Structure, optical spectroscopy properties and thermochromism of Sm₃Fe₅O₁₂ garnets

Reversible thermochromic inorganic materials change colour with temperature, which are used in temperature indicator devices, such as temperature sensors, laser marking and warning signals. In this paper, we report a series of thermochromic materials of rare-earth iron garnets Sm_3−xCa_xFe₅O₁₂ (x = 0, 0.1, 0.3, 0.5). All samples were prepared by a high temperature solid state reaction by repeated grinding and calcination of high purity Sm₂O₃, Fe₂O₃ and CaO. The sample structure and optical behaviour were characterized by powder X-ray diffraction (XRD), the Mössbauer spectrum, infrared (IR), Raman and UV-vis-NIR spectroscopy. All results showed the pure phase of Sm_3−xCa_xFe₅O₁₂ (x = 0, 0.1, 0.3, 0.5) with Ca²⁺ partially substituted Sm³⁺ at C sites. The Mössbauer spectrum indicates that Fe³⁺ shows mixed valence states in Ca²⁺ doped samples. Thermochromism of all Sm_3−xCa_xFe₅O₁₂ (x = 0, 0.1, 0.3, 0.5) samples showed the change in colour from green to brown and the introduction of Ca²⁺ ions shows gradual tunability of colour. Temperature-dependent XRD and UV-vis spectra indicated that a continuous structural evolution accompanied by Fe³⁺–O bond length stretching is the main reason for the thermochromic properties. Thus, this paper provides a family of new thermochromic materials for potential thermometric applications.