High-pressure polymerization of CS2 leads to structurally and stoichiometrically disordered solids, making characterization of the products and of reaction mechanisms challenging. This is due to the multiplicity of chemical pathways that CS2 is subject to in the free space. We reduced this multiplicity by constraining polymerization in a confined space. We inserted liquid CS2 in the one-dimensional pores of the all-siliceous zeolite Theta-One, subjected the mixture to maximum temperatures and pressures of 250 °C and 14 GPa, and recovered a darkened sample at ambient conditions. Synchrotron X-ray diffraction was performed on the materials. Full structural refinements, using ab initio calculated structures as starting models, show that CS2 in the zeolite undergoes selective topotactic polymerization, resulting in the ordered trans-isomer of the long-sought-after Bridgman's black polymer (-(C═S)-S-)n. We provide physical insight into the reaction mechanism by proposing that it consists of a molecular bending followed by addition. The trans-polymer forms spontaneously from the unstable, confined monomer upon relaxation in ab initio calculations.

Barium-cobaltate-based perovskite (BaCoO3-delta) and barium-cobaltate-based nanocomposites have been intensively studied in energy storage and conversion devices mainly due to flexible oxygen stoichiometry and tunable nonprecious transition metal oxidation states. Although a rich and complex family of structural polymorphs has already been reported for these perovskites in the literature, the potential structural evolution that may occur during the oxygen reduction reaction and the oxygen evolution reaction has not been investigated so far. In this study, we synthesized and characterized the lowest Co-oxidation state possible in the compound, BaCoO2, which exhibits a quartz-derived, trigonal structure with a helicoidally corner-sharing, CoO4-tetrahedral-framework as already proposed by Spitsbergen et al. Oxygen can reversibly be inserted in such a crystal structure to form BaCoO3-delta, i.e., with 0 <= delta <= 1, based on the results of an in situ coupled thermogravimetric - neutron diffraction study and which presents therefore giant oxygen capacity storage due to the extreme tunability of the electronic configuration of the cobalt cations which defines the fundamental origins of the materials performance. The reversible conversion of BaCoO2 to BaCoO3-delta associated with a similar electronic conductivity above 900 K permits to clarify the high potential of BaCoO3-delta-based energy storage and conversion devices.

In this article, we conduct comparative studies on the optical properties of metallic carbon nanotubes. First, we compare the complex dielectric constant predicted by an analytical model, the linear surface conductivity model, with ab initio calculations based on density functional theory. We highlight the similarities and differences between these two models, with the major discrepancy being the significantly different behavior of the plasma frequency with respect to the carbon nanotube diameter. In the second step, we compare the predictions of these models with experimental measurements of the dielectric function. We demonstrate that the screened plasma frequency serves as a reliable quantifier for distinguishing between the two models. In conclusion, we find that the ab initio calculations more accurately describe the optical properties of metallic carbon nanotubes compared with the commonly used linear surface conductivity model.

We report the existence in NaLa(SO4)2·H2O of a displacive phase transition under 200 K from the nonpolar P3121 to the polar P31 space group. This phase transition was predicted by density functional theory based calculations and experimentally confirmed from infrared spectroscopy and X-ray diffraction. The A2 polar irreducible representation is the primary order parameter. The structural water and hydrogen bonding are the mechanism driving the phase transition. The piezoelectric properties of this new P31 phase have been investigated by first principles based calculations. The highest piezoelectric-strain constants in the zero Kelvin limit are predicted for the d12 and d41 elements with values about 3.4 pC N-1. This compound could be interesting as piezoelectric actuator for cryogenic applications.

