South Africa: Gifted Education in South Africa

Neethling, Kobus

doi:10.1177/107621758500800402

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“…However, for lower dopant range 0 mol% x 20 mol% PbF 2 , no fast-ion conduction is predicted at T = 700 K, indicated by a constant variation of mean-square displacements with time t (see figure 3(b)). This observation indicates that the transition temperatures for samples x = 0 mol% PbF 2 and x = 20 mol% PbF 2 are well above T = 700 K, a result which is consistent with our previous observation on the transition temperature (T c ) from a nonconducting to a conducting state of pure CdF 2 [9]. Hence, substantial fluorine diffusion in mixed-cation superionic fluorites takes place below the transition temperature of pure CdF 2 , which is about 1000 K. In contrast, cations (both Pb 2+ and Cd 2+ ion species) do not contribute to diffusion even at 700 K (not shown here) in all samples, indicating that the simulated material is below its melting point.…”

Section: Resultssupporting

confidence: 93%

Spectroscopic and computer modelling studies of mixed-cation superionic fluorites

Netshisaulu¹,

Chadwick²,

Ngoepe³

et al. 2005

J. Phys.: Condens. Matter

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Mixed-cation superionic fluorites are exceptionally good F − ion conductors due to the mixed nature of the cation sublattice. In this study, we have used extended x-ray absorption fine structure (EXAFS) spectroscopy to gain insights into the local environments of the Cd and Pb cations (as a function of composition and temperature) in CdF 2 (xPbF 2 ) mixed-cation superionic fluorites. A high degree of disorder is shown around both cations. However, the extent of disorder is even larger around the Pb 2+ in the entire series of solid solutions, with the highest disorder noted for the x = 60 mol% PbF 2 sample. The large values of the Debye-Waller factors of the Pb-F interactions are consistent with the low amplitudes of their Fourier transforms. This can be correlated with relative ionic conductivities of each composition in these materials. Molecular dynamics (MD) has been used as a complementary technique on structural properties; systems were modelled at temperatures ranging from liquid nitrogen temperature, 77 K, to 300 K. Our MD results confirm that disorder is greatest at concentrations x ≈ 60 mol% PbF 2 .

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Section: Resultssupporting

confidence: 93%