Phase Transition in K3Na(MoO4)2 and Determination of the Twinned Structures of K3Na(MoO4)2 and K2.5Na1.5(MoO4)2 at Room Temperature

1997 ◽  
Vol 53 (4) ◽  
pp. 596-603 ◽  
Author(s):  
J. Fábry ◽  
V. Petrícek ◽  
P. Vanek ◽  
I. Císarová
Keyword(s):  
Phase Transition ◽  
Room Temperature ◽  
Domain Switching ◽  
Point Group ◽  
Atomic Displacement ◽  
Scanning Calorimetry ◽  
Sodium Potassium ◽  
Displacement Vectors ◽  
Reversible Phase Transition

The room-temperature phases of sodium potassium molybdates K3Na(MoO4)2 and K2.5Na1.5(MoO4)2 are isostructural with the monoclinic low-temperature phases of KaNa(SeO4)2 and K3Na(CrO4)2, which are twinned distorted glaserite structures. In the molybdates there are two crystallographically independent potassiums and their environment slightly differs from those in K3Na(SeO4)2 and K3Na(CrO4)2. The excessive Na in K2.5Na1.5(MoO4)2 occupies the position of the more firmly bound potassium. A reversible phase transition at 513 K was discovered in KaNa(MoO4)2 by DSC (differential scanning calorimetry), but no such transition in K2.5Na1.5(MoO4)2 was detected. Both samples used in the diffractometer experiment were found to be composed of six domains being related by twinning operations of the point group 6. The twinning may be considered as a combination of a merohedral and a pseudo-merohedral twinning with two- and threefold rotations as twinning operations, respectively. However, a reversible domain switching, which is observable in the related ferroelastic crystals of KaNa(SeO4)2 and K3Na(CrO4)2, was not observed either in K3Na(MoO4)2 or in K2.5Na1.5(MoO4)2, either due to semitransparency of the samples or high ferroelastic distortion. This distortion is manifested by the values of the atomic displacement vectors which are about twice as large as those in the selenate or the chromate.

scholarly journals Large electromechanical strain and unconventional domain switching near phase convergence in a Pb-free ferroelectric

2020 ◽  
Vol 3 (1) ◽  
Author(s):  
Sarangi Venkateshwarlu ◽  
Lalitha K. Venkataraman ◽  
Valentin Segouin ◽  
Frederick P. Marlton ◽  
Ho Chin Hin ◽  
...  
Keyword(s):  
Phase Transition ◽  
Room Temperature ◽  
Domain Switching ◽  
Polarization Vector ◽  
Orthorhombic Phase ◽  
Image Correlation ◽  
X Ray Diffraction ◽  
X Ray ◽  
Reversible Phase Transition ◽  
Domain Reorientation

Abstract In many ferroelectrics, large electromechanical strains are observed near regions of composition- or temperature- driven phase coexistence. Phenomenologically, this is attributed to easy re-orientation of the polarization vector and/or phase transition, although their effects are highly convoluted and difficult to distinguish experimentally. Here, we used synchrotron X-ray scattering and digital image correlation to differentiate between the microscopic mechanisms leading to large electrostrains in an exemplary Pb-free piezoceramic Sn-doped barium calcium zirconate titanate. Large electrostrains of ~0.2% measured at room-temperature are attributed to an unconventional effect, wherein polarization switching is aided by a reversible phase transition near the tetragonal-orthorhombic phase boundary. Additionally, electrostrains of ~0.1% or more could be maintained from room temperature to 140 °C due to a succession of different microscopic mechanisms. In situ X-ray diffraction elucidates that while 90° domain reorientation is pertinent below the Curie temperature (TC), isotropic distortion of polar clusters is the dominant mechanism above TC.

Reversible phase transition of pyridinium-3-carboxylic acid perchlorate

2010 ◽  
Vol 66 (3) ◽  
pp. 387-395 ◽  
Author(s):  
Heng-Yun Ye ◽  
Li-Zhuang Chen ◽  
Ren-Gen Xiong
Keyword(s):  
Phase Transition ◽  
Carboxylic Acid ◽  
Relative Displacement ◽  
Temperature Phase ◽  
Scanning Calorimetry ◽  
Cell Parameters ◽  
Dsc Measurements ◽  
Reversible Phase Transition ◽  
Reversible Phase ◽  
Hydrogen Bonded

