Rare earth based nanostructures constitute a type of functional materials widely used and studied in the recent literature. The purpose of this review is to provide a general and comprehensive overview of the current state of the art, with special focus on the commonly employed synthesis methods and functionalization strategies of rare earth based nanoparticles and on their different bioimaging and biosensing applications. The luminescent (including downconversion, upconversion and permanent luminescence) and magnetic properties of rare earth based nanoparticles, as well as their ability to absorb X‐rays, will also be explained and connected with their luminescent, magnetic resonance and X‐ray computed tomography bioimaging applications, respectively. This review is not only restricted to nanoparticles, and recent advances reported for in other nanostructures containing rare earths, such as metal organic frameworks and lanthanide complexes conjugated with biological structures, will also be commented on.

The crystal structures of Ca_0.6Sr_0.4TiO₃ and Ca_0.8Sr_0.2TiO₃ perovskites have been studied at high temperatures by X-ray powder diffraction. The most likely sequence of phase transitions with increasing temperature in Ca_0.6Sr_0.4TiO₃ is the same as that shown by the system CaTiO₃–SrTiO₃ with increasing Sr content at room temperature. This sequence is: orthorhombic Pbnm, orthorhombic Bmmb, tetragonal I4/mcm and cubic Pmm. The second orthorhombic phase could not be determined in Ca_0.8Sr_0.2TiO₃, either due to the poorer resolution of the diffractometer employed or because the Bmmb phase might not have a stability field at this composition. The transition temperatures decrease on going from Ca_0.8Sr_0.2TiO₃ to Ca_0.6Sr_0.4TiO₃. Calorimetric measurements on 12 samples spanning the system CaTiO₃–SrTiO₃ then allow one to draw, in combination with the RT XRD data, the temperature vs. composition phase diagram. Phase boundaries show a linear shift toward lower Sr content with the increase of temperature, implying a negligible plateau effect near the CaTiO₃ composition, but quantum saturation occurs at low temperatures near the SrTiO₃ composition.