Structural Variability versus Structural Flexibility. A Case Study of

Nov 20, 2014 - Data reduction and integration, together with global unit cell refinements, were done by the INTEGRATE program incorporated in APEX2 so...
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Structural Variability versus Structural Flexibility. A Case Study of Eu9Cd4+xSb9 and Ca9Mn4+xSb9 (x ≈ 1/2) Xiao-Cun Liu,† Zhen Wu,† Sheng-Qing Xia,*,† Xu-Tang Tao,† and Svilen Bobev*,‡ †

State Key Laboratory of Crystal Materials, Institute of Crystal Materials, Shandong University, Jinan, Shandong 250100, P. R. China Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716, United States

‡

S Supporting Information *

ABSTRACT: The focus of this article is on the synthesis and structural characterization of the new ternary antimonides Eu9Cd4+xSb9 and Ca9Mn4+xSb9 (x ≈ 1/2). Although these compounds have analogous chemical makeup and formulas, which may suggest isotypism, they actually belong to two different structure types. Eu9Cd4.45(1)Sb9 is isostructural with the previously reported Eu9Zn4.5Sb9 (Pbam), and its structure has unit cell parameters a = 12.9178(11) Å, b = 23.025(2) Å, and c = 4.7767(4) Å. Ca9Mn4.41(1)Sb9 crystallizes in the orthorhombic space group Pnma with unit cell dimensions a = 12.490(2) Å, b = 4.6292(8) Å, and c = 44.197(8) Å and constitutes a new structure type. The two structures are compared and contrasted, and the structural relationships are discussed. Exploratory work aimed at the arsenic-based analogues of either type led to the identification of Ca9Zn4.46(1)As9, forming with the latter structure [a = 11.855(2) Å, b = 4.2747(8) Å, and c = 41.440(8) Å]. Differential thermal analysis and electrical resistivity measurements, performed on single crystals of Ca9Zn4+xAs9, indicate high thermal stability and semiconducting behavior. Magnetic susceptibility measurements on Eu9Cd4+xSb9 samples confirm the expected Eu2+ ([Xe]4f7) ground state.

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INTRODUCTION Zintl phases constitute a special subset of polar intermetallic compounds, often viewed as the bridge between the typical salts (ionic compounds) and typical metals. The connotation of the term “polar” here signifies the differences in the electronegativities of the constituting elements (and not polarity arising from the lack of inversion symmetry, for instance). Thus, it is not surprising that the classic examples of Zintl phases contain the alkali metal or alkaline-earth metals, and the main-group elements from groups 13−15.1,2 In these compounds, the electronegative elements form (poly)anions, so that the bonding satisfies the valence rules, while the electropositive metals act as the charge-balancing cations. By doing so, complete charge transfer between the cationic and anionic substructures is assumed, rendering such compounds as valence-precise semiconductors. In recent years, many new compounds have pushed the traditional boundaries of the Zintl concept.3−5 It is now wellunderstood that the valence rules are an oversimplification, and the primary reason for achieving closed-shell configurations is that the cations in these compounds are not fully “ionized”. As such, their roles also change: they are not simple electron donors (or spacers/spectators), and they can participate in covalent bonding with the (poly)anions.6 Ternary pnictides are among the most polar Zintl phases. They have attracted considerable interest because of their diverse crystal structures and unusual electronic structures and physical properties.7−12 Arguably, the best known and frequently studied compound in this series is Yb14MnSb11, © 2014 American Chemical Society

which has become the benchmark thermoelectric material in the high-temperature range.7,8 Not long ago, our team reported another Zintl phase with similar chemical makeup, Yb9Mn4+xSb9 (x ≈ 0.2),13 which subsequently was also shown to have high thermoelectric efficiency.14,15 For Yb9Mn4+xSb39, a noteworthy feature is the very low, glasslike lattice thermal conductivity, which can be correlated with the complex structure of this material. The roots of this interesting structure can be traced back to the late 1970s, when the first compound with analogous formula Ca9Mn4Bi9, was synthesized.16 According to the initial report, the anionic substructure comprises one-dimensional [Mn4Bi9]19− ribbons, separated by nine Ca2+ cations. This description of the structure is inconsistent with the electron count per the Zintl concept, yet it persisted in the literature for more than 2 decades. In 2004, the notion of electron deficiency was refuted, and a previously overlooked interstitial (albeit partially occupied) position was uncovered in Ca9Zn4+xSb9 and Yb9Zn4+xSb9 (0.2 < x < 0.5).17 The latter report laid the groundwork for revisiting the crystal structures of the whole family with the general formula A9M4Pn9, where A = alkaline-earth (Ca and Sr) or the rare-earth metals Eu and Yb, M = d5 or d10 metals, and Pn = pnictogen, i.e., a group 15 element. This task was achieved in 2007 in a paper that showed that the degree of offSpecial Issue: To Honor the Memory of Prof. John D. Corbett Received: September 26, 2014 Published: November 20, 2014 947

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For the Ca−Mn−Sb phase, Sn flux was not suitable (the major product was CaMn2Sb223). Better results were obtained using Pb as a metal flux, although Ca9Mn4.41(1)Sb9 appeared to be a minor product (small needles); the major phase was Ca21Mn4Sb1824 (irregular, blockshaped crystals). Powder X-ray Diffraction. The outcomes of all syntheses were monitored by powder X-ray diffraction. The powder X-ray diffraction patterns were taken at room temperature on a Bruker AXS powder Xray diffractometer, equipped with a monochromated Cu Kα source. The data were recorded in a 2θ mode with a step size of 0.02° and a counting time of 10 s and were used for phase identification only. In Figure S1 in Supporting Information (SI), we provide the experimental and simulated patterns ascertaining the quality of the samples, which were used for subsequent property measurements. Single-Crystal X-ray Diffraction. Intensity data sets were collected on a Bruker SMART CCD-based diffractometer. Monochromated Mo Kα radiation was utilized with full coverage of the reciprocal space up to sin θ/λ ≈ 0.75 Å−1, as managed by the Bruker software. Data reduction and integration, together with global unit cell refinements, were done by the INTEGRATE program incorporated in APEX2 software.25 Semiempirical absorption corrections based on equivalent reflections were applied using the program SADABS.26 The structures were solved by direct methods and refined by full matrix least-squares methods on F2 using SHELX.27 All three structures were refined to convergence with anisotropic displacement parameters (Table 1). Specific details of the structure solutions and refinements follow.

