Structures and phase transition in the layered perovskite La0.6Sr0.1TiO3: a new orthorhombic structure solved from high-resolution diffraction in combination with group theoretical analysis

2003 ◽  
Vol 15 (26) ◽  
pp. 4543-4553 ◽  
Author(s):  
Christopher J Howard ◽  
Zhaoming Zhang
2015 ◽  
Vol 48 (1) ◽  
pp. 179-188 ◽  
Author(s):  
I. Belenkaya ◽  
A. Matvienko ◽  
A. Nemudry

A group-theoretical analysis was carried out to determine the possible orientation states of domains formed as a result of the `perovskite–brownmillerite' phase transition in SrCo0.8Fe0.2O2.5oxide with mixed ion–electron conductivity (MIEC). The results of the theoretical analysis agree with the experimental data obtained in the study of the SrCo0.8Fe0.2O2.5microstructure by means of transmission electron microscopy. Brownmillerite SrCo0.8Fe0.2O2.5(BM) has a lamellar texture composed of 90° twins 60–260 nm in size; the 〈010〉BMand 〈101〉BMdirections are linked through twinning in accordance with the predictions of the group-theoretical analysis. The presence of twins and their switching under mechanical load provide evidence that the perovskite–brownmillerite phase transition in SrCo0.8Fe0.2O2.5is ferroelastic. Comparative analysis of the phenomena observed for ferroelectrics and MIEC oxides indicates their similarity based on the common nature of ferroelectricity and ferroelasticity, and allows us to suppose that nonstoichiometric SrCo0.8Fe0.2O3−δwith compositional disorder may be considered (in terms of its microstructural features) a `relaxor ferroelastic'.


1983 ◽  
Vol 61 (9) ◽  
pp. 1271-1277 ◽  
Author(s):  
J. E. Eldridge ◽  
Normand Fortier ◽  
Frances E. Bates ◽  
J. S. Miller

We present the powder absorption spectrum of (TMA)(I)(TCNQ) in the far-infrared as a function of temperature. Below 160 K new absorption features appear and others increase in intensity. This behaviour supports the electron- and neutron-diffraction evidence of a distortion on the TCNQ chains below the first phase-transition temperature. Effects due to the second phase transition at 89 K are also observed. The results of a group-theoretical analysis are presented. These show that totally-symmetric internal modes ag are to be expected in the infrared spectra of this compound even in the undistorted phase.


2006 ◽  
Vol 62 (1) ◽  
pp. 60-67 ◽  
Author(s):  
Zhaoming Zhang ◽  
Christopher J. Howard ◽  
Kevin S. Knight ◽  
Gregory R. Lumpkin

The crystal structures of Nd0.7Ti0.9Al0.1O3, taken to represent the ideal Nd2/3TiO3, have been elucidated from 4 to 1273 K using high-resolution neutron powder diffraction in combination with group-theoretical analysis. The room-temperature structure is monoclinic in C2/m, on a cell with a = 7.6764 (1), b = 7.6430 (1), c = 7.7114 (1) Å, β = 90.042 (2)°. Pertinent features are the layered ordering of the A-site Nd cations/vacancies along the z axis and out-of-phase tilting of the (Ti/Al)O6 octahedra around both the x and z axes. From about 750 to 1273 K, the octahedra are tilted around just one axis (x axis) perpendicular to the direction of the cation ordering, giving rise to an orthorhombic structure with space-group symmetry Cmmm.


Nanomaterials ◽  
2021 ◽  
Vol 11 (2) ◽  
pp. 286
Author(s):  
Valery Davydov ◽  
Evgenii Roginskii ◽  
Yuri Kitaev ◽  
Alexander Smirnov ◽  
Ilya Eliseyev ◽  
...  

We report the results of experimental and theoretical studies of phonon modes in GaN/AlN superlattices (SLs) with a period of several atomic layers, grown by submonolayer digital plasma-assisted molecular-beam epitaxy, which have a great potential for use in quantum and stress engineering. Using detailed group-theoretical analysis, the genesis of the SL vibrational modes from the modes of bulk AlN and GaN crystals is established. Ab initio calculations in the framework of the density functional theory, aimed at studying the phonon states, are performed for SLs with both equal and unequal layer thicknesses. The frequencies of the vibrational modes are calculated, and atomic displacement patterns are obtained. Raman spectra are calculated and compared with the experimental ones. The results of the ab initio calculations are in good agreement with the experimental Raman spectra and the results of the group-theoretical analysis. As a result of comprehensive studies, the correlations between the parameters of acoustic and optical phonons and the structure of SLs are obtained. This opens up new possibilities for the analysis of the structural characteristics of short-period GaN/AlN SLs using Raman spectroscopy. The results obtained can be used to optimize the growth technologies aimed to form structurally perfect short-period GaN/AlN SLs.


2011 ◽  
Vol 84 (4) ◽  
Author(s):  
Chandra Shekhar Pati Tripathi ◽  
Patricia Losada-Pérez ◽  
Christ Glorieux ◽  
Alexandra Kohlmeier ◽  
Maria-Gabriela Tamba ◽  
...  

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