scholarly journals Orientational disorder and phase transitions in crystals of (NH4)2NbOF5

2008 ◽  
Vol 64 (5) ◽  
pp. 527-533 ◽  
Author(s):  
Anatoly A. Udovenko ◽  
Natalia M. Laptash
Keyword(s):  
Phase Transitions ◽  
Phase Ii ◽  
Central Atom ◽  
Crystal Form ◽  
Partial Ordering ◽  
Phase Iii ◽  
X Ray Diffraction ◽  
Dynamic Nature ◽  
Two Phase ◽  
X Ray

Ammonium oxopentafluoroniobate, (NH4)2NbOF5, was synthesized in a single-crystal form and the structures of its different phases were determined by X-ray diffraction at three temperatures: phase (I) at 297 K, phase (II) at 233 K and phase (III) at 198 K. The distorted [NbOF5]2− octahedra are of similar geometry in all three structures, with the central atom shifted towards the O atom. The structure of (I) is disordered, with three spatial orientations of the [NbOF5]2− octahedron related by a jump rotation around the pseudo-threefold local axis such that the disorder observed is of a dynamic nature. As the temperature decreases, the compound undergoes two phase transitions. The first is accompanied by full anionic ordering and partial ordering of the ammonium groups (phase II). The structure of (III) is completely ordered. The F and O atoms in the structures investigated were identified via the Nb—X (X = O and F) distances. The crystals of all three phases are twinned.

scholarly journals Crystallographic features of ammonium fluoroelpasolites: dynamic orientational disorder in crystals of (NH4)3HfF7 and (NH4)3Ti(O2)F5

2017 ◽  
Vol 73 (1) ◽  
pp. 1-9 ◽  
Author(s):  
Anatoly A. Udovenko ◽  
Alexander A. Karabtsov ◽  
Natalia M. Laptash
Keyword(s):  
Single Crystal ◽  
Electron Density ◽  
Diffraction Data ◽  
Central Atom ◽  
X Ray Diffraction ◽  
Dynamic Nature ◽  
Orientational Disorder ◽  
High Quality ◽  
Structural Units ◽  
X Ray

A classical elpasolite-type structure is considered with respect to dynamically disordered ammonium fluoro-(oxofluoro-)metallates. Single-crystal X-ray diffraction data from high quality (NH4)3HfF7 and (NH4)3Ti(O2)F5 samples enabled the refinement of the ligand and cationic positions in the cubic Fm \bar 3 m (Z = 4) structure. Electron-density atomic profiles show that the ligand atoms are distributed in a mixed (split) position instead of 24e. One of the ammonium groups is disordered near 8c so that its central atom (N1) forms a tetrahedron with vertexes in 32f. However, a center of another group (N2) remains in the 4b site, whereas its H atoms (H2) occupy the 96k positions instead of 24e and, together with the H3 atom in the 32f position, they form eight spatial orientations of the ammonium group. It is a common feature of all ammonium fluoroelpasolites with orientational disorder of structural units of a dynamic nature.

scholarly journals Thermal evolution of the crystal structure and phase transitions of KNbO 3

2018 ◽  
Vol 5 (6) ◽  
pp. 180368 ◽  
Author(s):  
S. L. Skjærvø ◽  
K. Høydalsvik ◽  
A. B. Blichfeld ◽  
M.-A. Einarsrud ◽  
T. Grande
Keyword(s):  
Crystal Structure ◽  
Phase Transitions ◽  
Thermal Evolution ◽  
Dynamical Instability ◽  
X Ray Diffraction ◽  
Two Phase ◽  
X Ray ◽  
Cell Parameters ◽  
First Order ◽  
Heating And Cooling

The thermal evolution of the crystal structure and phase transitions of KNbO 3 were investigated by high-temperature powder X-ray diffraction and Rietveld refinement of the diffraction data. Two phase transitions from orthorhombic ( Amm 2) to tetragonal ( P 4 mm ) and from tetragonal to cubic ( P m 3 ¯ m ) were confirmed, both on heating and cooling. Both phase transitions are first order based on the observed hysteresis. The mixed displacive and order–disorder nature of the tetragonal to cubic transition is argued based on symmetry and apparent divergence of the atomic positions from pseudo-cubic values. The transition between the orthorhombic and tetragonal phase shows no temperature-dependence for atomic positions and only thermal expansion of the unit cell parameters and is thus discussed in relation to a lattice dynamical instability.

