Violeta Koleva - Academia.edu (original) (raw)
Papers by Violeta Koleva
Batteries
This study aims to quantify the synergistic effect of Ni2+ and Mn2+ ions on the capacitive perfor... more This study aims to quantify the synergistic effect of Ni2+ and Mn2+ ions on the capacitive performance of oxide, hydroxide and phosphate electrodes in alkaline electrolytes. Three types of phases containing both nickel and manganese in a ratio of one-to-one were selected due to their stability in alkaline media: oxides with ilmenite and spinel structures (NiMnO3 and Ni1.5Mn1.5O4); hydroxides with layered structures (β-Ni1/2Mn1/2(OH)2); and phosphates with olivine and maricite structures (LiNi1/2Mn1/2PO4 and NaNi1/2Mn1/2PO4). In the mixed hydroxides and phosphates, Ni2+ and Mn2+ ions randomly occupied one crystallographic site, whereas in the ilmenite oxide, a common face was shared by the Ni2+ and Mn4+ ions. The electrochemical parameters of the Ni–Mn compositions were evaluated in asymmetric hybrid supercapacitor cells working with alkaline electrolytes and activated carbon as a negative electrode. A comparative analysis of oxides, hydroxides and phosphates enabled us to differenti...
Molten salt reactions between dittmarite-type NH4FePO4.H2O and different lithium eutectic composi... more Molten salt reactions between dittmarite-type NH4FePO4.H2O and different lithium eutectic compositions are systematically investigated in order to obtain at low temperature olivine-type LiFePO4 which is valuable electrode material for lithium-ion batteries. Nanosized LiFePO4 powder is successfully prepared by topotactic reactions using LiCl-LiOH eutectic compositions at 277–325 oC for short reaction time of 6090 min. It is well crystallized with a preferred crystallite orientation along a-crystallographic axis. The prepared LiFePO4 is characterized by a plate-like morphology inherited from the iron precursor. The micrometer LiFePO4 plates are composed of nanosized spherical particles with dimensions about 50 nm which is favorable for further electrochemical performance.
Chemical record (New York, N.Y.), Jan 17, 2018
The energy storage by redox intercalation reactions is, nowadays, the most effective rechargeable... more The energy storage by redox intercalation reactions is, nowadays, the most effective rechargeable ion battery. When lithium is used as intercalating agents, the high energy density is achieved at an expense of non-sustainability. The replacement of Li with cheaper monovalent ions enables to make greener battery alternatives. The utilization of polyvalent ions instead of Li permits to multiplying the battery capacity. Contrary to Li , the realization of quick and reversible intercalation of bigger monovalent and of polyvalent ions is a scientific challenge due to kinetic constraints, polarizing ion effects and Coulomb interactions. Herein we provide a vision how to make the intercalation of these ions feasible. The idea is to perform dual intercalation of ions having different charges, radii, preferred coordination and diffusion pathway topology. All these features are demonstrated by the recent knowledge on selective and non-selective intercalation properties of oxides and polyanion...
Electrochimica Acta, 2018
Hybrid supercapacitors with the composite electrode materials display high energy density at the ... more Hybrid supercapacitors with the composite electrode materials display high energy density at the expense of the reduced cycle stability. Herein, we provide new data on the electrochemical performance of hybrid electrodes based on mixed nickel hydroxides/manganese oxides in the form of structured and multiphase composites. As structured composites, two types of less known structure modifications are examined: interstratified modification of Ni(OH) 2 (i.e. α/β IS-Ni(OH) 2) and ε-modification of MnO 2. The multiphase hydroxide/oxide composites are prepared by the conventional grinding of α/β IS-Ni(OH) 2 and ε-MnO 2 and by the in-situ formation after the reaction of layered Na 0.5 Ni 0.5 Mn 0.5 O 2 with mixed LiOH-KOH electrolyte. The structure, morphology and porous texture properties of composites are analyzed by means of powder X-ray diffraction, scanning electron microscopy (SEM) and low-temperature nitrogen adsorption, respectively. The electrochemical performance of composites electrodes is determined by galvanostatic experiments in concentrated individual KOH and mixed LiOH-KOH electrolytes. The ex-situ X-ray diffraction is used to monitor the changes in composite electrodes during electrochemical cell function. It has been found that α/β IS-Ni(OH) 2 participates in electrochemical reaction concomitantly with H 2 O and Li + intercalation, while the electrochemical performance of ε-MnO 2 is determined by surface adsorption of electrolyte
