Structure, optical spectroscopy properties and thermochromism of Sm3Fe5O12 garnets

Abstract

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.