RESUMEN
The pyrochlore iridates,A2Ir2O7, show a wide variety of structural, electronic, and magnetic properties controlled by the interplay of different exchange interactions, which can be tuned by external pressure. In this work, we report pressure-induced iso-structural phase transitions at ambient temperature using synchrotron-based x-ray diffraction (up to â¼20 GPa) and Raman-scattering measurements (up to â¼25 GPa) of the pyrochlore series (Sm_{1-x}Bix)2Ir2O7(x= 0, 0.02, and 0.10). Our Raman and x-ray data suggest an iso-structural transition in Sm2Ir2O7atPcâ¼ 11.2 GPa, associated with the rearrangement of IrO6octahedra in the pyrochlore lattice. The transition pressure decreases to â¼10.2 and 9 GPa forx= 0.02 and 0.10, respectively. For all the samples, the linewidth of three phonons associated with Ir-O-Ir (A1gandEg) and Ir-O (T2g4) vibrations show anomalous decrease up toPc, due to decrease in electron-phonon interaction.
RESUMEN
The high pressure-high temperature structural stability of Zeolite A (ZA) has been studied using the X-ray diffraction (XRD) method. Structural studies at high temperatures show a reduction in the oxygen occupancy, belonging to the water molecule, indicating thermal dehydration and subsequent expulsion of water molecules from the pores of the structure. ZA does not undergo structural phase transition with temperature. However, structural transitions are observed in in situ XRD studies at high pressure and high temperature. At 1.3 GPa and 300 °C, the cubic ZA concomitantly transformed to cubic sodalite (SOD) and tetragonal zeolite NaP (ZNP). This transition was completely forbidden at 2.7 GPa, where a temperature-induced amorphization was favored at 250 °C. The thermal studies at higher pressure reveal the marginal influence of pressure on the thermal expansion coefficients of hydrated ZA. Pressure evolution of the high pressure-high temperature phases indicates no further phase transitions up to 5.9 GPa. The equation of state fit to the pressure-volume data of these phases show that ZNP is less compressible, followed by SOD and ZA. In contrast to the behavior at 0.1 MPa, SOD shows a pressure-induced negative thermal expansion (NTE) at 5.9 GPa. On the other hand, the positive thermal expansion (PTE) observed along the direction of c axis is compensated by the NTE along the a axis leading to a negligible volume thermal expansion for the ZNP structure. The bulk moduli and thermal expansion coefficients of all of the observed phases are reported. The outcomes of this study have been consolidated as a pressure-temperature phase diagram, which provides an insight into the technological and industrial applications of ZA at extreme conditions.
RESUMEN
The intriguing functional nature of ceramics containing rare earth sesquioxide (RES) is associated with the type of polymorphic structure they crystallize into. They prefer to be in the cubic, monoclinic or hexagonal structure in the increasing order of cation size, RRE. Since the functional properties of these ceramics varies with RRE, temperature and pressure, a systematic investigation delineating the cation size effect is indispensable. In the present work we report the structural stability and compressibility behaviour of the RES ceramics, (Eu1-xLax)2O3, of RESs with dissimilar structure and significant difference in cationic radii. The selected compositions of (Eu1-xLax)2O3 have been studied using the in-situ high pressure synchrotron X-ray diffraction and the structural parameters obtained through Rietveld refinement. The cubic structure, which is stable for 0.95 Å [Formula: see text] RRE [Formula: see text] Å at ambient temperature and pressure (ATP), prefers a cubic to hexagonal transition at high pressures. The biphasic region of cubic and monoclinic structure, which is stable for 0.98 Å [Formula: see text] RRE [Formula: see text] Å at ATP, prefers a cubic/monoclinic to hexagonal transition at high pressures. Further, in the biphasic region of monoclinic and hexagonal structure, observed for 1.025 Å [Formula: see text]RRE [Formula: see text] Å, the monoclinic phase is found to be progressing towards the hexagonal phase with increasing pressure. The pure hexagonal phase obtained for 1.055 Å [Formula: see text] RRE [Formula: see text] 1.10 Å is found to be structurally stable at high pressures. The bulk moduli are obtained from the Birch-Murnaghan equation of state fit to the compressibility data and its dependance on the cation size is discussed. The microstrain induced by the difference in cation size causes an internal pressure in the crystal structure leading to a reduction in the bulk modulus of [Formula: see text] and 0.6. A pressure-concentration (P-x) phase diagram upto a pressure of 25 GPa is constructed for (Eu1-xLax)2O3. This would provide an insight to the fundamental and technological aspects of these materials and the RESs in general.