Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
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K(Th0.75Sr0.25)2Se6: Structural Change Resulting from the Disorder of Differently Charged Cations Adel Mesbah,†,‡ Jai Prakash,†,§ Jessica C. Beard,† Christos D. Malliakas,† and James A. Ibers*,† †
Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208-3113, United States ICSM, UMR 5257 CEA, CNRS, ENSCM, Univ. Montpellier, Site de Marcoule-Bât. 426, BP 17171, 30207 Bagnols-sur-Cèze, France § Department of Chemistry, Indian Institute of Technology Hyderabad, Kandi, Sangareddy, Telangana 502285, India ‡
ABSTRACT: Single crystals of K(Th0.75Sr0.25)2Se6 were obtained by a standard solid-state chemistry route at 1173 K. This compound does not belong to the AAn2Q6 family (A = K, Rb, Cs, or Tl; An = Th, U, or Np; Q = S, Se, or Te) that possesses infinite Q−Q−Q chains and where a charge distribution of A+, 2 × An4+, 2 × Q2−, 2 × (Q22.5−) has been proposed and hence a charge of −1.25 on Q of the “dichalcogenide”. Rather in K(Th0.75Sr0.25)2Se6, where the Th and Sr cations randomly occupy the same site, incorporation of these differently charged cations breaks the infinite Se−Se−Se chains into a structure that has typical Se22− pairs.
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Immm. One site contains 3/1 Th4+/Sr2+. The incorporation of the divalent element breaks the infinite Se−Se−Se chains to yield classical Se−Se pairs, as observed in the structures of ZrSe3 and Ba3ThSe3(Se2)2.11
INTRODUCTION Because of the ability of the chalcogens (Q = S, Se, and Te) to form Q−Q bonds, a number of compounds that show remarkable structural features and complex electronic properties have been synthesized. Among them are compounds AAn2Q6 (A = K, Rb, Cs, or Tl; An = Th, U, or Np; Q = S, Se, or Te),1−6 Ba2MAnTe7 (M = Cr or Ti; An = U or Th),7 CsUTe6,8 and ATiU3Te9 (A = Rb or Cs).9 Some of these present difficulties in the assignment of formal oxidation states because of the presence of infinite Q−Q−Q chains. The tendency of chalcogen atoms to form Q−Q−Q chains increases in the following order: Te > Se > S. This is one reason why there are more examples of Te-based compounds that exhibit linear infinite Te−Te−Te chains with intermediate Te−Te interactions. In actinide chalcogenides, the stabilization of Q− Q−Q chains in combination with normal Q−Q bonding often gives rise to intricate crystal structures in which charge balance is difficult and somewhat arbitrary. As an example, for the AAn2Q6 compounds, a charge distribution of A+, 2 × An4+, 2 × Se2−, 2 × (Se22.5−) has been proposed and hence a charge of −1.25 on Se of the “diselenide”. Similar conclusions were reached for the “ditellurides” in the Te−Te−Te infinite chains in the CsAn2Te6 (An = Th3 and U6), Ba2MAnTe7,7 and ATiU3Te9 structures.9 The structures of the ternary BaThTe4 and BaUTe4 compounds show both normal single Te−Te bonds (∼2.76 Å) and linear infinite Te−Te−Te chains (∼3.2 Å) with an average charge of −1.5 e− on the Te atoms in the chain.10 In this work, single crystals of what turned out to be K(Th0.75Sr0.25)2Se6 were synthesized at 1173 K. The crystal structure was determined from single-crystal X-ray diffraction data collected at 100 K and found to crystallize in space group © XXXX American Chemical Society
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EXPERIMENTAL METHODS
