PEROVSKITES. FROM PIGMENTS TO MULTIFERROICS, TO THE DISCOVERY OF THE LOCKED-TILT PEROVSKITES

Superconductors, non-toxic inorganic pigments, sensitizers in dye-sensitized solar cells (DSSCs), semiconductors, and multiferroics are just a few examples of the broad range of uses for which ceramics with perovskite structure are employed. In its aristotype form the ABO3 perovskite structure is cubic (s.g. Pm-3m) with A cations located at the center of dodecahedral sites defined by a three-dimensional array of corner-sharing BO6 octahedra. This polyhedral arrangement makes the perovskite structure extremely flexible. Indeed, due to changes in the chemical nature of A and B cations as well as on temperature and pressure conditions, the ideal cubic symmetry is often lowered, and the resulting structure is described by a combination of tilts and distortion of the BO6 octahedra (Mitchell, 2002).
\nIn this contribution, a series of case studies on the interplay among structural characterizations, coloration mechanisms, and technological prospects of ceramics with perovskite structure are outlined. Starting from the Crdoped (Y,REE)AlO3 (YAP) case, i.e. the only system manufactured at the industrial scale and currently used as ceramic red pigment (Ardit et al., 2016), the first case study (i.e., a X-ray powder diffraction and electronic absorption spectroscopy combined investigation) is focused on the effects due to the Al↔Cr substitution at the B octahedral site of the YAP structure, which cause a degree of structural relaxation around Cr3+ that, whether compared with other Cr-bearing structures, is relatively low (Cruciani et al., 2009). The concept of crystal field stabilization energy CFSE, closely related with that of structural relaxation (Burns, 1993), will lead to the second case study where in situ high-pressure synchrotron XRD measurements coupled with ab initio simulations of the electronic population of the orthorhombic NdCrO3 perovskite are compared with the compressional feature of NdGaO3 (high-tech ceramics applied as interconnector of solid oxide fuel cells (SOFC), substrate for high-Tc superconductors (HTSC), colossal magnetoresistive (CMR) film epitaxy, and so on). The different electronic configuration of octahedrally coordinated Cr3+ and Ga3+ ions that leads to a redistribution of electrons at the 3d orbitals for Cr3+ allows the CFSE at octahedral sites to act as vehicle of octahedral softening in NdCrO3 or it turns octahedra into rigid units when CFSE is null as in NdGaO3. The third (and last) case study deals with the first finding of a perovskite characterized by absence of changes in the octahedral tilting and a volume reduction with pressure exclusively controlled by an isotropic polyhedral compression (Ardit et al., 2017). A synchrotron structural investigation at high-pressure sets YAl0.25Cr0.75O3 as the prototype of the so-called locked-tilt perovskites. Besides to reveal a new P-T thermodynamic scenario in the world of perovskite compounds, such a freezing of the octahedral rotations offers a new (and antithetic, keeping in mind that oxygen rotations are strongly coupled to the functional properties of perovskite compounds) perspective on the research of new functional materials (e.g., ferroelectrics and multiferroics). For example, the mutual interaction between layers of octahedrally tilted perovskites subjected to an external perturbation can be easily tuned whether one of the layers is a locked-tilt perovskite.
\nArdit, M., Cruciani, G., Dondi, M., Zanelli, C. (2016): Pigments based on perovskite. In: “Perovskites and related mixed oxides: Concepts and applications”, P. Granger, V. Parvulescu, S. Kaliaguine, W. Prellier, eds. Wiley-VCH, 259-288.
\nArdit, M., Dondi, M., Cruciani, G. (2017): Locked octahedral tilting in orthorhombic perovskites: At the boundary of the general rule predicting phase transitions. Phys. Rev. B, 95, 024110.
\nBurns, R.G. (1993): Mineralogical application of crystal field theory. 2nd Ed. Cambridge University Press, 551 p.
\nCruciani, G., Ardit, M., Dondi, M., Matteucci, F., Blosi, M., Dalconi, M.C., Albonetti, S. (2009): Structural relaxation around Cr3+ in YAlO3-YCrO3 perovskites from electron absorption spectra. J. Phys. Chem. A, 113, 13772-13778.
\nMitchell, R.H. (2002): Perovskites: Modern and Ancient. Almaz Press., Thunder Bay, Ontario, 316 p.

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