Structures and phase transitions in a new ferroelectric – pyridinium chlorochromate – studied by X-ray diffraction, DSC and dielectric methods

2012 ◽  
Vol 68 (2) ◽  
pp. 128-136 ◽  
Author(s):  
Hanna Małuszyńska ◽  
Piotr Czarnecki ◽  
Anna Czarnecka ◽  
Zdzisław Pająk
Keyword(s):  
Phase Transitions ◽  
Pyridinium Salt ◽  
Diffraction Method ◽  
Phase Iii ◽  
Pyridinium Chlorochromate ◽  
X Ray Diffraction ◽  
Scanning Calorimetry ◽  
X Ray ◽  
Intermediate Phases ◽  
Ferroelectric Hysteresis

Pyridinium chlorochromate, [C5H5NH]+[ClCrO3]− (hereafter referred to as PyClCrO3), was studied by X-ray diffraction, differential scanning calorimetry (DSC) and dielectric methods. Studies reveal three reversible phase transitions at 346, 316 and 170 K with the following phase sequence: R\bar 3m (I) → R3m (II) → Cm (III) → Cc (IV), c′ = 2c. PyClCrO3 is the first pyridinium salt in which all four phases have been successfully characterized by a single-crystal X-ray diffraction method. Structural results together with dielectric and calorimetric studies allow the classification of the two intermediate phases (II) and (III) as ferroelectric with the Curie point at 346 K, and the lowest phase (IV) as most probably ferroelectric. The ferroelectric hysteresis loop was observed only in phase (III). The high ionic conductivity hindered its observation in phase (II).

Low-temperature crystal structure of RS-thiocamphor

2004 ◽  
Vol 219 (9) ◽  
Author(s):  
E. Louise R. Robins ◽  
Michela Brunelli ◽  
Asiloé J. Mora ◽  
Andrew N. Fitch
Keyword(s):  
Crystal Structure ◽  
Phase Transitions ◽  
Low Temperature ◽  
Room Temperature ◽  
Cubic Phase ◽  
X Ray Diffraction ◽  
Orthorhombic Space Group ◽  
Two Phase ◽  
X Ray ◽  
Low Temperature Crystal Structure

AbstractDSC and high-resolution powder X-ray diffraction measurements in the range 295 K–100 K show that RS-thiocamphor undergoes two phase transitions. The first, at around 260 K on cooling, is from the room-temperature body-centred-cubic phase to a short-lived intermediate. At 258 K the low-temperature form starts to appear. The crystal structure of the latter is orthorhombic, space group

X-ray diffraction study of the phase transitions of (CH3)4NCdCl3 between 293 and 80 K: a quantitative analysis of the ferroelastic domains distribution below 118 K

2000 ◽  
Vol 56 (2) ◽  
pp. 215-225 ◽  
Author(s):  
I. Peral ◽  
G. Madariaga ◽  
A. Pérez-Etxebarria ◽  
T. Breczewski
Keyword(s):  
Phase Transitions ◽  
Structural Phase ◽  
X Ray Diffraction ◽  
Two Phase ◽  
Threefold Axis ◽  
X Ray ◽  
Organic Part ◽  
Small Distortion ◽  
Diffraction Patterns ◽  
Ferroelastic Domains

