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 measurenents 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 stes 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.