Browsing by Author "Yu, M"

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    Comparison of thermal expansion of Tc and Re salts
    (Materials Research Society (MRS), 2017-10-29) Thorogood, GJ; Kennedy, BJ; Reynolds, EM; Poineau, F; Yu, M; Injac, S
    99Tc is the most significant long-lived product of uranium fission, producing the largest fraction of the total long-lived radiation emissions of nuclear waste. Tc 7+ compounds are highly mobile in the environment. Relatively little is known regarding the solid state chemistry of Tc. Recently we studied the structural properties of (NH4)TcO4 (Tc7+) and confirmed that this is isostructural with (NH4)ReO4 adopting a tetragonal scheelite type structure in space group I41/a. The unit cell parameters of (NH4)TcO4 are strongly temperature dependent with the structure showing negative thermal expansion along both the a- and c-axis, albeit at different temperatures This behavior is significantly different to that previously reported for the isostructural oxide (NH4)ReO4, although we note that the data for (NH4)ReO4 was collected at much lower resolution. Nevertheless it is clear in the literature that the thermal expansion behavior of (NH4)ReO4 is highly anisotropic. Despite the difference in the thermal expansion between what we have observed for (NH4)TcO4 and that described by others for (NH4)ReO4 it is likely that the origin of the anomalous thermal expansion in is the same in both cases, namely it is a consequence of re-orientation of the ammonium ions in the surrounding cage of eight oxygen atoms. To verify this for (NH4)TcO4 requires we replace the ammonium cation with another small cation. Therefore we have compared the thermal behavior of AReO4 with ATcO4 to determine if Re oxides are suitable surrogates to predict the behavior of Tc oxides and how they may behave in the environment.
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    Structure and magnetism in Sr A_xTcO 3 perovskites: importance of the A-site cation
    (American Physical Society, 2017-02-27) Reynolds, EM; Avdeev, M; Thorogood, GJ; Poineau, F; Czerwinski, KR; Kimpton, JA; Yu, M; Kayser, P; Kennedy, BJ
    The Sr1−xBaxTcO3 (x=0, 0.1, 0.2) oxides were prepared and their solid-state and magnetic structure studied as a function of temperature by x-ray and neutron powder diffraction. The refined Tc moments at room temperature and Néel temperatures for Ba0.1Sr0.9TcO3 and Ba0.2Sr0.8TcO3 were 2.32(14)μβ and 2.11(13)μβ and 714∘C and 702∘C, respectively. In contrast to expectations, the Néel temperature in the series Sr1−xAxTcO3 decreases with increasing Ba content. This observation is consistent with previous experimental measurements for the two series AMO3 (M=Ru, Mn; A=Ca, Sr, Ba) where the maximum magnetic ordering temperature was observed for A=Sr. Taken with these previous results the current work demonstrates the critical role of the A-site cation in the broadening of the π∗ bandwidth and ultimately the magnetic ordering temperature.©2017 American Physical Society
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    Thermal expansion of ammonium pertechnetate and ammonium perrhenate
    (Elsevier, 2019-06) Reynolds, EM; Yu, M; Thorogood, GJ; Brand, HEA; Poineau, F; Kennedy, BJ
    The thermal expansion of NH4TcO4 and NH4ReO4 between 97 and 500 K has been established using variable temperature synchrotron X-ray diffraction. Both oxides display a tetragonal scheelite-type structure over the temperature range. The observed anomalous thermal expansion is associated with disorder in the orientation of the NH4 groups and the presence of anisotropic, equatorial and axial, interactions between the H atoms and the BO4 tetrahedra. The importance of the hydrogen interactions was verified by a study of the isostructural compound NaReO4 over a similar temperature range. The differences in the details of the Negative Thermal Expansion (NTE) behaviour of NH4TcO4 and NH4ReO4 is a consequence of differences in the H-BO4 interactions due to changes in the B-O bond lengths within the BO4 groups. Both ammonium salts decompose when heated above ∼500 K. ©2019 Elsevier Inc.