BaCoO2 with Tetrahedral Cobalt Coordination: The Missing Element to Understand Energy Storage and Conversion Applications in BaCoO3-δ-Based Materials.

Barium-cobaltate-based perovskite (BaCoO3-δ) 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-δ, i.e., with 0 ≤ δ ≤ 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-δ associated with a similar electronic conductivity above 900 K permits to clarify the high potential of BaCoO3-δ-based energy storage and conversion devices.

Strong Swelling and Symmetrization in Siliceous Zeolites due to Hydrogen Insertion at High Pressure.

Hydrogen and helium saturate the 1D pore systems of the high-silica (Si/Al>30) zeolites Theta-One (TON), and Mobile-Twelve (MTW) at high pressure based on x-ray diffraction, Raman spectroscopy and Monte Carlo simulations. In TON, a strong 22% volume increase occurs above 5 GPa with a transition from the collapsed P21 to a symmetrical, swelled Cmc21 form linked to an increase in H2 content from 12 H2/unit cell in the pores to 35 H2/unit cell in the pores and in the framework of the material. No transition and continuous collapse of TON is observed in helium indicating that the mechanism of H2 insertion is distinct from other fluids. The insertion of hydrogen in the larger pores of MTW results in a strong 11% volume increase at 4.3 GPa with partial symmetrization followed by a second volume increase of 4.5% at 7.5 GPa, corresponding to increases in hydrogen content from 43 to 67 and then to 93 H2/unit cell. Flexible 1D siliceous zeolites have a very high H2 capacity (1.5 and 1.7 H2/SiO2 unit for TON and MTW, respectively) due to H2 insertion in the pores and the framework, in contrast to other atoms and molecules, thereby providing a mechanism for strong swelling.

Preparation of Confined One-Dimensional Boron Nitride Chains in the 1-D Pores of Siliceous Zeolites under High-Pressure, High-Temperature Conditions.

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.

Effect of H2O on the Pressure-Induced Amorphization of Hydrated AlPO4-17.

The incorporation of guest species in zeolites has been found to strongly modify their mechanical behavior and their stability with respect to amorphization at high pressure (HP). Here we report the strong effect of H2O on the pressure-induced amorphization (PIA) in hydrated AlPO4-17. The material was investigated in-situ at HP by synchrotron X-ray powder diffraction in diamond anvil cells by using non- and penetrating pressure transmitting media (PTM), respectively, silicone oil and H2O. Surprisingly, in non-penetrating PTM, its structural response to pressure was similar to its anhydrous phase at lower pressures up to ~1.4 GPa, when the amorphization was observed to start. Compression of the structure of AlPO4-17 is reduced by an order of magnitude when the material is compressed in H2O, in which amorphization begins in a similar pressure range as in non-penetrating PTM. The complete and irreversible amorphization was observed at ~9.0 and ~18.7 GPa, respectively, in non- and penetrating PTM. The present results show that the insertion of guest species can be used to strongly modify the stability of microporous material with respect to PIA, by up to an order of magnitude.

Pressure-Induced Sublattice Disordering in SnO_{2}: Invasive Selective Percolation.

SnO_{2} powders and single crystal have been studied under high pressure using Raman spectroscopy and ab initio simulations. The pressure-induced changes are shown to drastically depend on the form of the samples. The single crystal exhibits phase transitions as reported in the literature, whereas powder samples show a disordering of the oxygen sublattice in the first steps of compression. This behavior is proposed to be related to the defect density, an interpretation supported by ab initio simulations. The link between the defect density and an amorphouslike Raman signal is discussed in terms of the invasive percolation of the anionic sublattice. The resistance of the cationic sublattice to the disorder propagation is discussed in terms of cation close packing. This result on SnO_{2} may be extended to other systems and questions a "traditional" crystallographic description based on polyhedra packing, as a decoupling between both sublattices is observed.

Mechanism of H2O insertion and chemical bond formation in AlPO(4)-54·xH2O at high pressure.

The insertion of H2O in AlPO4-54·xH2O at high pressure was investigated by single-crystal X-ray diffraction and Monte Carlo molecular simulation. H2O molecules are concentrated, in particular, near the pore walls. Upon insertion, the additional water is highly disordered. Insertion of H2O (superhydration) is found to impede pore collapse in the material, thereby strongly modifying its mechanical behavior. However, instead of stabilizing the structure with respect to amorphization, the results provide evidence for the early stages of chemical bond formation between H2O molecules and tetrahedrally coordinated aluminum, which is at the origin of the amorphization/reaction process.

