Bonding of Guest Molecules in the Tubes of Nanoporous Cetineite Crystals
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Bonding of Guest Molecules in the Tubes of Nanoporous Cetineite Crystals E.E. Krasovskii, O. Tiedje, S. Brodersen, W. Schattke, F. Starrost1, J. Jockel 2, U. Simon2
Institut fur Theoretische Physik und Astrophysik, Christian-Albrechts-Universitat Kiel, Leibnizstrae 15, 24098 Kiel, Germany 1 Department of Chemistry and Biochemistry, Box 951569, University of California, Los Angeles, CA 90095-1569, USA 2 Institut fur Anorganische Chemie, RWTH Aachen, Professor-Pirlet-Str.1, 52074 Aachen, Germany
ABSTRACT A theoretical study of the optical excitation processes in cetineites is presented. This new exciting class of crystals with tubular structures displays photoconductivity, which strongly depends on the presence of guest molecules within the tubes. Based on self-consistent electronic structure calculations we present calculated dielectric function and photoelectron spectra of the (Na;Se) cetineite with the tube lled with di erent guest molecules. Calculations are performed with the extended linear augmented plane wave kp method. The con guration and the arrangement of the lling molecules is derived from available crystallographic data and from heuristic arguments on chemical binding.
INTRODUCTION The electronic structure of various pure cetineite compounds has been previously studied extensively, see Ref. [1] and references therein. The general composition formula is A6[Sb12O18][SbX3]2 (6 ; mx)H2Ox[Bm+(OH;)m], where A = Na+ , K+ , Rb+; X = S2; , Se2;; and B = Na+, Sb3+. The crystal formulae are abbreviated by (A;X ). These materials present an extremely attractive and promising subject owing to their single crystal porous structures with tube diameter of about 0.7 nm (see Figure 1) and their semiconducting properties. Potential applications are similar to those of the zeolites with the additional feature of controling their function in an application by electric power. To date, only in the case of K6[Sb12O18][SbSe3] homogeneous single crystals can be grown entirely free of guest molecules within the pores (in the form of needles of about 0.5 mm diameter and 2 mm length along the axes of the pores). Geometrical and electronic properties of the cetineites are discussed in Ref. [1]. In that paper ab initio band structure calculations were presented for A = Na, K; X = S, Se. The bonding mechanisms have been explained by the hexagonal aggregation of the tubes consisting of SbO3 tetrahedra { netted to form the tube walls. The tubes are stabilized by the presence of singly ionized alkali atoms close to the walls inside the tubes and the negatively charged SbX3 pyramids between the tubes (see Figure 1). In Ref. [1] the anisotropy of the photoconductivity has been explained in terms of the band structure of the guest-free crystals. Two qualitatively di erent conduction processes can be deduced from the bandstructure results, namely, hopping between the SbO3 tetrahedra and GG4.9.1
c Sb(2)
O(1)
Sb(3)
Na
Sb(1) Se
O(2')
O(2)
a
b
Na6[Sb12O18][SbSe3]2
Figure 1: Crystal structure of the cetineites. (Figure from R
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