Synthesis, crystal structure and electronic structure of modulated Pd7-δSnTe2

Pd7-5SnTe2 was synthesized starting from the elements either by direct reaction at 970 K or by reaction in a tin flux at 1170 K. Depending on the reaction route and starting composition, single crystals of Pd7-deltaSnTe2 with slightly different compositions were found: 0.3 less than or equal to delta less than or equal to 0.8. The average structure of Pd7-deltaSnTe2 (space group I4/mmm, No. 139, a = 4.001(2) Angstrom, c = 20.929(3) Angstrom, Z = 2) is isotypic to those of the respective nickel compounds Ni(7-delta)SnQ(2) (Q = S, Se, Te) and consists Of Cu3Au-like Sn/Pd blocks and NaCl-like Pd/Te slabs. Additional satellite reflections that occur in the diffraction image of Pd7-6SnTe2 can be assigned to the modulation vectors q(1) = (0, -0.4, 0) and q(2) = (0.4, 0, 0). The modulated structure was refined in the (3+2) dimensional super-space group I4/mmm(0-beta0,alpha00)0.ss.mm to an agreement factor of wR(2) = 0.070 for 49 parameters and 483 reflections. The origin of the modulation can mainly be attributed to a variation of the occupancy of the Pd sites in the Pn/Te double slabs. Band structure calculations on the stoichiometric phase Pd7SnTe2 suggest metallic behavior for this compound.