The composition dependence of the structure of potassium-loaded zeolite A : evolution of a potassium superlattice
The composition dependence of the structure of potassium-loaded zeolite A : evolution of a potassium superlattice
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负载钾沸石 A 结构的组成依赖性：钾超晶格的演化
DOI：
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发表时间：
1994
期刊：
影响因子：
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通讯作者：
P. Edwards
中科院分区：
文献类型：
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作者：
A. R. Armstrong;P. Anderson;P. Edwards
摘要通过控制脱水钾分子筛A与校准的钾蒸汽量的反应，制备了三种钾沸石样品，每个原始晶胞中分别含有1、3和5个额外的钾原子。通过Rietveld粉末中子衍射数据的修正，确定了这些材料的结构，并确定了脱氢K12-A的结构。在Pm3m空间群中，假设Si和Al无序，对K12-A和最轻负载样品(K1/K12-A)的结构进行了修正。对于负载较重的样品K3/K12-A和K3/K12-A，观察到超晶格，导致FM3m空间群的晶胞倍增。这种超晶格是钾有序的结果。在钾包裹体上观察到方钠石笼中的K3位和α笼中的K4位。在低负荷时，方钠石笼中的场地开始被占用。这两个位置中的第一个与6环窗口相反的K1之间的间隔使得这两个位置不能同时占据，导致面对α笼子的4环的第二个新位置被占据。这两个新站点的占有率随着负载水平的增加而增加，直到发生排序。在极限载荷下，每个方钠石笼中形成一个K3四面体，交替的α笼分别包含8个K1和12个K4。讨论了K3+4在超晶格形成中的可能作用。
Abstract Three samples of potassium zeolite A, containing 1, 3, and 5 additional potassium atoms per primitive unit cell, were prepared through the controlled reaction of dehydrated potassium zeolite A with a calibrated amount of potassium vapor. The structures of these materials, and that of the host dehydrated K 12 -A, were determined by Rietveld refinement of powder neutron diffraction data. The structures of K 12 -A and the most lightly loaded sample with one extra potassium atom per unit cell (K 1 /K 12 -A) were refined in space group Pm 3 m assuming disorder of Si and Al. For the more heavily loaded samples K 3 /K 12 -A and K 3 /K 12 -A a superlattice was observed, resulting in a doubling of the unit cell in space group Fm 3 m . This superlattice arises as a result of potassium ordering. On potassium inclusion additional potassium sites K3 in the sodalite cage and K4 in the α cage are observed. At low loadings occupation of the site in the sodalite cages begins. The separation between the first of these two sites and K1 on the opposite of the 6-ring windows is such that the two sites cannot be occupied simultaneously, leading to occupation of the second new site facing the 4-rings of the α cage. The occupancies of these two new sites increase with increasing loading levels until ordering occurs. At limiting loading a tetrahedron of K3 forms in each sodalite cage, with alternate α cages containing 8 K1 and 12 K4, respectively. The possible role of K 3+ 4 in directing the formation of the superlattice is discussed.