Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
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Sr(M,Te)2O6 (M = Cr, Mn, Fe, Co, Ni): A Magnetically Dilute Family of Honeycomb Tellurates Alannah M. Hallas* and Emilia Morosan*
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Department of Physics and Astronomy and Rice Center for Quantum Materials, Rice University, Houston, Texas 77005, United States ABSTRACT: We report the synthesis and characterization of five new honeycomb tellurates with the PbSb2O6 structure type. These materials, SrM2/3Te4/3O6 (M = Cr, Mn, Fe) and SrM1/2Te3/2O6 (M = Co, Ni), are prepared by conventional solid state synthesis. Rietveld refinements of powder X-ray and neutron diffraction data reveal that these materials crystallize in the P3̅1m space group, in which M and Te are octahedrally coordinated and randomly distributed over the honeycomb sublattice. In three of these materials (M = Cr, Mn, Fe), the transition metal takes a 3+ oxidation state, and hence, charge neutrality necessitates that the honeycomb sublattice is 66% occupied by nonmagnetic Te. In the remaining two of these materials (M = Co, Ni), the transition metal takes a 2+ oxidation state, requiring that 75% of the honeycomb lattice is occupied by nonmagnetic Te. Thus, the honeycomb sublattice is highly diluted, as only 33% or 25% of the sites are occupied by a magnetic ion. These occupation values fall well-below the percolation threshold for the honeycomb lattice, pc = 70%. Accordingly, magnetic susceptibility measurements reveal no magnetic order or spin freezing down to 1.8 K.
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INTRODUCTION The crystal structure of PbSb2O6 was originally solved within the P312 space group in 1941 by Magneli1 and subsequently revised to the higher symmetry P3̅1m space group by Hill in 1987.2 This quasi-two-dimensional trigonal structure consists of alternating triangular layers of Pb and honeycomb layers of Sb. The triangular layers are formed by isolated trigonally distorted PbO6 octahedra, with point group symmetry D3d. The honeycomb layers are formed by an edge sharing network of trigonally distorted SbO6 octahedra, with point group symmetry D3. The layers are stacked such that the isolated PbO6 octahedra sit above and below the vacancies in the honeycomb layers. This structure type is maintained for a wide range of materials, such as the antimonates ASb2O6 (A = Ba, Sr, Ca, Hg, Cd, Pb),1 the arsenates AAs2O6 (A = Sr, Ca, Hg, Cd, Pb, Co, Ni),1,3 and the ruthenate SrRu2O6.4 SrRu2O6, an especially intriguing member of this family, orders antiferromagnetically at TN = 560 K.5,6 This Neel temperature is among the highest known, which is surprising given this material’s quasi two-dimensional crystal structure. Perhaps the largest class of materials that adopt the PbSb2O6 structure type are the tellurates, AMTeO6, where A can be either a divalent or trivalent ion and M is then correspondingly either tetravalent or trivalent. In these tellurates, a number of different structural modifications are possible. (i) The first of these occurs when the M and Te cations are randomly distributed over the honeycomb sublattice, in which case the P3̅1m (No. 162) space group symmetry of the parent compound is preserved. This structure type has not been reported for any tellurates prior to now. (ii) In the second modification, the random distribution of the M and Te cations © XXXX American Chemical Society
is maintained, but the oxygen environments are altered from octahedral to trigonal prismatic. This modification corresponds to space group P6̅2m (No. 189) and has been reported for SrMnTeO67 and PbMnTeO6.8 (iii) A third possibility occurs when there is an intralayer site ordering of the octahedrally coordinated M and Te cations such that all the nearest neighbors for each M site are Te and vice versa. This corresponds to two interpenetrating triangular sublattices. Then the layers are stacked identically, such that there are columns of M and Te cations along the c axis. This structural modification can be generated within the noncentrosymmetric P312 (No. 149) space group without requiring any expansion of the unit cell and is observed in AGeTeO6 (A = Sr, Mn, Cd, Pb)9,10 and BiMTeO6 (M = Al, Ge).11 (iv) Another scenario can be realized when the M and Te cations are intralayer ordered, as in scenario iii. However, the stacking of layers alternates such that there are M−Te−M−Te chains along the c axis, resulting in a doubling of the unit cell. This structural modification belongs to space group P3̅1c (No. 163) and is reported for ACrTeO6 (A = La − Yb,12 Bi13) and LaMTeO6 (M = Al, Cr, Ga, Fe, Rh13). It is interesting to note that BiMnTeO 6 forms in a structure related to this last modification, but with a large monoclinic distortion attributed to the Jahn−Teller effect of Mn3+, giving space group P21/c (No. 14).14 (v) A final modification has been recently put forward by Jia et al. for the structure of PbGeTeO6,15 in conflict with the original report of P312 by Kim et al., described in scenario iii.10 This new report, which is based on Received: March 2, 2019
