC magnetization M were
measured in the temperature interval from 2 to 300 K. The magnetization curves were measured at 2 K and 300
K in magnetic fields to 5 T.
Results and discussion
1. Experimental results and discussion
The data of thermogravimetric analysis of two modifications of anhydrous copper(II) formate and dehydrate are
presented in Figs. 1-3 as TG and DTA curves. Thermal decomposition of α-Cu(HCOO)2 and β-Cu(HCOO)2 is
an exothermic process as evidenced by the maxima on the DTA curves (Figs. 1 and 2). The mass loss of the
o
samples heated in air to 500 C is estimated from the TG curves to be 48.26% (Fig. 1) and 48.04% (Fig. 2),
which corresponds to the calculated value 48.19% provided that copper(II) oxide CuO is formed. The mass loss
o
in the minimum points on the TG curves at 223 and 236 C (~Δm = 54 %) corresponds to the formation of
copper(I) oxide Cu2O during decomposition of α- and β-Cu(HCOO)2 (Δm = 53.40 %), whose oxidation
determines the samples’ mass enhancement with further increase in temperature.
The formation of crystalline hydrate Cu(HCOO)2∙2H2O is confirmed by the TG and DTA curves (Fig. 3). The
o
mass loss of the sample at 120 C is 18.53 mass%, which corresponds to the estimated data (Δm = 18.99 %)
provided that two moles of H2O are removed. The decomposition of the dehydration product also proceeds with
heat release and according to mass-spectrometry data is accompanied by isolation of H2O and CO2 (Fig. 3). The
thermal properties of copper(II) formate tetrahydrate were not studied by DTA and TG methods because of its
extremely low stability in ambient conditions: this compound decomposes into Cu(HCOO)2∙2H2O and β-
Cu(HCOO)2 already at room temperature during preparation. In the cause of time, a light blue deposit of small
β-Cu(HCOO)2 crystals is formed on the surface of large crystals Cu(HCOO)2∙4H2O due to their exposure in an
open vessel. According to XPA data (Fig. 6S), upon one day exposure in an open vessel the sample of
Cu(HCOO)2∙4H2O contains 50.9 mass% of initial phase, 38.3 mass% of Cu(HCOO)2∙2H2O and 10.8 mass% of
o
β-Cu(HCOO)2. However, on heating to 500 C the mass loss of freshly prepared Cu(HCOO)2∙4H2O sample is
64.12 mass%, which agrees well with the estimated data (Δm = 64.73 mass%) for its decomposition with the
formation of copper oxide CuO. According to earlier studies [38], the water lass for Cu(HCOO)2∙4H2O
corresponds to four H2O molecules per formula unit and proceeds in one stage without intermediate formation
of dehydrate during both isothermal dehydration at 50 °C and continuous heating to 110 °С, when anhydrous
monoclinic β modification of copper(II) formate is formed. On the other hand, the authors of work [41] showed
with the use of thermogravimetric and X-ray phase analysis methods that the process of dehydration of
Cu(HCOO)2∙4H2O during heating in air takes place in two stages with successive formation of
Cu(HCOO)2∙2H2O and β-Cu(HCOO)2.
3

Fig. 1. TG and DTA curves for α-Cu(HCOO)2.
Fig. 2. TG and DTA curves for β-Cu(HCOO)2.
Fig. 3. TG, DTA curves and mass-spectrometry data for Cu(HCOO)2∙2H2O.
Thus, all the synthesized copper(II) formates are transformed into CuO during heating in air to 500 °С at a rate
of 10 oC/min. The minima observed on the TG curves are due apparently to the formation of copper in the form
of oxide Cu2O and metal Cu0 [26], whose transformation into CuO corresponds to enhancement and
stabilization of the sample mass at temperatures above 400 °С (Figs. 1-3). According to data of works [11, 18-
21], Cu(HCOO)2 decomposes with the formation of elemental copper on heating in inert gaseous atmosphere.
