In this critical review we consider the large literature that has accumulated in the past 5-10 years concerning solution-mediated crystallisation of complex oxide materials using hydrothermal, or more generally solvothermal, reaction conditions The aim is to show how the synthesis of dense, mixed-metal oxide materials, usually prepared using the high temperatures associated with solid-chemistry, is perfectly feasible from solution in one step reactions, typically at temperatures as low as 200 °C, and that important families of oxide materials have now been reported to crystallise using such synthetic approaches We will focus on two common structures seen in oxide chemistry, ABO(3) perovskites and A(2)B(2)O(6)O' pyrochlores, and include a systematic survey of the variety of chemical elements now included in these two prototypical structure types, from transition metals, in families of materials that include titanates, niobates, manganites and ferrites, to main-group elements in stannates, plumbates and bismuthates The significant advantages of solution-mediated crystallisation are well illustrated by the recent literature: examples are provided of elegant control of crystal form from the nanometre to the micron length scale to give thin films, anisotropic crystal morphologies, or hierarchical structures of materials with properties desirable for many important contemporary applications In addition, new metastable materials have been reported, not stable once high temperatures and pressures are applied and hence not amenable using conventional synthesis We critically discuss the possible control offered by solvothermal synthesis from crystal chemistry to crystal form and how the discovery of new materials may be achieved Computer simulation, combinatorial synthesis approaches and in situ methods to follow crystallisation will be vital in providing the predictability in synthesis that is needed for rational design of new materials (232 references)

Solvothermal synthesis of perovskites and pyrochlores: crystallisation of functional oxides under mild conditions

Big, bigger, biggest: Polyoxoniobate anions [HNb27O76]16− and [H10Nb31O92(CO3)]23−incorporate pentagonal Nb(Nb)5 building blocks; the central Nb ion is seven-coordinate within the clusters. The Nb27 species was observed using ESI-MS, thus demonstrating some solution stability; the Nb31 species is chiral and incorporates a carbonate ligand in the outer section of the cluster. The two species are the largest polyoxoniobates reported to date.

The Construction of High‐Nuclearity Isopolyoxoniobates with Pentagonal Building Blocks: [HNb27O76]16− and [H10Nb31O93(CO3)]23−

The tolerance factor of ABO3-type ilmenite by analyzing the ABO3 ilmenite crystal structure is established. Combining with the electronegativity difference and octahedral factor of ABO3 structure, regularities governing the formation and the stability of ilmenite-type compounds are discussed. The tolerance factor equation was proved appropriate for ilmenite structure by analyzing the structure stability of some ilmenite compounds. According to the results of statistically analyzing the tolerance factor and electronegativity difference of the present ABO3-type ilmenite, the experience tolerance factor value and experience electronegativity difference value to form stable ilmenite compound were obtained, that is, t > 0.80 and e > 1.465, and the lowest limit of the octahedral factor \( \left( {R_M /R_{O^{2 - } } } \right) \) for ilmenite formation is 0.48.

N. Kumata

Papers

A new allotropic form with ilmenite-type structure of NaNbO3