Ionic conduction in one dimension: A structural study of the hollandite K1.54Mg0.77Ti7.23O16 over the range 133≤T≤919 K

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 Å−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