Mirosław Mączka

Order–Disorder Transition and Weak Ferromagnetism in the Perovskite Metal Formate Frameworks of [(CH3)2NH2][M(HCOO)3] and [(CH3)2ND2][M(HCOO)3] (M = Ni, Mn)

We report the synthesis, crystal structure, thermal, dielectric, Raman, infrared, and magnetic properties of hydrogen and deuterated divalent metal formates, [(CH3)2NH2][M(HCOO)3] and [(CH3)2ND2][M(HCOO)3], where M = Ni, Mn. On the basis of Raman and IR data, assignment of the observed modes to respective vibrations of atoms is proposed. The thermal studies show that for the Ni compounds deuteration leads to a decrease of the phase transition temperature Tc by 5.6 K, whereas it has a negligible effect on Tc in the Mn analogues. This behavior excludes the possibility of proton (deuteron) movement along the N-H···O (N-D···O) bonds as the microscopic origin of the first-order phase transition observed in these crystals below 190 K. According to single-crystal X-ray diffraction, the dimethylammonium (DMA) cations are dynamically disordered at room temperature, because the hydrogen bonds between the NH2 (ND2) groups and the metal-formate framework are disordered. The highly dynamic nature of hydrogen bonds in the high-temperature phases manifests in the Raman and IR spectra through very large bandwidth of modes involving vibrations of the NH2 (ND2) groups. The abrupt decrease in the bandwidth and shifts of modes near Tc signifies the ordering of hydrogen bonds and DMA(+) cations as well as significant distortion of the metal-formate framework across the phase transition. However, some amount of motion is retained by the DMA(+) cation in the ferroelectric phase and a complete freezing-in of this motion occurs below 100 K. The dielectric studies reveal pronounced dielectric dispersion that can be attributed to slow dynamics of large DMA(+) cations. The low-temperature studies also show that magnetic properties of the studied compounds can be explained assuming that they are ordered ferrimagnetically with nearly compensated magnetic moments of Ni and Mn. IR data reveal weak anomalies below 40 K that arise due to spin-phonon coupling. Our results also show that due to structural phase transition more significant distortion of the metal-formate framework occurs for the deuterated samples.

Structure, Phonon Properties, and Order–Disorder Transition in the Metal Formate Framework of [NH4][Mg(HCOO)3]

We report the synthesis, crystal structure, thermal, dielectric, IR, and Raman studies of [NH4][Mg(HCOO)3] formate. Single-crystal X-ray diffraction shows that it crystallizes in the hexagonal space group P6322, with orientationally disordered NH4(+) ions located in the cages of the network. Upon cooling, [NH4][Mg(HCOO)3] undergoes a phase transition at around 255 K to the ferroelectric P63 structure. Raman and IR spectra show a strong increase in intensity of the N-H stretching bands as well as narrowing of the bands related to the NH4(+) ions upon cooling. These changes indicate that the phase transition is due to orientational ordering of the NH4(+) ions. Analysis of the Raman data show, however, that the rotational and translational motions of NH4(+) do not freeze completely at the phase transition but exhibit further slowing down below 255 K, and the motional freezing becomes nearly complete below 140 K.

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.

Temperature- and Pressure-Induced Phase Transitions in the Metal Formate Framework of [ND4][Zn(DCOO)3] and [NH4][Zn(HCOO)3]

Vibrational properties and the temperature-induced phase transition mechanism have been studied in [NH4][Zn(HCOO)3] and [ND4][Zn(DCOO)3] metal organic frameworks by variable-temperature dielectric, IR, and Raman measurements. DFT calculations allowed proposing the detailed assignment of vibrational modes to respective motions of atoms in the unit cell. Temperature-dependent studies reveal a very weak isotopic effect on the phase transition temperature and confirm that ordering of ammonium cations plays a major role in the mechanism of the phase transition. We also present high-pressure Raman scattering studies on [ND4][Zn(DCOO)3]. The results indicate the rigidity of the formate ions and strong compressibility of the ZnO6 octahedra. They also reveal the onset of a pressure-induced phase transition at about 1.1 GPa. This transition has strong first-order character, and it is associated with a large distortion of the metal formate framework. Our data indicate the presence of at least two nonequivalent formate ions in the high-pressure structure with very different C-D bonds. The decompression experiment shows that the transition is reversible.