We pressure tune the hydrogen bond in Fe-O--H\ensuremath{\cdot}\ensuremath{\cdot}\ensuremath{\cdot}O-P structural segments of mixed-valence barbosalite $({\mathrm{Fe}}^{2+}{\mathrm{Fe}}_{2}^{3+}){(\mathrm{P}{\mathrm{O}}_{4})}_{2}{(\mathrm{OH})}_{2}$. Infrared spectroscopy evidences changes in softening of O--H stretch modes and excessive profile broadening onset below 10 GPa. Single-crystal x-ray diffraction shows pseudosymmetrization of the original monoclinic unit cell concurs with these changes in the O--H vibrational mode. These are considered compelling indicators of proton delocalization onset below 10 GPa as hydrogen bonds are strengthened under pressure. Subsequently in the range $10\ensuremath{-}30$ GPa, Fe M\"ossbauer spectroscopy discerns ${\mathrm{Fe}}^{2+}\ensuremath{\Leftrightarrow}{\mathrm{Fe}}^{3+}$ valence fluctuations at proximate cations of the hydrogen bonds. When the original crystal potential at an ${\mathrm{Fe}}^{2+}$ site is perturbed by proton delocalization at a ligand, electron exchange is induced along ${\mathrm{Fe}}^{2+}\ensuremath{\rightarrow}L\ensuremath{\rightarrow}{\mathrm{Fe}}^{3+}$ pathways [ligand $L=\mathrm{O}$ or ${(\mathrm{OH})}^{\ensuremath{-}}$ of shared octahedral faces]. Thus, $({\mathrm{Fe}}^{2+}{\mathrm{Fe}}_{2}^{3+}){(\mathrm{P}{\mathrm{O}}_{4})}_{2}{(\mathrm{OH})}_{2}$ under pressure exemplifies the interplay between proton (THz) and electron (MHz) dynamics on two disparate timescales in the same condensed phase.

NaLa(SO4)2,H2O crystalline powder was obtained under hydrothermal conditions at 220 degrees C. A coupled TGA/DTA experiment of NaLa(SO4)2,H2O exhibits a weight loss at 260 degrees C corresponding to the dehydration and an endo-thermal peak at 774 degrees C. To elucidate the transformation mechanism as a function of temperature, single crystals have been grown at 80 degrees C, 300 and 800 degrees 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 degrees 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 coor-dination 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) angstrom; c = 8.1267(1) angstrom) with LaO12 icosahedrons. Millimeter size single crystals of this new phase have been grown under hydrothermal conditions (300 degrees C, 157 bars). After the endothermal peak at 774 degrees 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 degrees C and 800 degrees C is compatible with the expected Raman signatures. Finally, a complete transformation of NaLa(SO4)2,H2O up to 800 degrees C is proposed. After 1000 degrees C, the compound decomposes chemically with a large weight loss.

Low-dimensional boron nitride (BN) chains were prepared in the one-dimensional pores of the siliceous zeolites theta-one (TON) and Mobil-twelve (MTW) by the infiltration, followed by the dehydrocoupling and pyrolysis of ammonia borane under high-pressure, high-temperature conditions. High-pressure X-ray diffraction in a diamond anvil cell and in a large-volume device was used to follow in situ these different steps in order to determine the optimal conditions for this process. Based on these results, millimeter-sized samples of BN/TON and BN/MTW were synthesized. Characteristic B-N stretching vibrations of low-dimensional BN were observed by infrared and Raman spectroscopies. The crystal structures were determined using a combination of X-ray diffraction and density functional theory with one and two one-dimensional zig-zag (BN)x chains per pore in BN/TON and BN/MTW, respectively. These 1-D BN chains potentially have interesting photoluminescence properties in the far ultraviolet region of the electromagnetic spectrum.

Ti0.5Sn0.5O2 nanoparticles (∼5 nm and ∼10 nm) have been studied under high pressure by Raman spectroscopy. For particles with diameter ∼10 nm, a transformation has been observed at 20-25 GPa while for particles with ∼5 nm diameter no phase transition has been observed up to ∼30 GPa. The Ti0.5Sn0.5O2 solid solution shows an extended stability at the nanoscale, both of its cationic and anionic sublattices. This ultrastability originates from the contribution of Ti and Sn mixing: Sn stabilizes the cationic network at high pressure and Ti ensures a coupling between the cationic and anionic sublattices. This result questions a "traditional" crystallographic description based on polyhedra packing and this synergistic effect reported in this work is similar to the case of metamaterials but 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.

The siliceous zeolite Socony Mobil-twelve (ZSM-12) or MTW (Mobil-TWelve) with a one-dimensional pore system was studied at high pressure by synchrotron X-ray powder diffraction in nonpenetrating DAPHNE7474 oil and in a penetrating argon pressure medium. A phase transition from the space group C2/c to P2/n is observed in the nonpenetrating medium at close to 1.5 GPa with a 4% volume decrease and a strong increase in compressibility in the ac plane corresponding to partial collapse of the pores. Strong decreases in diffracted intensity are observed with further compression, and the diffraction pattern contains broad features characteristic of an amorphous material above 10 GPa. Distinct behavior is observed when this material is pressurized in argon. Argon fills the pores with 9 +/- 1 Ar atoms per unit cell. In this medium-pore zeolite, the quantity of inserted argon was not found to vary with pressure. Argon-filled MTW is 35% less compressible than the corresponding empty form.

