Structural phase transition to disorder low-temperature phase in [Fe(ptz)6](BF4)2 spin-crossover compounds

2012 ◽  
Vol 68 (1) ◽  
pp. 40-56 ◽  
Author(s):  
Joachim Kusz ◽  
Maciej Zubko ◽  
Reinhard B. Neder ◽  
Phillipp Gütlich
Keyword(s):  
Short Range ◽  
Short Range Order ◽  
Spin Crossover ◽  
Structural Phase ◽  
The Other ◽  
Special Technique ◽  
Temperature Phase ◽  
Disordered Phase ◽  
Novel Approach ◽  
Low Temperature Phase

In the spin-crossover compound [Fe(ptz)6](BF4)2 (where ptz=1-n-propyltetrazole) six different phases are observed. When a single crystal is slowly cooled from high temperatures to those below 125 K, the reflections broaden into diffuse maxima and split into two maxima along the c* direction [Kusz, Gütlich & Spiering (2004). Top. Curr. Chem. 234, 129–153]. As both maxima are broad along the c* direction, the short-range order exists only along the c direction and in the ab plane the structure remains long-range ordered. In this disordered phase additional satellite reflections appear. Upon heating above 135 K, the diffuse maxima return to their previous shape and this process is completely reversible. Rapidly cooled samples, on the other hand, do not show such splitting and the symmetry remains R\bar 3, despite a jump in lattice parameters. We use a special technique to analyse the disorder model of the slowly cooled samples, which consists of layered domains shifted in the hexagonal ab plane. The low-spin disordered phase was solved in a novel approach to accommodate the very unusual twinning and refined in the non-standard space group C\bar 1. In contrast to the ordered low-spin phase, the Fe ion is in a non-centrosymmetric coordination polyhedron and two of the six propyl groups change their conformation.

Structural Phase Transition in Tetrafluoroaluminates

1982 ◽  
Vol 21 ◽  
Author(s):  
J.M. Launay ◽  
J.Y. Laval ◽  
A. Gibaud ◽  
A. Bulou ◽  
J. Nouet
Keyword(s):  
Electron Diffraction ◽  
Room Temperature ◽  
Structural Phase ◽  
The Other ◽  
Electron Diffraction Data ◽  
Temperature Phase ◽  
X Ray ◽  
Quadratic Phase ◽  
Low Temperature Phase ◽  
Good Agreement

ABSTRACTIt is shown that the detailed investigation of an irreversible SPT implies the combination of X-ray, neutron and electron diffraction data. This diffraction analysis is applied to K Al F4 and Rb Al F4. There is a good agreement between the three diffraction techniques in the case of Rb Al F4 where the SPT is reversible. On the other hand, for K Al F4 room temperature quadratic phase, electron diffraction exhibits exta spots which are not evidenced by X-ray and neutron diffraction. Finally the assumption of an orthorombic lattice from neutron scattering for the low temperature phase is consistant with electron diffraction results.

Short-Range Order in α-Brass

1989 ◽  
Vol 166 ◽  
Author(s):  
L. Reinhard ◽  
B. Schoenfeld ◽  
G. Kostorz ◽  
W. Buehrer
Keyword(s):  
Short Range ◽  
Short Range Order ◽  
Pair Interaction ◽  
Linear Displacement ◽  
Range Order ◽  
Temperature Phase ◽  
Diffuse Intensity ◽  
Inverse Monte Carlo ◽  
Data Pair ◽  
Low Temperature Phase

ABSTRACTQuenched equilibrium states of Cu–31.1 at.% Zn and Cu–22.4 at.% Zn single crystals (prepared with the Cu-65 isotope) were investigated by elastic diffuse neutron scattering. The diffuse intensity showed maxima which are attributed to the flat portions of the Fermi surface in the <110> directions. Short–range order parameters and linear displacement parameters were obtained from a fit to the measured data. Pair interaction energies were determined based on the inverse Monte–Carlo method. An ordered low-temperature phase Cu3Zn with the DO23 structure is suggested.