The process occurs at low temperatures and is accompanied by release of H2, CO and CO2 creating conditions
for preserving copper in the form of metal [7,20]. Figure 4 displays the photographic images of
Cu(HCOO)2∙4H2O crystal and aggregates CuO and Cu0 formed as a result of its decomposition at 400 oC in air
and in helium atmosphere, respectively. It is seen that the aggregates of thermolysis products, in spite of
dramatic changes in the composition, preserve the appearance of Cu(HCOO)2∙4H2O crystal. The SEM images
in Fig. 5 demonstrate that the aggregates of the products of Cu(HCOO)2∙4H2O thermolysis in air and in helium
atmosphere actually have a loose internal structure and consist of CuO (Fig. 5a) and Cu0 (Fig. 5b) particles with
average size of about 150 nm. These aggregates easily decompose into constituent particles on exposure to
mechanical action or ultrasound. Figure 6 shows the morphology of the aggregates of the products of α-
o
Cu(HCOO)2 thermolysis in air and in helium atmosphere at 400 C. In this case, the decomposition of
anhydrous copper(II) formate gives rise to more dense aggregates built of CuO (Fig. 6a) and Cu0 (Fig. 6b)
nanoparticles.
4

Fig. 4. Photographic images of freshly prepared crystal Cu(HCOO)2∙4H2O (a) and the products of its
thermolysis in air (b) and in helium atmosphere (c) at 400 oC.
Fig. 5. SEM images of the morphology of the products of Cu(HCOO)2∙4H2O thermolysis in air (a) and in
helium atmosphere (b) at 400 oC.
Fig. 6. SEM images of the morphology of the products of α-Cu(HCOO)2 thermolysis in air (a) and in helium
atmosphere (b) at 400 oC.
5

As seen from Fig. 7 and Table 1, the IR spectra of the orthorhombic (α) and monoclinic (β) structural
modifications of anhydrous copper(II) formate are similar. The differences are in the splitting of some bands
into two components in the IR spectrum of α-Cu(HCOO)2. The absorption bands observed in the IR spectra of
the compounds are typical of metal-coordinated ion HCOO- [42]. A very intensive absorption band split into
two components, 1606 and 1555 cm-1, is responsible for the asymmetric stretching vibrations of C-O bonds of
OCO group in the IR spectrum of α-Cu(HCOO)2. Analogous vibrations of C-O bonds in the IR spectrum of β-
Cu(HCOO)2 are characterized by a very intensive band at 1566 cm-1. A very intensive narrow band at 1396
(shoulder), 1361 cm-1 (α) and 1367 cm-1 (β) is responsible for the symmetric stretching vibrations of C-O bonds
of OCO group. The stretching vibrations of C-H bonds manifest themselves as a weak broad band split into two
components, 2998, 2925 cm-1 (α) and 3000, 2924 cm-1 (β). A very weak band with absorption maximum at
1052 cm-1 (α) and 1063 cm-1 (β) is responsible for the bending vibrations of C-H bonds. The narrow band of
medium intensity at 819 cm-1 (α), 821 cm-1 (β) is due to the scissors vibrations of OCO [42]. The bands with
absorption maxima at 436, 399 cm-1 (α) and 426 cm-1 (β) can be attributed to the stretching vibrations of Cu-O
bonds [35,43]. The broad weak band at ~3500 cm-1 observed on the IR spectra (Fig. 7) is related to the
stretching vibrations of O-H bonds of sorbed H2O molecules. This is confirmed by the fact that this band is
absent on the IR (ATR) spectra (Fig. 7S, Table 1S).
Fig. 7. IR spectra of α-Cu(HCOO)2 (1) and β-Cu(HCOO)2 (2).
Table 1. Assignment of bands in IR spectra of α and β modifications of Cu(HCOO)2.
α-Cu(HCOO)2
β-Cu(HCOO)2
Assignment
2998
2925
1606
1555
3000
2924
1566
ν С-Н
νas СОО
νs СОО
1396 (sh.)
1361
1367
1052
1063
821
δ С-Н
δ OCO
ν Cu-O
819
436
399
426
6

Fig. 8. UV-Vis-NIR spectra of α-Cu(HCOO)2 (1) and β-Cu(HCOO)2 (2).