stoichiometry and the concomitant occupancy of the interstitial site can vary widely.18 For example, in Sr9Cd4.49(1)Sb9, nearly a quarter of the interstitial sites have been shown to be occupied, whereas in Yb9Cd4.01(1)Bi9, the interstitial positions have been shown to be nearly empty. A very delicate interplay between the sizes of the atoms of the constituent elements and their electronic properties (i.e., electronegativity) is believed to govern the formation of the structure and its electronic stability range. Following the hypothecation of the “A9M4+xPn9 structure map”, we have been trying to find more examples that support (or violate) the above-mentioned ideas. One of the initial targets was the hitherto unknown phase Eu9Cd4+xSb9, for which, because of the similar sizes of Sr2+ (1.17 Å) and Eu2+ (1.16 Å)19 and their similar electronegativities,20 we anticipated a large fraction of the interstitial sites to be occupied (close to a quarter), in analogy with Sr9Cd4.49(1)Sb9. With the same reasons in mind, we also aimed at Ca9Mn4+xSb9, expecting it to be analogous with Yb9Mn4+xSb9 (x ≈ 0.2)13 because Ca2+ (1.02 Å)19 and Yb2+ (1.00 Å)19 are very similar spatially, and have similar electronegativities.20 A third objective was probing the limits of our ideas by synthesizing arsenidies with this structure type: so far the only known representatives are antimonides and bismuthides.13,16−18,21,22 With this paper, we detail the results from our exploratory and directed work in the last 5−6 years, by recounting the synthesis and structural characterization of the new ternary antimonides Eu9Cd4+xSb9 and Ca9Mn4+xSb9 (x ≈ 1/2). Although these compounds have analogous chemical makeup and formulas, they belong to two different structure types. We also report on the first-of-a-kind arsenide Ca9Zn4.46(1)As9, forming with the newly identified Ca9Mn4+xSb9 structure type. The two structures are compared and contrasted, and the structural relationships are briefly discussed. Some basic physical properties of the title compounds are provided as well.

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Table 1. Selected Crystallographic Data for Eu9Cd4.45(1)Sb9, Ca9Mn4.41(1)Sb9, and Ca9Zn4.46(1)As9a empirical formula fw temperature (K) radiation space group Z a (Å) b (Å) c (Å) V (Å3) ρcal (g cm−3) μ (cm−1) collected reflns

EXPERIMENTAL SECTION

Synthesis. The elements were purchased from Alfa-Aesar (purity > 99.9%) and were used as received. To avoid oxidation of the reactive Ca and Eu, all manipulations of raw materials were performed in an argon-filled glovebox with an oxygen level below 0.1 ppm or under a vacuum. Eu9Cd4.45(1)Sb9. The crystal used in the structure determination resulted from the reaction of a mixture of the three elements in the proper molar ratio (2:1:2). The mixture was placed in a sealed Nb tube and then jacketed in an evacuated silica tube. The tube was heated up to 1000 °C at a rate of 100 °C h−1, and then it was kept at that temperature for 1 day and subsequently cooled to 800 °C over 100 h. The tube was kept at 800 °C for 3 days, followed by a slow cooling (−5 °C h−1) to the room temperature. The reaction product was not fully homogeneous, although small, needle-shaped crystals of the title compound appeared to be the bulk of it. The crystals were hand-picked and used for the structural analysis and magnetic susceptibility measurements. Reactions using metal fluxes were also tried with the idea to grow larger crystals, but they were unsuccessful. Ca9Mn4.41(1)Sb9 and Ca9Zn4.46(1)As9. Single crystals of these phases were grown using Sn and Pb as metal fluxes. The elements in the atomic ratio of 2:1:2 were loaded in alumina crucibles, which were topped off with flux (typically, in 20-fold excess over the 2:1:2 loading ratio). The alumina crucibles were subsequently sealed in evacuated fused silica tubes and were heated up to 1000 °C over 6 h, kept at that temperature for 20 h, and then cooled slowly at a rate of −3 °C h−1 to 600 °C, at which temperature the tube was quickly removed from the furnace, inverted, and spun in a centrifuge in order to remove the flux. Using this synthesis procedure and Sn flux, large (some up to 3−4 mm long) needlelike single crystals of Ca9Zn4.46(1)As9 could be synthesized.

data/param R1 [I > 2σ(I)]b wR2 [I > 2σ(I)]b

Eu9Cd4.45(1)Sb9

Ca9Zn4.46(1)As9

Ca9Mn4.41(1)Sb9

2963.01

1326.39 296(2)

1698.48

Pbam (No. 55) 2 12.9178(11) 23.025(2) 4.7767(4) 1420.8(2) 6.93 311.4 8450 (Rint = 0.030) 1824/77 0.0252 0.0567

Mo Kα, 0.71073 Å Pnma (No. 62) Pnma (No. 62) 4 4 11.855(2) 12.490(2) 4.2747(8) 4.6292(8) 41.440(8) 44.197(8) 2100.1(7) 2555.4(8) 4.20 4.42 212.8 132.3 24023 (Rint = 27444 (Rint = 0.097) 0.070) 2647/157 2954/156 0.0494 0.0465 0.0792 0.0868

a

For full details, see the SI. CIFs have also been deposited with Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany [fax (49) 7247-808-666; e-mail crysdata@fiz-karlsruhe. de) under depository numbers CSD 428524 (Eu9Cd4.45(1)Sb9), 428525 (Ca9Mn4.41(1)Sb9), and 428526 (Ca9Zn4.46(1)As9). bR1 = ∑||Fo| − |Fc||/ ∑|Fo| and wR2 = {∑[w(Fo2 − Fc2)2]/∑[w(Fo2)2]}1/2, where w = 1/ [σ2Fo2 + (AP)2 + BP] and P = (Fo2 + 2Fc2)/3; A and B are weight coefficients. In order to avoid possible decomposition by air/moisture, the single crystals were picked in an argon-filled glovebox fitted with a microscope. The crystals were cut to less than ca. 0.10 mm in all dimensions (as a way to mitigate potential issues stemming from high X-ray absorption) and mounted on glass fibers using Paratone N oil. After an initial screening for quality, the diffraction data for Eu9Cd4.45(1)Sb9 were worked out; the structure could be readily solved by direct methods in the space group of choice, Pbam (Table 948

