Between 5 °K and its m.p. (446.0 °K), tris, H2NC(CH2OH)3, exhibits one phase transition, at 407.3 °K, to form the orientationally disordered (plastic) phase I. The ordered form, phase II, is orthorhombic, Pn21a, a=8.844(1) Å, b=7.794(1) Å, c=8.795 (1) Å, Z=4, Dx=1.327 g/cm3, and Dm=1.320 g/cm3. The crystal structure can be described in terms of layers of molecules approximately normal to the c axis, with strong intralayer hydrogen bonding and weak interlayer hydrogen bonding. The crystal structure of phase II was studied using x-ray diffraction at 144, 295, 297, 348, 382, and 395 °K. Full-matrix least-squares refinement of single-crystal diffractometer data resulted in R values between 2–4% for all but the highest-temperature data sets. Although the integrity of the structure is maintained throughout this temperature range, subtle effects are noted that indicate the increased thermal motion as phase I is approached. Phase I is body-centered-cubic, Im3m, with a=6.876(6) Å at 423 °K, Z=2, and Dx=1.237 g/cm3. A comparison of the two structures and the experimental investigation of a single sample in both phases indicate that the mechanism of the phase transition involves a realignment of the molecules within the the layers and a lessening of the interlayer distance. The formation of a bcc lattice in phase I, rather than a fcc lattice as was found in the other members of this series, has been tentatively interpreted in terms of the ellipsoidal shape of the tris molecule.