Phys. Chem. Chem. Phys., 2017
In Mg-substituted NaMnPO4, Na+, Mg2+ and metal vacancies reside in the M1 olivine position, while... more In Mg-substituted NaMnPO4, Na+, Mg2+ and metal vacancies reside in the M1 olivine position, while Mn2+ ions are predominantly in the M2 position.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 2002
Synthetic malachite, hydrozincite and five monophasic mixed copper-zinc hydroxycarbonates have be... more Synthetic malachite, hydrozincite and five monophasic mixed copper-zinc hydroxycarbonates have been studied by Fourier transform infrared (FTIR) spectroscopy at ambient and liquid nitrogen temperature in the region of 4000-400 cm(-1). The analysis of the spectra reveals that the samples containing up to 20% zinc retain the malachite lattice, thus forming solid solutions. The inclusion of zinc ions in malachite reflects on the positions and intensity of the bands corresponding to the internal modes of the carbonate ion, to the OH librations and to the Me-O interactions. For example, the higher and the lower frequency components of v3 shift to higher and lower frequencies, respectively. The intensity of the bands corresponding to v2 decreases with the zinc content increase. The spectrum of the sample Cu1.31Zn0.69(OH)2CO3 become diffuse and ill-resolved in the region of the Me-O interactions (region below 600 cm(-1)) and the corresponding bands are shifted to lower frequencies due to the weaker Zn-O interactions as compared with those of the copper ions. The internal modes of the carbonate ions in hydrozincite and aurichalcite are assigned and discussed taking into account the site symmetry and factor group symmetry. The OH and OD stretches (matrix-isolated HDO molecules) and the hydrogen bond strengths are interpreted in terms of Me-O interactions (synergetic effect), hydrogen bond angles and different hydrogen bond acceptor strengths of the oxygen atoms from the carbonate ions. It proves that the hydrogen bonds in hydrozincite are stronger as compared with those in malachite, irrespective of both the larger hydrogen bond lengths and the weaker Zn-O interactions in hydrozincite due to the higher hydrogen bond acceptor strength of the non-coordinated oxygen atom and the formation of bifurcated hydrogen bonds.
European Journal of Inorganic Chemistry, 2010
Materials Chemistry and Physics, 2010
Bulgarian Chemical Communications
The phase formation in the systems NH4MnPO4.H2O-LiCl-LiNO3 and NH4MnPO4.H2O-NaCH3COO.3H2O is syst... more The phase formation in the systems NH4MnPO4.H2O-LiCl-LiNO3 and NH4MnPO4.H2O-NaCH3COO.3H2O is systematically investigated in order to obtain at low-temperature olivine-type LiMnPO4 and NaMnPO4 which are of great interest as cathode materials for lithium and sodium ion batteries. The experimental conditions such as molar ratio between reagents, temperature and reaction time are varied to find the more suitable procedure for the low temperature formation of pure phospho-olivines. It is established that the transformation of the dittmarite precursor into sodium manganese phospho-olivine proceeds between 200-250 oC with participation of large excess of the Na-salt more slowly in comparison with the fast ion exchange of NH4+ for Li+ leading to the formation of LiMnPO4 for 1.5 h only. Both prepared olivines are well-crystallized and exhibit nano-sized crystallites (50 - 60 nm).
Vibrational Spectroscopy, 2003
The infrared spectra of related compounds with kröhnkite-type chains Na 2 Me(SeO 4) 2 Á2H 2 O (Me... more The infrared spectra of related compounds with kröhnkite-type chains Na 2 Me(SeO 4) 2 Á2H 2 O (Me ¼ Mn, Co, Ni, Cu, Zn, Cd) containing matrix-isolated SO 4 2À guest ions are reported and discussed with respect to the SO stretching modes n 3 and n 1. Due to the low site symmetry C 1 of the SO 4 2À guest ions three bands for n 3 and one band for n 1 are seen in all spectra. When SO 4 2À guest ions are incorporated in the triclinic Na 2 Zn(SeO 4) 2 Á2H 2 O, Na 2 Co(SeO 4) 2 Á2H 2 O and Na 2 Ni(SeO 4) 2 Á2H 2 O host lattices the n 3 stretching region resembles a higher local symmetry (A 1 È E) of the SO 4 2À guest ions than the crystallographic one (i.e. Dn ab > Dn bc instead of Dn ab % Dn bc , a, b and c being three n 3 components). Hence, the ratio between Dn ab and Dn bc additionally to Dn max (the difference between the highest and the lowest wavenumbered SO stretching modes) has to be taken into account when the SO 4 2À guest ion distortions are considered (the higher the ratio Dn ab /Dn bc is, the weaker the distortion is). Both the site group splittings of the triplet component n 3 (Dn ac) and Dn max values are an adequate measure for the SO 4 2À guest ion distortion when the guest ions are incorporated in the monoclinic Na 2 Cu(SeO 4) 2 Á2H 2 O, Na 2 Mn(SeO 4) 2 Á2H 2 O and Na 2 Cd(SeO 4) 2 Á2H 2 O (i.e. Dn ab % Dn bc). In addition to the local potential at the lattice site of the host lattice, the metal environment of the SO 4 2À guest ions reflects on the guest ion distortions. A correlation between the SO 4 2À guest ion distortions and the metal electronic configurations has been found and discussed. Me 02þ guest ions incorporated additionally to the SO 4 2À guest ions in the selenate lattices do not influence practically on the vibrational behavior of the SO stretches.