Mn2TeO6: a Distorted Inverse Trirutile Structure - ACS Publications

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Mn2TeO6: a Distorted Inverse Trirutile Structure Nami Matsubara,*,†,‡ Françoise Damay,‡ Bénédicte Vertruyen,§ Nicolas Barrier,† Oleg I. Lebedev,† Philippe Boullay,† Erik Elkaïm,∥ Pascal Manuel,⊥ Dmitry D. Khalyavin,⊥ and Christine Martin† †

CRISMAT, 6 bvd Maréchal Juin, 14050 Caen Cedex, France Laboratoire Léon Brillouin, CEA-CNRS UMR12, Université Paris-Saclay, 91191 Gif sur Yvette Cedex, France § GREENMAT, CESAM Research Unit, Université de Liège, Institut de Chimie B6, 13 Allée du 6 août, 4000 Liege, Belgium ∥ Synchrotron Soleil, Saint-Aubin BP 48, 91192 Gif sur Yvette Cedex, France ⊥ ISIS Facility, Rutherford Appleton Laboratory-CCLRC, Chilton, Didcot, Oxfordshire OX11 0QX, United Kingdom ‡

S Supporting Information *

ABSTRACT: Inverse trirutile Mn2TeO6 was investigated using in situ neutron and X-ray powder diffraction between 700 °C and room temperature. When the temperature was decreased, a structural phase transition was observed around 400 °C, from a tetragonal (P42/mnm) to a monoclinic phase (P21/c), involving a doubling of the cell parameter along b. This complex monoclinic structure has been solved by combining electron, neutron, and synchrotron powder diffraction techniques at room temperature. It can be described as a distorted superstructure of the inverse trirutile structure, in which compressed and elongated MnO6 octahedra alternate with more regular TeO6 octahedra, forming a herringbone-like pattern. Rietveld refinements, carried out with symmetry-adapted modes, show that the structural transition, arguably of Jahn−Teller origin, is driven by a single primary mode.

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INTRODUCTION The well-known tetragonal structure of rutile, TiO2 (space group P42/mnm, with a ≅ 4.6 Å and c ≅ 2.9 Å),1 is built of chains of edge-sharing octahedra along the c direction, these chains being joined together through corners in the ab plane. In 1926, while studying tapiolite FeTa2 O 6 , Goldschmidt 2 described for the first time a superstructure of rutile, corresponding to a tripling along the c axis of the rutile structure, which he appropriately named trirutile. This superstructure originates in the ordering of the metal ions in the chains running along c, following the sequence FeTaTaFe··· Until the early 1960s, the only known trirutile-type structures were A2+B25+O6 compounds (A = Mg, Ni, Co, Fe, ... B = Nb, Ta, Sb; see refs 3 and 4 and references therein). Bayer5 was the first to synthesize a new compound with composition A23+B6+O6, i.e. Cr2WO6, and to propose a trirutile-type structure. The sites of the A and B cations being interchanged with respect to normal trirutile A2+B25+O6, he coined the term “inverse trirutile”. Afterward, several other compounds were synthesized with B = hexavalent tungsten or tellurium and A = Al, Ga, Cr, Fe.6−10 Like the trirutile structure, inverse trirutile (Figure 1) is therefore characterized by a tripling along c of the rutile unit cell, resulting from the 2A:1B ordering of the cations inside the chains of edge-sharing octahedra running along c. © 2017 American Chemical Society

Figure 1. Crystal structure of the tetragonal (P42/mnm) inverse trirutile structure showing the 2:1 cationic ordering of trivalent (orange) and hexavalent (green) species.

Recently a resurgence of activity has been observed for this class of materials due to their magnetoelectric properties, as reported for Cr2WO611,12 and Fe2TeO6.13−15 In contrast, the properties of Mn2TeO6, first studied by Hund,16 have not been explored since the preliminary results described in the 1970s. Received: May 17, 2017 Published: August 7, 2017 9742

DOI: 10.1021/acs.inorgchem.7b01269 Inorg. Chem. 2017, 56, 9742−9753

Article

Inorganic Chemistry

Figure 2. (left) Temperature evolution of the NPD patterns of Mn2TeO6, in the 2.65−3.17 Å−1 selected Q range (from 50 to 700 °C and then back to 50 °C). The horizontal black dotted lines indicate the structural transition at 400 °C during both the heating and cooling processes. (right) NPD data of Mn2TeO6 at 700 and 50 °C (S1 and S2). The insets indicate additional small peaks (black thin arrows) and the splitting of peaks (red thick arrows) in the 50 °C patterns.

