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.