The lattice symmetry and lattice parameters of the low-temperature solid forms I1 and I of 2-methyl-2-nitropropane (TBN), (CH,),CNO,, have been determined by X-ray powder diffraction and found to be of the orthorhombic and face-centred cubic type, respectively. The pVT data have been determined in the temperature range 228 to 353 K up to ca. 300 MPa, as well as the volume changes accompanying the phase transitions. The volume changes allow the calculation, via the Clausius-Clapeyron equation of the corresponding entropy changes. The volume changes on melting decrease markedly with increasing pressure, whereas those at the solid-solid transitions (I1 -, I and 111 + 11) exhibit a much less pronounced pressure dependence. With regard to the entropy changes, only the I1 -+ I transition shows a small dependence (decrease) on increasing pressure. The two mean parts of the entropy changes (configurational and volume-dependent terms) are discussed for each transition.

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The phase behaviour, the calorimetric and volumetric properties of 2,2,2-trichloro-ethanol (TCE) have been studied with differential thermal analysis (DTA) and pVT measurements in the pressure range 1 atm to 300 MPa and temperatures between 250 K to 355 K. TCE displays a metastable plastic phase (solid I') and a non-plastic solid phase II at atmospheric pressure. At least two pressure-induced solid phases have been detected: a stable plastic phase (solid I') and a non-plastic phase (solid III). There are two sets of triple points: a) 123 MPa and 308 K with the phases solid I, solid II and the liquid, b) 243 MPa and 316 K for solid I, II, III. Furthermore a metastable low-temperature brittle form (solid II') has been found, which transforms to solid I at a considerably lower temperature than solid II. The melting curve of solid I' can be pursued to higher pressures up to 260 MPa. On the other hand, the melting curve of the stable plastic phase solid I can be extrapolated beyond the triple point to pressures below 123 MPa. Volume and enthalpy changes are reported for all phase transitions.

Differential thermal analysis and dielectric studies under pressures up to 300 MPa and temperatures of about 200 to 350 K have been performed on 2-methyl-2-nitro-propane (TBN). TBN displays an orientationally disordered phase (ODIC), solid I, and two non-plastic phases, solids II and III. The coexistence region of the plastic phase I increases with increasing pressure, whereas the low-temperature phase II apparently vanishes at a triple point I, II, III, above 300 MPa. The static permittivity increases on freezing, characterizing the solid I as an ODIC phase. In the frame of the Kirkwood-Onsager-Fröhlich theory the g-factor is about unity, discounting specific dielectric correlations. The dielectric behaviour of TBN is similar to previously studied related compounds, such as 2-chloro-2-methyl-propane or 2-brome- 2-methyl-propane