The absorption spectra of anhydrous copper(II) formate samples were studied in UV, visible and NIR ranges
6
3
(Fig. 8). In octahedral crystal field, the main electronic state for Cu2+ (d9 (2D)) is t2g eg , which agrees with the
term 2Eg. The term 2T2g corresponds to the excited electronic state t2g eg [44]. In the octahedral crystal field of
the β-Cu(HCOO)2 there is only one electron transition 2Eg→2T2g for Cu2+, which is observed in the absorption
spectrum at 1.77 eV. Distortion of octahedron (by elongation or compression) leads to tetragonal symmetry.
The ground state 2Eg is split due to the Jahn-Teller effect into 2B1g (becomes the ground state) and 2A1g states in
5
4
2
2
tetragonal symmetry, which leads to symmetry reduction for Cu2+ ion. The excited term T2g is split into B2g
2
and Eg states. In the absorption spectrum of the orthorhombic phase α-Cu(HCOO)2, in whose structure the
copper atoms are located in tetragonal pyramids CuO5 [30], the bands typical of Cu2+ are registered: the band at
2
2
1.86 eV corresponds to B1g→2Eg transition and the band at 1.48 eV – to B1g→2B2g transition. The values of
the charge-transfer absorption band edge are 3.7 eV for α-Cu(HCOO)2 and 3.9 eV for β-Cu(HCOO)2.
The magnetic properties of anhydrous copper formate have long attracted the attention of researchers in
connection with the problems of low-temperature magnetism and magnetic transitions in simple metal organic
structures. It was shown [30] that α-Cu(HCOO)2 above 20 K is a paramagnetic compound with Curie constant
C = 0.438 and magnetic moment per copper ion μ = 1.87 μB. The paramagnetic Curie temperature Θ (Weiss
constant), was evaluated as 9.8 K, indicative of a ferromagnetic transition. As distinct from the α phase, β-
Cu(HCOO)2 is ordered at a higher temperature (30.4 K), and in the ordered phase it exhibits a state attributed to
weak ferromagnetism [32]. Our studies of the α and β modifications of anhydrous copper(II) formate confirmed
that their magnetic behaviors differ considerably. From Fig. 9 it is seen that α-Cu(HCOO)2 at ≈ 8.5 K
undergoes a ferromagnetic ordering, the magnetization grows at this temperature by several orders of
magnitude. The magnetization curve measured at 2 K reaches saturation in the field of 0.1 T. Extrapolation of
the saturation area to zero field makes it possible to determine the magnetic moment value per copper ion. It is
1.1 μB, which confirms the conclusion about the ferromagnetic character of ordering in this phase [32,45],
however, it is much lower than that obtained in these works at 5 and 7 K. From Fig. 9 it also follows that the
magnetization of the ferromagnetic phase α-Cu(HCOO)2 decreases abruptly when the temperature falls below 5
K. The nature of this effect requires special studies using magnetic neutron diffraction analysis methods. At
temperatures above 25 K, α-Cu(HCOO)2 behaves as a paramagnet; Figure 10 shows the temperature
dependence of magnetic susceptibility measured in the 0.01 T field.
Fig. 9. Magnetization of α-Cu(HCOO)2 vs temperature and magnetic field (inset).
7

Fig. 10. Temperature dependences of magnetic susceptibility for α- and β-Cu(HCOO)2.
Fig. 11. Temperature and field (inset) dependences of magnetization for β-Cu(HCOO)2.
The susceptibility of α-Cu(HCOO)2 at T > 25 K is described well by the Curie-Weiss law χ = A + C/(T - Θ)
(CW), where A is the temperature-independent part including the diamagnetic correction and Van Vleck
paramagnetism. Figure 10 displays the measured values of magnetic susceptibility and the result of their CW
approximation. It is seen that the experimental and calculated data are in good agreement. The following values
were obtained: A = -0.0017 cm3/mol, C = 0.658 cm3/mol*K, Θ = 15.8 K (R = 0.94). A positive value of Θ is
indicative of ferromagnetic exchange interactions between copper ions in α-Cu(HCOO)2, which at low
temperatures lead to ferromagnetic ordering. However, magnetic moment per copper ion calculated by formula
μ = √(8С) is 2.29 μB. This is much larger than the spin-only value for the d9 configuration, S = 1/2 (1.0 μB).
