More than 90% of the matter in the solar system exists at pressures above 1 million atmospheres, pressure and temperature shape our planets and extraterrestrial bodies and turn everyday liquids and gases into condensed solids, unexpected molecular compounds or exotic metals. Conditions must be considered to as extreme with respect to the ambient conditions being exposed as human being. The techniques of structural crystallography are the tools of choice in order to directly visualize the effects and externally induced changes, and apart from computational approaches it is the experimental investigation that provides insight to the limiting factors of stability of a phase and to the mechanisms of transformation induced by changing exterior conditions.
The technique of single-crystal diffraction receives more and more attention being implemented for in-situ crystallographic research at non-ambient conditions. Aiming for up and up higher pressures and temperatures, the available sample size are restricted to a few crystallites or in many cases to just a single crystal, which can exposed to monitored (multi)megabar pressures and several thousands of Kelvin on using laser-heating techniques. Implementations of synchrotron micro-beam techniques and the use of new detectors with short acquisition times allow even to develop in-situ measurements, which are resolved in space and time. In this context the newest advancements in high-pressure crystallography will be reported, which include the experience with new hybrid pixel detectors at synchrotron beamlines and in-house diffractometers. Within the scope of time-optimized experiments the possibilities of in-situ high-pressure-high-temperature single-crystal XRD will be shown, including the drawbacks and experimental challenges one has to consider working with HPHT devices on experimental operation of our-circle diffractometers.
The results of most recent in-situ investigations of the structural behavior, phase transformations and transformation mechanisms will be exemplified by experimental investigations e.g. on the carbonate phases. Moreover insight to future attempts as of using nano-polycrystalline diamond anvils for high-quality intensity data collection at high-pressure conditions will provide some outlook to upcoming developments and new challenges on the experimental pathway going to extremes in structural crystallography.