The wustite enigma

High-pressure energy dispersive X-ray diffraction of wustite has been obtained with three types of diamond cells; each one is designed to optimize a type of in situ study, namely single-crystal X-ray diffraction, deviatoric strain measurement, or simultaneous high-P-T experimentation. The results demonstrate that above 17 GPa at 300 K wustite undergoes a displacive transition from B1 to a rhombohedral structure, and above 90 GPa at 600 K, a second transition to the NiAs structure. Many of the earlier inconsistencies concerning the structure and properties of wiistite at high pressures can be attributed to the extreme softening of the C-44 elastic modulus at pressures above 10 GPa.