We have fabricated novel controlled‐geometry samples for the laser‐heated diamond‐anvil cell (LHDAC) in which a transparent oxide layer (SiO2) is sandwiched between two laser‐absorbing layers (Ni) in a single, cohesive sample. The samples were mass manufactured (>104 samples) using a combination of physical vapor deposition, photolithography, and wet and plasma etching. The double hot‐plate arrangement of the samples, coupled with the chemical and spatial homogeneity of the laser‐absorbing layers, addresses problems of spatial temperature heterogeneities encountered in previous studies where simple mechanical mixtures of transparent and opaque materials were used. Here we report thermal equations of state (EOS) for nickel to 100 GPa and 3000 K and stishovite to 50 GPa and 2400 K obtained using the LHDAC and in situ synchrotron X‐ray microdiffraction. We discuss the inner core composition and the stagnation of subducted slabs in the mantle based on our refined thermal EOS.