Crystal structures of zeolite linde type A incorporating K clusters: Dependence on the K atom loading density

Crystal structures of K-LTA (Linde Type A) incorporating K clusters of 0.35-6.4 K atoms per alpha-cage, which show ferromagnetism with the Curie temperature, T-C, up to 8 K, were determined by Rietveld analysis, followed by whole-pattern fitting on the basis of the maximum entropy method from X-ray and neutron powder diffraction data. Increasing the K content lowered space group symmetry without changing the double periodic ordering of a.-cages. We identified space group F (4) over bar 3c in the low K content region. The F (4) over bar 3c model means that K clusters in adjacent alpha-cages must be equivalent to each other. Space group F23 in the high K content region leads to an alternate arrangement of two kinds of K clusters in neighboring alpha-cages. No structural phase transition occurred at T-C. The relationship between the crystal structure and ferromagnetism is discussed on the basis of the crystal data obtained in this work. The difference in the number of K+ ions between the adjacent alpha-cages was estimated to be ca. 1.7 in K-loaded samples exhibiting ferromagnetism. Our results, coupled with magnetic data, support a spin-canting model of unpaired electrons in arrayed K clusters.