Partially collapsed cristobalite structure in the non molecular phase V in CO2.

Non molecular CO(2) has been an important subject of study in high pressure physics and chemistry for the past decade opening up a unique area of carbon chemistry. The phase diagram of CO(2) includes several non molecular phases above 30 GPa. Among these, the first discovered was CO(2)-V which appeared silica-like. Theoretical studies suggested that the structure of CO(2)-V is related to that of ß-cristobalite with tetrahedral carbon coordination similar to silicon in SiO(2), but reported experimental structural studies have been controversial. We have investigated CO(2)-V obtained from molecular CO(2) at 40-50 GPa and T > 1500 K using synchrotron X-ray diffraction, optical spectroscopy, and computer simulations. The structure refined by the Rietveld method is a partially collapsed variant of SiO(2) ß-cristobalite, space group I42d, in which the CO(4) tetrahedra are tilted by 38.4° about the c-axis. The existence of CO(4) tetrahedra (average O-C-O angle of 109.5°) is thus confirmed. The results add to the knowledge of carbon chemistry with mineral phases similar to SiO(2) and potential implications for Earth and planetary interiors.

A high-temperature molecular ferroelectric Zn/Dy complex exhibiting single-ion-magnet behavior and lanthanide luminescence.

Multifunctional molecular ferroelectrics are exciting materials synthesized using molecular chemistry concepts, which may combine a spontaneous electrical polarization, switched upon applying an electric field, with another physical property. A high-temperature ferroelectric material is presented that is based on a chiral Zn(2+) /Dy(3+) complex exhibiting Dy(3+) luminescence, optical activity, and magnetism. We investigate the correlations between the electric polarization and the crystal structure as well as between the low-temperature magnetic slow relaxation and the optical properties.

Giant negative linear compressibility in zinc dicyanoaurate.

The counterintuitive phenomenon of negative linear compressibility (NLC) is a highly desirable but rare property exploitable in the development of artificial muscles, actuators and next-generation pressure sensors. In all cases, material performance is directly related to the magnitude of intrinsic NLC response. Here we show the molecular framework material zinc(II) dicyanoaurate(I), Zn[Au(CN)(2)](2), exhibits the most extreme and persistent NLC behaviour yet reported: under increasing hydrostatic pressure its crystal structure expands in one direction at a rate that is an order of magnitude greater than both the typical contraction observed for common engineering materials and also the anomalous expansion in established NLC candidates. This extreme behaviour arises from the honeycomb-like structure of Zn[Au(CN)(2)](2) coupling volume reduction to uniaxial expansion, and helical Au…Au 'aurophilic' interactions accommodating abnormally large linear strains by functioning as supramolecular springs.

Iodine capture by Hofmann-Type clathrate Ni(II)(pz)[Ni(II)(CN)4].

The thermally stable Hofmann-type clathrate framework Ni(II)(pz)[Ni(II)(CN)4] (pz = pyrazine) was investigated for the efficient and reversible sorption of iodine (I2) in the gaseous phase and in solution with a maximum adsorption capacity of 1 mol of I2 per 1 mol of Ni(II)(pz)[Ni(II)(CN)4] in solution.

Confined H2O molecules as local probes of pressure-induced amorphisation in faujasite.

Confined H2O molecules act as local probes for depressurization phenomena during the pressure induced amorphisation of faujasite NaX at which the OH stretching frequency first decreases and then increases almost to its room pressure value upon further compression. Pair distribution function (PDF) analysis provides evidence that amorphisation corresponds to a collapse of the structure around hydrated sodium cations with strong distortion of the secondary building units (double six-membered rings, sodalite cages). Both the use of guest molecules as local probes in far- and mid-infrared spectroscopy, where we correlate intermolecular water H bonding vibrations and internal mode behaviour under confinement, and PDF analysis could be of great use to study the mechanical behaviour of other hydrated materials.

Silicon carbonate phase formed from carbon dioxide and silica under pressure.

