From cation flexibility to multifaceted industrial adoptability: a voyage to the resourceful spinel

Signature of cation flexibility associated with the order-disorder structure of three spinel families, namely aluminates (MgAl2O4/ZnAl2O4), ferrites (NiFe2O4/ZnxNi(1-x)Fe(2)O(4)), and titanate (Li4Ti5O12) was probed using the X-ray diffraction and Raman spectroscopy. Cation flexibility (also known as antisite defects or (dis) order structural transitions) is a unique feature of many oxide AB(2)O(4) spinel systems in which the cations exchange their sites under external influence. This feature generates functionality in many different ways. In some cases, such as ferrites, the flexibility of zinc to occupy tetrahedral sites enhances the magnetic properties, whereas the flexibility of lithium in lithium titanate brings exotic electrochemical feature with zero strain. Similarly, cation flexibility of magnesium and aluminium makes the structure tolerant towards nuclear irradiation. Some such salient features of spinel systems are described highlighting the underlying crystal structures. Enriched with enormous chemical versatility, order-disorder structure, vacancy, interstitials and mixed-cation occupancy, `Spinel is a world in itself'. This is why, exploring its huge accomplishments never gets enough. Ever wonder, where from this versatility stems in! This review endeavours to seek the answer by combining the flexible order-disorder structure, supporting diagnosis tools and tuneable properties of this versatile class: The Spinels