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- Edvardsson, Sverker, et al.
(author)
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A study of vibrational modes in Na(+)beta-Alumina by molecular-dynamics simulation
- 1994
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In: JOURNAL OF PHYSICS - CONDENSED MATTER. - 0953-8984 .- 1361-648X. ; 6:7, s. 1319-1332
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Journal article (peer-reviewed)abstract
- The vibrational properties of crystalline Na+ beta-alumina (Na1.22Al11O17.11) have been studied using the molecular dynamics simulation technique. The vibrational density of states was calculated from the velocity autocorrelation function, and the infrared spectrum from the dipole-dipole autocorrelation function. Knowledge of the vibrations in different crystallographic directions for the different atomic species facilitates the assignment of spectral peaks. The sodium in-plane vibrations are 59, 88 and 112 cm-1, and the out-of-plane vibrations are at 146 cm-1. The stoichiometric compound is also studied, and in this case the sodium in-plane vibrations are at 80 cm-1 and the out-of-plane vibrations at 140 cm-1. The density of states is used to calculate thermodynamic properties: heat capacity, entropy and internal and free energy. The values obtained at 300 K are C(upsilon) = 410 J K-1 mol-1, S(upsilon) = 300 J K-1 mol-1, U = 370 kJ mol-1 and F = 280 kJ mol 1. The heat capacity and entropy values are in good agreement with experiment, and thus strongly support the empirical force field used in the simulation
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| 4. |
- Edvardsson, Sverker, et al.
(author)
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Sensitivity of optical absorption intensities for rare-earth ions
- 1992
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In: Physical Review B. - 0163-1829. ; 45:19, s. 10918-10923
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Journal article (peer-reviewed)abstract
- A technique is presented that uses molecular-dynamics simulation to generate the atomic positions needed to calculate the crystal-field parameters used in a Judd-Ofelt (JO) analysis. A slightly modified theory is used that permits analysis of both pi- and sigma-polarized absorption spectra. Different temperatures and ranges of calculation (up to 8 angstrom) around one rare-earth ion are used in the simulation of our test compound, Nd2O3. It is shown that as many of the surrounding ions as possible should be included in the calculations, and also that it is possible to probe the sensitivity of the different transitions in rare-earth-ion spectra. It is further shown that the behavior of the so-called hypersensitive transition for a rare-earth ion is a result of the sensitivity of the JO parameter OMEGA-2, which is proportional to the square of the electric field created by the surrounding ions.
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| 5. |
- Wolf, Mats, et al.
(author)
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Molecular dynamics based analysis of the absorption of spectra of Nd3+-doped Na+ β’’-alumina.
- 1993
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In: Physical Review B. ; 48:14, s. 10129-10136
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Journal article (peer-reviewed)abstract
- The polarized absorption spectra of Nd3+-doped Na+ beta''-alumina have been treated theoretically for five neodymium concentrations. The perturbing crystal field, needed in the Judd-Ofelt analysis, is calculated from a molecular-dynamics simulation for each composition. Oscillator strengths and Judd-Ofelt intensity parameters are calculated using a point-charge model at each time step in the simulation for each of the Nd3+ ions in the simulation box. A study of the effect of the dynamics of the system on these parameters was also made. The theoretical results thus obtained are compared to experiment. It is shown that the absorption spectra can be explained essentially on the basis of the special structural situation of the Nd3+ ions in these materials.
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