Investigation on lattice constants of Mg-Al spinels

Abstract

Mg-Al spinel is the most typical material in the family of chemical compounds with spinel crystalline structure that exhibits many applications [1, 2]. In general, it can be noted as MgO-n·Al2O3. MgAl2O4 spinel is an fcc structure of oxygen ions with a lattice parameter of 0.808 nm. There are eight molecules in its unit cell, in which there are 64 tetrahedral symmetry sites and 32 octahedral ones. In the perfect case, magnesium ions occupy 8 tetrahedral positions and aluminum ions occupy 16 octahedral sites [3]. In many cases, this MgO-n·Al2O3 compound is nonstoichiometric, n is not equal to 1. Form 1 up to 7.3, the excess Al3+ ions occupying tetrahedral sites, substitute for Mg2+ ions [4]. This causes a proportional decrease of lattice parameter with the amount of excess Al3+ due to a smaller diameter of Al3+ than that of Mg2+. In the present paper, we show the result of the unit cell parameters of Mg-Al spinels solid solutions with XRD and provide a formula for calculating the lattice parameter of such Mg-Al spinel compounds, which has a good physical meaning compared with that given by Sigalovsky et al. [5]. In our experiments ultra-fine powders of Mg-Al spinel were prepared with a method of high temperature solid reaction. Reagent grade AlNH4(SO4)2·12H2O and MgSO4·7H2O were mixed based on different n in de-ionized water. These solutions were calcimined at 1080 ◦C for 4–5 h to obtain a single spinel phase powder with being high pure, good disperse and homogeneous. The obtained powders were measured by XRD using Cu-Kα radiation on a D/max rA diffractometer. The XRD patterns of the obtained powder samples are given in Fig. 1. The stoichiometric spinel XRD pattern (a pattern labelled n= 1.0) is the same as the standard powder XRD pattern [6]. For the non-stoichiometric spinel samples, the peaks shift to the direction in higher values of 2θ as n increases. That means the distance of (hkl) plane dhkl became smaller according to the following expression: