Variable temperature, $2\mathrm{K}<T<500\mathrm{K}$, powder neutron diffraction studies of the beta-pyrochlores $A{\mathrm{Os}}_2{\mathrm{O}}_6$ and $A\mathrm{Nb}\mathrm{Te}{\mathrm{O}}_6$, $A=\mathrm{K}$, Rb, and Cs, have been undertaken. Anomalous behaviors in the lattice parameter variations as a function of temperature for the superconducting osmates with $A=\mathrm{K}$, and to a lesser extent for $A=\mathrm{Rb}$, are associated with local, static displacements of the alkali metal cation, which occur below $\sim 80\mathrm{K}$. These displacements are random around the $8b$ position as modeled in the space group $Fd\text{−}3m$; no evidence for decrease in symmetry from $Fd\text{−}3m$ was observed for any sample above $2\mathrm{K}$. For the nonsuperconducting $A\mathrm{Nb}\mathrm{Te}{\mathrm{O}}_6$, $A=\mathrm{K}$, Rb, and Cs, phases, similar structural and lattice parameter behaviors are observed with localization of the potassium ions in $\mathrm{K}\mathrm{Nb}\mathrm{Te}{\mathrm{O}}_6$ below $\sim 30\mathrm{K}$.

• Received 8 January 2008

DOI:https://doi.org/10.1103/PhysRevB.77.104523