Hydrothermal conversion of amorphous NiFe2–xAlx(OH)8 into crystalline phases

Abstract

Mixtures of amorphous hydroxides prepared from Fe^IIIAl^III and Ni^II nitrate solutions in proportion to obtain a series of compounds with 0.1 (if necessary 0.05) increments for x in NiFe_2–xAl_x(OH)₈, have been subjected to hydrothermal ageing at 150 °C in mother-liquor for one or six months. X-Ray, IR, TG and magnetic studies revealed no influence of ageing time on the properties of the crystalline phases obtained. It was proved that for certain values of x, it is possible to obtain hydrothermally a pure phase of nickel ferrite-aluminate solid solutions, displaying magnetic characteristics similar to those of solid solutions produced by standard ceramic methods. At x= 0.5 the spinel phase deteriorates, the haematite structure appears and the nickel ions engage (till x= 2) in the formation of layered double hydroxides (LDHs), accompanied from x= 0.8 by the boehmite phase. At x= 1.8 the spinel phase appears again, having a lattice parameter corresponding to 0.72 Al³⁺ in the ferrite structure.