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.

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.

NQR study of the structural phase transitions in a system with reduced effective dimensionality: K2ReCl6

1983 ◽  
Vol 61 (10) ◽  
pp. 1374-1381 ◽  
Author(s):  
Marie D'Iorio ◽  
Robin L. Armstrong
Keyword(s):  
Fourier Transform ◽  
Phase Transitions ◽  
Hydrostatic Pressure ◽  
Magnetic Resonance ◽  
Structural Phase ◽  
Nonlinear Phenomena ◽  
Structural Phase Transitions ◽  
Quadrupole Resonance ◽  
Nuclear Quadrupole ◽  
Effective Dimensionality

Chlorine 35 nuclear quadrupole resonance (NQR) frequency and lineshape data, obtained with a Fourier transform magnetic resonance spectrometer, are presented for the antifluorite crystal K2ReCl6. In the temperature range investigated, 85 to 130 K, two structural phase transitions occur which reduce the crystal symmetry from cubic to tetragonal to monoclinic. The variation of the NQR spectrum with temperature and with hydrostatic pressure up to 2.64 kbar is documented. The structure of the spectrum and the intensity distribution within it are unexpected from previous studies of antifluorite systems. An explanation of the results is given which is based on recent discussions of nonlinear phenomena at phase transitions in crystals exhibiting reduced effective dimensionality.

Neutron-diffraction study of the structural phase transitions in the antifluorite K2PtBr6

1985 ◽  
Vol 63 (7) ◽  
pp. 988-996 ◽  
Author(s):  
Robin L. Armstrong ◽  
Rose M. Morra ◽  
Brian M. Powell
Keyword(s):  
Phase Transitions ◽  
Nuclear Quadrupole Resonance ◽  
Goodness Of Fit ◽  
Structural Phase ◽  
Spectral Characteristics ◽  
Neutron Diffraction Study ◽  
Structural Phase Transitions ◽  
Quadrupole Resonance ◽  
Nuclear Quadrupole

Neutron-powder-diffraction profiles are reported for the antifluorite K2PtBr6 at temperatures of 296, 160, 141, and 120 K. Bromine nuclear-quadrupole-resonance measurements have shown that each of these temperatures corresponds to a different crystal structure and have provided information on the most probable structures. A variety of structural models with symmetries obtained from a group theoretical classification of structural phase transitions are fitted to the diffraction data. A comparison of goodness-of-fit parameters for the models considered at each temperature allows the space group of each phase to be identified. The results provide (i) a definitive test of the validity of a classification scheme proposed for nuclear-quadrupole-resonance spectral characteristics, and (ii) support for the proposition that the allowable, continuous distortions of a crystal are given by the nondisruptive criterion rather than the more restrictive group–subgroup criterion.

scholarly journals NQR and DSC Studies on Structural Phase Transitions and Lattice Stability in Some Tetrabromozincate(II) Compounds A2ZnBr4

2002 ◽  
Vol 57 (6-7) ◽  
pp. 425-430 ◽  
Author(s):  
Keizo Horiuchi ◽  
Hideta Ishihara ◽  
Naoko Hatano ◽  
Syuntaroh Okamoto ◽  
Tohru Gushiken
Keyword(s):  
Phase Transition ◽  
Phase Transitions ◽  
Temperature Dependence ◽  
Single Phase ◽  
Structural Phase ◽  
Lattice Stability ◽  
Structural Phase Transitions ◽  
Melting Points ◽  
Dsc Studies ◽  
Disorder Type

The temperature dependence of 81Br NQR frequencies in some tetrabromozincate(II) compounds, pyridinium tetrabromozincate(II) (pyH)2ZnBr4, 4-picolinium tetrabromozincate(II) (4- piH)2ZnBr4, 2,6-lutidinium tetrabromozincate(II) (2,6-luH)2ZnBr4 and guanidinium tetrabromozincate( II) (guH)2ZnBr4,weremeasured between 77Kand temperatureswhere signals faded out.All compounds exhibited four NQR signals over the whole temperature range investigated. Moreover, DSC was measured between about 130 K and melting points. (4-piH)2ZnBr4 and (guH)2ZnBr4 showed no structural phase transition, while (pyH)2ZnBr4 and (2,6-luH)2ZnBr4 showed a single phase transition. The values of transition entropies obtained suggest that these transitions are of the order-disorder type. The nature of these transitions and the lattice stability in the present compounds were discussed

scholarly journals Suppression of phase transitions and glass phase signatures in mixed cation halide perovskites

2020 ◽  
Vol 11 (1) ◽  
Author(s):  
Mantas Simenas ◽  
Sergejus Balciunas ◽  
Jacob N. Wilson ◽  
Sarunas Svirskas ◽  
Martynas Kinka ◽  
...  
Keyword(s):  
Phase Transitions ◽  
Theoretical Study ◽  
Structural Phase ◽  
Solubility Limit ◽  
Cubic Phase ◽  
Structure And Properties ◽  
Structural Phase Transitions ◽  
Electric Dipoles ◽  
Halide Perovskites ◽  
Molecular Cations

Abstract Cation engineering provides a route to control the structure and properties of hybrid halide perovskites, which has resulted in the highest performance solar cells based on mixtures of Cs, methylammonium, and formamidinium. Here, we present a multi-technique experimental and theoretical study of structural phase transitions, structural phases and dipolar dynamics in the mixed methylammonium/dimethylammonium MA1-xDMAxPbBr3 hybrid perovskites (0 ≤ x ≤ 1). Our results demonstrate a significant suppression of the structural phase transitions, enhanced disorder and stabilization of the cubic phase even for a small amount of dimethylammonium cations. As the dimethylammonium concentration approaches the solubility limit in MAPbBr3, we observe the disappearance of the structural phase transitions and indications of a glassy dipolar phase. We also reveal a significant tunability of the dielectric permittivity upon mixing of the molecular cations that arises from frustrated electric dipoles.

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