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Figure 1. Idealized orthorhombic crystal structures of Eu9Cd4.45(1)Sb9 (a) and Ca9Mn4.41(1)Sb9 (b). The unit cells are outlined. Both structures are drawn in polyhedral representation, emphasizing the Cd- and Mn-based tetrahedral units. Coloring of the tetrahedra in Ca9Mn4.41(1)Sb9 is chosen to illustrate the notion that the two structures are interrelated and that the latter is made of two distinct types of slabs. The Eu or Ca atoms are shown as dark-red spheres, the Sb atoms are drawn as blue spheres, and the Cd or Mn atoms are shown in green or yellow, respectively. For both, the interstitial atoms, Cd3 and Mn3, are omitted for clarity. Three likely scenarios for the Ca1 positional disorder (small magenta sphere), which can be correlated with Mn4/Mn5 and Sb9A/Sb9B occupational disorder, are depicted in part b. 1). All fully occupied atomic positions were located by direct methods, suggesting isotypism with the Ca9Mn4Bi9-type structure (Pearson code oP44).28 Subsequent refinements showed a peak of significant height (25−30 e− Å−3) located at reasonable distances from the Sb atoms. When this site was assigned as partially occupied Cd, following the reasons already put forth in the article discussing Sr9Cd4.49(1)Sb9,18 the Fourier map flattened out and the structure was readily refined to convergence. All sites, including the interstitial position (ca. 25% occupation factor), were refined anisotropically. One might reason that the 1/4 occupancy of the Cd3 interstitial position could lead to a longrange-ordered structure in a unit cell with quadrupled volume, but the intensity data showed no weak unindexed reflections (see the SI). A schematic representation of the “average” structure is given in Figure 1. The diffraction data for Ca9Mn4.41(1)Sb9 and Ca9Zn4.46(1)As9 could not be processed in a straightforward way. First, the unit cell volume was nearly doubled compared to that of Ca9Zn4+xSb917 (Table 1; note that the c axes exceed 40 Å!). Second, the systematic absences were inconsistent with the Pbam space group, discussed above for Eu9Cd4.45(1)Sb9, used previously to refine all hitherto known A9M4+xPn9 structures. Instead, the reflection conditions indicated either Pnma (common centrosymmetric group) or Pna21 (its noncentrosymmetric subgroup). The former was favored based on the intensity statistics |E2 − 1| and was therefore chosen for structure solution by direct methods. Both compounds appeared to be isostructural, and the solutions confirmed this notion: the same models emerged from both solutions and exhibited the same problems. The structure is clearly a close relative of the “9-4-9” family, exhibiting the same building blocks (Figure 1). However, in the “supercell”, the well-ordered slabs that can be traced back to a known structure are stacked in an alternating fashion with disordered slabs of different topology. Several challenges (e.g., unphysical distances) were

encountered for both Ca9Mn4.41(1)Sb9 and Ca9Zn4.46(1)As9, and they warrant more attention here than a cursory remark. The easiest-to-handle disorder involved one of the Ca atoms: Ca1 had to be offset to a general position with 50% occupancy because it was not on a mirror plane. This positional disorder turned out to be concomitant with an occupation disorder at the Mn4 (Zn5) and Mn5 (Zn5) sites, both of which refined as nearly 50% occupied [an artifact of the ca. 2.5 Å distances between Ca1 and Mn4 (Zn5) and Mn5 (Zn5) or vice versa]. Small positional disorder was also uncovered at two other sites when they were refined anisotropically and showed very elongated thermal ellipsoids. After a suitable “split”, the corresponding positions were assigned as Mn6A/Mn6B and Sb9A/ Sb9B in the Ca9Mn4.41(1)Sb9 structure and as Zn6A/Zn6B and As9A/ As9B in the Ca9Zn4.46(1)As9 structure. None of these problems could be rectified if the structures were solved and refined in Pna21 or even in P1. Finally, the difference Fourier map showed an extra peak, bridging the two slabs together. The coordination environment of that site mirrors that of the Cd3 interstitial position in Eu9Cd4.45(1)Sb9 (above) and was assigned as partially occupied Mn3 (Zn3). Taking all of the above into consideration, the refined formulas become Ca9Mn4.41(1)Sb9 and Ca9Zn4.46(1)As9. Because there are no other compounds with this structure, we propose this new type to be named after Ca9Mn4.41(1)Sb9. The final positional and isotropic thermal parameters and important distances for Eu9Cd4.45(1)Sb9 and Ca9Mn4.41(1)Sb9 are listed in Tables 2 and 3, respectively. The tables for the third structure, Ca9Zn4.46(1)As9, as well as the combined crystallographic information file (CIF) of all three compounds are given in the SI. Electron Microscopy. To verify the refined compositions, energydispersive X-ray spectrometry analyses were carried out on selected single crystals. The instrument employed was an electron microscope (JEOL 7400 F) equipped with an Oxford INCA spectrometer. The 949

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Table 2. Atomic Coordinates and Equivalent Isotropic Displacement Parameters (Ueqa) of Eu9Cd4.45(1)Sb9 and Ca9Mn4.41(1)Sb9 atom

site

x

Eu1 Eu2 Eu3 Eu4 Eu5 Cd1 Cd2 Cd3b Sb1 Sb2 Sb3 Sb4 Sb5

2a 4h 4h 4h 4g 4g 4g 4h 2c 4h 4h 4g 4g

0 0.3612(1) 0.0554(1) 0.1187(1) 0.2849(1) 0.2224(1) 0.3767(1) 0.1208(4) 0 0.1896(1) 0.3707(1) 0.0371(1) 0.1855(1)

Ca1c Ca2 Ca3 Ca4 Ca5 Ca6 Ca7 Ca8 Ca9 Mn1 Mn2 Mn3b Mn4c Mn5c Mn6d Sb1 Sb2 Sb3 Sb4 Sb5 Sb6 Sb7 Sb8 Sb9e

