Ming Zhang

[(CH₃)₂NH₂][Mn(HCOO)₃], an organic–inorganic perovskite-like hybrid multiferroic, shows promising aspirations for the next generation magnetic storage devices, light-weight chips, optical devices, etc. However, the growth of centimeter-sized large single crystals of [(CH₃)₂NH₂][Mn(HCOO)₃] remains a big challenge. Herein, we report a facile, environmentally friendly approach to grow [(CH₃)₂NH₂][Mn(HCOO)₃] single crystals whose size could be up to 10 × 9.61 × 3.6 mm, the biggest one ever reported to date. In addition, high-pressure Raman spectra of the material exhibited the existence of three high-pressure polymorphs at 2.1, 4.1, and 6.7 GPa. The high-pressure phase transitions are associated with highly distorted frameworks, and the phase under a pressure larger than 6.7 GPa induces large structural changes in the hydrogen bond-related interactions, which is different from the highly ordered low-temperature monoclinic ferroelectric phase (Cc) observed at ambient pressure. Our studies may shed light on the phase transition mechanisms of the multiferroic metal–organic framework [(CH₃)₂NH₂][Mn(HCOO)₃] under high pressures.

The thermal behavior of optical phonons and OH vibrations of phlogopite (a trioctahedral mica) was examined at temperatures up to 1000 K using in situ infrared spectroscopy. The results showed that with increasing temperature, O–K bands in phlogopite exhibited a relatively strong variation in frequency in a manner similar to those in muscovite. The work revealed that different types of OH bands (fundamentals and combinations) have very different thermal behavior or temperature dependence, and their absorption coefficients are commonly not constant on heating. OH combination bands that are associated with summation processes of multi-phonon interactions commonly show a decrease in their intensities on heating, but in contrast combination bands due to difference processes generally exhibit an increase. This means that temperature dependencies of their absorption coefficients need to be considered when using the Beer-Lambert law to determine or estimate OH contents or hydrogen concentrations at high temperatures. The results showed a structural anomaly associated with a discontinuity in the temperature derivative of the wavenumber of Al–O and Si–O vibrations and O–H stretching near 600 K. However, framework-related phonon modes in the FIR and MIR regions do not suggest a break of the original monoclinic structural symmetry in the investigated temperature region. The complex changes are attributed to temperature-induced alteration of local configuration involving TO₄ tetrahedra and a possible change of the orientation of OH dipoles, in addition to a previously reported distortion of MO₆ octahedra. Increasing temperature to 1000 K also causes partial dehydroxylation, as evidenced by the disappearance of the OH band near 3623 cm⁻¹ and the decrease in OH band height and area of other OH bands. The study did not record the formation of H₂O inside phlogopite as a result of partial dehydroxylation. The work offers new data and findings that have important implications in understanding the complex structural modifications and the behavior of phonon modes and the thermal stability of hydroxyls on approaching the dehydroxylation, as well as the way hydrogen is released from micas at high temperatures. Our data also show that phologpite becomes less transparent with increasing temperature suggesting a change of radiative properties and ability to transmit heat, which could be of interest for modeling thermal-transmission in crustal rocks.

Electrocaloric effect in lead-free Ba_0.94Ca_0.06Ti_1−xSn_xO₃ ceramics is studied using an indirect method. The Ba_0.94Ca_0.06Ti_0.875Sn_0.125O₃ ceramic located near a multi-phase point shows best electrocaloric performance, which provides further experimental evidence for optimizing electrocaloric properties through constructing multiphase coexistence. Giant electrocaloric efficiency (∼0.4 K mm/kV) is achieved in this ceramic at about room temperature at a low electric field of 6 kV/cm. While large electrocaloric temperature (∼0.63 K) is obtained by further enhancing electric field (20 kV/cm), a decrease in electrocaloric efficiency (0.32 K mm/kV) is simultaneously observed, which is attributed to phase transition from first-order to more diffusive second-order under higher electric field.

Two-dimensional textured carbon fiber is an excellent electrode material and/or supporting substrate for active materials in fuel cells, batteries and pseudocapacitors owing to its large surface area, high porosity, ultra-lightness, good electric conductivity and excellent chemical stability in various liquid electrolytes. And Nickel hydroxide is one of the most promising active materials that have been studied in practical pseudocapacitor applications. Here we report a high-capacitance, flexible and ultra-light composite electrode that combines the advantages of these two materials for pseudocapacitor applications. Electrochemical measurements demonstrate that the 3D hybrid nanostructured carbon fiber–NiCo₂O₄–Ni(OH)₂ composite electrode shows high capacitance, excellent rate capability. To the best of our knowledge, the electrode developed in this work possesses the highest areal capacitance of 6.04 F cm⁻² at the current density of 5 mA cm⁻² among those employing carbon fiber as the conductor. It still remains 64.23% at 40 mA cm⁻². As for the cycling stability, the initial specific capacitance decreases only from 4.56 F cm⁻² to 3.35 F cm⁻² after 1000 cycles under a current density of 30 mA cm⁻².