Magnetic properties of the geometrically frustrated S=1/2 antiferromagnets, La_2LiMoO_6 and Ba_2YMoO_6, with the B-site ordered double perovskite structure: Evidence for a collective spin-singlet ground state

摘要

Two B-site ordered double perovskites, La_2LiMoO_6 and Ba_2YMoO_6, based on the S=1/2 ion, Mo~(5+), have been investigated in the context of geometric magnetic frustration. Powder neutron diffraction, heat capacity, susceptibility, muon-spin relaxation (μSR), and ~(89)Y NMR-including magic-angle spinning (MAS) NMR data have been collected. La_2LiMoO_6 crystallizes in P2_1 with α=5.59392(19) A, b=5.69241(16) A, c=7.88029(22) A, and β=90.2601(30)° at 299.7 K, while Ba_2YMoO_6 is cubic, Fm3m, with α=8.39199(65) A at 297.8 K. Ba_2YMoO_6 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. ~(89)Y NMR MAS data indicate about a 3% level of Y/Mo site mixing. La_2LiMoO_6 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 μ_(eff) = 1.42/μ_B, (spin only = 1.73μ_B) and a Weiss temperature, θ_C, 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 Ba_2YMoO_6 also deviate strongly from the C-W law below 150 K with a nearly spin only μ_(eff)= 1.72μ_B and θ_C=-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 ~(89)Y 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 / T_1. 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.