A CO₂ laser heating system for in situ high pressure-temperature experiments at HPCAT

We present a CO₂ laser heating setup for synchrotron x-ray diffraction inside a diamond anvil cell, situated at HPCAT (Sector 16, Advanced Photon Source, Argonne National Lab, Illinois, USA), which is modular and portable between the HPCAT experiment hutches. The system allows direct laser heating of wide bandgap insulating materials to thousands of degrees at static high pressures up to the Mbar regime. Alignment of the focused CO₂ laser spot is performed using a mid-infrared microscope, which addressed past difficulties with aligning the invisible radiation. The implementation of the mid-infrared microscope alongside a mirror pinhole spatial filter system allows precise alignment of the heating laser spot and optical pyrometry measurement location to the x-ray probe. A comparatively large heating spot (∼50 μm) relative to the x-ray beam (<10 μm) reduces the risk of temperature gradients across the probed area. Each component of the heating system and its diagnostics have been designed with portability in mind and compatibility with the various experimental hutches at the HPCAT beamlines. We present measurements on ZrO₂ at 5.5 GPa which demonstrate the improved room-temperature diffraction data quality afforded by annealing with the CO₂ laser. We also present in situ measurements at 5.5 GPa up to 2800 K in which we do not observe the postulated fluorite ZrO₂ structure, in agreement with recent findings.