Time and Frequency Resolved Hydrogen Dynamics in deuterated LiBH 4
- PDF / 462,874 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 57 Downloads / 161 Views
1216-W08-29
Time and frequency resolved hydrogen dynamics in deuterated LiBH4 Robin Gremaud1, Esben R. Andresen2, Anibal J. Ramirez Cuesta3, Keith Refson3, Zbigniew Łodziana1, Andreas Züttel1, Peter Hamm3, and Andreas Borgschulte1 1 Empa, Swiss Federal Laboratories for Materials Testing and Research, Hydrogen & Energy, Überlandstrasse 129, CH-8600 Dübendorf, Switzerland. 2 Physikalisch-Chemisches Institut, Universität Zürich, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland. 3 ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX, United Kingdom. ABSTRACT We study deuterated tetrahydroborate LiBH4 in the low-temperature (LT) phase, focusing on the B-H (B-D) stretching vibrations, by infrared (IR) and IR-pump-probe spectroscopy. Calculated infrared spectra by density functional theory allow us to disentangle the contributions coming from all the coexisting BH4-nDn (n = 0…4) molecular units. We report on anharmonicity and vibrational lifetimes of the B-H and B-D vibrations, as well as on the BH4 orientational relaxation by polarization anisotropy IR-pump-probe measurements. We find a constant anisotropy for at least 5 ps, confirming the low rotational relaxation and thermal disorder of the BH4 unit in the LT phase of this potential hydrogen storage material. INTRODUCTION LiBH4,[1] an ionic solid consisting of Li+ ions and BH- ions, has one of the highest known gravimetric hydrogen densities of 18 mass%, and is a promising candidate for hydrogen storage application. Despite considerable research effort, the conditions for hydrogen uptake and release by LiBH4 remain harsh.[2] Fundamental, microscopic studies of the BH bond and its interactions could yield valuable understanding to relate the microscopic to macroscopic parameters. Such studies are presently done by NMR [3-5], neutron [6,7], and optical spectroscopy [8-10]. Over the last decade, two-dimensional infrared (2D-IR) spectroscopy has been developed [11] which measures signal against two frequency axes (IR-pump and IR-probe [12-14]). As in linear spectroscopy, the (two-dimensional) lineshape in a 2D-IR spectrum contains information about the vibrational dynamics. It is, however, easier to deduce this information from a twodimensional lineshape, because of the extra frequency axis. In addition, 2D-IR spectroscopy directly provides information on anharmonicity, vibrational lifetimes, spectral diffusion, and energy exchange.[15] Here, we report on the measurement of polarization anisotropy [16] to conclude on the rotational relaxation process in LiBH4. THEORY The normal mode vibrational analysis is performed with ab initio lattice dynamics based on density-functional theory and the plane-wave pseudopotential method as implemented in the CASTEP code.[17,18] Raman tensors are calculated by numerical differentiation of polarizability tensors obtained using DFPT, and Raman intensities and activities were calculated using the formalism presented in Ref. [19]}. Infrared intensities were calculated by determination of the Born effective charge
Data Loading...
