43 research outputs found
Magnetic properties of Ag(2)VOP(2)O(7): an unexpected spin dimer system
Magnetic properties of the silver vanadium phosphate Ag(2)VOP(2)O(7) are
studied by means of magnetic susceptibility measurements and electronic
structure calculations. In spite of the layered crystal structure suggesting 1D
or 2D magnetic behavior, this compound can be understood as a spin dimer
system. The fit of the magnetic susceptibility indicates an intradimer
interaction of about 30 K in perfect agreement with the computational results.
Our study emphasizes the possible pitfalls in interpreting experimental data on
structural basis only and points out the importance of microscopic models for
the understanding of the magnetic properties of vanadium phosphates.Comment: 8 pages, 5 figures; to appear in PR
Sr2V3O9 and Ba2V3O9: quasi one-dimensional spin-systems with an anomalous low temperature susceptibility
The magnetic behaviour of the low-dimensional Vanadium-oxides Sr2V3O9 and
Ba2V3O9 was investigated by means of magnetic susceptibility and specific heat
measurements. In both compounds, the results can be very well described by an
S=1/2 Heisenberg antiferromagnetic chain with an intrachain exchange of J = 82
K and J = 94 K in Sr2V3O9 and Ba2V3O9, respectively. In Sr2V3O9,
antiferromagnetic ordering at T_N = 5.3 K indicate a weak interchain exchange
of the order of J_perp ~ 2 K. In contrast, no evidence for magnetic order was
found in Ba2V3O9 down to 0.5 K, pointing to an even smaller interchain
coupling. In both compounds, we observe a pronounced Curie-like increase of the
susceptibility below 30 K, which we tentatively attribute to a staggered field
effect induced by the applied magnetic field. Results of LDA calculations
support the quasi one-dimensional character and indicate that in Sr2V3O9, the
magnetic chain is perpendicular to the structural one with the magnetic
exchange being transferred through VO4 tetrahedra.Comment: Submitted to Phy. Rev.
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
New lead vanadium phosphate with langbeinite-type structure: Pb1.5V2(PO4)3
The new lead vanadium phosphate Pb1.5V2(PO4)3 was synthesized by solid state reaction and characterized by X-ray powder diffraction, electron microscopy, and magnetic susceptibility measurements. The crystal structure of Pb1.5V2(PO4)3 (a = 9.78182(8) Å, S.G. P213, Z = 4) was determined from X-ray powder diffraction data and belongs to the langbeinite-type structures. It is formed by corner-linked V3+O6 octahedra and tetrahedral phosphate groups resulting in a three-dimensional framework. The lead atoms are situated in the structure interstices and only partially occupy their positions. An electron microscopy study confirmed the structure solution. Magnetic susceptibility measurements revealed Curie–Weiss (CW) behavior for Pb1.5V2(PO4)3 at high temperature whereas at around 14 K an abrupt increase on the susceptibility was observed