Persistence of the valence bond glass state in the double perovskites Ba2-xSrxYMoO6

The Ba2-xSrxYMoO6 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 (theta) increases from -159 K (x=0) to -50 K (x=2) while the LT values are -2.3(3) K for x<2 and +10 K for x=2. Neutron powder diffraction does not evidence any Neel ordering. The x=2 sample is weakly ferromagnetic below 8 K (M-sat similar to 0.1 mu(B)/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 Ba2YMoO6, leaving a small fraction of the spins isolated or only weak ferromagnetically coupled to each other.