Spinel Structure

Features of the spinel

An ideal spinel structure is described by the general formula A²⁺B₂³⁺O4, where A²⁺ is a divalent metal ion. The unit cell of ideal spinel structure consists of a close-packed arrangement of oxygen ions with two types of interstices for the cations as shown in Fig. 1. Tetrahedral (A) sites are surrounded by or coordinated with four nearest neighboring oxygen ions whose lines connecting their centre form a tetrahedron. Octahedral (B) sites are coordinated by six nearest neighboring oxygen ions whose centre connecting lines describe an octahedron. The unit cell contains 32 anions forming 64 tetrahedral interstices and 32 octahedral interstices; of these 8 tetrahedral and 16 octahedral are occupied by cations. These are called A- and B-sites, respectively. Thus, an elementary spinel cell contains eight molecules, where A and B are the symbols for metal cation at least one of them belongs to the transition metal group. Here A is a tetrahedral surrounded cation, B an octahedral surrounded one and O an anion [1]

Fig. 1. Two octants of the spinel unit cell showing A ions on tetrahedral sites and B ions on octahedral sites

      Ferrites have a cubic spinel structure with eight formula units per unit cell. A unit cell comprises eight tetrahedral and 16 octahedral sites, with formula (A) [B] ₂O₄, where (A) and [B] refer to the tetrahedral and the octahedral cation sites respectively, in a FCC anion (oxygen) sub lattice. Due to the large electronegativity oxygen the ionic type of bonds prevails in almost all oxide spinels. In cubic spinel ferrites, while in inverse spinel structure, the tetrahedral (A) sites are occupied by the Fe³⁺ ions and the octahedral sites [B] are occupied by the divalent metal ions (M²⁺⁾ and Fe³⁺ in equal proportions. In a normal spinel structure cations occupy the tetrahedral (A) sites, where as Fe3+ cations occupy the octahedral [B] site. The types of cations and their distributions between the two interstitial sites in these ferrites determine the intrinsic magnetic properties of the ferrites [2-3].

[1] M. Kaiser, Effect of nickel substitutions on some properties of Cu–Zn ferrites, Journal of Alloys and Compounds xxx (2008) xxx–xxx

[2] Javed Ahmad, Muhammad Qadeer Awan, Muhammad Ehsan Mazhar, Muhammad Naeem Ashiq, “Effect of substitution of Co²⁺ ions on the structural and electrical properties of nanosized magnesium aluminate”, Physica B 406 (2011) 254-258

[3] Jing Jiang,Yang Min Yang,Liang Chao Li,  “Synthesis and magnetic properties of lanthanum substituted lithium-nikel ferrites via a soft chemistry route”, Physica B 399 (2007) 105-108