Polytypism in mcalpineite: a study of natural and synthetic Cu₃TeO₆

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.