Exploring dicyanamides with two different alkali-metal cations: phase separations, solid solutions and the new compound Rb1.667Cs0.333[N(CN)2]2

Abstract

Abstract Metathesis reactions designed with the tentative starting stoichiometry (A1)+ : (A2)+ : [N(CN)2]− = 1 : 1 : 2 (A1, A2: Li, Na, K, Rb and Cs) were used to get a complete picture of the compounds and structures that are formed in this regime of dicyanamides with two different alkali-metal cations. Next to solid solutions and phase separations of the respective pseudo-binary dicyanamides, the new compound Rb1.667Cs0.333[N(CN)2]2 was found and structurally characterized as a disorder variety of the KCs[N(CN)2]2 structure. Rb1.667Cs0.333[N(CN)2]2 adopts the monoclinic space group C2/c and the unit-cell parameters a = 1387.73(8), b = 1005.00(6), c = 1451.29(8) pm, β = 117.175(3)° for Z = 8. Its crystal structure is very closely related with those of β-Rb[dca] and KCs[dca]2 ([dca]− ≡ [N(CN)2]−) and so are the vibrational spectra. These three quasi-isostructural compounds are compared in detail. A synopsis of the crystal structures (either already reported or discussed here for the first time) for all combinations in the (A1)+: (A2)+: [N(CN)2]− systems is presented and the behavior of those not forming a distinct compound is discussed.