[en] Nearly all displacive transitions have been considered to be continuous or second order, and the rigid unit mode (RUM) provides a natural candidate for the soft mode. However, in-situ X-ray diffraction and Raman measurements show clearly the first-order evidences for the scheelite-to-fergusonite displacive transition in BaWO₄: a 1.6% volume collapse, coexistence of phases, and hysteresis on release of pressure. Such first-order signatures are found to be the same as the soft modes in BaWO₄, which indicates the scheelite-to-fergusonite displacive phase transition hides a deeper physical mechanism. By the refinement of atomic displacement parameters, we further show that the first-order character of this phase transition stems from a coupling of large compression of soft BaO₈ polyhedrons to the small displacive distortion of rigid WO₄ tetrahedrons. Such a coupling will lead to a deeper physical insight in the phase transition of the common scheelite-structured compounds. (condensed matter: structural, mechanical, and thermal properties)

[en] By in situ x-ray diffraction, an isostructural phase transition between two kinds of the cubic PbCrO_3perovskites at around 1.6 GPa and room temperature with a 9.8% volume change is discovered. Recently, we have synthesized this cubic PbCrO_3perovskite successfully. Here we report our high-pressure in situ electrical resistance measurements up to 4.1 GPa for this perovskite sample. At room temperature, the resistance shows special changes at 1.2 and 2.7GPa. They may indicate the starting and ending points of this transformation. At 4.1 GPa, the negative temperature resistance coefficient is observed, which means that phase II could be considered as a semiconductor according to our present measurement