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 2007
The IR and Raman spectra of the isostructural M'M'&#3... more The IR and Raman spectra of the isostructural M'M''PO4.H2O compounds (M'=K+, NH4+; M''=Mn2+, Co2+, Ni2+) are reported and discussed with respect to the normal vibrations of the PO(4)3- ions. The vibrational behavior of PO4(3-) is in agreement with its low site symmetry Cs in the lattices-the symmetric nu1 and nu2 modes are activated in the IR spectra and the degeneration of the asymmetric nu3 and nu4 modes is lifted. A relatively large unit-cell group splitting is observed for nu1 in both the IR and Raman spectra and for nu3 in Raman spectra. It has been established that the mean wavenumbers of the P-O stretches (nuPO) are not affected by the M2+ ions present, but they are lower for the NH4-series than for the K-one (predominant influence of both the smaller repulsion potential and the hydrogen bonds in the NH4-lattices over the influence of the M+-O interactions). The extent of the energetic distortion of the PO(4)3- ions has been estimated based on the spectroscopic data for the site group splitting of the asymmetric modes (Deltanu3 and Deltanu4), the separation between the highest and the lowest wavenumbered P-O stretches (Deltanumax) and the intensity of nu1 in the IR spectra. The data provide an evidence that the PO4(3-) ions in KM''PO4.H2O are more distorted regarding the P-O bond lengths than those in NH4M''PO4.H2O, but their angular distortion is the same in both series. The trends for the energetic distortion of the phosphate ions found from the spectroscopic data correspond to the data for their geometric distortion deduced from the values of the distortion indices DI(PO) and DI(OPO).
Materials Research Bulletin, 2006
ABSTRACT It has been established that M(H2PO4)2·2H2O (M = Mn, Co, Ni) are paramagnetics between 1... more ABSTRACT It has been established that M(H2PO4)2·2H2O (M = Mn, Co, Ni) are paramagnetics between 173 and 353 K with weak antiferromagnetic exchange interaction between the metal ions. In situ magnetic measurements during the thermal decomposition of the salts show that the oxidation state and the octahedral coordination of M2+ are preserved. From the data obtained it could be supposed that in M(H2PO4)2·2H2O (M = Co, Ni) this process is topotactic with no long-range diffusion transport. In Mn(H2PO4)2·2H2O, the formation of the large variety of intermediate products probably requires more drastic rearrangement and diffusion of the manganese ions during the complex transformations, which reflect on both the value and the sign of the θ constants. M2P4O12 (M = Mn, Co, Ni), which are the final decomposition products of the corresponding dihydrogen phosphates are paramagnetics in the temperature range of 295–573 K with antiferomagnetic interactions between the metal ions. The lattice parameters of Ni(H2PO4)2·2H2O have been calculated. It crystallizes in the monoclinic system with a = 7.228(1) Å; b = 9.778(1) Å; c = 5.306(1) Å; β = 94.50(1)°, SG P21/n with Z = 2.
Materials Chemistry and Physics, 2010
ABSTRACT The preparation of homogeneous phosphate–formate precursors by freeze-drying of solution... more ABSTRACT The preparation of homogeneous phosphate–formate precursors by freeze-drying of solutions containing lithium, Mn(II), phosphate and formate ions ensures production of stoichiometric, well crystallized, defectless and nano-sized LiMnPO4 powders at temperatures above 400◦C. A systematic study of the influence of the synthesis conditions on the structure and morphology of LiMnPO4 is performed. It is established that the concentration of the solutions subjected to freeze-drying and the annealing temperature strongly affect the structure, particle dimensions and morphology of LiMnPO4. LiMnPO4 with low extent of Li-to-Mn disorder (below1%) are formed between 500 and 600◦C.Uniform and regular particles with close sizes distribution in the 60–120nm range are prepared from diluted freeze-dried solutions and at relatively low annealing temperature.
