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Jürgens, Barbara; Irran, Elisabeth; Schneider, Julius; Schnick, Wolfgang (2000): Trimerization of NaC2N3 to Na3C6N9 in the Solid. Ab Initio Crystal Structure Determination of Two Polymorphs of NaC2N3 and of Na3C6N9 from X-ray Powder Diffractometry. In: Inorganic chemistry, Vol. 39, Nr. 4: S. 665-670
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Abstract

Sodium dicyanamide NaC2N3 was found to undergo two phase transitions. According to thermal analysis and temperature-dependent X-ray powder diffractometry, the transition of α-NaC2N3 (1a) to β-NaC2N3 (1b) occurs at 33°C and is displacive. 1a crystallizes in the monoclinic system, space group P21/n (no. 14), with a = 647.7(1), b = 1494.8(3), c = 357.25(7) pm, β = 93.496(1)°, and Z = 4. The structure was solved from powder diffraction data (Cu Kα1, T = 22°C) using direct methods and it was refined by the Rietveld method. The final agreement factors were wRp = 0.072, Rp = 0.053, and RF = 0.074. 1b crystallizes in the orthorhombic system, space group Pbnm (no. 62), with a = 650.15(5), b = 1495.1(2), c = 360.50(3) pm, and Z = 4. The structure was refined by the Rietveld method using the atomic coordinates of 1a as starting values (Mo Kα1, T = 150°C). The final agreement factors were wRp = 0.044, Rp = 0.034, RF = 0.140. The crystal structures of both polymorphs contain sheets of Na+ and N(CN)2- ions which are in 1a nearly and in 1b exactly coplanar. Above 340°C, 1b trimerizes in the solid to Na3C6N9 (2). 2 crystallizes in the monoclinic system, space group P21/n (no. 14), with a = 1104.82(1), b = 2338.06(3), c = 351.616(3) pm, β = 97.9132(9)°, and Z = 4. The structure was solved from synchrotron powder diffraction data (λ = 59.733 pm) using direct methods and it was refined by the Rietveld method. The final agreement factors were wRp = 0.080, Rp = 0.059, and RF = 0.080. The compound contains Na+ and the planar tricyanomelaminate C6N93-. The phase transition from 1b to 2 is reconstructive. It occurs in the solid-state without involvement of other phases or intermediates. The crystal structures of 1b and 2 indicate that there is no preorientation of the N(CN)2- in the solid before their trimerization to C6N93-.