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Jahn-Teller distortion [24, 27, 28], while the exchange interaction is of the order of 100 K [24]. The other com- pounds were found to be paramagnetic and with cubic symmetry at all temperatures. Ba$_{2}$YMoO$_{6}$ is the sim- plest of these compounds because the Y 3+ ion does not carry a magnetic moment. The magnetic exchange is mainly via the 90 ◦ B’-O-O-B’ π − π bonds [21, 24], giv- ing rise to 12 near-neighbour AF J$_{1}$ bonds for each spin, across the edges of the tetrahedra (Fig. 1). 

Polycrystalline Ba$_{2}$YMoO$_{6}$ was prepared by the solid state reaction of stoichiometric oxides of Y$_{2}$O$_{3}$, MoO$_{3}$ and BaCO$_{3}$ powders of at least 99.99% purity. These were ground, die-pressed into a pellet and heated under flowing 5% H$_{2}$/N$_{2}$. The final synthesis temperature was 1200-1250 $^{◦}$C with three intermediate regrinding steps to ensure phase homogeneity. It was found that a first heat- ing step of ∼ 2 hr at 900 $^{◦}$C in air and thorough homoge- nization helps to prevent the formation of BaMoO$_{4}$ and Y$_{2}$O$_{3}$ impurities. Phase purity was confirmed by labo- ratory X-ray powder diffraction. In a related paper [29] neutron powder diffraction results are discussed, which show that the Y/Mo site disorder is less than 1%. The diamagnetic analog, Ba$_{2}$YNbO$_{6}$, was prepared at 1200 $^{◦}$C in air from YNbO$_{4}$ and BaCO$_{3}$. The sample magnetisa- tion was measured on a Quantum Design magnetic prop- erty measurement system (MPMS) in fields up to 5 T. The heat capacity was measured on a Quantum Design physical property measurement system (PPMS), using 7 . 0 mg of a sintered pellet. The µ SR experiment was carried out at MUSR at ISIS, UK. 

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The DC magnetic susceptibility measured in a 1 T field 

is shown in Fig. 2. A Curie-Weiss fit to the high temper- ature susceptibility yields a Weiss temperature of -160 K and a Curie constant of 0.25 emu mol $_{−}$1 K $^{$_{−}$1}$, small com- pared to the 0.38 emu mol $_{−}$1 K $_{−}$1 expected for s = 1 / 2. This difference is attributed to strong quantum fluctu- ations common in low-spin antiferromagnets and previ- ously observed in double perovskites [26]. Below 25 K a second linear regime is observed in χ $^{$_{−}$1}$, corresponding (for a 1 T field) to a ∼ 10% fraction of all the s = 1 / 2 moments (or ∼ 5% if they have the full j = 3 / 2 where g$_{J}$ = 4 / 3) and a Weiss temperature of − 2 . 3 K indicat- ing weak AF exchange. This fraction is too large to be ascribed directly to either structural disorder or an im- purity phase in the sample. Furthermore, fits to M ( H ) measured at 2.3 and 5 K (inset of Fig. 2) with Brillouin functions lead to estimates of respectively 2 and 7% of all spins, compared to 10% for fits to the M ( T ) curve. This suggests that the apparently quasi-free spins are an emergent property of the (disorder free) system. 

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The AC susceptibility measured with a field amplitude of 5 Oe and zero DC offset field is shown in Fig. 3. The dispersive part of the AC susceptibility ( χ $^{′}$) is almost frequency independent and is comparable to the diverg- ing low temperature DC susceptibility. The dissipative part ( χ $^{′′}$) shows a frequency dependent maximum be- tween 26 and 70 K. Remarkably, the maximum gradually gets sharper as the frequency increases, instead of weaker as expected for a spin-glass transition. The agreement between the DC susceptibility and χ ′ below 20 K is a strong indication that the Curie-term can be ascribed to the weakly coupled spins. 5+ 

The heat capacity associated with the single Mo 4 d electron in Ba$_{2}$YMoO$_{6}$ (as shown in Fig. 4) was ob- tained from comparison with the heat capacity of the dia- magnetic analogue Ba$_{2}$YNbO$_{6}$. The heat capacity from phonons of Ba$_{2}$YMoO$_{6}$ is expected to be lower than for Ba$_{2}$YNbO$_{6}$ by a factor 0.991 due to the mass difference 

