Hongwu Xu

Aluminum in magnesium silicate perovskite: Formation, structure, and energetics of magnesium-rich defect solid solutions

[1]xa0MgSiO3-rich perovskite is expected to dominate Earths lower mantle (pressures >25 GPa) with iron and aluminum as significant substituents. The incorporation of trivalent ions, M3+, may occur by two competing mechanisms: MgA + SiB = MA + MB and SiB = AlB + 0.5 (vacancy)O. Phase synthesis studies show that both substitutions do occur and the nonstoichiometric or defect substitution is prevalent along the MgSiO3-MgAlO2.5 join. Lattice parameters associated with the first substitution (stoichiometric) show more rapid increases with increasing Al content than those for the second substitution (nonstoichiometric), consistent with the differences in size of substituting ions. Oxide melt solution calorimetry has been used to compare the energetics of both substitutions. The stoichiometric substitution, represented by the reaction 0.95 MgSiO3 (perovskite) + 0.05 Al2O3 (corundum) = Mg0.95Al0.10Si0.95O3 (perovskite), has an enthalpy of −0.8 ± 2.2 kJ/mol. The nonstoichiometric reaction, 0.90 MgSiO3 (perovskite) + 0.10 MgO (rocksalt) + 0.05 Al2O3 (corundum) = MgSi0.9Al0.1O2.95 (perovskite) has a small positive enthalpy of 8.5 ± 4.6 kJ/mol. Configurational T ΔS terms play a role in both substitutions. The defect substitution is not prohibitive in enthalpy, entropy, or volume, is favored in perovskite coexisting with magnesiowustite and may significantly affect the elasticity, rheology, and water retention of silicate perovskite in Earth.

Thermochemistry of stuffed quartz-derivative phases along the join LiAlSiO 4 -SiO 2

Abstract Enthalpies of drop-solution (∆Hdrop-soln) of a suite of stuffed quartz-derivative phases with the composition Li1-xAl1-xSi1+xO4 (0 ≤ x ≤ 1) have been measured in molten 2PbO·B2O3 at 974 K. Substitution of Si4+ for Li++Al3+ results in more exothermic enthalpies of drop-solution, which is consistent with behavior seen in other crystalline and glassy aluminosilicates. Al/Si ordering serves to stabilize these phases, and long-range ordering for compositions with x approximately <0.3 can be discerned in both calorimetric data and in structural data obtained by electron and synchrotron X-ray diffraction (XRD). In contrast, a structural but not an energetic discontinuity is apparent at x ≅ 0.65, which corresponds to a compositionally induced α-β quartz transition with a small enthalpy of transformation. An enthalpy for the Al/Si order-disorder reaction in β-eucryptite was measured as 25.9 ± 2.6 kJ/ mol. Standard molar enthalpies of formation of the stuffed quartz-derivative phases from constituent oxides (∆H0f,ox) and elements (∆H0f,el) at 298 K also are presented. ∆H0f,ox = -69.78 ± 1.38 kJ/mol and ∆H0f,el = -2117.84 ± 2.50 kJ/mol for β-eucryptite, which are in good agreement with results previously determined by HF solution calorimetry at 346.7 K (Barany and Adami 1966). The enthalpies of formation of other compositions are reported for the first time.

Enthalpies of formation of microporous titanosilicates ETS-4 and ETS-10

The energetics of microporous titanosilicates ETS-4 (K1.13Na3.92Ti3.07Si8.17O25·8.64H2O) and ETS-10 (K0.61Na1.09Ti1.10Si4.98O13·2.89H2O) has been investigated by high-temperature drop solution calorimetry using lead borate as the solvent at 974 K. Combining the measured heats of drop solution with the published enthalpies of drop solution and formation for the constituent oxides, the standard enthalpies of formation from the oxides (ΔH0f,ox) and from the elements (ΔHf,el0) for both phases were derived for the first time. The obtained values (in kJ/mol) are as follows: ΔHf,ox0(ETS-4)=−818.5±13.1, ΔHf,el0(ETS-4)=−14,642.8±15.9, ΔHf,ox0(ETS-10)=−262.2±3.1, and ΔHf,el0(ETS-10)=−6995.2±6.1. Comparison between the ΔHf,ox0 (or ΔHf,el0) values of the two phases suggests that ETS-4 is thermodynamically more stable than ETS-10 with respect to the oxides (or the elements) at 298 K and 1 atm. This behavior can largely be attributed to the higher degree of hydration of ETS-4 than that of ETS-10.

