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

LiEuPSe₄, the first quaternary lithium-containing selenophosphate, was synthesized as red polyhedra by reacting Eu with a molten mixture of Li₂Se/P₂Se₅/Se at 750 °C. Similarly, the reaction of Eu with a molten mixture of K₂Se/P₂Se₅/Se at 495 °C produced red polyhedral crystals of KEuPSe₄. Both compounds are unstable in moist air. In addition, both compounds were plagued with crystal twinning. Acceptable crystal structure refinements could only be obtained by identifying the type of twinning and taking it into account in the final refinement. LiEuPSe₄ crystallizes in the noncentrosymmetric space group Ama2 (no. 40) with a = 10.5592 (9) Å, b = 10.415 (1) Å, c = 6.4924 (7) Å, and Z = 4. The structure is three-dimensional and composed of EuSe₈ distorted square antiprisms and PSe₄ tetrahedral building blocks that create tunnels, running down the a axis, in which the Li ions reside. The Li ions are in a highly distorted tetrahedral coordination. KEuPSe₄ crystallizes in the space group P2₁/m (no. 11) with a = 6.8469(6) Å, b = 6.9521(6) Å, c = 9.0436(8) Å, β = 107.677(2)°, and Z = 2. The structure has two-dimensional character with layers composed of EuSe₆ trigonal prisms and PSe₄ tetrahedral units. Between the [EuPSe₄](n)(n-) layers the K ions reside in a bicapped trigonal 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.