Laser heating system at the Extreme Conditions Beamline, P02.2, PETRA III

A laser heating system for samples confined in diamond anvil cells paired with in situ X-ray diffraction measurements at the Extreme Conditions Beamline of PETRA III is presented. The system features two independent laser configurations (on-axis and off-axis of the X-ray path) allowing for a broad range of experiments using different designs of diamond anvil cells. The power of the continuous laser source can be modulated for use in various pulsed laser heating or flash heating applications. An example of such an application is illustrated here on the melting curve of iron at megabar pressures. The optical path of the spectroradiometry measurements is simulated with ray-tracing methods in order to assess the level of present aberrations in the system and the results are compared with other systems, that are using simpler lens optics. Based on the ray-tracing the choice of the first achromatic lens and other aspects for accurate temperature measurements are evaluated.

Melting line of calcium characterized by in situ LH-DAC XRD and first-principles calculations

In this work, the melting line of calcium has been characterized both experimentally, using synchrotron X-ray diffraction in laser-heated diamond-anvil cells, and theoretically, using first-principles calculations. In the investigated pressure and temperature range (pressure between 10 and 40 GPa and temperature between 300 and 3000 K) it was possible to observe the face-centred phase of calcium and to confirm (and characterize for the first time at these conditions) the presence of the body-centred cubic and the simple cubic phase of calcium. The melting points obtained with the two techniques are in excellent agreement. Furthermore, the present results agree with the only existing melting line of calcium obtained in laser-heated diamond anvil cells, using the speckle method as melting detection technique. They also confirm a flat slope of the melting line in the pressure range between 10 and 30 GPa. The flat melting curve is associated with the presence of the solid high-temperature body-centered cubic phase of calcium and to a small volume change between this phase and the liquid at melting. Reasons for the stabilization of the body-centered face at high-temperature conditions will be discussed.