Crystal Structure of Yttrium and Other Rare‐Earth Borates

Abstract

Crystal Structure of Yttrium and Other Rare-Earth Borates 253( C ) 1400" F Heat Treatment: Maximum crystallinity was developed during the 1400°F heat treatment as well as maximum resistance to thermal shock. An average AT of 925°F was required to cause failure. The phases lithium titanium silicate, sphene, and albite increased to a maximum whereas rutile remained constant. The structure exhibited no crystal orientation, as in the 1600°F heat treatment, and the crystals were much better developed, as shown in Fig. 11. Fracture characteristics were completely nonconchoidal, as shown in Fig. 12. There were also increases in transcrystalline failure and in secondary fracture planes intersecting the principal fracture surface.(D) 1100°F Heat Treatment: Heat treatment a t 1 100°F Fig. 12. Fracture surface of crystallized glass caused by failure in thermal shock at 925OF.structure is somewhat comparable to a titanium-opacified enamel. Thermal-shock failure occurred a t 550°F with the type of failure common to glassy materials. The conchoidal fracture surface resulting from the ineffective crystallization is shown in Fig. 8. ( B ) 1 600°F Heat Treatment: Thermal-shock resistance following the 1GOO"F heat treatment showed little improvement over the non-heat-treated specimen. The size of the rutile crystals increased slightly and traces of lithium titanium silicate, sphene, and albite developed. The typical structure of the specimen which failed a t 600°F is shown in Fig. 9. The orientation of the crystals in the center of the micrograph was caused by coating flow during heat treatment at too high a temperature. The fracture characteristics were similar to those of the non-heat-treated coating with a slight increase in discontinuities in the predominantly planar fracture (Fig. 10).was below the critical crystallization temperature, resulting in the same structure as the non-heat-treated specimen even though the time was extended to 65 hours. Fracture characteristics and thermal-shock resistance were also the same. The sequence of crystallization for the series started with spontaneous crystallization of rutile during firing. High-temperature heat treatment developed only trace amounts of lithium titanium silicate, sphene, and albite. At 1400°F the maximum development of crystals occurred, resulting in the maximum disturbance of the fracture pattern and increase in thermal-shock resistance. V. SummaryThe growth of crystals and their cumulative effect in developing internal stresses in glass coatings can be conveniently studied with the electron microscope. The information gathered in this manner can be related to the physical properties of the coatings. Heat treatments which developed maximum crystallinity with good distribution of the crystalline phase within the matrix resulted in materials with the best resistance to thermal shock. As resistance to thermal shock increased, the fracture paths became increasingly disrupted owing to the internal stresses developed between crystal and matrix. As i t became necessary for fra...

The vaterite-type ABO3rare-earth borates

“…They indicate that either model is approximately reconciled with the powder data, but do not afford a clear choice between them. Either level of agreement is superior to that possible in the model of Newnham et al (1963), in which only boron can contribute to amplitudes when h-k¢ 3n.…”

“…1. In each, oxygen atoms in twelvefold positions, designated O(1), are symmetrically displaced from the averaged subcell positions derived by Newnham, Redman & Santoro (1963) to provide nets of edge-sharing octahedra about Yb in two subcells. Extinction criteria and this model are consistent with the space group P~c2.…”

“…Bartram & Felten (1961) and Newnham, Redman & Santoro (1963), working on low-temperature rare earth borates, and Kamhi (1963) further noted that ordered arrays in the six-formula cell could exist, and cited one arrangement in the space group P 63/mcm which provides six vertical BO3 groups.…”

Gadolinium and Yttrium Borates: Thermal Behavior and Structural Considerations

“…2. The second structure proposed by R. E. Newnham et al (7) for YBO involved monomeric planar and triangular BO\ anions inclined to one another but parallel to the c axis with two possible arrangements, leading a disordered (space group P6 /mmc) or an ordered (space group P6 /mcm) hexagonal structure. In these structures, each rare-earth ion would be coordinated to a distorted cube with eight oxygen atoms.…”

Crystal structures and magnetic properties of lanthanide containing borates LnM(BO3)2 (Ln=Y, Ho–Lu; M=Sc, Cr)

“…Although the LnBO 3 for larger Ln ions adopt other crystal structures [18,19], the small average radius, (R Ln3+ + R Sc3+ )/2 = 0.804-0.823 Å for Ln = Y, Ho-Lu from the Shannon's ionic radius [Shannon], may stabilize the calcite structure. The calculated profile by using the calcite model is shown in Fig.…”