Ž . Vibrational Spectroscopy 22 2000 75–86 www.elsevier.comrlocatervibspec IR structural evidence of hydrotalcites derived oxidic forms Mihaela Jitianu ) , Andrei Jitianu, Maria Zaharescu, Dorel Crisan, Rodica Marchidan Institute of Physical Chemistry, 202 Splaiul Independentei, 77208 Bucharest, Romania Received 23 April 1999; received in revised form 24 August 1999; accepted 30 August 1999 Abstract w Ž . xŽ . Ni–Al hydrotalcite-like compounds with the general formula Ni Al OH CO Ø mH O, where 0.25 Fx F0.66, 1yx x 2 3 x r2 2 were synthesised using coprecipitation at a constant pH, and were treated hydrothermally. The structures of the oxidic forms obtained by calcination of the hydrotalcites at 4508C and 9008C, respectively, were investigated using X-ray diffraction and, mainly, IR and UV–VIS spectroscopy. A NiO phase was identified by XRD in all calcined samples; an additional oxidic phase — the nickel spinel, NiAl O — developed at 9008C. IR spectroscopy results gave supplementary information about 2 4 the incipient, local organisation of cations in the interstices of the oxygen atoms lattice. IR spectra were different, depending Ž . on the samples’ composition. In case of the HT precursors calcined at 4508C a structure like a transition alumina g-Al O 2 3 was found as a main oxidic phase in samples with a high Al-content; IR spectra of the samples with a high Ni content evidenced NiO as the main oxidic phase; in case of these latter samples, the formation of an oxidic structure with a spinel-type local order was identified at this temperature. This structure developed to an inverse nickel spinel oxidic phase at 9008C, as shown by the IR absorption bands. The NiO structure in the samples with a high Ni content at 4508C was found also in the oxides obtained by calcination at 9008C. The spinel-type local order was also observed by UV–VIS spectroscopy in case of the HT precursors calcined at 4508C, by the presence of both absorption bands of the tetrahedral and octahedral Ž . Ž . Ni II ions in the Al O lattice and of octahedral Ni II ions in the NiO lattice. The same absorption bands were found also 2 3 Ž . in the samples calcined at 9008C, proving that the NiAl O spinel identified has a partial inverse structure, with the Ni II 2 4 ions both in tetrahedral and octahedral crystalline fields. Our found structural data were in accord with the models proposed in the literature. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Hydrotalcite; Takovite; Mixed oxides; Spinel; IR spectroscopy; UV–VIS spectroscopy 1. Introduction Ž . The hydrotalcites HTs belong to the large class of the anionic clays, natural and synthetic layered Ž . double hydroxides LDHs . They are less spread in nature with respect to the cationic clays, but they can ) Corresponding author. Tel.: q40-1-312-1147; fax: q40-1- 312-1147; e-mail: ajitianu@chimfiz.icf.ro wx be synthesised in a wide range of compositions 1. A lot of papers concerning the synthesis of HTs with w x very different cations have been reported 2–15 . HT-like compounds have the general formula w Ž . Ž .Ž . x xq zy M II M III OH A Ø mH O and have a 1yx x 2 x r z 2 Ž . Ž . CdI type structure, with a partial M II rM III 2 substitution, the corresponding positive charge being w x balanced by the interlayer anions 16–18 . Ni–Al hydrotalcite, called takovite, is a pyroaurite-like min- w x eral related to hydrotalcite 16,17 . 0924-2031r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. Ž . PII: S0924-2031 99 00067-3