Journal of Solid State Chemistry, 1997
It has been established that two series of mixed crystals are formed in the Cu(HCOO) 2-Mn(HCOO) 2... more It has been established that two series of mixed crystals are formed in the Cu(HCOO) 2-Mn(HCOO) 2-2H 2 O system at 25°C. The thermal decompositions of Cu x Mn 1؊x (HCOO) 2 • 2H 2 O mixed crystals (04x40.5) have been studied. On the basis of the methods used (TG, DTA, and X-ray diffraction analysis), it has been established that homogeneous and heterogeneous oxide phases are obtained depending on the x-values. At about 300°C the formates decompose to Mn 5 O 8 (x ؍ 0); Cu 0.4 Mn 4.6 O 8 (x ؍ 0.08); a mixture of Cu 0.4 Mn 4.6 O 8 and Cu 1.5 Mn 1.5 O 4 with a different ratio (0.08 < x < 0.5), and a Cu 1.5 Mn 1.5 O 4 spinel (x ؍ 0.5). Cu 0.4 Mn 4.6 O 8 (Cu 2؉ 0.4 Mn 2؉ 0.4 Mn 4؉ 3 O 8) crystallizes in the monoclinic system with lattice parameters a ؍ 10.39(1) A s , b ؍ 5.729(4) A s , c ؍ 4.862(5) A s , ؍ 109.4°and space group C2/m. At about 600°C the oxides obtained transform into-Mn 2 O 3 (x ؍ 0) and a mixture of-Mn 2 O 3 and Cu 1.5 Mn 1.5 O 4 with a different ratio (0 < x < 0.5). The slowly cooled specimen shows a fine structure in the IR spectrum due to the 1 : 3 ordering of the ions on the B-sublattice, and the cation valences are Cu ؉ [Cu 2؉ 0.5 Mn 4؉ 4.5 ]O 4. The magnetic measurements show that the oxides obtained are paramagnetic in the temperature range of 25-300°C, and the 1/ versus T curves obey the Curie-Weiss law. The theoretical and experimental magnetic moments of the oxides are given. They coincide well in the case of the homogeneous samples.
Journal of Molecular Structure, 2002
We have investigated the vibrational behavior of the SO stretching modes n 3 and n 1 of the matri... more We have investigated the vibrational behavior of the SO stretching modes n 3 and n 1 of the matrix-isolated SO 4 22 guest ions isomorphously incorporated in selenate matrices of different crystal structure types: MeSeO 4 •6H 2 O (Me ¼ Mg, Co, Ni, Zn), MeSeO 4 •5H 2 O (Me ¼ Mn, Co, Cu, Zn) and MeSeO 4 •4H 2 O (Me ¼ Co, Ni). The spectra of the SO 4 22 guest ions exhibit less bands corresponding to the SO stretching modes in all selenate matrices under study than deduced from the site-group analysis. These findings indicate a higher symmetry of the guest ions than the crystallographic one (so-called spectroscopic effective symmetry). The spectroscopic data may be readily explained with effective symmetry D 2d of the SO 4 22 guest ions in MeSeO 4 •5H 2 O, MeSeO 4 •4H 2 O and in the monoclinic MeSeO 4 •6H 2 O lattices and with an effective symmetry close to ideal T d in the tetragonal selenate hexahydrate lattices. An adequate measure for the SO 4 22 guest ion distortions is the site group splittings of n 3 (Dn ab). The values of Dn ab are larger, i.e. the distortions increase on going from the hexahydrates to the tetrahydrates. The values of Dn ab correlate with the repulsion potential at the lattice sites of the host compounds, i.e. with the reduced cellvolumes.
Journal of Molecular Structure, 2000
The infrared (IR) spectra of Mg(HCOO)2·2H2O and Mg1−xCux(HCOO)2·2H2O mixed crystals (0&am... more The infrared (IR) spectra of Mg(HCOO)2·2H2O and Mg1−xCux(HCOO)2·2H2O mixed crystals (0&amp;amp;amp;amp;amp;amp;lt;x≤0.5) have been recorded and the internal modes of the formate groups and the water molecules are reported. The number of the bands corresponding to the fundamental vibrations of the HCOO groups and the H2O molecules in the Mg(HCOO)2·2H2O spectrum reflects the existence of two crystallographically different formate groups and
Journal of Alloys and Compounds, 2009
Mixed phosphate-formate precursors, LiFePO 4 H x (HCOO) x •yH 2 O (x ∼ 1, 1 < y < 1.3 and x < y),... more Mixed phosphate-formate precursors, LiFePO 4 H x (HCOO) x •yH 2 O (x ∼ 1, 1 < y < 1.3 and x < y), were prepared by freeze-drying of solutions containing Li + , Fe 2+ , PO 4 3− and HCOO − in a ratio of 1:1:1:2. Fe(HCOO) 2 •2H 2 O was used as an iron source. The concentration of the freeze-dried solutions allows affecting the morphology of phosphate-formate precursors. Powder XRD, IR spectroscopy, DTA, BET measurements, SEM and XPS analyses were used for the characterization of the precursors and LiFePO 4. IR spectroscopic study of the precursors shows that the deprotonated phosphate and formate ions are coordinated around the metal ions. The thermal heating of the lithium-iron phosphate-formate precursors at 350 • C for a short period yields stoichiometric defectless LiFePO 4 having a high-specific surface area of about 35 m 2 /g, a mesoporous structure and containing up to 1.5 mass% carbon. By variation of the solutions concentration subjected to freeze-drying, the morphology of LiFePO 4 and the carbon content are changed. The morphology of target LiFePO 4 comprises micrometric aggregates, which are composed of nanometric particles with close particle size distribution in the range of 60-140 nm. The nanometric particles are interconnected in a way to form mesoporous network. This method is suitable for the preparation of LiFePO 4 as cathode material for lithium-ion batteries since it allows affecting both the particle dimensions and carbon content.