A
DOI: 10.1021/acs.inorgchem.9b00617 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Figure 1. Three of the possible structure types for tellurates in the trigonal PbSb2O6 family, illustrated for the case of the transition metal exhibiting a 2+ oxidation state such that the honeycomb lattice is 75% occupied by Te. In the (a) P3̅1m structure, the metal atoms have an octahedral oxygen coordination and are randomly distributed. In the (b) P6̅2m structure, the metal atoms have a trigonal prismatic oxygen coordination and are randomly distributed. In the (c) P312 structure, the metal atoms have an octahedral oxygen coordination and are site-ordered such that one site is fully occupied by Te and the other has a 50/50 occupation of M and Te. NiO). All precursors were initially ground together into a fine powder using an agate mortar and pestle and then pressed into a pellet. The pellets were placed in alumina crucibles and reacted in air at 650 °C for approximately 300 h, with three intermediate regrindings and repelletings. The M = Co sample was first annealed at 72 h at 550 °C, then 96 h at 60 °C, before being reacted at 650 °C. The M = Ni sample was first annealed at 600 °C for 48 h prior to being reacted at 650 °C. The samples were washed with dilute HNO3 to remove minor SrTeO4 or SrTe2O5 impurities. The final products are green (M = Cr), violet (M = Mn), reddish brown (M = Fe), light brown (M = Co), and pale green (M = Ni) polycrystalline powders (shown in the insets of Figure 2). Powder X-ray Diffraction. The phase purity of all samples was assessed using powder X-ray diffraction. Measurements were performed in a Bruker D8 Advance using a copper anode with λKα1 = 1.54056 Å and λKα2 = 1.54439 Å at T = 295 K. Measurements were performed between 10 ≤ 2θ ≤ 120° with a step size of 0.02°. Rietveld refinements of the X-ray diffraction data were performed with TOPAS.16 Powder Neutron Diffraction. Further structural investigation was carried out using the neutron time-of-flight powder diffractometer POWGEN at the Spallation Neutron Source at Oak Ridge National Laboratory.17 The samples were sealed in 6 mm diameter vanadium cans, and measurements were collected at T = 300 K. Data were collected in Bank 2 with a median wavelength of λ = 1.500 Å, allowing momentum transfers from 0.5 ≤ Q ≤ 12 Å−1. Rietveld refinements of the neutron diffraction data were performed with FullProf.18 Magnetic Susceptibility. Magnetic susceptibility measurements were performed using a Quantum Design Magnetic Property Measurement System SQUID magnetometer. Each sample was measured between 1.8 ≤ T ≤ 300 K in an applied magnetic field of 0.1 T. Diamagnetic contributions to the susceptibility from the sample and the sample holder were found to be negligible.
single crystal X-ray diffraction of crystals grown using the flux method, suggests that PbGeTeO6 crystallizes in the P31m (No. 157) space group.15 In this modification, the oxygen ions are split onto two distinct crystallographic sites that are inequivalent distances from the metal cations, giving a polar structure. In this manuscript, we report the synthesis and characterization of five new transition metal and tellurium-based members of the PbSb2O6 family of materials, SrM2/3Te4/3O6 (M = Cr, Mn, Fe) and SrM1/2Te3/2O6 (M = Co, Ni). The ratio of M to Te in this structure type varies depending on whether the oxidation state of the transition metal is 3+ (M:Te = 1:2) or 2+ (M:Te = 1:3). We performed Rietveld refinements of powder X-ray and neutron diffraction data that show these new materials form within the P3̅1m space group, in which the M and Te atoms are randomly distributed over the honeycomb sublattice. Consequently, the magnetic occupation is at most 33%, well-below the percolation threshold for the honeycomb lattice, pc = 70%. This is confirmed by magnetic susceptibility measurements, which reveal Curie−Weiss behavior for the whole measured temperature range, with no magnetic ordering down to 1.8 K.