Synthesis. Caution! 232Th is an α-emitting radioisotope and as such is considered a health risk. Experiments using radioisotopes require appropriate inf rastructure and personnel trained in the handling of radioactive materials. The following reactants were used as supplied: Th powder (MP Biomedicals, LLC, 99.1%), Sr (Aldrich, 99%), Se (Cerac, 99.999%), and KCl (Aldrich, ≥99%). K(Th0.75Sr0.25)2Se6. Single crystals of K(Th0.75Sr0.25)2Se6 were obtained by the reaction of Th (19.72 mg, 0.085 mmol), Sr (22.34 mg, 0.255 mmol), and Se (40.13 mg, 0.501 mmol) in 100 mg of KCl as a flux. The mixture was loaded into a carbon-coated fused-silica tube in an Ar-filled glovebox to minimize the oxidation of the elements. The tube was evacuated to 10−4 Torr, flame-sealed, and placed in a computer-controlled furnace. The mixture was heated to 1173 K, kept at that temperature for 4 days, and cooled at a rate of 2.5 K/h to 673 K, and then the furnace was shut down. The reaction afforded orange single crystals in ∼60 wt % yield based on Th. These were analyzed by EDX using a Hitachi S3400 SEM microscope. The analysis of selected single crystals gave a ≈2:1:3:12 K:Sr:Th:Se ratio. No attempt was made to maximize the yield. However, the synthesis was repeated successfully but produced smaller single crystals. Other single crystals were analyzed and gave an ≈1:1 Th:Se ratio, probably from ThOSe. Structure Determination. The crystal structure of what turned out to be K(Th0.75Sr0.25)2Se6 was determined from single-crystal X-ray diffraction data collected at 100(2) K with the use of a Bruker APEX2 diffractometer12 equipped with graphite-monochromatized MoKα Received: April 12, 2018
A
DOI: 10.1021/acs.inorgchem.8b00925 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry radiation (λ = 0.71073 Å). The data collection strategy consisting of a series of 0.3° scans in ω and φ was determined by the algorithm COSMO implemented in APEX2. Each frame was collected for 10 s with a crystal-to-detector distance of 60 mm. Face-indexed absorption corrections were performed with the use of SADABS.13 The structure was determined and refined with the SHELX14 program package.13,14 STRUCTURE TIDY15 in PLATON16 was used to standardize the atomic positions. Further details are given in Table 1. The structure
constant [21.6305(11) Å] is smaller than in the KTh2Se6 structure [21.860(4) Å].5 The asymmetric unit of the K(Th0.75Sr0.25)2Se6 structure comprises one randomly mixed site An = (0.75Th + 0.25Sr) (mm2 symmetry), one K atom (mmm symmetry), and two independent Se atoms, Se1 and Se2 (m.. and mm2 symmetry sites, respectively). Each An atom is coordinated to eight Se atoms to form a distorted bicapped trigonal prism (Figure 1b). The AnSe8 polyhedra are connected to each other by face sharing in the [100] direction and edge sharing in the [010] direction to form [An2Se6]− infinite layers aligned perpendicular to the c axis and separated by K atoms. The substructure of [An2Se6]− infinite layers resembles that of the neutral [ZrSe3] layers of the ZrSe3 structure.19 A comparison of the K(Th0.75Sr0.25)2Se6 and KTh2Se65 structures shows that the former has Se22− dimers [Se−Se, 2.395(2) Å] whereas the latter has linear Se−Se−Se chains with alternating interactions of 2.727(2) and 2.907(2) Å.5 The two neighboring Se22− dimers in K(Th0.75Sr0.25)2Se6 are separated by a distance of 3.333(2) Å. Note that distances involving An are affected by the Th/Sr disorder. The An−Se interatomic distances are found to be 4 × 3.044(1) Å for Se1 and 2 × 2.951(1) and 2 × 2.996(4) Å for Se2. The An−Se2 distances are comparable to those found in the members of the ATh2Se6 family, such as 2 × 2.962(1) and 2 × 2.991(1) Å in the KTh2Se65 