SummaryThe combination of focused ion beam and scanning electron microscopy with a cryo-preparation/transfer system allows specimens to be milled at low temperatures. However, for biological specimens in particular, the quality of results is strongly dependent on correct preparation of the specimen surface. We demonstrate a method for deposition of a protective, planarizing surface layer onto a cryo-sample, enabling high-quality cross-sectioning using the ion beam and investigation of structures at the nanoscale.

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The structure of brine films in grain boundaries of halite has been the subject of much controversy over the past 20 years; although a number of innovative methods have been developed to study these structures, much is still unknown and fundamental information is missing. In this study, we investigated different methods of plungefreezing to vitrify the brine fill of grain boundaries for natural salt polycrystal. This was followed by a preliminary study of the 3D morphology of a vitrified grain boundary in a natural rock salt sample with a focused ion beam (FIB) excavation system. We have shown that brine-filled grain boundaries in rock salt can be efficiently well frozen when dimensions are less than about 1 mm. Coupled with an ion beam tool, cryo-SEM allows 3D observation of the well-frozen grain boundaries in large volumes and high resolution. Initial results of brine-filled natural halite grain boundaries show non-faceted crystal-brine interfaces and unexpectedly low dihedral angles at room temperature and pressure.

SummaryNaturally deformed ice contains subgrains with characteristic geometries that have recently been identified in etched surfaces using high-resolution light microscopy (LM). The probable slip systems responsible for these subgrain boundary types can be determined using electron backscattered diffraction (EBSD), providing the etch features imaged with reflected LM can be retained during EBSD data acquisition in a scanning electron microscope (SEM). Retention of the etch features requires that the ice surface is stable. Depending on the pressure and temperature, sublimation of ice can occur. The equilibrium temperature for a low pressure SEM operating at 1 × 10 −6 hPa is about −112 • C and operating at higher temperatures causes sublimation. Although charging of uncoated ice samples is reduced by sublimation, important information contained in the etch features are removed as the surface sublimes. We developed a method for collecting EBSD data on stable ice surfaces in a low pressure SEM. We found that operating at temperatures of <-112 • C reduced sublimation so that the original etch surface features were retained. Charging, which occurred at low pressures (<1.5 × 10 −6 to 2.8 × 10 −5 hPa) was reduced by defocusing the beam. At very low pressures (<1.5 × 10 −6 hPa) the spatial resolution with a defocused beam at 10 kV was about 3 µm in the x-direction at −150 • C and 0.5 µm at −120 • C, because at higher temperature charging was less and only a small defocus was needed to compensate it. Angular resolution was better than 0.7 • after orientation averaging. Excellent agreement was obtained between LM etch features and EBSD mapped

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