Cadmiumreiche Cadmide des Systems Na/K/Cd

Abstract

AbstractThe phase formation in the cadmium‐rich section of the ternary system Na/K/Cd was explored by syntheses from the elements. The crystal structures of the 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, were refined on the basis of single‐crystal X‐ray data. The structure of the sodium cadmide Na2Cd11, which was in principle known from early film experiments (Mg2Zn11 structure type, cubic, space group Pm$\bar3$, a = 958.75(9) pm, Z = 3, R1 = 0.042) was confirmed and refined using single crystal data. The border phases of the new series KNaxCd11–x (x = 0.87/0.53; tetragonal, space group I41/amd, a = 1222.98(6)/1215.17(12) pm, c = 766.05(4)/769.05(7) pm, Z = 4, R1 = 0.021/0.024) form the BaCd11 structure type, which was previously unknown for alkaline metal compounds. The barium site of the basic structure type is occupied by potassium, whereas one cadmium site with slightly higher coordination number and interatomic distances is statistically occupied by sodium and cadmium atoms, which leads 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 atoms inside the channels. The detailed inspection of single crystal image plate diffractometer data revealed satellite reflections indicating an almost commensurately modulated superstructure. In the new structural model with a quadrupled c axis (tetragonal, 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 structural factors (strongly changing radius of the countercations) and the electronic criteria, which can be assessed from the pseudo band gap calculated using FP‐LAPW‐DFT methods, are discussed.