Synthesis, structural characterization, and electronic structure of the novel Zintl phase Ba2ZnP2

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

The novel α-BaZn₂P₂ structural polymorph has been synthesized and structurally characterized for the first time. Its structure, elucidated from single crystal X-ray diffraction, indicates that the compound crystallizes in the orthorhombic α-BaCu₂S₂ structure type, with unit cell parameters a = 9.7567(14) Å, b = 4.1266(6) Å, and c = 10.6000(15) Å. With β-BaZn₂P₂ being previously identified as belonging to the ThCr₂Si₂ family and with the precedent of structural phase transitions between the α-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 ∼1123 K, whereby the low-temperature orthorhombic α-phase transforms to a high-temperature tetragonal β-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 α-BaZn₂P₂ is a p-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 β-BaZn₂P₂ phase. The Seebeck coefficient, S(T), for the material increases steadily from the room temperature value of 119 μV/K to 184 μV/K at 600 K. The electrical resistivity (ρ) of α-BaZn₂P₂ is relatively high, on the order of 40 mΩ·cm, and the ρ(T) dependence shows gradual decrease upon heating. Such behavior is comparable to those of the typical semimetals or degenerate semiconductors.