C-type Solid Solution Research Articles

Phase characterization of Sm–Lu mixed oxide powders prepared by sintering and radio frequency plasma spraying

Abstract Radio frequency plasma spraying is a promising technique to prepare rare earth oxide nano-particles. Applying this method after sintering of pellets at T = 1600 °C and subsequent grinding, Sm2O3—Lu2O3 mixed oxide powders of various compositions were synthesized. Analyzing the ground powders, the phase field can be divided into three regions: (1) Sm-rich monoclinic B-type solid solutions, (2) Lu-rich cubic C-type solid solutions, and (3) a diphasic region for Lu contents 0.33 ≤ x Lu ≤ 0.54. In the single phase areas, the volume for one formula unit of the rare earth sesquioxide is found to depend linearly on the cubed average cation radius. Plasma spraying of the Lu-rich powders leads to a second monoclinic B-type phase. Excluding the solubility gap, the monoclinic lattice parameters show quadratic dependencies on the Lu content and the molecular volume is a function of third order on the cation radius.

Phase relations, lattice thermal expansion in Ce 1− xEu xO 2− x/2 and Ce 1− xSm xO 2− x/2 systems and stabilization of cubic RE 2O 3 (RE: Eu, Sm)

The phase relations in CeO 2–Eu 2O 3 and CeO 2–Sm 2O 3 systems have been established under slow-cooled conditions from 1400 °C. The two-phase relations differ as the CeO 2–Eu 2O 3 system showed only two monophasic phase fields, namely F-type cubic and C-type cubic, whereas CeO 2–Sm 2O 3 system showed three phase fields namely F-type cubic, C-type cubic and a biphasic field comprising of C-type cubic and monoclinic phase. An interesting observation of this investigation is the stabilization of C-type rare-earth oxide after Ce 4+ substitution, which is attributed to decrease in average cationic size on Ce 4+ substitution at RE 3+ site. The lattice thermal expansion behavior of F-type solid solution and C-type solid solution in CeO 2–Eu 2O 3 system was investigated by high-temperature XRD.

Phase relation studies in the CeO 2–Gd 2O 3–ZrO 2 system

The phase relations in the CeO 2–Gd 2O 3–ZrO 2 system have been established after slowly cooling the samples from 1400 °C. Ceria has been used as a surrogate material in place of plutonia. About 80 compositions in Zr 1− x Gd x O 2− x /2, Ce 1− x Gd x O 2− x /2, Ce 1− x Zr x O 2.00, (Zr 0.5Ce 0.5) 1− x Gd x O 2− x /2, (Ce 0.5Gd 0.5) 1− x Zr x O 1.75+ x /4, (Zr 0.5Gd 0.5) 1− x Ce x O 1.75+ x /4, and (Ce 0.8Zr 0.2) x Gd 1− x O 1.5+ x / 2 were prepared by a three steps heating protocol. Based on the refinement of the XRD data, several phase regions namely; cubic fluorite type solid solution, C-type solid solution, and various biphasic regions could be delineated. This system showed the existence of a very wide cubic phase field. About 17.5 mol% GdO 1.5 was found to fully stabilize the cubic zirconia. On the other hand ceria did not stabilize the cubic zirconia. The anion-excess gadolinia, i.e., Gd 1− x Ce x O 1.5+ x was found to retain the C-type lattite unlike pure gadolinia. The ternary phase relations were mainly characterized by the presence of wide homogeneity ranges of fluorite type or C-type phases.

