352 research outputs found
Peierls Instabilities in Quasi-One-Dimensional Quantum Double-Well Chains
Peierls-type instabilities in quarter-filled () and half-filled
() quantum double-well hydrogen-bonded chain are investigated
analytically in the framework of two-stage orientational-tunnelling model with
additional inclusion of the interactions of protons with two different optical
phonon branches. It is shown that when the energy of proton-phonon coupling
becomes large, the system undergoes a transition to a various types of
insulator states. The influence of two different transport amplitudes on ground
states properties is studied. The results are compared with the pressure effect
experimental investigations in superprotonic systems and hydrogen halides at
low temperatures.Comment: 7 pages, RevTeX, 9 eps figure
Dielectric properties of BiFeO3 ceramics obtained from mechanochemically synthesized nanopowders
Dielectric behaviour of BiFeO3 ceramics,obtained by hot-pressing of nanopowders produced by mechanochemical synthesis from Bi2O3 and Fe2O3 oxides (weight ratio 2:1), was studied in the temperature range 125–575 K. The ceramics was found to exhibit step-like dielectric response ε*(T) with high permittivity values, similar to the behaviour of materials with giant dielectric permittivity. Three overlapping relaxation processes contribute to the dielectric response: i) relaxation in the lowtemperature range (220–420 K), characterized by activation energy of 0.4 eV, ii) relaxation in the temperature range 320–520 K with activation energy of 1.0 eV and iii) broad dielectric anomaly in the vicinity of 420 K, which disappears after 1 h annealing at 775 K. The lowtemperature relaxation is ascribed to the carrier hopping process between Fe2+ and Fe3+ ions. The presence of mixed valence of the Fe ions was proved by X-ray photoelectron spectroscopy. Dielectric relaxation in the middle-temperature range is considered as a result of grain boundary effect and internal barrier layers related to Bi25FeO40 phase as verified by X-ray diffraction. The high-temperature dielectric anomaly we relate to short-range hopping of ordered oxygen vacancies
Dielectric properties of BiFeO3 ceramics obtained from mechanochemically synthesized nanopowders
Dielectric behaviour of BiFeO3 ceramics,obtained by hot-pressing of nanopowders produced by mechanochemical synthesis from Bi2O3 and Fe2O3 oxides (weight ratio 2:1), was studied in the temperature range 125–575 K. The ceramics was found to exhibit step-like dielectric response ε*(T) with high permittivity values, similar to the behaviour of materials with giant dielectric permittivity. Three overlapping relaxation processes contribute to the dielectric response: i) relaxation in the lowtemperature range (220–420 K), characterized by activation energy of 0.4 eV, ii) relaxation in the temperature range 320–520 K with activation energy of 1.0 eV and iii) broad dielectric anomaly in the vicinity of 420 K, which disappears after 1 h annealing at 775 K. The lowtemperature relaxation is ascribed to the carrier hopping process between Fe2+ and Fe3+ ions. The presence of mixed valence of the Fe ions was proved by X-ray photoelectron spectroscopy. Dielectric relaxation in the middle-temperature range is considered as a result of grain boundary effect and internal barrier layers related to Bi25FeO40 phase as verified by X-ray diffraction. The high-temperature dielectric anomaly we relate to short-range hopping of ordered oxygen vacancies
Structural phase transitions and their influence on Cu+ mobility in superionic ferroelastic Cu6PS5I single crystals
The structural origin of Cu+ ions conductivity in Cu6PS5I single crystals is
described in terms of structural phase transitions studied by X-ray
diffraction, polarizing microscope and calorimetric measurements. Below the
phase transition at Tc=(144-169) K Cu6PS5I belongs to monoclinic, ferroelastic
phase, space group Cc. Above Tc crystal changes the symmetry to cubic
superstructure, space group F-43c (a=19.528); finally at 274K disordering of
the Cu+ ions increases the symmetry to F-43m, (a=9.794). The phase transition
at 274K coincides well with a strong anomaly in electrical conductivity
observed in the Arrhenius plot. Diffusion paths for Cu+ ions are evidenced by
means of the atomic displacement factors and split model. Influence of the
copper stechiometry on the Tc is also discussed.Comment: conference pape
Dy8SnS13.61O0.39 from single-crystal data
Crystals of the title dysprosium tin sulfide oxide, Dy8SnS13S1−xOx [x = 0.39 (4)], were obtained unintentionally from the Dy–Sn–S system. A statistical mixture of sulfur and oxygen was assumed for one position in the structure. S and O atoms surround each of the eight symmetrically non-equivalent dysprosium atoms. The Sn atoms are located in tetrahedral surroundings of sulfur atoms. Trigonal prisms and tetrahedra are connected to each other by their edges. All atoms are situated in mirror planes
Hydrogen transport in superionic system Rb3H(SeO4)2: a revised cooperative migration mechanism
We performed density functional studies of electronic properties and
mechanisms of hydrogen transport in Rb3H(SeO4)2 crystal which represents
technologically promising class M3H(XO4)2 of proton conductors (M=Rb,Cs, NH4;
X=S,Se). The electronic structure calculations show a decisive role of lattice
dynamics in the process of proton migration. In the obtained revised mechanism
of proton transport, the strong displacements of the vertex oxygens play a key
role in the establishing the continuous hydrogen transport and in the achieving
low activation energies of proton conduction which is in contrast to the
standard two-stage Grotthuss mechanism of proton transport. Consequently, any
realistic model description of proton transport should inevitably involve the
interactions with the sublattice of the XO4 groups.Comment: 11 pages, 11 figures, to appear in Physical Review
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