Mechanism of the Phase Change in PbK2LiNb5O15: Dielectric, structural, and Raman scattering studies

Experiments reveal that PbK2LiNb5O15 which belongs to the tetragonal tungsten bronze family presents paraelectric and ferroelectric phases and a complex structural change between them. High and low temperature phases are of symmetry P4/mbm and Pba2 respectively, so that this change is also of ferroelastic type. As presented here, crystallographic results hint at a displacive character of the ferroelectric ordering but show a more complex behaviour, with a clear order-disorder mechanism which accompanies the appearance of ferroelasticity. To complete our knowledge of this material, we have performed Raman experiments which exhibit a low frequency mode, but no clear soft mode is observed

MECHANISM OF THE PHASE CHANGE IN PbK2LiNb5O15: Dielectric, structural, and Raman scattering studies

Experiments reveal that PbK2LiNb5O15 which belongs to the tetragonal tungsten bronze family presents paraelectric and ferroelectric phases and a complex structural change between them. High and low temperature phases are of symmetry P4/mbm and Pba2 respectively, so that this change is also of ferroelastic type. As presented here, crystallographic results hint at a displacive character of the ferroelectric ordering but show a more complex behaviour, with a clear order-disorder mechanism which accompanies the appearance of ferroelasticity. To complete our knowledge of this material, we have performed Raman experiments which exhibit a low frequency mode, but no clear soft mode is observed

Ga substitution as an effective variation of Mn-Tb coupling in multiferroic TbMnO3

Ga for Mn substitution in multiferroic TbMnO$_{3}$ has been performed in order to study the influence of Mn-magnetic ordering on the Tb-magnetic sublattice. Complete characterization of TbMn$_{1-x}$Ga$_x$O$_{3}$ ($x$ = 0, 0.04, 0.1) samples, including magnetization, impedance spectroscopy, and x-ray resonant scattering and neutron diffraction on powder and single crystals has been carried out. We found that keeping the same crystal structure for all compositions, Ga for Mn substitution leads to the linear decrease of $T_{\rm N}^{\rm Mn}$ and $\tau^{\rm Mn}$, reflecting the reduction of the exchange interactions strength $J_{\rm Mn-Mn}$ and the change of the Mn-O-Mn bond angles. At the same time, a strong suppression of both the induced and the separate Tb-magnetic ordering has been observed. This behavior unambiguously prove that the exchange fields $J_{\rm Mn-Tb}$ have a strong influence on the Tb-magnetic ordering in the full temperature range below $T_{\rm N}^{\rm Mn}$ and actually stabilize the Tb-magnetic ground state.Comment: 9 pages, 8 figure

Transitions de phase a-b dans le quartz et FePO4 (relations avec la diffusion anomale de la lumière et mécanismes)

TOULON-BU Centrale (830622101) / SudocSudocFranceF

On the Complex Structural Picture of the Ionic Conductor Sr 6 Ta 2 O 11

Sr6Ta2O11 presents an interesting model system of a highly defective and disordered complex perovskite, exhibiting oxide ion conductivity at high temperatures as well as proton conductivity when hydrated by presence of water vapor. In this paper, we present a comprehensive structural study of Sr6Ta2O11 in its dry and hydrated state, based on DFT calculations and NPD measurements. At low temperatures, dry Sr6Ta2O11 has a tetragonal symmetry with ordered oxygen vacancies. The oxygen vacancy induces perturbations to its near surroundings, disturbing particularly the A site cations. As a consequence, the elaborate structural picture collapses at higher temperatures. The high temperature structure has a cubic symmetry with fractionally occupied structural oxygen sites and disorder on the oxygen and strontium sublattices. The structure of the fully hydrated compound Sr6Ta2O10 OH 2 was determined as monoclinic with significant distortions of the anion and metal cation site

Structure, Magnetism, and the Magnetocaloric Effect of MnFe4_{4}Si3_{3} Single Crystals and Powder Samples

Magnetic and structural properties of high quality magnetocaloric MnFe4Si3 single crystals are investigated macroscopically and on an atomic scale. Refinements of combined neutron and X-ray single crystal diffraction data introduce a new structural model in space group P6 characterized by partial ordering of Mn and Fe into layers perpendicular to c on one of the transition metal sites. A second transition metal site is exclusively occupied by iron. MnFe4Si3 has a phase transition to a ferromagnetically ordered phase at approximately 300 K and displays a strong anisotropy of the magnetization and the magnetocaloric effect with the easy axis of magnetization in the a,b-plane. This is confirmed by a refinement of the magnetic structure in the magnetic spacegroup Pm′, which shows that the spins on the sites with mixed occupancy of Mn and Fe are aligned in the a,b-plane. A significant magnetic moment for the site exclusively occupied by iron could not be refined. The thermal evolution of the lattice parameters exhibits an anisotropic behavior and clearly reflects the onset of magnetic ordering. A comparison of the ordered moment and the effective paramagnetic moment hints toward itinerant magnetism in the system