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Mn2TeO6 was then reported as an A2TeO6 inverse trirutile compound by Fruchart et al.,17 who also investigated its magnetic properties. The crystal structure was not determined precisely, but two polymorphs were mentioned: a tetragonal form, P42/mnm with a = 4.613 Å and c = 9.082 Å,16,17 and a monoclinic form, for which only lattice parameters were reported. This monoclinic form was synthesized under slight oxygen pressure at ≅730 °C (a = b = 4.591 Å, c = 9.125 Å, γ = 90.30°) or by starting from the tetragonal form at ≅530 °C under 40 kbars (a = b = 4.595 Å, c = 9.108 Å, γ = 90.51°).17 The lack of details concerning this monoclinic structure in addition to the fact that both polymorphs show different magnetic behaviors were strong incentives to revisit the Mn− Te−O system. This interest was reinforced by the recent announcement of multiferroicity in Mn3TeO6.18 In this article, we report on the first in-depth study of the crystal structure of Mn2TeO6, combining room-temperature precession electron diffraction tomography, synchrotron X-ray diffraction, advanced transmission electron microcopy (TEM) including high angle annular dark field and annular bright field scanning TEM, and in situ neutron powder and X-ray diffraction between 700 °C and room temperature. It is shown that, from 700 to 400 °C, Mn2TeO6 exhibits the known tetragonal inverse trirutile structure. Below 400 °C, a structural phase transition is observed, leading to a complex and distorted superstructure of the high-temperature inverse trirutile cell. Rietveld refinement results indicate that this transition is mainly triggered by a single displacement mode, leading to a coexistence of compressed and elongated MnO6 octahedra. Evidence of residual disorder in the sample is discussed in terms of the known problems of cationic ordering in this system.

EXPERIMENTAL SECTION

Synthesis. Mn2TeO6 was prepared by a solid-state reaction, using manganese oxalate (MnC2O4·2H2O) and commercial telluric acid (H6TeO6, Fluka, >99%), as proposed in ref 17. The precursor MnC2O4·2H2O was first synthesized by precipitation of manganese acetate (Mn(OOCCH3)2·4H2O, Sigma-Aldrich, >99%) by oxalic acid (H2C2O4·2H2O, Acros Organics, 99.5%), as reported by Huizing et al.19 The water content was ascertained by thermogravimetric analysis (TGA). Both precursors were weighed in a Mn/Te = 2 ratio and then mixed in an agate mortar, before being calcined in alumina crucibles at 550 °C for 12 h in air. The powder was then ground again, pressed with an uniaxial press in the shape of bars (≅2 × 2 × 12 mm) and sintered at 600 °C for 24 h in air (this sample is referred as S1 hereafter). S1 was then heated again at 700 °C for 24 h under an O2 flow (S2). The powder sample for synchrotron X-ray measurement was additionally annealed at 700 °C for 1 week under O2 (S3). TGA experiments confirmed that the oxygen atmosphere prevents the decomposition that would occur if the heat treatment at 700 °C were performed in air. In addition, according to TGA, the S1 into S2 transformation occurs without weight loss or gain, suggesting an oxygen content close to the expected O6 stoichiometry. X-ray and Neutron Powder Diffraction. Sample quality was checked by room-temperature X-ray powder diffraction (XRPD), using a D8 ADVANCE Vario1 (Bruker) diffractometer (Cu Kα1 radiation). In situ high-temperature XRPD measurements were also carried out using an Anton Paar HTK1200N chamber, from room temperature to 640 °C under an O2 flow, on a flat alumina sample holder. Synchrotron X-ray powder diffraction experiments were performed on the CRISTAL beamline at the Soleil synchrotron, at room temperature, with the wavelength λ = 0.7288 Å, using a rotating glass (i.d. 0.5 mm) capillary tube (S3). In situ neutron powder diffraction (NPD) was performed on the high-resolution time-of-flight WISH20 beamline at ISIS. The S1 powder was put in a quartz tube under O2 flow, and the temperature was increased from 50 °C up to 700 °C and then decreased back to 50 °C (S2). These diffraction data were analyzed using several methods and combined to take advantage 9743


