Judith K. Stalick

Phase composition and its changes during annealing of plasma-sprayed YSZ☆

Abstract The phase composition of plasma-sprayed yttria stabilized zirconia (YSZ), ZrO2 with 8% by mass Y2O3, was studied using neutron and X-ray diffraction. Comparison shows that neutron diffraction is superior for analysis of the phase composition as well as for the analysis of the yttria content of the tetragonal phase. The presence of large amounts of the cubic phase is probably often neglected or underestimated in standard XRD analysis due to scattering-related limitations and the inherent difficulty of the analysis. The importance of this fact needs to be addressed in future studies. The amount of monoclinic, tetragonal, and cubic phases was determined using neutron Rietveld refinement for feedstock powders, as-sprayed deposits and for samples annealed for 1 h at temperatures of 1100, 1200, 1300, and 1400°C. The two studied feedstock materials were manufactured by different production methods, and contained various amounts of the monoclinic phase depending on manufacturing method. While the tetragonal phase dominated, both feedstock powders also contained significant amounts of the cubic phase. The as-sprayed deposits were composed of mostly tetragonal phase, with only traces of the monoclinic phase; the amount of the cubic phase was reduced with respect to the feedstock. For one of the materials the cubic phase content remained significant (approx. 25% by mass), and for the other the cubic phase content was significantly lower (approx. 6% by mass). The cubic phase content after annealing at 1400°C was in both cases similar — approximately 40% by mass. There was no significant change in monoclinic phase content observed in this experiment. The yttria fraction within the tetragonal and cubic phases was followed and changes are discussed.

Thermal spray yttria-stabilized zirconia phase changes during annealing

Phase stability of thermal barrier deposits made from yttria partially stabilized zirconia (Y-PSZ) is a requirement for extended service lifetime. The response of Y-PSZ plasma-sprayed deposits to annealing at 1000, 1200, and 1400 °C with times from 1 to 1000 h has been evaluated using the Rietveld analysis of neutron diffraction data. Results show that the yttria concentration of the as-sprayed tetragonal zirconia component generally decreased with increasing annealing temperature and time. As the yttria content in the tetragonal phase approached a limiting concentration, about 3.5 mol.% of YO1,5, the tetragonal phase transformed into monoclinic phase on cooling. An increase in monoclinic phase content was clearly observed after annealing for 24 h at 1400 °C and was nearly 35% after 100 h at 1400 °C. A similar trend was observed at 1200 °C for longer annealing times, with monoclinic phase formation beginning after 400 h. At 1000 °C, experimental times were not sufficient for monoclinic phase to form, although a decrease in the yttria concentration in the tetragonal phase was observed.

Neutron diffraction study of the brown phase BaNd2CuO5

Abstract A neutron Rietveld refinement study of the “brown phase” BaNd 2 CuO 5 shows that this material is isostructural with the La analog, but adopts a totally different structure from that of the “green phase” BaR 2 CuO 5 formed for R = Y and most lanthanide elements. BaNd 2 CuO 5 is tetragonal, space group P4/mbm(126), with Z =2 and refined cell parameters a = 6.7015(1) and c =5.8211 (1) A. The BaNd 2 CuO 5 framework is built from edge- and face-sharing BaO 10 and NdO 8 polyhedra, while in the green phase structure the smaller R 3+ ion is surrounded by seven oxygen atoms. The square-planar CuO 4 groups are isolated in the structure.

Crystallographic structures and phase transformations in ZrPd

Abstract Phase transformations and crystal structures in the equiatomic compound ZrPd were examined using in-situ heating and cooling in a transmission electron microscope (selected-area diffraction and high resolution imaging) and Rietveld refinement of high resolution neutron powder diffraction data. Three phases were characterized: a high temperature cubic B2-type phase (Pm3m; Z = 1; a = 0.335 97(3) nm at 800°C); an orthorhombic CrB-type phase formed by martensitic transformation from the B2 phase (Cmcm; Z = 4; a = 0.333 19(3) nm, b = 1.030 1(1) nm and c = 0.441 11(4) nm at 400°C) and a monoclinic variant of the CrB-type phase at room temperature (assumed space group Cm; Z = 8; a = 0.666 11(6) nm, b = 0.874 99(7) nm, c = 0.542 35(6) nm; β = 108.21(1)°). The monoclinic ⇌ orthorhombic phase transformation was found to occur reversibly around 200°C.

Crystal structure of the Zr3Pd4 phase

Abstract A new binary ZrPd phase with Zr3Pd4 stoichiometry was found. The structure of the phase was investigated by transmission electron microscopy and powder neutron diffraction. The structure belongs to the Pu3Pd4 structure type, which has a rhombohedral R 3 space group with 42 atoms in a hexagonal cell. Modifications of the ZrPd phase diagram which accommodate the new phase are suggested.