Pyridinium-3-carboxylic acid perchlorate was synthesized and separated as crystals. Differential scanning calorimetry (DSC) measurements show that this compound undergoes a reversible phase transition at ∼ 135 K with a wide hysteresis of 15 K. Dielectric measurements confirm the transition at ∼ 127 K. Measurement of the unit-cell parameters versus temperature shows that the values of the c axis and β angle change abruptly and remarkably at 129 (2) K, indicating that the system undergoes a first-order transition at T c = 129 K. The crystal structures determined at 103 and 298 K are all monoclinic in P21/c, showing that the phase transition is isosymmetric. The crystal contains one-dimensional hydrogen-bonded chains of the pyridinium-3-carboxylic acid cations, which are further linked to perchlorate anions by hydrogen bonds to form well separated infinite planar layers. The most distinct differences between the structures of the higher-temperature phase and the lower-temperature phase are the change of the distance between the adjacent pyridinium ring planes within the hydrogen-bonded chains and the relative displacement between the hydrogen-bonded layers. Structural analysis shows that the driving force of the transition is the reorientation of the pyridinium-3-carboxylic acid cations. The degree of order of the perchlorate anions may be a secondary order parameter.

scholarly journals Crystalline forms of silver iodide II: Determination of phase transformations

2007 ◽  
Vol 72 (8-9) ◽  
pp. 857-868 ◽  
Author(s):  
Marija Vukic ◽  
Dragan Veselinovic ◽  
Vesna Markovic
Keyword(s):  
Phase Transition ◽  
Phase Transformations ◽  
Heating Rate ◽  
Silver Iodide ◽  
Room Temperature ◽  
Sample Heating ◽  
Crystalline Forms

In order to obtain appropriate forms of AgI (?- and ?-), a procedure was developed to synthesize AgI at room temperature (23?C), whereby samples of varying crystallographic purity and of varying crystallographic contents of the different forms were obtained. This paper presents the results of investigations of the influence of the manner of preparation of ?-AgI and ?-AgI samples and the sample heating rate on the phase transformations and their temperatures. During the heating of non-ground, ground and pressed synthesized AgI samples, the phase transformations and the corresponding temperatures for one ?-AgI and four ?-AgI samples with different ?-AgI contents (representing a crystallographic impurity) were identified. The following phase transformations were observed for the non-ground AgI samples: ?-AgI ? ?-AgI at 149.6?C (for the ?-AgI sample) and ?-AgI ? ?-AgI at 148.7?C or 148.2?C for the ?-AgI samples with a minimum content of ?-AgI (less than 7 %), as a crystallographic impurity. The phase transition ?-AgI ? ?-AgI was irreversible because ?-AgI was obtained whenever the samples were heated up to 260?C. Manual sample grinding, as well as pressing at p1 = 650 MPa and p2 = 900 MPa resulted in the ?-AgI ? ?-AgI phase transition in all the investigated cases. .

scholarly journals Identification of an Al–Ni–O precipitate in combustion-synthesized NiAl

2000 ◽  
Vol 33 (5) ◽  
pp. 1217-1222
Author(s):  
A. Biswas ◽  
Madangopal K. ◽  
J. B. Singh ◽  
S. K. Roy ◽  
S. Banerjee
Keyword(s):  
Room Temperature ◽  
Point Group ◽  
Composition Analysis ◽  
Second Phase ◽  
X Ray ◽  
Selected Area Electron Diffraction ◽  
Phase Precipitate ◽  
Transmission Electron ◽  
Chemical Composition Analysis

The complete identity of a second-phase precipitate detected by transmission electron microscopy in combustion-synthesized NiAl was established. The crystal structure, including the point group, the space group and the lattice parameters, was determined by convergent and selected-area electron diffraction techniques. Energy dispersive X-ray spectroscopy was used for the determination of the chemical composition. Analysis established the phase to be the solid solution of NiO in Al2O3and presented evidence of the hitherto unreported room-temperature solubility.

scholarly journals Above Room Temperature Reversible Phase Transition Induces Distinct Dielectric and Nonlinear Optical Switching Response Behavior in Crown-Ether-Based Supramolecular Clathrate

Crystals ◽  
2019 ◽  
Vol 9 (4) ◽  
pp. 184 ◽  
Author(s):  
Jing Jing ◽  
Fan Jiang ◽  
Yan-Li Wei ◽  
Chao Shi ◽  
Heng-Yun Ye ◽  
...  
Keyword(s):  
Phase Transition ◽  
Crown Ether ◽  
Optical Switching ◽  
Nonlinear Optical ◽  
Thermal Hysteresis ◽  
Switching Behavior ◽  
Stimuli Responsive ◽  
Scanning Calorimetry ◽  
Reversible Phase Transition ◽  
Reversible Phase