structure and 2 × 2.959(1) and 2 × 3.006(1) Å in the CsTh2Se6 structure.2 The An−Se1 distances are slightly longer than those found in these structures: 2.991(1) Å in KTh2Se6 and 2.985(1) Å in CsTh2Se6.3 The Se−Se distance in the K(Th0.75Sr0.25)2Se6 structure is 2.395(2) Å, comparable to the distances of 2.374(1) and 2.377(1) Å found in the Ba 3ThSe 3(Se2 )2 structure.11 Oxidation State. The assignment of formal oxidation states for the elements in the AAn2Q6 family is somewhat arbitrary. The presence of infinite Q−Q−Q chains seems incompatible with the oxidation state of the Q22− pairs. In the presence of one A+ and 2 × An4+, the six chalcogens should provide a charge of −9. The structure has two single Q2− anions, and hence, the charge on the four remaining anions should be −5. For the ATh2Se6 compounds,3 it has been postulated that the alkali metal provides an electron to the Se 4p σ* orbital, with a charge distribution for the selenides as 2 × Se2−, 2 × Se22.5−. Some modulated structures with Th and U were written (A+)2(An4+)4(Se2−)6(Se22−)3. Here the Se chain distances are still ∼2.7 Å (Table 3). These distances decrease in the structure of the disordered La/U compound, which has a proposed charge distribution of (Cs+)0.88(La3+)0.68(U3.85+)1.32(Se2−)2(Se22−)2.2 Here the Se distances are 2.422(3) and 3.266(2) Å. For the structure presented here, the assignment of formal oxidation states is straightforward: K+, 0.5 × Sr2+, 1.5 × Th4+, 2 × Se2−, 2 × Se22−. Remarkably, the insertion of a divalent element (Sr) reduces the cationic sum to 8+, and the infinite Se chains break into single Se22− pairs. For our K(Th0.75Sr0.5)2Se6 compound, the Se−Se distance of 2.395(2) Å is typical for a Se22− species with no infinite Se chains and thus could be compared to that of the Ba3ThSe3(Se2)2 compound with Se−Se distances of 2.374(1) and 2.377(1) Å.11 Optical Properties. Absorption spectra in the visible range were collected at 293 K on a thin platelike single crystal of K(Th0.75Sr0.25)2Se6. The crystal shows a broad absorption transition at 1.97(2) eV (Figure 2, left). Analysis of the square and square root of absorbance data as a function of energy (Figure 2, right) gives a direct band-gap component of 2.07(2)
Table 1. Crystallographic Dataa,b and Structure Refinement Details for K(Th0.75Sr0.25)2Se6 space group a (Å) b (Å) c (Å) V (Å3) ρ (g cm−3) μ (mm−1) R(F)c Rw(F02)d
Immm 4.2647(2) 5.7280(3) 21.6305(11) 528.39(5) 5.686 44.592 0.0358 0.091
λ = 0.71073 Å, T = 100(2) K, Z = 2. bR(F) = 0.0568 for constrained refinement. cR(F) = ∑||F0| − |Fc||/∑|F0| for F02 > 2σ(F02). dRw(F02) = {∑[w(F02 − Fc2)]2/∑wF04}1/2. For F02 < 0, w−1 = σ2(F02); for F02 ≥ 0, w−1 = σ2(F02) + (qFo2)2, where q = 0.058. a
features a disordered Th/Sr site. The precession images constructed from the data show no signs of modulation. Hence, the distribution of Th and Sr is random. The refinement of the Th:Sr ratio led to a 0.777(8):0.223(8) ratio. The agreement indices were not increased significantly when, in accordance with charge balance, this ratio was set at 0.75:0.25. Further details are given in Table 1. Absorption Spectra. Single-crystal absorption spectra were obtained at 293 K on a Hitachi U-6000 Microscopic FT spectrophotometer mounted on an Olympus BH2-UMA microscope. Five spectra were collected and averaged. A thin crystal of K(Th0.75Sr0.25)2Se6 was placed on a glass slide and positioned over the light source where the transmitted light was recorded. The background signal of the glass slide was subtracted from the collected intensity.
