Two Polymorphs of BaZn2P2: Crystal Structures, Phase Transition, and Transport Properties

The novel alpha-BaZn2P2 structural polymorph has been synthesized and structurally characterized for the first time. Its structure, elucidated from single crystal Xray diffraction, indicates that the compound crystallizes in the orthorhombic alpha-BaCu2S2 structure type, with unit cell parameters a = 9.7567(14) angstrom, b = 4.1266(6) angstrom, and c = 10.6000(15) angstrom. With beta-BaZn2P2 being previously identified as belonging to the ThCr2Si2 family and with the precedent of structural phase transitions between the alpha-BaCu2S2 type and the ThCr2Si2 type, the potential for the pattern to be extended to the two different structural forms of BaZn2P2 was explored. Thermal analysis suggests that a first-order phase transition occurs at similar to 1123 K, whereby the low-temperature orthorhombic a-phase transforms to a high-temperature tetragonal beta-BaZn2P2, the structure of which was also studied and confirmed by single-crystal X-ray diffraction. Preliminary transport properties and band structure calculations indicate that alpha-BaZn2P2 is a pi-type, narrow-gap semiconductor with a direct bandgap of 0.5 eV, which is an order of magnitude lower than the calculated indirect bandgap for the beta-BaZn2P2 phase. The Seebeck coefficient, S(T), for the material increases steadily from the room temperature value of 119 mu V/K to 184 mu V/K at 600 K. The electrical resistivity (rho) of alpha-BaZn2P2 is relatively high, on the order of 40 m Omega.cm, and the.(T) dependence shows gradual decrease upon heating. Such behavior is comparable to those of the typical semimetals or degenerate semiconductors.