8d 4c 4c 4c 4c 4c 4c 4c 4c 4c 4c 4c 4c 4c 4c 4c 4c 4c 4c 4c 4c 4c 4c 4c

y

Eu9Cd4.45(1)Sb9 0 0.3635(1) 0.4091(1) 0.2405(1) 0.1031(1) 0.455(1) 0.2590(1) 0.1092(2) 1 /2 0.0045(1) 0.1922(1) 0.1488(1) 0.3337(1) Ca9Mn4.41(1)Sb9 0.0152(4) 0.163(1) 1 0.0251(3) /4 1 /4 0.2419(3) 1 /4 0.2897(3) 1 /4 0.3015(3) 1 /4 0.3057(3) 1 /4 0.5528(3) 1 /4 0.5748(3) 1 /4 0.8480(4) 1 /4 0.4410(2) 1 /4 0.5918(2) 1 0.8201(6) /4 1 0.0697(4) /4 1 0.8487(4) /4 1 0.3982(4) /4 1 /4 0.2509(1) 1 /4 0.2751(1) 1 /4 0.5063(1) 1 /4 0.5523(1) 1 /4 0.6060(1) 1 /4 0.6210(1) 1 /4 0.8756(1) 1 /4 0.8123(1) 1 0.0556(2) /4

z

Table 3. Selected Interatomic Distances in Eu9Cd4.45(1)Sb9 and Ca9Mn4.41(1)Sb9 atom pair

Ueq (Å2)

0 1 /2 1 /2 1 /2 0 0 0 1 /2 0 1 /2 1 /2 0 0

0.015(1) 0.014(1) 0.014(1) 0.018(1) 0.024(1) 0.016(1) 0.018(1) 0.030(1) 0.013(1) 0.015(1) 0.016(1) 0.012(1) 0.011(1)

0.4193(1) 0.3272(1) 0.6714(1) 0.5794(1) 0.3749(1) 0.7524(1) 0.6910(1) 0.5511(1) 0.5011(1) 0.2540(1) 0.3595(1) 0.6835(2) 0.6096(1) 0.5713(1) 0.4974(1) 0.4496(1) 0.2984(1) 0.6204(1) 0.7797(1) 0.4683(1) 0.2952(1) 0.6297(1) 0.3755(1) 0.5459(1)

0.029(1) 0.024(1) 0.019(1) 0.024(1) 0.017(1) 0.029(1) 0.015(1) 0.034(1) 0.056(1) 0.017(1) 0.016(1) 0.016(2) 0.014(2) 0.010(1) 0.022(1) 0.017(1) 0.013(1) 0.015(1) 0.021 (1) 0.017(1) 0.012(1) 0.020(1) 0.016(1) 0.016(1)

Cd1−Sb1 Cd1−Sb2 (×2) Cd1−Sb5 Cd2−Sb3 (×2) Cd2−Sb4 Cd2−Sb5 Mn1−Sb2 Mn1−Sb4 (×2) Mn1−Sb6 Mn2−Sb3 (×2) Mn2−Sb6 Mn2−Sb8 Mn3a−Sb2 (×2) Mn3a−Sb4 Mn3a−Sb7

distance (Å)

atom pair

Eu9Cd4.45(1)Sb9 3.0525(9) Cd3a−Sb2 2.8793(6) Cd3a−Sb3 2.837(1) Cd3a−Sb4 (×2) 2.8413(6) 2.966(1) 3.010(1) Ca9Mn4.41(1)Sb9 2.854(3) Mn4a−Sb7 2.753(2) Mn4a−Sb8 (×2) 2.891(3) Mn4a −Sb9a 2.766(2) Mn5a−Sb1 (×2) 2.868(3) Mn5a−Sb7 2.844(3) Mn5a−Sb9a 2.722(4) Mn6b−Sb1 3.577(6) Mn6b−Sb5 (×2) 2.477(7) Mn6b−Sb9a

distance (Å) 2.570(4) 3.571(1) 2.776(2)

2.581(5) 2.823(3) 2.820(6) 2.785(3) 2.603(5) 2.817(6) 2.684(9) 2.773(5) 2.93(1)

a

Denotes a disordered/partially occupied site. In cases of positional disorder, only the physically meaningful distances are presented; the unphysical distances to neighboring atoms or symmetry equivalent sites are omitted. bThe minority site of Mn6 is coordinated to the fully occupied Sb1 and Sb5 with distances to Sb1 [2.804(5) Å] and to Sb5 [2.768(3) Å (×2) and 2.898(5) Å], respectively.

Magnetic Susceptibility. Direct-current magnetization measurements were carried out using a Quantum Design superconducting quantum interference device (SQUID) magnetometer. We used field cooling (from 300 to 5 K) with an applied field of 1000 Oe. Magnetization measurements for both the Eu9Cd4.45(1)Sb9 and Ca9Mn4.41(1)Sb9 samples were sought after, but reliable results could only be obtained for Eu9Cd4.45(1)Sb9 (7.25 mg); because the raw material was not a single phase, the measurement was performed on several, carefully selected single crystals. The large effective magnetic moment on Eu (following the Hund’s rules,29 Eu2+ will have μeff = 7.94 μB) afforded a reasonably strong signal from a very small amount of material. For Ca9Mn4.41(1)Sb9, however, this approach did not yield useful data. The raw magnetization data were corrected for the holder contribution and converted to molar susceptibility (χm = M/H). Electrical Resistivity. The electrical resistivity of a suitably sized Ca9Zn4.46(1)As9 crystal (needle, 0.15 × 0.2 × 3 mm) was measured using the four-probe method. Four Cu wires were attached to the sample with Epo-Tek Ag epoxy, and a constant current of 1 μA was applied through the two outer leads with a Keithley model 224 current source. The temperature range was varied from 100 to 300 K, and temperature control was done by the SQUID interface; for this purpose, a custom-made sample holder to fit the commercially available chamber was utilized.

a

Ueq is defined as one-third of the trace of the orthogonalized Uij tensor. bInterstitial sites: refined occupancy of 0.223(3) for Cd3 and 0.41(1) for Mn3. cCa1 is offset from the mirror plane with an occupancy of 50%. The closest Mn4 and Mn5 neighbors are also refined with half-occupancy. dMn6 undergoes positional disorder; the coordinates of the minority site are 0.3038(7), 1/4, 0.5084(2). eSb9 also undergoes positional disorder; the coordinates of the minority site are 0.0888(3), 1/4, 0.5348(1).

