Observation of nanostructured cluster formation of Tm ions in CaF2 crystals

Transmission electron microscopy, energy dispersive X-ray spectroscopy, and high-resolution scanning-transmission electron microscopy, with electron beam sizes ranging from 2 to 50 nm, were used to investigate the spatial distribution and homogeneity of doped Tm3+ ions in CaF2 host matrices with atomic resolution, in solid crystals grown from melts using the Bridgman–Stockbarger method. With the smallest size electron beam available of 2 nm, it was found that the Tm3+ ions were distributed inhomogeneously at the host sites. They took the form of sub-nm agglomerations of 3–5 atoms, rather than individual ions and the phase transition layer was 0.1 nm thick. The spatial extend of inhomogeneous Tm3+ concentration was 2.6–6 nm and originates from ionic density fluctuations in the liquid phase at the interface layer due to the local electrostatic field at the ionic sites.