Batteries
This study aims to quantify the synergistic effect of Ni2+ and Mn2+ ions on the capacitive perfor... more This study aims to quantify the synergistic effect of Ni2+ and Mn2+ ions on the capacitive performance of oxide, hydroxide and phosphate electrodes in alkaline electrolytes. Three types of phases containing both nickel and manganese in a ratio of one-to-one were selected due to their stability in alkaline media: oxides with ilmenite and spinel structures (NiMnO3 and Ni1.5Mn1.5O4); hydroxides with layered structures (β-Ni1/2Mn1/2(OH)2); and phosphates with olivine and maricite structures (LiNi1/2Mn1/2PO4 and NaNi1/2Mn1/2PO4). In the mixed hydroxides and phosphates, Ni2+ and Mn2+ ions randomly occupied one crystallographic site, whereas in the ilmenite oxide, a common face was shared by the Ni2+ and Mn4+ ions. The electrochemical parameters of the Ni–Mn compositions were evaluated in asymmetric hybrid supercapacitor cells working with alkaline electrolytes and activated carbon as a negative electrode. A comparative analysis of oxides, hydroxides and phosphates enabled us to differenti...
Molten salt reactions between dittmarite-type NH4FePO4.H2O and different lithium eutectic composi... more Molten salt reactions between dittmarite-type NH4FePO4.H2O and different lithium eutectic compositions are systematically investigated in order to obtain at low temperature olivine-type LiFePO4 which is valuable electrode material for lithium-ion batteries. Nanosized LiFePO4 powder is successfully prepared by topotactic reactions using LiCl-LiOH eutectic compositions at 277–325 oC for short reaction time of 6090 min. It is well crystallized with a preferred crystallite orientation along a-crystallographic axis. The prepared LiFePO4 is characterized by a plate-like morphology inherited from the iron precursor. The micrometer LiFePO4 plates are composed of nanosized spherical particles with dimensions about 50 nm which is favorable for further electrochemical performance.
Chemical record (New York, N.Y.), Jan 17, 2018
The energy storage by redox intercalation reactions is, nowadays, the most effective rechargeable... more The energy storage by redox intercalation reactions is, nowadays, the most effective rechargeable ion battery. When lithium is used as intercalating agents, the high energy density is achieved at an expense of non-sustainability. The replacement of Li with cheaper monovalent ions enables to make greener battery alternatives. The utilization of polyvalent ions instead of Li permits to multiplying the battery capacity. Contrary to Li , the realization of quick and reversible intercalation of bigger monovalent and of polyvalent ions is a scientific challenge due to kinetic constraints, polarizing ion effects and Coulomb interactions. Herein we provide a vision how to make the intercalation of these ions feasible. The idea is to perform dual intercalation of ions having different charges, radii, preferred coordination and diffusion pathway topology. All these features are demonstrated by the recent knowledge on selective and non-selective intercalation properties of oxides and polyanion...
Electrochimica Acta, 2018
Hybrid supercapacitors with the composite electrode materials display high energy density at the ... more Hybrid supercapacitors with the composite electrode materials display high energy density at the expense of the reduced cycle stability. Herein, we provide new data on the electrochemical performance of hybrid electrodes based on mixed nickel hydroxides/manganese oxides in the form of structured and multiphase composites. As structured composites, two types of less known structure modifications are examined: interstratified modification of Ni(OH) 2 (i.e. α/β IS-Ni(OH) 2) and ε-modification of MnO 2. The multiphase hydroxide/oxide composites are prepared by the conventional grinding of α/β IS-Ni(OH) 2 and ε-MnO 2 and by the in-situ formation after the reaction of layered Na 0.5 Ni 0.5 Mn 0.5 O 2 with mixed LiOH-KOH electrolyte. The structure, morphology and porous texture properties of composites are analyzed by means of powder X-ray diffraction, scanning electron microscopy (SEM) and low-temperature nitrogen adsorption, respectively. The electrochemical performance of composites electrodes is determined by galvanostatic experiments in concentrated individual KOH and mixed LiOH-KOH electrolytes. The ex-situ X-ray diffraction is used to monitor the changes in composite electrodes during electrochemical cell function. It has been found that α/β IS-Ni(OH) 2 participates in electrochemical reaction concomitantly with H 2 O and Li + intercalation, while the electrochemical performance of ε-MnO 2 is determined by surface adsorption of electrolyte
