15 Search Results

The high proclivity of x rays to destabilize and distort molecular structures has been previously utilized in the synthesis of novel compounds. Here, we show that x-ray induced decomposition of cadmium oxalate induces chemical and structural transformations only at 0.5 and 1 GPa. Using x-ray diffraction and Raman spectroscopy, the synthesized product is identified as cadmium carbonate with cadmium oxalate remnants, which is stable under ambient conditions. At ambient and >1 GPa pressures, only degradation of the electronic density distribution is observed. The transformation kinetics are examined in terms of Avrami’s model, which demonstrates that despite the necessity of highmore » pressure for efficient x-ray induced synthesis of cadmium carbonate, the rate and geometry of structural synthesis in the 0.5–1 GPa pressure range do not depend on the applied pressure. In addition, the possible role of intermolecular distance and molecular mobility in transformation yield is also discussed. Our experimental results indicate that x-ray induced photochemical synthetic pathways can be modulated and optimized by specific parameter selection such as high pressure.« less

We examined the high pressure behavior of stannous oxalate via Raman and X-ray absorption spectroscopy (XAS) inside a diamond anvil cell. Phase transitions were observed to occur near 2.6 and 15 GPa which were reversible upon decompression to ambient conditions. When further pressurized above 15 GPa, the colorless material sustains irreversible chemical alterations and becomes bright red colored – darkening at higher pressures. Another irreversible phase transition occurred above 20 GPa. Concomitant with color change of the sample, we observed a softening of the ν(C–C) modes of the C₂O₄^2- anion via Raman spectroscopy. We performed a separate XAS experiment whichmore » indicates that the Sn²⁺ cation undergoes a partial reduction of the 2+ oxidation state with pressure which persists when the sample was depressurized to ambient conditions. Thus, electron density within the C–C bond in the oxalate anion appears to migrate toward the tin cation with pressure. This observation suggests that pressure can offer a very controllable means to vary cation–anion and unit cell dimensions (and thus the electric interactions causing electron movement) and thus the pressure-induced synthesis of novel materials.« less

The pressure-induced cubic→orthorhombic transition occurring in HgF₂ was investigated here via X-ray diffraction and density functional theory. The predicted, ambient cubic (Fm-3m) fluorite structure of HgF₂, can be obtained using a high-pressure ramp purification process of HgF₂ mixed with XeF₂. Subsequently, the purified HgF₂ underwent a phase transition from the fluorite to the orthorhombic cotunnite-type structure near 4.7 GPa, persisting to 63 GPa. An equation of state fit yielded a bulk modulus K₀ of 94.4 GPa for the fluorite-type structure and zero-pressure volume V₀ of 168.6 ų, and K₀ = 92.9 GPa and V₀ = 156.4 ų for the cotunnite-typemore » structure.« less

The response of solids to x-ray irradiation is not well understood, in part, because the interactions between x-rays and molecules in solids depend on the intra- and/or inter-molecular electronic properties of the material. Our previous work demonstrated that x-ray induced damage of certain ionic salts depends on the irradiating photon energy, especially when irradiated with photons of energy near the cation’s K-edge. To advance understanding of the cationic dependence of x-ray photochemistry, we present studies of x-ray induced damage of barium nitrate and strontium nitrate. Polycrystalline samples of barium and strontium nitrate were irradiated with high flux monochromatic synchrotron x-raysmore » at selected energies near the K-edge of the respective cations. The damage processes were studied with powder x-ray diffraction, and irradiation products, NO₂ and O₂, were characterized via Raman spectroscopy. Our results demonstrate that irradiating barium and strontium nitrate with photons of energy greater than the K-edge of the cation promotes high rate of decomposition compared to that observed when irradiating with photons of energy below the K-edge. Additionally, differences in x-ray induced damage between the two compounds are examined and discussed, and evidence of the diffusion of irradiation products is presented.« less

In this work, the synthesis and characterization of a novel, low cost, amorphous wide-bandgap semiconductor via X-ray induced decomposition of strontium oxalate at high pressure have been demonstrated. By means of IR spectroscopy, the final product is identified as a mixture of strontium carbonate, strontium oxalate and CO-derived materials. Band gap measurements demonstrate that the final product exhibits a much lower band gap (2.45 eV) than the initial strontium oxalate powder (4.07 eV), suggesting that the synthesized material can be highly useful in electronic and optical applications.

A novel structural and chemical synthesis of Cs–O derived materials has been demonstratedviaX-ray irradiation of Cs₂C₂H₂O₅at high pressure.

A novel structural and chemical synthesis of Cs–O derived materials has been demonstrated via X-ray irradiation of Cs ₂ C ₂ H ₂ O ₅ at high pressure.

The synthesis of a Sr-based wide bandgap semiconductor via X-ray irradiation of strontium oxalate at high pressure has been demonstrated.

We report measurements of the X-ray-induced decomposition of crystalline strontium oxalate (SrC2O4) as a function of energy and high pressure in two separate experiments. SrC2O4 at ambient conditions was irradiated with monochromatic synchrotron X-rays ranging in energy from 15 to 28 keV. A broad resonance of the decomposition yield was observed with a clear maximum when irradiating with ~20 keV X-rays and ambient pressure. Little or no decomposition was observed at 15 keV, which is below the Sr K-shell energy of 16.12 keV, suggesting that excitation of core electrons may play an important role in the destabilization of the C2O42–more » anion. A second experiment was performed to investigate the high-pressure dependence of the X-ray-induced decomposition of strontium oxalate at fixed energy. SrC2O4 was compressed in a diamond anvil cell (DAC) in the pressure range from 0 to 7.6 GPa with 1 GPa increments and irradiated in situ with 20 keV X-rays. A marked pressure dependence of the decomposition yield of SrC2O4 was observed with a decomposition yield maximum at around 1 GPa, suggesting that different crystal structures of the material play an important role in the decomposition process. This may be due in part to a phase transition observed near this pressure.« less

Two experiments are reported on KClO₄ at extreme conditions. A static high pressure Raman study was first conducted to 18.9 GPa. Evidence for at least two new phases was observed: one between 2.4 and 7.7 GPa (possibly sluggish), and the second near 11.7 GPa. Then, the X-ray induced decomposition rate of potassium perchlorate (KClO₄ ^hv→ KCl + 2O₂) was studied up to 15.2 GPa. The time-dependent growth of KCl and O₂ was monitored. The decomposition rate slowed at higher pressures. As a result, we present the first direct evidence for O₂ crystallization at higher pressures, demonstrating that O₂ molecules aggregatemore » at high pressure.« less