Question

3. a. Classify the following minerals:CaZrO3, FeCr2O4, SrTiO3, ZnAl2O4; as spinels or perovskites. Show how you came to this classification.

b. Describe the important electrical and/or magnetic properties of spinels, perovskites and non-stoichiometric oxides. (Note: if you quote directly from the textbook or other source please use an appropriate citation).

Answer 1

a)

The general formula for Perovskites are   ABX3, and they are named after the mineral Perovskite, that has the composition CaTiO3. In its ideal form, the perovskite structure is a cubic closed pack structure. In this structure the Ca cations are cuboctahedrally surrounded by 12 oxygen ions and the Ti cations are octahedrally surrounded by 6 oxygen ions.

So here the molecules CaZrO3, and SrTiO3,are of ABX3 type and belongs to prevoskites structure.

In spinels the oxygen ions are forming a cubic close packed structure. There are two kinds of spinels: normal spinel with the general formula AB2X4 and inverse spinel with the general formula B[AB]O4.

So here the molecules FeCr2O4, ZnAl2O4 belongs to AB2X4 and have spinel structure

Spinels are important class of metal oxides. They exhibit two common forms of cooperative magnetic properties . Ferromagnetism and antiferromagnetism. Spinel ferrite is known to be good candidate for magnetic recording and microwave applications. Nano-sized grains of ferrite are being applied in permanent magnets, magnetic drug delivery, and high-density information storage. Spinels in general are very important in solid state electronic industry for their electric and magnetic properties.

Non-stoichiometric compounds are chemical compounds, , having elemental composition whose proportions cannot be represented by integers. In such materials, some small percentage of atoms are missing or too many atoms are packed into an otherwise perfect lattice work. Since the solids are overall electrically neutral, the defect is compensated by a change in the charge of other atoms in the solid, either by changing their oxidation state, or by replacing them with atoms of different elements with a different charge. Many metal oxides and sulfides have non-stoichiometry for example, stoichiometric iron(II) oxide, which is rare, has the formula FeO, whereas the more common material is nonstoichiometric, with the formula Fe_0.95O.( there is a defect consisting of missing Fe atoms).   Non-stoichiometric compounds exhibit special electrical or chemical properties because of the defects; for example, when atoms are missing, the other atoms can move through the solid more rapidly. Non-stoichiometric compounds have applications in ceramic and superconductive material and in electrochemical (i.e., battery) system designs.

The perovskites has an unusual cubic close packed lattice of oxide and calcium ions in the octahedral holes of which the much smaller Ti⁴⁺ ion rattle around. If the temperature is not too high the Ti⁴⁺ ion tend to be off the centre of the lattice unit cell, giving rise to an electric charge separation or dipole, such materials are ferroelectrics. Application of mechanical pressure on one side of perovskite crystal structure causes Ti⁴⁺ ions to migrate generating an electrical current, application of electrical current causes mechanical motion of the ions. The pressure effect known as piezoelectric effect makes perovskites useful in converting mechanical energy to electrical energy. The electrical effect makes these materials useful as capacitors and for deflecting laser beams in electronics

References:

1.) U. Müller, Inorganic structural chemistry, Wiley-VCH, 1999

2.) D. Shriver, P. Atkins, Inorganic chemistry, Wiley-VCH, second edition, 1997