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Inorganic Chemistry of each technique. Structural refinements were performed with the FullProf suite21 of programs, taking into account anisotropic strains using a spherical harmonics modeling of the Bragg peak broadening. EXPO200922 was also used after extraction of the structure factors from XRPD data by the Le Bail method with the JANA200623 program. Symmetry analysis was carried out using the Bilbao Crystallographic Server24 and its AMPLIMODES routine.25 Electron Microscopy. Electron diffraction tomography (EDT)26 combined with precession electron diffraction (PED)27 was performed using a JEOL 2010 (200 kV) transmission electron microscope equipped with a side-mounted Gatan Orius CCD camera and a Nanomegas Digistar precession unit (S2). Precession electron diffraction tomography (PEDT) data sets were collected in a tilt range of about 90° with a precession angle set to 1.2° and a goniometer tilt step below 1°. Lattice parameters and symmetry were deduced from the 3D reconstruction of the reciprocal space using the programs PETS28 and JANA2006.23 High angle annular dark field scanning transmission microscopy (HAADF-STEM)29 and annular bright field STEM (ABF-STEM)30 were performed using a JEM ARM200F cold FEG probe and image aberration corrected microscope, operated at 200 kV and equipped with a large angle CENTURIO EDX detector and QUANTUM GIF. Samples for TEM were prepared by grinding materials in an agate mortar in ethanol and then deposited on holey carbon Ni grids (S1 and S2). Simulations of the high-resolution TEM images were performed with the JEMS31 software.

Table 1. Rietveld Refinement Results and Corresponding Interatomic Distances in Mn2TeO6 (P42/mnm) at 700 °C, Derived from NPD Dataa structural params aHTT (Å) cHTT (Å) zMn xOI xOII zOII BMn (Å2) BTe (Å2) BOI (Å2) BOII (Å2) χ2 RBragg (%)

4.6421(1) 9.0750(2) 0.3350(3) 0.2981(3) 0.3115(2) 0.3385(2) 3.00(7) 3.43(7) 3.03(7) 3.28(6) 11.7 5.80

selected interatomic distances (Å) Mn−OI (×2) Mn−OII (×2) Mn−OII (×2) ⟨d⟩(Mn−O) Δ(MnO6) (×10−4) Te−OI (×2) Te−OII (×4) ⟨d⟩(Te−O) Δ(TeO6) (×10−4)

2.000(2) 2.045(1) 2.003(3) 2.016(2) 1.0 1.957(1) 1.918(2) 1.932(2) 0.9

Mn−Mn Mn−Te

2.995(4) 3.040(3)

a

Te atoms are in the (2a) Wyckoff position (WP) [000], Mn in (4e) [00z], OI in (4f) [xx0], and OII in (8j)[xxz]. The refined x and z parameters and isotropic thermal factors (B) are given for the different ions. The distortion parameter Δ is calculated for each MnO6 octahedron as Δ= (1/6)∑n=1,6{(dn − ⟨d⟩)/⟨d⟩}2, with ⟨d⟩ being the average Mn−O distance.

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Both types of octahedra are nearly regular and only slightly elongated: the longer Mn or Te−O distances are lying in the ab plane, with the O atoms linking two octahedra by the corners. All of the diffractograms recorded between 700 and 420 °C can be indexed in this tetragonal structure, which will be referred to as high-temperature tetragonal (HTT) in the following. Below 400 °C, and down to room temperature, a splitting of a few Bragg peaks is observed (red arrows, inset of Figure 2), in addition to the appearance of several small peaks (black arrows; see also the inset of Figure 4). To accurately determine the structure of the room-temperature phase, a combination of electron and X-ray synchrotron techniques was necessary, as detailed in the next section. Room-Temperature Structure of Mn2TeO6. The PEDT data confirm the existence of extra reflections inconsistent with the HTT form (Figure 5), which can be indexed with the new cell parameters a = cHTT ≅ 9.1 Å, b = 2√2 × aHTT ≅ 13.0 Å and c = √2 × aHTT ≅ 6.5 Å. These cell parameters were then refined using XRPD data, further evidencing a small but clear

RESULTS High-Temperature Structural Investigation of Mn2TeO6. Figure 2 shows the results of the high-temperature (50−700 °C) in situ neutron diffraction study. At 700 °C, the pattern is characteristic of a tetragonal unit cell (P42/mnm with a = 4.6421(1) Å, c = 9.0750(2) Å), which corresponds to the inverse trirutile structure previously reported for Cr2TeO67 and illustrated in Figure 1. As mentioned earlier, it can be seen as a stacking along c of three rutile type cells, resulting from the ordering of Mn and Te species in a 2:1 ratio. Both Mn and Te cations are in octahedral environments, sharing edges along c and corners in the ab plane. Figure 3 shows the result of the

Figure 3. Rietveld refinement of the NPD data of Mn2TeO6 at 700 °C with P42/mnm space group: experimental data, open circles; calculated profile, black continuous line; allowed Bragg reflections, green vertical marks. The difference between the experimental and calculated profiles is displayed at the bottom of the graph as a blue continuous line. Figure 4. Rietveld refinement of the synchrotron XRPD data of Mn2TeO6 at room temperature: experimental data, open circles; calculated profile, black continuous line; allowed Bragg reflections, green vertical marks. The difference between the experimental and calculated profiles is displayed at the bottom of the graph as a blue continuous line. The inset shows an enlarged view of the satellite peaks, indexed in the monoclinic structure (P21/c, a = 9.103 Å, b = 13.046 Å, c = 6.466 Å, β = 90.03°).

