Tuning porosity with electric fields: Many metal-organic framework (MOF) compounds exhibit soft porosity—i.e., their lattices can undergo considerable deformation. Knebel et al. formed membrane layers of the MOF ZIF-8 and found that it was converted into a polar polymorph with a stiffer lattice in response to an applied electric field (see the Perspective by Gascon). This change reduced gas transport but, for certain gas mixtures such as propane and propene, also improved their separation factor. Science , this issue p. 347 ; see also p. 303

Collective vibrational modes of crystal lattices, called phonons, determine fundamental material properties, such as their thermal and electrical conductivities. Bulk phonon spectra are influenced by point defects. More recently, the importance of phonons on nanostructures has come into the focus of attention. Here we show a spatially resolved phonon spectra of point defects that reveal distinctly different signatures for a cavity alone and an impurity atom fully integrated into the surface as opposed to one placed into a cavity. The spectra are indicative for delocalized phonons and localized vibrations, respectively, as confirmed by theory.

Li x Na2−x Ti₆O₁₃has received attention as 3d-metal oxide based anode material for possible battery application. Generally, titanium oxides represent excellent Li hosts due to their zero-strain behavior, cycling stability and high operating voltage. New developments choose Na as charge carrier, but less effort is put in the investigation of mixed cation conductors. Owing to the synthesis route of Li x Na2−x Ti₆O₁₃(0 ≤ x ≤ 2) the coordination of Na and Li in the channels is different which had been investigated by means of X-ray and neutron diffraction. Up to now, no Raman spectroscopy has been applied. This oxide is highly Raman-active, thus the local structure can also be analyzed in terms of vibrational spectroscopy. Micro-Raman spectroscopy carried out at room temperature with different cation contents (x = 0, 0.33, 1, 2) shows the continuous change from Na to Li by a superposition of the modes for Na₂Ti₆O₁₃ and Li₂Ti₆O₁₃. The only exceptions are two distinct modes. They appear either for Li (118 cm –1) or Na (135 cm –1). The results confirm the high-temperature phase stability of Na₂Ti₆O₁₃ as well as the anisotropic thermal expansion of the unit cell seen by in situ X-ray powder diffraction under two different gas atmospheres. Combining these results, we suppose that the anisotropic thermal expansion of the lattice parameters is affected by the normal vectors of the corresponding bond vibrations in Na₂Ti₆O₁₃ and Li₂Ti₆O₁₃. Crystalline-orbital calculations of the Raman shifts of Li x Na2−x Ti₆O₁₃were carried out for the cation contents x = 0, 1, 2 and Raman modes were assigned to specific bond vibrations supported by theory. Besides, this gives additionally information about the Li/Na ratio in a new and simple way.