Figure 1

(Color online) Top: The composite incommensurate structure of Rb-IV, as viewed down the $c$ axis. The upper part shows the tetragonal 16-atom host framework (in yellow) and the guest species (in gray) (Refs. 4, 7). The host atoms form columns of square antiprisms. The perspective lower drawing shows the body-centered tetragonal guest substructure (Ref. 7). Crystallographic axes are labeled.Reuse & Permissions

Figure 2

Integrated 1D diffraction profile from K-III at $22.1\mathrm{GPa}$, with the strongest peaks indexed on the tetragonal cell of Winzenick et al. (Ref. 17). For comparison, the inset shows a diffraction profile from Rb-IV at $19.3\mathrm{GPa}$, with the strongest peaks indexed on the tetragonal host unit cell. The arrows in the K-III profile identify those reflections not accounted for by the tetragonal cell of Winzenick et al. (Ref. 17).Reuse & Permissions

Figure 3

Rietveld refinement of K-III at $22.1\mathrm{GPa}$ in superspace group $I4∕mcm\left(00\gamma \right)0000$. The tick marks below the profile show the peak positions of (upper) the host and host/guest (Ref. 30) reflections, $hkl0$ and $hk00$, respectively, and (lower) the $hk0m$ guest-only reflections. Indices are given for some of the strongest reflections of each type. Below the tick marks is the difference between the observed and calculated profiles. The inset shows a perspective view of the K-III guest structure with crystallographic axes marked.Reuse & Permissions

Figure 4

Pressure dependence of the ratio $\gamma =c_H∕c_G$ in K-III to $24.9\mathrm{GPa}$. Filled symbols show data collected on pressure increase; unfilled symbols show data from another sample collected on pressure decrease. The commensurate value of $8∕5$ for $c_H∕c_G$ is marked. For comparison, the inset shows the pressure dependence of the $c_H∕c_G$ ratio in the composite structure of Rb-IV (from Ref. 7).Reuse & Permissions

Figure 5

Pressure dependence of the atomic volume of potassium to $25\mathrm{GPa}$. The data for K-I (bcc, filled circles) and K-II (fcc, filled triangles) (Ref. 34) have been fitted to a single equation of state, which is plotted as a solid line. Data for K-III collected on pressure decrease are shown using unfilled symbols. The volume per atom is calculated using the number of atoms in a volume, $V_H$, corresponding to that of a unit cell of the host substructure, as $V_H∕(16+2\times \gamma )$. The inset shows the pressure dependence of the ratio $c∕a$ of the K-III host substructure. In both plots, the estimated standard deviations are smaller than the symbols used to plot the points.Reuse & Permissions