Overestimated values of magnetic moments are often observed for compounds containing Cu2+ cations [5].
They can be interpreted as a result of incomplete orbital moment quenching possible for this electronic
configuration. On the other hand, it is known [32] that the structure of α-Cu(HCOO)2 contains Cu-Cu dimers,
which magnetically can be treated as exchange-coupled clusters, whose behavior is usually considered in the
framework of the Heisenberg-Dirac-Van Vleck model. For the description of the temperature dependence of
magnetic susceptibility, we used a particular case for dimer, the so-called Bleaney-Bowers equation: χ =
NAg2β2/2*3kBT *[1+1/3exp(-2J/kBT]-1 +A (BB), where J is an exchange interaction parameter in dimers and A
(as in CW) is a temperature-independent part including the diamagnetic correction and Van Vleck
paramagnetism. Fitting of the parameters J and A yielded J = +16.9 cm-1 and A = -0.0028 cm3/mol (R = 0.94).
The values of the R factor in the calculations by formulas (CW) and (BB) coincide, therefore the results of
approximation in Fig. 10 also completely coincide. The values of the temperature-independent contribution to
magnetic susceptibility obtained by two formulas are predictably similar. The obtained positive value of the
exchange parameter demonstrates the reasonableness of the employed model. The ferromagnetic character of α-
Cu(HCOO)2 at low temperatures is in line with the results of first-principle modeling of electronic band
structure discussed below.
β-Cu(HCOO)2 exhibits magnetism of different character. Magnetic ordering of β-Cu(HCOO)2 takes place at
30.5 K, which agrees well with data [32]. It is characterized by a rather large hysteresis (Fig. 11, inset).
8

Extrapolation of the field dependence of magnetization to zero field yields here 0.012 μB moment per copper
ion. This is two orders of magnitude smaller than in case of α-Cu(HCOO)2. At higher temperatures for β-
Cu(HCOO)2, as distinct from α modification, a more complex dependence of magnetic susceptibility is
observed. This dependence deviates considerably from the Curie-Weiss law: at ≈70 K a wide maximum is
observed on the temperature dependences of susceptibility and magnetization (Figs. 10, 11). In work [46], the
anomalies of this function are discussed in the framework of the antiferromagnetic Heisenberg model for spin S
= ½, allowing χ(T) to be expressed as a series in terms of powers of function kBTc/J. However, a more
demonstrative and no less successful model is the BB model offering a good explanation of the χ(T)
dependences with a broad maximum typical of exchange-coupled clusters with antiferromagnetic exchange.
Using the (BB) equation for the description of susceptibility of β-Cu(HCOO)2 at temperatures above 50 K, we
have the value of exchange parameter J = -39.4 cm-1. The results of approximation are shown by a black solid
line in Fig. 10. The negative sign of the exchange parameter confirms the antiferromagnetic bonding between
copper ions in clusters. When the temperature decreases, a transition to antiferromagnetic state occurs (Fig. 11).
The observed in [32] weak ferromagnetism, with very small values of copper magnetic moments, 0.06 µB, has
then to be attributed to canting effect. The antiferromagnetic character of β-Cu(HCOO)2 at low temperatures is
confirmed by the first-principles calculations of the electronic band structure performed by the authors.
2. Electronic band structure and magnetism of α- and β-Cu(HCOO)2
Formate α-Cu(HCOO)2 crystallizes in orthorhombic structure Pbca, whose parameters are reported in
[30], and β-Cu(HCOO)2 – in monoclinic structure P21/aс with parameters given in [32]. Figures 12 and 13
show the cells of these structures plotted with the use of the VESTA program [47], which for illustration
purposes are doubly broadened in the b direction for α-Cu(HCOO)2 and in the direction of с axis for β-
Cu(HCOO)2. The main structural elements of α-Cu(HCOO)2 are edge-sharing tetragonal pyramids CuO5
forming dimers. All 10 vertices of a dimer are connected with neighboring dimers through carboxylate groups.
Fig. 12. Unit cell of α-Cu(HCOO)2.