We report a complete investigation of the structural, electronic, vibrational, elastic and piezoelectric properties of the P 2 1 2 1 2 1 orthorhombic phase in cadmium diiodate (δ-Cd(IO 3 ) 2 ) by combining experiments and first-principles based calculations.

We pressure tune the hydrogen bond in Fe-O-H center dot center dot center dot O-P structural segments of mixed-valence barbosalite (Fe2+Fe23+)(PO4)(2)(OH)(2). Infrared spectroscopy evidences changes in softening of O-H stretch modes and excessive profile broadening onset below 10 GPa. Single-crystal x-ray diffraction shows pseudosymmetrization of the original monoclinic unit cell concurs with these changes in the O-H vibrational mode. These are considered compelling indicators of proton delocalization onset below 10 GPa as hydrogen bonds are strengthened under pressure. Subsequently in the range 10-30 GPa, Fe Mossbauer spectroscopy discerns Fe2+ double left right arrow Fe3+ valence fluctuations at proximate cations of the hydrogen bonds. When the original crystal potential at an Fe2+ site is perturbed by proton delocalization at a ligand, electron exchange is induced along Fe2+ -> L -> Fe3+ pathways [ligand L = O or (OH)(-) of shared octahedral faces]. Thus, (Fe2+Fe23+)(PO4)(2)(OH)(2) under pressure exemplifies the interplay between proton (THz) and electron (MHz) dynamics on two disparate timescales in the same condensed phase.

We report on the preparation and the X-ray crystal structure of colorless KSbGe3O9, its vibrational properties (Raman and infrared studies), and density functional theory (DFT) calculations. KSbGe3O9, grown by the high-temperature flux method from K2Mo4O13 flux, is thermally stable at least up to 1200 degrees C and is isostructural to the benitoite BaTiSi3O9 (space group P (6) over bar c2 (no. 188)). The hexagonal unit cell contains two formula units and the structure was refined to R1 = 0.0324 from single-crystal X-ray diffraction data. KSbGe3O9 is characterized with only one crystallographically independent Ge atom involved in three-member units [Ge3O9](6-) of regular germanate tetrahedra. The K+ ions are located in channels and, like Sb-V, are octahedrally surrounded by oxygen atoms. The KSbGe3O9 local structure and the planarity of Ge3O3 rings are also studied by a room-temperature vibrational investigation using non-polarized infrared and Raman spectroscopy. Both the infrared and Raman phonon modes have been assigned from the agreement observed between our experimental data and the corresponding DFT ones. In particular, two E'(TO) modes (both IR and Raman active) characterize the planarity of the Ge3O3 ring in the ab plane.

The one-dimensional structure of single-walled carbon nanotubes (NT) display optical absorption and near-infrared emission (thanks to van Hove singularities). Chromophore encapsulation into host single-walled carbon nanotubes allows to create hybrid nano-systems with tunable opto-electronic properties. Up to now, we have been confining different kinds of chromophores,1-4 absorbing from the blue/ green (400/500 nm) range (tetracyanoquinodimethane (TNCQ), quaterthiophene derivatives (4T) and tetramethyl-paraphenylenediamine (TMPD)) to the red (700 nm) range (phthalocyanine (MPc)). In addition then can be either electron donor (4T, TMPD) or acceptor (TNCQ). In this study, we investigate, at both the macroscopic and the individual scales, the electronic and the optical properties of our hybrid systems by means of Raman and photoluminescence spectroscopies. Photoluminescence experiments clearly demonstrate changes on the emission properties after encapsulation. The intensities can be increased or reduced depending on the nature of the confined chromophores (electron donor or acceptor) and on the NT diameter. From Raman measurements, a significant charge transfer from the confined dye to the nanotube is evidenced. The main relevant parameters that govern the charge transfer are the nanotube diameter and the nature of the chromophores (electron donor or acceptor). Therefore, Raman and photoluminescence experiments strongly suggest charge transfer between the confined molecules and the nanotubes, leading to a Fermi level shift which governs the radiative de-excitation efficiency. References [1] L. Alvarez et al, J. Phys. Chem. C, 119, (2015), pp. 5203−5210 [2] Y. Almadori et al, J. Phys. Chem. C; 118, (2014), pp. 19462−19468 [3] A. Belhboub et al, J. Phys. Chem. C; 120, (2016), pp. 28802−28807 [4] Y. Almadori et al, Carbon 149, (2019), pp. 772-780

