Low symmetry splittings due to phase transitions in the vibronic spectrum of ReCl62−and ReBr62−doped in K2PtCl6‐type crystals

1980 ◽  
Vol 72 (3) ◽  
pp. 1938-1944 ◽  
Author(s):  
Rainer Wernicke ◽  
Hans‐Herbert Schmidtke
Keyword(s):  
Phase Transitions ◽  
Vibronic Spectrum ◽  
Low Symmetry

scholarly journals Group-theoretical analysis of 1:3 A-site-ordered perovskite formation

2019 ◽  
Vol 75 (2) ◽  
pp. 379-397 ◽  
Author(s):  
Mikhail V. Talanov
Keyword(s):  
Phase Transitions ◽  
Theoretical Analysis ◽  
Perovskite Structure ◽  
Functional Materials ◽  
Order Parameters ◽  
Space Group ◽  
Symmetry Phase ◽  
Structural Mechanisms ◽  
Low Symmetry ◽  
Group Theoretical Analysis

The quadruple perovskites AA′3 B 4 X 12 are characterized by an extremely wide variety of intriguing physical properties, which makes them attractive candidates for various applications. Using group-theoretical analysis, possible 1:3 A-site-ordered low-symmetry phases have been found. They can be formed from a parent Pm{\bar 3}m perovskite structure (archetype) as a result of real or hypothetical (virtual) phase transitions due to different structural mechanisms (orderings and displacements of atoms, tilts of octahedra). For each type of low-symmetry phase, the full set of order parameters (proper and improper order parameters), the calculated structure, including the space group, the primitive cell multiplication, splitting of the Wyckoff positions and the structural formula were determined. All ordered phases were classified according to the irreducible representations of the space group of the parent phase (archetype) and systematized according to the types of structural mechanisms responsible for their formation. Special attention is paid to the structural mechanisms of formation of the low-symmetry phase of the compounds known from experimental data, such as: CaCu3Ti4O12, CaCu3Ga2Sn2O12, CaMn3Mn4O12, Ce1/2Cu3Ti4O12, LaMn3Mn4O12, BiMn3Mn4O12 and others. For the first time, the phenomenon of variability in the choice of the proper order parameters, which allows one to obtain the same structure by different group-theoretical paths, is established. This phenomenon emphasizes the fundamental importance of considering the full set of order parameters in describing phase transitions. Possible transition paths from the archetype with space group Pm{\bar 3}m to all 1:3 A-site-ordered perovskites are illustrated using the Bärnighausen tree formalism. These results may be used to identify new phases and interpret experimental results, determine the structural mechanisms responsible for the formation of low-symmetry phases as well as to understand the structural genesis of the perovskite-like phases. The obtained non-model group-theoretical results in combination with crystal chemical data and first-principles calculations may be a starting point for the design of new functional materials with a perovskite structure.

Evidence for precursor clusters associated with structural phase transitions in antifluorite crystals

1982 ◽  
Vol 60 (4) ◽  
pp. 397-398 ◽  
Author(s):  
Robin L. Armstrong ◽  
Marie D'Iorio ◽  
Maximo E. Ramia
Keyword(s):  
Phase Transitions ◽  
Temperature Dependence ◽  
Structural Phase ◽  
Indirect Evidence ◽  
Cubic Phase ◽  
Structural Phase Transitions ◽  
Type Phase ◽  
Symmetry Phase ◽  
Nuclear Quadrupole ◽  
Low Symmetry

Measurements of the temperature dependence of the intensities of the halogen nuclear quadrupole resonances in K2PtBr6 and K2ReCl6, in the cubic phase are reported. These results provide indirect evidence for the formation of dynamic clusters of the low symmetry phase as a precursor to the occurrence of rotative type phase transitions to tetragonal structures in these crystals.

AMPLIMODES: symmetry-mode analysis on the Bilbao Crystallographic Server

2009 ◽  
Vol 42 (5) ◽  
pp. 820-833 ◽  
Author(s):  
Danel Orobengoa ◽  
Cesar Capillas ◽  
Mois I. Aroyo ◽  
J. Manuel Perez-Mato
Keyword(s):  
Phase Transitions ◽  
Structural Phase ◽  
Mode Analysis ◽  
Higher Symmetry ◽  
Atomic Displacements ◽  
Driving Mechanisms ◽  
Distorted Structure ◽  
Symmetry Structure ◽  
Symmetry Mode ◽  
Low Symmetry

AMPLIMODESis a computer program available on the Bilbao Crystallographic Server that can perform a symmetry-mode analysis of any distorted structure of displacive type. The analysis consists in decomposing the symmetry-breaking distortion present in the distorted structure into contributions from different symmetry-adapted modes. Given the high- and the low-symmetry structures,AMPLIMODESdetermines the atomic displacements that relate them, defines a basis of symmetry-adapted modes, and calculates the amplitudes and polarization vectors of the distortion modes of different symmetry frozen in the structure. The program uses a mode parameterization that is as close as possible to the crystallographic conventions, expressing all quantities for the asymmetric unit of the low-symmetry structure. Distorted structures are often related to their higher-symmetry counterparts by temperature- and/or pressure-driven phase transitions, ferroic phase transitions being a particular example. The automatic symmetry-mode analysis performed byAMPLIMODEScan be very useful for establishing the driving mechanisms of such structural phase transitions or the fundamental instabilities at the origin of the distorted phases.

