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Apr 17, 2019 - Preparation, Crystal Chemistry, and Hidden Magnetic Order in the Family of Trigonal Layered Tellurates A2Mn(4+)TeO6 (A = Li, Na, Ag, or...
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Preparation, Crystal Chemistry, and Hidden Magnetic Order in the Family of Trigonal Layered Tellurates A2Mn(4+)TeO6 (A = Li, Na, Ag, or Tl) Vladimir B. Nalbandyan,*,† Igor L. Shukaev,† Grigory V. Raganyan,‡ Artem Svyazhin,§,∥ Alexander N. Vasiliev,‡,⊥,# and Elena A. Zvereva‡,⊥ †

Chemistry Faculty, Southern Federal University, Rostov-on-Don 344090, Russia Physics Faculty, M. V. Lomonosov Moscow State University, Moscow 119991, Russia § European Synchrotron Radiation Facility, Grenoble 38043, France ∥ M. N. Mikheev Institute of Metal Physics of Ural Branch of Russian Academy of Sciences, Yekaterinburg 620990, Russia ⊥ National Research South Ural State University, Chelyabinsk 454080, Russia # National University of Science and Technology “MISiS”, Moscow 119049, Russia

Inorg. Chem. Downloaded from pubs.acs.org by IDAHO STATE UNIV on 04/17/19. For personal use only.

‡

S Supporting Information *

ABSTRACT: We report the first four magnetic representatives of the trigonal layered A2M(4+)TeO6 (here, M = Mn) family. Na2MnTeO6 was synthesized from NaMnO2, NaNO3, and TeO2 at 650−720 °C, but analogues for which A = Li and K could not be obtained by direct synthesis. However, those for which A = Li, Ag, and Tl (but not K) were prepared by exchange reactions between Na2MnTeO6 and the corresponding molten nitrates. The oxygen content was verified by redox titration. According to the X-ray diffraction Rietveld analysis, the four new compounds are isostructural with Na2GeTeO6, trigonal (P3̅1c), based on ilmenite-like layers of edge-shared oxygen octahedra occupied by Mn(4+) and Te(6+) in an ordered manner. These layers are separated by cations A, also in a distorted octahedral coordination. However, off-center displacement of Tl+ is so strong, due to the lone-pair effect, that its coordination is better described as trigonal pyramid. Each MnO6 octahedron shares two opposite faces with AO6 octahedra, whereas TeO6 octahedra avoid sharing faces. Besides this double-layered structure, Na2MnTeO6 was often accompanied by a transient triple-layered rhombohedral polytype. However, it could not be prepared as a single phase and disappeared on annealing at 700−720 °C. All A2MnTeO6 samples (A = Ag, Li, Na, or Tl) revealed the unusual phenomenon of hidden magnetic order. Low-field magnetic susceptibility data exhibit a Curie− Weiss type behavior for all samples under study and do not show any sign of the establishment of long-range magnetic order down to 2 K. In contrast, both the magnetic susceptibility in sufficiently high external magnetic fields and the zero-field specific heat unambiguously revealed an onset of antiferromagnetic order at low temperatures. The frustration index f = Θ/TN takes values larger than the classical values for three-dimensional antiferromagnets and implies moderate frustration on the triangular lattice.

1. INTRODUCTION

In this respect, we are interested in the family of trigonal layered phases A2MTeO6 (A = Li or Na; M = Ge, Ti, or Sn).6 Of course, all of them are diamagnetic; however, identical octahedral radii7 of Ge4+ and Mn4+ suggest that Mn may be substituted for Ge (Figure 1) if we succeed in stabilizing the thermally unstable Mn4+ state. Besides magnetic properties, introduction of the 3d cation would modify electronic structure (with further bandgap modification by a variety of A cations and their combination in solid solutions) that may be favorable for, e.g., photocatalytic or photovoltaic performance. In this paper, we report the preparation, structure, and

The frustrated low-dimensional systems, primarily two-dimensional (2D) spin systems, today present the most challenging issues.1−5 The triangular lattice (TL) 2D antiferromagnets possess the natural geometrical frustration of the underlying magnetic lattice coupled with strong Coulomb correlation and spin−orbit interaction, which can conspire to produce novel exotic magnetic phases in these systems. As a result, TL magnets exhibit an amazingly rich range of complicated magnetic phase diagrams, and herein, studies of layered compounds play a central role in forming that opinion. In some cases, the establishment of long-range order is hampered in favor of a spin-liquid-like ground state with the remaining entropy. © XXXX American Chemical Society

Received: December 10, 2018

A

DOI: 10.1021/acs.inorgchem.8b03445 Inorg. Chem. XXXX, XXX, XXX−XXX

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2.4. X-ray Absorption Spectroscopy. The experiment was performed on beamline ID26 of the European Synchrotron Radiation Facility (Grenoble, France). The incident energy was monochromatized with Si(311) monochromator crystals. High-energy resolution fluorescence-detected X-ray absorption near edge structure (HERFDXANES) spectra of Mn were obtained by setting the emission energy to the maximum of the Kβ1,3 line (corresponding to the 3p → 1s transition) while scanning the incoming energy through the absorption edge. The spectra were obtained using four spherically bent Ge(440) analyzer crystals; the total experimental broadening, measured as the average of the full width at half-maximum of the profiles of elastic peaks, was 0.9 eV. 2.5. Magnetization, Specific Heat, and Electron Spin Resonance (ESR) Measurements. The temperature dependencies of the magnetic susceptibility were measured by varying the magnetic field (B) from 0 to 9 T in the temperature range of 2−300 K by means of a Quantum Design MPMS 9 system. In addition, the isothermal magnetization curves were obtained in external fields up to 9 T at various constant temperatures after cooling the sample in zero magnetic field. Specific heat measurements were taken by a relaxation method using a Quantum Design PPMS system. Data were collected at zero magnetic field and 9 T in the temperature range of 2−50 K. ESR studies were carried out at room temperature using X-band ESR spectrometer CMS 8400 (ADANI) (f ≈ 9.4 GHz; B ≤ 0.7 T). The effective g factor has been calculated with respect to a BDPA (a,gbisdiphenyline-b-phenylallyl) reference sample with get = 2.00359.