The discovery of nonmolecular carbon dioxide under high-pressure conditions shows that there are remarkable analogies between this important substance and other group IV oxides. A natural and long-standing question is whether compounds between CO(2) and SiO(2) are possible. Under ambient conditions, CO(2) and SiO(2) are thermodynamically stable and do not react with each other. We show that reactions occur at high pressures indicating that silica can behave in a manner similar to ionic metal oxides that form carbonates at room pressure. A silicon carbonate phase was synthesized by reacting silicalite, a microporous SiO(2) zeolite, and molecular CO(2) that fills the pores, in diamond anvil cells at 18-26 GPa and 600-980 K; the compound was then temperature quenched. The material was characterized by Raman and IR spectroscopy, and synchrotron X-ray diffraction. The experiments reveal unique oxide chemistry at high pressures and the potential for synthesis of a class of previously uncharacterized materials. There are also potential implications for CO(2) segregation in planetary interiors and for CO(2) storage.

Direct Single Crystal to Amorphous Transformation and Memory Effect in AlPO4-17.

Pressure induced amorphization provides a distinct route to prepare novel amorphous materials. Single crystals of the porous aluminophosphate AlPO4-17 directly transform to an amorphous state beginning at 0.6 GPa, without fragmentation into polycrystalline material. Apart from a reduction in dimensions, the amorphous material retains the form of the initial single crystal. Remnant crystalline domains in the amorphous material also preserve the initial orientation of the single crystal. X-ray diffraction indicates the compression of the structure around the empty pores in the xy plane and such an amorphization mechanism is consistent with a direct structural relationship between the single crystal and amorphous forms. The collapse of the initial pore volume is almost complete at 2.5 GPa. A memory effect is observed in the amorphous form, which strongly expands on decompression. The present process opens the way for the synthesis of topologically ordered amorphous materials approaching "perfect glasses" with improved mechanical properties.

Homologous critical behavior in the molecular frameworks Zn(CN)2 and Cd(imidazolate)2.

Using a combination of single-crystal and powder X-ray diffraction measurements, we study temperature- and pressure-driven structural distortions in zinc(II) cyanide (Zn(CN)2) and cadmium(II) imidazolate (Cd(im)2), two molecular frameworks with the anticuprite topology. Under a hydrostatic pressure of 1.52 GPa, Zn(CN)2 undergoes a first-order displacive phase transition to an orthorhombic phase, with the corresponding atomic displacements characterized by correlated collective tilts of pairs of Zn-centered tetrahedra. This displacement pattern sheds light on the mechanism of negative thermal expansion in ambient-pressure Zn(CN)2. We find that the fundamental mechanical response exhibited by Zn(CN)2 is mirrored in the temperature-dependent behavior of Cd(im)2. Our results suggest that the thermodynamics of molecular frameworks may be governed by considerations of packing efficiency while also depending on dynamic instabilities of the underlying framework topology.

Vibrational origin of the thermal stability in the highly distorted α-quartz-type material GeO2: an experimental and theoretical study.

We report an experimental and theoretical vibrational study of the high-performance piezoelectric GeO2 material. Polarized and variable-temperature Raman spectroscopic measurements on high-quality, water-free, flux-grown α-quartz GeO2 single crystals combined with state-of-the-art first-principles calculations allow the controversies on the mode symmetry assignment to be solved, the nature of the vibrations to be described in detail, and the origin of the high thermal stability of this material to be explained. The low-degree of dynamic disorder at high-temperature, which makes α-GeO2 one of the most promising piezoelectric materials for extreme temperature applications, is found to originate from the absence of a libration mode of the GeO4 tetrahedra.

Strongly Modified Mechanical Properties and Phase Transition in AlPO4-17 Due to Insertion of Guest Species at High Pressure.

The porous aluminophosphate AlPO4-17 with a hexagonal erionite structure, exhibiting very strong negative thermal expansion, anomalous compressibility, and pressure-induced amorphization, was studied at high pressure by single-crystal and powder X-ray diffraction in the penetrating pressure transmitting media N2, O2, and Ar. Under pressure, these guest species were confirmed to enter the pores of AlPO4-17, thus completely modifying its behavior. Pressure-induced collapse in the xy plane of AlPO4-17 no longer occurred, and this plane exhibited close to zero area compressibility. Pressure-induced amorphization was also suppressed as the elastic instability in the xy plane was removed. Crystal structure refinements at a pressure of 5.5 GPa indicate that up to 28 guest molecules are inserted per unit cell and that this insertion is responsible for the reduced compressibility observed at high pressure. A phase transition to a new hexagonal structure with cell doubling along the a direction was observed above 4.4 GPa in fluid O2.

Siliceous zeolite-derived topology of amorphous silica.