Changes of physical properties in multiferroic phase transitions

2014 ◽  
Vol 70 (4) ◽  
pp. 382-384 ◽  
Author(s):  
Daniel B. Litvin
Keyword(s):  
Phase Transition ◽  
Phase Transitions ◽  
Physical Properties ◽  
Physical Property ◽  
High Symmetry ◽  
High Symmetry Phase ◽  
Symmetry Phase ◽  
Low Symmetry

The physical property coefficients that arise in a phase transition which are zero in the high-symmetry phase and nonzero in the low-symmetry phase are calledspontaneous coefficients. For all 1601 Aizu species of phase transitions, matrices have been constructed which show the nonzero coefficients of a wide variety of magnetic and nonmagnetic physical properties including toroidal property coefficients in the high-symmetry phase and their corresponding spontaneous coefficients in the low-symmetry phase. It is also shown that these spontaneous coefficients provide for the distinction of and switching between nonferroelastic domain pairs.

Structural phase transitions of HfV2 at low temperatures

2000 ◽  
Vol 56 (4) ◽  
pp. 601-606 ◽  
Author(s):  
Yusheng Zhao ◽  
Fuming Chu ◽  
Robert B. Von Dreele ◽  
Qing Zhu
Keyword(s):  
Phase Transitions ◽  
Low Temperatures ◽  
Structural Phase ◽  
Orthorhombic Phase ◽  
Unit Cell ◽  
The Body ◽  
Cubic Phase ◽  
Face Centered Cubic ◽  
Structural Phase Transitions ◽  
Low Symmetry

We report a high-resolution synchrotron X-ray powder diffraction study on HfV2, hafnium divanadium, at low temperatures. In this work we show, for the first time, a complete sequence of structural phase transitions of HfV2 from cubic (Fd3m) to tetragonal (I41/amd) to orthorhombic (Imma) in succession as temperature decreases. Peak splitting and extra diffraction peaks owing to lattice distortion can be clearly distinguished for the low-symmetry phases. The atomic positions and lattice parameters were obtained by Rietveld refinement. The bond lengths and angles of the HfV2 crystal structure at the low-symmetry phases were correctly determined from the structure refinement. The face-centered cubic (Fd3m) unit cell (Z = 24) transforms to a body-centered tetragonal (I41/amd) phase with a 45° rotation relative to the cubic cell and with a reduced number of atoms (Z = 12) in the unit cell at a temperature of T = 112 K. The orthorhombic phase occurs at T = 102 K and it keeps the body-centered symmetry (Imma) and Z = 12 in the unit cell. The refinement results indicate that there may be a small amount of untransformed cubic phase left over in the lower symmetry phases. The abnormal thermal contraction of both tetragonal phase and orthorhombic phase marks the significance of structural change in HfV2.

Unique atom hyper-kagome order in Na4Ir3O8and in low-symmetry spinel modifications

2015 ◽  
Vol 71 (3) ◽  
pp. 301-318 ◽  
Author(s):  
V. M. Talanov ◽  
V. B. Shirokov ◽  
M. V. Talanov
Keyword(s):  
Experimental Data ◽  
Phase Transitions ◽  
Order Parameter ◽  
Landau Theory ◽  
Atomic Order ◽  
Structure Phase ◽  
Spinel Structures ◽  
Low Symmetry ◽  
Critical Order

Group-theoretical and thermodynamic methods of the Landau theory of phase transitions are used to investigate the hyper-kagome atomic order in structures of ordered spinels and a spinel-like Na4Ir3O8crystal. The formation of an atom hyper-kagome sublattice in Na4Ir3O8is described theoretically on the basis of the archetype (hypothetical parent structure/phase) concept. The archetype structure of Na4Ir3O8has a spinel-like structure (space group Fd\bar 3m) and composition [Na1/2Ir3/2]16d[Na3/2]16cO32e4. The critical order parameter which induces hypothetical phase transition has been stated. It is shown that the derived structure of Na4Ir3O8is formed as a result of the displacements of Na, Ir and O atoms, and ordering of Na, Ir and O atoms, orderingdxy,dxz,dyzorbitals as well. Ordering of all atoms takes place according to the type 1:3. Ir and Na atoms form an intriguing atom order: a network of corner-shared Ir triangles called a hyper-kagome lattice. The Ir atoms form nanoclusters which are named decagons. The existence of hyper-kagome lattices in six types of ordered spinel structures is predicted theoretically. The structure mechanisms of the formation of the predicted hyper-kagome atom order in some ordered spinel phases are established. For a number of cases typical diagrams of possible crystal phase states are built in the framework of the Landau theory of phase transitions. Thermodynamical conditions of hyper-kagome order formation are discussed by means of these diagrams. The proposed theory is in accordance with experimental data.

Sign in / Sign up

Export Citation Format

Share Document