Magnetoelectric ordering of BiFeO3 from the perspective of crystal chemistry

In this paper we examine the role of crystal chemistry factors in creating conditions for formation of magnetoelectric ordering in BiFeO3. It is generally accepted that the main reason of the ferroelectric distortion in BiFeO3 is concerned with a stereochemical activity of the Bi lone pair. However, the lone pair is stereochemically active in the paraelectric orthorhombic beta-phase as well. We demonstrate that a crucial role in emerging of phase transitions of the metal-insulator, paraelectric-ferroelectric and magnetic disorder-order types belongs to the change of the degree of the lone pair stereochemical activity - its consecutive increase with the temperature decrease. Using the structural data, we calculated the sign and strength of magnetic couplings in BiFeO3 in the range from 945 C down to 25 C and found the couplings, which undergo the antiferromagnetic-ferromagnetic transition with the temperature decrease and give rise to the antiferromagnetic ordering and its delay in regard to temperature, as compared to the ferroelectric ordering. We discuss the reasons of emerging of the spatially modulated spin structure and its suppression by doping with La3+.Comment: 18 pages, 5 figures, 3 table

Gold P Pin

Small Gold p pin. Awarded at Pacific College for athletic achievement. Reception of this pin meant membership in the Gold P Club.https://digitalcommons.georgefox.edu/museum_gfu/1108/thumbnail.jp

SINGLE-CRYSTAL POLARIZED FTIR SPECTROSCOPY AND NEUTRON DIFFRACTION REFINEMENT OF CANCRINITE

We relate here a combined single-crystal polarized-light FTIR (Fourier transform infrared) and neutron diffraction study of two natural cancrinites from Cameroun and Canada, respectively. Electron microprobe analyses show both samples to be almost end-member carbonate-cancrinites [ideal chemical formula Na6Ca2(Si6Al6O24)(CO3)2\u20222H2O]. The structural refinements show that the extra-framework content in the large 12-membered rings channels is represented by one independent Na-site (Na2) and two independent, and statistically distributed, CO3 groups. The geometry of the CO3 groups appears to be almost regular, with C1-OC1 ~ 1.298(7) \uc5 and C2-OC2 ~ 1.300(5) \uc5, in agreement with the previous studies [1,2,3]. The atoms of the carbonate-groups are not perfectly coplanar, being z(C1) 60 z(OC1) and z(C2) 60 z(OC2). The H2O molecules and a further Na-site (Na1) lie in the cancrinite-cage; the H2O oxygen site (OW) lies off from the triad axis, giving rise to a statistical configuration with three equivalent and mutually exclusive water molecules, as already suggested by [1,2]. The high-quality neutron data show that the water molecule is almost symmetric, with very similar Ow-H1 and Ow-H2 bond-distances, and is slightly tilted from the (0001) plane. It is involved in bifurcated hydrogen bridges, with two possible bonds for H1 (i.e. OW-H1\u2022\u2022\u2022O2 and OW-H1\u2022\u2022\u2022O4) and two for H2 (i.e. OW-H2\u2022\u2022\u2022O3 and OW-H2\u2022\u2022\u2022O2). The Ow\u2022\u2022\u2022O donor-acceptor distances are all > 2.7 \uc5. The polarised-light FTIR spectra show two main absorptions, at 3602 and 3531 cm-1, respectively. The former is polarised for E \uf05e c, while the latter is polarized for E \uf05e c. On the basis of the neutron diffraction data, the 3602 cm-1 band is assigned to the anti-symmetric stretching mode (\u3bd3), while the 3531 cm-1 band is assigned to the symmetric stretching mode (\u3bd1) of the same water molecule, in agreement with the presence of a single bending mode at 1630 cm-1. One additional weak component at 4108 cm-1 could possibly indicate the presence of low amounts of additional OH groups in the structure of cancrinite. Several overlapping bands in the 1300-1500 cm-1 range are strongly polarized for E \uf05e c, and are assigned to the vibrations of the CO3 group. --------------------------------------- References. [1] H.D. Grundy, I. Hassan, Can. Mineral., 20, 239-251, 1982; [2] P. Ballirano, A. Maras, Eur. J. Mineral., 16, 135-141, 2004; [3] I. Hassan, S.M. Antao, J.B. Parise, Am. Mineral., 91, 1117-1124, 2006

Single-crystal polarized FTIR spectroscopy and neutron diffraction refinement of cancrinite.

We relate a single-crystal FTIR (Fourier transform infrared) and neutron diffraction study of two natural cancrinites. The structural refinements show that the oxygen site of the H2O molecule lies off the triad axis. The water molecule is almost symmetric and slightly tilted from the (0001) plane. It is involved in bifurcated hydrogen bridges, with Ow...O donor\u2013acceptor distances [2.7 Aengstr . The FTIR spectra show two main absorptions. The first at 3,602 cm-1 is polarized for E perp. c and is assigned to the v3 mode. The second, at 3,531 cm-1, is also polarized for E perp. c and is assigned to v1 mode. A weak component at 4,108 cm-1 could possibly indicate the presence of additional OH groups in the structure of cancrinite. Several overlapping bands in the 1,300\u20131,500 cm-1 range are strongly polarized for E perp. c, and are assigned to the vibrations of the CO3 group

Atomistic insight into lithospheric conductivity revealed by phonon–electron excitations in hydrous iron-bearing silicates

Amphiboles are essential components of the continental crust and subduction zones showing anomalous anisotropic conductivity. Rock properties depend on the physical properties of their constituent minerals, which in turn depend on the crystal phonon and electron density of states. Here, to address the atomic-scale mechanism of the peculiar rock conductivity, we applied in situ temperature-dependent Raman spectroscopy, sensitive to both phonon and electron states, to Fe2+-rich amphiboles. The observed anisotropic resonance Raman scattering at elevated temperatures, in combination with density-functional-theory modelling, reveals a direction-dependent formation of mobile polarons associated with coupled FeO6 phonons and electron transitions. Hence, temperature-activated electron-phonon excitations in hydrous iron-bearing chain and layered silicates are the atomistic source of anisotropic lithospheric conductivity. Furthermore, reversible delocalization of H+ occurs at similar temperatures even in a reducing atmosphere. The occurrence of either type of charge carriers does not require initial mixed-valence state of iron or high oxygen fugacity in the system. Amphiboles are hydrous silicates occurring in many rock types in the continental crust and subduction zones. Here, in situ Raman spectroscopy of grunerite reveals temperature-activated electron-phonon excitations that provide an atomistic insight into the role of amphiboles in the anisotropic lithospheric conductivity