Enclathration of X@La4 Tetrahedra in Channels of Zn–P Frameworks in La3Zn4P6X (X = Cl, Br)

Two new quaternary lanthanum zinc phosphide-halides were synthesized via high-temperature solid-state reactions. Their complex crystal structures were determined by a combination of X-ray diffraction and advanced solid-state 31 P NMR spectroscopy. La 3 Zn 4 P 6 Cl and La 3 Zn 4 P 6.6 Br 0.8 share a common structural feature: a polyanionic Zn–P framework with large channels hosting complex one-dimensional cations. The cations are built from X@La 4 tetrahedral chains with X = Cl (La 3 Zn 4 P 6 Cl) or Br 0.8 P 0.2 (La 3 Zn 4 P 6.6 Br 0.8 ). The X@La 4 tetrahedra share two vertices forming one-dimensional chains. To accommodate larger bromine-containing cations the Zn–P framework is rearranged by breaking and forming several Zn–P and P–P bonds. This results in the formation of a unique [P 3 ] 3– cycle, which is isoelectronic to cyclopropane. Analysis of the electron localization and orbital overlaps confirmed the presence of different chemical bonding in the Zn–P networks in the Cl- and Br-containing compounds. La 3 Zn 4 P 6 Cl was predicted to be a narrow bandgap semiconductor, while the formation of the [P 3 ] 3– units in the structure of La 3 Zn 4 P 6.6 Br 0.8 was shown to lead to a narrowing of the bandgap. Characterization of the transport properties confirmed both La 3 Zn 4 P 6 Cl and La 3 Zn 4 P 6.6 Br 0.8 to be narrow bandgap semiconductors with electrons as dominating charge carriers at low temperatures. La 3 Zn 4 P 6 Cl exhibits a n-p transition around 250 K. Due to the complex crystal structure and segregation of the areas of different chemical bonding, both title compounds exhibit ultralow thermal conductivities of 0.7 Wm –1 K –1 and 1.5 Wm –1 K –1 at 400 K for La 3 Zn 4 P 6 Cl and La 3 Zn 4 P 6.6 Br 0.8, respectively.