Читать книгу Solid State Chemistry and its Applications - Anthony R. West - Страница 67

The oxide O²⁻, nitride N³⁻ and fluoride, F⁻ ions are of similar size and can readily substitute for each other in crystal structures, other although the number of mixed anion systems that have been studied and reported in the literature are far fewer than those of mixed cation‐based materials. The reasons for this are that first, oxynitrides do not usually have the same thermal stability as corresponding oxides since metal‐oxygen bonds are usually stronger than metal‐nitrogen bonds. Controlling the N stoichiometry presents difficulties during synthesis if N₂ gas is lost in an ‘open’ reaction. Second, the synthesis of oxyfluorides requires stringent safety precautions if F₂ gas is involved.

A range of oxynitride perovskites are known with a 2:1 ratio of O:N and an overall cation charge of 7+. These include II/V cation charge combinations in ATaO₂N: A=Ca, Sr, Eu and III/IV combinations in ATiO₂N: A=Ce, Pr, Nd. Oxynitride perovskites with a 1:2 ratio of O:N form, such as LaTaON₂ and EuWON_2. Given the similar sizes but different valencies of O²⁻ and N³⁻, there is also much scope for the preparation of non‐stoichiometric oxynitrides that have variable O:N ratios together with different cation combinations, including mixed valence cations, to maintain charge balance.

O and N are either disordered or ordered, fully or partially, in these perovskites and similar tilted or distorted structures occur to those found in oxide perovskites. For instance, LaZrO₂N, Ca(Ta,Nb)O₂N and NdTiO₂N adopt the GdFeO₃ structure with tilt system a ⁺ b ⁻ b ⁻ , whereas LaTiO₂N adopts the a ⁻ b ⁻ c ⁻ tilt system. In ordered 2:1 oxynitrides such as Sr(Nb,Ta)O₂N, the [(Nb,Ta)O₄N₂] octahedra show strong preference for a cis arrangement of the two N positions. Each N forms a linear B‐N‐B segment in the linkage of corner‐sharing octahedra; additional bonding interactions involving the transition metal d orbitals are believed to be responsible for the commonly observed cis configurations.

Oxide perovskites exhibit a very wide range of electrical, magnetic and optical properties and these may be modified by substitution of N for O in oxynitrides. The property modifications are associated with the lower electronegativity of N than O which leads to (i) more covalent metal‐nitrogen bonds compared with the more ionic metal‐oxygen bonds and (ii) a consequent reduction in the band gap of oxynitrides and nitrides. The band gap is frequently in the visible part of the electromagnetic spectrum and can be fine‐tuned by adjusting the O:N ratio, leading to a range of brightly coloured materials with applications as non‐toxic pigments and in visible light, photocatalytic water‐splitting processes.

A similar story can be told for oxyfluoride perovskites. The ones prepared so far have large A‐site cations, especially Ca, Sr and Ba, exhibit various tilted structures and the possibility of variable O:F ratio, especially in fluorinated cuprate phases that exhibit superconductivity.