Octahedral microporous phases Na2Nb2−xTixO6−x(OH)x·H2O and their related perovskites: Crystal chemistry, energetics, and stability relations

2005 ◽  
Vol 20 (3) ◽  
pp. 618-627 ◽  
Author(s):  
Hongwu Xu ◽  
Alexandra Navrotsky ◽  
May D. Nyman ◽  
Tina M. Nenoff
Keyword(s):  
Crystal Chemistry ◽  
Phase Stability ◽  
Solution Calorimetry ◽  
Enthalpies Of Formation ◽  
Dehydration Reaction ◽  
Calorimetric Data ◽  
Synthesis Conditions ◽  
Formation Enthalpies

A family of microporous phases with compositions Na2Nb2−xTixO6−x(OH)x⋅H2O (0 ≤ x ≤ 0.4) transform to Na2Nb2−xTixO6−0.5x 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 3Na2O·4MoO3 solvent at 974 K. As Ti/Nb increases, the enthalpies of formation for the microporous phases become less exothermic up to x = ∼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: Nb5+ + O2− → Ti4+ + OH− for the microporous phases and Nb5+ → Ti4+ + 0.5VO.. for the perovskites. From the calorimetric data for the two series, the enthalpies of the dehydration reaction, Na2Nb2−xTixO6−x(OH)x⋅H2O → Na2Nb2−xTixO6−0.5x + H2O, have been derived, and their implications for phase stability at the synthesis conditions are discussed.

scholarly journals Enthalpies of formation of lanthanide oxyapatite phases

2001 ◽  
Vol 16 (10) ◽  
pp. 2780-2783 ◽  
Author(s):  
A. S. Risbud ◽  
K. B. Helean ◽  
M. C. Wilding ◽  
P. Lu ◽  
A. Navrotsky
Keyword(s):  
Structural Formula ◽  
Solution Calorimetry ◽  
Enthalpies Of Formation ◽  
Enthalpies Of Solution ◽  
Thermodynamic Cycles ◽  
Oxide Melt ◽  
Formation Enthalpies ◽  
High Temperature Oxide ◽  
Standard Enthalpies Of Formation ◽  
Temperature Oxide

A family of lanthanide silicates adopts an oxyapatitelike structure with structural formula Ln9.33 0.67(SiO4)6O2 (Ln = La, Sm, Nd, Gd,   = vacancy). The enthalpies of solution, ΔHS, for these materials and their corresponding binary oxides were determined by high-temperature oxide melt solution calorimetry using molten 2PbO B2O3 at 1078 K. These data were used to complete thermodynamic cycles to calculate enthalpies of formation from the oxides, ΔH0 f-oxides (kJ/mol): La9.33 0.67(SiO4)6O2 = 776.3 ± 17.9, Nd9.33 0.67(SiO4)6O2 = 760.4 ± 31.9, Sm9.33 0.67(SiO4)6O2 = 590.3 ± 18.6, and Gd9.33 0.67(SiO4)6O2 = 446.9 ± 21.9. Reference data were used to calculate the standard enthalpies of formation from the elements, ΔH0 f (kJ/mol): La9.33 0.67(SiO4)6O2 = 14611.0 ± 19.4, Nd9.33 0.67(SiO4)6O2 = 14661.5 ± 32.2, Sm9.33 0.67(SiO4)6O2 = -14561.7 ±; 20.8, and Gd9.33 0.67(SiO4)6O2 = -14402.7 ± 28.2. The formation enthalpies become more endothermic as the ionic radius of the lanthanide ion decreases.

Thermochemistry of Substituted Perovskites in the NaTixNb1-xO3-0.5x System

2002 ◽  
Vol 718 ◽  
Author(s):  
Hongwu Xu ◽  
Alexandra Navrotsky ◽  
M. Lou Balmer ◽  
Yali Su
Keyword(s):  
Perovskite Structure ◽  
Sol Gel ◽  
Rietveld Analysis ◽  
Solution Calorimetry ◽  
Enthalpies Of Formation ◽  
Charge Imbalance ◽  
Cell Parameters ◽  
Formation Enthalpies ◽  
First Time ◽  
Linear Manner

