Structural transformation in magnetite below the Verwey transition

Abstract

The magnetite structure was studied with synchrotron x-ray powder diffraction above and below the Verwey transition. A symmetry-mode analysis was performed to obtain the atomic displacements from the amplitudes of condensing modes. The main contributing modes that drive the structural phase transition at the Verwey temperature correspond to the irreducible representations 5 , X 1 , X 4 , W 1 , and W 2 . The W modes, neglected so far, must be taken into account so a reliable description of the low-temperature crystal structure can be obtained. This is refined in the nonpolar space group C2/c with ten nonequivalent octahedral irons. The condensation of the mentioned modes leads to a wide distribution of local environments around the octahedral iron atoms, whose valences range between 2.53 and 2.84. This finding rules out any bimodal charge disproportionation of the octahedral iron atoms, i.e., an Fe 2+ -like/Fe 3+ -like ordering.

Magnetic properties of glass-ceramics obtained by crystallization of iron-rich borosilicate glasses

“…The Rietveld full pattern fitting yields the following lattice parameters: a = 8.3954(12) Å and 8.3976(28) Å for BSFC24 and BSFP24, respectively. These values are slightly higher than the data for ideal magnetite (a = 8.39457(1) Å [37]). The attempt to calculate the degree of crystallinity using the amorphous background and the large peaks located at 23, 45, and 60, is basically questionable because the amorphous glassy matrix, which consists mainly in light elements (Si, B, Na), and the crystalline magnetite have different atomic scattering factors.…”

Strong local lattice instability in hexagonal ferritesRFe2O4(R= Lu,Y,Yb) revealed by x-ray absorption spe

“…Below T CO this electron is localized on the iron sites, impeding its mobility. However, it has been later demonstrated that this Verwey CO model is at odds with the experimental facts in Fe 3 O 4 because the charge distribution is not bimodal and the maximum charge disproportionation found is much lower than one electron [13,14]. On the other hand, the CO transition in LuFe 2 O 4 was considered to originate electrical dipoles in the geometrically frustrated triangular Fe lattice that would give rise to ferroelectricity [15].…”

Charge order at magnetite Fe3O4(0 0 1): surface and Verwey phase transitions

“…The decrease of the conductivity is due to a freezing of the electron hopping between different octahedral Fe sites, causing a charge disproportionation that results in two types of Fe B atoms acting with a slightly enhanced (Fe 3+ ) or reduced (Fe 2+ ) valence. The distribution of the different Fe B atoms at the unit cell configures the charge order (CO), intimately linked to the orbital order 5,6 , and determines the full monoclinic Cc symmetry [7][8][9][10][11] . Decades of efforts have been devoted to the understanding of the VT 12,13 , and yet some puzzling fundamental aspects remain unanswered, such as the structural or electronic origin of the transition, or the extent of the short range order above T V .…”