scholarly journals Direct observation of excitonic instability in Ta2NiSe5

2021 ◽  
Vol 12 (1) ◽  
Author(s):  
Kwangrae Kim ◽  
Hoon Kim ◽  
Jonghwan Kim ◽  
Changil Kwon ◽  
Jun Sung Kim ◽  
...  
Keyword(s):  
Phase Transition ◽  
Direct Observation ◽  
Structural Phase ◽  
Temperature Phase ◽  
Phonon Coupling ◽  
Strong Electron ◽  
Weiss Temperature ◽  
Excitonic Insulator ◽  
Electron Phonon Coupling ◽  
Low Temperature Phase

AbstractCoulomb attraction between electrons and holes in a narrow-gap semiconductor or a semimetal is predicted to lead to an elusive phase of matter dubbed excitonic insulator. However, direct observation of such electronic instability remains extremely rare. Here, we report the observation of incipient divergence in the static excitonic susceptibility of the candidate material Ta2NiSe5 using Raman spectroscopy. Critical fluctuations of the excitonic order parameter give rise to quasi-elastic scattering of B2g symmetry, whose intensity grows inversely with temperature toward the Weiss temperature of TW ≈ 237 K, which is arrested by a structural phase transition driven by an acoustic phonon of the same symmetry at TC = 325 K. Concurrently, a B2g optical phonon becomes heavily damped to the extent that its trace is almost invisible around TC, which manifests a strong electron-phonon coupling that has obscured the identification of the low-temperature phase as an excitonic insulator for more than a decade. Our results unambiguously reveal the electronic origin of the phase transition.

scholarly journals Phase transition and proton ordering at 50 K in 3-(pyridin-4-yl)pentane-2,4-dione

2017 ◽  
Vol 73 (6) ◽  
pp. 1172-1178 ◽  
Author(s):  
Khai-Nghi Truong ◽  
Carina Merkens ◽  
Martin Meven ◽  
Björn Faßbänder ◽  
Richard Dronskowski ◽  
...  
Keyword(s):  
Phase Transition ◽  
Low Temperature ◽  
Single Crystal ◽  
Structural Phase ◽  
Pyridyl Ring ◽  
Temperature Phase ◽  
Crystallographic Axis ◽  
Temperature Phase Transition ◽  
Intramolecular Hydrogen ◽  
Low Temperature Phase

Single-crystal neutron diffraction experiments at 100 and 2.5 K have been performed to determine the structure of 3-(pyridin-4-yl)pentane-2,4-dione (HacacPy) with respect to its protonation pattern and to monitor a low-temperature phase transition. Solid HacacPy exists as the enol tautomer with a short intramolecular hydrogen bond. At 100 K, its donor···acceptor distance is 2.450 (8) Å and the compound adopts space group C2/c, with the N and para-C atoms of the pyridyl ring and the central C of the acetylacetone substituent on the twofold crystallographic axis. As a consequence of the axial symmetry, the bridging hydrogen is disordered over two symmetrically equivalent positions, and the carbon–oxygen bond distances adopt intermediate values between single and double bonds. Upon cooling, a structural phase transition to the t 2 subgroup P\bar 1 occurs; the resulting twins show an ordered acetylacetone moiety. The phase transition is fully reversible but associated with an appreciable hysteresis in the large single crystal under study: transition to the low-temperature phase requires several hours at 2.5 K and heating to 80 K is required to revert the transformation. No significant hysteresis is observed in a powder sample, in agreement with the second-order nature of the phase transition.

scholarly journals Magnetism in spin crossover systems: Short-range order and effects beyond the Heisenberg model

2019 ◽  
Vol 100 (14) ◽  
Author(s):  
V. I. Kuz'min ◽  
Yu. S. Orlov ◽  
A. E. Zarubin ◽  
T. M. Ovchinnikova ◽  
S. G. Ovchinnikov
Keyword(s):  
Heisenberg Model ◽  
Short Range ◽  
Short Range Order ◽  
Spin Crossover ◽  
Range Order

Differential Thermal Analysis and Dielectric Studies on 2-Methyl-2-Nitro-Propane under High Pressure

