The detailed temperature-dependent IR and Raman spectra were used to study and understand the mechanism of structural phase transition occurring at 175 K in manganese hypophosphite templated with ...formamidinium (FA+) ions, FAMn(H2POO)3, which adopts a perovskite-like architecture. The structural transformation between the C2/c and the P21/c monoclinic phases has a complicated nature and is mainly driven by re-orientational motions of the FA+ cations but it is also accompanied by a significant distortion of the MnO6 octahedral units as well as steric-forced changes of the PH2 groups determining the off-center shifts of FA+ cations in the cages. The re-orientational motions of formamidinium cations at 175 K are followed by slight changes of their geometry and re-arrangement of hydrogen bonds (HBs). The strong temperature-dependences of bands corresponding to vibrations involving hydrogen bonding reveal the highly-dynamic character of this phase transition and strong nature of created HBs. The most pronounced changes are observed for the modes corresponding to the formamidinium cation, proving that the phase transition has an order-disorder character.
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•The Mn2+-hypophosphite framework with FA+ cations has been studied using vibrational spectroscopy.•The assignment of observed IR and Raman bands has been proposed.•The mechanism of PT at 175 K has been explained.•PT involves order-disorder process along with deformation of the framework.
We report on temperature-dependent infrared (IR) and Raman studies of (CH3)2NH2M(HCOO)3 metal–organic frameworks (MOFs) with M=Zn, Fe. Based on Raman and IR data, an assignment of the observed modes ...to respective vibrations of atoms is proposed. Temperature-dependent studies revealed abrupt changes below 160K that are attributed to the onset of first-order structural phase transition. The most pronounced changes are observed for the modes corresponding to the dimethylammonium cation, especially those involving motion of hydrogen atoms. This behavior proves that the phase transition has an order–disorder character and is associated with the ordering of protons. The abrupt splitting of some modes related to the formate ion indicates that this transition is also associated with significant distortion of the metal-formate framework.
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•Raman and IR spectra of multiferroic (CH3)2NH2Ni(HCOO)3 were recorded.•DFT calculations for the model compounds were performed.•Assignment of vibrational modes has been ...proposed.•Modes sensitive to formation of hydrogen bonds have been identified.
Experimental Raman and IR spectra of multiferroic (CH3)2NH2Ni(HCOO)3 were recorded at room temperature. The three-parameter hybrid B3LYP density functional method has been used with the 6-31G(d, p) basis set to derive the equilibrium geometry, atomic spin densities, vibrational wavenumbers, infrared intensities and Raman scattering activities. Based on these calculations, the assignment of the observed bands to the respective internal and lattice modes is proposed. The performed calculations revealed that the ν(NH2) stretching, ρ(NH2) rocking and τ(CH3) torsional modes are very sensitive to formation of the hydrogen bond between the DMA+ cation and Ni-formate framework. Therefore, these modes are suitable probes for strength of hydrogen bonds in this family of metal-formate frameworks and study of their temperature dependence may provide significant information on a role of the hydrogen bonds in mechanism of the ferroelectric phase transition occurring in these compounds at low temperatures.
The fundamental aspects of the relaxation dynamics in niccolite-type, mixed valence metal-organic framework, multiferroic (CH3)2NH2Fe(3+)Fe(2+)(HCOO)6 single crystals have been reported using ...dielectric relaxation spectroscopy covering eight decades in frequency (10(-2) ≤ f ≤ 10(6)) in the temperature range 120 K ≤ T ≤ 250 K. The compound shows antiferroelectric to paraelectric phase transition near T = 154 K with the relaxor nature of electric ordering. The temperature dependent dielectric response in modulus representation indicates three relaxation processes within the experimental window. The variable range hopping model of small polarons explains the bulk non-Debye type conductivity relaxation. The fastest relaxation with activation energy Ea = 0.17 eV is related to progressive freezing of the reorientation motions of DMA(+) cations. X-ray diffraction data revealed that complete freezing of orientational and translational motions of DMA(+) cations occurs well below phase transition temperature. These experimental observations are fundamentally important for the theoretical explanation of relaxation dynamics in niccolite-type metal-organic frameworks.
Two methods for calculating transport parameters in semiconductor superlattices by applying Green’s functions are compared in the paper. For one of the methods, the Wannier functions method, where ...computations in the complex space and Wannier functions base are required, the Hamiltonian matrix is small in size and its elements depend solely on the energy. For the real space method, as it operates in the floating point domain and uses the Hamiltonian containing the elements dependent both on energy and position, the Hamiltonian matrix is larger in size. The size makes the method computationally challenging. To find the consequences of choosing one of the methods, a direct comparison between the computations, obtained for both methods with the same input parameters, was undertaken. The differences between the results are shown and explained. Selected simulations allowed us to discuss advantages and disadvantages of both methods. The calculations include transport parameters such as the density of states and the occupation functions, with regard to scattering processes where the self-consistent Born approximation was used, as well as the spatial distribution of electron concentration for two superlattices structures. The numerical results are obtained within the non-equilibrium Green’s functions formalism by solving the Dyson and the Keldysh equations.
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•Imidazolium manganese-hypophosphite perovskite has been studied.•The pressure-dependent Raman data have been analyzed.•Three pressure-induced phase transitions have been ...discovered.•Insights on the phase transition mechanisms are provided.