Figure 1. Structure of (MTeO6)2− layers in Na2GeTeO66 (left) and Na2MnTeO6, (this work) (right). Dashed lines in the right panel illustrate the triangular magnetic lattice of Mn4+ ions, which provides the conditions for geometrical frustration of antiferromagnetic exchange coupling.

thermodynamic properties of the four new compounds of the A2MnTeO6 (A = Li, Na, Ag, or Tl) structural family.

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. All starting materials were of reagent grade, and all syntheses were performed in air. For Na2MnTeO6, two preparation routes were tested. The “direct” variant was based on the following equation:

3. RESULTS AND DISCUSSION 3.1. Sample Preparation and Analysis. 3.1.1. Na2MnTeO6. Na2MnTeO6 was synthesized several times under similar conditions with similar but not identical results (see section 3 of the Supporting Information for details). Some representative examples are illustrated in Figure 2. In most cases, the XRD patterns (Figure 2a,c,d) were almost identical to that of the double-layered polytype of Na2GeTeO6,6 space group P3̅1c (2H type for short). In some preparations (Figure 2a,b), it was accompanied by a phase analogous to the triple-layered rhombohedral polytype of Na2GeTeO6 (3R for short). Woodward et al.6 reported that the almost pure 3R sodium germanotellurate (with only 4% of the 2H phase) was prepared at 700 °C but completely transformed to the 2H phase after being annealed for 36 h at 750 °C. Quite similarly, the 3R phase of Na2MnTeO6 was only observed in 600−650 °C preparations and completely transformed to the 2H phase after a short heating at 700− 750 °C. However, it was never obtained as a sufficiently pure phase, even at the lowest preparation temperatures. Figure 2b shows the XRD pattern with the largest fraction of the 3R polytype, and Figure 2c its transformation to the 2H polytype. As an attempt to prepare the pure 3R phase, we annealed another portion of the same 3R/2H mixture at lower temperatures: for 15 h at 600° and then for 4 h at 500 °C. However, this did not change the previously obtained 3R/2H ratio. Of course, hexagonal parameters a of both polytypes are very similar whereas their parameters c are in a 2/3 ratio (see Table S1). Then, in the mixture of polytypes, hkl reflections from the 3R phase with l = 3n overlap with those from the 2H phase with l = 2n, and parameters of each phase could be measured independently only with non-overlapping reflections. To study thermal stability, portions of Na2MnTeO6 in small calcined porcelain crucibles were placed into a furnace with a preliminarily established temperature, held for 20 min, and quenched. Up to 800 °C, changes in the XRD pattern were

0.5Mn2O3 + 0.5NaNO3 + 0.75Na 2CO3 + TeO3 → Na 2MnTeO6 + 0.75CO2 + 0.25NO + 0.25NO2 The mixture was reacted first at 600 °C and then, after regrinding, at 650−700 °C. Otherwise, NaNO3 and TeO2 reacted under the same conditions with NaMnO2 that was preliminarily obtained at 800 °C: NaMnO2 + NaNO3 + TeO2 → Na 2MnTeO6 + NO The latter variant was found to be more effective and was used for all experiments. These preparations had some non-obvious details as described in sections 1−3 of the Supporting Information. For ion exchange, powdered Na2MnTeO6 was treated with molten nitrates: for 2 h at 300 °C with LiNO3, AgNO3, or TlNO3 and at 335 °C with the (KNO3+KCl) eutectic. The salts were taken in 5-, 2-, 3-, and 5-fold excess with respect to the theoretical amount. The products were washed with water and dried at 150 °C. 2.2. Chemical Analyses. To verify the supposed oxidation states, redox analysis was employed. Two or three weighed portions of each product were dissolved, on gentle heating, in aliquots of a 2 mol/L H2SO4 solution containing 0.1 mol/L FeSO4 in excess sufficient to reduce all Te(6+) to Te(4+) and all Mn(4+) to Mn(2+). After cooling, the resulting solutions were titrated with a standard KMnO4 solution together with identical aliquots to which no tellurate was added. The difference between the volumes of the KMnO4 solution was then equivalent to a Mn oxidation state in excess of 2+. Ag+ was determined in the same solutions by thiocyanate titration. With the thallium compound, however, the analysis was complicated as discussed in section 3.1.3. For EDX analysis, an INCA ENERGY 450/XT instrument with an X-Act ADD detector was used. Due to considerable scatter, the Na/ Te, Mn/Te, and Tl/Te molar ratios were averaged for 6−10 measurements at various points of a sample. 2.3. Phase Analysis and Structural Studies. For X-ray diffraction (XRD) measurements, we used an ARL X’TRA diffractometer with an Si(Li) detector and Cu Kα radiation. Lattice parameters were refined with CELREF 3 (J. Laugier and B. Bochu), using corundum as an internal standard for Bragg angles. Structural studies performed via the Rietveld method were performed with the GSAS + EXPGUI suite.8,9 To reduce the preferred orientation of grains, an amorphous powder (instant coffee) was admixed before measurements. B

