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

Abstract

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.

Characterization of the high-pressure and high-temperature phase diagram and equation of state of chromium

“…This confirms the unreliability of the direct observation of movement on the sample surface (also known as the ‘speckle technique’) as melting diagnostic. In fact, there are elements for which the melting lines obtained by this technique and by in situ XRD show good agreement (e.g Ca 54 , Pt 18 , Al 55 and Cu 56 ). However, in most cases, the melting line obtained via the speckle technique underestimates the true melting line of the studied elements (e.g Fe 2 , Ni 38 , Ta 13 , Mo 16 , etc.…”