By contrast, the main structural elements of β-Cu(HCOO)2 are distorted CuO6 octahedra, each of which shares
edges with two adjacent octahedra forming chains in the direction of the с axis. The neighboring chains are also
connected with each other by formate groups. In the case of α-Cu(HCOO)2, the electronic structure calculations
were carried out for a primitive unit cell of atomic composition Cu8O32C16H16 with parameters reported in [30],
whose dimensions are large enough for modeling both ferro- and antiferromagnetic ordering. The calculations
have been performed for the totally ferromagnetic ordering of the copper magnetic moments, whereas for the
antiferromagnetic ordering the opposite orientation of copper moments inside the adjacent pyramids of the
dimers has been accepted. In the case of β-Cu(HCOO)2, the calculations were performed for a doubled cell
(Fig. 13) with atomic composition Cu4O16C8H8. Doubling in the c axis also makes it possible to model ferro-
and antiferromagnetic ordering in the chains of octahedrons. For the ferromagnetic ordering the equal
orientation of all the moments has been accepted whereas for the antiferromagnetic model the opposite
orientation of the copper atoms moments nearest inside the chains was supposed. An important detail of the
calculation is the choice for the initial magnetic moments of the self-consistency process. We have detected that
taking these values less than 2 µB we can achieve the self-consistent values of magnetic moment in good
agreement with experiment data, ~1 µB, whereas with higher initial moments the final moments are strongly
overestimated.
9

The electronic band structure calculations were carried out using the pseudo-potential augmented projector
waves (PAW) method of the electronic density functional theory as implemented in the Vienna ab-initio
simulation package VASP [48]. Program supplied PAW-type atomic exchange-correlation potentials were
used. The electronic band states were calculated on a grid containing 24 wave vectors per the irreducible part of
the Brillouin zone in case of α-Cu(HCOO)2 and 68 vectors in case of β-Cu(HCOO)2. Band states wave
functions were expanded in plane waves with energy cutoff 600 eV. The densities of band states were plotted
using the energy level broadening of 0.05 eV.
Fig. 13. Unit cell of β-Cu(HCOO)2.
The known error of the computational methods based on the GGA approximation is underestimation of band
gap in semiconductors containing d or f elements, which sometimes leads even to an incorrect character of
conductivity. For copper oxides, the methods for elimination of this error were analyzed in greater detail in
work [49]. It was shown that the effective method for band structure correction is the so-called GGA+U
approximation, in which single-center Anderson exchange-correlation corrections are introduced for d or f
states; the exchange corrections depend on the U parameter and the correlation correction – on the J parameter.
The most frequently used version of GGA+U approximation is proposed by Dudarev et al. [50], in which
corrections depend on the value of only one parameter U-J determining both exchange and correlation. For
copper oxides it was shown [49] that the optimal value is U-J = 7.5-0.98 eV; this value of parameter U-J was
used as a standard value also in works [51-53].
Fig. 14. Total and 3d Cu densities of states for ferromagnetic α-Cu(HCOO)2 phase in GGA+U approximation
with U-J=6.5 eV. The Fermi level counts as the zero of energy. The sign of the DOS,s corresponds to sign of
the z spin projection.
We analyzed the effectiveness of application of the GGA+U and GGA approximations to copper formates.
Figure 14 displays the total and 3dCu densities of states (DOS) for α-Cu(HCOO)2 in the case of GGA+U
approximation at U-J = 6.5 eV. Comparison of the total and partial DOS shows that the occupied 3dCu states
with the positive spin projection are located in the interval from –6 to -3 eV, the states with the negative spin
projection have the energy between -4.5 and -3 eV, and the empty states with the negative spin have the energy
of about 2.2 eV. It follows that the minimal excitation energy between copper states is about 5 eV, which does
10

not agree with our absorption spectra indicating that the maximal energy of transitions between 3d states does
not exceed ~2.5 eV (λ ~ 400 nm).
