Structural evolution of La2Ti2O7 at elevated temperatures

2019 ◽  
Vol 75 (2) ◽  
pp. 257-272 ◽  
Author(s):  
Nobuo Ishizawa ◽  
Keisuke Ninomiya ◽  
Jun Wang
Keyword(s):  
Low Temperature ◽  
Flat Plate ◽  
Elevated Temperatures ◽  
Structural Evolution ◽  
High Temperature Phase ◽  
Temperature Phase ◽  
Two Phase ◽  
A Cell ◽  
Perovskite Type ◽  
Modulation Vector

Structural evolution of a La2Ti2O7 ferroelectric compound possessing perovskite-type slabs at elevated temperatures was investigated using the single-crystal X-ray diffraction technique. The monoclinic low-temperature phase (L) transformed into the orthorhombic high-temperature phase (H) via an incommensurately modulated phase (IC) between ∼989 and ∼1080 K. The L–IC transition was considered to be of the first order, with the L+IC two-phase co-existing region between ∼989 and ∼1027 K. The structure of IC was determined from the (3+1)-dimensional superspace representation with a modulation vector q = αa o (α ≃ 0.49), where a o is the a-axis vector of the basic cell. The structural modulation originated from the variation of the tilt angle of the TiO6 octahedra in the perovskite-type slab in association with small positional displacements of La atoms. The IC–H transition took place at ∼1080 K and was close to the second order. During the IC–H transition, nanoscale flat plate domains having either a cell twin of the L-type structural modules or a cell twin of the alternating H- and L-type structural modules began to appear in the approximant structure of IC. The thickness of the flat plate domains then grew rapidly along the modulation vector in proportion to (½ − α)−1 as α approached ½ with decreasing temperature. In the two-phase L+IC co-existing region, the IC phase consisting of the two types of cell twins was gradually replaced with the low-temperature monoclinic phase L, which is not cell twinned but rather twinned macroscopically by the L-type structural modules.

scholarly journals Synthesis of by the Flux Method

2012 ◽  
Vol 2012 ◽  
pp. 1-5 ◽  
Author(s):  
Yuki Maruyama ◽  
Chihiro Izawa ◽  
Tomoaki Watanabe
Keyword(s):  
High Temperature ◽  
Solid State ◽  
Low Temperature ◽  
Diffuse Reflectance ◽  
High Temperature Phase ◽  
Temperature Phase ◽  
Flux Method ◽  
Phase Crystal ◽  
Euhedral Crystal ◽  
Low Temperature Phase

has been successfully synthesized using Bi2O3–B2O3 eutectic flux. In particular, we succeeded in synthesizing a low-temperature-phase crystal (α-) at 1073 K as well as high-temperature-phase crystal (β-). The morphology of α- and β- particles prepared by the flux method is a euhedral crystal. In contrast, the morphology of particles prepared by solid state reaction differs: α- is aggregated and β- is necked. Ultraviolet-visible diffuse reflectance spectra indicate that the absorption edge is at a longer wavelength for β- than for α- with β- absorbing light of wavelengths up to nearly 400 nm.

Superprotonic high temperature phase and refinement of the low temperature structure of CsHSeO4

2001 ◽  
Vol 216 (3) ◽  
Author(s):  
M. A. Zakharov ◽  
Sergej I. Troyanov ◽  
Erhard Kemnitz
Keyword(s):  
Crystal Structure ◽  
High Temperature ◽  
Low Temperature ◽  
High Temperature Phase ◽  
Temperature Phase ◽  
Temperature Structure ◽  
Superprotonic Phase

AbstractThe crystal structure of the high temperature superprotonic phase of CsHSeO

Magnetic Phase Diagram and Magnetization Dynamics of Frustrated Spin-Chain Compounds

2009 ◽  
Vol 152-153 ◽  
pp. 233-236 ◽  
Author(s):  
Yu. B. Kudasov
Keyword(s):  
Phase Diagram ◽  
Low Temperature ◽  
Nearest Neighbor ◽  
Magnetic Phase ◽  
Magnetic Phase Diagram ◽  
High Temperature Phase ◽  
Temperature Phase ◽  
Magnetization Dynamics ◽  
Ising Spin ◽  
Glauber Theory

The magnetic phase diagram of Ising spin chains packed into the frustrated triangular lattice is discussed. A structure of a low-temperature phase depends strongly on interactions between the next-to-nearest-neighbor chains because they lift the degeneracy of the triangular AFM Ising model. That is why, a variety of low-temperature phases is observed in CsCoCl3, Ca3Co2O6, and Sr5Rh4O12. On the contrary, the high-temperature phase (honeycomb AFM structure) is unique. The frustrated Ising chain systems demonstrate an unusual and very slow magnetization dynamics. A model of the magnetization dynamics similar to the Glauber theory is developed.