Phys. Chem. Chem. Phys., 2017
In Mg-substituted NaMnPO4, Na+, Mg2+ and metal vacancies reside in the M1 olivine position, while... more In Mg-substituted NaMnPO4, Na+, Mg2+ and metal vacancies reside in the M1 olivine position, while Mn2+ ions are predominantly in the M2 position.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 2002
Synthetic malachite, hydrozincite and five monophasic mixed copper-zinc hydroxycarbonates have be... more Synthetic malachite, hydrozincite and five monophasic mixed copper-zinc hydroxycarbonates have been studied by Fourier transform infrared (FTIR) spectroscopy at ambient and liquid nitrogen temperature in the region of 4000-400 cm(-1). The analysis of the spectra reveals that the samples containing up to 20% zinc retain the malachite lattice, thus forming solid solutions. The inclusion of zinc ions in malachite reflects on the positions and intensity of the bands corresponding to the internal modes of the carbonate ion, to the OH librations and to the Me-O interactions. For example, the higher and the lower frequency components of v3 shift to higher and lower frequencies, respectively. The intensity of the bands corresponding to v2 decreases with the zinc content increase. The spectrum of the sample Cu1.31Zn0.69(OH)2CO3 become diffuse and ill-resolved in the region of the Me-O interactions (region below 600 cm(-1)) and the corresponding bands are shifted to lower frequencies due to the weaker Zn-O interactions as compared with those of the copper ions. The internal modes of the carbonate ions in hydrozincite and aurichalcite are assigned and discussed taking into account the site symmetry and factor group symmetry. The OH and OD stretches (matrix-isolated HDO molecules) and the hydrogen bond strengths are interpreted in terms of Me-O interactions (synergetic effect), hydrogen bond angles and different hydrogen bond acceptor strengths of the oxygen atoms from the carbonate ions. It proves that the hydrogen bonds in hydrozincite are stronger as compared with those in malachite, irrespective of both the larger hydrogen bond lengths and the weaker Zn-O interactions in hydrozincite due to the higher hydrogen bond acceptor strength of the non-coordinated oxygen atom and the formation of bifurcated hydrogen bonds.
European Journal of Inorganic Chemistry, 2010
Materials Chemistry and Physics, 2010
Bulgarian Chemical Communications
The phase formation in the systems NH4MnPO4.H2O-LiCl-LiNO3 and NH4MnPO4.H2O-NaCH3COO.3H2O is syst... more The phase formation in the systems NH4MnPO4.H2O-LiCl-LiNO3 and NH4MnPO4.H2O-NaCH3COO.3H2O is systematically investigated in order to obtain at low-temperature olivine-type LiMnPO4 and NaMnPO4 which are of great interest as cathode materials for lithium and sodium ion batteries. The experimental conditions such as molar ratio between reagents, temperature and reaction time are varied to find the more suitable procedure for the low temperature formation of pure phospho-olivines. It is established that the transformation of the dittmarite precursor into sodium manganese phospho-olivine proceeds between 200-250 oC with participation of large excess of the Na-salt more slowly in comparison with the fast ion exchange of NH4+ for Li+ leading to the formation of LiMnPO4 for 1.5 h only. Both prepared olivines are well-crystallized and exhibit nano-sized crystallites (50 - 60 nm).
Vibrational Spectroscopy, 2003
The infrared spectra of related compounds with kröhnkite-type chains Na 2 Me(SeO 4) 2 Á2H 2 O (Me... more The infrared spectra of related compounds with kröhnkite-type chains Na 2 Me(SeO 4) 2 Á2H 2 O (Me ¼ Mn, Co, Ni, Cu, Zn, Cd) containing matrix-isolated SO 4 2À guest ions are reported and discussed with respect to the SO stretching modes n 3 and n 1. Due to the low site symmetry C 1 of the SO 4 2À guest ions three bands for n 3 and one band for n 1 are seen in all spectra. When SO 4 2À guest ions are incorporated in the triclinic Na 2 Zn(SeO 4) 2 Á2H 2 O, Na 2 Co(SeO 4) 2 Á2H 2 O and Na 2 Ni(SeO 4) 2 Á2H 2 O host lattices the n 3 stretching region resembles a higher local symmetry (A 1 È E) of the SO 4 2À guest ions than the crystallographic one (i.e. Dn ab > Dn bc instead of Dn ab % Dn bc , a, b and c being three n 3 components). Hence, the ratio between Dn ab and Dn bc additionally to Dn max (the difference between the highest and the lowest wavenumbered SO stretching modes) has to be taken into account when the SO 4 2À guest ion distortions are considered (the higher the ratio Dn ab /Dn bc is, the weaker the distortion is). Both the site group splittings of the triplet component n 3 (Dn ac) and Dn max values are an adequate measure for the SO 4 2À guest ion distortion when the guest ions are incorporated in the monoclinic Na 2 Cu(SeO 4) 2 Á2H 2 O, Na 2 Mn(SeO 4) 2 Á2H 2 O and Na 2 Cd(SeO 4) 2 Á2H 2 O (i.e. Dn ab % Dn bc). In addition to the local potential at the lattice site of the host lattice, the metal environment of the SO 4 2À guest ions reflects on the guest ion distortions. A correlation between the SO 4 2À guest ion distortions and the metal electronic configurations has been found and discussed. Me 02þ guest ions incorporated additionally to the SO 4 2À guest ions in the selenate lattices do not influence practically on the vibrational behavior of the SO stretches.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 2007
