Partial replacement of Ti4+ by Te4+ ions in calcium copper titanate lattice improved its dielectric behaviour mostly due to cubic-to-tetragonal structural transformation and associated distortion in TiO6 octahedra. The relative permittivity values (23–30 x 103) of Te4+ doped ceramics is more than thrice that of un-doped ceramics (8 x 103) at 1 kHz. A decreasing trend in relative permittivity with increasing temperature (50–300 K) is observed for all the samples. Barrett’s formula, as a signature of incipient ferroelectricity, is invoked to rationalize the relative permittivity variation as a function of temperature. A systematic investigation supported by temperature dependent Raman studies reveal a possible ferroelectric transition in Te4+ doped ceramic samples below 120 K. The possible ferroelectric transition is attributed to the interactions between quasi-local vibrations associated with the micro-clusters comprising TiO6 and TeO6 structural units and indirect dipole-dipole interactions of off-center B–cations (Ti4+ and Te4+) in double perovskite lattice.
One of the key challenges of fuel cell technology is to find solid electrolytes which are cheap and environmentally friendly with high proton (H+) conductivities. In this context, designing new materials based on organic cocrystals/salts appears very promising to expand the scope of H+ ion conductors. In our approach, we have synthesized a quaternary organic salt consisting of gallic acid, isoniazid, sulfate (SO4 2–) ion, and water by a slow evaporation method which exhibits high proton conductivity of 0.2 mS cm–1 at 293 K to serve as a model system. The proton conductivity value observed in our system is comparable and in some cases better than recently published coordination polymers, metal organic frameworks, and covalent organic frameworks. The system crystallizes as monoclinic with space group P21/c (Z′ = 3; Z = 12), which depicts a layered structure with extensive O–H···O and N–H···O hydrogen bonding networks. Further, it exhibits interesting order–disorder phase transitions at both high and low temperatures. Calculation of the activation energy (∼0.39 eV) from conductivity plots for the system reveals the mechanism of proton conduction to be Grotthuss type. Thus, our novel design strategy of preparing an organic salt for proton conduction applications opens up a pathway to generate easy synthesis of cheap and environmentally friendly materials.
Graphene holds promises for exploring exotic superconductivity with Dirac-like fermions. Making graphene a superconductor at large scales is however a long-lasting challenge. A possible solution relies on epitaxially-grown graphene, using a superconducting substrate. Such substrates are scarce, and usually destroy the Dirac character of the electronic band structure. Using electron diffraction (reflection high-energy, and low-energy), scanning tunneling microscopy and spectroscopy, atomic force microscopy, angle-resolved photoemission spectroscopy, Raman spectroscopy, and density functional theory calculations, we introduce a strategy to induce superconductivity in epitaxial graphene via a remote proximity effect, from the rhenium substrate through an intercalated gold layer. Weak graphene–Au interaction, contrasting with the strong undesired graphene–Re interaction, is demonstrated by a reduced graphene corrugation, an increased distance between graphene and the underlying metal, a linear electronic dispersion and a characteristic vibrational signature, both latter features revealing also a slight p doping of graphene. We also reveal that the main shortcoming of the intercalation approach to proximity superconductivity is the creation of a high density of point defects in graphene (1014 cm−2). Finally, we demonstrate remote proximity superconductivity in graphene/Au/Re(0001), at low temperature.
Van der Waals magnetic materials are building blocks for novel kinds of spintronic devices and playgrounds for exploring collective magnetic phenomena down to the two-dimensional limit. Chromium−tellurium compounds are relevant in this perspective. In particular, the 1T phase of CrTe 2 has been argued to have a Curie temperature above 300 K, a rare and desirable property in the class of lamellar materials, making it a candidate for practical applications. However, recent literature reveals a strong variability in the reported properties, including magnetic ones. Using electron microscopy, diffraction, and spectroscopy techniques, together with local and macroscopic magnetometry approaches, our work sheds new light on the structural, chemical, and magnetic properties of bulk 1T-CrTe 2 exfoliated in the form of flakes having a thickness ranging from few to several tens of nanometers. We unambiguously establish that 1T-CrTe 2 flakes are ferromagnetic above room temperature, have an in-plane easy axis of magnetization, and low coercivity, and we confirm that their Raman spectroscopy signatures are two modes: E 2g (103.5 cm −1 ) and A 1g (136.5 cm −1 ). We also prove that thermal annealing causes a phase transformation to monoclinic Cr 5 Te 8 and, to a lesser extent, to trigonal Cr 5 Te 8 . In sharp contrast with 1T-CrTe 2 , none of these compounds have a Curie temperature above room temperature, and they both have perpendicular magnetic anisotropy. Our findings reconcile the apparently conflicting reports in the literature and open opportunities for phaseengineered magnetic properties.
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