Phase Equilibria and Crystal Chemistry of Rubidium Niobates and Rubidium Tantalates

The phase equilibria relations and crystal chemistry of portions of the Rb₂O-Nb₂O₅ and Rb₂O-Ta₂O₅ systems were investigated for structures potentially useful as ionic conductors. A hexagonal tungsten bronze-type (HTB) structure was found in both systems as well as three hexagonal phases with mixed HTB-pyrochlore type structures. Ion exchange experiments between various alkali ions are described for several phases. Unit cell dimensions and x-ray diffraction powder patterns are reported.

Investigation of Calcium Aluminate Cement Phases Under High Gaseous Pressure

The chemical degradation of refractory cement castable liners is important in the coal gasification process. The system CaO-Al₂O₃-H₂O has been investigated at high temperature and pressure. A pseudobinary reaction diagram was constructed for the system CaO-Al₂O₃-H₂O in steam at 1000 psig. Several experiments were conducted in the CaO-Al₂O₃-H₂O system in CO₂, CH₄, and CO.

Personality correlates of EEG change during meditation

15 seminary students who regularly practiced a form of Christian meditation were given the Minnesota Multiphasic Personality Inventory (MMPI). Electroencephalographic (EEG) signals were recorded and analyzed for percent time alpha during performance of a series of mental arithmetic tasks and also during a period of meditation. Higher scores on the Hypochondriasis scale of the MMPI were associated with increase in alpha as the subject proceeded from the mental arithmetic tasks to the meditation period. This result is discussed in terms of previous research, the pre-meditation level of alpha, and the implications for the use of meditation as a therapeutic procedure.

Phase Equilibria and Crystal Growth in the Alkali Antimonate Systems Sb2O4-NaSbO3/ Sb2O4-KSbO3/, and Sb2O4-NaSbO3-NaF

Phase equilibrium diagrams have been constructed from experimental data for the systems Sb₂O₄–NaSbO₃, Sb₂O₄–KSbO₃, and Sb₂O₄–NaSbO₃–NaF. The system Sb₂O₄–NaSbO₃ contains only an intermediate pyrochlore type solid solution with a maximum melting point of 1490 °C at a Na:Sb atom ratio of 3:5. The Sb₂O₄–KSbO₃ system contains in addition to the pyrochlore phase a compound 3K₂O • 5Sb₂O₅ which melts congruently at about 1450 °C and two polymorphs of K₂O • 2Sb₂O₅. The low temperature form of K₂O • 2Sb₂O₅ was found to be monoclinic P2₁/c with a = 7.178, b = 13.378, c = 11.985 A, β = 124°10′. The melting point of Sb₂O₄ was found to be 1350 ± 5 °C and NaSbO₃ and KSbO₃ both melt congruently at 1555 ± 5 °C and 1410 ± 5 °C respectively. The previously reported cubic form of KSbO₃ was found to be a K⁺ deficient phase stabilized by reaction with atmospheric moisture. A similar cubic phase which appears to be a good Na⁺ ion conductor can be synthesized in the ternary system NaSbO₃–Sb₂O₄–NaF.

Temperature-Pressure Phase Relationships in Niobium Pentoxide

A pressure-temperature (P-T) section of the phase equilibrium diagram for Nb2O5 has been determined. Four single phase regions, H-Nb2O5, B-Nb2O5, and two L-Nb2O5 areas, were characterized by x-ray powder diffraction data. The superstructure of L-Nb2O5 was indexed on the basis of similarities to the known superstructures of L-Ta2O5 and confirmed with single crystal electron diffraction data. The correct unit cell is orthorhombic with a = 6.168, b = 29.312, c = 3.938 Å, with the b axis 8 times the subcell (8 × 3.664 Å).

Effect of Oxide Additions on the Polymorphism of Tantalum Pentoxide: III. "Stabilization" of the Low Temperature Structure Type

The "low temperature structure type" of Ta2O5 has been found to occur in two distinct forms with the lowest temperature form having a unit cell 14 times the subcell and an intermediate temperature form with a unit cell 11 times the subcell. The two types form intermediate partially ordered mixtures which are apparently in thermal equilibrium at various temperatures between ~ 1000 and 1350 °C. The addition of MoO3, WO3, SiO2, GeO2, ZrO2, TiO2, B2O3 and Al2O3 each affect the multiplicity of the true unit cell in different ways. WO3, SiO2, GeO2, B2O3, and Al2O3 form phases structurally similar to "low-Ta2O5" which are stable up to the solidus temperatures of the corresponding systems.

