Nattamai S. P. Bhuvanesh

Solid-state chemistry of early transition metal- oxides containing d0 and d1 cations

This paper presents a brief survey of the structures and properties of early transition-metal oxides containing d0 and d1–d0 electronic configurations. The metal–oxygen (MO6) octahedron, which is the essential structure building unit of these materials, exhibits a characteristic out-of-centre distortion for the d0 configuration in many instances, the degree of distortion increasing with decreasing HOMO–LUMO gap. Several characteristic properties of d0 oxides, which include low-dimensional structures (that give rise to intercalation, ion-exchange and acidity), ferroelectricity and non-linear optical response, arise from the out-of-centre distortion of d0 MO6 octahedra. Oxides containing d1–d0 electronic configurations exhibit an equally impressive array of electronic properties that owe their origin to the nature of d states near the Fermi level. While three-dimensional (3-D) oxides containing 5d and 4d electrons exhibit itinerant electron properties, 3d1 oxides, especially with low-dimensional (low-D) structures, display localized electron magnetism and semiconduction. Low-dimensional oxides containing 4d electrons, typified by molybdenum bronzes and Mo4O11 , exhibit charge-density-wave (CDW)-driven electronic instabilities arising from electron–phonon interactions.

Synthesis and structure of novel layered perovskite oxides: Li2La1.78Nb0.66Ti2.34O10, and a new family, Li2[A0.5nBnO3n + 1]

Several new members (n = 2, 3, 4 members) of the family of layered perovskites Li2[AxBnO3n + 1] (A = Ca, Sr, La; B = Nb, Ta, Ti, Fe) have been synthesized for the first time. The structure analysis of Li2SrNb2O7 and Li2SrTa2O7 determined by powder X-ray diffraction (XRD), and of Li2La1.78Nb0.66Ti2.34O10 and Li2Sr2Nb3.97Fe0.03O12.97 determined by single crystal XRD data, shows that these novel phases are related to the Ruddlesden–Popper series of compounds, A′2[An – 1MnO3n + 1]. While the structures of n = 2 members are identical to the RP series, in the higher members the A sites are partially occupied. Interestingly, the compounds for A = Ca, Sr and B = Nb, Ta, Fe indicate the formation of a new family of oxides of general formula Li2A0.5nBnO3n + 1 where different members of the family can be synthesized with the same set of A and B atoms.

Towards rational synthesis of inorganic solids

There have been major advances in the past couple of years in the rational synthesis of inorganic solids: synthesis of mercury-based superconducting cuprates showing transition temperatures up to 150 K; ZrP2-xVxO7 solid solutions showing zero or negative thermal expansion; copper oxides possessing ladder structures such as La1-xSrxCuO2.5; synthesis of mesoporous oxide materials having adjustable pore size in the range 15-100 Angstrom; and synthesis of a molecular ferromagnet showing a critical temperature of 18.6 K. Despite great advances in probing the structures of solids and measurement of their physical properties, the design and synthesis of inorganic solids possessing desired structures and properties remain a challenge today. With the availability of a variety of mild chemistry-based approaches, kinetic control of synthetic pathways is becoming increasingly possible, which, it is hoped, will eventually make rational design of inorganic solids a reality.

Acid leaching of LiMW2O8(M Al, Fe) in aqueous HNO3: formation of WO3·H2O and WO3·1/3H2O

WO3·H2O and WO3·1/3H2O are obtained by acid leaching of LiMW2O8(M Al, Fe). The monohydrate is obtained by leaching in 6 mol I–1 HNO3, and the one-third hydrate by leaching in concentrated HNO3. On dehydration, the monohydrate transforms to the monoclinic ReO3-like WO3, whereas the one-third hydrate transforms to hexagonal WO3. The present work, together with our previous work on the leaching of LiVWO6(Chem. Mater., 1994, 6, 373), shows that acid leaching is not likely to be a topochemical reaction, since the structure of the solid products obtained is not determined by the structure of the starting material.

Soft-chemical routes to synthesis of solid oxide materials

We describe three different families of metal oxides, viz., (i) protonated layered perovskites, (ii) framework phosphates of NASICON and KTiOPO4 (KTP) structures and (iii) layered and three-dimensional oxides in the H-V-W-O system, synthesized by ‘soft-chemical’ routes involving respectively ion-exchange, redox deintercalation and acid-leaching from appropriate parent oxides. Oxides of the first family, HyA2B3O10(A = La/Ca; B = Ti/Nb), exhibit variable Bronsted acidity and intercalation behaviour that depend on the interlayer structure. V2(PO4)3 prepared by oxidative deintercalation from Na3 V2(PO4)3 is a new host material exhibiting reductive insertion of lithium/hydrogen, while K0.5Nb0.5 M0.5OPO4 (M = Ti, V) are novel KTP-like materials exhibiting second harmonic generation of 1064nm radiation. HxVxW1-xO3 for x = 0–125 and 0.33 possessing α-MoO3 and hexagonal WO3 structures, prepared by acid-leaching of LiVWO6, represent functionalized oxide materials exhibiting redox and acid-base intercalation reactivity.