Polytypism in mcalpineite: a study of natural and synthetic Cu₃TeO₆

Synthetic and naturally occurring forms of tricopper orthotellurate, Cu^II₃Te^VIO₆ (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^II₃Te^VIO₆ is shown to occur in two polytypes. The higher-symmetric Cu^II₃Te^VIO₆-1C polytype is cubic, space group Ia3, with a = 9.537 (1) Å and V = 867.4 (3) ų as reported in previous studies. The 1C polytype is a well characterized structure consisting of alternating layers of Cu^IIO₆ octahedra and both Cu^IIO₆ and Te^VIO₆ octahedra in a patchwork arrangement. The structure of the lower-symmetric orthorhombic Cu^II₃Te^VIO₆-2O 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^II₃Te^VIO₆-2O 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^IIO₆ and Te^VIO₆ octahedral layer by (1/4, 1/4, 0), leading to an offset of Te^VIO₆ and Cu^IIO₆ octahedra in every second layer giving an ABAB* stacking arrangement. Syntheses of Cu^II₃Te^VIO₆ 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^II₃Te^VIO₆. In Raman spectroscopy, Cu^II₃Te^VIO₆-1C has a single strong band around 730 cm⁻¹, whereas Cu^II₃Te^VIO₆-2O shows a broad double maximum with bands centred around 692 and 742 cm⁻¹.

Computing details

Program(s) used to refine structure: FULLPROF for Cu3TeO6. Molecular graphics: Olex2 1.5 (Dolomanov et al., 2009) for (I). Software used to prepare material for publication: Olex2 1.5 (Dolomanov et al., 2009) for (I).

Mcalpineite (I)

Crystal data

Cu3TeO6	Dx = 5.928 Mg m^−3
Mr = 414.2	Electron radiation, λ = 0.0197 Å
Orthorhombic, Pcca	Cell parameters from 206 reflections
a = 9.745 (3) Å	θ = 0.1–0.7°
b = 9.749 (2) Å	µ = 0 mm^−1
c = 9.771 (2) Å	T = 293 K
V = 928.3 (4) Å^3	Irregular, green
Z = 8	0.00002 × 0.00002 × 0.00001 × 0.00002 (radius) mm
F(000) = 1496

Data collection

Transmission electron microscope diffractometer	Rint = 0.18
Radiation source: Cu3TeO6 nanocrystal	θmax = 0.7°, θmin = 0.1°
stepwise toration scans	h = −12→12
4206 measured reflections	k = −12→12
935 independent reflections	l = −12→12
631 reflections with I > 3σ(I)

Refinement

Refinement on F^2	96 parameters
R[F^2 > 2σ(F^2)] = 0.141	0 restraints
wR(F^2) = 0.303	0 constraints
S = 6.55	Weighting scheme based on measured s.u.'s w = 1/(σ^2(I) + 0.0004I^2)
935 reflections	(Δ/σ)max = 0.001

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Te1	0.5	0.5	0.5	0.0239 (14)
Te2	0.75	0	0.3028 (6)	0.0356 (18)
Cu1	0.5	1	0.5	0.028 (2)
Cu2	0.5	0.7169 (8)	0.25	0.020 (2)
Cu3	0.75	0.5	0.2166 (9)	0.021 (2)
Cu4	0.2191 (5)	0.7482 (6)	0.5046 (10)	0.0355 (17)
Cu5	0.5	0.2208 (11)	0.25	0.044 (4)
O1	0.6189 (14)	0.5762 (14)	0.3532 (17)	0.022 (4)
O2	0.4304 (16)	0.3500 (15)	0.3824 (19)	0.030 (5)
O3	0.3546 (16)	0.8778 (18)	0.5641 (18)	0.034 (6)
O4	0.3555 (15)	0.6174 (14)	0.4220 (17)	0.023 (5)
O5	0.4000 (17)	0.8580 (15)	0.140 (2)	0.033 (6)
O6	0.643 (2)	0.091 (2)	0.169 (2)	0.055 (8)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Te1	0.020 (2)	0.024 (2)	0.027 (3)	−0.001 (3)	0.001 (2)	0.001 (3)
Te2	0.033 (3)	0.033 (3)	0.041 (4)	−0.004 (3)	0	0
Cu1	0.021 (4)	0.019 (3)	0.043 (5)	−0.001 (4)	−0.002 (4)	0.001 (6)
Cu2	0.023 (4)	0.012 (3)	0.025 (5)	0	−0.005 (4)	0
Cu3	0.010 (3)	0.028 (3)	0.025 (4)	0.012 (4)	0	0
Cu4	0.014 (2)	0.025 (2)	0.067 (4)	−0.002 (3)	0.007 (5)	−0.016 (2)
Cu5	0.067 (8)	0.025 (4)	0.040 (7)	0	0.024 (6)	0
O1	0.018 (6)	0.019 (6)	0.028 (9)	0.011 (5)	−0.003 (6)	0.007 (6)
O2	0.028 (8)	0.019 (7)	0.043 (11)	−0.005 (6)	0.012 (7)	0.016 (7)
O3	0.018 (8)	0.048 (10)	0.035 (11)	0.001 (8)	0.011 (8)	−0.020 (9)
O4	0.020 (8)	0.013 (6)	0.035 (10)	0.004 (6)	0.008 (7)	0.003 (7)
O5	0.033 (9)	0.011 (7)	0.056 (13)	−0.003 (6)	−0.008 (8)	−0.001 (8)
O6	0.077 (16)	0.044 (10)	0.042 (13)	0.032 (10)	0.012 (12)	0.002 (9)

