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

Na-4(3+) and K-4(3+) clusters arrayed in aluminosilicate sodalite are known to show antiferromagnetism below respective Neel temperatures T-N = 48 and similar or equal to 70K. We newly prepared K-Rb alloy (K3Rb)(3+) clusters in addition to Na-4(3+) and K-4(3+) ones, and performed muon-spin relaxation (mu SR) measurements. We find a large amplitude of precession signal in zero-field mu SR spectra, indicating that antiferromagnetism takes place in the major volume of the samples. The T-N is estimated from the temperature dependence of the precession signal to be similar or equal to 80K for (K3Rb)(3+). The internal field at the muon stopping site is estimated from the precession frequency to be similar or equal to 92, similar or equal to 142, and similar or equal to 155 Oe for Na-4(3+), K-4(3+), and (K3Rb)(3+), respectively. Therefore, T-N 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.