Thermochromic solids are highly valuable smart materials that can act as temperature sensors and filters and find applications in many diverse fields like infrastructure coating, aerospace, and anticounterfeiting. Rarely presented in chalcogenides, far scarcer are accounts of this behavior in the absence of transition metals. Herein we explore the photophysical properties and high-temperature thermochromic behavior of Ln10S14O (Ln = La, Ce, Pr, Nd, and Sm). Through Tauc plot and derivation of absorption spectrum fitting analyses we conclude that these visible light absorbers are direct band gap semiconductors, ranging from 2.02 eV to 2.84 eV, from Ce to La. Moreover, surface photovoltage spectroscopy indicates contact potential differences larger than -1 V in La, Pr, Nd and Sm oxysulfides, with electrons as the majority carriers. Ln10S14O demonstrate dramatic changes in their coloration within the temperature range of 250 – 600 °C. In situ x-ray powder diffraction studies indicate that these materials exhibit an anisotropic lattice expansion with no change in crystallographic phase. Additionally, the observed thermochromism shows high reversibility under inert atmosphere.

Exploring the photophysical and thermochromic properties of lanthanoid oxysulfides

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