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At 300 K ultrafine approximately 6 nm grain-size nanoanatase retains its structural integrity up to 18 GPa. There is progressive pressure-induced structural disorder to a highly disordered state at P>18 GPa. Signatures of short-range order persist to well beyond 18 GPa in both the synchrotron-x-ray diffraction and Raman data. Molecular dynamics simulations suggest disorder initiated in the surface shell of a nanograin with crystallinity being retained in the core. A bulk modulus B0=237+/-3 GPa for the nanoanatase (approximately 30% higher than the bulk value) is derived from the P-V data, concordant with the MD calculations.

Polyethylene is of paramount importance both from the fundamental and technological points of view, since it is the most common polymeric material and a model low-dimensional bead springs system. It is also a crystalline polymer, and the amount of crystallinity is related to the density. High pressure studies of polyethylene have been performed in the last four decades, in a limited P-T range. The room temperature solid phase of polyethylene is assessed to be orthorhombic Pnam (two chains per cell) [1]. A monoclinic structure has also been revealed at room pressure in stressed samples, having metastable character [2], while an high P-T reversible modification has been found to an hexagonal phase above 0.35 GPa and 500 K [3][4][5]. Knowledge of the extreme P-T conditions phase diagram of crystalline polyethylene, beyond 3.2 GPa and 770 K, was still lacking up today.We investigated the pressure evolution of the infrared spectra of polyethylene in the 0-20 GPa and 300-620 K P-T range, in order to discover new possible high pressure modifications of this material, to characterize its extended phase diagram and investigate its overall chemical and mechanical stability. An extra peak appears above 4 GPa centred at about 1450 cm -1 , e.g. close to the bending mode doublet of the orthorhombic phase lying at 1463-1482 cm -1 , whose intensity steeply increases with pressure. This modification is reversible, in the sense that the new peak disappears on releasing pressure below 3-4 GPa and the spectrum of the orthorhombic phase is recovered. We suggest that a proper first order thermodynamic phase transition does occur in crystalline polyethylene, at about 4 GPa, between the orthorhombic crystal and a new phase, having lower symmetry. Also, the phase transition boundary has been determined up to 620 K, and it appears to be nearly temperature independent, at least below 470 K, indicating that the two phases are isentropic. Measurements by XRD and Raman spectroscopy are in progress.[1] Bunn C.W., Structure of long-chain normal paraffin hydrocarbons. "Shape" of CH 2 groups, Trans. Faraday Soc., 1939, 75, 6113-6118. [2] Kobayashi M., J. Chem. Phys., 1979, 70, 505. [3] Bassett D.C., Block S., Piermarini G.J., J. Appl. Phys., 1974, 45, 4146. [4] Yasuniwa M., Enoshita R., Takemura T., Jap. J. Appl. Phys., 1976, 15, 1421 [5] Yamamoto T., Miyaji H., Asai K., Jap. J. Appl. Phys., 1977, 16, The phase diagram of KNbO 3 was investigated with Raman scattering up to 30 GPa and in the 10 to 500 K temperature range. Macroscopic ferroelectricity vanishes on compression and the various ferroelectric solids, rhombohedral, orthorhombic and tetragonal (R-O-T), were found to exist in close domains of the pressure-temperature plane. Specifically the T solid vanishes below 200 K and the R and O solids exits very likely down to 0 K. The transition to the cubic paraelectric phase is of the displacive type at low temperature whereas it is of the order-disorder type at high temperature. In the cubic solid, first-order Raman scattering, assigned to...

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