New Method to Calculate the Sign and Relative Strength of Magnetic Interactions in Low-Dimensional Systems on the Basis of Structural Data

Antiferromagnetic spin-frustrated layers of corner-sharing Cu4 tetrahedra on the kagome lattice in volcanic minerals Cu5O2(VO4)2(CuCl), NaCu5O2(SeO3)2Cl3, and K2Cu5Cl8(OH)4·2H2O

The objective of the present work was to analyze the possibility of realization of quantum spin liquids in three volcanic minerals-averievite (Cu₅O₂(VO₄)₂(CuCl)), ilinskite (NaCu₅O₂(SeO₃)₂Cl₃), and avdononite (K₂Cu₅Cl₈(OH)₄·2H₂O)-from the crystal chemistry point of view. Based on the structural data, the sign and strength of magnetic interactions have been calculated and the geometric frustrations serving as the main reason of the existence of spin liquids have been investigated. According to our calculations, the magnetic structures of averievite and ilinskite are composed of antiferromagnetic (AFM) spin-frustrated layers of corner-sharing Cu₄ tetrahedra on the kagome lattice. However, the direction of nonshared corners of tetrahedra is different in them. The oxygen ions centering the OCu₄ tetrahedra in averievite and ilinskite provide the main contribution to the formation of AFM interactions along the tetrahedra edges. The local electric polarization in averievite and the possibility of spin configuration fluctuations due to vibrations of tetrahedra-centering oxygen ions have been discussed. The existence of structural phase transitions accompanied with magnetic transitions was assumed in ilinskite because of the effect of a lone electron pair by Se⁴⁺ ions. As was demonstrated through comparison of averievite and avdoninite, at the removal of centering oxygen ions from tetrahedra, the magnetic structure of the pyrochlore layer present in averievite transformed into an openwork curled net with large cells woven from corner-sharing open AFM spin-frustrated tetrahedra ('butterflies') in avdoninite.

The importance of the on-site electron-electron interaction for the magnetic coupling in the zigzag spin-chain compound In2VO5

We present first-principles electronic structure calculations for the zigzag spin-chain compound In(2)VO(5) using the generalized gradient approximation both with and without inclusion of an on-site Coulomb interaction. It has been proposed that In(2)VO(5) is characterized by itinerant V 3d electrons at high temperature and localized electrons at low temperature. Consequently, it is to be expected that electronic correlations play an important role for the magnetic transition from ferromagnetic to antiferromagnetic exchange around 120 K. In this context, we study the electronic and magnetic properties of a set of possible spin configurations. Our calculations show that inclusion of an on-site Coulomb interaction in fact changes the ground state from ferromagnetic to antiferromagnetic.

Crystal chemistry aspects of the magnetically induced ferroelectricity in TbMn(2)O(5) and BiMn(2)O(5)

The origin of magnetic frustration was stated and the ions, whose shift is accompanied by emerging magnetic ordering and ferroelectricity in TbMn(2)O(5) and BiMn(2)O(5), were determined on the basis of calculating the 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 in the crystal structure. The approximation of the bond valence to the initial value (charge ordering) under magnetic ordering conditions is only possible again due to polar displacement of Mn2 (or O1) and O4 ions along the b axis which is the cause of the ferroelectric transition.

Search of low-dimensional magnetics on the basis of structural data: spin-1/2 antiferromagnetic zigzag chain compounds In(2)VO(5), β-Sr(VOAsO(4))(2), (NH(4),K)(2)VOF(4) and α-ZnV(3)O(8)

A new technique for searching low-dimensional compounds on the basis of structural data is presented. The sign and strength of all magnetic couplings at distances up to 12 Å in five predicted new antiferromagnetic zigzag spin-1/2 chain compounds In(2)VO(5), β-Sr(VOAsO(4))(2), (NH(4))(2)VOF(4), K(2)VOF(4) and α-ZnV(3)O(8) were calculated. It was stated that in the compound In(2)VO(5) zigzag spin chains are frustrated, since the ratio (α = J(2)/J(1)) of competing antiferromagnetic (AF) nearest- (J(1)) and AF next-to-nearest-neighbour (J(2)) couplings is equal to 1.68 that exceeds the Majumdar-Ghosh point by 1/2. In other compounds the zigzag spin chains are AF magnetically ordered single chains as [Formula: see text]. The interchain couplings were analysed in detail.