Magnetic properties of the geometrically frustrated S= 1 2 antiferromagnets, La2 LiMoO6 and Ba2 YMoO 6, with the B-site ordered double perovskite structure: Evidence for a collective spin-singlet ground state

Tomoko Aharen, John E. Greedan, Craig A. Bridges, Adam A. Aczel, Jose Rodriguez, Greg MacDougall, Graeme M. Luke, Takashi Imai, Vladimir K. Michaelis, Scott Kroeker, Haidong Zhou, Chris R. Wiebe, Lachlan M.D. Cranswick

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Two B-site ordered double perovskites, La2 LiMoO6 and Ba2 YMoO6, based on the S= 1 2 ion, Mo5+, have been investigated in the context of geometric magnetic frustration. Powder neutron diffraction, heat capacity, susceptibility, muon-spin relaxation (μSR), and Y 89 NMR-including magic-angle spinning (MAS) NMR data have been collected. La2 LiMoO6 crystallizes in P 21 /n with a=5.59392 (19) Å b=5.69241 (16) Å c=7.88029 (22) Å and β=90.2601 (30)°at 299.7 K, while Ba2 YMoO6 is cubic, Fm3m, with a=8.39199 (65) Å at 297.8 K. Ba2 YMoO6 shows no distortion from cubic symmetry even at 2.8 K in apparent violation of the Jahn-Teller theorem for a t 2g 1 ion. Y 89 NMR MAS data indicate about a 3% level of Y/Mo site mixing. Ba2 YMoO6 deviates strongly from simple Curie-Weiss (C-W) paramagnetic behavior below 150 K and zero-field-cooled/field- cooled (ZFC/FC) irreversibility occurs below 20 K with a weak, broad susceptibility maximum near 5 K in the ZFC data. A Curie-Weiss fit shows a reduced μeff =1.42 μB, (spin only=1.73 μB) and a Weiss temperature, θC, which depends strongly on the temperature range of the fit. Powder neutron diffraction and heat capacity show no evidence for long-range magnetic order to 2 K. On the other hand oscillations develop below 20 K in μSR indicating at least short-range magnetic correlations. Susceptibility data for Ba2 YMoO6 also deviate strongly from the C-W law below 150 K with a nearly spin only μeff =1.72 μB and θC =-219 (1) K. There is no discernible ZFC/FC irreversibility to 2 K. Heat capacity, neutron powder diffraction, and μSR data show no evidence for long-range order to 2 K but a very broad, weak maximum appears in the heat capacity. The Y 89 NMR paramagnetic Knight shift shows a remarkable local spin susceptibility behavior below about 70 K with two components from roughly equal sample volumes, one indicating a singlet state and the other a strongly fluctuating paramagnetic state. Further evidence for a singlet state comes from the behavior of the relaxation rate, 1/ T1. These results are discussed and compared with those from other isostructural S= 1 2 materials and those based on S=3/2 and S=1.

Original languageEnglish (US)
Article number224409
JournalPhysical Review B - Condensed Matter and Materials Physics
Issue number22
StatePublished - Jun 4 2010
Externally publishedYes

ASJC Scopus subject areas

  • Electronic, Optical and Magnetic Materials
  • Condensed Matter Physics


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