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EXPERIMENTAL SECTION
Reagents. The reagents used in this study were SrCO3 (SigmaAldrich, > 99.9%), Cr2O3 (Alfa-Aesar, 99.97%), MnO (Alfa-Aesar, 99.99%), Fe 2 O 3 (Alfa-Aesar, 99.945%), Co 3 O 4 (Alfa-Aesar, 99.9985%), NiO (Alfa-Aesar, 99.998%), TeO2 (Alfa-Aesar, 99.99%), and telluric acid hydrate H2TeO4·2H2O (Alfa-Aesar, >99%). Synthesis. Polycrystalline samples of Sr(M,Te)2O6 (M = Cr, Mn, Fe, Co, Ni) were prepared via conventional solid state synthesis. The SrM2/3Te4/3O6 (M = Cr, Mn, Fe) samples were synthesized by combining SrCO3, TeO2, and H2TeO4·2H2O in a 3:2:2 ratio, with a stoichiometric quantity of the transition metal oxide (Cr2O3, MnO, or Fe2O3). The SrM1/2Te3/2O6 (M = Co, Ni) samples were synthesized by combining SrCO3, TeO2, and H2TeO4·2H2O in a 4:3:3 ratio, with a stoichiometric quantity of the transition metal oxide (Co3O4 or
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RESULTS AND DISCUSSION Crystal Structure. The title compounds in this work were initially obtained as the majority phase in an attempt to substitute nickel for copper in the SrCuTe2O7 structure.19 The
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DOI: 10.1021/acs.inorgchem.9b00617 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
structure successfully yielded SrCo1/2Te3/2O6, where cobalt is also in its 2+ oxidation state. However, our attempts to replace Co2+ by Mn2+ did not succeed, giving samples with large impurity phases. Instead, in this structure, the earlier transition metals (M = Cr, Fe, and Mn) take a 3+ oxidation state, requiring the starting composition to be modified, for example, in the case of SrCr2/3Te4/3O6: SrCO3 +
1 2 2 Cr2O3 + TeO2 + H 2TeO4 ·2H 2O 3 3 3
⇒ SrCr2/3Te4/3O6
Previously, Wulff and Müller-Buschbaum reported the flux growth of single crystalline SrMnTeO6, with manganese in a 4+ oxidation state.7 Our attempt to obtain this phase via solid state reaction in air was not successful. Similarly, Kim et al. also report a failed attempt to synthesize SrMnTeO6 via solid state reaction in an evacuated quartz tube, which, interestingly, is a method that succeeds for PbMnTeO6.8 Thus, it is possible that higher temperature synthetic routes are required to stabilize this phase. It is also possible that the crystals originally obtained by Wulff and Müller-Buschbaum were in fact SrMn2/3Te4/3O6, as their identification relies purely on single crystal X-ray diffraction and does not include an elemental analysis or any secondary confirmation of the oxidation state of Mn.7 The X-ray diffraction patterns for the five Sr(M,Te)O6 (M = Cr, Mn, Fe, Co, Ni) compounds are presented in Figure 2. In each case, all of the Bragg reflections can all be indexed within a trigonal PbSb2O6-type space group. As discussed in the introduction, there are several possible modifications of the PbSb2O6 structure-type. Site-ordering of the M and Te atoms in the P3̅1c structure leads to a doubling of the c axis. As we do not observe any superlattice reflections, we can immediately rule out this structure type. The P31m structure type was also ruled out as refinements in that space group gave negative thermal parameters. Thus, we are left to consider the P3̅1m (nonsite ordered, octahedral coordination), P6̅2m (nonsite ordered, trigonal prismatic coordination) and P312 (site ordered, octahedral coordination) structures. The differences between these three structures are illustrated in Figure 1 for the case of the transition metal having a 2+ oxidation state, where the honeycomb lattice is 75% occupied by tellurium. In each of these structures, the original PbSb2O6 unit cell is preserved. In our Rietveld refinements, the minimum values of χ2 for all five compounds are obtained with the P3̅1m structure. For example, in the case of SrCo1/2Te3/2O6 the χ2 values for P3̅1m, P6̅2m, and P312 are 2.18, 2.29, and 2.50, respectively. We can likely exclude P312, which despite having more adjustable structural coordinates gives the worst χ2 value. However, distinguishing between P3̅1m and P6̅2m is more challenging, as these structures differ exclusively in their oxygen positions, and laboratory X-rays are not sensitive to oxygen. In order to more definitively solve the crystal structures of Sr(M,Te)O6 (M = Cr, Mn, Fe, Co, Ni), we turned to powder neutron diffraction. The coherent scattering cross-section for oxygen is large, and thus, neutron diffraction allows us to distinguish between crystal structures that differ in only their oxygen positions. This is illustrated in Figure 3, which shows a comparison of Rietveld refinements for SrCo1/2Te3/2O6 in the P3̅1m and P6̅2m space groups with X-rays and neutrons. While the X-ray refinement is reasonable in both space groups, as