Irreversible single-crystal to polycrystal and reversible single-crystal to single-crystal phase transformations in cyanurates

2001 ◽  
Vol 57 (6) ◽  
pp. 791-799 ◽  
Author(s):  
Menahem Kaftory ◽  
Mark Botoshansky ◽  
Moshe Kapon ◽  
Vitaly Shteiman
Keyword(s):  
Phase Transformation ◽  
High Temperature ◽  
Low Temperature ◽  
Single Crystal ◽  
High Temperature Phase ◽  
Monoclinic Space Group ◽  
Temperature Phase ◽  
Space Group ◽  
Reversible Phase ◽  
Low Temperature Phase

4,6-Dimethoxy-3-methyldihydrotriazine-2-one (1) undergoes a single-crystal to single-crystal reversible phase transformation at 319 K. The low-temperature phase crystallizes in monoclinic space group P21/n with two crystallographically independent molecules in the asymmetric unit. The high-temperature phase is obtained by heating a single crystal of the low-temperature phase. This phase is orthorhombic, space group Pnma, with the molecules occupying a crystallographic mirror plane. The enthalpy of the transformation is 1.34 kJ mol−1. The small energy difference between the two phases and the minimal atomic movement facilitate the single-crystal to single-crystal reversible phase transformation with no destruction of the crystal lattice. On further heating, the high-temperature phase undergoes methyl rearrangement in the solid state. 2,4,6-Trimethoxy-1,3,5-triazine (3), on the other hand, undergoes an irreversible phase transformation from single-crystal to polycrystalline material at 340 K with an enthalpy of 3.9 kJ mol−1; upon further heating it melts and methyl rearrangement takes place.

35Cl NQR and Structural Studies of Chloroacetanilides C6H3Cl2NHCOCH3-xClx, 1 ≤ x ≤ 3

1992 ◽  
Vol 47 (1-2) ◽  
pp. 160-170
Author(s):  
Dirk Groke ◽  
Shi-Qi Dou ◽  
Alarich Weiss
Keyword(s):  
High Temperature ◽  
Low Temperature ◽  
Temperature Dependence ◽  
Phenyl Group ◽  
High Temperature Phase ◽  
Temperature Phase ◽  
X Ray Diffraction ◽  
Chlorine Atoms ◽  
Space Group ◽  
Low Temperature Phase

AbstractThe temperature dependence of 35Cl NQR frequencies and the phase transition behaviour of chloroacetanilides (N-[2,6-dichlorophenyl]-2-chloroacetamide, -2,2-dichloroacetamide, -2,2,2-trichloroacetamide) were investigated. The crystal structure determination of N-[2,6-dichlorophenyl]- 2-chloroacetamide leads to the following: a = 1893.8 pm, b = 1110.7 pm, c = 472.1 pm, space group P212121 = D24 with Z = 4 molecules per unit cell. The arrangement of the molecules and their geometry is comparable to the high temperature phase of the acetyl compound N-[2,6-dichlorophenyl]- acetamide. For N-[2,6-diclorophenyl]-2,2,2-trichloroacetamide it was found: a = 1016.6 pm, b = 1194.3 pm, c = 1006.7 pm, ß= 101.79°, space group P21/c = C52h, Z = 4. The structure is similar to the low temperature phase of N-[2,6-dichlorophenyl]-acetamide. Parallelism between the temperature dependence of the 35C1 NQR lines of the CCl3 group and the X-ray diffraction results concerning the different behaviour of the chlorine atoms was observed. The structures of the compounds show intermolecular hydrogen bonding of the N - H • • • O - C type. The phenyl group and the HNCO function are nearly planar. A bleaching out of several 35Cl NQR lines at a temperature far below the melting point of the substances was observed. The different types of chlorine atoms (aromatic, chloromethyl) can be distinguished by their temperature coefficients of the 35Cl NQR frequencies. All the resonances found show normal "Bayer" temperature behaviour. N-[2,6-dichlorophenyl]-2,2-diehloroacetamide shows several solid phases. One stable low temperature phase and an instable high temperature phase (at room temperature) were observed. The different phases were detected by means of 35Cl NQR spectroscopy and thermal analysis

Dielectric Constant, Density, and Enthalpy of Transition of Cyclohexane and Neopentane at 500 Atmospheres

1973 ◽  
Vol 51 (13) ◽  
pp. 2141-2146 ◽  
Author(s):  
Hemming Art-Ming Chew ◽  
Raymond Kai-Chow Chan
Keyword(s):  
Dielectric Constant ◽  
Solid Phase ◽  
High Temperature Phase ◽  
Dielectric Constants ◽  
Temperature Phase ◽  
Constant Density ◽  
Volume Changes ◽  
Dielectric Data ◽  
A Cell ◽  
Enthalpy Of Transition

A cell has been described for the simultaneous measurement of the dielectric constant and volume changes at the phase transition of solids at low temperatures and pressures up to 500 atm. The dielectric constants of the high temperature phase of cyclohexane and neopentane at 10 kHz increase with decreasing temperature according to the Clausius–Mossotti equation. The low temperature solid phase of both non-polar substances has considerably high shear strength and it is suggested that the dielectric data of these substances in the literature may have considerable errors. The enthalpies of fusion and solid–solid transition at 500 atm have been calculated from the experimental results.

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