X-RAY STRUCTURAL ANALYSIS OF COMPOUNDS WITH SPINEL STRUCTURE

L.A. Aleshina¹, I.S. Inina², E.V. Moshkina¹, E.A. Repnikova¹, I.S. Hamraeva¹

¹ Petrozavodsk State University, pr. Lenina, 33, Petrozavodsk, Karelia, Russia. e-mail: erepn@mainpgu.karelia.ru
²Karelian Scientific Center, Pushkinskaya sreet, 11, Petrozavodsk, Karelia, Russia.

Keywords: X-Ray diffraction, structure, spinel.

Magnetite and magnesiochromite are basic raw materials for metallurgical industry. This minerals belong to the sub-class of compound oxides with the spinel structure and expressd by general formula Me²⁺Me₂³⁺O₄. , where Me is cation of the metal. Magnetite (Fe₃O₄) has the invers-spinel structure with space group Fd3m. Magnesiochromite (MgCr₂O₄) classified as normal spinel with the same space group. Physical properties of natural spinels are controlled by isomorphic impurities content and distribution of substitution cations between different crystallographical positions in the structure.

In our work we investigated magnetite and magnesiochromite samples from different regions of the deposit, which possess different magnetic properties. It^,s known that structure of magnetite samples of this deposit may include Al³⁺, Ti²⁺, Cr²⁺ and Mg²⁺ impurities, substituting for Fe²⁺ and Fe³⁺ cations. In the structure of magnesichromite Mg²⁺ and Cr²⁺ cations may substitute for Al³⁺, Fe²⁺, Fe³⁺, Mn²⁺ cations. But the quantity of this isomorphic impurities in samples weren^,t known.

For the investigation of the character of isomorphic impurities distribution between octahedral and tetrahedral positions in the spinel structure of samples we used X-Ray structural analysis with combination of computer modelling of the spinel structure. Analysing variations of unit cell parameters from sample to sample and character of diffracted intensity re-distribution on powder X-Ray patterns have been made suppositions about impurities type and their distribution between tetrahedral and octahedral positions in the structure. Thus, on the base of experimental data have been constructed models of structure for different samples.

Then, measured X-Ray pattern by experiment for each sample has been compeared with X-Ray patterns, calculated for different structure models. Have been selected the model for which observed agreement between experimental and theoretical X-Ray patterns. For obtaining a significant solution by Ritvield method have been refined some structure parameters: terminal position parameters, the character of the distribution of impurity cations in octahedral and tetrahedral positions and occupacy of sites coefficient.