Thermochemistry of microporous silicotitanate phases in the Na{sub 2}O-Cs{sub 2}O-SiO{sub 2}-TiO{sub 2}-H{sub 2}O system

Microporous silicotitanates can potentially be used as ion exchangers for removal of Cs{sup +} from radioactive waste solutions. The enthalpies of formation from constituent oxides for two series of silicotitanates at 298 K have been determined by drop-solution calorimetry into molten 2PbO{center_dot}B{sub 2}O{sub 3} at 974 K: the (Na{sub 1-x}Cs{sub x}){sub 3}Ti{sub 4}Si{sub 3}O{sub 15}(OH){center_dot}nH{sub 2}O (n=4 to 5) phases with a cubic structure (P4(bar sign)3m), and the (Na{sub 1-x}Cs{sub x}){sub 3}Ti{sub 4}Si{sub 2}O{sub 13}(OH){center_dot}nH{sub 2}O (n=4 to 5) phases with a tetragonal structure (P4{sub 2}/mcm). The enthalpies of formation from the oxides for the cubic series become more exothermic as Cs/(Na+Cs) increases, whereas those for the tetragonal series become less exothermic. This result indicates that the incorporation of Cs in the cubic phase is somewhat thermodynamically favorable, whereas that in the tetragonal phase is thermodynamically unfavorable and kinetically driven. In addition, the cubic phases are more stable than the corresponding tetragonal phases with the same Cs/Na ratios. These disparities in the energetic behavior between the two series are attributed to their differences in both local bonding configuration and degree of hydration. (c) 2000 Materials Research Society.

Thermochemistry of guest-free melanophlogite

Abstract Melanophlogite is a naturally occurring clathrasil possessing a framework of linked silicate tetrahedra surrounding small, isolated cages, which can host small molecules. The energetics of a guest-free natural sample was determined by oxide-melt solution calorimetry. Melanophlogite is energetically metastable with respect to α-quartz by 9.5 ± 0.5 kJ/mol, a value similar to that for amorphous silica and for synthetic small-pore zeolitic silicas (Petrovic et al. 1993, Piccione et al. 2001). Thus, its occurrence in nature, for example in environments where it can occlude volcanic gases, is reasonable on energetic grounds. Molecular modeling of the internal pore volume of melanophlogite confirms that this enthalpy follows the trend previously established for a variety of silica zeolites, which defines an internal surface energy of 0.093 ± 0.010 J/m2, similar to that of the external surface energy of amorphous silica. Thus melanophlogite, despite its unique topology and isolated cages, behaves energetically as predicted from the enthalpies of more-open zeolitic frameworks.

A new polymorph of eucryptite (LiAlSiO4), ε-eucryptite, and thermal expansion of α- and ε-eucryptite at high pressure

Abstract X-ray diffraction experiments have been carried out on b-eucryptite (LiAlSiO4) at pressures up to 2.5 GPa and temperatures up to 1073 K in a large-volume apparatus. With room-temperature compression, we observed a phase transition to a new polymorph between 0.83 and 1.12 GPa. This transition is reversible in character. The new phase, referred to here as e-eucryptite, can be indexed according to an orthorhombic unit cell with a = 10.217(4) Å, b = 8.487(4), Å, c = 5.751(3) Å, and V = 498.7(4) Å3 for XRD data at 2.2 GPa and 298 K. On heating at 2.2 GPa, e-eucryptite and beucryptite were metastable over the temperature interval 298-873 K; at higher temperatures they underwent an irreversible phase transition to a-eucryptite. Both hexagonal α-eucryptite and ε-eucryptite show anisotropic thermal expansion. For α-eucryptite, we obtained αa = 6.71(±0.25) × 10-6 K-1, αc = 1.07(±0.05) × 10-5 K-1, and αv = 2.42(±0.1) x 10-5 K-1 at 1.94(2) GPa over the temperature range 298-1073 K. For ε-eucryptite at 2.32(8) GPa, we find larger thermal expansion in a smaller temperature range 298-773 K, with αa = 1.47(±0.15) × 10-5 K-1, αb = 6.65(±1.33) x 10-6 K-1, αc = 7.83(±0.88) × 10-6 K-1, and αv = 2.99(±0.15) × 10-5 K-1. In combination with a previous determination of thermal expansion at ambient pressure, the pressure effect on volume thermal expansion of a-eucryptite is determined to be -2.68 × 10-6 GPa-1 K-1, and the temperature derivative of the bulk modulus is estimated to be -0.015 GPa/K.

