M. Maczka

STRUCTURE AND VIBRATIONAL PROPERTIES OF TETRAGONAL SCHEELITE NABI(MOO4)2

The structure of the tetragonal NaBi(MoO4)2 crystal was determined. The space group is I4=S42 with cell dimensions a=5.267, c=11.565 A and Z=2. The molybdenum atoms occupy two different sites of 4 symmetry. The Na+ and Bi3+ cations are situated on two positions with different occupancy factors and are short-range ordered. The IR and Raman polarized spectra are discussed on the basis of the factor group analysis and phonon calculations. The modified Urey–Bradley force field and potential energy distribution were applied for three molecular models of local disorder. Polarization properties are explained and related to the crystal structure.

Raman scattering study of NaAl(MoO4)2 crystal under high pressures

The high-pressure Raman spectra of ferroelastic NaAl(MoO4)2 have been measured at room temperature. The studies indicated that this crystal exhibits two pressure-induced phase transitions at about 1.1 and 3.3 GPa. The first transition is connected with slight rotation of the MoO4−2 tetrahedra with loss of the inversion centre. The second transition is connected with significant distortion of the MoO42− tetrahedra and Al3+ coordination sphere. By performing the lattice dynamics calculations in the starting phase (monoclinic C2h6) we have been able to make an assignment of the Raman modes of this material. The deep knowledge of the modes helped us to get fundamental insights into the mechanism driving the structural changes occurring in NaAl(MoO4)2. The two phase transitions observed in the 0.0–4.2 GPa pressure range are completely reversible.

Observation of attractive intermolecular C–H⋯O interaction in the crystal packing of 3-chloro- and 3-bromo-2,6-dimethyl-4-nitropyridine N-oxide

Abstract The X-ray crystal structure determination of 3-chloro-2,6-dimethyl-4-nitropyridine N-oxide (ClDMNPNO) and 3-bromo-2,6-dimethyl-4-nitropyridine N-oxide (BrDMNPNO) shows that the two pyridine derivatives are isomorphic with monoclinic space group P21/c, and four formula units in a cell with the following dimensions: a=7.933(1), b=9.721(1), c=11.419(1) A, β=107.70(1)° and a=7.981(1), b=9.817(2), c=11.515(1) A, β=106.54(1)° for ClDMNPNO and BrDMNPNO, respectively. The shortest intermolecular contacts form medium strong hydrogen bonds of the type C–H⋯O. Another relatively short intermolecular contact is established for CH3⋯Cl/Br. The planar pyridine rings do not show the C–C bonds alternation typical for resonance forms in aromatic rings. The IR and Raman spectra, measured in the 50–3500 cm−1 region at ambient temperature, are correlated with X-ray structural data. The assignment of IR and Raman bands is given. Comparison of the spectra of dissolved samples with the ones obtained from polycrystalline samples shows the attractive character of the intermolecular C–H⋯O contact for these molecules. No strong spectroscopic support is found for the existence of a significant CH3⋯Cl/Br intermolecular interaction.

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.

NaAl(MoO4)2: a rare structure type among layered yavapaiite-related AM(XO4)2 compounds

NaAl(MoO4)2, sodium aluminium dimolybdate(VI), represents a rare pseudo-orthorhombic structure type among AM(XO4)2 compounds with yavapaiite-related sheet structures. It is isostructural with NaFe(MoO4)2. Infinite (001) sheets composed of AlO6 octahedra, corner-linked to MoO4 tetrahedra, are separated by eight-coordinated Na atoms. Average Na—O, Al—O and Mo—O bond lengths are 2.648, 1.890, and 1.762 A, respectively. All atoms are on general positions except for Na and Al (both of which have site symmetry \overline 1).

Local Transition of MnO4 2 − Molecular Impurity Ion in Proper Ferroelastic K3Na(CrO4)2

Temperature dependences of EPR spectrum for Mn 6+ paramagnetic center embodied in MnO 4 2 − molecular impurity ion in proper ferroelastric K 3 Na(CrO 4 ) 2 were studied. Local transition due to local electron-lattice instability appearing at low temperature region T ≤ 18 K has been detected and identified for this system in the first time. Model of the phenomenon is based on the transformation of Jahn-Teller effect to pseudo-Jahn-Teller effect with its the following complete suppressing under the action of ferroelastic order parameter increase at low temperatures.

Electron paramagnetic resonance and optical spectroscopy of K3Na(CrO4)2 ferroelastics activated by MnO 4 2− molecular impurity ions

Crystals of a proper ferroelastic K3Na(CrO4)2 containing molecular impurity ions MnO42− are studied using electron paramagnetic resonance (EPR) and optical spectroscopy. The EPR spectrum of the Mn6+ ion contained in the molecular impurity ion MnO42− is identified at low temperatures (T ≤ 20 K). The intensity of this spectrum decreases unusually fast as the temperature increases. A broad IR luminescence band with a vibronic structure well resolved at a temperature of 8 K is revealed. Theoretical treatment of the Mn6+ ion involved in the molecular impurity ions MnO42− of the K3Na(CrO4)2 ferroelastic crystal suggests that an important role in this case is played by the pseudo-Jahn-Teller. The pseudo-Jahn-Teller effect offers an explanation for the appearance of a fine structure in the vibronic replicas in the luminescence spectrum, on the one hand, and accounts for the fast decrease in the intensity of the EPR spectrum of K3Na(CrO4)2: MnO42− with increasing temperature, on the other.

CsAl(MoO4)2

The title compound, caesium aluminium dimolybdate(VI), CsAl(MoO4)2, belongs to the glaserite type family of double molybdates and tungstates. The crystal structure was studied by in situ X-ray single-crystal and powder diffraction at room temperature. The temperature dependence of the lattice parameters at low temperatures is also presented.