We conducted X-ray diffraction (XRD) measurements of a pyrolitic mantle material up to 4480 K at 122-166 GPa in a laser-heated diamond-anvil cell (DAC). Results demonstrate that the phase transition between bridgmanite and post-perovskite occurs in pyrolite within the lowermost mantle pressure range even at >4000 K. It suggests the ubiquitous occurrence of post-perovskite above the core-mantle boundary (CMB), which may be consistent with recent high-quality seismology data that non-observations of D” reflections are exceptions. Combining with earlier experiments performed at and below the normal lower-mantle geotherm, our data show that the bridgmanite + post-perovskite two-phase region is ~5 GPa thick and the Clapeyron slope of the boundary is +7(+2/-3) MPa/K in agreement with previous theoretical calculations. The global presence of rheologically weak post-perovskite at the bottom of the mantle has profound implications in seismology, geodynamics, and heat transfer from the core.