The effect of molecular dynamics on basic parameters of cw-epr spectrum of pyridinium tetrafluoroborate single crystal

This paper presents an investigation of the phase transitions and molecular dynamics of radicals in pyridinium tetrafluoroborate single crystals by CW-EPR combined with the Computer Resolution Enhancement Method. We present the advantages following from the computer method for enhancement of resolution by the example of applying it in the analysis of complex CW-EPR spectra of organic radicals. The method enabled determination of basic parameters such as spectroscopic splitting factor, hyperfine coupling constants and widths of individual spectral lines in CW-EPR spectra of pyridinium tetrafluoroborate recorded in a wide range of temperatures. Phase transitions and changes in molecular dynamics of radicals may imply changes in their electronic structure, which may affect the above-mentioned parameters of CW-EPR spectra.     

Phase Transitions in Three New Hybrid Crystals: (Me3NR)4[Ni(NCS)6] (R=Ethyl,Propyl, and Butyl)

Understanding the microscopic origin of phase transitions in hybrid crystals is of significant important but challenging for designing novel phase-transition materials. Here, three new hybrid crystals, (Me₃NR)₄[Ni(NCS)₆] (R=ethyl for 1 , propyl for 2 , and butyl for 3 ) were synthesized and comprehensively studied on their crystal structures, intermolecular interactions, and phase transitions. They possess a same anti-XeF₄ structure mode but exhibit different phase transitions arising from their subtly changed alkyl groups. Specifically, 1 undergoes four-step P2₁/c-P2₁/c-P2₁/c-Pbca-Cmce crystalline transitions at 165, 203, 244, and 280 K, respectively; 2 undergoes three-step P2₁2₁2₁-Pbca-P2₁/n-P 1c crystalline transitions at 167, 393, and 410 K, respectively, following by a solid-liquid transition at 453 K; 3 undergoes an iso-space-group P2₁/n-P2₁/n crystalline phase transition at 257 K and a solid-liquid transition at 410 K. These instances well demonstrate the key roles of delicate and complicated intermolecular interactions on inducing nontrivial phase transitions in hybrid crystals.     

Electromagnetically induced transparency in a non-uniform plasma

Electromagnetically induced transparency in a non-uniform plasma is investigated. We discuss the possible frequency ranges in which a weak beam below cutoff can propagate transparently in this plasma with the presence of a strong high frequency field. We also analyze the effect of driving intensity on the frequency range of the probe. Received 3 March 2000 and Received in final form 21 April 2000     

Add a Pinch of Tetrel: The Transformation of a Centrosymmetric Metal into a Nonsymmorphic and Chiral Semiconductor

Centrosymmetric skutterudite RhP₃ was converted to a nonsymmorphic and chiral compound RhSi_0.3P_2.7 (space group P2₁2₁2₁) by means of partial replacement of Si for P. The structure, determined by a combination of X-ray crystallography and solid state ³¹P NMR, exhibits branched polyanionic P/Si chains that are unique among metal phosphides. A driving force to stabilize the locally noncentrosymmetric cis-RhSi₂P₄ and fac-RhSi₃P₃ fragments is π-electron back-donation between the Rh t_2g-type orbitals and the unoccupied antibonding Si/P orbitals, which is more effective for Si than for P. In situ studies and total energy calculations revealed the metastable nature of RhSi_0.3P_2.7. Electronic structure calculations predicted centrosymmetric cubic RhP₃ to be metallic which was confirmed by transport properties measurements. In contrast, the electronic structure for chiral orthorhombic RhSi_0.3P_2.7 contained a bandgap, and this compound was shown to be a narrow gap semiconductor.     

Phonon dispersion and Raman spectra of wurtzite InAs under pressure

The authors have systematically studied the vibrational properties of wurtzite InAs at high pressure within the Density Functional Theory scheme. It is found that pressure significantly affects the phonon dispersion curves and Raman spectra. We observed an indication of phase transition for WZ-InAs at about 10 GPa. The elastic constants calculation show mechanical stability for WZ-InAs. The calculated values of structural parameters are in good agreement with available data. There is a quadratic increase in optical modes with pressure while the LO–TO splitting and effective charge decrease linearly with pressure.     

Detection of a P-induced liquid ⇌ solid-phase transformation using multiple acoustic transducers in a multi-anvil apparatus

A technique for detecting and measuring phase transitions in a multi-anvil apparatus by measuring the change in travel time for a longitudinal sound wave as a function of pressure is reported. The system measures the time for pulsed ultrasonic signals to travel through a high pressure assembly with a sample in the center. Upon phase change from liquid to solid, the travel time shows an abrupt decrease due to the intrinsic increase in velocity in the sample and a reduced delay between the triggering of an amplitude threshold and the arrival of the waveform. As a proof of concept, results are shown for mercury as it undergoes pressure-induced liquid ? solid transitions at room temperature. We propose that this non-destructive technique may be valuable in situations where other in situ probing techniques cannot be readily used to provide information about changes of state and potentially to study transition kinetics at high pressures as well.