Phase Transformations of Baddeleyite induced by Compression

Baddeleyite.whose composition is denoted as zro_2 .is found in rocks of pyroxenite or syenite.Itscrystal symmetry是典型的monoclinic.No other allotropy has been known in nature.Thismaterial has two high temperature and two high pressure polymorphs. tetragonal and cubic.andOrtho(I)and(II)respectively.This substance.known under the name zirconia.possesses practicallyuseful properties such as high strength and fracture toughness.These are strongly depend on theinterrelations among the phases mentioned above.To promote these peculiarities.impurity-dopedsubstances.which are stabilized to be one of high temperature forms.are now popular in technologicalapplications.In this project.the stability fields of high pressure phases and a mechanism of phasetransformation generated by compression have been investigated.As for Ortho(I)phase.its existencehad not been clear since its structure was difficult to be maintained at . the presentexperiment succeeded to quench this phase…More when fine starting powder of zro_2 was prepared.In the similar processing.Ortho (I)phaseof 2 mol% y_2_3 doped zro_2 was also quencheable but the tetragonal phase.which had originallycoexisted in the starting powder.was always found in the product.This is explained by considering afact that the stability of Ortho(I)and the tetragonal phases against compression is almost same andlatter survives after the removal of pressure since the difference in density between these twophases is small.The crystal structure of quenched Ortho(I)phase was refined by the Rietveldanalisis .being confirmed to be the same as that determined under pressure.Ortho(II)phases of bothzro_2 and 2mo1 % y_2o_3 were synthesized under conditions above 1000 - C and 10gata . the Rietveld analysisof the product showed that it is isostructural with cotunnite.A peculiar phenomena found in thecase of y_2_3 -doped sample depressed from pressures slightly below 10gusa . the product onlyconsisted of the monoclinic phase, but neither Ortho(I)nor the tetragonal was observed.This isprobably due to a mechanism in which the tetragonal phase becomes unstable by compression close toGPa and turns to Ortho(I).As is already seen.Ortho(I)phase is unstable on decreasing pressure andthe only monoclinic phase is found in the treated samples. Less

Baddeleyite.whose composition is denoted as ZrO_2.is found in rocks of pyroxenite or syenite.Its crystal symmetry is classified as monoclinic.No other allotropy has been known in nature.This material has two high temperature and two high pressure polymorphs. tetragonal and cubic.and Ortho(I)and(II)respectively.This substance.known under the name zirconia.possesses practically useful properties such as high strength and fracture toughness.These are strongly depend on the interrelations among the phases mentioned above.To promote these peculiarities.impurity-doped substances.which are stabilized to be one of high temperature forms.are now popular in technological applications.In this project.the stability fields of high pressure phases and a mechanism of phase transformation generated by compression have been investigated.As for Ortho(I)phase.its existence had not been clear since its structure was difficult to be maintained at 1 atm.The present experiment succeeded to quench this phase … More when fine starting powder of ZrO_2 was prepared.In the similar processing.Ortho (I)phase of 2 mol%Y_2O_3 doped ZrO_2 was also quencheable but the tetragonal phase.which had originally coexisted in the starting powder.was always found in the product.This is explained by considering a fact that the stability of Ortho(I)and the tetragonal phases against compression is almost same and the latter survives after the removal of pressure since the difference in density between these two phases is small.The crystal structure of quenched Ortho(I)phase was refined by the Rietveld analysis.being confirmed to be the same as that determined under pressure.Ortho(II)phases of both ZrO_2and 2 mo1%Y_2O_3were synthesized under conditions above 1000゜C and 10GPa.The Rietveld analysis of the product showed that it is isostructural with cotunnite.A peculiar phenomena were found in the case of Y_2O_3-doped sample depressed from pressures slightly below 10GPa.The product only consisted of the monoclinic phase but neither Ortho(I)nor the tetragonal was observed.This is probably due to a mechanism in which the tetragonal phase becomes unstable by compression close to 10GPa and turns to Ortho(I).As is already seen.Ortho(I)phase is unstable on decreasing pressure and the only monoclinic phase is found in the treated samples. Less