Stimuli-responsive materials with coexisting nonlinear optical (NLO) and dielectric properties are technologically important, which enable simultaneous conversion of optoelectronic properties between different states under external stimuli. By rationally screening guest cations (C6H5NF2)+ in the crown-ether inclusion system, we synthesized a crown-ether supramolecular compound [(C6H5NF2)(18-crown-6)][PF6] (1). Differential scanning calorimetry (DSC) showed that 1 undergoes a reversible phase transition above room temperatures (305 K/292 K), with a thermal hysteresis of 13 K. Temperature-dependent dielectric and NLO measurements show that the compound exhibits two distinct switching response behaviors. Structural analysis indicates that the order–disorder change of the host molecule 18-crown-6 and the guest organic cation during the phase transition induces the dielectric and NLO switching behavior of the compound.

Structure and phase transition of the Se-rich variety of antimonpearceite, [(Ag,Cu)6(Sb,As)2(S,Se)7][Ag9Cu(S,Se)2Se2]

2006 ◽  
Vol 62 (5) ◽  
pp. 768-774 ◽  
Author(s):  
Michel Evain ◽  
Luca Bindi ◽  
Silvio Menchetti
Keyword(s):  
Phase Transition ◽  
Room Temperature ◽  
Atomic Displacement ◽  
X Ray Diffraction ◽  
X Ray ◽  
Dimensional Diffusion ◽  
Ionic Conducting ◽  
Local Arrangement ◽  
Diffusion Paths ◽  
Linear Coordination

The crystal structure of a Se-rich antimonpearceite has been solved and refined by means of X-ray diffraction data collected at temperatures above (room temperature) and below (120 K) an ionic conductivity-induced phase transition. Both structure arrangements consist of the stacking of [(Ag,Cu)6(Sb,As)2(S,Se)7]2− A (A′) and [Ag9Cu(S,Se)2Se2]2+ B (B′) module layers in which Sb forms isolated SbS3 pyramids typically occurring in sulfosalts; copper links two S atoms in a linear coordination, and silver occupies sites with coordination ranging from quasi-linear to almost tetrahedral. In the ionic-conducting form, at room temperature, the silver d 10 ions are found in the B (B′) module layer along two-dimensional diffusion paths and their electron densities described by means of a combination of a Gram–Charlier development of the atomic displacement factors and a split-atom model. The structure resembles that of pearceite, except for the presence of both specific (Se) and mixed (S, Se) sites. In the low-temperature `ordered' phase at 120 K the silver d 10 ions of the B (B′) module layer are located in well defined sites with mixed S—Se coordination ranging from quasi-linear to almost tetrahedral. The structure is then similar to that of 222-pearceite but with major differences, specifically its cell metric, symmetry and local arrangement in the B (B′) module layer.

Photoacoustic determination of phase transition in BaTiO3 induced by high pressure at room temperature

2003 ◽  
Vol 74 (1) ◽  
pp. 732-734 ◽  
Author(s):  
M. Villagrán-Muniz ◽  
M. Navarrete ◽  
E. V. Mejı́a-Uriarte
Keyword(s):  
Phase Transition ◽  
High Pressure ◽  
Room Temperature

A Reversible Non-disruptive Phase Transition Shown by the Zinc Iodide Dimethylformamide Complex ZnI2(dmf)2

1998 ◽  
Vol 54 (5) ◽  
pp. 663-670 ◽  
Author(s):  
R. A. Edwards ◽  
A. J. Easteal ◽  
O. P. Gladkikh ◽  
W. T. Robinson ◽  
M. M. Turnbull ◽  
...  
Keyword(s):  
Phase Transition ◽  
Transition Temperature ◽  
Intermolecular Forces ◽  
Scanning Calorimetry ◽  
Space Group ◽  
Cell Parameters ◽  
Reversible Phase Transition ◽  
Higher Temperature ◽  
High Temperature Form ◽  
Reversible Phase

At 228 K crystals of ZnI2(dmf)2 show a reversible phase transition which does not disrupt the lattice. Above the transition temperature the space group is C2/c and the cell contains eight symmetrically equivalent molecules. Cooling to below the transition temperature has little effect on the cell parameters or on the Zn- and I-atom positions, but the space group is now P21/n and the asymmetric unit comprises two conformationally different molecules. These arise from cooperative rotations of either ca +25 or −43° about the Zn—O bond of one of the dmf ligands in the high-temperature form. This displacive transition involves large movements of some atoms. The corresponding chloride and bromide are isomorphous with the higher temperature C2/c form, but it is only with the iodide that the weaker intermolecular forces permit the unusual phase change. The transition has been followed by differential scanning calorimetry, which gives an enthalpy change of 1.44 (5) kJ mol−1.

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