Persistence of the valence bond glass state in the double perovskites Ba_2-xSr_xYMoO₆

The Ba_2-xSr_xYMoO₆ double perovskites with a geometrically frustrated face centered Mo⁵⁺ (s= 1/2) sublattice have been investigated using neutron powder-diffraction, magnetic- susceptibility, and heat-capacity measurements. Reduction in the average A -site radius results in the distortion of the crystal structure from cubic for x=0 to monoclinic for x=2 without affecting the Y/Mo inversion (<1%). Magnetic-susceptibility measurements evidence two paramagnetic regimes for all samples. The high-temperature (HT) Curie constants are independent of composition x [0.26 (1) emu mol^-1 K^-1] while the low-temperature (LT) value gradually increases from 0.04 (1) emu mol ^-1 K^-1 for x=0 to -0.10 (1) emu mol^-1 K ^-1. The HT Weiss temperature (?) increases from -159K (x=0) to -50K (x=2) while the LT values are -2.3 (3) K for x<2 and +10K for x=2. Neutron powder diffraction does not evidence any Neel ordering. The x=2 sample is weakly ferromagnetic below 8 K (Msat ~0.1 µB /Mo in 1 T at 5 K) which is consistent with the small positive LT Weiss temperature. The magnetic data are consistent with a valence bond glass state where the bulk of the spins condense into spin singlets as previously observed for Ba2 YMoO6, leaving a small fraction of the spins isolated or only weak ferromagnetically coupled to each other. © 2010 The American Physical Society.