The synthesis of unconventional stoichiometric compounds in the K-Br system at high pressures

Recently, the search for and synthesis of unconventional stoichiometric compounds have become one of the most active areas of high pressure research. Here, we report the synthesis of two new stoichiometric compounds, namely KBr₃ and KBr₅, at high pressures in the K-Br system. Until now, KBr was the only known compound in this system. Two independent experimental techniques, namely Raman spectroscopy and X-ray diffraction measurements, were employed to detect and confirm the formation of the new compounds. A room temperature chemical reaction between KBr and Br₂ resulted in the formation of orthorhombic KBr₃ at ∼2.0 GPa. Further compression led to the formation of monoclinic KBr₅ at ∼6.0 GPa. This was accompanied by an anomalously large pressure (>2 GPa) increase inside the sample chamber and it remained stable up to the highest pressure, 24 GPa, of our study. Upon decompression, KBr₅ remained stable down to 5.0 GPa. High-pressure (14-20 GPa) and high-temperature (>1500 K) laser heating experiments showed the decomposition of KBr₅ into KBr₃ (trigonal) and Br₂ with a large volume reduction. First-principles structural searches were carried out to solve the composition and related crystal structures. The proposed structures give good description of the experimental Raman spectra and X-ray diffraction data. The electronic structure calculations reveal semiconducting behaviour for these compounds.