X-ray diffraction patterns of [N(CH_{3})_{4}][CdCl_{3}], tetramethylammonium trichlorocadmate(II), have been investigated in the temperature range 80–293 K, which includes two phase transitions at 118 and 104 K, respectively. The main interest in this compound is to establish the mechanism of the structural phase transitions common to other members of the isostructural family [(CH_{3})_{4}N][MX_{3}]. It is supposed to be related to the ordering of the organic part together with some small distortion of the inorganic chains. The origin of the order–disorder mechanism would be the orientationally disordered distribution of the tetramethylammonium tetrahedra at room temperature. Maximum Entropy Methods suggest that the most probable distribution of the organic groups can be described through the so-called two-well model, in which one threefold axis of the tetramethylammonium tetrahedron coincides with the crystallographic threefold axis of the structure. Below 118 K the reflections are split. However, the splitting cannot be fully explained by the ferroelastic domains expected to appear after the phase transitions. Recent NMR results [Mulla-Osman et al. (1998). J. Phys. Condensed Matter, 10, 2465–2476] corroborate the existence of more domains than expected from symmetry considerations. A model of ferroelastic domains which is in agreement with both X-ray diffraction diagram and NMR measurements is proposed.

Disorder in crystals of dioxofluorotungstates, (NH4)2WO2F4 and Rb2WO2F4

2008 ◽  
Vol 64 (6) ◽  
pp. 645-651 ◽  
Author(s):  
Anatoly A. Udovenko ◽  
Natalia M. Laptash
Keyword(s):  
Phase Transition ◽  
Phase Transitions ◽  
Symmetry Axis ◽  
Crystal Form ◽  
X Ray Diffraction ◽  
Twin Structure ◽  
Static Disorder ◽  
X Ray ◽  
Independent States ◽  
Lower Temperature

Dioxotetrafluorotungstates (NH4)2WO2F4 [(I) at 297 K and (II) at 133 K] and Rb2WO2F4 (III) were synthesized in a single-crystal form and their structures were determined by X-ray diffraction. Two independent states of the cis-WO2F4 octahedron are characteristic of static and dynamic disorder in structure (I). Dynamically disordered W2 is displaced from the symmetry axis producing four possible orientations of anion that permits O and F atoms to be identified in separate orientations owing to the inherent differences between W—O and W—F bonding. After the phase transition at lower temperature (201 K), (I) transforms into the twin structure (II) with complete O/F ordering. Structure (III) is characterized by full O/F static disorder without any phase transitions at lower temperature.

scholarly journals High-Pressure Behaviors of Ag2S Nanosheets: An in Situ High-Pressure X-Ray Diffraction Research

Nanomaterials ◽  
2020 ◽  
Vol 10 (9) ◽  
pp. 1640
Author(s):  
Ran Liu ◽  
Bo Liu ◽  
Quan-Jun Li ◽  
Bing-Bing Liu
Keyword(s):  
High Pressure ◽  
Phase I ◽  
Phase Ii ◽  
Structural Phase ◽  
Phase Iii ◽  
X Ray Diffraction ◽  
X Ray ◽  
Structure Phase ◽  
Ag2s Nanoparticles

An in situ high-pressure X-ray diffraction study was performed on Ag2S nanosheets, with an average lateral size of 29 nm and a relatively thin thickness. Based on the experimental data, we demonstrated that under high pressure, the samples experienced two different high-pressure structural phase transitions up to 29.4 GPa: from monoclinic P21/n structure (phase I, α-Ag2S) to orthorhombic P212121 structure (phase II) at 8.9 GPa and then to monoclinic P21/n structure (phase III) at 12.4 GPa. The critical phase transition pressures for phase II and phase III are approximately 2–3 GPa higher than that of 30 nm Ag2S nanoparticles and bulk materials. Additionally, phase III was stable up to the highest pressure of 29.4 GPa. Bulk moduli of Ag2S nanosheets were obtained as 73(6) GPa for phase I and 141(4) GPa for phase III, which indicate that the samples are more difficult to compress than their bulk counterparts and some other reported Ag2S nanoparticles. Further analysis suggested that the nanosize effect arising from the smaller thickness of Ag2S nanosheets restricts the relative position slip of the interlayer atoms during the compression, which leads to the enhancing of phase stabilities and the elevating of bulk moduli.

