A Cs-NMR Study of the Chemical Bonding of the Solid Compound Semiconductor Cs3Sb

The alkali-antimonide compounds are of some technical importance because of their high photoelectric quantum efficiency in the visible region. For this reason the electronic properties of these semiconductors have been studied relatively intensively in the past — for comprehensive reviews see Ref. [1] and [2] —. However, the understanding of the nature of chemical bonding of these materials is less clear. The 3 : 1 compounds crystallize either in a hexagonal structure isotypic with that of NaßAs or in a cubic modification of either the BiF3 or NaTl structure. Several of these compounds show a transition from cubic to hexagonal or viceversa as the temperature is raised — see e.g. [2] — indicating only slight energy differences between these structures. In the case of Cs3Sb only the cubic structure has been reported so far [3], where below 260 K De Munari et al. [4] suggest that there is an ordered arrangement of the Cs atoms with Oh5 symmetry, although at room temperature it has been assumed by Jack and Wachtel [3] that the Cs-Sb sublattice is partially disordered. The chemical bonding of all these compounds based upon these structures has until recently been described by resonating sp-hybridized bonds [5], [6]. However, in a recent series of XPS studies Bates et al. [7] have suggested that the bonding in CssSb is predominantly ionic in character in conflict with the previous assumptions of covalency in these compounds. In order to obtain further information on these questions we report here measurements of the 133Cs NMR chemical shift, the spin-lattice relaxation time T\, and the quadrupolar splitting of solid CssSb.