Figure 2. Rietveld refinement in the P3̅1m space group of the powder X-ray diffraction patterns for Sr(M,Te)2O6 (a) M = Cr, (b) M = Mn, (c) M = Fe, (d) M = Co, and (e) M = Ni measured at room temperature with Cu Kα radiation. The inset shows the color of each powder.
starting composition was altered until a single phase material was obtained, corresponding to SrNi1/2Te3/2O6: 1 3 3 SrCO3 + NiO + TeO2 + H 2TeO4 ·2H 2O 2 4 4 ⇒ SrNi1/2Te3/2O6
(2)
(1)
With this composition, charge neutrality requires that tellurium is in its 6+ oxidation state while nickel is in its 2+ oxidation state. Direct replacement of nickel by cobalt in this C
DOI: 10.1021/acs.inorgchem.9b00617 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Figure 3. Comparison of the sensitivity of powder (a) X-ray and (b) neutron diffraction to the oxygen positions in the PbSb2O6 structure type. Whereas it is difficult to distinguish between P3̅1m and P6̅2m using X-ray diffraction, neutrons allow a definitive determination. Note that the residual for P6̅2m in the neutron refinement has been scaled down by 80% to fit within the axes.
evidenced by the similar χ2 values, the neutron refinements are dramatically different giving χ2 values of 9.6 and 373 for P3̅1m and P6̅2m, respectively. Note that the residual for P6̅2m presented in Figure 3b has been multiplied by 0.2 to fit on the scale (i.e., scaled down by 80%). Refinements within P312 were also attempted and this lower symmetry structure did not yield any improvements to the goodness-of-fit indicators. Thus, neutron diffraction allows us to definitively assign the crystal structures of the Sr(M,Te)O6 (M = Cr, Mn, Fe, Co, Ni) materials as belonging to the P3̅1m space group. The Rietveld refinements of the powder neutron diffraction patterns are presented in Figure 4, and the lattice parameters and goodnessof-fit indicators are summarized in Table 1 and Table 2. It is interesting that previous studies have shown BiMnTeO6 to have a very large monoclinic distortion due to the Jahn− Teller effect of Mn3+.14 No such distortion would be expected, nor is it observed, in the case of PbMnTeO6, where Mn has a 4+ oxidation state. 8 However, in our new material SrMn2/3Te4/3O6, Mn does take a 3+ oxidation state and no Jahn−Teller distortion is observed. This can be likely be explained by the fact that in BiMnTeO6, the Mn3+ are siteordered, while in the case of SrMn2/3Te4/3O6, they are randomly distributed. It is also possible that a small admixture of Mn4+ is present in our material, which could explain why the unit cell does not expand upon replacing Cr3+ by Mn3+. Magnetic Properties. The magnetic susceptibility, χ(T), for the five Sr(M,Te)2O6 (M = Cr, Mn, Fe, Co, and Ni) compounds between 1.8 and 300 K is presented in Figure 5 (filled symbols, left-hand axis). The magnetism in these materials originates only from the transition metal cations. There was no splitting observed between measurements performed under field-cooled and zero field-cooled conditions. The inverse susceptibility, χ−1(T), is also plotted in Figure 5 (open symbols, right-hand axis). For each of these compounds except M = Co, Curie−Weiss behavior, χ−1 = (T − ΘCW)/C, was observed over a wide temperature interval, from 10 to 300
Figure 4. Rietveld refinement in the P3̅1m space group of the powder neutron diffraction patterns for Sr(M,Te)2O6 (a) M = Cr, (b) M = Mn, (c) M = Fe, (d) M = Co, and (e) M = Ni measured at room temperature with the time-of-flight diffractometer POWGEN, with median wavelength λ = 1.5 Å.