Octahedral microporous phases Na2Nb2-xTixO6-x(OH)xH2O and their dehydrated perovskites : crystal chemistry, energetics and stability relations.

A family of microporous phases with compositions Na{sub 2}Nb{sub 2-x}Ti{sub x}O{sub 6-x}(OH){sub x} {center_dot} H{sub 2}O (0 {le} x {le} 0.4) transform to Na{sub 2}Nb{sub 2-x}Ti{sub x}O{sub 6-0.5x} perovskites upon heating. In this study, we have measured the enthalpies of formation of the microporous phases and their corresponding perovskites from the constituent oxides and from the elements by drop solution calorimetry in 3Na{sub 2}O {center_dot} 4MoO{sub 3} solvent at 974 K. As Ti/Nb increases, the enthalpies of formation for the microporous phases become less exothermic up to x = {approx}0.2 but then more exothermic thereafter. In contrast, the formation enthalpies for the corresponding perovskites become less exothermic across the series. The energetic disparity between the two series can be attributed to their different mechanisms of ionic substitutions: Nb{sup 5+} + O{sup 2-} {yields} Ti{sup 4+} + OH{sup -} for the microporous phases and Nb{sup 5+} {yields} Ti{sup 4+} + 0.5 V{sub O}** for the perovskites. From the calorimetric data for the two series, the enthalpies of the dehydration reaction, Na{sub 2}Nb{sub 2-x}Ti{sub x}O{sub 6-x}(OH){sub x} {center_dot} H{sub 2}O {yields} Na{sub 2}Nb{sub 2-x}Ti{sub x}O{sub 6-0.5X} + H{sub 2}O, have been derived, and their implications for phase stability at the synthesis conditionsmorexa0» are discussed.«xa0less

New Crystalline Silicotitanate (CST) Waste Forms: Hydrothermal Synthesis and Characterization of CS-SI-TI-O Phases

The radioactivity of the Hanford site waste tanks is primarily from {sup 137}Cs and {sup 90}Sr, of which can both be selectively removed from solution using a crystalline silicotitanate (CST) ion exchanger. The authors are currently seeking waste forms alternative to borosilicate glass for Cs-CSTs. In order to obtain a fundamental basis for the development of an alternative waste form, they are investigating synthesis and characterization of CST component phases, namely Cs-Si-Ti-O phases. Two novel Cs-Ti-Si-O phases (one porous, one condensed) have been hydrothermally synthesized, characterized and evaluated as waste form candidates based on chemical and thermal stability, leachability, and ion exchange capabilities.

Aluminum in Magnesium Silicate Perovskite: Synthesis and Energetics of Defect Solid Solutions

MgSiO 3 - rich perovskite is expected to dominate the Earths lower mantle (pressures > 25 GPa), with iron and aluminum as significant substituents. The incorporation of trivalent ions, M 3+ , may occur by two competing mechanisms: M gA+ Si B = M A + M B and Si B = Al B + 0.5 VO. Phase synthesis studies show that both substitutions do occur, and the nonstoichiometric or defect substitution is prevalent along the MgSiO 3 - MgAlO 2.5 join. Oxide melt solution calorimetry has been used to compare the energetics of both substitutions. The stoichiometric substitution, represented by the reaction 0.95 MgSiO 3 (perovskite) + 0.05 Al 2 O 3 (corundum) = Mg 0.95 Al 0.10 Si 0.95 O 3 (perovskite), has an enthalpy of -0.8±2.2 kJ/mol. The nonstoichiometric reaction, 0.90 MgSiO 3 (perovskite) + 0.10 MgO (rocksalt) + 0.05 Al 2 O 3 (corundum) = MgSi 0.9 Al 0.1 O 2.95 (perovskite) has a small positive enthalpy of 8.5±4.6 kJ/mol. The defect substitution is not prohibitive in enthalpy, entropy, or volume, is favored in perovskite coexisting with magnesiowustite, and may significantly affect the elasticity, rheology and water retention of silicate perovskite in the Earth.