  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 magnetic
frustration. Powder neutron diffraction, heat capacity, susceptibility, muon
spin relaxation(_SR), and 89Y NMR- including MAS NMR- data have been collected.
La2LiMoO6 deviates strongly from simple Curie-Weiss paramagnetic behavior below
150K and zero-field cooled/ field cooled (ZFC/FC)irreversibility occurs below
20K with a weak, broad susceptibility maximum near 5K in the ZFC data. A
Curie-Weiss fit shows a reduced mu_eff=1.42\mu_B, (spin only = 1.73 muB) and a
Weiss temperature, \theta_c, which depends strongly on the temperature range of
the fit. Powder neutron diffraction, heat capacity and 7Li NMR show no evidence
for long range magnetic order to 2K. On the other hand oscillations develop
below 20K in muSR indicating at least short range magnetic correlations.
Susceptibility data for Ba2YMoO6 also deviate strongly from the C-W law below
150K with a similarly reduced mu_eff = 1.72\mu_B and \theta_c = - 219(1)K. Heat
capacity, neutron powder diffraction and muSR data show no evidence for long
range order to 2K but a very broad maximum appears in the heat capacity. The
89Y NMR paramagnetic Knight shift shows a remarkable local spin susceptibility
behavior below about 70K 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.
