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The elastic properties and thermal behavior of synthetic zincochromite (Zn[Cr.sub.2][O.sub.4]) have been studied by combining room-temperature high-pressure (0.0001-21 GPa) synchrotron radiation powder diffraction data with high-temperature (298-1240 K) powder diffraction data. Elastic properties were obtained by fitting two Equations of State (EoS) to the P-V data. A third-order Birch-Murnaghan model, which provides results consistent with those from the Vinet EoS, yields: [K.sub.0] = 183.1([+ or -] 3.5) GPa, K' = 7.9([+ or -] 0.6), K' = -0.1278 G[Pa.sup-1] (implied value), at [V.sub.0] = 577.8221 [[Angstrom].sup.3] (fixed). Zincochromite does not exhibit order-disorder reactions at high temperature in the thermal range explored, in agreement with previous studies. The volume thermal expansion was modeled with [[alpha].sub.v] = [[alpha].sub.0] + [[alpha].sub.1]T + [[alpha].sub.2]/[T.sup.-2], where only the first coefficient was found to be significant [[[alpha].sub.0] = 23.0(4) [10.sup.-6] [K.sup.-1]. Above 23 GPa diffraction patterns hint at the onset of a phase transition; the high pressure phase is observed at approximately 30 GPa and exhibits orthorhombic symmetry. The elastic and thermal properties of zincochromite were then used to model by thermodynamic calculations the P-T stability field of Zn[Cr.sub.2][O.sub.4] with respect to its oxide constituents ([Cr.sub.2][O.sub.3] and rocksalt-like ZnO). Spinel is expected to decompose into oxides at about 18 GPa and room temperature, in absence of sluggish kinetics.

The lattice parameters of natural spessartine and uvarovite garnets were measured as a function of pressure up to 25 and 35 GPa, respectively, by synchrotron radiation powder diffraction. The bulk modulus and its first and second derivatives were determined by fitting three equation of state (EoS) models to the pressure-volume data: the Birch-Murnaghan EoS, the Vinet EoS, and the Poirier-Tarantola logarithmic EoS. The third-order Birch-Murnaghan EoS, assuming [V.sub.0] at its experimental value, yields [K.sub.0] = 171 (1) GPa and [K.sub.0]' = 5.4 (2) with [V.sub.0] = 1573 [[Angstrom].sup.3] for spessartine, and [K.sub.0] = 160(1) GPa and [K.sub.0] = 5.8(1) with [V.sub.0] = 1697.5 [[Angstrom].sup.3] for uvarovite. The results obtained by the EoS models are consistent with each other within the experimental uncertainties. A comparison is made with earlier results from the literature.

The HP behavior of the natural zeolite yugawaralite and of the synthetic zeolite Na-A was studied by in situ synchrotron X-ray powder diffraction, using a non-penetrating P-transmitting medium. The unit-cell parameters of yugawaralite were refined up to the pressure of 10 GPa, at which reductions were found of about 7, 2.4, 7, 1.3. and 15% for a, b, c, [beta], and V, respectively. Contractions of 6.5 and 18.4% were found for a and V, respectively, for zeolite Na-A in the range [10.sup.-4] to 6.8 GPa. Diffraction patterns collected during decompression show that the effects induced by high pressure on both samples are almost completely reversible. These results are compared with those obtained under similar experimental conditions for other natural zeolites, with the aim of rationalizing the deformation mechanisms of these porous materials and comparing their flexibility under high-pressure and high-temperature conditions.

Pressure-induced structural modifications in scolecite were studied by means of in situ synchrotron X-ray powder diffraction and density functional computations. The experimental cell parameters were refined up to 8.5 GPa. Discontinuities in the slope of the unit-cell parameters vs. pressure dependence were observed; as a consequence, an increase in the slope of the linear pressure-volume dependence is observed at about 6 GPa, suggesting an enhanced compressibility at higher pressures. Weakening and broadening of the diffraction peaks reveals increasing structural disorder with pressure, preventing refinement of the lattice parameters above 8.5 GPa. Diffraction patterns collected during decompression show that the disorder is irreversible. Atomic coordinates within unit cells of different dimensions were determined by means of Car-Parrinello simulations. The discontinuous rise in compressibility at about 6 GPa is reproduced by the computation, allowing us to attribute it to re-organization of the hydrogen bonding network, with the formation of water dimers. Moreover we found that, with increasing pressure, the tetrahedral chains parallel to c rotate along their elongation axis and display an increasing twisting along a direction perpendicular to c. At the same time, we observed the compression of the channels. We discuss the modification of the Ca polyhedra under pressure, and the increase in coordination number (from 4 to 5) of one of the two Al atoms, resulting from the approach of a water molecule. We speculate that this last transformation triggers the irreversible disordering of the system.

The elastic properties and thermal behavior of synthetic zincochromite (ZnCr2O4) have been studied by combining room-temperature high-pressure (0.0001-21 GPa) synchrotron radiation powder diffraction data with high-temperature (298-1240 K) powder diffraction data. Elastic properties were obtained by fitting two Equations of State (EoS) to the P-V data. A third-order Birch-Murnaghan model, which provides results consistent with those from the Vinet EoS, yields: K0 = 183.1 (+3.5) GPa, K' = 7.9(±0.6), K" = -0.1278 Gpa-1 (implied value), at V0 = 577.8221 Å (fixed). Zincochromite does not exhibit order-disorder reactions at high temperature in the thermal range explored, in agreement with previous studies. The volume thermal expansion was modeled with αv = α0 + α1T + α2/T-2, where only the first coefficient was found to be significant [α0 = 23.0(4) 10-6 K-1]. Above 23 GPa diffraction patterns hint at the onset of a phase transition; the high pressure phase is observed at approximately 30 GPa and exhibits orthorhombic symmetry. The elastic and thermal properties of zincochromite were then used to model by thermodynamic calculations the P-T stability field of ZnCr2O4 with respect to its oxide constituents (Cr2O3 and rocksalt-like ZnO). Spinel is expected to decompose into oxides at about 18 GPa and room temperature, in absence of sluggish kinetics. [ABSTRACT FROM AUTHOR]