In My Element: Oxygen

Abstract

The In My Element series celebrates the personal accounts from Chemistry – A European Journal Editorial Board members for the 2019 International Year of the Periodic Table. In this contribution, C. N. R. Rao gives his story on oxygen. My love for oxygen can be traced to the very beginning of my research career. This is not because oxygen was discovered by the Father of Chemistry, Lavoisier, and it is essential for life as O2 and as H2O, but because it forms a variety of metal oxides with interesting structures and a wide range of properties1 (Table 1). Oxygen can exist as O21−, O22− and O2− in solids and as O1− on certain surfaces. Oxygen stoichiometry is an important aspect of oxides, oxygen deficiency often giving rise to new types of ordered structures (TinO2n−1 Magneli phases, Pr2O2n−2). Metallic: ReO3, NbO, LaNiO3 Dielectric: BaTiO3 Antiferromagnetic: NiO, LaCrO3 Ferromagnetic: CrO2, La0.5Sr0.5CoO3, SrRuO3 Ferroelectric: BaTiO3, KNbO3 Ferroelastic: Gd2(MoO4)3 Superconducting: BaPb0.75Bi0.25O3, YBa2Cu3O7 Insulator-Metal Transition: V2O3, VO2, La1−xSrxCoO3 Colossal Magnetoresistance (CMR): La0.7Ca0.3MnO3 Multiferroic: BiFeO3, YMnO3, TbMnO3 I have worked on metal oxides for a long time,2 and my very first research paper in materials chemistry in the late 1950′s was on the transformation of the anatase form of TiO2 to the rutile form. I also studied the so-called nonstoichiometric oxides of rare earths of the type Pr6O11, Pr7O12 and Tb4O7. Soon I realized how perovskite oxides (ABO3) exhibit a variety of phenomena and properties such as ferroelectricity and magnetism, and how the pervoskite motif is present in several families of oxides. I have been working on perovskite oxides including two-dimensional oxides for many years. I worked on La2CuO4, as early as 1971, mother of cuprate superconductors. My group was one of the first to characterize YBa2Cu3O7, the first liquid nitrogen superconductor, which also has the perovskite motif. The number of cuprate families exhibiting high temperature superconductivity is impressive.3 The discovery of colossal magnetoresistance in rare earth manganites of the type La1−xCaxMnO3 which show both orbital and charge-ordering led to a variety of investigations and to the observation of the phenomenon of electronic phase separation.4 We observed major effects of phase separation in Nd0.5Sr0.5MnO3, which is ferromagnetic at ordinary temperatures, and occurs in three phases (FM+2AFMs) at liquid helium temperatures. I started working on multiferroics in the 1990’s, the well-known examples being BiFeO3 and YMnO3. Cation substitution in oxides is commonly carried out to cause changes in structures and properties. An impressive example is that of Sr substituted LaCoO3 where a paramagnetic insulator becomes a ferromagnetic metal on substitution of Sr in place of La. Cation substitution generally affects the unoccupied states and anion substitution, specially aliovalent anions on the other hand, cause drastic changes in the valence bands. In recent months, I have been working on oxide analogues wherein oxygen is substituted by nitrogen and fluorine, resulting is novel electronic and catalytic properties.5 For example, Zn2NF and TiNF are isoelectronic with ZnO and TiO2 respectively, but exhibit entirely different electronic structures and properties. Position Linus Pauling Research Professor E-mail cnrrao@jncasr.ac.in Homepage http://www.jncasr.ac.in/cnrrao Education B.Sc (Mysore, 1951), M.Sc (Banaras, 1953) Ph.D (1957, Prof R.L. Livingston, Purdue, “Molecular structure investigations by gas phase electron diffraction)” D.Sc (1960) Mysore “Studies of Structural Chemistry and Nitrogen Chemistry” Post-doctoral fellowship (1958-1959; Berkeley, Prof K.S. Pitzer) Awards Dan David Prize for Science in the Future Dimension (2005) Ernesto Illy Trieste Science Prize (2011) for materials research The Royal Medal (The Queens Medal) of the Royal Society, London, U.K (2009) The August-Wilhelm-von-Hoffmann Medal for outstanding contributions to chemistry by the German Chemical Society (2010) Von Hippel Award for materials research by Materials Research Society, US (2017) Chevalier de la Légion D′Honneur by the President of the French Republic (2005) Order of the Rising Sun, Gold and Silver Star by the Emperor of Japan (2015) Bharat Ratna (Jewel of India), Highest Civilian Award of India (2014) Research interests Chemistry of Materials, in particular, oxide materials, 2D materials, solar water-splitting Hobbies and interests Listening to classical music and cooking