Interionic potential energy studied by far-infrared spectroscopy

Ionic potential models for tetra-alkylammonium halides are proposed, the repulsive energy being evaluated from crystal properties. By assuming a CsCl type structure, such potentials give a qualitative agreement with far-infrared (FIR) spectroscopic data (lattice bands and ion-pair vibration in solution) for a series of salts. For Pr₄NBr, the true structure is known and a more realistic potential has been elaborated. It is shown that the ionic potential is largely determined by the number of first neighbours and the shortest anion-cation distance in the solid. A possible way to estimate the repulsive energy for any R₄NX salt is suggested by using ion-pair dipole moment and vibration frequency data.     

Luminescence kinetics characteristics of lead-containing aggregates dispersed in Rb0.95Cs0.05Cl solid state solution

The relaxation of the electron excitations in an insulating matrix such as the Rb_0.95Cs_0.05Cl solid solution containing CsPbCl₃ nanocrystals and single lead centers is studied. Modification of the spectral kinetic parameters of the CsPbCl₃ nanocrystals due to the quantum confinement is explored. Mechanism of the energy transfer from matrix to the nanocrystals is established. The luminescence of the Rb_0.95Cs_0.05C–Pb crystal is characterised by means of measurements with subnanosecond time resolution under synchrotron radiation excitation in the energy range of 3.7–20 eV at T=10 K.     

Magnetic properties of MnO nanocrystals dispersed in a silica matrix

Magnetic nanocrystalline MnO particles have been synthesized in a silica glass matrix by the sol-gel method at calcination temperatures up to 1000 °C. EPR spectra of 0.1 mol% MnO doped silica gel and glasses studied in the temperature range 10-290 K show with the exception of those samples calcined at 900 and 1000 °C 6-line characteristic Mn(II) hyperfine (HF) lines. Additionally five spin-forbidden doublets have been observed at 100 K and below. Small spreads in spin Hamiltonian parameters (D and E) imply that the ligand field environments of Mn(II) ions embedded in the silica glass are nearly uniform. Monotonous decrease in HF linewidth in going from 120 °C gel to 800 °C calcined glass has been interpreted as the continuous decrease in population of isolated Mn²⁺ ions in silica glass matrix resulting in the decrease of magnetic dipolar interactions leading to the observed decrease in HF linewidth. XRD and TEM of sample calcined at 1000 °C shows the presence of nanocrystals of MnO having orthorhombic crystalline phase and sizes about 10 nm. The thermal behavior of magnetization (zero-field-cooled and field-cooled) and magnetic hysteresis of MnO nanocrystals in the 5-300 K temperature interval have demonstrated that the MnO nanocrystals display superparamagnetic-ferromagnetic transition at low temperatures. X-band EPR linewidth data plotted versus inverse of temperature (1/T) for samples calcined at 900 and 1000 °C (EPR recorded in the vicinity of 0.35 T applied field) depict similar transitions.     

Caesium and hydrated lead chlorates studied by35Cl NQR spectroscopy

Molecular dynamics of caesium and lead chlorates (hydrated form) has been studied by³⁵Cl nuclear quadrupole resonance (NQR) spectroscopy. The occurrence of only one resonance line for the two compounds in the whole temperature range studied testifies to the crystallographically equivalent positions of their molecules in elementary cells. The relaxation processes in the two chlorates have been found to be analogous to those in the other chlorates not undergoing phase transitions. Analysis of the temperature dependence of the spin-lattice relaxation time has indicated that at low temperatures the dominant relaxation mechanism consists of torsion vibrations (ClO₃⁻ anion libration), whereas at 260 K the hindered rotation of the chlorate ion about the threefold axis sets on and its activation energy is 50.0 kJ/mol. The temperature dependence of the resonance frequency for the two chlorates studied is best described by the Brown model extended over the low-temperature range, which indicates large anharmonicity of the vibrations.     

Photoinduced phase transitions by time-resolved far-infrared spectroscopy in V2O3

Using time-resolved far-infrared spectroscopy, we observe multiple routes for photoinduced phase transitions in V(2)O(3). This includes (i) a photothermal antiferromagnetic to paramagnetic transition and (ii) an incipient strain-generated paramagnetic metal to paramagnetic insulator transition, which manifests as coherent oscillations in the far-infrared conductivity. The ～100 ps conductivity oscillation results from coherent acoustic phonon modulation of the bandwidth W. Our results indicate that poor metals are particularly amenable to coherent strain control of their electronic properties.     

