LiEuPSe4 and KEuPSe4: Novel selenophosphates with the tetrahedral [PSe4](3-) building block

LiEuPSe4, the first quaternary lithium-containing selenophosphate, was synt hesized as red polyhedra by reacting Eu with a molten mixture of Li2Se/P2Se 5/Se at 750 degrees C. Similarly, the reaction of Eu with a molten mixture of K2Se/P2Se5/Se at 495 degrees C produced red polyhedral crystals of KEuPS e4. Both compounds are unstable in moist air. In addition, both compounds w ere plagued with crystal twinning. Acceptable crystal structure refinements could only be obtained by identifying the type of twinning and taking it i nto account in the final refinement. LiEuPSe4 crystallizes in the noncentro symmetric space group Ama2 (no. 40) with a = 10.5592 (9) Angstrom, b = 10.4 15 (1) Angstrom, c = 6.4924 (7) Angstrom, and Z = 4. The structure is three -dimensional and composed of EuSe8 distorted square antiprisms and PSe4 tet rahedral building blocks that create tunnels, running down the a axis, in w hich the Li ions reside. The Li ions are in a highly distorted tetrahedral coordination. KEuPSe4 crystallizes in the space group P2(1)/m (no. 11) with a = 6.8469(6) Angstrom, b = 6.9521(6) Angstrom, c = 9.0436(8) Angstrom, be ta = 107.677(2)degrees, and Z = 2. The structure has two-dimensional charac ter with layers composed of EuSes(6) trigonal prisms and PSe4 tetrahedral u nits. Between the [EuPSe4](n)(n-) layers the K ions reside in a bicapped tr igonal prism of Se atoms. The structure of the [EuPSe4](n)(n-) framework is similar to that found in CsPbPSe4. Both compounds are semiconductors with band gaps of 2.00 and 1.88 eV, respectively. Differential thermal analysis and infrared spectroscopic characterization are also reported.