True Composition and Structure of Hexagonal “YAlO3”, Actually Y3Al3O8CO3

The discovery of a brilliant-blue color upon the introduction of Mn(3+) to the trigonal-bipyramidal (TBP) sites in YInO(3) has led to a search for other hosts for Mn(3+) in TBP coordination. An obvious choice would be YAlO(3). This compound, which has only been prepared through a citrate precursor route, has long been considered isostructural with YInO(3). However, Mn(3+) substitutions into YAlO(3) have failed to produce a product with the anticipated color. We find that the hexagonal structure for YAlO(3) with Al in TBP coordination proposed in 1963 cannot be correct based on its unit cell dimensions and bond-valence sums. Our studies indicate instead that all, or nearly all, of the Al in this compound has a coordination number (CN) of 6. Upon heating in air, this compound transforms to YAlO(3), with the perovskite structure liberating CO(2). The compound long assumed to be a hexagonal form of YAlO(3) is actually an oxycarbonate with the ideal composition Y(3)Al(3)O(8)CO(3). The structure of this compound has been characterized by powder neutron and X-ray diffraction data obtained as a function of temperature, magic-angle-spinning (27)Al NMR, Fourier transform infrared, and transmission electron microscopy. Refinement of neutron diffraction data indicates a composition of Y(3)Al(3)O(8)CO(3). We find that the hexagonal structures of YGaO(3) and YFeO(3) from the citrate route are also stabilized by small amounts of carbonate. Surprisingly, Y(3)Al(3)O(8)CO(3) forms a complete solid solution with YBO(3) having tetrahedral borate groups. Other unlikely solid solutions were prepared in the YAlO(3)-YMnO(3), YAlO(3)-YFeO(3), YAlO(3)-YBO(3), YBO(3)-YMnO(3), YBO(3)-YFeO(3), and YBO(3)-YGaO(3) systems.

Structural studies and electrical properties of Cs/Al/Te/O phases with the pyrochlore structure.

A series of polycrystalline and single crystal cesium aluminum tellurates with the pyrochlore structure have been prepared and characterized. The variations in cell edge for the Cs/Al/Te/O phases range from 10.06 Å for the Al rich limit to 10.14 Å for the Te rich limit. Rietveld structural analyses based on both X-ray and neutron diffraction data were performed on 5 different compositions. Single crystals of 3 compositions were prepared and studied by X-ray diffraction. The anharmonic component of the thermal motion for Cs was small but became significant on replacing Cs with Rb. A maximum in the electrical conductivity of about 0.1 S/cm is found in the middle of this range close to the ideal composition of CsAl(1/3)Te(5/3)O(6). The conductivity is attributed to filled Te 5s states associated with Te(4+) lying just below the conduction band based on empty Te 5s states associated with Te(6+). The relatively large Te(4+) ion is compressed by the lattice, and as this compression increases the filled 5s states approach the conduction band and thereby increases conductivity.

Electronic Structure of Sr2MoO4

The band structure for Sr2MoO4, which is isostructural with the unconventional spin-triplet superconductor Sr2RuO4, has been calculated within the local-density approximation. The results of the neutron-diffraction structure refinement used in the calculation are also presented. The elecronic structure of Sr2MoO4 resembles that of Sr2RuO4, while in Sr2MoO4 the anisotropy of Fermi velocity is considerably larger than that in Sr2RuO4. The neutron diffraction results and the structure optimization result suggest that the MoO6 octahedron is less elongated than the previously reported value.

CsTe2O6–x: Novel Mixed-Valence Tellurium Oxides with Framework-Deficient Pyrochlore-Related Structure

Structures of CsTe₂O(6-x) phases were investigated by single-crystal X-ray diffraction and neutron powder diffraction. Stoichiometric CsTe₂O₆ is a mixed-valence Cs₂Te⁴⁺Te₃⁶⁺O₁₂ compound with a rhombohedral pyrochlore-type structure where there is complete order of Te⁴⁺ and Te⁶⁺. On heating, this compound develops significant electrical conductivity. As CsTe₂O₆ becomes oxygen deficient above 600 °C, the rhombohedral pyrochlore-type structure is replaced by a cubic pyrochlore-type structure with disordered Te⁴⁺/Te⁶⁺ and oxygen vacancies. However, for CsTe₂O(6-x) phases prepared at 500 °C, the observed pyrochlore-type structure has symmetry. The Te⁴⁺ and O vacancies are all on chains running along the b axis, and the maximum value of x observed is about 0.3. At still higher values of x a new compound was discovered with a structure related to that reported for Rb₄Te₃⁴⁺Te₅⁶⁺O₂₃.

One-dimensional disorder in Zr9M11 (M = Ni, Pd, Pt) and low-temperature atomic mobility in Zr9Ni11

The Zr9M11 structure type (M = Ni, Pd, Pt) has been investigated using transmission electron microscopy and powder neutron diffraction in the temperature range 4‐1273 K. The crystal structures are tetragonal, P4/m, Z = 2, with a = 9.882(1) ˚