Relations of crystal structure to magnetic properties in the quasi-one-dimensional compound PbNi 1.88Mg 0.12V 2O 8

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The crystal structure of the quasi-one-dimensional oxide PbNi 1.88Mg 0.12V 2O 8 has been studied by Rietveld analysis of combined high-resolution neutron and X-ray powder diffraction data at 300 K and at low temperatures. The (Ni/Mg)O 6 octahedral units share a common edge and form spiral chains along the c-axis of the tetragonal unit cell, without deviating the I4 1 cd ( Z=8) symmetry upon cooling. DC magnetic susceptibility measurements show that the system undergoes a magnetic phase transition below T N≅3.4 K. 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 Ni 2+ moment [〈 μ〉=0.98(3) μ B] suggests important zero-point spin fluctuations. Structural considerations are consistent with small changes in the interatomic distances around the bridging tetrahedral VO 4 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.