DOI: 10.1021/acs.inorgchem.8b03445 Inorg. Chem. XXXX, XXX, XXX−XXX

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in the empirical formula Na2MnTeO5.709(5) indicating a markedly reduced oxidation state of Mn and/or Te. In agreement with this, both unit cell parameters were considerably enlarged (Table S1, sample 8A). It is clear, therefore, that Na2MnTeO6 exhibits an extended homogeneity range. However, a detailed study of its nonstoichiometry was outside the scope of this work. 3.1.2. Other A2MnTeO6. Attempts to prepare A2MnTeO6 (A = Li or K) in a manner similar to that of Na2MnTeO6 were unsuccessful: we obtained unrecognized phases with average oxidation number of Mn of ≪4 (see sections 4 and 5 of the Supporting Information for details). These results could not be explained by kinetic problems associated with oxidation by air or the low reactivity of Mn2O3 because we used precursors combining, in a single phase, the alkali and manganese in elevated oxidation states: Li2MnO3 and KMnO4, respectively. We, thus, conclude that neither K2MnTeO6 nor Li2MnTeO6 can be prepared by direct synthesis in an ambient atmosphere and, probably, require a higher pressure of oxygen. It is well-known that elevated oxidation states of an element are stabilized in alkaline environments. In aqueous solutions, this means high pH; in the solid state, this means the presence of large low-valence cations. For example, the highest Mn oxidation state attainable in air at elevated temperatures is 3+ in the absence of large cations (Mn2O3), 4+ with small Li+ (Li2MnO3), but 6+ (A2MnO4) with a larger A of Na+ or K+. Therefore, the instability of Li2MnTeO6 and the stability of Na2MnTeO6 seem to be quite logical. However, K2MnTeO6 might then be expected to be even more stable under the same conditions, but our experiments show the opposite results. This problem is discussed from the structural point of view in section 3.3. Ion exchange substitution of K for Na in Na2MnTeO6 was also unsuccessful. Despite the 5-fold excess of the potassium salts, the XRD pattern of the product was identical to that of the starting sodium compound. This, again, is analogous to the behavior of Na2GeTeO6.6 In contrast to the experiments described above, ion exchange preparation of A2MnTeO6 (A = Li, Ag, or Tl) was successful. Analytical and XRD data (sections 3.1.3 and 3.2, respectively) confirm essentially complete substitution of sodium. Comparison of the unit cell volumes (Figure 3) shows a reasonable correlation with corresponding ionic radii.

Figure 2. Low-angle parts of XRD patterns for some representative Na2MnTeO6 samples. Highest preparation temperatures are indicated in parentheses: (a) sample 4 (650 °C), 2H polytype with only a trace of 3R (marked with asterisks); (b) sample 5 (650 °C), mixture of 2H and 3R polytypes, black and red hkl, respectively (arrows point to reflections from Na2TeO4); (c) sample 5A (700 °C), pure 2H phase; (d) sample 6 (700 °C), 2H with a trace of 3R; (e) sample 6A (800 °C), 2H; (f) sample 6B (850 °C), Na2TeO3, Mn2O3, 3R, and unknown; and (g) sample 8A (700 °C), 2H, trace Na2TeO4, and trace of 3R. Patterns e and f are weak because of the very small samples scanned on a zero-background holder.

negligible (Figure 2e) and the weight loss at 800° was only 0.9%, probably due to adsorbed moisture and/or the volatility of tellurium oxide. A short heating at 850 °C, however, resulted in the formation of Na2TeO3 and Mn2O3 (Figure 2f). This may be expressed as Na 2MnTeO6 → Na 2TeO3 + 0.5Mn2O3 + 0.75O2

The weight loss calculated from this equation is 7.40%, whereas the experimental value is slightly lower, 6.6%. In accordance with this, the decomposition is incomplete, and weak reflections from Na2MnTeO6 are visible in Figure 2f (together with some weak non-identified reflections). Unexpectedly, they correspond to the 3R polytype rather than the starting 2H phase. This result confirms that the 3R phase cannot be classified as a low- or high-temperature polymorph. The two polytypes with identical coordination should have almost identical thermodynamic stability. Then, formation of 2H or 3R may be occasional, due to minor uncontrolled variations in composition and/or preparation conditions. All of the experiments described above were based on preparations using the NaMnO2 precursor. The “direct” use of Mn2O3 was found to be less effective. Even after a longer heating at 650 °C, the sample contained larger amounts of foreign phases; a relatively pure phase was obtained only after an additional annealing at 700 °C. However, weak reflections from Na2TeO4 and the 3R polytype were still visible, and diffraction peaks were broadened with respect to the preceding samples (Figure 2g). More importantly, redox titration resulted

Figure 3. Correlation between unit cell volumes of A2MnTeO6 and octahedral radii of A+ ions.7 For better linearization, cubic roots of the volumes are plotted. C

