(b) T ′- La 4 Ni 3 O 8

1. High-pressure study on the crystal structure and the spin-state of Ni ions in the triple-layer T′-La4Ni3O8 John B. Goodenough, University of Texas at Austin, DMR 0904282 The T′-type nickelates with mixed-valence Ni1+/Ni2+ ions stabilized in a square-coplanar coordination are of particular interest because they have been argued theoretically as the direct analog of the high-Tc cuprates with Cu2+/Cu3+. One of such compounds, the triple-layer T′- La4Ni3O8,can be obtained via a low-temperature reduction of the corresponding Ruddlesden-Popper phase T-La4Ni3O10. Instead of a metallic or superconducting ground state, T′- La4Ni3O8 was found to undergo a sharp transition at Tt = 105 K into a low-temperature insulating state, which has been interpreted as a temperature-driven low-to-high spin-state transition of Ni1.33+ ions. A recent density-function-theory calculation predicted that a moderate pressure of 5 GPa can suppress the spin-state transition to the lowest temperature so as to achieve a metallic ground state. Our comprehensive high-pressure resistivity measurements by using different techniques covering a broad pressure range up to 50 GPa confirm that the transition at Tt can be suppressed by pressure. However, the predicted low-spin metallic state was not observed under pressure. This discrepancy was then resolved by our in-situ high-pressure structural study, which revealed that a new T†-type structure was stabilized under P > 21 GPa. The presence of isolated Ni sites with apical oxygen in the T′ structure leads to a variable range-hopping conductivity in the low-spin phase in which the transition temperature is suppressed under pressure. H2 flow 350°C,72h 295 K 21 GPa (c) T†- La4Ni3O8 (b) T′- La4Ni3O8 • T/O- • La4Ni3O10

2. High-pressure study on the crystal structure and the spin-state of Ni ions in the triple-layer T′-La4Ni3O8 John B. Goodenough, University of Texas at Austin, DMR 0904282 Education and Outreach. This program provides a comprehensive training to graduate and postdoc students in high-pressure techniques for structural and physical-property characterizations, which is an important aspect in establishing the correlation between the physical properties and the crystal structure of transition-metal oxides. These techniques include the high-pressure structural determination with a gas-loading diamond anvil cell performed with high-energy synchrotron radiation, and high-pressure resistivity measurements with a self-clamped BeCu piston-cylinder cell up to 2 GPa, a cubic anvil cell up to 8 GPa, and a diamond anvil cell up to 50 GPa. Students also have opportunities to access the domestic synchrotron source and the state-of-the-art high-pressure facilities in laboratories around the world. Results from this laboratory and collaboration with other groups have led to publications in high impact Journals like Physical Review Letters and Physical Review B. The HPCAT 16BM-D beam line in Advance Photon Source (APS), Argonne National Lab is dedicated to high-pressure research, where we have carried out the in-situ angle-dispersive synchrotron X-ray powder diffraction under pressure to reveal the pressure-induced T′-T† structural transition of La4Ni3O8.