Sn₂P₂S₆: A lead-free reversible thermochromic ferroelectric with high near-infrared reflectance

Multifunctional thermochromic materials integrated optical, ferroelectric, and/or magnetic properties have been long envisioned and played an invaluable role in signal processing and non-contact optical information storage in smart windows. However, poor materials stability, together with high discoloration temperature, absence of ferroelectric, or magnetic behaviors, impedes most thermochromic materials for practical applications. Here, the thermochromic behavior of all-inorganic lead-free ferroelectric Sn₂P₂S₆ (SPS) is extensively studied, which exhibits a reversible color change between light orange and red around the ferroelectric-paraelectric phase transition temperature (∼ 67 °C). In situ powder X-ray diffraction (PXRD) and Raman spectroscopy indicate the change of P-S and Sn-S bond distance, and thus result in the modification of (P₂S₆) octahedral geometry and the coordination environment of Sn²⁺ ions along with the evolution of thermochromism (TCM). Additionally, the first-principles calculations reveal that, the Sn 5s² stereochemically active lone pair electrons evoke negative lattice expansion comparable to that of the known ZrW₂O₈ at warming and is responsible for the reversible TCM of SPS. SPS also possesses high near-infrared reflectance (R > 80 %, 1100-2500 nm). The integration of ferroelectricity, high near-infrared reflectance, and TCM below 100 °C renders potential applications of the titled compound in multifunctional optical smart windows.