Polytypism in mcalpineite: a study of natural and synthetic Cu₃TeO₆
Abstract
Synthetic and naturally occurring forms of tricopper orthotellurate, Cu^(II)₃Te^(VI)O₆ (the mineral mcalpineite) have been investigated by 3D electron diffraction (3D ED), X-ray powder diffraction (XRPD), Raman and infrared (IR) spectroscopic measurements. As a result of the diffraction analyses, Cu^(II)₃Te^(VI)O₆ is shown to occur in two polytypes. The higher-symmetric Cu^(II)₃Te^(VI)O₆-1C polytype is cubic, space group Ia3̅, with a = 9.537 (1) Å and V = 867.4 (3) Å3 as reported in previous studies. The 1C polytype is a well characterized structure consisting of alternating layers of Cu^(II)O₆ octahedra and both Cu^(II)O₆ and Te^(VI)O₆ octahedra in a patchwork arrangement. The structure of the lower-symmetric orthorhombic Cu^(II)₃Te^(VI)O₆-2O polytype was determined for the first time in this study by 3D ED and verified by Rietveld refinement. The 2O polytype crystallizes in space group Pcca, with a = 9.745 (3) Å, b = 9.749 (2) Å, c = 9.771 (2) Å and V = 928.3 (4) ų. High-precision XRPD data were also collected on Cu^(II)₃Te^(VI)O₆-2O to verify the lower-symmetric structure by performing a Rietveld refinement. The resultant structure is identical to that determined by 3D ED, with unit-cell parameters a = 9.56157 (19) Å, b = 9.55853 (11) Å, c = 9.62891 (15) Å and V = 880.03 (2) ų. The lower symmetry of the 2O polytype is a consequence of a different cation ordering arrangement, which involves the movement of every second Cu^(II)O₆ and Te^(VI)O₆ octahedral layer by (1/4, 1/4, 0), leading to an offset of Te^(VI)O₆ and Cu^(II)O₆ octahedra in every second layer giving an ABAB* stacking arrangement. Syntheses of Cu^(II)₃Te^(VI)O₆ showed that low-temperature (473 K) hydrothermal conditions generally produce the 2O polytype. XRPD measurements in combination with Raman spectroscopic analysis showed that most natural mcalpineite is the orthorhombic 2O polytype. Both XRPD and Raman spectroscopy measurements may be used to differentiate between the two polytypes of Cu^(II)₃Te^(VI)O₆. In Raman spectroscopy, Cu^(II)₃Te^(VI)O₆-1C has a single strong band around 730 cm⁻¹, whereas Cu^(II)₃Te^(VI)O₆-2O shows a broad double maximum with bands centred around 692 and 742 cm⁻¹.
Additional Information
© 2022 IUCr. Received 1 October 2021. Accepted 8 December 2021. We thank Professor Janusz Lipkowski for handling our manuscript and two anonymous reviewers for their helpful and insightful comments. Support funding has been provided to OPM by an Australian Government Research Training Program (RTP) Scholarship, a Monash Graduate Excellence Scholarship (MGES) and a Robert Blackwood Monash–Museums Victoria scholarship. The authors acknowledge use of the facilities within the Monash Centre for Electron Microscopy. OPM thanks TU Wien for extending the invitation to spend a portion of his PhD research in Vienna. Part of this study has been funded by the Ian Potter Foundation grant "Tracking Tellurium" (to SJM) and by the John Jago Trelawney Endowment to the Mineral Sciences Department of the Natural History Museum of Los Angeles County (to ARK).Attached Files
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Additional details
- Alternative title
- Polytypism in mcalpineite: a study of natural and synthetic Cu3TeO6
- Eprint ID
- 113321
- Resolver ID
- CaltechAUTHORS:20220207-90466000
- Australian Government
- Monash University
- Robert Blackwood Monash–Museums Victoria
- Potter Foundation
- John Jago Trelawney Endowment
- Created
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2022-02-08Created from EPrint's datestamp field
- Updated
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2022-02-08Created from EPrint's last_modified field