Rietveld refinement, and corresponding parameters and agreement factors are gathered in Table 1 (and CSD 432878). The obtained interatomic distances are comparable with those reported for other inverse trirutiles such as Cr2TeO6 and Fe2TeO6.32 In particular, the TeO6 octahedra are smaller than the FeO6, CrO6, or MnO6 octahedra; the average distances are ⟨dTe−O⟩ = 1.931 Å and ⟨dMn−O⟩ = 2.016 Å for Mn2TeO6. 9744


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Figure 5. (a) [hk0], (b) [0kl], (c) [h0l], and (d) [h1l] reciprocal space sections reconstructed from the PEDT data obtained at room temperature on an S2 single crystal. Rows of extra reflections (green arrows) show the existence of a superstructure with respect to the tetragonal trirutile cell (highlighted by a red dashed grid). These extra reflections can be accounted for by considering lattice parameters a ≅ 9.1 Å, b ≅ 13.0 Å, and c ≅ 6.5 Å. When (c) and (d) are compared, the reflection condition h0l, l = 2n is clear.

distortion, leading to a monoclinic cell with a = 9.1000(1) Å, b = 13.0424(1) Å, c = 6.4643(1) Å, and β = 90.015(1)°. Considering the conditions limiting the reflection (h0l, l = 2n; 0k0,k = 2n; 00l, l = 2n) identified in the sections reconstructed from PEDT, the centrosymmetric space group P21/c was retained to conduct the ab initio structure determination. The obtained room-temperature structure, with its large unit cell of monoclinic symmetry (RTM), contains 18 atoms, including 12 oxygen atoms, on general Wyckoff sites, leading to 54 atomic positional parameters. A symmetry analysis was thus performed to analyze the structural transition from the HTT form. Rietveld refinements using the synchrotron X-ray data and based on symmetry-adapted modes (constructed with AMPLIMODES) were then undertaken. In this approach, the structural distortion is seen as a superposition of the parent structure (P42/mnm) with symmetry-breaking modes, associated with different irreducible representations (Irreps) of the parent space group. The advantage is that the refinement deals with mode amplitudes instead of individual atomic coordinates, so that the number of free parameters is lower once the main distortion modes are identified (a mode amplitude corresponds to atomic displacements, in relative units, of the atoms of the asymmetric unit of the distorted phase, with respect to the parent phase). The results of this analysis are given in Table 2. There are six Irreps of the P42/mnm space group, in addition to the isostructural mode Γ1+, that can be used to achieve the P21/ c distortion: Γ4+, Γ5+, Σ2, Σ3, M1−M4−, and M5−. The monoclinic structure of Mn2TeO6 can be derived therefore from a maximum of seven modes, whose distribution is given in Table 3. A single Irrep distortion is not sufficient to explain the full symmetry breaking of the transformation from P42/mnm to P21/c; nevertheless, during the symmetry mode amplitude refinements, it became clear that one mode was persistently

Table 2. Summary of the Mode Decomposition, Showing the Different Distortion Components, Following the ISOTROPY Formalism25,46 k vector

Irrep

direction

isotropy subgroup P42/mnm (No. 136) Cmmm (No. 65) C2/m (No. 6) Pbam (No. 55) Pbcm (No. 57) Pban (No. 50) Pmma (No. 51)

(0,0,0)

Γ1+

(a)

(0,0,0) (0,0,0) (1/4,1/4,0) (1/4,1/4,0) (1/2,1/2,0) (1/2,1/2,0)

Γ4+ Γ5+ Σ2 Σ3 M1−M4− M5 −

(a) (a,a) (a,0,0,a) (a,0,0,−a) (a,b) (a,a)

no. of modes 4 4 6 13 16 2 9

Table 3. Summary of the Basis Modes Involved in the Distortion of Mn2TeO6, Distributed per Type of Wyckoff Position in P42/mnma atom

WP

modes

O2 O1 Mn Te

8j 4f 4e 2a

Γ1+ (2), Γ4+ (2), Γ5+ (3), Σ2 (5), Σ3 (7), M1−M4− (2), M5− (3) Γ1+ (1), Γ4+ (1), Γ5+ (1), Σ2 (4), Σ3 (3), M5− (2) Γ1+ (1), Γ4+ (1), Γ5+ (2), Σ2 (2), Σ3 (4), M5− (2) Σ2 (2), Σ3 (2), M5− (2)

a

The values in parentheses indicate the number of modes for each Irrep.

