Sodium dicyanamide NaC2N3 was found to undergo two phase transitions. According to thermal analysis and temperature-dependent X-ray powder diffractometry, the transition of alpha-NaC2N3 (1a) to beta-NaC2N3 (1b) occurs at 33 degrees C and is displacive. 1a crystallizes in the monoclinic system, space group P2(1)/n (no. 14), with a = 647.7(1), b = 1494.8(3), c = 357.25(7) pm, beta = 93.496(1)degrees, and Z = 4. The structure was solved from powder diffraction data (Cu K alpha(1), T = 22 degrees C) using direct methods and it was refined by the Rietveld method. The final agreement factors were wR(p) = 0.072, R-p = 0.053, and R-F = 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 la as starting values (Mo K alpha(1), T = 150 degrees C). The final agreement factors were wR(p) = 0.044, R-p = 0.034, R-F = 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 degrees C, 1b trimerizes in the solid to Na3C6N9 (2). 2 crystallizes in the monoclinic system, space group P2(1)/n (no. 14), with: a = 1104.82(1), b = 2338.06(3), c = 351.616(3) pm, beta = 97.9132(9)degrees, and Z = 4. The structure was solved from synchrotron powder diffraction data (lambda = 59.733 pm) using direct methods and it was refined by the Rietveld method. The final agreement factors were wR(p) = 0.080, R-p = 0.059, and R-F = 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-.