Phase Diagram Studies on Neopentylglycol and Pentaerythritol-Thermal Energy Storage Materials

1989 ◽  
Vol 33 ◽  
pp. 445-452 ◽  
Author(s):  
Dhanesh Chandra ◽  
Renee A. Lynch ◽  
Wei Ding ◽  
John J. Tomlinson
Keyword(s):  
Phase Diagram ◽  
Phase Transitions ◽  
Molecular Crystals ◽  
Energy Storage Materials ◽  
X Ray Diffraction ◽  
Scanning Calorimetry ◽  
Two Phase ◽  
X Ray ◽  
Organic Molecular Crystals ◽  
Chain Type

AbstractPentaerythritol (PE) and neopentylglycol (NPG) are organic molecular crystals which have hydrogen-bonded lattices with layered- and chain-type structures, respectively. The PE undergoes solid-state phase transitions at 416K, from tetragonal to a cubic (γ’) structure and NPG also undergoes this transition at 317K from a monoclinic to a cubic (γ) structure. The phase transitions in binary PE-NPG solid solutions show more than one solid-solid transition in which the NPG-rich β transforms to γ at a constant temperature but the transition of the PE-rich a phase to γ’ passes through a two-phase field at a temperature that varies as a function of composition. Structural analyses by high-temperature x-ray diffractometry have revealed some new, interesting transitions. A phase diagram has been constructed for 0-30 mol% NPG in the PE-NPG system, using x-ray diffraction and differential scanning calorimetry results, and work is in progress to complete this diagram. The variation of lattice parameters as a function of temperature in the α, γ and γ’ phases for PE+20 mol% NPG are presented here, together with a discussion of phase transitions for certain compositions.

X-ray studies of the phase transitions of bis(guanidinium)zirconium bis(nitrilotriacetate) hydrate, (C(NH2)3)2Zr(N(CH2COO)3)2 · H2O

1999 ◽  
Vol 214 (3) ◽  
Author(s):  
J. Schreuer ◽  
E. Haussühl
Keyword(s):  
Phase Transition ◽  
Phase Transitions ◽  
Phase I ◽  
Phase Iii ◽  
Group Symmetry ◽  
Translational Symmetry ◽  
X Ray Diffraction ◽  
Space Group Symmetry ◽  
X Ray ◽  
Structural Differences

AbstractThe structural differences of phase I (at 193 K) and phase III (293 K) of bis(guanidinium)zirconium bis(nitrilotriacetate) hydrate were investigated by means of X-ray diffraction. The phase transition III → I is characterised by a loss of translational symmetry as it is indicated by the change of space group symmetry from

Orientational disorder in crystals of (NH4)3MoO3F3 and (NH4)3WO3F3

2008 ◽  
Vol 64 (3) ◽  
pp. 305-311 ◽  
Author(s):  
A. A. Udovenko ◽  
N. M. Laptash
Keyword(s):  
Vibrational Spectra ◽  
Central Atom ◽  
Crystal Form ◽  
Equilibrium Structure ◽  
Ammonium Cation ◽  
X Ray Diffraction ◽  
X Ray ◽  
The Face ◽  
Cubic System ◽  
Atomic Coordinates

Triammonium trioxotrifluoromolybdate (NH4)3MoO3F3 (I) and trioxotrifluorotungstate (NH4)3WO3F3 (II) were synthesized in a single-crystal form and their structures were refined by X-ray diffraction. These two isostructural compounds belong to the elpasolite-type structure (cubic system, space group Fm3m, Z = 4). O and F atoms are randomly distributed in two independent positions [24(e) and 96(j)] of the cubic unit cell, and the central atoms and the ammonium cation containing N2 are shifted from the symmetry centers into the 32(f) position. As a consequence, O and F atoms in the equilibrium structure were identified on a local scale by the metal–ligand distance and hydrogen atomic coordinates of the disordered ammonium cation N2H4 were determined. The slightly compressed MX 6 (M = Mo, W; X = O, F) octahedron has a fac configuration with the central atom shifted toward the face occupied by three O atoms. The true geometry of this polyhedron permits us to explain the observed vibrational spectra of the compounds examined and to eliminate the contradiction in interpretation of the vibrational spectra of [MO3F3] n− species reported in the literature. Both complexes reveal a dynamic disordering: the [MX 6]3− anions are disordered on 48 equivalent orientations and the N2H4 groups have eight orientations.

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