K (solid lines in Figure 5). The results of these Curie−Weiss fittings are summarized in Table 3. Deviations from Curie− Weiss behavior below 10 K can be attributed to a minor Curie tail contribution arising from impurity spins. Only one electronic configuration is possible for M = Cr3+ and Ni2+, which is t32g (S = 3/2) for Cr3+ and t62ge2g (S = 1) for Ni2+, respectively. The computed paramagnetic moments, μeff = 8C μB, are in good agreement with the values typically observed for these ions. For each of M = Mn3+, Fe3+, and Co2+, D
DOI: 10.1021/acs.inorgchem.9b00617 Inorg. Chem. XXXX, XXX, XXX−XXX
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Table 1. Crystallographic Properties of the Title Compounds Including Lattice Parameters and Goodness-of-Fit Indicators M (g/mol)
S.G.
SrCr2/3Te4/3O6
388.42
P3̅1m
SrMn2/3Te4/3O6
390.38
P3̅1m
SrFe2/3Te4/3O6
390.98
P3̅1m
SrCo1/2Te3/2O6
404.48
P3̅1m
SrNi1/2Te3/2O6
404.36
P3̅1m
a
X-ray neutronb X-ray neutron X-ray: neutron: X-ray: neutron: X-ray: neutron:
a (Å)
c (Å)
V (Å3)
Z
ρ (g/cm3)
Rp
Rwp
Rexp
χ2
5.1698(5) 5.1683 5.1720(2) 5.1732 5.2019(5) 5.1979 5.2260(2) 5.2247 5.2041(4) 5.2056
5.3791(5) 5.3788 5.3689(2) 5.3701 5.3594(5) 5.3543 5.3670(2) 5.3653 5.3797(5) 5.3812
124.50(3) 124.42 124.37(1) 124.46 125.60(3) 125.28 126.94(1) 126.84 126.17(2) 126.29
1 1 1 1 1 1 1 1 1 1
5.180(1) 5.185 5.212(1) 5.234 5.169(1) 5.183 5.291(1) 5.296 5.322(1) 5.317
6.8 20.3 7.7 21.4 8.8 18.9 7.9 17.9 7.1 18.8
9.3 12.9 10.6 12.4 12.8 12.4 10.8 11.7 9.9 12.1
5.7 3.2 6.8 4.1 6.7 3.2 7.3 3.7 5.6 3.9
2.7 16.7 2.4 9.2 3.6 14.8 2.2 9.8 3.2 9.8
X-ray measurements were collected at room temperature using Cu Kα radiation (λKα1 = 1.54056 Å, λKα2 = 1.54439 Å). bNeutron measurements were collected at room temperature using a time-of-flight diffractometer with median wavelength λ = 1.500 Å. Refined error bars on the lattice parameters were smaller than one part in ten thousand. a
Table 2. Atomic Coordinates and Thermal Parameters for the Title Compounds Refined from the Powder Neutron Diffraction Measurements atom
x
site
Sr Cr Te O
1a 2d 2d 6k
Sr Mn Te O
1a 2d 2d 6k
Sr Fe Te O
1a 2d 2d 6k
Sr Co Te O
1a 2d 2d 6k
Sr Ni Te O
1a 2d 2d 6k
0 /3 1 /3 0.3769(1) 1
0 /3 1 /3 0.3785(1) 1
0 /3 1 /3 0.3792(1) 1
0 /3 1 /3 0.3774(1) 1
0 /3 1 /3 0.3763(1) 1
y
z
SrCr2/3Te4/3O6 0 0 2 1 /3 /2 2 1 /3 /2 0 0.2996(2) SrMn2/3Te4/3O6 0 0 2 1 /3 /2 2 1 /3 /2 0 0.2992(2) SrFe2/3Te4/3O6 0 0 2 1 /3 /2 2 1 /3 /2 0 0.2976(2) SrCo1/2Te3/2O6 0 0 2 1 /3 /2 2 1 /3 /2 0 0.2974(2) SrNi1/2Te3/2O6 0 0 2 1 /3 /2 2 1 /3 /2 0 0.2980(2)
