ISSN:
1089-7690
Source:
AIP Digital Archive
Topics:
Physics
,
Chemistry and Pharmacology
Notes:
Joint probability density functions (pdf) of the mobile K atom (and their corresponding pseudopotentials) have been obtained from seven x-ray (MoKα) intensity data sets collected on a one-dimensional conductor with the hollandite structure (K1.54Mg0.77Ti7.23O16; space group I4/m; Z=1). The data were collected at high resolution [(sin θ/λ)max ≥0.9 A(ring)−1] and at several temperatures (133, 298, 387, 459, 540, 707, and 919 K) in order to determine the conduction mechanism. Two models of the K distribution within the strand of cavities making up the conduction tunnel have been refined. In the most complex model B [Rw(F) =0.02], up to four atomic positions are needed to describe the observed electron density. The pdfs show that—already at room temperature and within undamaged segments of the conduction tunnel—potassium ions diffuse at a high rate across the bottleneck between two neighboring cavities. Below about 430 K the potassium ions preferentially occupy sites shifted off the cavity center in the direction of the bottleneck; above this temperature the cavity center becomes the preferred location. At 298 K the potential barrier at the bottleneck amounts to 0.032(4) eV. In a plot of the cell constants vs T an inflection shows up at 387〈T〈459 K, indicating a second order transition. Within the same temperature interval we also observe a sharp break in a plot of U33 (refined from the one-atom model A) vs T. A comparative study of two other hollandites K1.92V8O16 and (Ba0.98Ca0.03 Zr0.02) [Al1.1 Ni0.48 Ti6.4] with different tunnel stoichiometry underscores the importance of the values of ionic density and charge in the conduction column to the conductivity.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1063/1.451820
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