Effect of oxide additions on polymorphism of tantalum pentoxide (system Ta2O5-TiO2)

Abstract

The phase eq uilibrium re lation ships of th e Ti02·Ta20 , system were determined in air. An equi· molar co mpound , TiTa2O" was found to me lt congru ently at about 1662 °e and have a monoclinic unit cell a = 20.297 A, b = 3.804 A, c = 11 .831 A, f3 = 120°14', apparently isostnictural with TiNb20,.In addition, two othe r compound s are postulated to occ ur at about Ti02:49Ta20 , and Ti02: 7Ta20 s and to dissociate at about 1230 and 1190 °e, respec tively. Ti02 apparently accepts a maximum of 9 mole pe rce nt T a20s in solid solution at 1630 0c. Two eutecti cs occ ur in the syste m at 54 mole perce nt Ta20, and 1650 °e and 31 mole percent TazOs and 1630 0c.

Effect of oxide additions on the polymorphism of tantalum pentoxide. II. ""Stabilization"" of the high temperature structure type

“…1), has been used as a model and the other systems have been drawn in accordance with Ta20 5-Ti0 2 , with the appropriate modifications necessary to fit the limited experimental data. No attempt has been made to delineate the metastable equilibrium relations of the H-Ta205 solid solution phase transi· tions as was previously done for Ta205-Ti0 2 [4].…”

“…The reason for this is that no s uitable crystals of pure Ta 20 s have been prepared due to the peculiar polymorphic behavior of the material. The polymorphism of Ta 20 5 has been di scussed by several authors [1 , 2, 3)1 and fully reviewed by the present authors in a previous publication [4] togethe r with the changes which occurred in the polymorphic b ehavior with thA addition of Ti0 2 • The low-temperature form of Ta20 5 has an, as yet, unknown structure related to the low-temperature form of Nb 20 5 , as well as to a-U30 S and a-U0 3• On heating to a temperature of about 1360°C [2] Ta20 5 exhibits an e nantiotropic, sluggishly reversible , phase transition to a high-te mperature form. This form apparently has a unique structure unlike any other reported compound.…”

“…The general quantitative spectrochemical analyses for the Ta20 5 used in this study has been previously reported [4]. All other oxides used were of reagent grade or better, as described in previous publications the additive, for 6 to 12 hr.…”

“…impossible task. Th e addition of Ti0 2 lowers the equilibrium tran sition temperature from about 1360 °C to about 1150 to 1200°C and the metastable te tragonal ~ monoclinic tran sition of th e high-te mpe rature form from a bout 950°C to as low as 600°C [4]. However, the quality of the room te mperature single crystals is not improve d by this addition.…”

“…This form apparently has a unique structure unlike any other reported compound. Whe n que nc hed from above the equilibrium phase transition, th e apparently te tragonal high-te mperature form transforms , through a monoclinic polymorph, to a third metastable polymorph which has an x-ray powder pattern which appears to be tri clini c at room te mperature [4]_ These transformations res ult in a mechanical disruption of the cr ystals, making an accurate measureme nt of the intensities of the x-ray diffraction spots essentially an I Figures in brackets indi ca te th e lit eralUre refe rences al th e end of thi s paper. impossible task.…”