NOVEL CRYSTAL PHYSICS UNDER PRESSURE. Y. Fujii, Institute for Solid State Physics, The University of Tokyo, 7-22-1 Roppongi, Minato-ku, Tokyo 106, Japan

Application of pressure to a crystal provides a unique opportunity to study interatomic interactions controlling its crystal-, magnetic-, and electronic-structures. A crystal lattice shrunk by pressure causes a significant change in these interactions resulting in phase transitions through reconfiguration of electrons, atoms, and molecules. By following a brief overview of the state-of-the-art in "high pressure crystallography" highlighted in six consecutive Microsymposia, we present several novel crystal- and magnetic-phases stabilized under pressure which have been investigated structurally and dynamically by x-ray and neutron scattering techniques: (1) Superconducting Metallic Halogens - A molecular-to-monatomic transition preceeded by the gradual metallization takes place in elemental diatomic molecules I₂(21GPa), Br₂(80GPa), and IBr(39GPa). A scaling rule with respect to their crystal lattices holds upon metallization, resulting from electron delocalization process directly observable by a MEM method using reliable intensity data. Also observed in iodine are further successive phase transitions in its monatomic superconducting phase, ultimately leading to an fcc lattice stabilized above 55GPa. Pressure collapses molecules and creates exotic materials. [H. Fujihisa et al., J. Phys. Chem. Solids 56, 1439 (1995).] (2) Devil's Flower- Dielectric compounds, [N(CH₃)₄]₂MC_l4 (M=Zn,Fe,Mn) having frustrating inter-radical interactions, display a large number of high-order commensurate phases intervening in nominally incommensurate phase in its P-T phase diagram in shape of an infinite number of petals. Also observed is a universal phase diagram for these compounds. Pressure controls a delicate balance of interactions. [S. Shimomura et al., Submitted to Phys. Rev. B.] (3) Spin-Peierls System - An inorganic spin-Peierls compound CuGeO₃ with a one-dimensional chain of S=1/2 spins on Cu displays a remarkable pressure effect on its dimerized lattice and spin-gap resulting from a singlet ground state below 14K at atmospheric pressure. Pressure controls a system dimensionality. [M. Nishi et al., Phys. Rev. B52, 6959 (1995).]