ALKALINE CAVITIES AND TETRAHEDRA IN RBXCS2-XZNCL4 AND KXCS2-XZNCL4 SOLID-SOLUTIONS

Seven different RbxCs2-xZnCl4 crystals were grown with 0 < x < 2 while seven different KxCs2-xZnCl4 crystals were grown with 0 < x less than or equal to 1, the limit value x = 1 corresponding to the clearly def ined KCsZnCl4 compound. A crystallographic study of these solid soluti ons was performed by comparison with the pure Cs2ZnCl4 crystal. Struct ures were solved in the Pnma space group for the rubidium solid soluti ons and in the P2(1)2(1)2(1) space group for the potassium ones. it wa s shown that the distribution of Cs and Rb, or Cs and K, among the two cationic sites called A(1) and A(2). was not random: Rb and K exhibit a higher affinity for the smallest cavity, the A(2) Site. While the p hase transition of Rb2ZnCl4 and K2ZnCl4 can be interpreted as rotation s of the rigid ZnCl4 tetrahedra mainly around the a direction, the sub stitution of Cs by Rb or K not only supplies the a rotation of ZnCl4 t etrahedra but, as shown by a first-order approximation model, also cha nges their size and shape.