Effect of laser annealing of pressure gradients in a diamond-anvil cell using common solid pressure media

Pressure media are one of the most effective deterrents of pressure gradients in diamond-anvil cell (DAC) experiments. The media, however, become less effective with increasing pressure, particularly for solid pressure media. One of the most popular ways of alleviating the increase in pressure gradients in DAC samples is through laser annealing of the sample. We explore the effectiveness of this technique for six common solid pressure media that include: alkali metal halides LiF, NaCl, KCl, CsCl, KBr, as well as amorphous SiO2. Pressure gradients are determined through the analysis of the first-order diamond Raman band across the sample before and after annealing the sample with a near-infrared laser to temperatures between ~2000 and 3000 K. As expected, we find that in the absence of sample chamber geometrical changes and diamond anvil damage, laser annealing reduces pressure gradients, albeit to varying amounts. We find that under ideal conditions, NaCl provides the best deterrent to pressure gradients before and after laser annealing, at least up to pressures of 60 GPa and temperatures between ~2000 and 3000 K. Amorphous SiO2, on the other hand, transforms in to harder crystalline stishovite upon laser annealing at high pressures resulting in increased pressure gradients upon further compression without laser annealing.