Probing ferroelectricity by X-ray absorption spectroscopy in molecular crystals

We carry out X-ray absorption spectroscopy experiment at the oxygen K edge in croconic acid (C5H2O5) crystal as a prototype of ferroelectric organic molecular solid, whose electric polarization is generated by proton transfer. The experimental spectrum is well reproduced by the electron-hole excitation theory simulations from configuration generated by ab initio molecular dynamics simulation. When inversion symmetry is broken in the ferroelectric state, the hydrogen bonding environment on the two bonded molecules become inequivalent. Such a difference is sensitively probed by the bound excitation in the pre-edge, which is strongly localized on the excited molecules. Our analysis shows that a satellite peak in the pre-edge will emerge at higher excitation energy, which serves as a clear signature of ferroelectricity in the material

Water adsorption on and desorption from crystalline copolymers of vinylidene fluoride with trifluoroethylene

Water adsorption and absorption on crystalline polyvinylidene fluoride with 30% trifluoroethylene, P(VDF-TrFE, 70:30), was examined by thermal desorption spectroscopy. Two distinctly different water adsorption sites are identified: one adsorbed species that resembles ice and another species that interacts more strongly with the polymer thin film. The existence of the latter species is consistent with X-ray diffraction studies of water absorbed into the bulk of copolymers of polyvinylidene fluoride with trifluoroethylene crystalline thin films. There are strong steric effects observed in the angle-resolved thermal desorption that may be a result of the large polymer thin film surface dipoles

The elimination of the influence of ambient environmental effects on the structure of 'inert' polymers

The construction and use of a vacuum chamber suitable for conventional X-ray diffraction has revealed the influence of the ambient environment, including moisture, on the bulk structure of very thin polymer films. It is concluded that studies of thin film organic systems, even those thought not to be perturbed by ambient water vapor and other contaminants, may benefit from undertaking some studies in such small sample vacuum chambers

The Electronic Structure Signature of the Spin Cross-Over Transition of [Co(dpzca)2]

The unoccupied electronic structure of the spin crossover molecule cobalt (II) N-(2-pyrazylcarbonyl)-2-pyrazinecarboxamide, [Co(dpzca) ] was investigated, using X-ray absorption spectroscopy (XAS) and compared with magnetometry (SQUID) measurements. The temperature dependence of the XAS and molecular magnetic susceptibility χ T are in general agreement for [Co(dpzca) ], and consistent with density functional theory (DFT). This agreement of magnetic susceptibility and X-ray absorption spectroscopy provides strong evidence that the changes in magnetic moment can be ascribed to changes in electronic structure. Calculations show the choice of Coulomb correlation energy U has a profound effect on the electronic structure of the low spin state, but has little influence on the electronic structure of the high spin state. In the temperature dependence of the XAS, there is also evidence of an X-ray induced excited state trapping for [Co(dpzca) ] at 15 K. 2 m 2

The Electronic Structure Signature of the Spin Cross-Over Transition of [Co(dpzca)2]

The unoccupied electronic structure of the spin crossover molecule cobalt (II) N-(2-pyrazylcarbonyl)-2-pyrazinecarboxamide, [Co(dpzca)2] was investigated, using X-ray absorption spectroscopy (XAS) and compared with magnetometry (SQUID) measurements. The temperature dependence of the XAS and molecular magnetic susceptibility χmT are in general agreement for [Co(dpzca)2], and consistent with density functional theory (DFT). This agreement of magnetic susceptibility and X-ray absorption spectroscopy provides strong evidence that the changes in magnetic moment can be ascribed to changes in electronic structure. Calculations show the choice of Coulomb correlation energy U has a profound effect on the electronic structure of the low spin state, but has little influence on the electronic structure of the high spin state. In the temperature dependence of the XAS, there is also evidence of an X-ray induced excited state trapping for [Co(dpzca)2] at 15 K

Evidence for multiple polytypes of semiconducting boron carbide (C2B10) from electronic structure

Boron carbides fabricated via plasma enhanced chemical vapour deposition from different isomeric source compounds with the same C2B10H12 closo-icosahedral structure result in materials with very different direct (optical) band gaps. This provides compelling evidence for the existence of multiple polytypes of C2B10 boron carbide and is consistent with electron diffraction results

Double cation formation from the Photo-Fragmentation of the closo-Carboranes

Time-of-flight mass analysis with multi-stop coincidence detection has been used to study the multi-cation ionic fragmentation via fission of three isomeric carborane icosahedral cage compounds closo-1,2-orthocarborane, closo-1,7-metacarborane, closo-1,12-paracarborane (C2B10H12) following inner-shell excitation at or above the B is regime. Photoelectron-photoion-photoion coincidence (PEPIPICO) spectroscopy was used to study the dominant fission routes in the core level excitation regime. Series of ion pairs are identified, where asymmetric fission dominates, leading to ion pairs of different mass. The fragmentation yields and charge separation mass spectra of all three isomers are generally quite similar in that the ion pairs (H+, Y+), Y-11(+)), and (Y-3(+), Y-9(+)) dominate, where denotes the mass region from B+ to CH. Slight differences are observed at the B 1s-threshold, where the H+ and BH2+/CH+ ion pairs dominate for ortho-and metacarborane, while (Y+, Y-11(+)) ion pairs dominate the multi-photofragment ion yield of paracarborane. These similarities and distinct differences in charge separation are discussed by considering the energetics of these three major species of ion pairs, as well as charge distributions in closo-carboranes and charge distributions in the carborane cage. It is shown that product formation via charge separation is driven by electronic relaxation, so that the lowest energy products are not necessarily formed

Probing ferroelectricity by X-ray absorption spectroscopy in molecular crystals

We carry out X-ray absorption spectroscopy experiment at the oxygen K edge in croconic acid (C5H2O5) crystal as a prototype of ferroelectric organic molecular solid, whose electric polarization is generated by proton transfer. The experimental spectrum is well reproduced by the electron-hole excitation theory simulations from configuration generated by ab initio molecular dynamics simulation. When inversion symmetry is broken in the ferroelectric state, the hydrogen bonding environment on the two bonded molecules become inequivalent. Such a difference is sensitively probed by the bound excitation in the pre-edge, which is strongly localized on the excited molecules. Our analysis shows that a satellite peak in the pre-edge will emerge at higher excitation energy, which serves as a clear signature of ferroelectricity in the material