**Crystal Structures and High-Temperature Vibrational Spectra for Synthetic Boron and Aluminum Doped Hydrous Coesite**

**Yunfan Miao 1, Youwei Pang 1, Yu Ye 1,\*, Joseph R. Smyth 2, Junfeng Zhang 1,3, Dan Liu 1, Xiang Wang <sup>1</sup> and Xi Zhu <sup>1</sup>**


Received: 5 November 2019; Accepted: 3 December 2019; Published: 5 December 2019

**Abstract:** Coesite, a high-pressure SiO2 polymorph, has drawn extensive interest from the mineralogical community for a long time. In this study, we synthesized hydrous coesite samples with different B and Al concentrations at 5 and 7.5 GPa (1273 K). The B concentration could be more than 400 B/106Si with about 300 ppmw H2O, while the Al content can be as much as 1200 to 1300 Al/106Si with *C*H2O restrained to be less than 10 ppmw. Hence, B-substitution may prefer the mechanism of Si4<sup>+</sup> = B3<sup>+</sup> + H+, whereas Al-substitution could be dominated by 2Si4<sup>+</sup> = 2Al3<sup>+</sup> + OV. The doped B3<sup>+</sup> and Al3<sup>+</sup> cations may be concentrated in the Si1 and Si2 tetrahedra, respectively, and make noticeable changes in the Si–O4 and Si–O5 bond lengths. In-situ high-temperature Raman and Fourier Transformation Infrared (FTIR) spectra were collected at ambient pressure. The single crystals of coesite were observed to be stable up to 1500 K. The isobaric Grüneisen parameters (γ*iP*) of the external modes (<350 cm−1) are systematically smaller in the Al-doped samples, as compared with those for the Al-free ones, while most of the OH-stretching bands shift to higher frequencies in the high temperature range up to ~1100 K

**Keywords:** coesite; high-temperature Raman; FTIR spectrum; single crystal structure; isobaric Grüneisen parameters; OH-stretching modes