Effect of Oxide Additions on the Polymorphism of Tantalum Pentoxide: II. "Stabilization" of the High Temperature Structure Type

The high temperature, apparently tetragonal, polymorph of tantalum pentoxide can be obtained at room temperature by quenching a specimen containing 2-5 mole percent of the following oxides:SnO2, Ga2O3, Cr2O3, Fe2O3, Sc2O3, or MgO. All the x-ray patterns can be indexed on a body centered tetragonal cell with a ≈ 3.830 Å, c ≈ 35.68 Å. However, P2O5, V2O5, Nb2O5, ZrO2, Lu2O3, NiO, or ZnO do not stabilize the tetragonal form at room temperature. Single crystals of scandium "stabilized" Ta2O5 have been grown by the Czochralski technique.

High Temperature Dehydroxylation of Apatitic Phosphates

Sr₁₀P₆O₂₄(OH)₂, (Hex. a = 9.765 Å, c = 7.280 Å), and Ba₁₀P₆O₂₄(OH)₂, (Hex. a = 10.177 Å, c = 7.731 Å), undergo cell parameter contractions at elevated temperatures in air. These can be correlated with progressive dehydroxylation, although neither can be completely dehydrated. Pb₁₀P₆O₂₄(OH)₂, (Hex. a = 9.878 Å, c = 7.432 Å), also does not completely dehydrate in air. A new apatite-like strontium phosphate, (Hex. a = 9.872 Å, c = 7.199 Å), was prepared at elevated temperatures in vacuum.

Effect of Oxide Additions on the Polymorphism of Tantalum Pentoxide (System Ta2O5-TiO2)

The phase equilibrium relationships of the TiO2-Ta2O5 system were determined in air. An equimolar compound, TiTa2O7, was found to melt congruently at about 1662 °C and have a monoclinic unit cell a = 20.291 Å, b = 3.804 Å, c = 11.831 Å, β = 120°14', apparently isostructural with TiNb2O7. In addition, two other compounds are postulated to occur at about TiO2:49Ta2O5 and TiO2:7Ta2O5 and to dissociate at about 1230 and 1190 °C, respectively. TiO2 apparently accepts a maximum of 9 mole percent Ta2O5 in solid solution at 1630 °C. Two eutectics occur in the system at 54 mole percent Ta2O5 and 1650 °C and 31 mole percent Ta2O5 and 1630 °C. The high temperature form of Ta2O5 is apparently triclinic at room temperature with a = 3.801 Å, b = 3.785 Å, c = 35.74 Å, α = 90°54.4', β = 90°11.5', and γ = 89°59.9'. A metastable transition occurs at 320 °C to a monoclinic form which inverts to tetragonal at 920 °C with a = 3.81 Å, c = 36.09 Å. Several intermediate morphotropic phases are found with up to 9 mole percent TiO2 in solid solution.

Phase Equilibria as Related to Crystal Structure in the System Niobium Pentoxide-Tungsten Trioxide

The phase equilibrium diagram for the binary system niobium pentoxide-tungsten trioxide has been constructed from results of x-ray diffraction studies on both single crystals and powders and from fusion characteristics. Twelve stable compounds have been found in the system. The exact composition of eight of these compounds has been established by single crystal analyses at the Nb2O5:WO3 ratios of 6:1, 13:4, 7:3, 8:5, 9:8, 1:1, 4:9, and 2:7. The approximate compositions of the remaining four other phases are 30:1, 6:11, 1:11, and 1:15. The 6:1, "6:11", 4:9, and 2:7 phases melt eongruently at 1476, 1378, 1380, and 1357 °C, respectively. The "30:1", 7:3, 8:5, 9:8, "1:11", and "1:15" phases melt incongruently at 1470, 1440, 1385, 1375, 1356, and 1358 °C, respectively; and the 13:4 and 1:1 phases decompose before melting at 1435 and 1115 °C, respectively. The 8:5, 9:8, "6:11", 2:7, "1:11", and "1:15" compounds are shown on the phase diagram as having minimum temperatures of stability. One metastable phase having a narrow range of composition near the 3:8 ratio was also encountered. Although Nb2O5 apparently exhibits no solid solution, WO3 was found to accept a maximum of three mole percent niobia in solid solution enabling all the reported polymorphs of WO3 to be obtained at room temperature.