Transmission electron microscopy study of optically anisotropic and isotropic haüyne

Optically anisotropic and isotropic haüyne has been observed in phonolite from Niangniang Hill, Jiangsu Province, southeastern China. Selected-area electron diffraction (SAED) and transmission electron microscopy (TEM) studies show the anisotropic haiiyne has a one-dimensional superstructure with a period of 6d_110sub along one of the (110) directions of the ideal cubic haüyne subcell. The superstructure may be considered as a periodic arrangement of (110) layer domains with P23 symmetry along [110]. Neighboring domains are related by a twin relationship. The unit-cell parameters for this haiiyne superstructure are thus metrically orthorhombic, with a_sup = 2d_{1 10sub}, b_sup = 6d_110sub, c_sup = c_sub, but the most likely space group is monoclinic, Pn. SAED patterns and TEM images of two optically isotropic haiiyne samples show they have either a three-dimensional superstructure or no superstructure. The average structure of the isotropic haiiyne with the superstructure is cubic, even though it is composed of twin domains with one-, two-, and three-dimensional superstructures. The superstructure may have formed by ordering of [Na₃Ca(SO₄)]³⁺ and [(Na,K)₄(OH)]³⁺ clusters in the cubo-octahedral cages of the haiiyne framework during an arrested phase transition from P43n to P23 symmetry.