A Single Crystal X-Ray Study of the Fluorite-Related "Solid Solutions" of CeO 2-YO 1.5

A study of the (1 - δ)CeO 2 · δYO 1.5 solid solution has been undertaken to understand the origin of the two-phase region that occurs between the fluorite-type, 0-∼50 mole% YO 1.5, and C-type, ∼73-100 mole% YO 1.5, solid solutions. Single crystal X-ray diffraction data have been collected at 49.7, 73.0, and 79.8 mole% YO 1.5(cubic, Ia3̄, a = 10.757(2), 10.698(4), 10.678(3) Å, Z = 32) and the structures refined to R 1 = 0.0841, 0.0146, and 0.0202 for 29, 25, and 72 independent reflections, respectively. The deviations from the ideal fluorite structure were examined and an apparent discontinuity observed for the metal atom shift allowed by Ia3̄ symmetry. Previously suggested metal atom ordering was not observed in the C-type structure. Models have been proposed to understand the limiting compositions of the fluorite-type and C-type solid solutions.

Cation Interdiffusion in Polycrystalline Cubic C‐Type Yttria–Zirconia–Hafnia Solid Solutions

Cation inter diffusion in cubic C‐type solid solutions was investigated for the polycrystalline systems Y2O3—ZrxHf1‐x‐1O2 and Y2O3–ZrO2 in the small‐grain, deep‐penetration condition at 1615° to 1822°C. Lattice and grain‐boundary diffusion parameters were calculated from cation concentration distributions by using Oishi and Ichimura's grain‐boundary diffusion equation. The results indicated that Zr–Y and Zr–Hf interdiffusion coefficients decreased with an increase in Y content. The cation diffusivities were lower than anion diffusivities, and the interdiffusion parameters were lower in the C‐type cubic than in fluorite‐type cubic solid solutions. The results were compared with diffusivities in other C‐type cubic oxides.

Phase equilibria and ordering in the system HfO2-Yb2O3

The system HfO2-Yb2O3 was investigated in the 0 to 100 mol % Yb2O3 range using X-ray diffraction analysis, linear thermal expansion measurements and melting point studies. At high temperatures, the system is dominated by wide regions of solid solutions based on HfO2 and Yb2O3 separated by a two-phase field which appears to extend to the solidus. The extent of the cubic hafnia and ytterbia C-type solid solution fields was established using the precision lattice parameter method. At low temperature (< 1800° C) two ordered phases were found in the system, one at 40 mol % ytterbia with ideal formula Yb4Hf3O12, and another at ∼ 70 mol % ytterbia with formula Yb6HfO11. Four eutectoid reactions and a peritectic reaction cubic ytterbia solid solution → cubic hafnia solid solution + liquid at ∼ 67 mol % and ∼ 2380° C have been established in the system. By incorporating the known tetragonal-cubic hafnia and C-type-hexagonal ytterbia transition temperatures, and the melting points data in the system, a tentative phase diagram is given for the system HfO2-Yb2O3.

Subsolidus Phase Equilibria and Ordering in the System ZrO2‐Y2O3

The subsolidus phase relations in the entire system ZrO2‐Y2O3 were established using DTA, expansion measurements, and room‐ and high‐temperature X‐ray diffraction. Three eutectoid reactions were found in the system: (a) tetragonal zirconia solid solution→monoclinic zirconia solid solution+cubic zirconia solid solution at 4.5 mol% Y2O3 and ∼490°C, (b) cubic zirconia solid solutiow→δ‐phase Y4Zr3O12+hexagonalphase Y6ZrO11 at 45 mol% Y2O3 and ∼1325°±25°C, and (c) yttria C‐type solid solution→wcubic zirconia solid solution+ hexagonal phase Y6ZrO11 at ∼72 mol% Y2O3 and 1650°±50°C. Two ordered phases were also found in the system, one at 40 mol% Y2O3 with ideal formula Y4Zr3O12, and another, a new hexagonal phase, at 75 mol% Y2O3 with formula Y6ZrO11. They decompose at 1375° and &gt;1750°C into cubic zirconia solid solution and yttria C‐type solid solution, respectively. The extent of the cubic zirconia and yttria C‐type solid solution fields was also redetermined. By incorporating the known tetragonal‐cubic zirconia transition temperature and the liquidus temperatures in the system, a new tentative phase diagram is given for the system ZrO2‐Y2O3.