Browsing by Author "Poineau, F"

Now showing 1 - 7 of 7
  • Thumbnail Image
    Item
    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.
  • No Thumbnail Available
    Item
    High temperature magnetic ordering in the 4d perovskite SrTcO3
    (American Physical Society, 2011-02-07) Rodriguez, EE; Poineau, F; Llobet, A; Kennedy, BJ; Avdeev, M; Thorogood, GJ; Carter, ML; Seshadri, R; Singh, DJ; Cheetham, AK
    We present evidence for possibly the highest magnetic ordering temperature in any compound without 3d transition elements. Neutron powder diffraction measurements, at both time-of-flight and constant wavelength sources, were performed on two independently prepared SrTcO3 powders. SrTcO3 adopts a distorted perovskite structure with G-type antiferromagnetic ordering and has a moment of 1.87(4)μB per Tc cation at room temperature with an extraordinarily high Néel point close to 750°C. Electronic structure calculations reveal extensive mixing between the technetium 4d states and oxygen states proximal to the Fermi level. This hybridization leads to a close relationship between magnetic ordering temperature and moment formation in SrTcO3. © 2011, American Physical Society
  • No Thumbnail Available
    Item
    Magneto-structural coupling in SrTcxRu1-xO3 (x = 0.25,0.5) perovskites
    (Elsevier, 2020-07) Reynolds, E; Romao, CP; Brand, HEA; Thorogood, GJ; Poineau, F; Czerwinski, KR; Kennedy, BJ
    The structure and magnetic properties of the mixed technetium-ruthenium perovskites SrTcxRu1-xO3 (x ​= ​0.25 and 0.5) have been investigated using variable temperature synchrotron X-ray diffraction, magnetic susceptibility measurements, and DFT calculations. Both oxides are isostructural with SrRuO3 and SrTcO3, and there is no evidence of long-range cation order. Magnetostriction is observed along the c-axis for both oxides. The inverse susceptibility and DFT calculations suggest a cation-disordered ferrimagnetic ground state for SrTc 0.5 Ru 0.5 O3. ©2020 Elsevier Inc.
  • No Thumbnail Available
    Item
    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
  • No Thumbnail Available
    Item
    Structures and phase transitions in pertechnetates
    (American Chemical Society, 2019-07-07) Kennedy, BJ; Injac, S; Thorogood, GJ; Brand, HEA; Poineau, F
    The temperature dependence of the structures of four pertechnetates (ATcO4A = Ag, Tl, Rb, Cs) from 90 K to their melting points is described. Synchrotron X-ray diffraction measurements show that RbTcO4 undergoes a I41/a to I41/amd transition near 530 K that is associated with a change in the orientation of the TcO4– tetrahedra about the scheelite b axis. AgTcO4 also exhibits a tetragonal scheelite type structure, and this is retained between 90 and 750 K, above which it melted. CsTcO4 has an orthorhombic pseudo-scheelite structure at room temperature and this undergoes a first-order orthorhombic to tetragonal transformation (Pnma to I41/a) near 430 K. TlTcO4 is isostructural with CsTcO4 at 90 K, but the orthorhombic to tetragonal transformation proceeds via an intermediate orthorhombic phase. The different behavior found here and described previously for the analogous Re oxide TlReO4 highlights the differences in the chemistry of these two systems. © 2019 American Chemical Society
  • No Thumbnail Available
    Item
    Thermal expansion behavior in TcO2. Toward breaking the Tc–Tc bond
    (American Chemical Society, 2017-07-24) Reynolds, EM; Zhang, Z; Avdeev, M; Thorogood, GJ; Poineau, F; Czerwinski, KR; Kimpton, JA; Kennedy, BJ
    The structure of TcO2 between 25 and 1000 °C has been determined in situ using X-ray powder diffraction methods and is found to remain monoclinic in space group P21/c. Thermal expansion in TcO2 is highly anisotropic, with negative thermal expansion of the b axis observed above 700 °C. This is the result of an anomalous expansion along the a axis that is a consequence of weakening of the Tc–Tc bonds. © 2017 American Chemical Society
  • No Thumbnail Available
    Item
    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.