Relations of crystal structure to magnetic properties in the quasi-one-dimensional compound PbNi1.88Mg0.12V 2O8

Abstract

The crystal structure of the quasi-one-dimensional oxide PbNi 1.88Mg0.12V2O8 has been studied by Rietveld analysis of combined high-resolution neutron and X-ray powder diffraction data at 300K and at low temperatures. The (Ni/Mg)O6 octahedral units share a common edge and form spiral chains along the c-axis of the tetragonal unit cell, without deviating the I41cd (Z=8) symmetry upon cooling. DC magnetic susceptibility measurements show that the system undergoes a magnetic phase transition below TN≅3.4K. Rietveld analysis of the medium resolution neutron powder diffraction data confirms that impurity-induced antiferromagnetic order (with propagation vector, κ=[000]) takes over from the Haldane ground state of the parent compound. The power-law [β=0.31(3)] temperature evolution of the strongest magnetic Bragg peak intensity indicates three-dimensional Ising-type magnetic interactions, while the reduced magnitude of the Ni2+ moment [〈μ〉=0.98(3)μB] suggests important zero-point spin fluctuations. Structural considerations are consistent with small changes in the interatomic distances around the bridging tetrahedral VO4 entities separating the chains. However, no bulk structural phase transition concurrent to the Néel ordering is found. We show that the modification of intra- and inter-chain Ni-Ni distances upon cooling promotes the magnetic coupling of the end-of-chain liberated S=1/2 spins and leads to antiferromagnetic ordering. © 2004 Elsevier Inc. All rights reserved.