NaLa(SO4)2,H2O crystalline powder was obtained under hydrothermal conditions at 220°C. A coupled TGA/DTA experiment of NaLa(SO4)2,H2O exhibits a weight loss at 260°C corresponding to the dehydration and an endothermal peak at 774°C. To elucidate the transformation mechanism as a function of temperature, single crystals have been grown at 80°C, 300 and 800°C. For each phase, single crystals have been isolated and structure determination was performed. As already published, NaLa(SO4)2,H2O crystallizes in a P3121 space group. However, the dehydration at 260°C is not a simple loss of the water molecule but a radical change in the structure. The removal of the water molecules inside the tunnels formed by the framework leads to a change in the coordination of the LaO9 Lanthanum-based polyhedrons. The compound obtained after dehydration is a new triple sulfate of the formula Na3La(SO4)3 crystallizing in the R-3 space group (a = 14.0976(1) Å; c = 8.1267(1) Å) with LaO12 icosahedrons. Millimeter size single crystals of this new phase have been grown under hydrothermal conditions (300°C, 157 bars). After the endothermal peak at 774°C, Na3La(SO4)3 decomposes by forming the anhydrous double sulfate NaLa(SO4)2 crystallizing in the P-1 space group with LaO10 polyhedrons. The structure of the three (NaLa)-compounds at RT, 300°C and 800°C is compatible with the expected Raman signatures. Finally, a complete transformation of NaLa(SO4)2,H2O up to 800°C is proposed. After 1000°C, the compound decomposes chemically with a large weight loss.

We report a complete investigation of the structural, electronic, vibrational, elastic and piezoelectric properties of the P2 1 2 1 2 1 orthorhombic phase in cadmium diiodate (δ-Cd(IO 3) 2) by combining experiments and first-principles based calculations. We revisited the nature of the electronic band gap and suggest an indirect band gap with a value of 4.6 eV. The infrared and Raman responses were measured and the different phonon modes assigned. To date, the δ-Cd(IO 3) 2 piezoelectric response remains unknown. We reported the different mechanisms involved in its piezoelectric response from the density functional perturbation theory. The highest value of the piezoelectric-stress and piezoelectric-strain constants in the zero Kelvin limit is predicted for e 41 =-0.27 C/m 2 and d 41 =-10.32 pC/N. These sizable values associated with the thermal stability (no phase transition up to the thermal decomposition at 550°C) and a relative large electronic band gap make δ-Cd(IO 3) 2 a potential candidate for piezoelectric applications.

Double-walled carbon nanotubes (DWNTs), often regarded as the simplest one-dimensional (1D) van der Waals moiré superlattices, are the ideal structures to explore the interlayer coupling, including mechanical and electronical coupling, at the nanoscale in 1D systems. By combining spatial modulation spectroscopy (SMS) and resonant Raman scattering (RRS) experiments on well-identified individual free-standing DWNTs with the calculations of their optical absorption by using the recursion method with non-orthogonal tight-binding (NTB) basis functions, we are able to unambiguously assign the nature, namely mirror transition or cross transition, of each optical transitions in DWNTs. Our study identifies and allows to determine the conditions of the mixing of the electronic states leading to the appearance of cross transitions in the optical absorption of DWNTs, beyond of the so-called Koshino's criteria previously derived. On the other hand, the striking behavior of the resonant excitation profile (REP) of the Raman active modes around the cross transition reveals the strong sensitivity of the modes intensity on the electronic interlayer coupling in DWNTs, emphasizing the relevance of Raman spectroscopy for studying the electronic and mechanical coupling at the nanoscale.

Photoluminescence of single-walled carbon nanotubes is monitored at the individual scale by molecule encapsulation into their hollow core. Depending on the electronic character (electron donor or acceptor) of the confined molecule, enhancement or quenching of the photoluminescence intensity is demonstrated. This behavior is assigned to a charge transfer, evidenced by the shift of the Raman G-band, and a correlated Fermi level shift shown by photoemission experiments. Our experimental results are supported by DFT calculations. A consistent picture of the physical interactions taking place in the hybrid systems and their effects on the optical and electronic properties is given. Our results indicate that the electron affinity or ionization potential of the encapsulated molecules and the diameter of the nanotube are relevant parameters to tune the light emission properties of the hybrid systems at the nanoscale.