New inorganic structures and modulated materials

Perovskite-type compounds demonstrate enormouschemical flexibilityandversatility of functional properties. We were pioneering in further expanding the perovskite family exploring hitherto unknown distortion modes and building principles. Among the mixed perovskites and elpasolites A₂BB'X₆ (X = O, F) we established the existence of very complex structures with broken corner-sharing connectivity of octahedral perovskite framework due to flexibility of coordination environment of the B cations. From rigorous structure analysis based on a combination of transmission electron microscopy, synchrotron X-ray and neutron powder diffraction we derived the crystal chemistry conditions favouring formation of such structures.

We demonstrated the existence of a whole family of perovskites, where the anion deficiency is accommodated through crystallographic shear (CS) planes, which were generally assumed to be incompatible with the perovskite structure. We have discovered the (Pb,Bi)_1-xFe_1+xO_3-y perovskites modulated by CS planes and unravelled their structures using crystallography in (3+1)-dimensional space [1]. By manipulating the CS planes we discovered the novel perovskite-based homologous series A_nBnO₃n-2 (A = Pb,Bi,Ba,Sr, B = Fe, Ti, Mn, Sn, Sc) [2]. 

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We explore the ability of the lone pair cations (Pb²⁺, Bi³⁺) to generate modular structures by relieving the stress at the interfaces between the structure modules through their flexible coordination environment. BiMnFe₂O₆ is an example of a quasi-2D structure where the blocks of a hypothetical hexagonal close packed MO structure are stabilized by the Bi³⁺ cations. [3] This material demonstrates a frustrated (Mn,Fe)-O sublattice, giving rise to an acentric spiral magnetic ground state. 

Recently we obtained the novel Bi₄Fe₅O₁₃F compound by using Bi₄F tetrahedra as separators between pentagonal Cairo Fe-O layers [4]. Bi₅Fe₄O₁₃F features a non-collinear AFM ground state and an intricate sequence of magnetic phase transitions. 

Competing interactions causing intricate modulated structures are very common in inorganic materials. We apply a combination of superspace crystallography, electron diffraction, transmission electron microscopy and powder diffraction techniques to investigate incommensurately modulated structures. So far we successfully resolved the modulated structures of A2BB'O5 brownmillerites, mixed electronic-ionic conductors A₄B₆O_13-d (A = Sr,Ba, B = Fe,In,Mg), todorokite [SrF_0.8(OH)_0.2]_2.526[Mn₆O₁₂], multiferroic perovskite Bi_0.75La_0.25FeO₃, Li⁺ ionic conductors Li_3xNd_2/3-xTiO₃ and others.