AbstractA suite of perovskite phases with the compositions NaTixNb1-xO3-0.5x, 0 ≤ x ≤ 0.2, has been synthesized for the first time using the sol-gel method. Rietveld analysis of powder XRD data reveals that with increasing Ti content, the orthorhombic perovskite structure becomes more cubic-like, as evidenced by the smaller differences among its three cell parameters. Enthalpies of formation of the synthesized phases from the constituent oxides and from the elements have been determined by drop solution calorimetry into molten 3Na2O·4MoO3 solvent at 974 K. As Ti4+ substitutes for Nb5+, the formation enthalpies become less exothermic in a nearly linear manner. This behavior suggests that the Ti→Nb substitution destabilizes the perovskite structure, presumably because of the concomitant occurrence of O2- vacancies, which compensate the charge imbalance between Ti4+ and Nb5+ in the structure.

Thermochemistry of rare-earth orthophosphates

2001 ◽  
Vol 16 (9) ◽  
pp. 2623-2633 ◽  
Author(s):  
S. V. Ushakov ◽  
K. B. Helean ◽  
A. Navrotsky ◽  
L. A. Boatner
Keyword(s):  
Ionic Radius ◽  
Sodium Molybdate ◽  
Solution Calorimetry ◽  
Enthalpies Of Formation ◽  
Lead Borate ◽  
Oxide Melt ◽  
Calorimetric Measurements ◽  
Formation Enthalpies ◽  
Tetravalent Actinide ◽  
Precipitation Formation

The enthalpies of formation for the compounds (RE3+)PO4, (where RE = Sc, Y, La–Nd, Sm–Lu) were determined by oxide-melt solution calorimetry. Calorimetric measurements were performed in a Calvet-type twin microcalorimeter in sodium molybdate (3Na2O · 4MoO3) and lead borate (2PbO · 2B2O3) solvents at 975 K. The experiments were carried out using both powdered single crystals grown by a flux technique and powders synthesized by precipitation. Formation enthalpies were derived from the drop-solution enthalpies for (RE)PO4, RE oxides, and P2O5. Enthalpies of formation for the (RE)PO4 compounds with respect to the oxides at 298 K become more negative with increasing RE3+ ionic radius; i.e., in going from ScPO4 (−209.8 ± 1.0 kJ/mol), to LuPO4 (−263.9 ± 1.9 kJ/mol), to LaPO4 (−321.4 ± 1.6 kJ/mol). From structural considerations, a similar trend is expected for the isostructural RE vanadates and arsenates, as well as for the tetravalent actinide orthosilicates.

Thermodynamic instability of the YBa2Cu3O7−x phase at the 1:2:3 composition

1991 ◽  
Vol 6 (5) ◽  
pp. 885-887 ◽  
Author(s):  
Fernando H. Garzon ◽  
Ian D. Raistrick ◽  
D.S. Ginley ◽  
John W. Halloran
Keyword(s):  
Phase Composition ◽  
Thermodynamic Analysis ◽  
High Precision ◽  
Solution Calorimetry ◽  
Enthalpies Of Formation ◽  
Calorimetric Data ◽  
Pure Phase ◽  
Thermodynamic Instability ◽  
Cation Ratio

We have measured enthalpies of formation of YBa2Cu3O7−x, Y2BaCuO5, YBa2Cu4O8, and BaCuO2 using high precision isothermal solution calorimetry. Thermodynamic analysis of the calorimetric data indicates that YBa2Cu3O7−x at the pure-phase composition (1:2:3 cation ratio) is metastable at ≍873 K and 1 atm Po2.

Enthalpy of formation of cubic yttria-stabilized hafnia

2004 ◽  
Vol 19 (6) ◽  
pp. 1855-1861 ◽  
Author(s):  
Theresa A. Lee ◽  
Alexandra Navrotsky
Keyword(s):  
Enthalpy Of Formation ◽  
Solution Calorimetry ◽  
Enthalpies Of Formation ◽  
Fluorite Phase ◽  
Complex Solution ◽  
Calorimetric Data ◽  
Solution Theory ◽  
Anion Sublattice ◽  
Oxide Melt ◽  
The Enthalpy Of Formation

