Crystal chemical and dielectric properties of Pb_{0.5(5\mbox{-}x)} Lax_x Nb_{5\mbox{-}x} Fex_x O15_{15} system with “tetragonal tungsten bronze” structural type

The lanthanum-lead and iron-niobium coupled substitution in PbNb$_{2}$O$_{6}$ have allowed to isolate new materials of a general formula Pb$_{0.5(5-{\rm x})}$La$_{\rm x}$Nb$_{5-{\rm x}}$Fe$_{\rm x}$O$_{15}$ crystallising with the "tetragonal tungsten bronze” structural type. The resulting influence on the lattice parameters, the ferroelectric Curie temperature and Cole - Cole plots are presented. A typical ferroelectric - paraelectric behaviour occurs in the composition range 0 $\le$ x $\le$ 0.33 while, a relaxor behaviour is observed in the composition range 0.33 $<$ x $\le$ 1

Structural study and luminescence of TlSrLa(AsO4)2

This work presents the crystal structure and luminescent properties of TlSrLa(AsO4)2. In this phase Tl+ ions are located in large tunnels delimited by chains of alternating (AsO4) and (Sr,La)O8 polyhedra. Thallium atoms are eightfold coordinated with C1 symmetry. Large Tl---O distances are observed revealing a low stereochemical activity of the 6s2 lone pair..

Preparation and Magnetic Properties of (La1−xSr1+x)(Mn0.5Co0.5)O4

100學年度研究獎補助論文[[abstract]]A series of single phase (La1−x Sr1+x ) (Mn0.5Co0.5)O4 (0≤x≤0.40) materials with a tetragonal K2NiF4 structure was prepared by a solid state reaction method at 1400 or 1450 °C with a short period of heating time. Powder X-ray diffraction (XRD) and the Rietveld refinement method were employed for the structural analysis. Unit cell a- and c-axes decrease with increasing amount of Sr substitution. A discrepancy between the zero-field-cooled and the field-cooled magnetization is observed in all samples investigated below a characteristic temperature, T *, which likely arises from the canted nature of spins or the random freezing of spins. It appears that T * decreases with increasing amount of Sr substitution, which is possibly due to the enhancement of chemical pressure induced by Sr and a corresponding increases of the valence of Mn and/or Co.[[journaltype]]國外[[incitationindex]]SCI[[incitationindex]]EI[[booktype]]紙本[[countrycodes]]US

Reduction of Sr2MnO4 investigated by high temperature in situ neutron powder diffraction under hydrogen flow.

International audienceThis experiment emphasizes the first example of two-phase sequential Rietveld refinements throughout a solid/gas chemical reaction monitored by Neutron Powder Diffraction (NPD) at high temperature. The reduction of the n = 1 Ruddlesden-Popper (RP) oxide Sr(2)MnO(4) heated under a flow of 5% H(2)-He has been investigated throughout two heating/cooling cycles involving isothermal heating at 500 and 550 °C. Oxygen loss proceeds above T ∼ 470 °C and increases with temperature and time. When the oxygen deintercalated from the "MnO(2)" equatorial layers of the structure results in the Sr(2)MnO(3.69(2)) composition, the RP phase undergoes a first order I4/mmm → P2(1)/c, tetragonal to monoclinic phase transition as observed from time-resolved in situ NPD. The phase transition proceeds at 500 °C but is incomplete; the weight ratio of the P2(1)/c phase reaches ∼41% after 130 min of isothermal heating. The fraction of the monoclinic phase increases with increasing temperature and the phase transition is complete after 80 min of isothermal heating at 550 °C. The composition of the reduced material refined to Sr(2)MnO(3.55(1)) and does not vary on extended heating at 550 °C and subsequent cooling to room temperature (RT). The symmetry of Sr(2)MnO(3.55(1)) is monoclinic at 550 °C and therefore consistent with the RT structure determined previously for the Sr(2)MnO(3.64) composition obtained from ex situ reduction. Consequently, the stresses due to phase changes on heating/cooling in reducing atmosphere may be minimized. The rate constants for the reduction of Sr(2)MnO(4.00) determined from the evolution of weight ratio of the tetragonal and monoclinic phase in the time-resolved isothermal NPD data collected on the isotherms at 500 and 550 °C are k(500) = 0.110 × 10(-2) and k(550) = 0.516 × 10(-2) min(-1) giving an activation energy of ∼163 kJ mol(-1) for the oxygen deintercalation reaction

Unprecedented High Solubility of Oxygen Interstitial Defects in La1.2Sr0.8MnO4+δ up to δ ∼ 0.42 Revealed by In Situ High Temperature Neutron Powder Diffraction in Flowing O2.

International audienceThe structural behavior of the n = 1 Ruddlesden-Popper (R-P) La1.2Sr0.8MnO4+δ phases has been monitored in situ in flowing O2 on heating over the temp. range 65 \textless T/°C \textless 550 by means of neutron powder diffraction (instrument D20, ILL/Grenoble). Sequential Rietveld refinement showed that the I4/mmm nearly stoichiometric phase undergoes a first oxygen uptake at 300-400 °C with phase sepn. into an oxygen-rich orthorhombic (Bmab) La1.2Sr0.8MnO4.30(1) phase and a stoichiometric tetragonal (I4/mmm) La1.2Sr0.8MnO4.00(2) phase. At T ∼ 410 °C, the first oxidn. step is complete and only the Bmab oxidized phase is present. The orthorhombicity decreases progressively on further heating, up to 510 °C with minor variation in the oxygen content. Above ∼510 °C, the system undergoes a second abrupt oxidn. step involving a single phase process to reach after prolonged isothermal heating at 550 °C, δ ∼ 0.42(2). Such a high soly. for the excess-oxygen defects has never been reported so far for a K2NiF4-type structure. The La1.2Sr0.8MnO4+δ system is thus interesting for the fundamental studies of structural distortions induced by the intercalation of a large amt. of oxygen defects owing to the flexibility of the R-P structure and high stability of the Mn4+ oxidn. state in an oxygen octahedral environment. [on SciFinder(R)

Crystal chemistry and selected physical properties of inorganic fluorides and oxide-fluorides

importance in the development of many new technologies, andare impacting various key points of modern life, that is, energyproduction and storage, microelectronics and photonics,catalysis, automotive, building, etc. Many research fields andapplications are indeed concerned by a better knowledge of therelationships occurring between the structure of suchcompounds and some pertinent physical properties. ThisReview deals with the structural chemistry of solid-stateinorganic fluorides and oxide-fluorides, mostly transitionmetal-based, including rare-earth elements. Such a Review hasnot been published for a long time.1 Articles that recentlyappeared on inorganic fluorinated compounds were mostlyfocused on material science characteristics: morphology, surfacefunctionalization, nanostructuration of the materials andapplications, rather than on the description of characteristicstructural features.2−5 Detailed reviews focused on rare earthbasedinorganic fluorides have also appeared some yearsago..