Unexpected semi-metallic BiS2 at HP and HT - Dr. Lin Wang

New work from a team of scientists led by Dr. Ling Wang from HPSTAR and Prof. Yanming Ma of Jinlin University predicted new compounds in the Bi-S system and experimentally synthesized the predicted BiS₂ compound under high pressure and temperature conditions. Their further electronic calculations indicate that this new found BiS₂ is a a promising semi-metallic material. This work is published in recent JPCL.

For the group V-VI compounds, there are just conventional A₂B₃-type, such as Bi₂S₃, Bi₂Se₃, Bi₂Te₃, and Sb₂Te₃ compounds at normal conditions and no other stable stoichiometric compound has been found.

Now, an international team of researchers co-led by Dr. Lin Wang, a staff scientist of HPSTAR has predicted another Bi-S compound—BiS₂ in the Bi-S system, a standard system in group V-VI compounds. Then the scientists experimentally synthesized and confirmed BiS₂.

The team first used the crystal structure prediction method to conduct a systematic search for stable Bi-S compounds in variable chemical compositions. They found two new stoichiometric BiS₂ and BiS which are energetically stable above 9 and 19 GPa, respectively. Unexpectedly, Bi₂S₃— stable at ambient conditions, was found to decompose into two new stoichiometric phases BiS₂ and BiS above 24 GPa.

“Our theoretical predication indicates pressure will lead to new valence states for the two elements just like that in Na-Cl system”, said Dr. Lin Wang.

In order to confirm the theoretically predicted stoichiometries, the team of scientists carried out following high-pressure experiments to synthesize the BiS₂ compound which was predicted to have lower energy compared to BiS and thus more easily to be synthesized.

They tried to obtain BiS₂ compound at extreme conditions along two pathways: from decomposition of Bi₂S₃ and composition of Bi and S mixture.

First they compressed Bi₂S₃ up to 38 GPa and heated it up to 2000 K, they found the formation of BiS₂ and extra Bi. To obtain pure BiS2 sample, the scientists directly use Bi and S as starting material and heat the mixture at almost the same pressure. They also successfully synthesized high quality BiS₂ compound.

"Since BiS becomes stable at higher pressure, we believe that we could also synthesize it while much higher pressure is required to overcome higher energy barrier”, said Dr. Guangtao Liu, the first author of the work and a postdoc worked at Dr. Lin Wang’s group.

Their following calculations reveal that the new compound BiS₂ is a semi-metal with interesting behaviors and potential applications.

"The diverse electronic properties can be understood intuitively in terms of the different concentrations of nonmetallic S in these phases, which can lead to an increase in valence electron transfer from Bi to S in BiS₂”, Dr. Lin Wang explained.

“Our work on the Bi-S system encourages us to believe that, as well as the well-known A2B3-structure type, other undiscovered phases with interesting and potentially useful properties should exist among the group V-VI compounds, which can be engineered or controlled by tuning these compositions under compression and have been confirmed by our primary calculations”, stated in the paper.

近年来，V-VI族化合物(Bi₂Te₃，Bi2Se3，Sb₂Te₃和Bi₂S₃)的拓扑绝缘性质、超导性质和热电性质等在基础研究和实际应用领域，已引起广泛的研究兴趣。已有的研究表明高压可以显著地调控这些性质。同样，其他化学计量配比的化合物可能在极端条件下存在，并具有反常的性质和行为。北京高压科学研究中心的王霖研究员与吉林大学马琰铭教授小组合作以V-VI族化合物中典型的Bi-S体系为研究对象，分别对该体系的不同化学配比的化合物的稳定性和晶体结构开展了系统的理论和实验研究。通过晶体结构预测方法和基于密度泛函理论第一性原理计算，他们发现了具有新价态的化合物BiS₂和BiS分别在9和19GPa后能量稳定。更意外的是，计算表明在常压下稳定存在的Bi2S3在24GPa后由于能量不稳定而趋于分解。