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.

Rare earth-doped lead borate glasses and transparent glass–ceramics: Structure–property relationship

Correlation between structure and optical properties of rare earth ions in lead borate glasses and glass-ceramics was evidenced by X-ray-diffraction, Raman, FT-IR and luminescence spectroscopy. The rare earths were limited to Eu(3+) and Er(3+) ions. The observed BO(3)↔BO(4) conversion strongly depends on the relative PbO/B(2)O(3) ratios in glass composition, giving important contribution to the luminescence intensities associated to (5)D(0)-(7)F(2) and (5)D(0)-(7)F(1) transitions of Eu(3+). The near-infrared luminescence and up-conversion spectra for Er(3+) ions in lead borate glasses before and after heat treatment were measured. The more intense and narrowing luminescence lines suggest partial incorporation of Er(3+) ions into the orthorhombic PbF(2) crystalline phase, which was identified using X-ray diffraction analysis.

Raman and IR Studies of Pressure- and Temperature-Induced Phase Transitions in [(CH2)3NH2][Zn(HCOO)3]

Temperature- and pressure-dependent studies of Raman and IR spectra have been performed on azetidinium zinc formate, [(CH2)3NH2][Zn(HCOO)3]. Vibrational spectra showed distinct anomalies in mode frequencies and bandwidths near 250 and 300 K, which were attributed to structural phase transitions associated with the gradual freezing of ring-puckering motions of the azetidinium cation. Pressure-dependent studies revealed a pressure-induced transition near 0.4 GPa. Raman spectra indicate that the structure of the room-temperature intermediate phase observed near 0.4 GPa is the same as the monoclinic structure observed at ambient pressure below 250 K. The second phase transition was found near 2.4 GPa. This transition has strong first-order character and is associated with strong distortion of both the zinc formate framework and azetidinium cations. The last phase transition was found near 7.0 GPa. This transition leads to lowering of the symmetry and further distortion of the zinc formate framework, whereas the azetidinium cation structure is weakly affected.

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.

Vibrational characteristics of the double oxygen bridge in the NaIn(WO4)2 and NaSc(WO4)2 tungstates with wolframite structure

Abstract The normal coordinate analysis of the tungsten–oxygen core in NaIn(WO 4 ) 2 and NaSc(WO 4 ) 2 crystals was performed. The Urey–Bradley force field and potential energy distribution (PED) were applied in the internal and external phonon calculations for the W 4 O 22 Na 2 In 2 molecular system. The dynamics of the asymmetric WO 2 W oxygen bridge as well as terminal WO bonds were analyzed and discussed. The vibrational characteristics of the oxygen double bridge bond, i.e. vibration energy, symmetry, force constants, PED, atomic displacements, direction of the transition dipole moments and mean square amplitudes were obtained and discussed. The theoretical considerations were based on the polarized IR and Raman spectra of the materials studied.

Brillouin scattering study of ferroelectric transition mechanism in multiferroic metal-organic frameworks of [NH4][Mn(HCOO)3] and [NH4][Zn(HCOO)3]

Temperature dependence of acoustic properties of [NH4][Mn(HCOO)3] and [NH4][Zn(HCOO)3] metal-organic frameworks was investigated by high-resolution micro-Brillouin scattering. Clear anomalies in the Brillouin shift and damping were observed near the transition temperature Tc upon cooling for the acoustic phonon corresponding to the c11 elastic constant. Analysis of the acoustic anomalies showed that the order parameter exhibits critical slowing down near Tc with the relaxation time of similar order of magnitude as for other order-disorder ferroelectrics.