DOI: 10.1021/acs.inorgchem.8b03445 Inorg. Chem. XXXX, XXX, XXX−XXX

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Of course, redox titration provides only an overall result and cannot determine oxidation states of each element separately. Therefore, some initial suggestions were necessary. Here, calculations are based on the quite natural postulation of oxidation states 6+ for Te, 4+ for Mn, and 1+ for Tl. Then, weight fractions of Mn and Tl are calculated and listed in Table 1. Otherwise, we might postulate an ideal Mn/Te molar ratio of 1 according to the starting composition and then calculate the average oxidation state of Mn. Fortunately, the results are rather close to the desired stoichiometry making the two approaches very similar. For example, assuming an ideal Ag2MnTeO5+x cationic composition, redox analysis gives x = 0.96, i.e., oxidation state of 3.92 for Mn. The apparent Mn4+ deficiency is small and almost identical for the three compounds with A = Li, Na, and Ag (3−4% of the theoretical content). This is quite natural because the ion exchange products should retain the rigid lattice of the Na precursor. We assume that the same should be valid for the Tl compound. The apparent Mn(4+) deficiency may be explained by at least three reasons or their combinations: actual Mn deficiency due to the insufficient quantity of starting NaMnO2 (see sections 2 and 3 of the Supporting Information), actual oxygen deficiency, with slightly reduced average oxidation states of Mn or Te, and the systematic error of the analysis. In the thallium compound, partial charge transfer (Tl+ + 2Mn4+ → Tl3+ + 2Mn3+) cannot be excluded by the analytical data. However, bond lengths and bond valence sums reported in section 3.3 are in reasonable agreement with the left side of this scheme; XANES results (section 3.2) also confirm an oxidation state of 4+ for Mn. In addition, the strongly asymmetrical local environment of Tl is typical of the lone-pair ion Tl+ and absolutely inappropriate for Tl3+. EDX analysis was performed for only three of the four compounds. Although the Mn/Te ratio in the Na compound agrees with the titration results, the overall accuracy of EDX is poor. Both the Mn/Te ratio in Tl2MnTeO6 and the Na/Te ratio in Na2MnTeO6 are impossibly large. Then, we may assume that the residual sodium content in the thallium compound is also overestimated. Because the surface of the sample becomes negatively charged under an electron beam, there may be some electromigration of Na+ ions, their reduction, and accumulation of sodium at the surface. 3.2. X-ray Absorption Near Edge Spectroscopy. Because analytical data for Tl2MnTeO6 are somewhat questionable (see section 3.1.3), we performed XANES measurements (Figure 5) to check the oxidation state of manganese taking the silver analogue as a reference because its composition was established most reliably with a Mn oxidation state very close to 4+. The positions of both the main absorption edge and the pre-edge feature (dashed rectangle in Figure 5) are known to be sensitive to the valence state of a

As an example of preparing solid solutions between different A 2 MnTeO 6 forms, we reacted equimolar amounts of Na2MnTeO6 and AgNO3, targeting 50% silver substitution for sodium. As in all similar cases,10 the exchange was completely irreversible: there was no Ag+ in the washings, and XRD of the washed product, AgNaMnTeO6, showed both lattice parameters and intensities intermediate between those for Na2MnTeO6 and Ag2MnTeO6 (Figures 3 and 4).

Figure 4. Comparison of XRD patterns of NaAgMnTeO6 and its end members: (a) Na2MnTeO6, (b) NaAgMnTeO6, and (c) Ag2MnTeO6.

Incomplete homogeneity of the solid solution is manifested in broadening of the Bragg reflections, especially those with a high index l, because lattice parameter c is more sensitive to the size of A+ ions. 3.1.3. Analytical Data. Results of elemental analyses of Na2MnTeO6 (sample 6) and its exchange products are listed in Table 1. As mentioned in section 2.2, there was a problem with redox analysis of Tl2MnTeO6, because Tl+ was also oxidized by permanganate but slowly, so that the equivalence point could not be determined definitively. Then, after addition of KMnO4 to oxidize all Te(4+), all residual Fe2+, and some Tl+, the solution was heated, NaCl was added, and the titration was completed with a standard KBrO3 solution to effectively oxidize Tl+ to Tl3+. However, from thus obtained data, neither Mn nor Tl content could be determined. Therefore, Tl was determined gravimetrically as TlBr after addition of Na2SO3 (to reduce Tl3+ back to Tl+) and NaBr. Precipitation of less soluble TlI might be preferable, but this was precluded by side reactions with Te compounds resulting in a black precipitate (presumably, amorphous Te). Due to the considerable solubility of TlBr in water, the Tl content should be underestimated. Then, the Mn content calculated from the redox titration data on the basis of this Tl content should also be underestimated. These data in Table 1 are questionable.

Table 1. Color and Analytical Data for A2MnTeO6 (A = Na, Li, Ag, or Tl)a chemical analysis results A+ (wt %)

Mn(4+) (wt %) A

color

found

calcd

Na Li Ag Tl

red orange black black

16.29(1) 18.24(1) 10.67(9) >7.2

16.93 18.79 11.12 7.99

found

EDX results (molar ratios) calcd

43.78(13) >56.2

43.70 59.47

Mn/Te

A/Te

0.95(5) 0.910(13)

2.5(5)

1.5(8)

1.90(5)

