Two different polymorphs of the new selenosilicate Na4Si2Se6 were synthesized by solid-state reactions. The hightemperature polymorph Na4Si2Se6-tP24 crystallizes in the tetragonal space group P42/mcm (No. 132) with lattice parameters a = 7.2793(2) A, c = 12.4960(4) A, and V = 662.14(3) A(3). The main structural motifs are isolated Si2Se6 units of two edge-sharing SiSe4 tetrahedra. The high- pressure/low-temperature polymorph Na4Si2Se6-oP48 crystallizes in the orthorhombic space group Pbca (No. 61) with lattice parameters a = 12.9276(1) A, b = 15.9324(1) A, c = 6.0349(1) A, and V = 1243.00(2) A(3) showing zweier single chains 8 1 [Si2Se6]4-. The lattice parameters of Na4Si2Se6-tP24 were determined by single-crystal X-ray diffraction, whereas those of Na(4)Si(2)Se6-oP48 were investigated by powder X-ray diffraction. Both modifications crystallize in new structure types. An energetic comparison of the two polymorphs and further hypothetical structure types was carried out by density functional theory modeling. Calculations reveal that the polymorphs are very close in energy (.E = 3.4 kJ mol-1). Impedance spectroscopic measurements show ionic conductivity (sspec = 1.4 x 10-8 S cm-1 at 50 degrees C and 6.8 x 10(-6) S cm(-1) at 200 degrees C) with an activation energy of EA = 0.54(2) eV for Na4Si2Se6-(o)P48.