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

The novel α-BaZnP 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-BaCuS structure type, with unit cell parameters = 9.7567(14) Å, = 4.1266(6) Å, and = 10.6000(15) Å. With β-BaZnP being previously identified as belonging to the ThCrSi family and with the precedent of structural phase transitions between the α-BaCuS type and the ThCrSi type, the potential for the pattern to be extended to the two different structural forms of BaZnP 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-BaZnP, 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-BaZnP 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-BaZnP phase. The Seebeck coefficient, (), 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-BaZnP is relatively high, on the order of 40 mΩ·cm, and the ρ() dependence shows gradual decrease upon heating. Such behavior is comparable to those of the typical semimetals or degenerate semiconductors.