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Two B-site ordered double perovskites, La2LiMoO6 and Ba2YMoO6, based on the S= 1/2 ion, Mo5+, have been investigated in the context of geometric magneticfrustration. Powder neutron diffraction, heat capacity, susceptibility, muonspin relaxation(_SR), and 89Y NMR- including MAS NMR- data have been collected.La2LiMoO6 deviates strongly from simple Curie-Weiss paramagnetic behavior below150K and zero-field cooled/ field cooled (ZFC/FC)irreversibility occurs below20K with a weak, broad susceptibility maximum near 5K in the ZFC data. ACurie-Weiss fit shows a reduced mu_eff=1.42\mu_B, (spin only = 1.73 muB) and aWeiss temperature, \theta_c, which depends strongly on the temperature range ofthe fit. Powder neutron diffraction, heat capacity and 7Li NMR show no evidencefor long range magnetic order to 2K. On the other hand oscillations developbelow 20K in muSR indicating at least short range magnetic correlations.Susceptibility data for Ba2YMoO6 also deviate strongly from the C-W law below150K with a similarly reduced mu_eff = 1.72\mu_B and \theta_c = - 219(1)K. Heatcapacity, neutron powder diffraction and muSR data show no evidence for longrange order to 2K but a very broad maximum appears in the heat capacity. The89Y NMR paramagnetic Knight shift shows a remarkable local spin susceptibilitybehavior below about 70K with two components from roughly equal sample volumes,one indicating a singlet state and the other a strongly fluctuatingparamagnetic state. Further evidence for a singlet state comes from thebehavior of the relaxation rate, 1/T1. These results are discussed and comparedwith those from other isostructural S = 1/2 materials and those based on S =3/2 and S = 1.

physical review. b, condensed matter and materials physics

Magnetic properties of the geometrically frustrated S=1/2  antiferromagnets, La2LiMoO6 and Ba2YMoO6, with the B-site ordered double  perovskite structure: Evidence for a Collective Spin Singlet Ground State