Baiso 0.59(3) 0.23(2) 0.23(2) 0.66(2) 0.99(3) 1.29(5) 1.29(5) 1.12(2) 0.83(2) 0.26(2) 0.26(2) 0.75(1) 0.81(3) 0.52(3) 0.52(3) 1.11(2) 0.82(3) 0.37(2) 0.37(2) 0.90(2)
occ. 1 /3 2 /3 1 1
1 /3 2 /3 1 1
1 /3 2 /3 1 1
1 /4 3 /4 1 1
1 /4 3 /4 1 1
a
Biso values for M and Te were jointly refined.
the calculated paramagnetic moments, μeff, are consistent with high-spin electron configurations, which are t32ge1g (S = 2), t32ge2g (S = 5/2), and t52ge2g (S = 3/2), respectively. The Curie−Weiss temperatures, ΘCW, are all negative, indicating net antiferromagnetic interactions. In the cases of M = Cr and Fe, the magnitude of ΘCW is larger than one might expect for such magnetically diluted systems, − 21.6 and −14.2 K, respectively. Figure 6 presents the temperature dependence of the paramagnetic moment, 8χ (T − ΘCW ) , where the dashed lines represent the computed values for the spin-only moments, μcalc = g S(S + 1) . Very good agreement is observed in the cases of M = Cr3+, Fe3+, and Mn3+. The experimentally observed moments in both M = Co2+ and Ni2+
Figure 5. Magnetic susceptibility for Sr(M,Te)2O6, (a) M = Cr, (b) M = Mn, (c) M = Fe, (d) M = Co, and (e) M = Ni, measured in an H = 0.1 T field (filled symbols, left-hand axis). The inverse susceptibility for each compound is fitted to the Curie−Weiss law (open symbols, right-hand axis).
exceed the calculated values due to moderate spin−orbit coupling, which leads to incomplete quenching of the orbital angular momentum, as is commonly observed for these ions. It should be noted that the observed moments for M = Co2+ and Ni2+ still fall significantly short of the strong spin−orbit Hund’s rules limit, which would be μcalc = 6.63 μB (J = 9/2) and μcalc = 3.55 μB (J = 5/2), respectively. E
DOI: 10.1021/acs.inorgchem.9b00617 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry Table 3. Magnetic Properties of the Title Compounds Derived from Curie−Weiss Fitsa SrCr2/3Te4/3O6 SrMn2/3Te4/3O6 SrFe2/3Te4/3O6 SrCo1/2Te3/2O6 SrNi1/2Te3/2O6
C (emu/mol-M K)
ΘCW (K)
μeff (μB)
1.75 2.75 4.04 2.96 1.27
−21.6 −4.2 −14.2 −9.1 −2.4
3.75 4.71 5.69 4.86 3.19
config. 3d3: 3d4: 3d5: 3d7: 3d8:
t32g t32ge1g t32ge2g t52ge2g t62ge2g
S 3
/2 2 5 /2 3 /2 1
The Curie−Weiss fits were performed between 10 and 300 K, except in the case of M = Co, which was fit between 200 and 300 K.