Anatase TiO2 nanocrystals prepared by mechanochemical synthesis and their photochemical activity studied by EPR spectroscopy

Anatase TiO₂ has been prepared by mechanochemical synthesis using TiOSO₄·xH₂O and Na₂CO₃ as starting reactants. The reaction was performed in high-energy ball mill using steel and corundum jars, respectively. The final products were obtained by annealing the milled powder in the temperature range of 300–700 °C and subsequently by washing out the water-soluble byproduct Na₂SO₄·xH₂O. When steel jars were used, the annealing in the range of 300–600 °C led to anatase. For products milled in corundum, the stability of anatase increased up to 700 °C. Transition electron microscopy (TEM) showed that crystallites with a size in the range of 20–50 nm with equiaxed morphology were obtained after milling in corundum and annealing at 600 and 700 °C. The process of photoinduced reactive hydroxyl radical generation in aerated aqueous titania suspensions was studied by EPR spectroscopy using spin trapping technique. The presence of iron impurities in the samples milled in steel substantially decreases the radical formation. The rate of radical formation is substantially affected by particle size development of TiO₂ nanopowders. The product milled in corundum and annealed at 700 °C outperforms more than twice the photochemical activity of TiO₂ Degussa P25 standard.     

Exploring the dynamics of superconductors by time-resolved far-infrared spectroscopy

We have examined the recombination of excess quasiparticles in superconducting Pb by time-resolved far-infrared spectroscopy using a pulsed synchrotron source. The energy gap shift calculated by Owen and Scalapino [Phys. Rev. Lett. 28, 1559 (1972)] is directly observed, as is the associated reduction in the Cooper pair density. The relaxation process involves a two-component decay; the faster ( approximately 200 ps) is associated with the actual (effective) recombination process, while the slower ( approximately 10 to 100 ns) is due to heat transport across the film/substrate interface. The temperature dependence of the recombination process between 0. 5T(c) and 0.85T(c) is in good agreement with theory.     

Luminescence of CsPbCl3 microcrystals in CsCl:Pb and PbCl2:Cs crystals under synchrotron excitation

This paper reports on a study of the luminescence kinetics in CsPbCl₃ microcrystals dispersed in a CsCl or PbCl₂ host matrix, which are excited by synchrotron radiation at energies E _exc=4–20 eV. The luminescence decay kinetics of CsPbCl₃ microcrystals is found to contain different time components generated by direct excitation of the microcrystals or reabsorption of the luminescence emitted by other centers. It is conjectured that the decreased luminescence decay constant of CsPbCl₃ microcrystals dispersed in these hosts as compared to its counterpart of CsPbCl₃ single crystals is accounted for by size quantization.     

Molecular motions in chloramphenicol studied by35Cl nuclear quadrupole and1H nuclear magnetic resonance spectroscopy

The³⁵Cl nuclear quadrupole resonance (NQR) frequency (v_Q), nuclear quadrupole spinlattice relaxation time (T_1Q),¹H nuclear magnetic resonance second moment (M₂) and nuclear magnetic spin-lattice relaxation timeT ₁) were measured for polycrystalline chloramphenicol (drug) as a function of temperature. Hindered rotation of the CHC1₂ group and the phenyl ring was detected, the relevant activation energies were determined. The rotations are discussed in detail.     

Ar 3s, 3p satellite lines studied by fluorescence spectroscopy

The fluorescent radiation emitted by the Ar 3s, 3p satellite states (observed by photoelectron spectroscopy) has been recorded in the wavelength range between 400 and 780 Å with an energy resolution ranging from 50 to 100 mV. The fluorescence was stimulated by use of undispersed synchrotron radiation.     

Photo-and X-ray luminescence spectra of CsPbX3 microcrystals dispersed in a PbX2 (X=Cl, Br) matrix

The CsPbX₃ (X=Cl, Br) microcrystals are obtained in a PbX₂ (X=Cl, Br) matrix doped with Cs ions (C _Cs=0.1–1 mol %). Spectral and kinetic parameters of the intrinsic luminescence of the CsPbX₃ single crystals are presented. For the CsPbCl₃ microcrystals, a quantum-confinement effect is detected, which reveals itself as a decrease in the lifetime of the free exciton luminescence. A mechanism of formation of a CsPbX₃ microcrystal in a PbX₂ matrix is proposed.     

Cooperative Jahn-Teller phase transition in LaMnO3 studied by X-ray absorption spectroscopy

The local structure of LaMnO3 across the Jahn-Teller (JT) transition at T(JT)=750 K was studied by means of x-ray absorption near edge structure and extended x-ray absorption fine structure at the Mn K-edge. Our results indicate a similar electronic local structure for Mn atoms above and below T(JT) and a dynamical tetragonal JT distortion of MnO6 octahedra above T(JT). The structural transition is originated by the ordering of tetragonally distorted octahedra. The entropy content of the transition is analyzed within the framework of the three-state Potts model with nearest neighbor antiferrodistortive coupling.