The topology of amorphous materials can be affected by mechanical forces during compression or milling, which can induce material densification. Here, we show that densified amorphous silica (SiO2) fabricated by cold compression of siliceous zeolite (SZ) is permanently densified, unlike densified glassy SiO2 (GS) fabricated by cold compression although the X-ray diffraction data and density of the former are identical to those of the latter. Moreover, the topology of the densified amorphous SiO2 fabricated from SZ retains that of crystalline SZ, whereas the densified GS relaxes to pristine GS after thermal annealing. These results indicate that it is possible to design new functional amorphous materials by tuning the topology of the initial zeolitic crystalline phases.

Rational design of materials with extreme negative compressibility: selective soft-mode frustration in KMn[Ag(CN)2]3.

We show that KMn[Ag(CN)(2)](3) exhibits the strongest negative linear compressibility (NLC) effect over the largest pressure range yet observed. Variable pressure neutron powder diffraction measurements reveal that its crystal lattice expands along the c axis of its trigonal cell under increasing hydrostatic pressure, while contracting along the a axis. This corresponds to a "wine-rack"-like mechanism for NLC that we find also results in anisotropic negative thermal expansion (NTE) in the same material. Inclusion of extra-framework K(+) counterions has minimal effect on framework flexibility (and hence the magnitude of NTE/NLC) but selectively frustrates the soft phonon modes responsible for destroying NLC in the related material Ag(3)[Co(CN)(6)].

Probing high-pressure phase transitions in Ti-based perovskite-type ferroelectrics using visible resonance Raman spectroscopy.

We report unprecedented dramatic changes in the 647.1 nm Raman signal of PbZr(0.6)Ti(0.4)O(3) occurring in the same pressure ranges as the critical pressures of the antiferrodistortive and ferroelectric-paraelectric phase transitions. This huge decrease in intensity of both the Raman modes and the background, observed for both pressure transmitting media used (glycerol or 4:1 methanol ethanol mixture), is shown to originate from the two-step loss of a resonance Raman effect and the concomitant fluorescence. Changes in the local titanium environment (first with the onset of octahedral tilting and then with the removal of polar cation displacements) alter the electronic band structure and modify the resonance conditions. Furthermore, the optimal resonance conditions are found to be particularly narrow, as shown by the fluorescence spectrum of PbZr(0.6)Ti(0.4)O(3) at atmospheric pressure characterized by the presence of a very well-defined sharp peak (fwhm = 8 nm) centered around 647.1 nm. These results thus demonstrate that visible resonance Raman spectroscopy can be used as a quick and efficient technique for probing phase transitions in PbZr(1-x)Ti(x)O(3) (PZT) and other technologically important perovskite-type materials such as PMN-xPT, PZN-xPT relaxors, lead free piezoelectrics, and ferroelectric nanopowders. This technique appears also a good alternative to UV Raman spectroscopy for probing the polar order at the nanoscale in ultrathinfilms and superlattices.

Study of Ga(3+)-induced hydrothermal crystallization of an α-quartz type Ga(1-x)Fe(x)PO4 single crystal by in situ X-ray absorption spectroscopy (XAS).

The dissolution of α-FePO(4) and the α-Ga(0.75)Fe(0.25)PO(4) solid solution with α-quartz-type structures under hydrothermal conditions in 1 M HNO(3) aqueous solution was investigated by in situ X-ray absorption spectroscopy (XAS) at the Fe K-edge. The solubility of α-FePO(4) increases with temperature and is higher at 25 MPa than at 50 MPa. The Fe(3+) cation in solution is 6-fold coordinated with an average Fe-O distance close to 2.0 Å. A similar experiment was performed with a solid solution of α-quartz-type Ga(0.75)Fe(0.25)PO(4) as the starting phase under a pressure of 25 MPa. By varying the temperature from 303 K up to 573 K a single crystal was grown with 23% Fe(3+) with the α-quartz-type structure. These results show that the crystallization of pure α-quartz-type FePO(4) by the hydrothermal method is not possible due to the formation of very stable Fe(3+) hexa-aquo complexes [Fe(H(2)O)(6)](3+) and to the absence of FeO(4) tetrahedra in solution. Ga(3+) cations in solution induce the formation of gallophosphate complexes at the solid-liquid interface, which are at the origin of the nuclei for crystallization. We propose a crystallization mechanism in which the Fe(3+) substitutes Ga(3+) with a 4-fold coordination in mixed (iron/gallo)-phosphate complexes that leads to the growth of an α-quartz-type Ga(0.77)Fe(0.23)PO(4) single crystal.