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RESULTS AND DISCUSSION Structural Relationships. The “average” structures of the title compounds are presented in Figure 1. Although they have the same nominal formulas, the structure of Eu9Cd4.45(1)Sb9, on one hand, and the structures of Ca9 Mn 4.41(1) Sb 9 and Ca9Zn4.46(1)As9, on the other hand, are different. Eu9Cd4.45(1)Sb9 is the missing member of the A9M4+xPn9 family,13,16−18 and its existence was predicted by the structure map that we had derived in an earlier publication.18 As stipulated therein, because of the similar sizes of Sr2+ and Eu2+ (combined with their similar electronegativities), Eu9Cd4.45(1)Sb9 ought to be an analogue of Sr9Cd4.49(1)Sb9, with a large fraction of the interstitial sites occupied.

microscope was operated at 10 μA beam current and 15 kV accelerating potential. The obtained data confirmed the elemental makeup of the crystals; the averaged results were in good agreement with the refinements. Thermal Analysis. The thermal stability of Ca9Zn4.46(1)As9 was evaluated from 300 to 1370 K using a Mettler-Toledo TGA/DSC/ 1600HT instrument. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) experiments were performed on polycrystalline samples under the protection of high-purity argon gas with a heating rate of 10 K min−1. 950

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Figure 2. Representation of the crystal structure of Ca9Mn4.41(1)Sb9 with thermal ellipsoids, drawn at the 90% probability level. All atomic positions devoid of disorder are shown as full ellipsoids with equatorial axes and connected to each other with cylinders. The atomic positions are subject to either positional or occupational disorder, as represented with hollow ellipsoids and connected to their closest neighbors with lines. All sites are shown, and some are impossibly close to their neighbors (or themselves), cases in which no bonds are drawn. The Ca atoms are in dark red, except the disordered Ca1, which is in bright red. The Sb atoms are drawn in blue, and the Mn atoms are shown in green/yellow. The interstitial Mn3 atoms are depicted in gray. Several related structures such as Ca3In2As4, Ba3Cd2Sb4, Ca9Mn4Bi9, and Ca3InP3, which feature fragments with very similar topologies, are presented in the insets.

It is worth noting that, in spite of the disorder already described in the Experimental Section, there are slabs that can be “cut out” from the Ca9Mn4.41(1)Sb9 structure, which have the exact same topology as those in Yb9Mn4+xSb9 (x ≈ 0.2).13 These slabs are made up of Mn1, Mn2, Sb2, Sb3, Sb4, Sb6, and Sb8, all well-behaved positions. All Ca cations that “solvate” this portion of the polyanionic substructure are also refined very well (Table 2). The Mn1 and Mn2 atoms center tetrahedra of antimony atoms, which are interlinked through common corners to form ribbons along the direction of the b axis. Note that corner sharing is the only mode of connectivity here, while the remainder of the structure is mostly based on edgeshared units. The Mn−Sb distances within the ordered slabs are in the range 2.753(2)−2.891(3) Å, matching very well with the Mn−Sb distances in Yb9Mn4+xSb9 (x ≈ 0.2),13 CaMn2Sb2,23 Ca21Mn4Sb18,24 Sr2MnSb2,31 and YbMn2Sb2,32 to name just a few. The corresponding angles deviate somewhat from the ideal tetrahedral value of 109.5°, ranging from 97.02(9)° to 114.5(1)°, a range that is a few degrees wider compared to Yb9Mn4+xSb9 [ranging from 102.04(6)° to 115.44(5)°].13 The interstitial Mn3 site is also very much like the interstitial site in Yb9Mn4+xSb9 (x ≈ 0.2).13 The interatomic distances involving the partially occupied Mn3 atom are on the short side, ranging from 2.477(7) to 2.722(4) Å. This is an issue that is an apparent artifact of the low occupancy (below 50%), and it is also seen in the structures of all other members of the A9M4+xPn9 family.13,16−18 The other half of the structure is quite disordered, but fragments that are known from other ternary phases can be easily recognized. For example, the Ca3In2As4 structure type33 shows the very same double chain of edge/corner-shared tetrahedra, as highlighted in light blue in Figure 2. Recall that the Ca1 site, which is offset to a general position with 50% occupancy, comes too close to Mn4 and Mn5, both of which

Currently, of the three d metals capable of forming compounds with this structureMn, Zn, and Cdonly Cd appears to “work” for the europium and strontium antimonides.30 To this date, neither Sr9Zn4+xSb9/Eu9Zn4+xSb9 nor Sr9Mn4+xSb9/Eu9Mn4+xSb9 is known to exist. The exact opposite circumstances surround the smaller and more electronegative Ca and Yb; with the latter metals, both Zn and Mn afford about half-a-dozen compounds, while Ca9Cd4+xSb9 and Yb9Cd4+xSb9 do not exist. Ba, the largest and least electronegative alkaline-earth metal, does not appear to form any compounds (Zn-, Mn-, or Cd-based) with the same or even related structures. These observations pertaining to the Eu9Cd4.45(1)Sb9 deserved a special mention; for a full structure description, the reader is referred to several earlier publications.17,18 The remainder of the discussion section here will be focused on the new structure type, epitomized by Ca9Mn4.41(1)Sb9. Positional and equivalent isotropic displacement parameters for Ca9Mn4.41(1)Sb9 are listed in Table 2; relevant crystallographic information for the isostructural Ca9Zn4.46(1)As9 can be found in the SI. Ca9Mn4.41(1)Sb9 quite unexpectedly is not isotypic with Yb9Mn4+xSb9 (x ≈ 0.2)13 and crystallizes in its own type instead (Figure 1). Notice too that, because Ca9Zn4+xSb9 and Yb9Zn4+xSb9 are isostructural, one could expect that it would be possible to make the pair Ca9Zn4+xAs9/ Yb9Zn4+xAs9; however, our preliminary data do not indicate that such compounds can be formed. Up until now, there is no evidence for the existence of other examples. The schematic representation of the relationship between the two “9−4−9” structure types is shown in Figure 1. Clearly, the polyanionic backbone of the new structure type is much more complex than the [Cd4Sb9] ribbons in Eu9Cd4.45(1)Sb9. A more detailed representation in a ball-and-stick mode is shown in Figure 2, which shows the [Mn≈9Sb18] layers. 951