Na/Te 0.010(16) 0.10(7)

a

Uncertainties in the last decimal digit are shown in parentheses. D

DOI: 10.1021/acs.inorgchem.8b03445 Inorg. Chem. XXXX, XXX, XXX−XXX

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distances and corresponding bond valence sums are listed in Table 2. The crystal structures are illustrated in Figures 7 and 8. The structure of Na2MnTeO6 effectively represents analogues with A = Li and Ag, whereas the Tl counterpart requires special discussion and is shown separately. In the isostructural Na2MTeO6 phases,6 NaO6 octahedra share one face with the smaller of two other cations: Ge(4+) in Na2GeTeO6 and Te(6+) in Na2SnTeO6. This is exactly what we see in our A2MnTeO6: AO6 octahedra share one face with the smaller of the two higher-valence cations, Mn(4+), whereas TeO6 octahedra avoid sharing faces (Figure 7). Strictly speaking, these “octahedra” are trigonal antiprisms; A+ ions are shifted off-center to avoid short contacts with Mn(4+) and have three short and three long A−O distances. Their ratios are 1.19, 1.07, 1.11, and 1.58 for A = Li, Na, Ag, and Tl, respectively. It is evident that Tl+ differs drastically from the other three cations. This is due to its lone-pair effect. Actually, the six oxygen atoms closest to Tl are all from the same plane, and the three oxygens shared with Mn are only seventh, eighth, and ninth neighbors (Figure 8). The three nearest oxygens provide an essentially correct bond valence sum of 0.99 for Tl, whereas six others add only 0.11 (Table 2). Therefore, the actual coordination number for Tl is only three, and TlO6 polyhedra of Figure 7 should not be taken seriously. As in other layered structures, substitution of larger univalent cations A affects mostly interlayer spacing whereas intralayer lattice parameters depend mostly on the nature of higher-valence cations, here Mn and Te. Within the group of

Figure 5. Mn K edge HERFD-XANES spectra of A2MnTeO6 (A = Ag or Tl).

probed element, shifting toward higher energies with an increase in oxidation state.11−13 Figure 5 reveals no detectable shifts with respect to each other in the pre-edge region, and insignificant variations in the main absorption edge, thus indicating no or negligible difference in the Mn oxidation state in these compounds. 3.3. Crystal Structures. The Rietveld refinement confirmed that all four new phases are isostructural to each other and to 2H Na2GeTeO6. Experimental and calculated profiles are compared in Figure 6, and refinement details and atomic parameters are listed in Table S2. Principal interatomic

Figure 6. Results of Rietveld refinement for A2MnTeO6 with (a) A = Na, (b) A = Ag, (c) A = Li, and (d) A = Tl. Legend: stars, experimental points; line, calculated profile; line at the bottom, difference profile; vertical bars, Bragg positions. E

DOI: 10.1021/acs.inorgchem.8b03445 Inorg. Chem. XXXX, XXX, XXX−XXX

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Table 2. Principal Interatomic Distances (L), Sums of Ionic Radii,7 Bond Valences (S14), and Their Sums (BVS) in A2MnTeO6 (A = Li, Na, Ag, or Tl) Li2MnTeO6

Na2MnTeO6

Tl2MnTeO6

L (Å)

S

S

L (Å)

S

A−O

2.019(7) × 3 2.395(13) × 3

0.225 × 3 0.126 × 3

2.322(4) × 3 2.484(4) × 3

0.225 × 3 0.153 × 3

2.374(6) × 3 2.631(6) × 3

0.249 × 3 0.122 × 3

sum of radii/BVS A−A Mn−O sum of radii/BVS Te−O sum of radii/BVS BVS for O

2.12 3.092(11) × 3 1.876(4) × 6 1.89 1.960(4) × 6 1.92

1.05

2.38 3.0646(13) × 3 1.844(4) × 6 1.89 1.974(4) × 6 1.92

1.13

2.51 3.0747(4) × 3 1.851(7) × 6 1.89 1.980(7) × 6 1.92

1.11

0.713 × 6 4.28 0.909 × 6 5.45 1.97

L (Å)

Ag2MnTeO6

0.778 × 6 4.67 0.875 × 6 5.25 2.03

0.763 × 6 4.57 0.861 × 6 5.17 1.99

L (Å) 2.532(8) × 3.578(7) × 3.989(9) × 2.86 3.5862(12) 1.864(8) × 1.89 1.930(8) × 1.92

S 3 3 3 ×3 6 6

0.329 × 3 0.028 × 3 0.010 × 3 1.10 0.738 × 6 4.43 0.979 × 6 5.87 2.08

off-center displacement should not be as strong as with Tl+. Then, K−K distances should be much smaller than Tl−Tl distances (∼3.1 Å), probably too short for such a large cation. Therefore, we conclude that the instability of Na2GeTeO6 type K2GeTeO6 and K2MnTeO6 may be mainly due to strong K−K repulsion at unavoidably short distances. For pyrochlore type tellurate reported as “K2GeTeO6”,15 an even shorter K−K distance of 2.83 Å may be calculated, but this composition is definitely wrong.16 The reported values of its experimental and calculated density, 4.40 and 4.46 g/cm3, respectively,15 agree only assuming an incorrect Z of 7. With a correct Z of 8 for space group Fd3̅m, the calculated density should be 5.10. This discrepancy with the experimental density means that the chemical composition was incorrect and actual K−K distances might be larger. In the Li and Tl compounds, partial Mn/Te inversion was found (Table S2). However, we suppose that this is not actual intralayer disorder but results from faulty stacking of completely ordered layers. Note that the largest degree of inversion (p = 14.6%) is found in the thallium compound where the largest interlayer spacing results in the weakest interlayer coupling. This is further supported by anomalous broadening of certain reflections of the thallium compound (Figure 6d) directly indicating stacking faults. 3.4. Magnetic Properties. From the point of view of the magnetic sublattice, all compounds under study have a similar organization. The magnetic layers of Mn4+ and Te6+ cations alternate with nonmagnetic layers of metal cations A+ (A = Ag, Li, Na, or Tl) along the c axis. Magnetic Mn4+ ions in an octahedral oxygen environment form a triangular network (Figure 1). The MnO6 octahedra are isolated, so main intralayer exchange coupling takes place via superexchange paths Mn−O−Te−O−Mn. The structural arrangement provides conditions for reducing the dimension of magnetic exchange interactions and frustration of the magnetic subsystem. The temperature dependencies of the magnetic susceptibility (χ = M/B) of four manganese tellurates A2MnTeO6 (A = Ag, Li, Na, or Tl), measured at B = 0.1 T, are shown in Figure 9. In weak magnetic fields, the samples show Curie−Weiss type behavior in terms of the magnetic susceptibility and do not show obvious signs of the establishment of long-range magnetic order. The magnetic susceptibility, measured in the field-cooled (FC) and zero-field-cooled (ZFC) modes, does not show any discrepancy. In the high-temperature range, the magnetic susceptibility can be satisfactorily described within the framework of the

