Scanning electron micrographs were obtained with a Jeol JSM7500F. Images were acquired at an acceleration voltage of 2 kV and a working distance (WD) of about 8 mm, using a through-the-lens secondary electron detector. The freshly exposed surfaces of the samples were coated with a layer of 2–3 nm of Pt prior to the investigations. Analytical information was obtained
at 15 kV and at the same WD used for imaging through an Oxford Inca Energy Dispersive Spectroscopy System (EDS) using an XMax™ silicon drift detector (spatial resolution 2 lm). Using this equipment we were able to detect less than 2 mol% of P. Particle size measurements on SEM-images were performed using ImageJ™ software