The IR and Raman spectra of the isostructural M&amp;#39;M&amp;#39;&amp;#3... more The IR and Raman spectra of the isostructural M&amp;#39;M&amp;#39;&amp;#39;PO4.H2O compounds (M&amp;#39;=K+, NH4+; M&amp;#39;&amp;#39;=Mn2+, Co2+, Ni2+) are reported and discussed with respect to the normal vibrations of the PO(4)3- ions. The vibrational behavior of PO4(3-) is in agreement with its low site symmetry Cs in the lattices-the symmetric nu1 and nu2 modes are activated in the IR spectra and the degeneration of the asymmetric nu3 and nu4 modes is lifted. A relatively large unit-cell group splitting is observed for nu1 in both the IR and Raman spectra and for nu3 in Raman spectra. It has been established that the mean wavenumbers of the P-O stretches (nuPO) are not affected by the M2+ ions present, but they are lower for the NH4-series than for the K-one (predominant influence of both the smaller repulsion potential and the hydrogen bonds in the NH4-lattices over the influence of the M+-O interactions). The extent of the energetic distortion of the PO(4)3- ions has been estimated based on the spectroscopic data for the site group splitting of the asymmetric modes (Deltanu3 and Deltanu4), the separation between the highest and the lowest wavenumbered P-O stretches (Deltanumax) and the intensity of nu1 in the IR spectra. The data provide an evidence that the PO4(3-) ions in KM&amp;#39;&amp;#39;PO4.H2O are more distorted regarding the P-O bond lengths than those in NH4M&amp;#39;&amp;#39;PO4.H2O, but their angular distortion is the same in both series. The trends for the energetic distortion of the phosphate ions found from the spectroscopic data correspond to the data for their geometric distortion deduced from the values of the distortion indices DI(PO) and DI(OPO).
Materials Research Bulletin, 2006
ABSTRACT It has been established that M(H2PO4)2·2H2O (M = Mn, Co, Ni) are paramagnetics between 1... more ABSTRACT It has been established that M(H2PO4)2·2H2O (M = Mn, Co, Ni) are paramagnetics between 173 and 353 K with weak antiferromagnetic exchange interaction between the metal ions. In situ magnetic measurements during the thermal decomposition of the salts show that the oxidation state and the octahedral coordination of M2+ are preserved. From the data obtained it could be supposed that in M(H2PO4)2·2H2O (M = Co, Ni) this process is topotactic with no long-range diffusion transport. In Mn(H2PO4)2·2H2O, the formation of the large variety of intermediate products probably requires more drastic rearrangement and diffusion of the manganese ions during the complex transformations, which reflect on both the value and the sign of the θ constants. M2P4O12 (M = Mn, Co, Ni), which are the final decomposition products of the corresponding dihydrogen phosphates are paramagnetics in the temperature range of 295–573 K with antiferomagnetic interactions between the metal ions. The lattice parameters of Ni(H2PO4)2·2H2O have been calculated. It crystallizes in the monoclinic system with a = 7.228(1) Å; b = 9.778(1) Å; c = 5.306(1) Å; β = 94.50(1)°, SG P21/n with Z = 2.
Materials Chemistry and Physics, 2010
ABSTRACT The preparation of homogeneous phosphate–formate precursors by freeze-drying of solution... more ABSTRACT The preparation of homogeneous phosphate–formate precursors by freeze-drying of solutions containing lithium, Mn(II), phosphate and formate ions ensures production of stoichiometric, well crystallized, defectless and nano-sized LiMnPO4 powders at temperatures above 400◦C. A systematic study of the influence of the synthesis conditions on the structure and morphology of LiMnPO4 is performed. It is established that the concentration of the solutions subjected to freeze-drying and the annealing temperature strongly affect the structure, particle dimensions and morphology of LiMnPO4. LiMnPO4 with low extent of Li-to-Mn disorder (below1%) are formed between 500 and 600◦C.Uniform and regular particles with close sizes distribution in the 60–120nm range are prepared from diluted freeze-dried solutions and at relatively low annealing temperature.
