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A constant pH precipitation method has been applied to obtain solids with Ni/Fe molar ratios of 2/1, 3/2, 1/1, 2/3, and 1/2. In all cases, a phase with the hydrotalcite-like structure is obtained, containing Ni II and Fe III in the brucite-like layers and carbonate in the interlayer, and, for samples with a Ni/Fe molar ratio lower than 2/1, amorphous hydrated iron oxides, undetected by X-ray diffraction, are also formed. The solids have been characterized by element chemical analysis, powder X-ray diffraction, differential thermal analysis, thermogravimetric and differential thermogravimetric analysis, FT-IR spectroscopy, temperature-programmed reduction and assessment of specific surface area by nitrogen adsorp-* Prof. V. Rives tion at Ϫ196°C. In all cases reduction leads to zero-valent state for the metals, reduced nickel particles probably favouring reduction of Fe III species; the specific surface area increases with the iron content, probably due to the amorphous nature of the hydrated iron oxides formed. Calcination at 1200°C in air leads to well crystallized solids, formed by NiFe 2 O 4 spinel and, additionally, rocksalttype NiO for Ni/Fe ratios larger than 1/2. In this way, solids with tailored compositions of these two phases can be prepared.

Lanthanide (Eu and Nd)-doped hydrocalumite has been prepared by a coprecipitation method. All samples, with Eu 3þ and Nd 3þ contents up to 4% (molar ratio), show a single crystallographic phase, without segregation of secondary lanthanide-containing phases. Calcination affords lime and mayenite, with lanthanide cations embedded in the structure. The photoluminescence spectra of the noncalcined samples consist of Eu 3þ (in the visible region) and Nd 3þ (in the NIR) intra-4f transitions and a broad band ascribed to Al-related defects. The Eu 3þ5 D 0 lifetime values and quantum efficiency of the Eu 3þ /Nd 3þ codoped hydrocalumite decrease ca. 10%, relative to the single doped material, suggesting that the Nd 3þ incorporation activates nonradiative channels for the 5 D 0 depopulation. High-temperature stable ceramic pigments, prepared from Eu 3þ and Nd 3þ doped hydrocalumite precursors, afford multiwavelength luminescent materials emitting in a large vis/NIR spectral region, with potential applications as barcodes and in broadband amplifiers. The emission spectra of the calcined samples give unequivocal evidence for the presence of two Eu 3þ local environments, attributed to Eu 3þ sites in CaO and mayenite. Upon calcination, the absolute emission quantum yield of the Eu 3þ -doped sample increases from less than 0.01 to 0.06.

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