preponderant: the Σ3 mode, whose 16 atomic displacements correspond to a symmetry breaking to the Pbcm isotropy subgroup, induced by the doubling of the cell along b. Indeed, in the refinement shown in Figure 4, only the Σ3 mode has been refined, leading to a satisfactory profile (see in particular the superstructure peaks in the inset) and to good agreement factors. 9745


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Inorganic Chemistry Table 4 shows the normalized polarization vectors for the Σ3 mode, in terms of components of the basis modes (note that,

The details of the atomic positions and thermal displacement factors (B), obtained after the single-mode refinement of the structure, are given in Table 5 (and CSD 432879). The atomic

Table 4. Normalized Polarization Vectors for the Σ3 Distortion Components of Mn2TeO6, Expressed as Displacements in Relative Units for the Asymmetric Cell of the Structure (Normalization Unit: 1 Å) atom

δx

δy

δz

Mn1 Mn2 Mn3 Mn4 Te1 Te2 O1 O2 O3 O4 O5 O6 O7 O8 O9 O10 O11 O12

−0.0131(2) 0.0043(2) 0.0043(2) −0.0131(2) −0.0087(2) 0.0080(2) −0.0056(12) −0.0071(12) 0.0182(7) 0.0182(7) −0.0170(7) −0.0015(7) 0.0039(5) 0.0039(5) 0.0039(5) 0.0039(5) −0.0170(7) −0.0015(7)

−0.0007(2) 0.0057(2) −0.0057(2) 0.0007(2) 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0086(6) −0.0124(6) 0.0023(4) 0.0023(4) −0.0023(4) −0.0023(4) −0.0086(6) 0.0124(6)

0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0020(11) −0.0020(11) −0.0020(11) 0.0020(11) 0.0000 0.0000

Table 5. Atomic Coordinates and Thermal Displacement Parameters of Mn2TeO6 at Room Temperaturea type

x

y

z

B (Å2)

Mn1 Mn2 Mn3 Mn4 Te1 Te2 O1 O2 O3 O4 O5 O6 O7 O8 O9 O10 O11 O12

0.3206(2) 0.8390(2) 0.1700(2) 0.6516(2) −0.0092(2) 0.5085(2) −0.0059(12) −0.0076(12) 0.5193(7) 0.5193(7) 0.3210(8) 0.3375(7) 0.8433(5) 0.8433(5) 0.1651(5) 0.1651(5) 0.6428(8) 0.6593(8)

0.1243(2) 0.8810(2) 0.8690(2) 0.1257(2) 0.1250 0.8750 0.2736 0.9764 0.8750 0.8750 0.2895(6) 0.9565(6) 0.8774(4) 0.8774(4) 0.8726(4) 0.8726(4) 0.2712(6) 0.9828(6)

0.2500 0.2500 0.2500 0.2500 0.2500 0.2500 0.2500 0.2500 0.5472 0.9528 0.2500 0.2500 0.5628(11) 0.9372(11) 0.5586(11) 0.9414(11) 0.2500 0.2500

0.309(11) 0.309(11) 0.309(11) 0.309(11) 0.349(5) 0.349(5) 0.115(26) 0.115(26) 0.115(26) 0.115(26) 0.115(26) 0.115(26) 0.115(26) 0.115(26) 0.115(26) 0.115(26) 0.115(26) 0.115(26)

a From synchrotron data. Crystal data: P21/c space group with a = 9.1030(1) Å, b = 13.0463(1) Å, c = 6.4659(1) Å, β = 90.0331(4)° and all atoms are in the (4e) general WP. Agreement factors: RBragg = 3.29%, χ2 = 8.45.

for the calculated mode amplitudes to be more meaningful in the refinement, the parameters of the reference tetragonal P42/ mnm structure were determined from NPD data at 420 °C). A representation of the polarization vectors corresponding to this Σ3 mode is also shown in Figure 6, using the setting of the

