Synthesis and structural characterization of the new Zintl phases Ba3Cd2P4 and Ba2Cd2P3. Rare example of small gap semiconducting behavior with negative thermopower within the range 300 ​K–700 ​K

Abstract

The new Zintl phases Ba3Cd2P4 and Ba2Cd2P3 have been synthesized using Pb flux, which allowed for the growth of 4–5 ​mm large crystals. The structures were determined utilizing single-crystal X-ray diffraction methods. Both compounds crystalize in the monoclinic crystal system (space group C2/m (No. 12)) and their structures are closely related. The structure of Ba3Cd2P4 can be seen as being comprised of divalent Ba atoms and conjoined CdP4 tetrahedra in the form of [Cd2P4]6– layers. Within the layers, homoatomic P–P bonds are present, which if cleaved, leave two infinite [CdP3]7– chains running along the crystallographic b-axis. The other structure, that of Ba2Cd2P3, can be rationalized as also having divalent Ba atoms and conjoined CdP4 tetrahedra in the form of [Cd2P3]6– layers. These layers, again, can be visualized as chains that run down the crystallographic b-axis, which are further connected by P–P dimers. Electronic band structure calculations show that each structure has an optimal number of valence electrons, and therefore conform to the Zintl-Klemm concept. Accordingly, the two compounds can be considered small band gap semiconductors, with band gaps of ca. 0.1 ​eV and 0.6 ​eV for Ba3Cd2P4 and Ba2Cd2P3, respectively. Electrical resistivity measurements show that Ba3Cd2P4 displays a large resistivity value at room temperature and an experimental band gap of ca. 0.05 ​eV, which fits reasonably well with the theoretical predictions. Thermopower measurements show that throughout the temperature range 300 ​K–700 ​K, Ba3Cd2P4 displays a negative Seebeck coefficient. The extremum value of −84 μV is reached at 630 ​K, suggestive of an n-type semiconductor, a rarity among Zintl phases.