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.
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.
The temperature-dependent Raman studies of A 2 Ti 2 O 7 (A = Dy, Er, Gd) were performed on single crystals and polycrystalline samples in the 4.2-295 K temperature range. The Raman spectra showed softening of the majority of phonon modes upon cooling in the whole temperature range studied and large decrease of linewidths. These changes have been analyzed in terms of strong third-order phonon-phonon anharmonic interactions. Moreover, the 312 and 330 cm −1 modes of Er 2 Ti 2 O 7 (Gd 2 Ti 2 O 7 ) showed hardening upon cooling down to about 130 K (100 K) and then anomalous softening below this temperature. The observed anomalous behavior of the Raman modes indicates that some important changes occur in these materials at low temperatures. However, the origin of this behavior is still not clear.
Nanosized Bi 2 WO 6 was synthesized by a mild hydrothermal crystallization process. This method allowed obtaining plate-like crystallites of very small thickness (down to 3 nm). The effect of particle size on the structure and properties of Bi 2 WO 6 was studied by X-ray diffraction (XRD), transmission electron microscopy, and Raman and infrared spectroscopies. It has been shown that the orthorhombic distortion decreases with decreasing particle size, but the structure of the smallest crystallites is still orthorhombic. Raman studies have also revealed a very strong intensity decrease for those modes that appear mainly for incident and scattered light polarized perpendicular to the layers. This behavior has been attributed to a decrease in the orthorhombic distortion and a plate-like shape of the nanocrystallites.
We report synthesis, thermal, dielectric, Raman, IR and luminescence studies of chromiumdoped multiferroic MOF, [(CH 3 ) 2 NH 2 ][Mn(HCOO) 3 ] (DMMn). These studies reveal that doping with chromium(III) leads to lowering of the ferroelectric phase transition temperature T c . The doping also changes the character of the phase transition from strongly first-order for undoped sample to partially diffused one for 3.1% of chromium doping. This behavior resembles behavior of inorganic ABO 3 perovskite ferroelectrics where doping often leads to decrease of T c and diffuse character of a phase transition. We also show that the chromium-doped sample exhibits efficient luminescence. Additional studies demonstrated that [(CH 3 ) 2 NH 2 ][M II (HCOO) 3 ] formates (M II =Mg, Mn, Co) may also be doped with other trivalent cations such as Al 3+ , In 3+ , Eu 3+ or Er 3+ .Doping with these ions also leads to decrease of T c and diffuse character of the phase transition.Additional optical studies show that europium-doped DMMn sample also exhibit luminescent properties. Thus our discovery opens up a new and simple route for synthesis of various multifunctional amine-templated metal formate frameworks with tunable multiferroic and luminescent properties by doping these frameworks with wide range of trivalent cations. 160 K. Firstly, many bands exhibit significant narrowing. This behavior is especially pronounced for the ρ(NH 2 ) IR mode observed near 920 cm -1 (Figure 6(b)) and the lattice modes ( Figure S7).Secondly, new bands appear and some bands split. For instance, the ν 3 (HCOO -) IR mode near 795 cm -1 and the ν 5 (HCOO -) mode near 1370 cm -1 split into doublets (Figure 6a and 6c). Third, some bands exhibit significant changes in intensity. This behavior is very clearly observed, for instance, for the Raman bands in the range 2800-2900 cm -1 (Figure 5a). Figure 5. (a) Detail of the Raman spectra results corresponding to the spectral range 3100-2750 cm -1 for DMMn: 3.1% Cr 3+ . For the comparison sake, data for previously reported DMMn are presented in (b). 18 Arrows indicate the bands that exhibit pronounced increase in intensity below the phase transition temperature.data shows also some significant differences. Firstly, the observed changes in DMMn: 3.1% Cr 3+ are not so sharp as in DMMn but they are smeared over 100-160 K range. This behavior is consistent with shift of the phase transition temperature to lower value and its diffuse character.Secondly, the observed frequency shifts of the νas(CNC) and νs(CNC) modes when going from room-temperature to 5 K are smaller for DMMn: 3.1% Cr 3+ compared to DMMn. Furthermore, Figures 5 and 6 show that bands of DMMn: 3.1% Cr 3+ are broader and splitting smaller when compared to DMMn. This behavior points to weaker distortion of the framework and smaller changes in the C-N bond lengths at T c upon doping this MOF with chromium ions.
Optical Studies of DMMn: 3.1 % Cr 3+Emission measured at 250 K is broad, centered at 12909 cm -1 (787.6 nm) and its FWHM is equal 2841 cm -1 (Figure 9)...
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