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 measure-
ments. 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 0.04͑1͒ emu mol−1 K−1 for x = 0 to −0.10(1) emu mol−1 K−1. The HT Weiss temperature 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 ~ 0.1 muB / 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.