Figure 7. Polyhedral view of crystal structures of Na2MnTeO6 (left) and Tl2MnTeO6 (right). Legend: cyan spheres, oxygen; pink octahedra, MnO6; yellow octahedra, TeO6; green octahedra, NaO6; gray polyhedra, TlO6.

Figure 8. Local environment of a monovalent cation in Na2MnTeO6 (left) and Tl2MnTeO6 (right). Legend: thick bonds, shortest contacts; thin bonds, next short A−O contacts.

four A2MnTeO6 compounds, lattice parameter c changes by 54% from A = Li to A = Tl whereas a varies by only 2%. In accordance with this, the A−A distances in A2MnTeO6 for A = Li, Na, and Ag are very similar, 3.09, 3.06, and 3.07 Å, respectively, and only slightly longer than a/√3 values of ≈2.89−2.96 Å. However, in the Tl compound, the alternation of strong up and down displacements along the 3-fold axis (Figure 8) makes Tl−Tl distances much longer (3.59 Å). At this point, we can explain why K2MnTeO6 could not adopt this structure type, although the ionic radius of K+ is intermediate between those of Ag+ and Tl+. For K+ having no lone pair, a coordination number of three is impossibly small. Like Na+ and Ag+, it should have six similar distances to oxygen; i.e., its F

DOI: 10.1021/acs.inorgchem.8b03445 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 9. Temperature dependencies of the magnetic susceptibility for A2MnTeO6 (A = Ag, Li, Na, or Tl) compounds at B = 0.1 T. The solid red curves are approximations in accordance with the Curie−Weiss law. The insets shows the specific heat at zero field in comparison with magnetic susceptibility at strong magnetic fields highlighting the establishment of long-range order at low temperatures.

Table 3. Main Parameters of the Magnetic Subsystem of A2MnTeO6 (A = Ag, Li, Na, or Tl), Obtained from the Approximation According to the Curie−Weiss Law and the Values of Tmax and TN Deduced from M/B(T) at High Magnetic Fields and C(T) Data, Respectively χdia (emu/mol)

sample Ag2MnTeO6 Li2MnTeO6 Na2MnTeO6 Tl2MnTeO6

−1.28 −0.94 −1.06 −1.63

× × × ×

−4

10 10−4 10−4 10−4

μeff (μB/Mn4+)

g factor

Θ (K)

Tmax (K)

TN (K)

f = Θ/TN

3.8 4.7 3.9 4.2

1.97 1.97 1.97 1.96

−14 −51 −19 −32

3.1 9.2 7.5 11

2.6 8.5 6.6 10.2

5.4 6 2.9 3.2

indicates the predominance of the antiferromagnetic exchange interactions in the samples under study. Effective g factors have been measured at room temperature by an ESR technique. Curie constants C were used to estimate effective magnetic moment μeff:

Curie−Weiss law with addition of the temperature-independent contribution χ0: χ = χ0 +

C T−Θ

(1)

where C is the Curie constant and Θ is the paramagnetic Weiss temperature. To reduce the number of variable parameters, the diamagnetic contribution was determined independently by summing the Pascal constants17 for the diamagnetic contributions of atoms constituting the formula unit and fixed during fitting. The main parameters of the magnetic subsystem, obtained from the approximation of the experimental data in the temperature range of 150−300 K according to the Curie− Weiss law, are listed in Table 3. As one can see from this table, for all samples the Weiss temperature is negative, which

μeff 2 = 3kBC /μB 2 NA

(2)

where kB is the Boltzmann constant, μB is the Bohr magneton, and NA is Avogadro’s number. For samples with sodium and silver, the experimental values of μeff are in good agreement with theoretical estimations in accordance with μtheor = ng 2S(S + 1) μB ≈ 3.8 μB for Mn4+ ions (S = 3/2) and using the g factor experimentally determined from the ESR data, while μeff takes slightly higher value for the samples with lithium and thallium. Such a discrepancy may be tentatively G

DOI: 10.1021/acs.inorgchem.8b03445 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 10. Field dependencies of the magnetization of A2MnTeO6 (A = Ag, Li, Na, or Tl) samples at 2 K (left). Field dependencies of the magnetization of Tl2MnTeO6 upon temperature variation (right).