a
BiMTeO6 with M = Cr3+, Fe3+, and Mn3+ are ordered at 17, 11, and 9.5 K respectively.14 However, in this case, the M and Te cations are partially or fully ordered onto two triangular sublattices, where the percolation threshold is significantly smaller, pc = 50%. Kim et al. claim an antiferromagnetic ordering transition at TN = 20 K in PbMnTeO6,8 where the honeycomb sublattice has a random 50% magnetic occupancy. This transition is marked by a very small deviation from paramagnetic behavior in magnetic susceptibility measurements,8 rather than the prominent cusp that would typically be expected. Furthermore, long-range magnetic order in PbMnTeO6 is not corroborated by any other property measurement. We note that Kim et al. do report that repeated washings with dilute HNO3 were required to remove a Pb2MnTeO6 impurity.8 The double perovskite Pb2MnTeO6 has an antiferromagnetic ordering transition at TN = 20 K, marked by a prominent cusp in magnetic susceptibility measurements.24 We therefore propose that it is possible that a Pb2MnTeO6 impurity (≤2%, typical limit of detection for laboratory X-ray diffractometers) is responsible for the observed deviation in the magnetic susceptibility of PbMnTeO6. More definitive evidence on the presence or absence of magnetic order in this material could be obtained from heat capacity measurements or neutron diffraction.
Figure 6. Temperature dependence of the effective moment, μeff = 8χ (T − ΘCW ) , for Sr(M,Te)2O6 (M = Cr, Fe, Mn, Co, Ni). Deviations from a constant value indicate the breakdown of Curie−Weiss behavior. The dashed lines give the calculated moment for the associated spin value.
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CONCLUSIONS Using solid-state techniques, we have synthesized five new members of the tellurate PbSb2O6 family: SrM2/3Te4/3O6 (M = Cr, Mn, Fe) and SrM1/2Te3/2O6 (M = Co, Ni). These materials crystallize in the P3̅1m space group, such that the M and Te atoms are randomly distributed over the honeycomb sublattice. Magnetic susceptibility measurements reveal Curie− Weiss behavior over the whole measured temperature range, except for M = Co, which has moderately strong spin−orbit coupling. No long-range magnetic ordering is observed down to 1.8 K, consistent with the expectations for percolation theory on a honeycomb lattice.
No magnetic ordering or spin freezing transitions are observed in any of these five materials down to 1.8 K. The absence of magnetic ordering is unsurprising as the honeycomb lattice is heavily diluted, where the magnetic transition metal occupies only 33% (M = Cr, Mn, Fe) or 25% (M = Co, Ni) of the sites while nonmagnetic Te6+ occupies the remaining 67% or 75% of the sites. The classical percolation threshold for the honeycomb lattice, which considers only interactions between nearest neighbors, is pc = 70%, which means that the ability of the system to exhibit collective magnetic phenomena will be lost when fewer than 70% of the sites are occupied by the magnetic ion. Some materials do show rather good agreement with this classical limit; for example when nonmagnetic Mg2+ is substituted for Ni2+ in Ba(NipMg1−p)2V2O8 the destruction of magnetic order is consistent with pc = 70%.20,21 However, this percolation threshold does not account for any further neighbor interactions or quantum effects. Thus, in real materials, longrange order can be preserved to significantly higher levels of dilution. For example, dilution of magnetic Mn2+ with nonmagnetic Zn2+ in MnpZn1−pPS3 results in the loss of magnetic order at p = 46%.22,23 Thus, while the true percolation limit for these compounds may be smaller than the nearest neighbor value, pc = 70%, it is clearly still higher than our maximum magnetic occupancy of pc = 33%. Long-range magnetic order is observed in several other members of the tellurate PbSb2O6-family. For example,
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ASSOCIATED CONTENT
Accession Codes
CCDC 1877527−1877531 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 Authors
*(A.M.H.) E-mail:
[email protected]. *(E.M.) E-mail:
[email protected]. F
DOI: 10.1021/acs.inorgchem.9b00617 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry ORCID
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Alannah M. Hallas: 0000-0002-0892-0982 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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REFERENCES
The authors acknowledge support from the Gordon and Betty Moore Foundation EPiQS Initiative through Grant GBMF 4417. Research at ORNL’s Spallation Neutron Source was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy.
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DOI: 10.1021/acs.inorgchem.9b00617 Inorg. Chem. XXXX, XXX, XXX−XXX