Geometric parameters (Å, º)

Te1—O2	1.979 (16)	O1—Cu3	1.991 (16)
Te1—O4	1.969 (14)	O2—Te1	1.979 (16)
Te2—O6	1.90 (2)	O2—Cu5	1.929 (18)
Cu1—O3	1.954 (17)	O3—Cu1	1.954 (17)
Cu2—O5	2.002 (18)	O3—Cu4	1.918 (18)
Cu3—O1	1.991 (16)	O4—Te1	1.969 (14)
Cu4—O3	1.918 (18)	O4—Cu4	2.011 (16)
Cu4—O4	2.011 (16)	O5—Cu2	2.002 (18)
Cu5—O2	1.929 (18)	O6—Te2	1.90 (2)
O2—Te1—O4	87.7 (6)	O3—Cu4—O4	94.8 (7)
O4—Te1—O4	180	O2—Cu5—O2	98.4 (8)
O6—Te2—O6	93.1 (9)	Te1—O2—Cu5	138.7 (9)
O3—Cu1—O3	180	Cu1—O3—Cu4	143.5 (10)
O5—Cu2—O5	93.2 (7)	Te1—O4—Cu4	133.3 (9)
O1—Cu3—O1	95.9 (7)

(Cu3TeO6)

Crystal data

Orthorhombic, Pcca	c = 9.62891 (15) Å
Hall symbol: -P 2a 2ac	V = 880.03 (2) Å^3
a = 9.56156 (19) Å	X-ray radiation
b = 9.55853 (11) Å

Data collection

2θmin = 4.090°, 2θmax = 69.961°, 2θstep = 0.014°

Refinement

Rp = 2.266	4706 data points
Rwp = 2.553	73 parameters
Rexp = 0.687	0 restraints
RBragg = 62.739

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Te1	0.50000	0.50000	0.50000	0.0112 (8)
Te2	0.75000	0.00000	0.30388 (17)	0.0159 (8)
Cu1	0.50000	1.00000	0.50000	0.01011 (19)*
Cu2	0.50000	0.7205 (5)	0.25000	0.01011 (19)*
Cu3	0.75000	0.50000	0.2180 (3)	0.01011 (19)*
Cu4	0.2157 (3)	0.7484 (4)	0.4998 (3)	0.01011 (19)*
Cu5	0.50000	0.2244 (5)	0.25000	0.01011 (19)*
O1	0.6176 (14)	0.5795 (15)	0.3482 (12)	0.01011 (19)*
O2	0.4254 (16)	0.3544 (11)	0.3857 (15)	0.01011 (19)*
O3	0.3623 (13)	0.8642 (12)	0.5666 (15)	0.01011 (19)*
O4	0.33984	0.6067 (11)	0.4283 (14)	0.01011 (19)*
O5	0.3965 (15)	0.85823	0.1547 (11)	0.01011 (19)*
O6	0.6298 (14)	0.0906 (13)	0.1614 (12)	0.01011 (19)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Te1	0.0184 (11)	0.0010 (7)	0.0141 (8)	−0.0033 (10)	−0.0131 (9)	0.0020 (10)
Te2	0.0123 (8)	0.0312 (11)	0.0041 (6)	0.0234 (8)	0.00000	0.00000