attributed to the presence of the impurities, for example, Mn3+ (S = 2) or Mn2+ (S = 5/2). Remarkably, the application of a sufficiently high external magnetic field leads to significant changes in the behavior of the magnetic susceptibility. One can detect a clear maximum on χ(T) at low temperatures (see insets in Figure 9), which is characteristic of long-range antiferromagnetic order, instead of the monotonous Curie−Weiss type behavior of χ(T) observed at low magnetic fields. Moreover, the specific heat C(T) curves (see the insets of Figure 9) demonstrate distinct λ-shaped anomalies that give direct confirmation of the establishment of long-range magnetic order in all samples under study. Thus, all members of the new A2MnTeO6 (A = Ag, Li, Na, or Tl) magnetic family reveal a very unusual phenomenon of hidden magnetic order. A similar unique magnetically hidden order phenomenon has been recently suggested for frustrated 2D square lattice compound Sr2VO418,19 and for the mixed dimensionality system CuP2O6.20 It might be worth comparing A2MnTeO6 2D compounds studied in the work presented here with another Bi3Mn4O12(NO3) 2D magnet based on Mn4+ (S = 3/2) forming a honeycomb lattice, with which one can track both common and distinctive features in magnetic behavior. Bi3Mn4O12(NO3) was found to have a disordered ground state despite the large AF Weiss constant of −257 K.21,22 In contrast to A 2 MnTeO 6 compounds, the specific heat of Bi3Mn4O12(NO3) does not show any peaks between 0.4 and 300 K, confirming the absence of the LRO at zero magnetic field. At the same time, magnetization and neutron scattering studies have shown that applied magnetic fields induce a magnetic transition, in which the short-range order abruptly expands into a long-range order. The frustration index f = Θ/TN takes values of 3−6, which are larger than the standard values of 2−3 for 3D antiferromagnets and imply moderate frustration on the triangular lattice. Note also that the absolute value of the Néel temperature (TN), deduced from C(T) data at B = 0 T, is slightly lower than the Tmax in M/B(T) (see Table 3), whereas it correlates well with a maximum on the magnetic susceptibility derivative. Indeed, as Fisher has shown,23,24 the temperature dependence of specific heat C(T) for antiferromagnets with strong short-range interactions should follow the derivative of the magnetic susceptibility.

The magnetization isotherms at 2 K recorded in static magnetic fields in ZFC mode for the A2MnTeO6 (A = Ag, Li, Na, or Tl) family are presented in the left panel of Figure 10. The magnetization curves have an S type shape with no evidence of the spin-reorientation transitions induced by the magnetic field and are quite typical for paramagnets or easyplane antiferromagnets with strong short-range order correlations. With an increase in temperature, the magnetization isotherms become more linear (right panel in Figure 10), indicating a decrease in the contribution of the short-range exchange interactions.

4. CONCLUSIONS Na2MnTeO6 has been prepared by direct synthesis, and three of its analogues, A2MnTeO6 (A = Li, Ag, or Tl), have been obtained by ion exchange from the sodium precursor. They are isostructural and the first four magnetic representatives of the trigonal layered A2M(4+)TeO6 family. Because of the equal ionic radii of Mn(4+) and Ge(4+), there is a profound analogy between Na2MnTeO6 and Na2GeTeO6 in crystal geometry, polytype formation, and the absence of potassium analogues. Of course, the analogy vanishes as far as electronic properties are concerned: color, bandgap, redox behavior (including thermal decomposition), and magnetism. An increase in ionic radii in the series Li+, Na+, Ag+, and Tl+ results in a monotonous increase in the interlayer distance, hexagonal parameter c, and unit cell volume, whereas a remains almost unchanged. Magnetic studies at low magnetic fields do not show any features indicating an onset of long-range order down to 2 K. However, the specific heat data unambiguously reveal an establishment of long-range magnetic order at low temperatures. The changes in character of magnetic susceptibility in external magnetic fields are also in agreement with antiferromagnetic order. Relatively large negative values of the Weiss temperature indicate moderate frustration on the triangular lattice, which can be a reason for the unusual phenomenon of hidden magnetic order. H

