**1. Introduction**

Coesite, a high-pressure polymorph of SiO2, was firstly synthesized at 3.5 GPa (773–1073 K) in 1953 [1], and subsequently discovered in many locations, such as in the shocked sandstone ejecta samples from craters [2,3] as well as in eclogite [4–6]. Coesite is a very important index silica mineral for ultrahigh-pressure metamorphism [7,8], which provides key clue for the continental dynamics such as lithospheric subduction, exhumation, and reentry in extreme depths of more than 100 km. Furthermore, the physical and chemical properties of coesite at high-pressure and high-temperature conditions also attract a lot of interest from the community of mineral physics, like thermo-elasticity [9–12], phase transitions [13–15], and vibrational spectra under high-pressure (*P*) and high-temperature (*T*) conditions [16–20].

Water (OH−) incorporation into coesite has a significant impact on the stability of coesite at high-*P*/*T* conditions [21,22], which is important for exploring preservation of coesite in the deep mantle. There could be up to 200–300 ppm ppmw H2O in synthetic coesite samples [19,23–26] resulted from hydro-garnet substitution (Si4<sup>+</sup> + 4O2<sup>−</sup> = V + 4OH−) as well as electrostatically coupled substitution

with M3<sup>+</sup> incorporation (Si4<sup>+</sup> = M3<sup>+</sup> + H+; M = B, Al), although natural coesite has been found to be nearly dry so far [27,28].

In this study, we synthesized hydrous coesite samples with various compositions (Si-pure, B-doped, Al-doped, as well as B plus Al-doped), and explored the effects of B and Al on the hydration mechanism and internal structure of coesite. Taking advantage of in-situ high-temperature Raman and FTIR vibrational spectra, we have also studied thermal response of lattice vibration with the contributions from the trace elements of H, Al, and B, which may provide important constraints on thermodynamic properties of coesite (such as heat capacities and entropy) under deep mantle conditions.
