Exploring dicyanamides with two different alkali-metal cations. Phase separations, solid solutions and the new compound Rb${}_{1.667}$Cs${}_{0.333}$[N(CN)${}_2$]${}_2$

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 Rb${}_{1.667}$Cs${}_{0.333}$[N(CN)${}_2$]${}_2$ was found and structurally characterized as a disorder variety of the KCs[N(CN)${}_2$]${}_2$ structure. Rb${}_{1.667}$Cs${}_{0.333}$[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.