Search

We present an in situ powder X-ray diffraction study on the phase stability and polymorphism of the metal-organic framework ZIF-4, Zn(imidazolate)2, at simultaneous high pressure and high temperature, up to 8 GPa and 600 °C. The resulting pressure-temperature phase diagram reveals four, previously unknown, high-pressure-high-temperature ZIF phases. The crystal structures of two new phases-ZIF-4-cp-II and ZIF-hPT-II-were solved by powder diffraction methods. The total energy of ZIF-4-cp-II was evaluated using density functional theory calculations and was found to lie in between that of ZIF-4 and the most thermodynamically stable polymorph, ZIF- zni. ZIF-hPT-II was found to possess a doubly interpenetrated diamondoid topology and is isostructural with previously reported Cd(Imidazolate)2 and Hg(Imidazolate)2 phases. This phase exhibited extreme resistance to both temperature and pressure. The other two new phases could be assigned with a unit cell and space group, although their structures remain unknown. The pressure-temperature phase diagram of ZIF-4 is strikingly complicated when compared with that of the previously investigated, closely related ZIF-62 and demonstrates the ability to traverse complex energy landscapes of metal-organic systems using the combined application of pressure and temperature.

Absolute measurements of the radiolytic yield of Fe3+ in a ferrous sulphate dosimeter formulation (6 mM Fe2+), with a 20 keV x-ray monoenergetic beam, are reported. Dose-rate suppression of the radiolytic yield was observed at dose rates lower than and different in nature to those previously reported with x-rays. We present evidence that this effect is most likely to be due to recombination of free radicals radiolytically produced from water. The method used to make these measurements is also new and it provides radiolytic yields which are directly traceable to the SI standards system. The data presented provides new and exacting tests of radiation chemistry codes.

21st IEEE ISAF, held Jointly with 11th ECAPD and 4th Conference on PFM and Nanoscale Phenomena in Polar Materials -- JUL 09-13, 2012 -- Univ Aveiro, Aveiro, PORTUGAL, WOS: 000313016400097, The solid solution between bismuth ferrite and lead titanate (xBiFeO(3)-(1-x)PbTiO3 or BFPT) possesses a morphotropic phase boundary (MPB) between the rhombohedral and tetragonal forms at x=0.7. It is of interest to investigate the influence of field-driven rhombohedral-tetragonal phase transitions across the MPB, to determine whether correctly oriented BFPT can provide both giant piezoelectric properties and significant magnetoelectric coupling. Here, we used the reactive templated grain growth (RTGG) technique to prepare crystallographically textured 0.6BiFeO(3)-0.4PbTiO(3) (60: 40 BFPT) ceramics. Both Aurivillius structure templates (Bi4Ti3O12 and PbBi4Ti4O15) and perovskite templates (BaTiO3 and SrTiO3) were used to prepare textured 60:40BFPT. Synchrotron radiation experiments were used to determine the degree of texture. A current data suggests that only BaTiO3 templates survive the sintering process, and other candidate template materials reacted with the 60: 40BFPT matrix at high temperature. In the case of SrTiO3, this reaction results in a low Curie temperature (T-c = 350 degrees C) due to the substitution of Sr2+. Aurivillius templates resulted in high Curie temperatures (610 degrees C) and may be chemically suitable if they could be stabilized during the sintering process. However, the resulting ceramics show low remanent polarization (P-r = 3 mu c/cm(2)), while SrTiO3 and BaTiO3 templated ceramics show higher remanent polarizations of 36 and 30 mu c/cm(2), respectively. Because of their high chemical stability in this system, BaTiO3 templates appear to be the best candidate for fabricating textured BFPT by the TGG method, displaying a high degree of crystallographic texture., IEEE, IEEE, Ultrason, Ferroelect & Frequency Control (UFFC) Soc