In order to evaluate the application efficiency of the GGA+U approach at small values of U, we also calculated
optical absorption for the ferromagnetic α-Cu(HCOO)2. According to the theory [54], the absorption
coefficient, determined as reduction of electric vector of light wave per a unit of propagation length in crystal,
depends mostly on the imaginary part of the dielectric function ε2. In Fig. 15, the function ε2 calculated by
using the GGA and GGA+U approximations with U-J = 1 and 2 eV and the corresponding absorption
coefficient K are compared with our experimental data. In the experimental spectrum, the interval from 1 to 2.5
eV corresponds to transitions between 3d states of copper, and fundamental absorption begins at energies above
3 eV. In the GGA approximation, the 3dCu excitations are also within the interval 1 – 2.5 eV. The introduction
of exchange-correlation corrections leads to a considerable increase of the energy of 3dCu excitations, at U-J =
2 eV the band of 3dCu excitations merges with the band of fundamental excitations, which does not agree with
the experimental data.
Fig. 15. The calculated values of the imaginary part of dielectric function ε2, absorption coefficient K, and
experimental absorption coefficient for α- and β-Cu(HCOO)2.
Thus, at any values of U-J parameter, the description of the low-temperature excitation spectrum on the basis of
GGA+U calculations is likely to be worse than in the “classical” GGA approach. The total DOS in the GGA
approximation for ferromagnetic α-modification of Cu(HCOO)2 and several partial DOS's are given in Fig. 16.
The states in the vicinity of -10 eV are built mainly of hydrogen 1s orbitals. Above them, in the interval from -
8.5 to -5 eV, weakly spin-polarized hybrid states built of almost similar contributions of 2pOpl and 2рСpl
orbitals are located, where Opl are atoms making up square bases of pyramidal clusters CuO4 and Cpl are atoms
of formate groups coupled with Opl atoms (we omit the discussion of DOS's of oxygen atoms forming vertices
of pyramids and coupled with them carbon atoms since they differ insignificantly from analogous DOS in Fig.
16). Similarly, there are strongly hybridized 2pOpl-2pCpl states above the Fermi level at energy of about 4 eV. It
follows then that the states between -8.5 and -5 eV are bonding oxygen-carbon states and at about 4 eV are their
antibonding counterparts. In the range from -4.5 eV to the Fermi level, the bands of 2pOpl states that are not
hybridized with the states of carbon atoms, i.e. nonbonding states of formate groups, are located. Above this
group of states, from -2 eV to the Fermi level and over it, there are 3dCu states, which are also partially
hybridized with the 2p states of Opl atoms. The density of 2pOpl states is relatively small, however, because of
their large number they considerably contribute to the cell magnetization. Near the Fermi level there are two
3dx2-y2 states, one occupied and one empty, with the energy interval between them equal to 1.1 eV,
corresponding to exchange splitting. From the shape of the band structure it follows that copper atoms have the
electron configuration Cu 3d9 and magnetic moment close to 1 μB, which corresponds to our experimental
estimates of copper magnetic moment, 1.1 μB, and differs considerably from the estimates of work [30]. The
11

calculations also show that the lower fundamental absorption edge at energy above 3 eV corresponds to
transitions from the 3d states of copper to the 2p states of carbon.
Fig. 16. Total and partial densities of states for ferromagnetic α-modification of Cu(HCOO)2 in GGA
approximation. The Fermi level counts as the zero of energy. Total and 3dCu DOS are given for positive and
negative spin projections, and DOS summed over spin projections are presented for oxygen and carbon atoms.
Such calculations for ferromagnetic ordering in β-Cu(HCOO)2 yield the densities of states close to the DOS of
α-Cu(HCOO)2; this contradicts the results of work [32] pointing to small values of magnetic moments for this
phase, 0.066 μB, which according to our data are 0.012 μB. Hence, in accordance with our measurements, β-
Cu(HCOO)2 at low temperatures is likely to be antiferromagnetic, and small values of its magnetic moments
are due to the canting effect at small deviations of the direction of magnetic moments from the axis of their
antiferromagnetism. For β-Cu(HCOO)2, 2 models of antiferromagnetic ordering are possible. In the first, the
antiferromagnetic ordering can take place inside chains of copper-oxygen octahedra, while in the second model
the ordering inside the chains is ferromagnetic, but the directions of magnetic moments in the neighboring
chains are antiparallel. Some characteristics of the electronic structure of α- and β-Cu(HCOO)2 obtained from
the GGA approximation calculations are listed in Table 2.
