Adam Sieradzki

Perovskite metal formate framework of [NH2-CH(+)-NH2]Mn(HCOO)3]: phase transition, magnetic, dielectric, and phonon properties.

We report the synthesis, crystal structure, and thermal, dielectric, phonon, and magnetic properties of [NH2-CH(+)-NH2][Mn(HCOO)3] (FMDMn). The anionic framework of [(Mn(HCOO)3(-)] is counterbalanced by formamidinium (FMD(+)) cations located in the cavities of the framework. These cations form extensive N-H···O hydrogen bonding with the framework. The divalent manganese ions have octahedral geometry and are bridged by the formate in an anti-anti mode of coordination. We have found that FMDMn undergoes a structural phase transition around 335 K. According to the X-ray diffraction, the compound shows R3̅c symmetry at 355 K and C2/c symmetry at 295 and 110 K. The FMD(+) cations are dynamically disordered in the high-temperature phase, and the disorder leads to very large bandwidths of Raman and IR bands corresponding to vibrations of the NH2 groups. Temperature-dependent studies show that the phase transition in FMDMn is associated with ordering of the FMD(+) cations. Detailed analysis shows, however, that these cations still exhibit some reorientational motions down to about 200 K. The ordering of the FMD(+) cations is associated with significant distortion of the anionic framework. On the basis of the magnetic data, FMDMn is a weak ferromagnet with the critical temperature Tc = 8.0 K.

Effect of aliovalent doping on the properties of perovskite-like multiferroic formates

We report the synthesis, as well as the thermal, dielectric, Raman, IR and luminescence studies of a chromium-doped multiferroic MOF, [(CH3)2NH2][Mn(HCOO)3] (DMMn). These studies reveal that doping with chromium(III) leads to a lowering of the ferroelectric phase transition temperature Tc. The doping also changes the character of the phase transition from strongly first-order for an undoped sample to a partially diffused one for 3.1% of chromium doping. This behavior resembles the behavior of inorganic ABO3 perovskite ferroelectrics where doping often leads to a decrease of the Tc and the diffuse character of a phase transition. We also show that the chromium-doped sample exhibits efficient luminescence. Additional studies demonstrated that the [(CH3)2NH2][MII(HCOO)3] formates (MII = Mg, Mn, or Co) may also be doped with other trivalent cations such as Al3+, In3+, Eu3+ or Er3+. Doping with these ions also leads to a decrease of the Tc and the diffuse character of the phase transition. Additional optical studies show that the europium-doped DMMn sample also exhibits luminescence properties. Thus our discovery opens up a new and simple route for the synthesis of various multifunctional amine-templated metal formate frameworks with tunable multiferroic and luminescent properties by doping these frameworks with a wide range of trivalent cations.

Structural, magnetic and dielectric properties of two novel mixed-valence iron(II)–iron(III) metal formate frameworks

Two novel mixed-valence iron(II)–iron(III) formate frameworks templated by ethylammonium and diethylammonium cations have been prepared and characterized by DSC, X-ray diffraction and spectroscopic methods. We also report dielectric and magnetic properties of the obtained samples. Both MOFs crystallize in the P1c structure and exhibit magnetic order at 39 K. The analogue with diethylammonium cations undergoes a structural phase transition near 240 K into a triclinic phase. This transition has an order–disorder character and it is associated with pronounced dielectric anomaly. This compound is therefore the second discovered mixed-valence metal formate exhibiting multiferroic properties.

Effect of solvent, temperature and pressure on the stability of chiral and perovskite metal formate frameworks of [NH2NH3][M(HCOO)3] (M = Mn, Fe, Zn)

We report the synthesis, crystal structure, and thermal, Raman, infrared and magnetic properties of [NH2NH3][M(HCOO)3] (HyM) compounds (M = Mn, Zn, Fe). Our results show that synthesis from methanol solution leads to perovskite polymorphs while that from 1-methyl-2-pyrrolidinone or its mixture with methanol allows obtaining chiral polymorphs. Perovskite HyFe, chiral HyFe and chiral HyMn undergo phase transitions at 347, 336 and 296 K, respectively, with symmetry changes from Pnma to Pna21, P63 to P212121 and P63 to P21. X-ray diffraction and Raman studies show that the phase transitions are governed by dynamics of the hydrazinium ions. Low-temperature magnetic studies show that these compounds exhibit magnetic ordering below 9-12.5 K. Since the low-temperature structures of chiral HyMn and perovskite HyFe are polar, these compounds are possible multiferroic materials. We also report high-pressure Raman scattering studies of chiral and perovskite HyZn, which show much larger stiffness of the latter phase. These studies also show that the ambient pressure polar phases are stable up to at least 1.4 and 4.1 GPa for the chiral and perovskite phase, respectively. Between 1.4 and 2.0 GPa (for chiral HyZn) and 4.1 and 5.2 GPa (for perovskite HyZn) pressure-induced transitions are observed associated with changes in the zinc-formate framework. Strong broadening of Raman bands and the decrease in their number for the high-pressure phase of chiral HyZn suggest that this phase is disordered and has higher symmetry than the ambient pressure one.

Calorimetric Investigations of Phase Transitions in KNO3 Embedded Into Porous Glasses

Results of the Differential Scanning Calorimetry (DSC) studies of porous glasses filled by KNO3 ferroelectric in wide temperature range are presented. Anomalous heat capacity of the potassium nitrate confined in nanoscale porous glasses with two different radius of pore 23 and 160nm show that the phase I-III transition temperature was slightly affected by particle size, whereas the phase III-II transition temperature significantly decreased. During the heating, the phase II-I transition temperature increased up to 12.5°C comparing to the bulk material. The lowering of the potassium nitrate particle size significantly reduced the phase I–liquid transition temperature.

