Ultrasonic studies on charge transfer complexes of iodine with certain aromatic compounds in n-hexane solution

17-23<span style="font-size: 15.5pt;mso-bidi-font-size:8.5pt;font-family:" times="" new="" roman","serif""="">Ultrasonic velocities, densities and viscosities have been measured in n-hexane solutions containing iodine (acceptor) and diphenyl (DP), diphenyl amine (DPA) or diphenyl ether (OPE) (donor) at various equimolar concentrations at 303 K. Acoustical parameters such as adiabatic compressibility <span style="font-size: 17.0pt;mso-bidi-font-size:10.0pt;font-family:" times="" new="" roman","serif""="">(β). linear free length (Lf), relaxation time <span style="font-size: 14.0pt;mso-bidi-font-size:7.0pt;font-family:" arial","sans-serif""="">(τ<span style="font-size:14.0pt;mso-bidi-font-size: 7.0pt;font-family:" arial","sans-serif""="">), <span style="font-size:15.5pt; mso-bidi-font-size:8.5pt;font-family:" times="" new="" roman","serif""="">free volume (Vf), molecular interaction parameter (<span style="font-size:15.5pt; mso-bidi-font-size:8.5pt;font-family:" ms="" mincho";mso-bidi-font-family:"times="" new="" roman""="">χ), acoustical impedance <span style="font-size:14.5pt;mso-bidi-font-size: 7.5pt;font-family:" times="" new="" roman","serif""="">(Z). absorption coefficient <span style="font-size:16.0pt;mso-bidi-font-size:9.0pt; font-family:" times="" new="" roman","serif""="">(<span style="font-size:16.0pt; mso-bidi-font-size:9.0pt;font-family:" ms="" mincho";mso-bidi-font-family:"times="" new="" roman""="">α/f2), Rao's constant <span style="font-size:14.5pt;mso-bidi-font-size:7.5pt; font-family:" arial","sans-serif""="">(R), <span style="font-size:15.5pt; mso-bidi-font-size:8.5pt;font-family:" times="" new="" roman","serif""="">cohesive <span style="font-size: 15.5pt;mso-bidi-font-size:8.5pt;font-family:" times="" new="" roman","serif""="">energy (CE), internal pressure <span style="font-size:14.5pt;mso-bidi-font-size: 7.5pt;font-family:" times="" new="" roman","serif""="">(πi) and Van der Waal's constant <span style="font-size:14.0pt;mso-bidi-font-size: 7.0pt;font-family:" arial","sans-serif""="">(a) <span style="font-size: 15.5pt;mso-bidi-font-size:8.5pt;font-family:" times="" new="" roman","serif""="">are evaluated. These parameters indicate the formation of charge- transfer complexes between iodine and the aromatic compounds. The stability constant (K), <span style="font-size:15.5pt;mso-bidi-font-size:8.5pt;font-family: " times="" new="" roman","serif""="">the free energy of activation (Δ<i style="mso-bidi-font-style: normal">G*). the standard free energy of formation (Δ<i style="mso-bidi-font-style: normal">G<span style="font-size:11.5pt;mso-bidi-font-size:4.5pt; font-family:" times="" new="" roman","serif""="">° )<span style="font-size:15.5pt; mso-bidi-font-size:8.5pt;font-family:" times="" new="" roman","serif""="">values are reported for the donor-acceptor complexes. These investigations reveal that the stability of the charge transfer complex depends on the structure of the aromatic compound. </span

Equilibrium phases in the multiferroic BiFeO3-PbTiO3 system - a revisit

The(1-x) BiFeO3-(x) PbTiO3 solid solution exhibiting a Morphotropic Phase Boundary (MPB) has attracted considerable attention recently because of its unique features such as multiferroic, high Curie point (T-C similar to 700 degrees C) and giant tetragonality (c/a -1 similar to 0.19). Different research groups have reported different composition range of MPB for this system. In this work we have conclusively proved that the wide composition range of MPB reported in the literature is due to kinetic arrest of the metastable rhombohedral phase and that if sufficient temperature and time is allowed the metastable phase disappears. The genuine MPB was found to be x=0.27 for which the tetragonal and the rhombohedral phases are in thermodynamic equilibrium. In-situ high temperature structural study of x=0.27 revealed the sluggish kinetics associated with the temperature induced structural transformation. Neutron powder diffraction study revealed that themagnetic ordering at room temperature occurs in the rhombohedral phase. The magnetic structure was found to be commensurate G-type antiferromagnetic with magnetic moments parallel to the c-direction (of the hexagonal cell). The present study suggests that the equilibrium properties in this solid solution series should be sought for x=0.27

