The first observation of a pressure-induced metallization accompanied by the spontaneous collapse of magnetic ordering is reported. This isostructural concurrence is observed in the layered antiferromagnetic insulator Nih. Electrical transport, x-ray-diffraction, and ,29 I Mossbauer studies were carried out in diamond-anvil cells. The evidence points to a mechanism by which the originally filled 5p-iodide band shifts and overlaps with the partially filled nickel-3d states. The formation of a hybridized conduction band at P > 19 GPa results in the onset of a diamagnetic metallic ground state.

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We report on the difference in microstructure of conventionally prepared YBa2(Cu[ "Fe")306+y (x=0.06 and 0.10) compared to materials that have been cooled under N2 (reducing atmosphere preparation) and subsequently reoxygenated near 350'C. The microstructure of conventionally prepared tetragonal materials shows a complex structure (with small-scale orthogonal striations). Materials prepared under reducing conditions, however, are orthorhombic and show a microstructure that is a 0 mixture of a twin structure and a structure with small-scale striations of order 100 A only along the [110] direction, of which about every sixth appears to be supporting a twin boundary. This can be related to Mossbauer evidence for the formation of extended Fe-0 clusters. We propose that the ordered striations in the Cu(1) plane are regions of Fe enrichments. This dilutes Fe from the matrix and leaves T, relatively unaffected, compared with the parent compound.