Structural frustration in a rod packing - An analogy to the disordered triangular Ising net

Single crystals of Na22Ba14(Ca,Sr)N-6 show diffuse scattering which arranges in Kagome nets in layers hkl with l = integer not equal 0. The hexagons of the nets are centered by sharp Bragg spots. The structure represents a hexagonal rod packing, and in the averaged structure a superposition of the rods in two different orientations, type A and B, at equal weight is found. Due to a periodic alternation of bond character in the rod frustration effects occur which closely correspond to those in a triangular two-dimensional antiferromagnetic Ising spin system and give rise to diffuse scattering. The computer simulated diffuse scattering is in very good agreement with experiment, particularly, when the ideally frustrated system is slightly modified towards some randomness in the rod packing.