Equilibrium phases in the multiferroic BiFeO3-PbTiO3 system – a revisit

The(1-x) BiFeO3-(x) PbTiO3 solid solution exhibiting a Morphotropic Phase Boundary (MPB) has attracted considerable attention recently because of its unique features such as multiferroic, high Curie point (TC ~ 700°C) and giant tetragonality (c/a -1 ~ 0.19). Different research groups have reported different composition range of MPB for this system. In this work we have conclusively proved that the wide composition range of MPB reported in the literature is due to kinetic arrest of the metastable rhombohedral phase and that if sufficient temperature and time is allowed the metastable phase disappears. The genuine MPB was found to be x=0.27 for which the tetragonal and the rhombohedral phases are in thermodynamic equilibrium. In-situ high temperature structural study of x=0.27 revealed the sluggish kinetics associated with the temperature induced structural transformation. Neutron powder diffraction study revealed that themagnetic ordering at room temperature occurs in the rhombohedral phase. The magnetic structure was found to be commensurate G-type antiferromagnetic with magnetic moments parallel to the c-direction (of the hexagonal cell). The present study suggests that the equilibrium properties in this solid solution series should be sought for x=0.27

Metastable morphotropic phase boundary state in the multiferroic BiFeO3-PbTiO3

Temperature-time study of the magnetoelectric multiferroic (1-x)BiFeO3-(x)PbTiO3 by x-ray and electron diffraction on the reported morphotropic phase boundary (MPB) compositions revealed that this MPB does not correspond to the equilibrium state. The MPB like state is rather of metastable nature and arise due to kinetic arrest of metastable rhombohedral (R3c) phase, along with the equilibrium tetragonal (P4mm) phase. The life time of the metastable R3c nuclei is very sensitive to composition and temperature, and nearly diverges at x -> 0.27. The MPB like state appears only if the system is cooled before the metastable R3c nuclei could vanish. These findings resolve the long standing controversy with regard to seemingly erratic phase formation behaviour reported by different groups and provides a rational basis for developing genuine equilibrium MPB compositions in this system for better piezoelectric properties. (C) 2013 AIP Publishing LLC

Stabilization of metastable tetragonal phase in a rhombohedral magnetoelectric multiferroic BiFeO3-PbTiO3

The phase formation behaviour of the magnetoelectric multiferroic 0.8BiFeO(3)-0.2PbTiO(3) was studied as a function of heat treatment at different temperatures of a sol-gel derived powder. While under ordinary synthesis conditions this composition exhibits antiferromagnetic ordering and a rhombohedral structure; the sol-gel-enabled low-temperature synthesis could stabilize a tetragonal metastable phase along with the stable rhombohedral phase, mimicking a morphotropic phase boundary state. The phase coexistence state exhibits relatively enhanced ferromagnetic correlation. The same system with a relatively higher PbTiO3 concentration, 0.65BiFeO(3)-0.35PbTiO(3), on the other hand, exhibits a rhombohedral metastable phase. These results suggest that the occurrence of metastable phases is a very common feature in the BiFeO3-PbTiO3 magnetoelectric ferroelectric system and that it affects the ferroelectric and magnetic properties of system quite remarkably

Neutron diffraction study of the coupling between spin, lattice, and structural degrees of freedom in 0.8BiFeO(3)-0.2PbTiO(3)

A powder neutron diffraction study was carried out on 0.8BiFeO(3)-0.2PbTiO(3) in the temperature range 27-1000 degrees C. The system exhibits magnetic transition at similar to 300 degrees C and a rhombohedral (R3c)-cubic (Pm3m) ferroelectric phase transition at similar to 650 degrees C. Anomalous variation in the lattice parameters and the octahedral tilt angle is observed across the magnetic transition temperature. In the magnetic phase, the c parameter is contracted and the octahedral tilt angle is slightly increased. The results suggest coupling between the spin, lattice and structural degrees of freedom. (C) 2011 American Institute of Physics. doi:10.1063/1.3555093

Ferroelectric phase coexistence by crystallite size reduction in BiFeO3-PbTiO3

Size effect in ferroelectrics has been extensively investigated in the past with the general consensus that the long-range ferroelectric ordering gradually disappears with decreasing crystallite size, eventually leading to the paraelectric state. In this paper, we show that the compositions exhibiting giant tetragonality (c/a similar to 1.18) of the ferroelectric alloy system BiFeO3-PbTiO3 transform from a pure tetragonal phase to a state comprising tetragonal and rhombohedral phases as the average crystallite size is reduced from similar to 10 to similar to 1 mu m. It is argued that the increased surface energy in the smaller sized crystallites creates an equivalent compressive stress that drives the system towards tetragonal-rhombohedral criticality

Phases in the (1-x)Na0.5Bi0.5TiO3-(x)CaTiO3 system

The structures of (1 - x) Na0.5Bi0.5TiO3-(x) CaTiO3 at room temperature have been investigated using neutron powder diffraction and dielectric studies. The system exhibits an orthorhombic (Pbnm) structure for x &gt;= 0.15 and rhombohedral (R3c) for x &lt;= 0.05. For x = 0.10, though the neutron diffraction pattern shows features of the orthorhombic (Pbnm) structure, Rietveld refinement using this structure shows a drastic reduction in the in-phase tilt angle (similar to 4 degrees) as compared to the corresponding value (similar to 8 degrees) for a neighbouring composition x = 0.15. The neutron diffraction pattern of x = 0.10 could be fitted equally well using a two-phase model (R3c + Pbnm) with orthorhombic as the minor phase (22%), without the need for a drastic decrease in the in-phase tilt angle. The dielectric studies of x = 0.10 revealed the presence of the polar R3c phase, thereby favouring the phase coexistence model, instead of a single-phase Pbnm structure, for this composition