Encapsulation of photoactive organic molecules inside single-walled carbon nanotubes (SWNTs) appears to be of great interest in terms of high power conversion efficiency and long-term stability for a commercial application of organic solar cells (OSCs). In this paper, we report a charge transfer (CT) evidence in donor-acceptor SWNTs filled with Sexithiophene oligomers (6T) by Raman spectroscopy. To compute the optimal diameter and demonstrate the most stable structure of the hybrid systems with either a single 6T molecule encapsulated into SWNTs (6T@SWNTs), or two 6T chains encapsulated (6T-6T@SWNTs), we have performed structural geometry optimization on the hybrid encapsulated systems using a convenient Lennard-Jones (LJ) expression of the van der Waals (vdW) intermolecular potential. Combining the density functional theory (DFT), molecular mechanics, bond polarizability model, and the spectral moment method (SMM), we computed the polarized nonresonant Raman spectra of 6T molecule and SWNTs (metallic and semiconducting) before and after encapsulation. The influence of the encapsulation on the Raman-active modes of the 6T molecule and those of the nanotube (radial breathing modes and tangential modes) are analyzed. In particular, significant changes observed in the G-band wavenumber. The possibility (or not) of an eventual CT between the 6T oligomer and the nanotube in both hybrid systems (6T@SWNTs and 6T-6T@SWNTs) is discussed. We show that there is a dependence of the CT with respect to the diameter of SWNTs, the CT vanish with increasing diameter of the nanotubes. Our finding of CT behavior in the filled SWNTs with respect to SWNT diameter will provide a useful guidance for enhancing the performance of OSCs by SWNTs.

Chromium disilicide CrSi2 is an interesting compound for thermoelectric applications. In order to decrease its lattice thermal conductivity that mainly limits its performance, two main routes have been investigated thus far, either increasing the unit cell disorder or creating multiple interfaces through nanostructuring. Here, we explore the effect of the latter route by investigating in detail the effect of the grain size reduction and residual microstrains on the lattice dynamics and lattice thermal conductivity. The phonon dispersion curves were measured on single-crystalline CrSi2 using inelastic neutron scattering, while the generalized vibrational density of states (GVDOS) was determined on bulk and nanostructured CrSi2. All experimental results are consistent with our density functional theory calculations. The results show that the optical phonons contribute from 50 to 70% of the lattice thermal conductivity. The temperature variations in the GVDOS of CrSi2 follow a quasi-harmonic behavior, which explains its rather large lattice thermal conductivity measured on the single-crystalline specimen. In addition, the GVDOS of nanocrystalline CrSi2 evidences a spectral weight transfer at low energy, which is related to a decrease in both the Debye temperature and the sound velocities that may be ascribed to an increase in both the interface density and internal strain. These observations explain the strong decrease in the lattice thermal conductivity observed in our prior study on densified nanostructured CrSi2 pellets.

The components of the frequency-dependent complex refractive index were determined indirectly for the new non-centrosymmetric alpha-GeO2 crystal using polarized Fourier transform infrared reflectivity spectra measured in the far- and mid-infrared spectral region at room temperature. All the longitudinal- and transverse-optical infrared active modes with E and A(2) symmetry, according to the D-3 point group, were identified and localized within the 100-1000 cm(-1) range in very good agreement with a previous first-principles based calculation. For the A(2)- and E-type modes, both the longitudinal- and transverse-optical splitting were detected. The refractive indices n(o) ((E) over right arrow perpendicular to c) and n(e) ((E) over right arrow //c) in the infrared domain present considerably higher values than the ones observed in the visible light range, and the high birefringence would find application in many optical devices.

Raman spectra of nitrogen-doped single-walled carbon nanotubes are calculated using the spectral moment’s method combined with the bond polarizability model. The influence of the nanotube diameter and chirality is investigated. We also address the important question of the effect of the N-doping concentration, and we propose an equation to estimate the doping concentration from the knowledge of the tube diameter and the frequency of the radial breathing mode.