Table 2. The characteristics of the band structure and magnetism in α and β phases of Cu(HCOO)2
Characteristic
Band gap (eV)
α phase
3.34
β phase
3.58
(3.70)*
(4.01)
Experimental band gap (eV)
3.67
3.87
Magnetic moment of copper atom (μB)
Total energy (Efm) of ferromagnetic cell (eV)
Total energy (Eafm) of antiferromagnetic cell (eV)
0.59
-446.281
-446.264
0.54
-222.765
-222.949
(-221.864)**
*Band gap in GGA+U approximation at U-J=1 eV.
**Total energy value for the second model of antiferromagnetic ordering.
It follows that the GGA approach leads as usual to an underestimated value of the fundamental absorption onset
energy; however, such calculations correctly reproduce the increase of the fundamental absorption edge from
to modification of anhydrous copper(II) formate. Note that the introduction of exchange-correlation
α
β
corrections in the GGA+U approximation at small U values leads only to partial improvement: for α-
Cu(HCOO)2 the calculated energy gap width corresponds better to the experimental value, but for β-
12

Cu(HCOO)2 it is overestimated. A lower energy Efm of ferromagnetic cell of α-Cu(HCOO)2 corresponds to
experimentally observed ferromagnetism, whereas a lower value of Eafm for the antiferromagnetic cell of β-
Cu(HCOO)2 corresponds to a very small experimental value of copper magnetic moment. Moreover, the
difference Efm-Eafm, -0.0217 eV for α-Cu(HCOO)2 and 0.184 eV for β-Cu(HCOO)2, in accordance with
experiment, is indicative of a higher value of the critical temperature for β modification of copper(II) formate.
In the case of β-Cu(HCOO)2, the total energy value for the first model of antiferromagnetism turns out to be
much smaller than that for the second model. We conclude then that the antiferromagnetic ordering in β-
Cu(HCOO)2 at T < 9 K is probably realized along the octahedra chains. This result contradicts the data of the
work [32] offering analysis of magnetism above 9 K, in which it is concluded that weak ferromagnetic ordering
is realized in the paramagnetic region inside the chains, whereas antiferromagnetic ordering occurs between the
chains. However, our theoretical conclusions on the nature of antiferromagnetism agree with our results of
modeling the temperature dependence of susceptibility based on the antiferromagnetic ordering in the chains.
Note also that the doubled value of the total energy for the first model of antiferromagnetic cell of β-
Cu(HCOO)2, -445.898 eV, is higher than the total energy for α-Cu(HCOO)2. Consequently, the β modification
of Cu(HCOO)2 is metastable, which agrees with both our observations and the results of work [32], bearing
witness to slow transformation of β-Cu(HCOO)2 into α-Cu(HCOO)2 during weak heating.
A certain disagreement with the experimental data lies in the fact that for a ferromagnetic cell of α-Cu(HCOO)2
the calculated magnetic moment of the copper atom, 0.59 μB, is smaller than our experimental value, 1.1 μB, and
besides it does not correspond to the conclusions following from analysis of the densities of states. However,
this contradiction can be explained based on the effect of magnetization delocalization throughout the cell. The
calculations show that all oxygen atoms have the same spin polarization direction as copper atoms. Their
magnetic moment values are small, from 0.07 to 0.1 μB, but together they make a considerable contribution to
magnetization. Since the experimental value of copper atom magnetic moment is found from magnetization
values of the whole cell, then referring our estimated value of magnetization of the whole cell of α-
Cu(HCOO)2, 7.64 μB, to copper atoms, we have M(Cu) = 0.95 μB, in good agreement with the experiment.
Conclusion
Thus, as a result of the studies performed we have developed a universal technique for the synthesis of
anhydrous copper(II) formate in the form of orthorhombic (α) and monoclinic (β) modifications, as well as
dehydrate and tetrahydrate. The technique is based on the interaction of copper(II) nitrate with formic acid,
which is represented in the form of two possible reactions (1 and 2). The structure and composition of the
synthesized compound were specified by varying the concentration of formic acid, temperature and
crystallization conditions. Reaction (1) is possible only when Cu(NO3)2∙3H2O is dissolved in concetrated
formic acid; reaction (2) is more environmentally friendly since it proceeds without emission of toxic NO2
vapors, however, in that case much more formic acid and water are consumed. Moreover, reaction (2) is
specific for solving the problem of synthesis of Cu(HCOO)2·2H2O and Cu(HCOO)2·4H2O hydrates.