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greater for Ca9Zn4+xAs9, and an occupancy of the interstitial site higher than 1/4 could be possible. However, according to the electron count based on the Zintl concept,1,2 the maximum occupancy of the 4g site should not exceed 25% (x = 0.5). In that sense, it seems that, to satisfy the electronic requirements, nature achieves structural stability in Ca9Zn4.46(1)As9 (nearperfect valence electron count) by “modifying” the structure in a subtle way. As far as the Mn-containing compounds are concerned, for reasons that are still poorly understood, a slight electron deficiency appears to be favored; perhaps the Mn d open shell is responsible for the electronic factors, taking lesser priority in this subset of the structural family. Recall that the interstitial sites in both Ca9Mn4Bi9 and Yb9Mn4Bi9 are empty, for example. Yb9Mn4+xSb9 (x ≈ 0.2) also shows only about 10% occupancy of the interstitial site, and a possible way to increase it is via Mn−Zn solid solutions, as demonstrated for Yb9(Mn,Zn)4+xSb9 (x ≈ 0.3).13 It seems that, in the case of Ca9Mn4.41(1)Sb9, the Mn content is approaching the “theoretical maximum”, which could be the driving force behind the structural variation. However, to shed more light on this hypothesis, one needs more structural data for samples with lower Mn content, ideally similar to that in Yb9Mn4+xSb9 (x ≈ 0.2). We have attempted numerous reactions with different nominal compositions, yet they all appear to produce Ca9Mn4+xSb9 in a very narrow homogeneity range (x ≈ 0.4).37 Thermal Stability. Simultaneous DSC analyses and TGA were performed in order to evaluate the thermal stability of the studied materials. Reproducible results were obtained only for Ca9Zn4.46(1)As9, and the results are shown in Figure 4. The data

are also 50% occupied. This means that half of the time there is a Ca atom in an octahedral hole of Sb atoms and the other half of the time there is an isolated Mn2Sb6 unit, which is actually more akin to the Al2Sb6 dimers in the Sr6Al2Sb6 structure.34 One can also make associations with the single chain of cornershared tetrahedra, which is the hallmark of the Ca3InP3 structure type,35 as highlighted in yellow in Figure 2. The edge/corner-sharing mode, where an individual Pn atom with three conjoined tetrahedra is very common, as epitomized in the PbO-type layers and many structures that can be derived from them by “cutting-and-pasting”. We have chosen to show one such structure, that of Ba3Cd2Sb4,36 which boasts [Cd2Sb4] ribbons, closely resembling the cut-out from the disordered layer in Ca9Mn4.41(1)Sb9. Another fragment also seems very familiar: the double chain that is shown in green in Figure 2. At first glance, it appears to be the same fragment, making up the backbone of the fully ordered slabs, as that discussed in the previous paragraph. Those all-cornered shared units are isosteric with the [Mn4Bi9] double chains in the Ca9Mn4Bi9 structure type.16 A more careful look at what the double chains are made of shows that the connectivity fashion is subtly different. In the former, four corner-shared tetrahedra form eight-membered rings. However, in the chains, we cut out of the disordered layers in the Ca9Mn4.41(1)Sb9 structure, one three-bonded Sb atom is shared by three MnSb4 tetrahedra, which results in the six-membered rings, as shown in Figure 3. This contention, of course, is made with the understanding that the structural fragment described here is not well-ordered, and all structural analogies might be fictitious.

Figure 3. Detailed representations of the double chains, excised from the disordered [Mn4Sb9] slabs in the Ca9Mn4.41(1)Sb9 structure (a) and the analogous unit from the ordered [Mn4Sb9] slabs (b). In the former, the tetrahedra are connected to form six-membered rings, and in the latter, they form eight-membered rings, just like in the structure of Yb9Mn4+xSb9 (x ≈ 0.2).13 Figure 4. Results from TGA−DSC measurement on a sample of Ca9Zn4.46(1)As9 over a temperature range of 300−1370 K. The data are recorded under a constant flow of argon gas. A weight loss of about 13% between 1200 and 1370 K is indicative of decomposition and subsequent evaporation of Zn and/or As.

Regardless of the extensive disorder, the majority of the refined Mn−Sb distances are reasonable (Table 3), matching very well the range of distances within the ordered slabs (vide supra). The refinements turn up some unphysical distances to neighboring atoms [e.g., Ca1−Mn4 = 2.531(6) Å] or to symmetry-equivalent sites [e.g., Ca1−Ca1 = 0.81(1) Å], which are easily avoided because all “problematic” sites are 1/2occupied. Last, we will try to address the question, why does Ca9Mn4.41(1)Sb9 (and Ca9Zn4.46(1)As9) not crystallize with the same structures as Yb9Mn4+xSb9 (x ≈ 0.2) and Ca9Zn4+xSb9 (x ≈ 0.5)?13,17 According to the structure map proposed for A9Zn4+xPn9 (0 < x < 0.5; A = Ca, Sr, Eu, Yb; Pn = Sb, Bi),18 the occupancy of the interstitial site correlates with the ratio rA/rPn (Pauling’s radii). Hence, extrapolating based on the antimonide Ca9Zn4+xSb9 (x ≈ 0.5), one should arrive at the conclusion that, because of the smaller radius of As, the ratio rA/rPn will be

from Eu9Cd4.45(1)Sb9 and Ca9Mn4.41(1)Sb9 were not conclusive and are not presented. From the figure, it is clearly seen that a small, sharp peak appearing at around 500 K should be designated as the melting process of the remaining Sn flux. A blunt endothermic peak and an obvious weight loss appear at about 1200 K, which indicates that the compound starts to decompose irreversibly at around this temperature. The experimental powder X-ray diffraction pattern after the TGA−DSC experiment confirms this notion (see the SI). Electrical Resistivity. Figure 5 shows the temperaturedependent resistivity (ρ) of a single crystal of Ca9Zn4.46(1)As9, measured with a current applied parallel to the c axis. Suitable 952

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Figure 5. Temperature-dependent resistivity of single crystals of Ca9Zn4.46(1)As9, measured with a current parallel to the c axis. The inset shows ln ρ versus 1/T in the high-temperature region and linear regression.