Ultrason., Ferroelectr. Freq. Control;(UFFC) Soc. Inst. Electr. Electron. Eng. (IEEE), 2012 21st IEEE Int. Symp. on Applications of Ferroelectrics Held with 11th IEEE European Conf. on Applications of Polar Dielectrics and 4th IEEE Int. Symp on Piezoresponse Force Microscopy and Nanoscale Phenomena in Polar Materials, ISAF/ECAPD/PFM -- 9 July 2012 through 13 July 2012 -- Aveiro -- 93333, The solid solution between bismuth ferrite and lead titanate (xBiFeO 3-(1-x)PbTiO 3 or BFPT) possesses a morphotropic phase boundary (MPB) between the rhombohedral and tetragonal forms at x=0.7. It is of interest to investigate the influence of field-driven rhombohedral-tetragonal phase transitions across the MPB, to determine whether correctly oriented BFPT can provide both giant piezoelectric properties and significant magnetoelectric coupling. Here, we used the reactive templated grain growth (RTGG) technique to prepare crystallographically textured 0.6BiFeO 3-0.4PbTiO 3 (60:40 BFPT) ceramics. Both Aurivillius structure templates (Bi 4Ti 3O 12 and PbBi 4Ti 4O 15) and perovskite templates (BaTiO 3 and SrTiO 3) were used to prepare textured 60:40BFPT. Synchrotron radiation experiments were used to determine the degree of texture. A current data suggests that only BaTiO 3 templates survive the sintering process, and other candidate template materials reacted with the 60:40BFPT matrix at high temperature. In the case of SrTiO 3, this reaction results in a low Curie temperature (T c = 350 °C) due to the substitution of Sr 2+. Aurivillius templates resulted in high Curie temperatures (610 °C) and may be chemically suitable if they could be stabilized during the sintering process. However, the resulting ceramics show low remanent polarization (P r = 3 µc/cm 2), while SrTiO 3 and BaTiO 3 templated ceramics show higher remanent polarizations of 36 and 30 µc/cm 2, respectively. Because of their high chemical stability in this system, BaTiO 3 templates appear to be the best candidate for fabricating textured BFPT by the TGG method, displaying a high degree of crystallographic texture.

Accurate electron density mapping is quite a common practice for crystals cooled at low temperature and accurately measured. This is not true for species under external perturbation, due to complicated experimental conditions. Studying molecular crystals in excited states is a challenge, Coppens(2009), and a purely experimental electron density mapping is not possible at present. So far, the same limitation affected molecular crystals at high pressure, with only few attempts to use theoretical multipoles to fit experimental data, Fabbiani (2011). Here we report on the first unconstrained multipolar model, refined for syn-l,6;8,13 biscarbonyl[14]annulene (BCA) at P=7.7 GPa. BCA was the subject of a low temperature data collection by Destro (1995). The molecule (close to C2v symmetry) has a fair aromaticity, but it progressively localizes double and single bonds as a function of pressure. At 7.7 GPa the geometrical distortion is quite evident and mirrored by the electron density. The experiment, carried out at Diamond Light Source, was possible combining: a) high energy (40 Kev) to overcome the resolution problems caused by diamond anvil cells and reduce absorption and extinction; b) microfocused beam (30 micron) to minimize spurious X-ray diffusion; c) two crystals in the DAC, to increase data coverage; d) sufficient pressure to quench atomic motion. The final agreement is obviously worse than what typically obtained at ambient pressure. However, the model is satisfactory because: a) the deformation density is sensible and in agreement with the calculated one; b) the distribution of residuals is normal and no significant error is evident. The study proves that aromatic molecules are more reactive when squeezed, in keeping with the recent theoretical study by Hoffmann et al. on benzene. The Figure shows the static deformation density of BCA in 3D, obtained from a multipolar model refined against the experimental structure factors.

Scientific reports 8(1), 4735 (2018). doi:10.1038/s41598-018-21813-z, Published by Nature Publishing Group, London