Single crystals of Ca4SiN4 were found in the product prepared by heating Ba, Ca, Si, NaN3, and Na at 900 degrees C. Ca4SiN4 [space group P2(1)/c (No. 14), Z = 4, a = 9.1905(4) angstrom, b = 5.9775(3) angstrom, c = 11.0138(7) angstrom, beta = 116.4054(17)degrees] is isotypic with Ca4GeN4 and K4SiO4. Isolated [SiN4](8-) tetrahedra were identified in the structure by single-crystal X-ray diffraction. After reheating the product at 900 degrees C, a new polymorph of Ca5Si2N6 crystallized. The space group of the polymorph [C2/m (No. 12), Z = 4, a = 6.2712(5) angstrom, b = 10.0175(8) angstrom, c = 12.0287(8) angstrom, beta = 99.303(2)degrees] is different from C2/c previously reported for Ca5Si2N6, while both polymorphs are composed of Ca2+ and edge-sharing double tetrahedra [Si2N6](10-).