displacement parameters reported in this table are constrained to be equal for atoms of the same species to decrease the number of free parameters (refining them separately did not significantly improve the agreement factors). Refining cationic occupancies converged systematically to a composition in excellent agreement with the expected Mn2TeO6. The corresponding structure, illustrated in Figure 7, shows that Mn2TeO6 retains an inverse trirutile structure, albeit distorted, with Mn/Te columns of edge-sharing octahedra running along a (Figure 7a). There are two types of chains of edge-sharing octahedra, either consisting of Mn1Mn4Te1Mn1 (chain I) or Te2Mn3Mn2Te2 (chain II), which are connected through oxygen atoms (Figure 7c). In contrast with the parent P42/mnm structure, these chains are puckered, particularly the second one. As will be further discussed in the last section, this is connected with different distortions of the octahedra: Mn1O6 and Mn3O6 are elongated but Mn2O6 and Mn4O6 are compressed, and Te1O6 and Te2O6 are nearly regular (Figure 7d). The waving of both chains mainly originates from oxygen displacements and appears larger for chain II, although the more distorted octahedra are observed in chain I, around Mn1. In each chain, there is a pair of one elongated and one compressed MnO6 octahedron, sandwiched between two TeO6 octahedra. The unique axes of the octahedra are along b for Mn1 and Mn4 and in the ab plane for Mn2 and Mn3, thus forming a herringbone-like pattern, as shown in Figure 7b. This also leads to slight differences in the cation−cation distances along the chains (Table 6): although the Mn−Mn distances are similar (3.013 and 3.017 Å), in each chain there is a long and a short Mn−Te distance (3.088 vs 3.002 Å and 3.080 vs 3.010 Å in chains I and II, respectively).

Figure 6. Description of the Σ3 distortion components of Mn2TeO6 through the polarization vectors of the Mn and Te cations, projected along [001]. The atomic displacements correspond to the direction of the arrows, and their relative length is proportional to the mode amplitude.

RTM structure. Displacements are observed along a for the six cationic sites, but a significant b component is only seen for Mn2 and Mn3 sites. There is no displacement along c. As shown in Figure 6, in each TeMnMnTe row (that is, with the same y value), all of the cations move in the same direction along a; two rows with opposite y coordinates (y and − y) move in the opposite direction. An equivalent drawing is given for the oxygen lattice in Figure S1 in the Supporting Information. 9746


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Figure 7. Polyhedra representation of the P21/c distorted inverse trirutile structure of Mn2TeO6 at room temperature projected (a) along [100], (b) along [001] and (c) along [010] and [001]. The parent tetragonal HTT cell is shown in (a) as a black line. In (b) both types of chains are represented, and the full/dotted black lines highlight the compressed/elongated Mn−O distances of the MnO6 octahedra. For clarity, only one type of chain is shown in each projection in (c). (d) Orientation of the drawing of the structure chosen to show the Mn− or Te−O interatomic distances (Å) in the six octahedra building Mn2TeO6.

Occasionally, the position of oxygen atoms deviates from the refined structure over several periods. This is highlighted by blue ellipses in Figure 9d, whose relative orientations along the b axis differ from those calculated from the X-ray structural model (blue ellipses in Figure S2 in the Supporting Information). Some level of disorder in oxygen positions might be explained by possible occupancy variations within cation columns, inducing a tilt of octahedra, or by oxygen vacancies. In any case, this deviation is small, as it cannot be refined from X-ray or neutron data meaningfully. Nevertheless, the assumption about cationic distribution is supported by another phenomenon evidenced in the ED pattern and corresponding HAADF-STEM image presented in Figure 10. Surprisingly strong 020 diffraction spots are observed in this [001] ED pattern, in contrast with the [001] ED pattern of Figure 8. Corresponding FT patterns taken from HAADFSTEM images also show clearly that the intensity of the 020 diffraction spots varies from one crystal to another and that often both types of [001] ED patterns (Figures 10 and 8) coexist in the same crystallite. This is associated with a variation in the intensity of the brightness of Te columns along the [010] direction, as revealed by a close inspection of the HAADF-

Figure 8, combining ED patterns and HRTEM images, confirms this structural model. Indeed, the [001] ED pattern exhibits superstructure spots, in comparison with the HTT unit cell; all patterns are thus indexed using the large RTM cell previously determined (Table 5). Moreover, the regular contrast of the corresponding HRTEM images shows that the sample is well crystallized and free of extended defects on large areas. [010] HAADF-STEM and [100] HAADF-STEM images (Figures 9a,b) and corresponding Fourier transform (FT) patterns are also in agreement with the RTM structure, as shown by the positions of Te (Z = 52) and Mn (Z = 25), appearing as very bright and less bright dots, respectively. No additional featuressuch as unindexed spots and modulationare observed along these zone axes. The large bRTM parameter, i.e. = 2√2 aHTT parameter, is clearly corroborated by the [001] HAADF-STEM and ABF-STEM images (Figures 9c,d), with the presence of the spots highlighted by the arrows in the corresponding FT pattern. In agreement with the RTM structure determined by synchrotron XRPD data, the cations and oxygen atoms are no longer aligned along [010] (in comparison with the HTT phase), as can be seen in HAADFSTEM (Figure 9c) and ABF-STEM images (Figure 9d). 9747


