Temperature-dependent X-ray Study Research Articles

Pressure and magnetic field effects on ferroelastic and antiferromagnetic orderings in honeycomb-lattice Mn2V2O7

The multiferroic nature of the honeycomb-lattice $\mathrm{M}{\mathrm{n}}_{2}{\mathrm{V}}_{2}{\mathrm{O}}_{7}$ was investigated through detailed temperature, high-pressure, and magnetic-field-dependent measurements. A first-order martensiticlike structural phase transition with the thermal hysteresis associated with magnetic, heat-capacity, and dielectric anomalies was observed between ${T}_{Mh}$ (303 K) and ${T}_{Mc}$ (291 K). External pressure up to 15.41 kbar suppresses the thermal hysteresis in the magnetization data, indicating that the high-temperature \ensuremath{\beta}-phase persists down to the lower temperature under 15.41 kbar. Furthermore, isothermal capacitance-stress hysteresis loops along with crystallographic Aizu notation of $2/mF\overline{1}$ supports a martensitic phase transition driving ferroelastic ordering near or below room temperature. At low temperature, a long-range antiferromagnetic ordering was observed at ${T}_{N}\ensuremath{\sim}17\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. With increasing the external pressure up to 15.41 kbar, 100% enhancement of ${T}_{N}$ was observed and a metamagnetic transition at 5 K was enhanced near 3 T. High-field magnetization study up to 60 T induces multiple metamagnetic transitions below ${T}_{N}$. Below ${T}_{N}$, a magnetostriction induced magnetoelectric coupling was observed and further supported by the temperature-dependent x-ray studies. Taking these comprehensive research findings into account, we established that $\mathrm{M}{\mathrm{n}}_{2}{\mathrm{V}}_{2}{\mathrm{O}}_{7}$ is a unique multifunctional material with the coexistence of ferroelastic and antiferromagnetic orderings and with weak magnetoelectric coupling.

Exchange-striction induced giant ferroelectric polarization in copper-based multiferroic materialα−Cu2V2O7

We report $\ensuremath{\alpha}\ensuremath{-}{\mathrm{Cu}}_{2}{\mathrm{V}}_{2}{\mathrm{O}}_{7}$ to be an improper multiferroic with the simultaneous development of electric polarization and magnetization below ${T}_{C}=35\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. The observed spontaneous polarization of $0.55\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{C}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$ magnitude is highest among copper-based improper multiferroic materials. Our study demonstrates a sizable amount of magnetoelectric coupling below ${T}_{C}$, even with a low magnetic field. The theoretical calculations based on density functional theory indicate magnetism in $\ensuremath{\alpha}\ensuremath{-}{\mathrm{Cu}}_{2}{\mathrm{V}}_{2}{\mathrm{O}}_{7}$ is a consequence of ferro-orbital ordering driven by a polar lattice distortion due to the unique pyramidal (${\mathrm{CuO}}_{5}$) environment of Cu. Spin-orbit coupling further stabilizes orbital ordering and is crucial for magnetism. The calculations indicate that the origin of the giant ferroelectric polarization is primarily due to the symmetric exchange-striction mechanism and is corroborated by temperature-dependent x-ray studies.

Discotic liquid crystals of transition metal complexes, part 36: syntheses and mesomorphic properties of very large discotic liquid crystals based on triphenylenocyanine and 1,4-diazatriphenylenocyanine

Two series of very large discotic complexes of tetrakis (2,3,6,7-tetraalkoxy)triphenylenocyaninato copper(II) (abbreviated as ( C n O )16 TcCu ; n = 8, 10, 12 and 14) and tetrakis(2,3,6,7-tetraalkoxy)-1,4-diazatriphenylenocyaninato copper(II) (abbreviated as ( C n O )16 TzCu ; n = 8, 10, 12 and 14), were synthesized to investigate their mesomorphic properties by differential scanning calorimetry, thermogravimetry analysis, polarizing optical microscope observation and temperature-dependent X-ray studies. The results show that each of the ( C n O )16 TcCu derivatives exhibits two or three disordered rectangular columnar ( Col rd ) mesophases from room temperature to the decomposition temperature at ca. 330-340°C under nitrogen gas. However, each of the ( C n O )16 TzCu complexes exhibits only one ordered tetragonal columnar ( Col tet.o ) mesophase in an extremely wide temperature region from room temperature to the decomposition temperature at ca 300°C and ca. 340-350°C under the atmosphere and nitrogen gas, respectively. It is very interesting that the Col tet.o mesophase of ( C n O )16 TzCu for n = 10, 12 and 14 shows remarkable spontaneous homeotropic alignment in a large area, without disclinations and polydomains, in the temperature range from room temperature to the decomposition temperature. As far as we know, these discotic liquid crystals are the first examples to show homeotropic alignment at room temperature. Furthermore, the difference between the mesomorphic properties of ( C n O )16 TcCu and ( C n O )16 TzCu originated only from the additional eight nitrogen atoms in the central core in ( C n O )16 TzCu .