1995 ◽  
Vol 50 (4-5) ◽  
pp. 502-504 ◽  
Author(s):  
D. Büsing ◽  
M. Jenau ◽  
J. Reuter ◽  
A. Würflinger ◽  
J. Li. Tamarit
Keyword(s):  
Thermal Analysis ◽  
Differential Thermal Analysis ◽  
Temperature Phase ◽  
Dielectric Studies ◽  
Static Permittivity ◽  
Coexistence Region ◽  
Disordered Phase ◽  
Plastic Phase ◽  
Related Compounds ◽  
Low Temperature Phase

Abstract Differential thermal analysis and dielectric studies under pressures up to 300 MPa and temperatures of about 200 to 350 K have been performed on 2-methyl-2-nitro-propane (TBN). TBN displays an orientationally disordered phase (ODIC), solid I, and two non-plastic phases, solids II and III. The coexistence region of the plastic phase I increases with increasing pressure, whereas the low-temperature phase II apparently vanishes at a triple point I, II, III, above 300 MPa. The static permittivity increases on freezing, characterizing the solid I as an ODIC phase. In the frame of the Kirkwood-Onsager-Fröhlich theory the g-factor is about unity, discounting specific dielectric correlations. The dielectric behaviour of TBN is similar to previously studied related compounds, such as 2-chloro-2-methyl-propane or 2-brome- 2-methyl-propane

Substitution Effect in the Ion Conductor Li3InBr6, Studied by Nuclear Magnetic Resonance

2002 ◽  
Vol 57 (6-7) ◽  
pp. 447-450 ◽  
Author(s):  
Yasumasa Tomita ◽  
Hiroshi Yonekura ◽  
Yasuo Yamauchi ◽  
Koji Yamada ◽  
Kenkichiro Kobayashi
Keyword(s):  
Nmr Spectra ◽  
Substitution Effect ◽  
The Other ◽  
Temperature Phase ◽  
Cation Substitution ◽  
Ion Conductor ◽  
X Ray ◽  
Lattice Constants ◽  
Low Temperature Phase ◽  
Cation Substitution Effect

Li3-2x Mg InBr6 (x= 0.02 - 0.4) was synthesized, and the cation substitution effect on the conductivity was investigated by means of 7Li and 115In NMR, and X-ray diffraction.With increasing x the lattice constants a and c increased, but b and β did not show significant changes. The conductivity of the low temperature phase increased with x, associated with a narrowing of the 7Li NMR spectra. In the high temperature superionic phase, on the other hand, the conductivity decreased with x accompanied by a broadening of the 115In NMR spectra

Structural phase transition and hydrogen ordering of TlH2PO4 at low temperature

2006 ◽  
Vol 62 (5) ◽  
pp. 719-728 ◽  
Author(s):  
I. H. Oh ◽  
M. Merz ◽  
S. Mattauch ◽  
G. Heger
Keyword(s):  
Crystal Structure ◽  
Phase Transition ◽  
Low Temperature ◽  
Structural Phase ◽  
Crystal Structure Analysis ◽  
Temperature Phase ◽  
Structure Model ◽  
Triclinic Space Group ◽  
Space Groups ◽  
Low Temperature Phase

The crystal structure of TlH2PO4 (TDP) has been studied at low temperature. The lattice parameters were derived from high-resolution X-ray powder diffraction in the temperature range between 8 and 300 K. A detailed crystal structure analysis of the antiferroelectric low-temperature phase TDP-III has been performed based on neutron diffraction data measured at 210 K on a twinned crystal consisting of two domain states. The structure model in the triclinic space group P\bar 1 is characterized by a complete ordering of all the H atoms in the asymmetric O—H...O hydrogen bonds. The phase transition from the ferroelastic TDP-II to the antiferroelectric TDP-III phase at 229.5 ± 0.5 K is only slightly of first order and shows no detectable hysteresis effects. Its mechanism is driven by the hydrogen ordering between the partially ordered TDP-II state and the completely ordered TDP-III state. The polymorphism of TDP and the fully deuterated TlD2PO4 (DTDP) is presented in the form of group–subgroup relations between the different space groups.

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