Crystallographic Changes Accompanying the Verwey Transition in a Magnetite from Steatite

Fernando Da Silva,Sandra Nogueira,Antônio Goulart,José Fabris,Paulo Couceiro

doi:10.21577/0103-5053.20200073

Abstract

The crystallographic structural analysis of the magnetite of a steatite rock at 298 K, (saturation magnetization, σ = 95.0 J T-1 kg-1) showed two cubic networks and, below Verwey temperature (TV ca.120 K) a monoclinic structure, basing on their cell dimensions and different transitional behaviors when reducing the temperature. A monoclinic structure was identified from their cell dimensions and different transition behaviors when reducing the temperature. The average chemical formula of this almost stoichiometric magnetite was deduced from the chemical analysis, 298 K-Mossbauer and the structural refinement. The Rietveld fitting of the 298 K X-ray pattern is Fe3+Fe0.953+Fe0.882+Cr0.043+Al0.043+Mg0.052+Ni0.022+⊗0.02O4, where [ ] and { } stand for cations in tetrahedral and octahedral coordination symmetries, respectively. The crystallographic structure below TV in this magnetite was observed from synchrotron X-ray diffraction data (XRD), collected at 15 K. It was identified three structures: two cubic (space group, Fd3m), with significantly different lattice parameters, and one monoclinic (P2/c).

Highlights

Magnetite is a ferrimagnetic iron oxide with an inverted spinel structure.[1,2] In nature, magnetite is often oxidized to hematite,[3] a thermodynamically more stable ferric oxide, either directly or via an intermediate formation of maghemite,[4] which crystallizes in space group Fd3m
As a result of this complexity, powder X-ray diffraction (XRD) patterns usually show a great deal of superposed reflections that can be better addressed with the Rietveld method.[15]
The measured value of saturation magnetization for the magnetic extract from the rock sample was σ = 95.0 J T-1 kg‐1, which is only slightly below the characteristic value for a pure magnetite,[21] σ ca. 100 J T-1 kg‐1, which suggests a sample in excellent condition (Figure 1)

Summary

Introduction

Based on the Mössbauer spectroscopy, we conclude that the existence of two hyperfine octahedral sites was the result of a slight crystallographic distortion of one of them, which leads to a small, but still significant, change in the value of the quadrupole displacement, ε = –0.04(1) (Table 2), in relation to the characteristic magnetite (ε = +0.01(1)). The structural Rietveld refinement[25] of XRD data for the sample at 298 K was performed by taking into account two crystallographic dimensions for the cubic cells.

Objectives

Results

Conclusion