• Mills, Stuart J.
• Hadermann, Joke
• Missen, Owen P.
• Libowitzky, Eugen
• Rumsey, Michael S.
• Artner, Werner
• Housley, Robert M.
• Canossa, Stefano
• Weil, Matthias
• Dunstan, Maja
• Spratt, John
• Nénert, Gwilherm

Abstract

<jats:p>Synthetic and naturally occurring forms of tricopper orthotellurate, Cu<jats:sup>II</jats:sup><jats:sub>3</jats:sub>Te<jats:sup>VI</jats:sup>O<jats:sub>6</jats:sub> (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<jats:sup>II</jats:sup><jats:sub>3</jats:sub>Te<jats:sup>VI</jats:sup>O<jats:sub>6</jats:sub> is shown to occur in two polytypes. The higher-symmetric Cu<jats:sup>II</jats:sup><jats:sub>3</jats:sub>Te<jats:sup>VI</jats:sup>O<jats:sub>6</jats:sub>-1<jats:italic>C</jats:italic> polytype is cubic, space group <jats:italic>Ia</jats:italic><jats:overline>3</jats:overline>, with <jats:italic>a</jats:italic> = 9.537 (1) Å and <jats:italic>V</jats:italic> = 867.4 (3) Å<jats:sup>3</jats:sup> as reported in previous studies. The 1<jats:italic>C</jats:italic> polytype is a well characterized structure consisting of alternating layers of Cu<jats:sup>II</jats:sup>O<jats:sub>6</jats:sub> octahedra and both Cu<jats:sup>II</jats:sup>O<jats:sub>6</jats:sub> and Te<jats:sup>VI</jats:sup>O<jats:sub>6</jats:sub> octahedra in a patchwork arrangement. The structure of the lower-symmetric orthorhombic Cu<jats:sup>II</jats:sup><jats:sub>3</jats:sub>Te<jats:sup>VI</jats:sup>O<jats:sub>6</jats:sub>-2<jats:italic>O</jats:italic> polytype was determined for the first time in this study by 3D ED and verified by Rietveld refinement. The 2<jats:italic>O</jats:italic> polytype crystallizes in space group <jats:italic>Pcca</jats:italic>, with <jats:italic>a</jats:italic> = 9.745 (3) Å, <jats:italic>b</jats:italic> = 9.749 (2) Å, <jats:italic>c</jats:italic> = 9.771 (2) Å and <jats:italic>V</jats:italic> = 928.3 (4) Å<jats:sup>3</jats:sup>. High-precision XRPD data were also collected on Cu<jats:sup>II</jats:sup><jats:sub>3</jats:sub>Te<jats:sup>VI</jats:sup>O<jats:sub>6</jats:sub>-2<jats:italic>O</jats:italic> 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 <jats:italic>a</jats:italic> = 9.56157 (19) Å, <jats:italic>b</jats:italic> = 9.55853 (11) Å, <jats:italic>c</jats:italic> = 9.62891 (15) Å and <jats:italic>V</jats:italic> = 880.03 (2) Å<jats:sup>3</jats:sup>. The lower symmetry of the 2<jats:italic>O</jats:italic> polytype is a consequence of a different cation ordering arrangement, which involves the movement of every second Cu<jats:sup>II</jats:sup>O<jats:sub>6</jats:sub> and Te<jats:sup>VI</jats:sup>O<jats:sub>6</jats:sub> octahedral layer by (1/4, 1/4, 0), leading to an offset of Te<jats:sup>VI</jats:sup>O<jats:sub>6</jats:sub> and Cu<jats:sup>II</jats:sup>O<jats:sub>6</jats:sub> octahedra in every second layer giving an <jats:italic>ABAB</jats:italic>* stacking arrangement. Syntheses of Cu<jats:sup>II</jats:sup><jats:sub>3</jats:sub>Te<jats:sup>VI</jats:sup>O<jats:sub>6</jats:sub> showed that low-temperature (473 K) hydrothermal conditions generally produce the 2<jats:italic>O</jats:italic> polytype. XRPD measurements in combination with Raman spectroscopic analysis showed that most natural mcalpineite is the orthorhombic 2<jats:italic>O</jats:italic> polytype. Both XRPD and Raman spectroscopy measurements may be used to differentiate between the two polytypes of Cu<jats:sup>II</jats:sup><jats:sub>3</jats:sub>Te<jats:sup>VI</jats:sup>O<jats:sub>6</jats:sub>. In Raman spectroscopy, Cu<jats:sup>II</jats:sup><jats:sub>3</jats:sub>Te<jats:sup>VI</jats:sup>O<jats:sub>6</jats:sub>-1<jats:italic>C</jats:italic> has a single strong band around 730 cm<jats:sup>−1</jats:sup>, whereas Cu<jats:sup>II</jats:sup><jats:sub>3</jats:sub>Te<jats:sup>VI</jats:sup>O<jats:sub>6</jats:sub>-2<jats:italic>O</jats:italic> shows a broad double maximum with bands centred around 692 and 742 cm<jats:sup>−1</jats:sup>.</jats:p>

Topics

• impedance spectroscopy
• mineral
• electron diffraction
• Raman spectroscopy
• space group