Local Sn Dipolar-Character Displacements behind the Low Thermal Conductivity in SnSe Thermoelectric.

The local atomic structure of SnSe was characterized across its orthorhombic-to-orthorhombic structural phase transition using x-ray pair distribution function analysis. Substantial Sn displacements with a dipolar character persist in the high-symmetry high-temperature phase, albeit with a symmetry different from that of the ordered displacements below the transition. The analysis implies that the transition is neither order-disorder nor displacive but rather a complex crossover. Robust ferrocoupled SnSe intralayer distortions suggest a ferroelectriclike instability as the driving force. These local symmetry-lowering Sn displacements are likely integral to the ultralow lattice thermal conductivity mechanism in SnSe.

Ultrathin Y2O3:Eu3+nanodiscs: spectroscopic investigations and evidence for reduced concentration quenching.

Here, we report the synthesis and spectral properties of ultrathin nanodiscs (NDs) of Y2O3:Eu3+. It was found that the NDs of Y2O3:Eu3+ with a thickness of about 1 nm can be fabricated in a reproducible, facile and self-assembling process, which does not depend on the Eu3+ concentration. The thickness and morphology of these NDs were determined with small angle x-ray scattering and transmission electron microscopy. We found that the crystal field in these nanoparticles deviates from both the cubic and monoclinic characteristics, albeit the shape of the 5D0 â†’ 7F J (J = 0, 1, 2) transitions shows some similarity with the transitions in the monoclinic material. The Raman spectra of the non-annealed NDs manifest various vibration modes of the oleic acid molecules, which are used to stabilise the NDs. The annealed NDs show two very weak Raman lines, which may be assigned to vibrational modes of Y2O3 NDs. The concentration quenching of the Eu3+ luminescence of the NDs before annealing is largely suppressed and might be explained in terms of a reduction of the phonon density of states.

Absence of significant structural changes near the magnetic ordering temperature in small-ion rare earth perovskite RMnO3.

Detailed structural measurements on multiple length scales were conducted on a new perovskite phase of ScMnO3, and on orthorhombic LuMnO3 as a benchmark. Complementary density functional theory (DFT) calculations were carried out, and predict that ScMnO3 possesses E-phase magnetic order at low temperature with displacements of the Mn sites (relative to the high temperature state) of ∼0.07 Å, compared to ∼0.04 Å predicted for LuMnO3. However, detailed local, intermediate and long-range structural measurements by x-ray pair distribution function analysis, single crystal x-ray diffraction and x-ray absorption spectroscopy, find no local or long-range distortions on crossing into the low temperature E-phase of the magnetically ordered state. The measurements place upper limits on any structural changes to be at most one order of magnitude lower than DFT predictions and suggest that this theoretical approach does not properly account for the spin-lattice coupling in these oxides and may possibly predict the incorrect magnetic order at low temperatures. The results suggest that the electronic contribution to the electrical polarization dominates and should be more accurately treated in theoretical models.