35 research outputs found
Crystal chemistry aspects of the magnetically induced ferroelectricity in TbMn2O5 and BiMn2O5
The origin of magnetic frustration was stated and the ions whose shift is
accompanied by emerging magnetic ordering and ferroelectricity in TbMn2O5 and
BiMn2O5 were determined on the basis of calculation of magnetic coupling
parameters by using the structural data. The displacements accompanying the
magnetic ordering are not polar, they just induce changes of bond valence
(charge disordering) of Mn1 and Mn2, thus creating instability of the crystal
structure. To approximate again the bond valence to the initial value (charge
ordering) under magnetic ordering conditions is possible only due to polar
displacement of Mn2 (or O1) and O4 ions along the b axis that is the cause of
ferroelectric transition.Comment: 17 pages, 3 figures, 5 table
Crystal chemistry search of multiferroics with the stereochemically active lone pair
On the basis of our previous studies of magnetoelectric ordering of BiFeO3,
TbMnO3, TbMn2O5 and BiMn2O5 we formulate the crystal chemistry criteria for the
search of multiferroics and reveal potential multiferroics Pb2Cu(OH)4Cl2,
Pb5Cr3F19, Mn(SeO3){\dot}H2O and BiPbSr2MnO6 each containing the ion with a
lone pair.Comment: 4 pages, 8 figures,submitted to J Supercond Nov Mag
Prediction of inorganic superconductors with quasi-one-dimensional crystal structure
Models of superconductors having a quasi-one-dimensional crystal structure
based on the convoluted into a tube Ginzburg sandwich, which comprises a
layered dielectric-metal-dielectric structure, have been suggested. The
critical crystal chemistry parameters of the Ginzburg sandwich determining the
possibility of the emergence of superconductivity and the Tc value in layered
high-Tc cuprates, which could have the same functions in quasi-one-dimensional
fragments (sandwich-type tubes), have been examined. The crystal structures of
known low-temperature superconductors, in which one can mark out similar
quasi-one- dimensional fragments, have been analyzed. Five compounds with
quasi-one-dimensional structures, which can be considered as potential parents
of new superconductor families, possibly with high transition temperatures,
have been suggested. The methods of doping and modification of these compounds
are provided.Comment: 22 pages, 14 figures and 2 table
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