By using Raman spectroscopy, we demonstrate that IMMn(H2POO)3 is a highly compressible material that undergoes three pressure-induced phase transitions. Using a diamond anvil cell we performed high-pressure experiments up to 7.1 GPa, using paraffin oil as the compression medium. The first phase transition, which occurs near 2.9 GPa, leads to very pronounced changes in the Raman spectra. This behavior indicates that this transition is associated with very large reconstruction of the inorganic framework and collapse of the perovskite cages. The second phase transition, which occurs near 4.9 GPa, is associated with subtle structural changes. The last transition takes place near 5.9 GPa and it leads to further significant distortion of the anionic framework. In contrast to the anionic framework, the phase transitions have weak impact on the imidazolium cation. Pressure dependence of Raman modes proves that compressibility of the high-pressure phases is significantly lower compared to the ambient pressure phase. It also indicates that the contraction of the MnO6 octahedra prevails over that of the imidazolium cations and hypophosphite linkers. However, compressibility of MnO6 strongly decreases in the highest pressure phase. Pressure-induced phase transitions are reversible.
Metal-organic frameworks (MOFs), in which metal clusters are coupled by organic moieties, exhibit inherent porosity and crystallinity. Although these systems have been examined for vast potential ...applications, the elementary proton conduction in anhydrous MOFs still remains elusive. One of the approaches to deal with this problem is the utilization of protic organic molecules, to be accommodated in the porous framework. In this work we report the temperature-dependent crystal structure and proton conduction in C
H
NH
Na
Fe
(HCOO)
metal-organic frameworks using X-ray diffraction and broadband dielectric spectroscopic techniques. The detailed analysis of the crystal structure reveals disorder of the terminal ethylene groups in the polar phase (space group Pn). The structural phase transition from Pn to P2
/n at T ≈ 363 K involves the distortion of the metal formate framework and ordering of EtA
cations due to the reduction of the cell volume. The dielectric data have been presented in the dynamic window of permittivity formalism to understand the ferroelectric phase transition. The relaxation times have been estimated from the Kramers-Kronig transformation of the dielectric permittivity. A Grotthuss type mechanism of the proton conduction is possible at low temperatures with the activation energy of 0.23 eV. This type of experimental observation is expected to provide new prospective on the fundamental aspect of elementary proton transfer in anhydrous MOFs.
Nanoparticles of cobalt(II) chromite, CoCr2O4, have been prepared by hydrothermal synthesis at 230 °C and firing of the obtained nanosized sample at various temperatures. The effect of particle size ...on the structure and properties of CoCr2O4 nanoparticles has been investigated using the XRD, TEM, diffuse reflectivity, Raman and infrared spectroscopy. The results show that simple hydrothermal synthesis yields material composed of very small crystalline particles with the average size of about 7 nm. Annealing of this sample allows obtaining particles with different sizes. Raman and IR studies show that in addition to the bands characteristic for normal spinel structure, a few additional broad bands appear due to the cation redistribution among the tetrahedral and octahedral sites. Intensity of these bands significantly increases for the samples annealed at lower temperatures. This result shows that with decreasing particle size the cation redistribution is tending towards more inverse spinel configuration compared to bulk, which is predominantly normal spinel. This conclusion is supported by the optical spectra of the obtained samples. Raman and IR spectra also indicate weak phonon confinement effect in CoCr2O4. Significantly larger phonon confinement effect is observed only for the 379 cm−1 IR-active mode involving motions of Cr(III) atoms located at the octahedral sites.
► Nanoparticles of cobalt(II) chromite, CoCr2O4, have been prepared. ► The obtained data revealed cation redistribution among octahedral and tetrahedral sites. ► With decreasing particle size the structure is tending towards inverse spinel configuration. ► Significant phonon confinement effect is observed for the 379 cm−1 IR-active mode.
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•Coexistence of chiral and perovskite phases in NH2NH3Mn(HCOO)3(HyMn) metal-organic compound.•Dielectric two-power law, generalized Mittag-Leffler response of HyMn.•The presence of ...polar nanoregions in HyMn suggesting the relaxor nature of the ferroelectricity.
We report the synthesis, thermal and dielectric measurements of NH2NH3Mn(HCOO)3 (HyMn) compound. Above room temperature this polymeric material undergoes two phase transitions at ∼360 K and ∼298 K, as observed via DSC and BDS spectra. The first high temperature phase transition is associated with the paraelectric to ferroelectric transition of perovskite HyMn. The low temperature phase transition corresponds to the paraelectric to antiferroelectric transition of chiral HyMn. However, mechanisms hidden behind the observed two polymorphic phases are not completely clear yet. Dielectric spectroscopy measurements have revealed formation of clusters and superclusters as well as the relaxor-like behavior of this compound in wide temperature range resulting from both chiral and perovskite phases. Analysis of dielectric permittivity spectra obtained for the investigated material showed the generalized Mittag-Leffler two-power-law relaxation pattern that was interpreted by means the stochastic scenario of correlated-clusters. The proposed approach brought into light the presence of polar nanoregions in the material suggesting the relaxor nature of the existing ferroelectricity.
Two metal–organic frameworks of (CH3)2NH2M(HCOO)3, where M=Mg and Cd, have been investigated by temperature-dependent IR and Raman methods in order to determine the nature of the phase transition. ...Our results indicate that phase transition in the Mg-compound is driven by ordering of the dimethylammonium cations. Additional X-ray diffraction and spectroscopic studies on Cd-compound as the function of temperature reveal that this compound does not undergo any structural phase transition. We attribute this behavior to the large size of the cavity occupied by the dimethylammonium cations and thus weak hydrogen bonding between these cations and formate ions.
Temperature-dependent IR and Raman studies of metal–organic frameworks (CH3)2NH2M(HCOO)3, M=Mg and Cd. Display omitted
•Temperature-dependent Raman scattering and IR measurements for (CH3)2NH2Mg(HCOO)3 and (CH3)2NH2Cd(HCOO)3 were performed.•The phase transition in Mg-compound at 270 K is driven by ordering of the dimethylammonium cations.
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