Stability Field of Baddeleyite (ZrO_2) and Its Change Due to Additive

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The mineral baddeleyite, ZrO_2 is known to have three polymorphs in high temperature, monoclinic, tetragonal, cubic and two in high pressure, orthorhombic I and II. Since some of these phases are not quenchable to ambient conditions of temperature and pressure, uncertainties have been left unsolved on their equilibrium relations. The present experiment was performed with the aim to determine the stability fields of ortho I and II phases and to clear the influence of additives on these fields. The obtained results are summarized as follows : 1. Starting materials with different grain sizes were treated at 400^ﾟC・6GPa and only the finest grains was found to be quenched. In situ observation using a diamond anvil cell revealed that the transition pressure from monoclinic to ortho I phases shifted towards higher pressure side as the grain size decreased, indicating a clear size effect. 2. Quenched sample of ortho I and II phases were submitted to thermodynamical experiments in which measurements of bulk modulus, thermal expansion and heat of formation were included. When these data were combined to the equilibrium values of temperature-pressure of adjacent two phases obtained by the synthesis, the phase boundaries between monoclinic and ortho I and also between ortho I and II were drawn, that is, T (^ﾟC) = 335 - 102P (GPa) (fine grain) and T (^ﾟC) = 300P (GPa) - 4300 respectively. 3. A sample, which contained 2mo1%Y_2O_3, were prepared to invest igate change of stability relations among the phases. This starting material consisted of monoclinic and tetragonal phases. When it was compressed, only monoclinic phases converted to ortho I. In the case of higher pressure, the sample was found to transform to cotunnite-type completely and no tetra gonal phase remained.