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Inorganic Chemistry Table 6. Selected Interatomic Distances (in Å) in Mn2TeO6 (P21/c) at Room Temperature cation

O

distance

⟨d⟩

10−4Δd

cation

O

distance

⟨d⟩

10−4Δd

Mn1

O3 O4 O5 O6 O7 O8 O1 O2 O5 O7 O8 O12 O1 O2 O7 O8 O9 O10 Mn−Mn

1.960(5) 1.961(5) 2.156(8) 2.194(8) 1.922(6) 1.921(6) 2.067(8) 1.870(8) 1.883(8) 2.023(7) 2.023(7) 2.107(8) 1.939(1) 1.939(1) 1.935(6) 1.936(6) 1.884(6) 1.883(6)

2.019(6)

30.9

Mn3

18.9

19.7

Mn4

1.998(6)

17.6

1.919(4)

1.8

Te2

1.944(9) 2.139(8) 1.905(8) 1.996(7) 1.996(7) 2.129(8) 2.035(5) 2.034(5) 2.077(6) 2.078(6) 1.900(8) 1.866(8) 1.924(1) 1.924(1) 1.911(8) 1.885(8) 1.931(8) 1.965(8)

2.018(8)

1.996(7)

O1 O2 O6 O9 O10 O11 O3 O4 O9 O10 O11 O12 O3 O4 O5 O6 O11 O12 Mn−Te

1.923(5)

1.5

Mn2

Te1

Mn1−Mn4 Mn3−Mn2

distance 3.013(2) 3.017(3)

Mn1−Te1 Mn4−Te1 Mn2−Te2 Mn3−Te2

distance 3.002(2) 3.088(2) 3.010(2) 3.082(2)

Figure 8. ED patterns along the main zone axes [100], [001], and [010] and corresponding [100] and [001] HRTEM images and associated FT patterns. The indexation corresponds to the RTM (white) and HTT (yellow) structures.

distribution of Mn in Te columns might be also correlated with the existence of dislocations. These are rather unusual in bulk oxides, and they are associated with antiphase boundaries (APBs) as illustrated in the HAADF-STEM image viewed along [001] (Figure 11). The APB appears as bright Te layers shifted with respect to each other by one-third along the [100] direction (region A), as schematized in the lower part of Figure 11. Such APBs originate from dislocations, which consist of the

STEM images. The corresponding intensity profile (plotted in Figure 10) suggests a chemical disordering within Te columns, incorporating Mn or vacancies: the bright dots correspond to 100% Te columns, while less bright dots correspond to mixed Te/Mn columns. This is further confirmed by structure factor calculations, which show that the intensity of the 020 spot is directly linked with Te occupancy, full Te occupancy corresponding to zero intensity of the 020 spot. Such a 9748


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Figure 9. High-resolution HAADF-STEM images and corresponding FT pattern of main zone axes along (a) [010], (b) [100], and (c) [001]. On the enlarged images, color-coded atoms are overlapped (Mn, Te, and O in green, orange, and blue, respectively), and simulated patterns are given in the inset. (d) [001] ABF-STEM image acquired simultaneously with HAASDF-STEM image given in (c) and its FT pattern. For simplicity, only O atoms are shown in the enlarged image.

°C, when the structural transition occurs. This could suggest that preliminary atomic reorganizations, premises of the structural transition, could start as high as 550 °C. This negative thermal expansion of cHTT below 400 °C does not affect the temperature evolution of the cell volume; it is not actually understood yet, but it could be linked with the increase in the average Mn−O distance of the distorted Mn1O6 and Mn3O6 octahedra at the transition.33 Jahn−Teller Effect and Distortion of the MnO 6 Octahedra. To better understand the structural transition, it is indeed interesting to focus on the behavior of the MnO6 octahedra. One useful factor to quantify the relative distortion of an octahedron is the distortion parameter Δd, which corresponds to the deviation of Mn−O distances with respect to the average ⟨Mn−O⟩ value. The Δd values given in Tables 1 and 6 visibly show that the distortion of the MnO6 octahedra increases drastically (by a factor 10 at least) on going from the HTT to RTM: whereas in the HTT phase MnO6 octahedra are regular, with an average distance of 2.016(2) Å, in RTM the Σ3 mode displacements of the oxygen atoms lead to elongated Mn1O6 and Mn3O6 octahedra, while the Mn2O6 and Mn4O6 octahedra are compressed (Figure 7d). These distortions originate from cooperative Jahn−Teller effects of Mn3+, but the sequence “two Mn between two Te” in each chain limits the degree of freedom to distort the structure: indeed, the octahedra are connected by edges and TeO6 units exhibit shorter bonds and could be seen as rather rigid,34 in contrast with the more flexible MnO6 units. Moreover, both types of chains have to adapt their distortion to each other. All of these

splitting of one Te plane in two, between which extra Te/Mn planes are inserted. The corresponding model is also drawn in Figure 11 (region B). In view of these local defects, the roomtemperature crystal structure of Mn2TeO6 deduced from the synchrotron diffraction data is certainly somewhat idealized, as it does not take into account local scale disorder, which probably explains the anisotropic broadening of the Bragg peaks mentioned in the experimental part.