One of the most important properties of compounds of the general composition Cu(HCOO)2·nH2O is their
ability to transform into Cu0 metal during heating in inert atmosphere, which can be used for producing copper
contacts, powders and composites based thereon. In this respect, of much interest is tetrahydrate, the
thermolysis of which leads to the production of copper powder with particle size of the order of 150 nm.
The obtained temperature dependences of magnetization of anhydrous modifications of Cu(HCOO)2 are
described well in the framework of the Bleaney-Bowers model. The values of critical temperatures found in this
model correspond to low-temperature ferromagnetic ordering of
α-Cu(HCOO)2 and antiferromagnetic ordering
of -Cu(HCOO)2. For modification, the magnetic moment values obtained from the BB model increase
β
α
appreciably with a rise in temperature from 1 to 7 K, which is probably due to the known effect of orbital
moments tempering.
For the electronic band structure modeling we tested two approaches: the GGA+U approximation usually used
for the calculation of the band structure of copper oxides and the “classical” GGA approximation for exchange-
correlation potential. It was shown that a more correct description of absorption spectra is achieved in the GGA
approximation, whereas the GGA+U approach is inadequate since absorption between copper ion terms is
superimposed on the fundamental absorption with charge transfer. The GGA approach indicates that absorption
with charge transfer takes place between the band states of copper and carbon atoms; this approach describes
correctly the increase of the edge of this adsorption from
α to β modification. On the other hand, the GGA
approach gives a description of low-temperature magnetic ordering corresponding to the results of our
experiments. For
β
-Cu(HCOO)2, the modeling indicates an antiferromagnetic ordering of moments in chains of
13

copper-oxygen octahedra, in agreement with the Bleaney-Bowers model. A higher calculated value of the total
energy for -Cu(HCOO)2 suggests the instability of this structural modification compared with -Cu(HCOO)2,
β
α
which also agrees with the experimental data.
Acknowledgment
This work was carried out in accordance with the scientific and research plans and state assignment of the
Institute of Solid State Chemistry of the Ural Branch of the Russian Academy of Sciences (grant No. AAAA-
A19-119031890025-9). The UV-Vis spectra were recorded on equipment of the Center for Joint Use
“Spectroscopy and Analysis of Organic Compounds” at the Postovsky Institute of Organic Synthesis, UB RAS.
The calculations were performed in URAN cluster at the Institute of Mathematics and Mechanics, UB RAS.
E.V.C. acknowledges support from the Saint Petersburg State University (Grant No. 51126047) and Tomsk
State University competitiveness improvement programme (project 8.1.01.2018).
Conflict of Interest
The authors declare that they have no conflict of interest.
All authors made substantial contributions to the conception of the work, analysis and interpretation of data,
and drafted the work and revised it critically for important intellectual content. Material preparation, data
collection and analysis were performed by [V.P. Zhukov], [E.V. Chulkov], [I.V. Baklanova], [D.G. Kellerman],
[О.I. Gyrdasova], [Т.V. Dyachkova] and [А.P. Tyutyunnik]. The first draft of the manuscript was written by
[V.N. Krasil’nikov] and all authors commented on previous versions of the manuscript. All authors read and
approved the final manuscript.
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16

Highlights
A new method has been developed to produce different types of copper (II) formates.
Magnetic properties of α and β modifications of Cu(HCOO)2 are very different.
Electronic structure of α- and β-Cu(HCOO)2 is investigated using first-principle calculations.

Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships
that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered
as potential competing interests:
Inna Baklanova, on behalf of the authors*
* Corresponding author: Dr. Inna Baklanova,
Institute of Solid State Chemistry, Ural Branch, Russian Academy of Sciences,
620990, Ekaterinburg, Russia
E-mail: baklanova_i@ihim.uran.ru
Ekaterinburg, Russia
April 8, 2020