Journal of Solid State Chemistry, 1997
It has been established that two series of mixed crystals are formed in the Cu(HCOO) 2-Mn(HCOO) 2... more It has been established that two series of mixed crystals are formed in the Cu(HCOO) 2-Mn(HCOO) 2-2H 2 O system at 25°C. The thermal decompositions of Cu x Mn 1؊x (HCOO) 2 • 2H 2 O mixed crystals (04x40.5) have been studied. On the basis of the methods used (TG, DTA, and X-ray diffraction analysis), it has been established that homogeneous and heterogeneous oxide phases are obtained depending on the x-values. At about 300°C the formates decompose to Mn 5 O 8 (x ؍ 0); Cu 0.4 Mn 4.6 O 8 (x ؍ 0.08); a mixture of Cu 0.4 Mn 4.6 O 8 and Cu 1.5 Mn 1.5 O 4 with a different ratio (0.08 < x < 0.5), and a Cu 1.5 Mn 1.5 O 4 spinel (x ؍ 0.5). Cu 0.4 Mn 4.6 O 8 (Cu 2؉ 0.4 Mn 2؉ 0.4 Mn 4؉ 3 O 8) crystallizes in the monoclinic system with lattice parameters a ؍ 10.39(1) A s , b ؍ 5.729(4) A s , c ؍ 4.862(5) A s , ؍ 109.4°and space group C2/m. At about 600°C the oxides obtained transform into-Mn 2 O 3 (x ؍ 0) and a mixture of-Mn 2 O 3 and Cu 1.5 Mn 1.5 O 4 with a different ratio (0 < x < 0.5). The slowly cooled specimen shows a fine structure in the IR spectrum due to the 1 : 3 ordering of the ions on the B-sublattice, and the cation valences are Cu ؉ [Cu 2؉ 0.5 Mn 4؉ 4.5 ]O 4. The magnetic measurements show that the oxides obtained are paramagnetic in the temperature range of 25-300°C, and the 1/ versus T curves obey the Curie-Weiss law. The theoretical and experimental magnetic moments of the oxides are given. They coincide well in the case of the homogeneous samples.
Journal of Molecular Structure, 2002
We have investigated the vibrational behavior of the SO stretching modes n 3 and n 1 of the matri... more We have investigated the vibrational behavior of the SO stretching modes n 3 and n 1 of the matrix-isolated SO 4 22 guest ions isomorphously incorporated in selenate matrices of different crystal structure types: MeSeO 4 •6H 2 O (Me ¼ Mg, Co, Ni, Zn), MeSeO 4 •5H 2 O (Me ¼ Mn, Co, Cu, Zn) and MeSeO 4 •4H 2 O (Me ¼ Co, Ni). The spectra of the SO 4 22 guest ions exhibit less bands corresponding to the SO stretching modes in all selenate matrices under study than deduced from the site-group analysis. These findings indicate a higher symmetry of the guest ions than the crystallographic one (so-called spectroscopic effective symmetry). The spectroscopic data may be readily explained with effective symmetry D 2d of the SO 4 22 guest ions in MeSeO 4 •5H 2 O, MeSeO 4 •4H 2 O and in the monoclinic MeSeO 4 •6H 2 O lattices and with an effective symmetry close to ideal T d in the tetragonal selenate hexahydrate lattices. An adequate measure for the SO 4 22 guest ion distortions is the site group splittings of n 3 (Dn ab). The values of Dn ab are larger, i.e. the distortions increase on going from the hexahydrates to the tetrahydrates. The values of Dn ab correlate with the repulsion potential at the lattice sites of the host compounds, i.e. with the reduced cellvolumes.
Journal of Molecular Structure, 2000
The infrared (IR) spectra of Mg(HCOO)2·2H2O and Mg1−xCux(HCOO)2·2H2O mixed crystals (0&am... more The infrared (IR) spectra of Mg(HCOO)2·2H2O and Mg1−xCux(HCOO)2·2H2O mixed crystals (0&amp;amp;amp;amp;amp;amp;lt;x≤0.5) have been recorded and the internal modes of the formate groups and the water molecules are reported. The number of the bands corresponding to the fundamental vibrations of the HCOO groups and the H2O molecules in the Mg(HCOO)2·2H2O spectrum reflects the existence of two crystallographically different formate groups and
Journal of Alloys and Compounds, 2009
Mixed phosphate-formate precursors, LiFePO 4 H x (HCOO) x •yH 2 O (x ∼ 1, 1 < y < 1.3 and x < y),... more Mixed phosphate-formate precursors, LiFePO 4 H x (HCOO) x •yH 2 O (x ∼ 1, 1 < y < 1.3 and x < y), were prepared by freeze-drying of solutions containing Li + , Fe 2+ , PO 4 3− and HCOO − in a ratio of 1:1:1:2. Fe(HCOO) 2 •2H 2 O was used as an iron source. The concentration of the freeze-dried solutions allows affecting the morphology of phosphate-formate precursors. Powder XRD, IR spectroscopy, DTA, BET measurements, SEM and XPS analyses were used for the characterization of the precursors and LiFePO 4. IR spectroscopic study of the precursors shows that the deprotonated phosphate and formate ions are coordinated around the metal ions. The thermal heating of the lithium-iron phosphate-formate precursors at 350 • C for a short period yields stoichiometric defectless LiFePO 4 having a high-specific surface area of about 35 m 2 /g, a mesoporous structure and containing up to 1.5 mass% carbon. By variation of the solutions concentration subjected to freeze-drying, the morphology of LiFePO 4 and the carbon content are changed. The morphology of target LiFePO 4 comprises micrometric aggregates, which are composed of nanometric particles with close particle size distribution in the range of 60-140 nm. The nanometric particles are interconnected in a way to form mesoporous network. This method is suitable for the preparation of LiFePO 4 as cathode material for lithium-ion batteries since it allows affecting both the particle dimensions and carbon content.