Figure 6. Magnetic susceptibility (χ) versus temperature (T) for Eu9Cd4.45(1)Sb9, measured under an applied field of 1 kOe in the temperature range 5−300 K. The inset shows 1/χ(T) and its linear fit.

ordering below this temperature. The susceptibility drops sharply at the lowest temperature, which suggests antiferromagnetic (AFM) behavior with a Néel temperature of about 9 K (determined by dχ/dT). The presence of AFM ordering at such low temperature is somewhat expected given that the other two known Eu−Cd−Sb phases, Eu11Cd6Sb1242 and EuCd2Sb2,43 both exhibit signs of AFM ordering with Neel temperatures on the order of 7−8 K. However, the Weiss constant, being nearly zero, may indicate that, in addition to the inferred AFM coupling, ferromagnetic correlations (possibly shorter-range) might also be present within the structure and compete/coexist with the AFM ground state.

single crystals of Eu9Cd4.45(1)Sb9 and Ca9Mn4.41(1)Sb9 were not available. The resistivity value at room temperature is rather high, about 0.04 Ω·m. This should not be surprising, given the nature of the chemical bonding in this material; recall that, on the basis of the structure description, Ca9Zn4.5As9 should be a salt-like compound with a perfect charge balance. Because the refined formula is pretty close to the “limiting” composition, a low carrier concentration is to be expected. The resistivity increases exponentially with decreasing temperature, i.e., thermally activated behavior, which is suggestive of a semiconductor. According to the thermal activation model [ρ(T) = ρ0 exp(Eg/2kBT), where Eg is the energy barrier, i.e., the band gap, and kB is the Boltzmann constant],38 in the hightemperature region, the logarithmic plot of ρ versus the inverse temperature should show linearity, following the equation ln ρ = Eg/2kBT + f. Fitting the experimental data in this manner shows excellent agreement between ln ρ and 1/T from 240 to 300 K, allowing us to calculate Eg = 0.523(6) eV. Such an experimental band gap compares very well with previously reported values for pnictide compounds, which are rationalized as Zintl phases, such as Ca5Al2Sb6,39 Sr5Al2Sb6,40 and Yb14AlSb11.41 This conjecture is in agreement with the semiconducting properties, expected from the Zintl concept.1,2 Magnetic Susceptibility. χ(T) data for Eu9Cd4.45(1)Sb9 are plotted in Figure 6. From the figure, it is clearly seen that the Curie−Weiss law is followed above ca. 20 K [χ(T) = C/(T − θp), where C is the Curie constant (C = NAμeff2/3kB, with NA being Avogadro’s number and μeff the effective magnetic moment) and θp is the paramagnetic Weiss temperature].29 θp and μeff were obtained from the linear regression, as shown in the inset. The results are μeff = 8.18 μB and θp = −0.4 K. The μeff value calculated from experimental data is slightly larger than the theoretical value of 7.94 μB, expected for a Eu2+ free ion (electronic configuration [Xe]4f7) with a total angular momentum J = 7/2 and the Landé factor g = 2.29 The small discrepancy is most likely due to an error in determining the sample mass (ca. 7 mg), or could be a sign for a secondary phase present in the sample. These could be Eu11Cd6Sb12,42 EuCd2Sb2,43 Eu11Sb10,44 or Eu16Sb11,45 but we have no solid crystallographic evidence to help with the identification of the side product(s). From the cusplike feature in the χ(T) plot near 10 K, it is evident that the compound undergoes spontaneous magnetic

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CONCLUSIONS In conclusion, we have discussed the synthesis and structures of three new compounds from the nominal “9−4−9” family: Eu9Cd4.45(1)Sb9, Ca9Zn4.46(1)As9, and Ca9Mn4.41(1)Sb9. These compounds are all substoichiometric, and they belong to two different structure types. Eu9Cd4.45(1)Sb9 is isostructural with Sr9Cd4.49(1)Sb9, and the structure boasts a [Cd≈9Sb18] framework based on corner-shared CdSb4 tetrahedra. The refined occupancy of the Cd atom at the interstitial site is about 1/4, as expected based on the structure map for this structure. According to the experimentally determined magnetic susceptibility, Eu is found in the typical divalent state for such systems. The structures of Ca9Zn4.46(1)As9 and Ca9Mn4.41(1)Sb9 are related to that of Eu9Cd4.45(1)Sb9 but arguably more complex and exhibiting extensive disorder. Their polyanionic backbones can be viewed as being two-dimensional layers, which are based on both corner- and edge-shared tetrahedral units. Evidence suggests that, although nonstoichiometric, the phase widths of the two compounds are very small, leaning toward the idealized “2−1−2” stoichiometry and satisfying the Zintl concept. Resistivity data on Ca9Zn4.46(1)As9 confirm the expected semiconducting behavior. A delicate balance between the atomic sizes and corresponding electronic requirements is believed to be the reason for the realization of the new structure type.

