Ni‐Ti Codoped Hibonite Ceramic Pigments by Combustion Synthesis: Crystal Structure and Optical Properties

Abstract

Hibonite (CaAl12O19, space group [s.g.] P63/mmc) has the structural formula A[XII]M1[VI] M2[V]M32[IV]M42[VI]M56[VI]O19, where Ca is 12‐fold coordinated at site A and Al3+ ions are distributed over five different sites: three distinct octahedra [M1 (point symmetry D3d), M4 (C3v) and M5 (Cs)], the M3 tetrahedron (C3v), and the unusual fivefold coordinated trigonal bipyramid M2 (D3h). Hibonite is able to accommodate a wide range of ions with different valence and coordination, making its structure a promising ceramic pigment. One of the main challenges is to understand and control incorporation mechanisms and the threshold of chromophores solubility. It is known that M2+ ions tend to be hosted at the M3 site, while M4+ ions are preferentially accommodated at the M4 site: the introduction of divalent ions might be promoted by the associated incorporation of tetravalent cations, which ensure the lattice electroneutrality and are ordered over the M4 face‐sharing octahedral dimers. In this work, the mechanism of the coupled substitution 2Al3+→(Ni2++Ti4+) was investigated by combining X‐ray powder diffraction and diffuse reflectance spectroscopy techniques. Hibonite turquoise pigments with increasing Ni+Ti doping (CaAl12‒2xNixTixO19 where x = 0.1‒2.0 apfu) were prepared by combustion synthesis, utilizing a fuel mixture (urea, glycine, β‐alanine) set up according to their compatibility with metal nitrates used as raw materials. The ignition temperature of combustion reaction was 400°C, but samples underwent an additional annealing at 1200°C. Samples up to x = 0.4 are monophasic; for higher doping, hibonite is the main component accompanied by growing percentages of spinel and perovskite phases. The Ni and Ti addition induced a regular increase in the hibonite unit‐cell parameters until x = 1.0, that is proportional to the amount and difference in ionic radii of dopants. In particular, an elongation of the 〈M–O〉 bond distances of both M3 and M4 sites was observed. In terms of optical parameters, Ni2+ is preferentially incorporated in tetrahedral coordination, up to 0.3 apfu at the M3 site, and at the M4 octahedron as well (up to 0.19 apfu). The crystal field strength of fourfold coordinated Ni2+ is regularly decreasing, implying an elongation of the local Ni–O bond that is coherent with the volume increasing from AlO4 to NiO4 tetrahedra registered by XRD. Ti4+ ions are accommodated at both the M2 and M4 octahedra which expand proportionally to the amount of dopants. Pigment purity and color strength vary with doping depending on the multistep mechanism of Ni and Ti incorporation in the hibonite lattice.

Dynamic Disorder of Fe3+ Ions in the Crystal Structure of Natural Barioferrite

“…The present study confirms the incorporation of Mg into the tetrahedral Fe3 site as suggested in [43,48] The mean Fe3-O bond length of 1.914 Å was slightly higher than that in the synthetic barioferrite of 1.888 Å [2]. The incorporation of Ti into magnetoplumbite-like compounds are widely discussed in the literature; according to this data Ti orders into M2 and M4 [49,50] or Fe4 > Fe3 > Fe2 [5]. In our case, Ti is allocated in the Fe2 and Fe4 sites.…”

Colored oxides with hibonite structure II: Structural and optical properties of CaAl12O19-type pigments with chromophores based on Fe, Mn, Cr and Cu

“…In addition to standard solid state synthesis [2,7,8,20,23,24,38,44,49,[51][52]56] other preparation methods were also applied to obtain hibonite-type oxides. Solution combustion synthesis using metal nitrates was performed [25,32,50]. Powder samples were made through wet techniques: sol-gel [8,21] and hydrolysis of alkoxydes followed by thermal decomposition and reaction [8].…”

“…X-ray diffraction (XRD) was the most common technique applied for structure studies of hibonite-type single crystals and powder samples [1,[3][4][5][6]8,12,14,16,18,23,37,44,49,50,51]. In some cases neutron Rietveld refinement was performed [2,3,19,20,22,44,56].…”

“…Elemental distribution was studied by electron microprobe analysis (EMP) [3][4][5]41]. Spectroscopic investigations (absorbance and fluorescence spectra) [4,6,7,34,42,55], nearinfrared (IR) spectra [7,37], emission spectra [9], luminescence [10], color description by L*a*b* parameters [53][54][55] and other optical property characterizations [34,35,41,43,50,51,[53][54][55] were employed. Additional information of the hibonite-based oxides was collected using electron paramagnetic resonance (EPR) [7], Mössbauer technique [4,35,38,40,[42][43], magnetic susceptibility measurements [6,21,56] magnetic structure refinement [19,29,33,47], thermal analysis [33,55], along with measurements of electrical properties [24,25], dielectric properties [56] chemical reactivity…”

“…Recently colored oxides with hibonite structure have attracted attention as an appealing host for inorganic pigment design. There have been several reports on turquoise pigments with Ni 2+ as a chromophore in tetrahedral coordination of CaAl 12 O 19 [50][51][52]. Blue hibonite compounds based on tetrahedrally coordinated Co 2+ have also been investigated [53], and Co-and Nicontaining solid wastes were proposed to be used as raw materials for pigment preparation [54,55].…”