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Phase separation and frustrated square lattice magnetism of Na1.5VOPO4F0.5

A. A. Tsirlin, R. Nath, A. M. Abakumov, Y. Furukawa, D. C. Johnston, M. Hemmida, H.-A. Krug von Nidda, A. Loidl, C. Geibel, and H. Rosner
Phys. Rev. B 84, 014429 – Published 28 July 2011
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Abstract

Crystal structure, electronic structure, and magnetic behavior of the spin-12 quantum magnet Na1.5VOPO4F0.5 are reported. The disorder of Na atoms leads to a sequence of structural phase transitions revealed by synchrotron x-ray powder diffraction and electron diffraction. The high-temperature second-order αβ transition at 500 K is of the order-disorder type, whereas the low-temperature βγ+γ transition around 250 K is of the first order and leads to a phase separation toward the polymorphs with long-range (γ) and short-range (γ) order of Na. Despite the complex structural changes, the magnetic behavior of Na1.5VOPO4F0.5 probed by magnetic susceptibility, heat capacity, and electron spin resonance measurements is well described by the regular frustrated square lattice model of the high-temperature α-polymorph. The averaged nearest-neighbor and next-nearest-neighbor couplings are J¯13.7 K and J¯26.6 K, respectively. Nuclear magnetic resonance further reveals the long-range ordering at TN=2.6 K in low magnetic fields. Although the experimental data are consistent with the simplified square-lattice description, band structure calculations suggest that the ordering of Na atoms introduces a large number of inequivalent exchange couplings that split the square lattice into plaquettes. Additionally, the direct connection between the vanadium polyhedra induces an unusually strong interlayer coupling having effect on the transition entropy and the transition anomaly in the specific heat. Peculiar features of the low-temperature crystal structure and the relation to isostructural materials suggest Na1.5VOPO4F0.5 as a parent compound for the experimental study of tetramerized square lattices as well as frustrated square lattices with different values of spin.

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  • Received 11 April 2011

DOI:https://doi.org/10.1103/PhysRevB.84.014429

©2011 American Physical Society

Authors & Affiliations

A. A. Tsirlin1,*, R. Nath1,2,3, A. M. Abakumov4, Y. Furukawa2, D. C. Johnston2, M. Hemmida5, H.-A. Krug von Nidda5, A. Loidl5, C. Geibel1, and H. Rosner1,†

  • 1Max Planck Institute for Chemical Physics of Solids, Nöthnitzer Str. 40, 01187 Dresden, Germany
  • 2Ames Laboratory and Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011 USA
  • 3School of Physics, Indian Institute of Science Education and Research, Trivandrum-695016 Kerala, India
  • 4EMAT, University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium
  • 5Experimental Physics V, Center for Electronic Correlations and Magnetism, University of Augsburg, 86135 Augsburg, Germany

  • *altsirlin@gmail.com
  • helge.rosner@cpfs.mpg.de

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Vol. 84, Iss. 1 — 1 July 2011

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