HIGH-PRESSURE RAMAN AND OPTICAL-ABSORPTION STUDIES ON LEAD PYRONIOBATE (PB2NB2O7) AND PRESSURE-INDUCED PHASE-TRANSITIONS

High-pressure Raman scattering and optical absorption studies have been carried out on lead pyroniobate (Pb2Nb2O7) up to 33 GPa, using a gasketed diamond anvil cell. The Raman study reveals the occurrence of two, possibly three, pressure-induced phase changes; a rather subtle change is indicated near 4.5 GPa. The transition near 13 GPa is attributed to a structural transition from the rhombohedral to the cubic pyrochlore structure. The third phase change occurs near 20 GPa. From the broad Raman feature that is observed at about 800 cm-1, it is concluded that the system turns amorphous at pressures above 20 GPa. The amorphous phase recrystallizes to the original rhombohedral phase, on release of pressure. The broad Raman peaks of the recrystallized phase indicate a high degree of disorder in the material. Lead pyroniobate turns deep red near 30 GPa, from light yellow at ambient pressure. Semiquantitative absorption measurements show that the energy gap shifts red at a rate of 30 meV/GPa. This shift is attributed to the downward motion of the 5d (eg) conduction band of Pb. © 1990 American Chemical Society.