Alternating-current internally resistive heating in diamond anvil cells

Internally resistance-heated diamond anvil cell (IHDAC) enables long-duration, thermally stable experiments at high pressure and high temperature conditions and is, therefore, well suited for physical property measurements implicating the Earth's deep interior. A potential limitation of conventional direct current (DC) operation is the formation of a steady electric potential gradient within the sample, which can induce electromigration and thereby drive time-dependent compositional changes during heating. To address this issue, here we introduce alternating current (AC) operation in IHDAC experiments. We demonstrate the influence of constant voltage bias by comparing DC- and AC-IHDAC for Fe-H and Fe-C alloys. In the Fe-H system, ramp laser heating under constant voltage bias produces a reduction in lattice volume, consistent with substantial hydrogen loss. In contrast, 1-kHz AC-internal heating does not show apparent hydrogen depletion up to ∼2350 K and allows stable tracking of the hcp-(hcp + fcc)-fcc transition, consistent with reported phase relations. For Fe-4 wt. % C, cross-sectional chemical analyses of recovered samples reveal carbon inhomogeneity after constant-voltage-biased heating, whereas AC heating yields uniform Fe and C distributions. These results demonstrate that AC-IHDAC can effectively reduce the net electrical bias in IHDAC experiments and suppress electromigration-driven redistribution of light elements, thereby improving chemical stability in high-P-T measurements on Fe-light element alloys.