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ASSOCIATED CONTENT

S Supporting Information *

CIF files for all three structures, four additional CIF files for structure refinement of Ca9Mn4+xAs9, details of the structure 953

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McQueeney, R. J.; Canfield, P. C.; Goldman, A. I.; Argyriou, D. N. Nat. Mater. 2009, 8, 471−475. (11) Payne, A. C.; Olmstead, M. M.; Kauzlarich, S. M. Chem. Mater. 2001, 13, 1398−1406. (12) Goforth, A. M.; Hope, H.; Condron, C. L.; Kauzlarich, S. M.; Jensen, N.; Klavins, P.; MaQuilon, S.; Fisk, Z. Chem. Mater. 2009, 21, 4480−4489. (13) Xia, S.-Q.; Bobev, S. Chem. Mater. 2010, 22, 840−850. (14) Bux, S. K.; Zevalkink, A.; Janka, O.; Uhl, D.; Kauzlarich, S.; Snyder, J. G.; Fleurial, J.-P. J. Mater. Chem. A 2014, 2, 215−220. (15) Ohno, S.; Zevalkink, A.; Takagiwa, Y.; Bux, S. K.; Snyder, G. J. J. Mater. Chem. A 2014, 2, 7478−7483. (16) Brechtel, E.; Cordier, G.; Schäfer, H. Z. Naturforsch. 1979, 34B, 1229−1233. (17) Bobev, S.; Thompson, J. D.; Sarrao, J. L.; Olmstead, M. M.; Hope, H.; Kauzlarich, S. M. Inorg. Chem. 2004, 43, 5044−5052. (18) Xia, S.-Q.; Bobev, S. J. Am. Chem. Soc. 2007, 129, 10011−10018. (19) Shannon, R. D. Acta Crystallogr., Sect. A 1976, 32, 751−767. (20) Pauling, L. The Nature of the Chemical Bond, 3rd ed.; Cornell University Press: Ithaca, NY, 1960. (21) Brechtel, E.; Cordier, G.; Schäfer, H. Z. Naturforsch. 1981, 36B, 1099−1104. (22) Kim, S.-J.; Salvador, J.; Bilc, D.; Mahanti, S. D.; Kanatzidis, M. G. J. Am. Chem. Soc. 2001, 123, 12704−12705. (23) Bobev, S.; Merz, J.; Lima, A.; Fritsch, V.; Thompson, J. D.; Sarrao, J. L.; Gillessen, M.; Dronskowski, R. Inorg. Chem. 2006, 45, 4047−4054. (24) Holm, A. P.; Olmstead, M. M.; Kauzlarich, S. M. Inorg. Chem. 2003, 42, 1973−1981. (25) APEX2; Bruker AXS Inc.: Madison, WI, 2005. (26) Sheldrick, G. M. SADABS; University of Göttingen: Göttingen, Germany, 2003. (27) Sheldrick, G. M. SHELXTL; University of Gö ttingen: Göttingen, Germany, 2001. (28) Villars, P., Calvert, L. D., Eds. Pearson’s Handbook of Crystallographic Data for Intermetallic Phases, 2nd ed.; ASM International: Materials Park, OH, 1991; desktop ed., 1997. (29) Kittel, C. Introduction to Solid-State Physics, 7th ed.; John Wiley & Sons, Inc.: Hoboken, NJ, 1996. (30) While this manuscript was undergoing peer review, a new publication appeared (dx.doi.org/10.1021/ic502061w), detailing the structures of the related coinage metal containing Eu9Cd4−xCM2+x−y□ySb9 (CM = Cu, Ag, Au). (31) Park, S. M.; Kim, S.-J.; Kanatzidis, M. G. Inorg. Chem. 2005, 44, 4979−4982. (32) Morozkin, A. V.; Isnard, O.; Henry, P.; Granovsky, S.; Nirmala, R.; Manfrinetti, P. J. Alloys Compd. 2006, 420, 34−36. (33) Cordier, G.; Schäfer, H.; Stelter, M. Z. Naturforsch. 1986, 41b, 1416−1419. (34) Cordier, G.; Stelter, M.; Schäfer, H. J. Less-Common Met. 1984, 98, 285−290. (35) Cordier, G.; Schäfer, H.; Stelter, M. Z. Naturforsch. 1985, 40b, 868−871. (36) Saparov, B.; Xia, S.-Q.; Bobev, S. Inorg. Chem. 2008, 47, 11237− 11244. (37) CIF files for four additional refinements are provided in the SI. The crystals all come from different reactions with different Mn loadings. Crystal 1: a = 12.467(2) Å, b = 4.6227(9) Å, c = 44.067(8) Å, V = 2539.7(8) Å3. Crystal 2: a = 12.4617(18) Å, b = 4.6169(7) Å, c = 44.051(6) Å, V = 2534.4(6) Å3. Crystal 3: a = 12.453(2) Å, b = 4.6206(8) Å, c = 44.078(8) Å, V = 2536.2(8) Å3. Crystal 4: a = 12.4668(16) Å, b = 4.6177(6) Å, c = 44.110(6) Å, V = 2539.4(6) Å3. All data collections are done at the same temperature, 120 K. As can be inferred from the very close unit cell volumes, the Ca9Mn4+xSb9 phase has a narrow homogeneity range with x ≈ 0.4. (38) Shklovskii, B. I.; Efros, A. L. Electronic Properties of Doped Semiconductors; Springer: Berlin, 1984. (39) Toberer, E. S.; Zevalkink, A.; Crisosto, N.; Snyder, G. J. Adv. Funct. Mater. 2010, 20, 4375−4380.

solution and refinement of Ca9Zn4.46(1)As9, tables with refined atomic coordinates and isotropic displacement parameters and important interatomic distances in Ca9Zn4.46(1)As9, powder Xray diffraction patterns, experimental and simulated, of Eu9Cd4.45(1)Sb9 and Ca9Zn4.46(1)As9, composed of zone images from X-ray diffraction data from Eu9Cd4.45(1)Sb9, and powder Xray diffraction patterns, experimental and simulated, of Ca9Zn4.46(1)As9, following a TGA−DSC experiment. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Fax: (531) 883-62519. *E-mail: [email protected]. Fax: (302) 831-6335. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S.B. acknowledges financial support from the United States Department of Energy through Grant DE-SC0008885. S.-Q.X. acknowledge financial support from the National Natural Science Foundation of China (Grants 51271098, 51021062, and 51272129), the Independent Innovation Foundation of Shandong University, and the Program of Introducing Talents of Disciplines to Universities in China (111 Program b06017).

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DEDICATION This paper is dedicated to the memory of Prof. John D. Corbett, whose zeal for new knowledge and lifelong commitment to research, gave all of us new appreciation for the meaning and importance of fundamental science. Prof. Corbett often offered unflagging support and wise advice to many junior researchers, ourselves included. He generously shared his insights, which supported and expanded our own work on exploratory solid-state syntheses.

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