■

DISCUSSION Tetragonal to Monoclinic Transition. The phase transition from HTT to RTM is illustrated in Figure 12 through the evolution of the refined lattice parameters as a function of temperature. The aHTT cell parameter decreases with decreasing temperature, and its splitting in bRTM and cRTM is clearly observed below 400 °C. This splitting increases with decreasing temperature, along with the monoclinic distortion. The symmetry of the primary mode Σ3 is orthorhombic, but the actual monoclinic cell symmetry requires an additional Γ5+ mode, which is not coupled by symmetry to Σ3: as a result, they can occur at different temperatures. However, as the monoclinic angle remains small at room temperature (close to 90°), it is not possible to distinguish such a possibility from the temperature evolution of the cell parameters, as they seem to occur roughly at the same temperature. In contrast with aHTT (=bHTT) cell parameters, the temperature evolution of cHTT is not monotonous: from 700 to 550 °C the usual decrease due to thermal contraction is observed, but below 550 °C cHTT starts to expand slightly, before increasing more sharply around 400 9749


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Figure 10. [001] HADDF-STEM image highlighting the area corresponding to the lighted 020 spots in the FT and ED patterns (red arrow). The intensity plot profile along the Te row of the selected area (white rectangle) is also given as an insert.

Figure 11. [001] HAADF-STEM image of an area with two types of defects. The (A) and (B) regions are enlarged (right panels), and corresponding structural models are proposed in the bottom left panel.

about the trivalent state of manganese, confirmed by the μeff value extracted from the 1/χ(T) curve in the paramagnetic

parameters do not facilitate the establishment of the Jahn− Teller distortion at long range. Nevertheless, there is no doubt 9750


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Figure 12. Temperature evolution of lattice parameters and cell volume of trirutile Mn2TeO6, from NPD data.

synthesis (600 and 700 °C), is an additional parameter that renders difficult a deeper interpretation correlating the degree of disorder with the cation−cation distances, as proposed in trirutiles.42 As a final remark, this disorder (distribution of cations and distortion of the polyhedra) could explain the difficulties encountered by Fruchard et al.17 to solve the Mn2TeO6 structure. In the samples synthesized at low temperature (≅600 °C), pronounced peak broadening is expected in the XRPD patterns, preventing a reliable determination of the cell parameters and symmetry.

domain, in agreement with the hexavalent state of tellurium evidenced by nuclear forward scattering.35 It would be interesting to follow the structural evolution at lower temperature to go further in the understanding of this transition, in particular to explain the origin of the unusual compressed MnO6 octahedra. Several scenarios are possible, such as an occupation of dx2−y2 orbitals, which would be a peculiar example of inverted occupancy of eg orbitals, or as disordered apically elongated MnO6 octahedra, a phenomenon previously discussed in manganites.36 The crucial role of the Jahn−Teller effect is also highlighted in CuSb2O6, which exhibits a tetragonal to monoclinic transition and a unique orbital ordering that drives the magnetic behavior.3,37−39 Mn/Te Cationic Ordering. On the other hand, cationic ordering in trirutile has always been an issue, well exemplified by FeNb2O6, for which a rutile structure with random cation distribution has been reported,40 as well as a trirutile structure, the tapiolite (Fe,Mn)(Nb,Ta)2O6, in which metal ions are considered ordered.41 Nevertheless, in this tapiolite case, different cationic distributions are proposed, covering a continuum from perfect order to total disorder. For instance, a single-crystal study of FeNb2O642 describes a structure which is intermediate between disordered rutile and ordered trirutile, while in a natural tapiolite sample the coexistence of both phases is observed.43 To take into account this partial cation ordering, the structure of FeTa2O6 was described as modulated: that is, a basic structure plus an occupancy/displacive wave.44 There is no such extensive literature about cationic disorder in inverse trirutiles, but tellurates are known to adopt cation distributions ranging from fully ordered to disordered.45 The cationic distribution is probably also at play in the investigated Mn2TeO6 system and has an effect on the O sublattice, but our X-ray and neutron diffraction experiments suggest that such disorders would be limited to the standard deviations (

Mn2TeO6: a Distorted Inverse Trirutile Structure - ACS Publications

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