The heavy alkali metals exhibit a rich variety of high-pressure phase transitions. The pressure-driven electronic s d transition is believed to be the driving force for destabilising the common high-symmetry crystal structures adopted near ambient conditions (bcc and fcc) with respect to lower-coordination structures. In particular at intermediate densities, both Rb and Cs adopt a body centred tetragonal structure, where the atoms are only eightfold coordinated. Solving the crystal structures of several other high-pressure modifications of the alkalis has remained a long-standing problem. Hereafter, we briefly summarise results of recent high-pressure structural studies of Rb. [1] Based on angle-dispersive synchrotron powder X-ray diffraction diagrams measured at the ESRF, we have solved the structure of Rb-IV which is found to be surprisingly complex.

Diffraction experiments were carried out at the beamline ID9 using diamond anvil pressure cells. The X-ray diffraction patterns were recorded at 300 K using a wavelength near 0.45 Å and an image plate detection system. Both the good angular resolution and the high sensitivity of the experimental setup at ID9 were essential for solving the crystal structures.

Figure 82 shows the integrated diffraction pattern of the phase Rb-IV at a pressure of 16.9 GPa. The pattern can be indexed on the basis of a cubic cell with ac ~ 10.35 Å. However, weak peak splittings observed at higher pressures indicate that the symmetry of the Rb-IV structure is in fact tetragonal. Solutions of the crystal structure were performed in the centrosymmetric space group I4/mcm. Application of direct methods reveals that one set of Rb atoms occupies the Wyckoff positions 16k (x,y,0; x ~ 0.79, y ~ 0.08). The resulting arrangement of Rb1 atoms (see Figure 83) consists of columns of face-sharing square antiprisms interconnected by short Rb1-Rb1 contacts (red lines in Figure 83). The closest separation between Rb1 atoms (3.04 Å at 16.9 GPa) corresponds to twice the ionic radius of Rb+ (1.52 Å).

The Rb1 framework hosts chains of a second set of Rb atoms as evidenced by electron density maxima in the difference Fourier map (see Figure 84). Maxima occur at the 8g (0.5,0,z) site. Thus, the Fourier map in combination with the maximum possible number of 20 atoms per unit cell ­ as inferred from atomic volumes of neighbouring phases ­ suggest an average Rb2 arrangement with statistical occupation of 8g sites and an occupation factor of 0.5. However, for this atomic arrangement, within the chains the average interatomic distance would drop to 2.6 Å, which is 15% smaller than the ionic radius of Rb and therefore difficult to accept.

We therefore conclude that the number of Rb2 atoms in chain sites is less than four per unit cell. A refinement, in which occupation factors of sites along the chains are treated as free parameters, converges to occupation numbers corresponding to an average chain atom separation of about 2.97 Å. We have not observed any supercell reflections which would indicate a commensurate ordering of the chain atoms with respect to the framework of Rb1 atoms. However, some diffraction patterns of Rb-IV show an extra reflection corresponding to a d-value of about 3.0 Å (marked by a triangle in Figure 82). If interpreted as a reflection arising from the intra-chain ordering, the absence of other additional reflections would indicate that chains are not correlated with respect to each other.

There is a quite surprising resemblance of the Rb-IV structure to the metal atom sublattice of the W5Si3-type structure. Furthermore, it has been pointed out earlier by Nesper and v. Schnering, that the Cs-IV-type structure, also adopted by the phase Rb-V, corresponds to the metal sublattice in the ThSi2-type structure. This leads to a more general concept, namely that at intermediate densities the alkali metals adopt crystal structures, which represent cation sublattices of binary intermetallics.

The results for Rb-IV demonstrate that the pressure-driven breakdown of the nearly-free electron character of a simple metal induces a phase transition to a rather complex structure. The present work partly closes the gap in our knowledge about the phase transition sequence in heavy alkali metals during the early stages of the s d transition. Furthermore, structure solutions for Rb-VI [2], Cs-V [3] and Cs-VI [4] are important steps for an understanding of the structural evolution of heavy alkali metals, when they are fully turned into monovalent d-transition metals by the application of pressure.

References
[1] U. Schwarz, A. Grzechnik, K. Syassen, I. Loa, M. Hanfland, Phys. Rev. Lett., 83, 4085 (1999).
[2] U. Schwarz, K. Syassen, A. Grzechnik, M. Hanfland, Solid State Commun., 112, 319 (1999).
[3] U. Schwarz, K. Takemura, M. Hanfland, K. Syassen, Phys. Rev. Lett., 81, 2711 (1998).
[4] K. Takemura, N.E. Christensen, D.L. Novikov, K. Syassen, U. Schwarz, M. Hanfland, Phys. Rev. B, submitted.

Authors
U. Schwarz (a,b), K. Syassen (a), M. Hanfland (c), A. Grzechnik (a), I. Loa (a), K. Takemura (d).

(a) MPI für Festkörperforschung, Stuttgart (Germany)
(b) MPI für Chemische Physik fester Stoffe, Dresden (Germany)
(c) ESRF
(d) NIRIM, Tsukuba (Japan)