日時：2019年 1月16日（水） 14:00～
講師：Prof. Daniel Khomskii
所属：II.Physikalisches Institut, Universitaet zu Koeln, Germany
題目：Negative charge transfer gaps and ligand holes: Oxides vs peroxides
In this talk I will discuss some effects occurring in transition metal compounds with small or negative charge transfer gap and with large contribution of ligand (e.g. oxygen) holes. In this case, when a lot of holes are transferred to oxygens (or in general to ligands, e.g. S, Se, Te) one of the options is that instead of the usual oxides, like say Ti4+(O2-)2 one could form peroxides, e.g. with pyrite structure, such as for example Mg2+(O2)2- or Fe2+(S2)2-. This is a very interesting class of compounds, having nontrivial magnetic and sometimes orbital properties. Specifically we consider the recently synthesized material FeO2 , which, according to our theoretical calculations , is a system “in between” the usual dioxides like TiO2, VO2, and peroxides M2+(O2)2- : in FeO2 the valence of Fe is neither 4+ as in dioxides nor 2+ as in pyrite, but 3+. This specific material can play a very important role in the physics of the deep Earths mantle, especially at the early stages of the Earths history. Ligand holes and anion bonding also plays significant role in the properties of gold telluride AuTe2 – one of rare cases of minerals with incommensurate crystal structure . Peroxides can also be important ingredients in the attempts to make better cathode materials for rechargeable batteries, and in many other applications.
 Hu, Q. et al. ‘’FeO2 and FeOOH under deep lower-mantle conditions and Earth’s oxygen–hydrogen cycles’’, Nature 534, 241–244 (2016).
 S.V. Streltsov, A.O. Shorikov, S.L. Skornyakov, A.I. Poteryaev, D.I. Khomskii, ‘’ Unexpected 3+ valence of iron in FeO2 , a geologically important material lying “in between” oxides and peroxides ‘’, Sci. Rep. 7, 13005 (2017)
 S.V. Streltsov, V.V. Roizen, A.V. Ushakov, A.R. Oganov, and D. I. Khomskii, “Solving the old puzzle of incommensurate crystal structure of calaverite AuTe2 and predicted stability of novel AuTe compound ”, Proc. Natl. Acad. Sci. USA, 115, 9945-9950 (2018)