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Coating thermal noise is one of the dominant noise sources in current gravitational wave detectors and ultimately limits their ability to observe weaker or more distant astronomical sources. This Letter presents investigations of mixed with () as a coating material. We find that, after heat treatment for 100 h at , thermal noise of a highly reflective coating comprising of and reduces to 76% of the current levels in the Advanced LIGO and Advanced Virgo detectors—with potential for reaching 45%, if we assume the mechanical loss of state-of-the-art layers. Furthermore, those coatings show low optical absorption of and optical scattering of . Notably, we still observe excellent optical and thermal noise performance following crystallization in the coatings. These results show the potential to meet the parameters required for the next upgrades of the Advanced LIGO and …