No abstract

We report the synthesis, structural characterization, and functionality (framework interconversions together with proton conductivity) of an open-framework hybrid that combines Ca(2+) ions and the rigid polyfunctional ligand 5-(dihydroxyphosphoryl)isophthalic acid (PiPhtA). Ca2[(HO3PC6H3COOH)2]2[(HO3PC6H3(COO)2H)(H2O)2]·5H2O (Ca-PiPhtA-I) is obtained by slow crystallization at ambient conditions from acidic (pH ≈ 3) aqueous solutions. It possesses a high water content (both Ca coordinated and in the lattice), and importantly, it exhibits water-filled 1D channels. At 75 °C, Ca-PiPhtA-I is partially dehydrated and exhibits a crystalline diffraction pattern that can be indexed in a monoclinic cell with parameters close to the pristine phase. Rietveld refinement was carried out for the sample heated at 75 °C, Ca-PiPhtA-II, using synchrotron powder X-ray diffraction data, which revealed the molecular formula Ca2[(HO3PC6H3COOH)2]2[(HO3PC6H3(COO)2H)(H2O)2]. All connectivity modes of the "parent" Ca-PiPhtA-I framework are retained in Ca-PiPhtA-II. Upon Ca-PiPhtA-I exposure to ammonia vapors (28% aqueous NH3) a new derivative is obtained (Ca-PiPhtA-NH3) containing 7 NH3 and 16 H2O molecules according to elemental and thermal analyses. Ca-PiPhtA-NH3 exhibits a complex X-ray diffraction pattern with peaks at 15.3 and 13.0 Å that suggest partial breaking and transformation of the parent pillared structure. Although detailed structural identification of Ca-PiPhtA-NH3 was not possible, due in part to nonequilibrium adsorption conditions and the lack of crystallinity, FT-IR spectra and DTA-TG analysis indicate profound structural changes compared to the pristine Ca-PiPhtA-I. At 98% RH and T = 24 °C, proton conductivity, σ, for Ca-PiPhtA-I is 5.7 × 10(-4) S·cm(-1). It increases to 1.3 × 10(-3) S·cm(-1) upon activation by preheating the sample at 40 °C for 2 h followed by water equilibration at room temperature under controlled conditions. Ca-PiPhtA-NH3 exhibits the highest proton conductivity, 6.6 × 10(-3) S·cm(-1), measured at 98% RH and T = 24 °C. Activation energies (Ea) for proton transfer in the above-mentioned frameworks range between 0.23 and 0.4 eV, typical of a Grothuss mechanism of proton conduction. These results underline the importance of internal H-bonding networks that, in turn, determine conductivity properties of hybrid materials. It is highlighted that new proton transfer pathways may be created by means of cavity "derivatization" with selected guest molecules resulting in improved proton conductivity.

Yeelimite, Ca 4 [Al 6 O 12 ]SO 4 , is outstanding as an aluminate sodalite, being the framework of these type of materials flexible and dependent on ion sizes and anion ordering/disordering. On the other hand, yeelimite is also important from an applied perspective as it is the most important phase in calcium sulfoaluminate cements. However, its crystal structure is not well studied. Here, we characterize the room temperature crystal structure of stoichiometric yeelimite through joint Rietveld refinement using neutron and X-ray powder diffraction data coupled with chemical soft-constraints. Our structural study shows that yeelimite has a lower symmetry than that of the previously-reported tetragonal system, which we establish to likely be the acentric orthorhombic space group Pcc2, with a √2a×√2a×a superstructure based on the cubic sodalite structure. Final unit cell values were a=13.0356(7) Å, b=13.0350(7) Å, and c=9.1677(2) Å. We determine several structures using density functional theory calculations, with the lowest energy structure being Pcc2 in agreement with our experimental result. Yeelimite undergoes a reversible phase transition to a higher-symmetry phase which has been characterized to occur at 470ºC by thermodiffractometry. The higher-symmetry phase is likely cubic or pseudo-cubic possessing an incommensurate superstructure, as suggested by our theoretical calculations which show a phase transition from an orthorhombic to a tetragonal structure. Our theoretical study also predicts a pressure-induced phase transition to a cubic structure of space group I43m. Finally, we show that our reported crystal structure of yeelimite enables better mineralogical phase analysis of commercial calcium sulfoaluminate cements, as shown by R F values for this phase, 6.9% and 4.8% for the previously published orthorhombic structure and for the one reported in this study, respectively.

Several oxy-apatite materials La 102x Sr x (TO 4 ) 6 O 320.5x (T = Ge, Si; 102x = 9.00, 8.80, 8.65 and 8.00) and La 9.33 (Si 12x Ge x O 4 ) 6 O 2 (x = 0, 0.5, 0.67) have been prepared as highly crystalline phases. The impedance study showed that all samples are oxide ion conductors. However, bulk conductivities changed by more than 2 orders of magnitude at a given temperature for some compositions. A thorough study on the oxygen sublattices for oxygen-stoichiometric oxy-apatites has been carried out. Constant-wavelength neutron powder diffraction data have been collected for La 9.33 (SiO 4 ) 6 O 2 . Time-of-flight neutron data have been collected for La 9.

The structures and electrical properties of four NASICON compositions, Na1.4M1.6In0.4(PO4)3 (M = Ti, Sn, Hf, Zr), have been determined and compared. Rietveld refinement of powder X-ray diffraction data confirmed the basic rhombohedral NASICON structure with random occupancy of the octahedral In/M sites, full occupancy of the Na(1) sites and partial occupancy of the Na(2) sites. For three compositions, M = Zr, Sn, and Hf, the 31P MAS NMR peak intensities of the four detected signals, attributed to four different phosphorus environments [P(OM)4 - n (OIn) n (n = 0−3)], were close to the ratios expected for a random distribution of In/M. For M = Ti, some departures from statistical occupancy were apparent. 23Na MAS NMR data gave evidence for two Na+ positions at room temperature for M = Ti, Sn, attributable to occupation of Na(1) and Na(2) sites. For M = Hf, Zr, only a single signal could be resolved at room temperature, which splits into two signals on cooling to − 50 °C, indicating high Na mobility at room temperature. Impedance data obtained on pressed sintered pellets over the range 25−300 °C showed that bulk ionic conductivities increased and activation energies decreased in the sequence Ti, Sn, Hf, Zr. The geometry of the M1M2 bottleneck has been determined from structural data, and a direct correlation found between activation energy for ion conduction and the bottleneck size.