The origin of colour in Cr-doped ceramic pigments with AIIIBIIIO3 perovskite structure was investigated by high resolution X-ray diffraction and UV-Vis-NIR spectroscopy. The development of red colour is related to different phenomena: symmetry lowering from hexagonal (Nd) to orthorhombic (Sm-Yb); crystal field strength decrease due to an octahedral tilting distortion with longer (Al,Cr)-O distances in HREE perovskites; shift of a broad absorption band in the bluegreen spectrum depending on the lanthanide ion.

The development of new red ceramic pigments is a pressing problem, as the current products (e.g. malayaite CaSn1-xCrxSiO5, corundum Al2-xMnxO3, zircon ZrSiO4-Fe2O3) give hues far away from the pure red [1]. Besides, greenockite-cadmoselite included in zircon (CdSexS1-x-ZrSiO4) – though developing an intense red colour – presents environmental problems due to the toxicity of Cd and Se.

This study was aimed at understanding the mechanism of colouration of red pigments with perovskite structure of type AIIIBIIIO3 of composition Ax(Al2-x-yCr3+y)O3 with 0.90<x<1 and 0.03<y<0.12, where A=yttrium or lanthanides. These new pigments have already been extensively characterized from the technological and colourimetrical points of view, but there are still many unsolved questions regarding the mechanism involved in the colour development [2].

The perovskite structure is composed of a three dimensional framework of corner sharing BX6 octahedra, while the A cation fills the twelve-fold coordinate cavities formed by the BX3 network and is surrounded by 12 equidistant anions [3,4]. The presence and magnitude of an octahedral tilting distortion has an important influence on different physical properties, such as colour. This study is an attempt to combine accurate crystal structure data – obtained through high resolution Xray diffraction measured at the European Synchrotron Radiation Facility (SNBL beamline) – with UV-visible-NIR spectroscopy studies, on the basis of the crystal field theory.

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