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.