K. Hermanowicz

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.

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.

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.

Vibrational properties of KLn(MoO4)2 crystals for light rare earth ions from lanthanum to terbium

Abstract Laser Raman and Fourier transform-IR spectra of potassium—rare earth double molybdates for lanthanides from La to Tb were recorded. Factor group analysis (FGA) was carried out and a vibrational assignment is proposed for the internal and external optic modes. The effect of 92Mo/100Mo isotope exchange on the IR and Raman transitions is discussed.

Temperature-dependent ESR studies on the Cr3+ ion-doped KAl(MoO4)2 crystal

Abstract A temperature-dependent ESR study of Cr 3+ ion in potassium aluminium double molybdate KAl(MoO 4 ) 2 crystal is reported. The trigonal chromium complex possesses spin Hamiltonian parameters: g || =1.9781, g ⊥ =1.9727 and | D |=0.4798 cm −1 . The temperature behaviour of the resonant lines indicates the possibility of two phase transitions which take place below 80 and 50 K.

Lattice dynamics and phase transitions in KAl(MoO4)2, RbAl(MoO4)2 and CsAl(MoO4)2 layered crystals

Raman and IR studies of KAl(MoO4)2, RbAl(MoO4)2 and CsAl(MoO4)2 are reported. The assignments of modes are given on the basis of lattice dynamics calculations. The temperature dependence of the KAl(MoO4)2 vibrational modes shows that this compound exhibits a second-order phase transition around 90 K from the to most probably a monoclinic and ferroelastic phase.

Temperature-dependent Raman and IR studies of multiferroic MnWO4 doped with Ni2+ ions.

Temperature-dependent Raman and IR studies of MnWO(4) crystal doped with Ni(2+) ions were performed in the 4.2-300 K range. These studies were complemented by magnetization and specific heat measurements in the 2-100K range, which revealed that MnWO(4) crystal doped with Ni(2+) ions exhibits two phase transitions at 13.9 and 12.5K. Temperature evolution of Raman wavenumbers and linewidths revealed anomalous behaviour at low temperatures. These anomalies have been attributed to spin-phonon coupling, which appear due to onset of antiferromagnetic spin ordering. The observed anomalies extend above T(N)=13.9 K. This behaviour is consistent with the fact that MnWO(4) is a moderately magnetically frustrated material.

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.

Raman Spectra of Molecules Adsorbed on Ag Centers in Sol-Gel Matrices

Silica monoliths and submicron spheres containing silver nanoparticles have been obtained using the sol gel technology. The Ag inclusions were synthesized via the counterdiffusion method. The silver-doped matrices were immersed in solutions of an organic dye (indocyanine green) enabling the solute molecules to interact with surface of the Ag-doped silica matrices. Raman spectra of free solutions of the organic molecules under investigation, the impregnated Ag-doped matrices and the impregnated Ag-free matrices have been measured. The impregnated silica matrices which did not contain silver nanoparticles were used as a reference. These experiments have been performed in order to establish if Raman signal enhancement could be obtained by adsorption of organic molecules on the surface of Ag inclusions in the sol-gel matrices analogously to the standard surface-enhanced Raman spectroscopy (SERS) method.

Emission and absorption properties of the eight-coordinate [Pr(C7H9NO)8](ClO4)3 complex with 3,4-lutidine N-oxide

Abstract The eight-coordinate complex of praseodymium(III) with 3,4-lutidine N -oxide, [Pr(C 7 H 9 NO) 8 ](ClO 4 ) 3 , has been synthesised and characterised by means of several spectroscopic methods. The IR and Raman spectra have been measured and are discussed. The electronic absorption, emission and excitation spectra have been recorded and are analysed in terms of possible structures. The 3 P 0 → 3 H 5,6 , 3 F 2,4 emissions have been observed and selective excitation was used to assign the energy levels of Pr 3+ . The potential application of this material as a precursor to light-conversion molecular schemes is discussed.