Application of DUT-4 MOF structure switching for optical and electrical humidity sensing.

The threshold structural transformation of the DUT-4 metal-organic framework (MOF) from an ordered to distorted phase during exposure to ambient conditions has been revealed. The in situ X-ray diffraction analysis, in situ Raman and FTIR spectroscopy, scanning electron microscopy and synchronous thermal analysis have been used for investigation. The reversible effect of exposure time and humidity on such a phase transition has been confirmed. We also demonstrated that the observed phase transition correlated well with changes in the optical and electronic properties of DUT-4, paving the way to a new family of MOF-based phase change materials for optoelectronic applications.

Thermal evolution of LiCoO2 structure and Raman spectra below 400 °C.

Lithium cobalt oxide is a convenient model material for the vast family of cathode materials with a layered structure and still retains some commercial perspectives for microbatteries and some other applications. In this work, we have used ab initio calculations, x-ray diffraction, Raman spectroscopy, and a theoretical physical model, based on quasi-harmonic approximation with anharmonic contributions of the three-phonon and four-phonon processes, to study a temperature-induced change of Raman spectra for LiCoO2. The obtained values of shift and broadening for Eg and A1g bands can be used for quantitative characterization of temperature change, for example, due to laser-induced heating during Raman spectra measurements. The theoretical analysis of the experimental results lets us conclude that Raman spectra changes for LiCoO2 can be explained by the combination of thermal expansion of the crystal lattice and phonon damping by anharmonic coupling with comparable contributions of the three-phonon and four-phonon processes. The obtained results can be further used to develop Raman-based quality control tools.

Temperature phase transitions in silver niobate and lithium tantalate-modified silver niobate ceramics.

The temperature behaviour of 0.955 AgNbO3-0.045 LiTaO3 and AgNbO3 ceramics was studied in the range from 10 to 415 K using Raman spectroscopy. Ab initio calculations of the Raman spectra in the Pmc21 phase of AgNbO3 were carried out using three potentials (A-PZ, PBE, and PBEsol) for spectral interpretation. The peculiarities in the Raman spectra in AgNbO3 ceramics are observed and explained. The differences in the spectra of the 0.955 AgNbO3-0.045 LiTaO3 and AgNbO3 ceramics are shown. The temperatures of the structural changes in the 0.955 AgNbO3-0.45 LiTaO3 and AgNbO3 ceramics were discussed. A structural phase transition below 120 K was observed in silver niobate. A phase transition was observed at 310 K and below 150 K in 0.955 AgNbO3-0.045 LiTaO3.

A Challenge toward Novel Quaternary Sulfides SrLnCuS3 (Ln = La, Nd, Tm): Unraveling Synthetic Pathways, Structures and Properties.

We report on the novel heterometallic quaternary sulfides SrLnCuS3 (Ln = La, Nd, Tm), obtained as both single crystals and powdered samples. The structures of both the single crystal and powdered samples of SrLaCuS3 and SrNdCuS3 belong to the orthorhombic space group Pnma but are of different structural types, while both samples of SrTmCuS3 crystallize in the orthorhombic space group Cmcm with the structural type KZrCuS3. Three-dimensional crystal structures of SrLaCuS3 and SrNdCuS3 are formed from the (Sr/Ln)S7 capped trigonal prisms and CuS4 tetrahedra. In SrLaCuS3, alternating 2D layers are stacked, while the main backbone of the structure of SrNdCuS3 is a polymeric 3D framework [(Sr/Ln)S7]n, strengthened by 1D polymeric chains (CuS4)n with 1D channels, filled by the other Sr2+/Ln3+ cations, which, in turn, form 1D dimeric ribbons. A 3D crystal structure of SrTmCuS3 is constructed from the SrS6 trigonal prisms, TmS6 octahedra and CuS4 tetrahedra. The latter two polyhedra are packed together into 2D layers, which are separated by 1D chains (SrS6)n and 1D free channels. In both crystal structures of SrLaCuS3 obtained in this work, the crystallographic positions of strontium and lanthanum were partially mixed, while only in the structure of SrNdCuS3, solved from the powder X-ray diffraction data, were the crystallographic positions of strontium and neodymium partially mixed. Band gaps of SrLnCuS3 (Ln = La, Nd, Tm) were found to be 1.86, 1.94 and 2.57 eV, respectively. Both SrNdCuS3 and SrTmCuS3 were found to be paramagnetic at 20-300 K, with the experimental magnetic characteristics being in good agreement with the corresponding calculated parameters.