DOI: 10.1021/acs.inorgchem.8b03445 Inorg. Chem. XXXX, XXX, XXX−XXX

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Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Gen. Crystallogr. 1976, A32, 751−767. (8) Larson, A. C.; Von Dreele, R. B. General Structure Analysis System (GSAS). Los Alamos National Laboratory Report LAUR 86748; Los Alamos National Laboratory: Los Alamos, NM, 2004. (9) Toby, B. H. EXPGUI, a graphical user interface for GSAS. J. Appl. Crystallogr. 2001, 34, 210−213. (10) Nalbandyan, V. B. Ion exchange as a simple and effective tool for screening possible cation conductors. J. Solid State Electrochem. 2011, 15, 891−900. (11) White, E. W.; McKinstry, H. A. Chemical Effect on X-ray Absorption-Edge Fine Structure. In Advances in X-ray Analysis; Springer: Boston, 1966; Vol. 9, pp 376−392. (12) Manceau, A.; Gorshkov, A. I.; Drits, V. A. Structural chemistry of Mn, Fe, Co, and Ni in manganese hydrous oxides: Part I. Information from XANES spectroscopy. Am. Mineral. 1992, 77, 1133−1143. (13) Manceau, A.; Marcus, M. A.; Grangeon, S. Determination of Mn valence states in mixed-valent manganates by XANES spectroscopy. Am. Mineral. 2012, 97, 816−827. (14) Gagné, O. C.; Hawthorne, F. C. Comprehensive derivation of bond-valence parameters for ion pairs involving oxygen. Acta Crystallogr., Sect. B: Struct. Sci., Cryst. Eng. Mater. 2015, B71, 562−578. (15) Amarilla, M.; Veiga, M. L.; Pico, C.; Gaitan, M.; Jerez, A. Synthesis and Characterization of the New Mixed Oxides M2(GeTe)O6 (M = K, Rb, Cs). Inorg. Chem. 1989, 28, 1701−1703. (16) Novikova, A. A.; Nalbandyan, V. B. Existence of minimum molar volumes (maximum packing densities) in morphotropic series of mixed oxides and fluorides. Crystallogr. Rev. 2013, 19, 125−148 (Supporting Information) . (17) Bain, G. A.; Berry, J. F. Diamagnetic Corrections and Pascal’s Constants. J. Chem. Educ. 2008, 85, 532. (18) Jackeli, G.; Khaliullin, G. Magnetically Hidden Order of Kramers Doublets in d1 Systems: Sr2VO4. Phys. Rev. Lett. 2009, 103, 067205. (19) Kim, B.; Khmelevskyi, S.; Mohn, P.; Franchini, C. Competing magnetic interactions in a spin-1/2 square lattice: Hidden order in Sr2VO4. Phys. Rev. B: Condens. Matter Mater. Phys. 2017, 96, 180405. (20) Nath, R.; Ranjith, K. M.; Sichelschmidt, J.; Baenitz, M.; Skourski, Y.; Alet, F.; Rousochatzakis, I.; Tsirlin, A. A. Hindered magnetic order from mixed dimensionalities in CuP2O6. Phys. Rev. B: Condens. Matter Mater. Phys. 2014, 89, 014407. (21) Smirnova, O.; Azuma, M.; Kumada, N.; Kusano, Y.; Matsuda, M.; Shimakawa, Y.; Takei, T.; Yonesaki, Y.; Kinomura, N. Synthesis, Crystal Structure, and Magnetic Properties of Bi3Mn4O12(NO3) Oxynitrate Comprising S = 3/2 Honeycomb Lattice. J. Am. Chem. Soc. 2009, 131, 8313. (22) Matsuda, M.; Azuma, M.; Tokunaga, M.; Shimakawa, Y.; Kumada, N. Disordered Ground State and Magnetic Field-Induced Long-Range Order in an S = 3/2 Antiferromagnetic Honeycomb Lattice Compound Bi3Mn4O12(NO3). Phys. Rev. Lett. 2010, 105, 187201. (23) Fisher, M. E.; Randall, J. T. Lattice statistics in a magnetic field, I. A two-dimensional super-exchange antiferromagnet. Proc. R. Soc. A 1960, 254, 66−85. (24) Fisher, M. E. Relation between the specific heat and susceptibility of an antiferromagnet. Philos. Mag. 1962, 7, 1731−1743.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b03445. Starting materials, synthesis and characterization of NaMnO 2 , synthesis and characterization of Na 2 MnTeO 6 , attempts to directly synthesize K2MnTeO6, attempts to directly synthesize Li2MnTeO6, and structural data for A2MnTeO6 (A = Li, Na, Ag, or Tl) (PDF) Accession Codes

CCDC 1882767−1882770 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Vladimir B. Nalbandyan: 0000-0002-8624-0165 Alexander N. Vasiliev: 0000-0003-3558-6761 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Russian Foundation for Basic Research through Grant 18-02-00326. The authors thank Drs. S. I. Shevtsova and Yu. V. Popov (SFU’s Shared Use Centre “Research in Mineral Resources and Environment”) for the EDX analyses. The XRD study was supported by Grant-in-Aid 00-15 from the International Centre for Diffraction Data. The XAS study was carried out within the state assignment of Minobrnauki of Russia (theme “Electron” No. AAAA-A18118020190098-5). A.N.V. is thankful for the support by the Ministry of Education and Science of the Russian Federation through NUST “MISiS” Grant K2-2017-084 and by Act 211 of the Government of Russia (Contracts 02.A03.21.0004 and 02.A03.21.0011).

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REFERENCES

(1) Misguich, G.; Lhuillier, C. Frustrated Spin Systems; World Scientific: Singapore, 2004. (2) Richter, J.; Schulenburg, J.; Honecker, A. In Quantum Magnetism, Lecture Notes in Physics; Schollwöck, U., Richter, J., Farnell, D. J. J., Bishop, R. F., Eds.; Springer: Berlin, 2004; Vol. 645. (3) Lacroix, C.; Mendels, P.; Mila, F. Introduction to Frustrated Magnetism: Materials, Experiments, Theory; Springer: Berlin, 2011. (4) Witczak-Krempa, W.; Chen, G.; Kim, Y. B.; Balents, L. Correlated Quantum Phenomena in the Strong Spin-Orbit Regime. Annu. Rev. Condens. Matter Phys. 2014, 5, 57−82. (5) Vasiliev, A.; Volkova, O.; Zvereva, E.; Markina, M. Milestones of low-D quantum magnetism. npj Quantum Materials 2018, 3, 18. (6) Woodward, P. M.; Sleight, A. W.; Du, L.-S.; Grey, C. P. Structural Studies and Order-Disorder Phenomenon in a Series of New Quaternary Tellurates of the Type A2+M4+Te6+O6 and A1+2M4+Te6+O6. J. Solid State Chem. 1999, 147, 99−116. (7) Shannon, R. D. Revised Effective Ionic Radii and Systematic Studies of Interatomic Distances in Halides and Chalcogenides. Acta I

DOI: 10.1021/acs.inorgchem.8b03445 Inorg. Chem. XXXX, XXX, XXX−XXX