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RESULTS AND DISCUSSION Synthesis. Compound K(Th0.75Sr0.25)2Se6 was obtained in an attempt to synthesize the Sr/Th analogue of the Ba3MUQ617,18 compounds. Orange single crystals were obtained in a yield of ∼60 wt % based on Th content. Crystal Structure. Compound K(Th0.75Sr0.25)2Se6 crystallizes with two formula units in space group Immm of the orthorhombic system. Metrical data are listed in Table 2. The Table 2. Selected Interatomic Distances for K(Th0.75Sr0.25)2Se6 atom pair
distance (Å)
An−Se2 × 2 An−Se2 × 2 An−Se1 × 4 Se1−Se1
2.951(1) 2.996(1) 3.044(1) 2.395(2)
crystal structure of K(Th0.75Sr0.25)2Se6 is derived from the KTh2Se6 structure.5 A comparison of these two structures is shown in Figure 1. The a and b lattice constants of K(Th0.75Sr0.25)2Se6 [a = 4.2647(2) Å, and b = 5.7280(3) Å; 100 K] are larger than those in the KTh2Se6 structure [a = 4.1899(5) Å, and b = 5.6337(5) Å; 298 K], whereas the c lattice B
DOI: 10.1021/acs.inorgchem.8b00925 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
Figure 1. Comparison of the (a) KTh2Se65 and (b) K(Th0.75Sr0.25)2Se6 structures viewed down the a axis. For the sake of clarity, Se−Se short interactions and bonding are shown as red colored dashed and solid lines, respectively.
Table 3. An−Se and Se−Se Distances (angstroms) in Some Selected Selenidesa
a
compound
structure
An−Se
Se−Se
KTh1.5Sr0.5Se6 KTh2Se6 CsTh2Se6 RbTh2Se6 Cs0.88(La0.68U1.32)Se6 Ba3ThSe3(Se2)2
layered layered layered layered layered one-dimensional
2.951(1)−3.044(1) 2.962(1)−3.015(1) 2.959(1)−3.006(1) 2.966(1)−3.015(1) 2.908(1)−2.978(1) 2.930(1)−3.050(1)
2.395(2) 2.727(2)−2.907(2) 2.698(3)−2.924(3) 2.728(3)−2.907(3) 2.422(3)−3.266(2) 2.374(1)−2.377(1)
An geometryb 8, 8, 8, 8, 8, 8,
bctp bctp bctp bctp bctp poct
ref this work 5 2 5 2 11
Interatomic distances have been rounded for comparison. bbctp, bicapped trigonal prismatic; poct, pseudooctahedral.
an average oxidation state of −1.25 per chalcogen. Rather, in K(Th0.75Sr0.25)2Se6, the incorporation of a divalent element (Sr) breaks these infinite chains into a structure that has typical Se22− pairs. From optical measurements, K(Th0.75Sr0.25)2Se6 has a direct band-gap component of 2.07(2) eV and an indirect band-gap value of 1.98(2) eV suggesting that K(Th0.75Sr0.25)2Se6 is an indirect wide-gap semiconductor.
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ASSOCIATED CONTENT
Accession Codes
CCDC 1837536 contains 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 data_
[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
Figure 2. Analysis of the square and square root of absorbance data as a function of energy in K(Th0.75Sr0.25)2Se6.
eV and an indirect band-gap value of 1.98(2) eV suggesting that K(Th0.75Sr0.25)2Se6 is an indirect wide-gap semiconductor.
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CONCLUSIONS Single crystals of K(Th0.75Sr0.25)2Se6 were obtained by solidstate chemistry routes at 1173 K. The structure crystallizes in space group Immm of the orthorhombic system. The Th and Sr occupy the same site. This compound does not belong to the AAn2Q6 family that possesses infinite Q−Q−Q chains having
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
James A. Ibers: 0000-0002-5418-3645 C
DOI: 10.1021/acs.inorgchem.8b00925 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry Notes
Chalcogenides Related to the 2H Hexagonal Perovskite Family. Inorg. Chem. 2015, 54, 2851−2857. (18) Mesbah, A.; Malliakas, C. D.; Lebègue, S.; Sarjeant, A. A.; Stojko, W.; Koscielski, L. A.; Ibers, J. A. Syntheses, Structures, and Electronic Properties of Ba3FeUS6 and Ba3AgUS6. Inorg. Chem. 2014, 53, 2899−2903. (19) Krönert, W.; Plieth, K. Die Struktur des Zirkontriselenids ZrSe3. Z. Anorg. Allg. Chem. 1965, 336, 207−218.
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Use was made of the IMSERC X-ray Facility at Northwestern University, which has received support from the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (National Science Foundation Grant ECCS-1542205), the State of Illinois, and the International Institute for Nanotechnology (IIN).
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REFERENCES
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DOI: 10.1021/acs.inorgchem.8b00925 Inorg. Chem. XXXX, XXX, XXX−XXX