Temperature- and pressure-induced phase transitions in the niccolite-type formate framework of [H3N(CH3)4NH3][Mn2(HCOO)6]

We report the synthesis, crystal structure, thermal, pyroelectric, Raman, infrared and magnetic properties of [NH3(CH2)4NH3][Mn2(HCOO)6] niccolite. Our results show that this compound crystallizes in a trigonal structure (space group P1c) with dynamically disordered [NH3(CH2)4NH3]2+ cations. It undergoes a phase transition near Tc = 350 K. The low-temperature structure is polar (space group Cc) and pyroelectric measurements confirm that it exhibits ferroelectric properties. Detailed analysis of the structural changes shows that both the spatial arrangement of the [NH3(CH2)4NH3]2+ dipole moments and distortion of the manganese formate framework contribute to the spontaneous polarization within the (a, c) plane. Based on Raman and IR data, assignment of the observed modes to the respective vibrations of atoms is also proposed. Dynamic disorder of organic cations in the high-temperature phase manifests in the vibrational spectra through very large width of bands corresponding to vibrations of the NH3 groups. Ordering of these cations is clearly observed in the spectra through a pronounced decrease in their bandwidths below the phase transition temperatures. Low-temperature magnetic studies show that this compound is a weak ferromagnet below 9.0 K. We also report high-pressure Raman scattering studies of this compound, which reveal the presence of two pressure-induced phase transitions between 0.5 and 0.9 GPa and between 1.3 and 1.9 GPa.

Phase transitions and chromium(III) luminescence in perovskite-type [C2H5NH3][Na0.5CrxAl0.5−x(HCOO)3] (x = 0, 0.025, 0.5), correlated with structural, dielectric and phonon properties

We report the synthesis, crystal structure, dielectric, vibrational and emission spectra of heterometallic MOFs, [C2H5NH3][Na0.5Cr0.5(HCOO)3] (EtANaCr), [C2H5NH3][Na0.5Al0.5(HCOO)3] (EtANaAl) and [C2H5NH3][Na0.5Al0.475Cr0.025(HCOO)3] (EtANaAlCr). These compounds crystallize in non-centrosymmetric monoclinic polar structures (space group Pn) and undergo order-disorder phase transitions upon heating to the monoclinic centrosymmetric structure (space group P21/n) at 369 (EtANaAl) and 373 K (EtANaCr). In principle, they are ferroelectric below these temperatures. In the high-temperature phase, ethylammonium (EtA+) cations are dynamically disordered over two symmetrically independent positions while upon cooling they begin to order. The ordering is accompanied by distortion of the metal formate framework. The hydrogen bonds (HBs) between the NH3+ group and NaO6 octahedral units are more robust than between the NH3+ group and CrO6 (AlO6) octahedral units and this feature explains a much stronger distortion of the former units and a weak effect of a trivalent cation type on the phase transition temperature. The dielectric studies have confirmed the occurrence of phase transitions of dipolar character and dipole relaxation processes. The optical studies show that EtANaCr and EtANaAlCr exhibit efficient Cr(iii)-based emission characteristics for intermediate-ligand field strength.

Size effects in KDP-porous glass ferroelectric nanocomposites

An influence of the size effect on the phase transition temperature in potassium dihydrogen phosphate (KDP)-porous glass nanocomposites was studied in this work. Results of the dielectric measurements in a wide temperature range for KDP crystals embedded in porous glasses are presented. The phase transition temperature shift toward higher temperatures was observed for KDP embedded in porous glasses with the mean value of the pore size of 312 and 160 nm, whereas for smaller pore sizes (71 and 23 nm) the phase transition temperature decreased. The phase transition temperature dependence of the mean values of the pores dimensions of KDP-porous glass nanocomposites is the experimental evidence showing the non-monotonical character of the size effect on the phase transition temperature in ferroelectrics particles. It was found that the dielectric permittivity of KDP embedded into the porous glasses with the mean values of pore sizes of 160 nm and 312 nm practically did not depend on frequency, whereas for KDP embedded into the pores with dimensions of 71 nm and 23 nm a dispersion of the dielectric permittivity is observed in both para- and ferro-electric phases.

Investigation of the crystal structure and influence of hydrostatic pressure on phase transition in Li2TiGeO5 ferroelastics

The crystal structure of the layered perovskite-like ferroelastic lithium titanium germanate (Li2TiGeO5) was determined by X-ray diffraction. At room temperature the crystal is tetragonal, space group P4/nmm (a = 6.6320(10), c = 4.4420(10) Å) and at 180 K is orthorhombic, space group Pmmn (a = 6.6270(10), b = 6.6290(10), c = 4.4270(10) Å). Measurements of hydrostatic pressure influence on phase transition (T 0 = 235.5 K) in the material have been carried out. The pressure derivative dT 0/dp is equal to −0.23 K/MPa for pressure range of 0-200 MPa.

Lattice dynamics calculations and temperature dependence of vibrational modes of ferroelastic Li2TiGeO5

Raman and polycrystalline IR spectra were obtained for Li2TiGeO5 and the assignment of the observed bands to the respective internal and external phonons has been proposed on the basis of lattice dynamics calculations. Temperature dependences of Raman- and IR-active phonons are also reported to probe the paraelastic–ferroelastic phase transition that takes place at 233.5 K. This study shows that the phase transition is continuous and that this phase transition leads to significant distortion of the unit cell.