Persistence of the valence bond glass state in the double perovskitesBa2−xSrxYMoO6

Abstract

The ${\text{Ba}}_{2\ensuremath{-}x}{\text{Sr}}_{x}{\text{YMoO}}_{6}$ double perovskites with a geometrically frustrated face centered ${\text{Mo}}^{5+}$ $(s=\frac{1}{2})$ 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 $(<1\mathrm{%})$. Magnetic-susceptibility measurements evidence two paramagnetic regimes for all samples. The high-temperature (HT) Curie constants are independent of composition $x$ $[0.26(1)\text{ }\text{emu}\text{ }{\text{mol}}^{\ensuremath{-}1}\text{ }{\text{K}}^{\ensuremath{-}1}]$ while the low-temperature (LT) value gradually increases from $0.04(1)\text{ }\text{emu}\text{ }{\text{mol}}^{\ensuremath{-}1}\text{ }{\text{K}}^{\ensuremath{-}1}$ for $x=0$ to $\ensuremath{-}0.10(1)\text{ }\text{emu}\text{ }{\text{mol}}^{\ensuremath{-}1}\text{ }{\text{K}}^{\ensuremath{-}1}$. The HT Weiss temperature $(\ensuremath{\theta})$ increases from $\ensuremath{-}159\text{ }\text{K}$ $(x=0)$ to $\ensuremath{-}50\text{ }\text{K}$ $(x=2)$ while the LT values are $\ensuremath{-}2.3(3)\text{ }\text{K}$ for $x<2$ and $+10\text{ }\text{K}$ for $x=2$. Neutron powder diffraction does not evidence any Neel ordering. The $x=2$ sample is weakly ferromagnetic below 8 K (${M}_{\text{sat}}\ensuremath{\sim}0.1\text{ }{\ensuremath{\mu}}_{\text{B}}/\text{Mo}$ 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 ${\text{Ba}}_{2}{\text{YMoO}}_{6}$, leaving a small fraction of the spins isolated or only weak ferromagnetically coupled to each other.