**4. Conclusions**

Using a DAC, we have found that the phase transition and metallization of orpiment occurred at about 25.0 and 42.0 GPa, respectively. The results were acquired by a combination of experimental and theoretical methods, including Raman scattering, impedance spectroscopy, electrical conductivity measurements at variable temperature, AFM, HRTEM, and first-principles calculations. The variable temperature electrical conductivity measurements and first-principles theoretical calculations provided strong evidence for a pressure-induced metallization at ~42.0 GPa. The images of the decompressed sample, from AFM and HRTEM, confirmed the well-preserved crystalline structure, which agreed well with the electrical conductivity and Raman spectroscopy measurements. Based on the observed compressibility change and the variation in pressure effects on the electrical conductivity, a second-order isostructural phase transition occurred at ~25.0 GPa. The observed high-pressure properties of orpiment will aid in the understanding of the universal crystal structure and electrical properties of *A*2*B*3-type materials. And furthermore, all of these high-pressure behaviors of orpiment will present some crucial information in the structural phase transition, metallization, amorphization and superconductivity for the *A*2*B*3-type of engineering materials at high pressure.

**Author Contributions:** L.D. (designing the project). K.L. and L.D. (writing the initial draft of the work and the final paper). K.L., L.D., H.L., H.H., L.Y., C.P., M.H. and P.L. (interpreting the results). L.D. (correcting and recognizing the final paper). K.L. and L.Y. (performing and interpreting the high-P experiments, the HRTEM and AFM images). K.L. and P.L. (performing the first-principles calculations). All authors discussed the results and commented on the manuscript.

**Funding:** This research was financially supported by the strategic priority Research Program (B) of the Chinese Academy of Sciences (Grant No. 18010401), Key Research Program of Frontier Sciences of CAS (Grant No. QYZDB-SSW-DQC009), "135" Program of the Institute of Geochemistry of CAS, Hundred Talents Program of CAS and NSF of China (Grant Nos. 41474078, 41774099 and 41772042).

**Acknowledgments:** The support of the Supercomputer Center of Fujian Institute of Research on the Structure of Matter (FJIRSM) is acknowledged.

**Conflicts of Interest:** The authors declare no conflict of interest.
