Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userAn innovative approach of structural instabilities in tetragonal tungsten bronze crystals through the concept of rigid unit modes
89
0
0.0
(
0
)
Added by
Pierre Saint-Gregoire
Publication date
2013
fields
Physics
and research's language is
English
Authors
Mikhail Smirnov
Ask ChatGPT about the research
No Arabic abstract
Tetragonal tungsten bronze (TTB) oxides are one of the most important classes of ferroelectrics. Many of these framework structures undergo ferroelastic transformations related to octahedron tilting deformations. Such tilting deformations are closely related to the Rigid Unit Modes (RUMs). This paper discusses the whole set of RUMs in an ideal TTB lattice and possible crystal structures which can emerge owing to the condensation of some of them. Analysis of available experimental data for the TTB-like niobates lends credence to the obtained theoretical predictions.
rate research
Read More
Several Niobium oxides of formula Ba2LnFeNb4O15 (Ln = La, Pr, Nd, Sm, Eu, Gd) with the Tetragonal Tungsten Bronze (TTB) structure have been synthesised by conventional solid-state methods. The Neodymium, Samarium and Europium compounds are ferroelectric with Curie temperature ranging from 320 to 440K. The Praseodymium and Gadolinium compounds behave as relaxors below 170 and 300 K respectively. The Praseodymium, Neodymium, Samarium, Europium and Gadolinium compounds exhibit magnetic hysteresis loops at room temperature originating from traces of a barium ferrite secondary phase. The presence of both ferroelectric and magnetic hysteresis loops at room temperature allows considering these materials as composites multiferroic. Based on crystal-chemical analysis we propose some relationships between the introduction of Ln3+ ions in the TTB framework and the chemical, structural and physical properties of these materials.
We report the superconducting properties of the K$_{x}$WO$_{3}$ tetragonal tungsten bronze. The highest superconducting transition temperature ($T_{c}=2.1$K) was obtained for K$_{0.38}$WO$_{3}$. $T_{c}$ decreases linearly with increasing K content. Using the measured values for the upper critical field $H_{c2}$, and the specific heat $C$, we estimate the orbital critical field $H_{c2}$(0), coherence length $xi$(0), Debye temperature $Theta _{D}$ and coupling constant $lambda _{ep}$. The magnitude of the specific heat jump at $T_{c}$ suggests that the K$_{x}$WO$_{3}$ tetragonal tungsten bronze is a weakly-coupled superconductor. The superconducting phase diagram of the doped tungsten bronze family is presented.
All optical femtosecond relaxation dynamics in a single crystal of mono-phosphate tungsten bronze (PO$_{2}$)$_{4}$(WO$_{3}$)$_{2m}$ with alternate stacking m=6 of WO$_{3}$ layers was studied through the three consequent charge density wave (CDW) transitions. Several transient coherent collective modes associated to the different CDW transitions were observed and analyzed in the framework of the time dependent Ginzburg-Landau theory. Remarkably, the interference of the modes leads to an apparent rectification effect in the transient reflectivity response. A saturation of the coherent-mode amplitudes with increasing pump fluence well below the CDWs destruction threshold fluence indicates a decoupling of the electronic and lattice parts of the order parameter under strong optical drive.
We have synthesised ceramic specimens of the tetragonal tungsten bronze K3Li2Ta5O15 (KLT) and characterized its phase transition via X-ray, dielectric permittivity, ultrasonic spectroscopy and heat capacity measurements. The space group of KLT is reported as both P4/mbm or Cmmm with the orthorhombic distortion occurring when there are higher partial pressures of volatile K and Li used within the closed crucibles for the solid state synthesis. The data show strong relaxor behaviour, with the temperature at which the two dielectric relative permittivity peaks decreasing with 104 K $geqslant$ Tm1 $geqslant$ 69 K and 69 K $geqslant$ Tm2 $geqslant$ 46 K as probe frequency f is reduced from 1 MHz to 316 Hz. The data satisfy a Vogel-Fulcher model with an extrapolated freezing temperature for {epsilon} and {epsilon} of Tf1 = + 15.8 and - 11.8 K and Tf2 = - 5.0 and - 15.0 K for f $rightarrow$ 0 (tending to dc). Therefore by tuning frequency, the transition could be shifted to absolute zero suggesting KLT has a relaxor-type quantum critical point. In addition, we have reanalysed the conflicting literature for Pb2Nb2O7 pyrochlore which suggests that this is also a relaxor-type quantum critical point as the freezing temperature from Vogel-Fulcher fitting is below absolute zero. Since the transition temperature evidenced in the dielectric data at ca. 100 kHz shifts below zero Kelvin for very low frequencies, heat capacity data collected in the zero-frequency (dc) limit, should not indicate a transition. Both of these materials show promise as possible new relaxor-type quantum critical points within non-perovskite based structures as multiple compounds are reported with low-temperature transitions.
Hexagonal BaTiO_3 undergoes a structural phase transition to an orthorhombic C222_1 phase at T_0 = 222 K. The transition is driven by a soft optical mode with E_2u symmetry whose couplings force the appearance of a spontaneous E_2g strain (improper ferroelastic character). Staying within the same E_2u subspace, the system could in principle settle into a second (not observed) orthorhombic phase (Cmc2_1). We have carried out a first-principles investigation of these questions, studying the structure of the existing C222_1 and the virtual Cmc2_1 phases, and describing the spontaneous E_2g strain in accord with the experimental observations. In addition, we show that the occurrence of C222_1 instead of Cmc2_1 cannot be explained by the E_2u soft modes themselves and, therefore, must be related to their couplings with secondary order parameters. A more detailed analysis proves that the E_2g strains do not account for the experimental preference.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