The enthalpy of formation of cubic yttria-stabilized hafnia from monoclinic hafnia and C-type yttria was measured by oxide melt solution calorimetry. The enthalpies of formation fit a function independent of temperature and quadratic in composition. The enthalpies of transition from m-HfO2 and C-type YO1.5, to the cubic fluorite phase are 32.5 ± 1.7 kJ/mol and 38.0 ± 13.4 kJ/mol, respectively. The interaction parameter in the fluorite phase is strongly negative, -155.2 ± 10.2 kJ/mol, suggesting even stronger short range order than in ZrO2–YO1.5. Regular solution theory or any other model assuming random mixing on the cation and /or anion sublattice is not physically reasonable. A more complex solution model should be developed to be consistent with the new calorimetric data and observed phase relations.

scholarly journals Thermodynamics of Molybdenum Trioxide (MoO3) Encapsulated in Zeolite Y

2021 ◽  
Author(s):  
Xianghui Zhang ◽  
Andrew Strzelecki ◽  
Cody Cockreham ◽  
Vitaliy Goncharov ◽  
Houqian Li ◽  
...  
Keyword(s):  
Molybdenum Trioxide ◽  
Zeolite Y ◽  
Formation Enthalpy ◽  
Solution Calorimetry ◽  
Metal Species ◽  
Enthalpies Of Formation ◽  
Host Interactions ◽  
Oxide Melt ◽  
High Temperature Oxide ◽  
Temperature Oxide

Zeolites with encapsulated transition metal species are extensively applied in the chemical industry as heterogenous catalysts for favorable kinetic pathways. To elucidate the energetic insights into formation of subnano-sized molybdenum trioxide (MoO) encapsulated/confined in zeolite Y (FAU) from constituent oxides, we performed a systematic experimental thermodynamic study using high temperature oxide melt solution calorimetry as the major tool. Specifically, the formation enthalpy of each MoO/FAU is less endothermic than corresponding zeolite Y, suggesting enhanced thermodynamic stability. As Si/Al ratio increases, the enthalpies of formation of MoO/FAU with identical loading (5 Mo-wt%) tend to be less endothermic, ranging from 61.1 ± 1.8 (Si/Al = 2.9) to 32.8 ± 1.4 kJ/mol TO (Si/Al = 45.6). Coupled with spectroscopic, structural and morphological characterizations, we revealed intricate energetics of MoO – zeolite Y guest – host interactions likely determined by the subtle redox and/or phase evolutions of encapsulated MoO.

Titration Calorimetry

2008 ◽  
Author(s):  
José A. Martinho Simões ◽  
Manuel Minas da Piedade
Keyword(s):  
Heat Flow ◽  
Temperature Change ◽  
Solution Calorimetry ◽  
Enthalpies Of Formation ◽  
Titration Calorimetry ◽  
Enthalpy Of Reaction ◽  
Thermodynamic Cycles ◽  
First Case ◽  
Reaction Solution ◽  
Constant Rate

Titration calorimetry is a method in which one reactant inside a calorimetric vessel is titrated with another delivered from a burette at a controlled rate. This technique has been adapted to a variety of calorimeters, notably of the isoperibol and heat flow types. The output of a titration calorimetric experiment is usually a plot of the temperature change or the heat flow associated with the reaction or physical interaction under study as a function of time or the amount of titrant added. A primary use of titration calorimetry is the determination of enthalpies of reaction in solution. The obtained results may of course lead to enthalpies of formation of compounds in the standard state by using appropriate thermodynamic cycles and auxiliary data, as described in chapter 8 for reaction-solution calorimetry. Moreover, when reactions are not quantitative, both the equilibrium constant and the enthalpy of reaction can often be determined from a single titration run. This also yields the corresponding ΔrGo and ΔrSo through equations 2.54 and 2.55. Extensive use has been made of titration calorimetry as an analytical tool. These applications, which are outside the scope of this book, have been covered in various reviews. The historical development of titration calorimetry has been addressed by Grime. The technique is credited to have been born in 1913, when Bell and Cowell used an apparatus consisting of a 200 cm3 Dewar vessel, a platinum stirrer, a thermometer graduated to tenths of degrees, and a volumetric burette to determine the end point of the titration of citric acid with ammonia from a plot of the observed temperature change against the volume of ammonia added. The capabilities of titration calorimetry have enormously evolved since then, and the accuracy limits of modern titration calorimeters are comparable to those obtained in conventional isoperibol or heat-flow instruments. The titration procedures described in the literature can be classified as continuous or incremental, depending on the mode of titrant addition. In the first case the titrant is continuously introduced in the reaction vessel at a programmed (not necessarily constant) rate during a run.

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