Scanning Electron Microscopy. Lattices were systematically imaged in the XY plane and as XZ and YZ cross-sections using a field-emission scanning electron microscope (Ultra55 Zeiss) at a 10kV voltage. For cross-section imaging, samples were cryo-fractured in a liquid N2 bath along selected planes. All samples were sputter-coated with a 10 nm layer of a Pt/Pd alloy before imaging