Cd-reiche Cadmide des Systems Na-K-Cd
Résumé
The phase formation in the cadmium-rich section of the ternary system Na-K-Cd has been explored by syntheses from melts of the elements. The crystal structures of all compounds in the region of Cd:(Na/K) ratios around 1:5 to 1:6, comprising the known binary compounds Na2Cd11 and K3Cd16 as well as ternary cadmides of the series KNaxCd11-x, have been refined using single crystal X-ray data. The structure of the binary border sodium cadmide Na2Cd11, which was in principle known from early film experiments (Mg2Zn11 structure type, space group Pm3, a=958.75(9) pm, Z=3, R1=0.0417) was confirmed and refined using single crystal data. The border phases of the new series KNaxCd11-x (x=0.87/0.53; space group I41/amd, a=1222.98(6)/1215.17(12), c=766.05(4)/769.05(7) pm, Z=4, R1=0.0211/0.0235) form the BaCd11 structure type, which was previously unknown for alkaline metal compounds. The Ba site of the basic structure type is occupied by K, whereas one Cd site with slightly higher coordination number and interatomic distances is statistically occupied by Na and Cd leading to the phase width given above. In the structure, the atoms of the two pure Cd positions form channels running along the tetragonal c axis, in which the K and Na/Cd atoms alternate. A similar structural motif is found in the second border compound K3Cd16, which has been described formerly as K0.37Cd2 with a complete disorder of potassium inside the channels. The detailed inspection of image plate diffractometer data revealed satellite reflections indicating an almost commensurately modulated superstructure. In the new structural model with a quadrupled c axis (space group P4/nnc, a=915.2(6), c=1159.0(4) pm, Z=2) three potassium cations per channel are partially ordered. Both the significant geometric factors and the electronic criteria, which can be assessed from the pseudo bandgap calculated using FP-LAPW-DFT methods, are discussed.
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