Probing Residual Structural changes in Shock Compressed Cryptomalene and Goethite

Laser-driven flyer plates (LDFP) were employed to conduct shock compression in cryptomelane (K(Mn4+, Mn2+)8O16) and goethite (α-FeOOH). This work focuses on LDFP experiments with subsequent postmortem confocal Raman spectroscopy (RS), powder x-ray diffraction (XRD), and transmission electron microscopy (TEM), essentially aiming to capture residual effects after the shock unloaded. Each single-shot experiment used Al flyer plates for an impact speed of 3.5 km/s. Hugoniot calculation and impedance matching for flyer speed 3.5 km/s, cryptomelane was found to be shocked at ~36.8 GPa, and goethite was shocked at ~ 41.6 GPa. Confocal RS indicates that cryptomelane underwent possible structural changes due to 2 peaks at 570 cm^-1 and 641 cm^-1 inverting in addition to peak shifts attributed to changes in Mn-O bond lengths. Changes of intensity also occurred on the post shocked samples indicating irreversible phase transitions. Amorphous regions near the edges and surface of the cryptomelane and goethite were detected after examining the lattice fringes of each mineral. Electron diffraction patterns also identified a change in crystallinity in both minerals and can be observed by streaking in the patterns. These studies have led to important conclusions and future research directions regarding structural integrity and potential mechanochemical applications of the minerals. Future research will investigate metal-doped versions of cryptomelane and goethite systematically.