µSR Study on Antiferromagnetism of Alkali-Metal Clusters Incorporated in Zeolite Sodalite

Na₄³⁺ and K₄³⁺ clusters arrayed in aluminosilicate sodalite are known to show antiferromagnetism below respective Néel temperatures TN = 48 and ≃70K. We newly prepared K-Rb alloy (K₃Rb)³⁺ clusters in addition to Na₄³⁺ and K₄³⁺ ones, and performed muon-spin relaxation (μSR) measurements. We find a large amplitude of precession signal in zero-field μSR spectra, indicating that antiferromagnetism takes place in the major volume of the samples. The TN is estimated from the temperature dependence of the precession signal to be ≃80K for (K₃Rb)³⁺. The internal field at the muon stopping site is estimated from the precession frequency to be ≃92, ≃142, and ≃155Oe for Na₄³⁺, K₄³⁺, and (K₃Rb)³⁺, respectively. Therefore, TN and the internal field are found to increase in the increasing order of atomic weight. These results are explained by the increase in the size of the s-electron wave function of clusters and the decrease in their potential depth.