Soft–Rigid Structural Integration in KMgBO3: A Pathway to Negative Thermo-optic Coefficient via Positive Thermal Expansion and Negative Temperature-Dependent Optical Absorption Edge

Borate materials have garnered significant attention in the field of optical devices owing to their superior optical properties. Achieving a negative thermo-optic coefficient in materials is essential for the development of advanced optical devices. In this work, we performed a detailed analysis of the atomic displacement parameter, Grüneisen parameter, chemical bond strength, and compressibility of KMgBO₃, revealing its distinct behaviors of K–O, Mg–O, and B–O bonds. This characteristic leads to an integration of soft and rigid features in KMgBO₃, resulting in a large positive thermal expansion coefficient and a temperature-induced reduction of the optical absorption edge. These two factors together are identified as the driving force for the KMgBO₃’s negative thermo-optic coefficient based on the Lorentz oscillator model.