Highly Disordered Crystal Structure and Thermoelectric Properties of Sn₃P₄

The crystal structure of Sn 3 P 4, a long-known tin phosphide, has been determined. It crystallizes in the trigonal space group R 3̄ m with unit-cell parameters a = 4.4315(1) Å and c = 28.393(1) Å ( Z = 3). The crystal structure of Sn 3 P 4 is disordered. It consists of alternating layers of phosphorus and tin atoms that are combined into five-layer blocks and propagate along the c -axis. The major structural feature is the disordered orientation of the P 2 4– dumbbells, which link the tin atoms. The latter possess two types of coordination. One third of the tin atoms reside inside the block and are octahedrally coordinated by phosphorus atoms. The rest of the tin atoms confine the block and possess a [3 + 3] environment made up of three close phosphorus neighbors and three rather distant (3.4 Å) tin atoms of the adjacent block. The coordination of the tin atoms is confirmed by 119 Sn Mössbauer spectroscopy. The electron diffraction and high-resolution electron microscopy data reveal ordered regions at the microscopic level, which do not result in any superstructure formation for the bulk sample. Physical property measurements show that Sn 3 P 4 is a narrow-gap semiconductor. Upon cooling to 150 K it undergoes a remarkable transition from n- type conduction with electrons as charge carriers to p- type conduction with holes as charge carries. Despite the large amount of disorder in the crystal structure, Sn 3 P 4 has a relatively high thermal conductivity of about 8 W m –1 K –1 .