Synthesis, Crystal Structure, Theoretical, and Resistivity Study of

Jul 11, 2016 - Each octahedral unit shares all six corners with neighboring octahedra, forming a three-dimensional network. BaUSe3 can be charge balan...
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Synthesis, Crystal Structure, Theoretical, and Resistivity Study of BaUSe3 Jai Prakash,† Maria S. Tarasenko,† Adel Mesbah,†,‡ Sébastien Lebègue,§ Christos D. Malliakas,† and James A. Ibers*,† †

Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208-3113, United States ICSM, UMR 5257 CEA/CNRS/UM2/ENSCM, Site de Marcoule - Bât. 426, BP 17171, 30207 Bagnols-sur-Cèze cedex, France § Laboratoire de Cristallographie, Résonance Magnétique et Modélisations (CRM2, UMR CNRS 7036), Institut Jean Barriol, Université de Lorraine, BP 239, Boulevard des Aiguillettes, 54506 Vandoeuvre-lès-Nancy, France ‡

S Supporting Information *

ABSTRACT: The black-colored compound BaUSe3 has been synthesized at 1173 K by a stoichiometric reaction of the elements in a CsCl flux. BaUSe3 crystallizes in the GdFeO3 structure type. There is no change in structure between 100 and 298 K. The U atoms in this structure are octahedrally connected to six Se atoms. Each octahedral unit shares all six corners with neighboring octahedra, forming a three-dimensional network. BaUSe3 can be charge balanced as Ba2+U4+(Se2−)3. DFT electronic structure calculations found BaUSe3 to be antiferromagnetic in its ground state and to be a semiconductor with a band gap of 2.5 eV. The band gap is inconsistent with the black color of the material and with the small activation energy of 0.12(1) eV obtained from resistivity measurements. A UV−vis spectrum indicated that there was no band gap above 1 eV. It is possible that, for BaUSe3, intrinsic and extrinsic impurities from the flux create midgap states that lead to the experimentally measured narrow optical gap. More likely, BaUSe3 presents a challenge to DFT calculations as applied to 5f materials.

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INTRODUCTION Actinide (An) chalcogenides (Q) are well-known for their rich structural chemistry and interesting physical properties owing to the involvement of the 5f electrons in chemical bonding. A wide range of physical properties, such as superconductivity,1,2 narrow band-gap semiconductors,3 and magnetism,4−6 result from the diverse crystal structures shown by these compounds. In the past decade, our group has been extensively involved in the exploratory syntheses of new actinide chalcogenides with the result that a large number of new binary, ternary, and quaternary compounds are now known along with many of their physical properties. Examples of such compounds are AAn2Q6 (An = Th, U, Np; Q = S, Se, and Te),7−9 CsUTe6,10 MU8Q17 (M = transition metals, Q = S, Se),11,12 MUQ3 (M = Sc, Fe; Q = S, Se),13,14 AMUQ3,15,16 AMNpS3 (M = Cu, Ag),17 A2M4U6Q17 (M = Pd, Pt),18 A2M3UQ6 (M = Pd, Pt),19 ATiU3Te9,20 Cs2Hg2USe5,21 and AZrUTe5.10,22 Recently, we have focused on the syntheses of new alkalineearth (Ak) metal containing actinide chalcogenides, such as Ba2AnS2(S2)2 (An = U, Th),23 Ba3ThSe3(Se2)2,24 BaUS3,25 AkAn2Q5 (Ak = Ca, Sr, Ba, or Pb; An = U, Th; Q = S, Se, Te),26,27 BaUTe6,28 BaAnTe4 (An = Th, U),28 Ba4UQ6 (Q = S,25 Se29), Ba3MUQ6 (M = Mn, Fe, Ag; Q = S, Se),29,30 Ba2MAnTe7 (M = Ti, Cr; An = Th, U),31 Ba8PdU2Se12(Se2)2,32 Ba2Cu4USe6,33 and Ak2CuAnQ5.33,34 These Ak-based compounds often show new structure types, and hence their crystal © XXXX American Chemical Society

chemistry is different from their alkali-metal counterparts. Here, we present the synthesis and characterization of the new ternary compound, BaUSe3, obtained by an exploratory study of the ternary Ba/U/Se system.

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EXPERIMENTAL METHODS

Syntheses. Caution! Depleted U is an α-emitting radioisotope and as such is considered a health risk. Its use requires appropriate inf rastructure and personnel trained in the handling of radioactive materials. The starting materials Ba (Johnson Matthey, 99.5%), CsCl (Aldrich, 99%+), and Se (Cerac, 99.999%) were used as obtained. Reactions were performed in sealed 6 mm carbon-coated fused-silica tubes. Depleted U powder was obtained by hydridization and decomposition of U turnings (IBI Laboratories) in a modification35 of a previous literature method.36 All the chemical manipulations were performed either inside an Ar-filled dry box or under vacuum. The reactants were weighed and transferred into tubes that were then evacuated to 10−4 Torr, flame sealed, and placed in a computer-controlled furnace for heat treatment. Semiquantitative EDX analyses of the products of the reactions were obtained with the use of a Hitachi S-3400 SEM microscope. Synthesis of BaUSe3. Black block-shaped crystals of BaUSe3 were obtained by reaction of Ba (11.67 mg, 0.0850 mmol), U (20.2 mg, 0.0849 mmol), Se (20.2 mg, 0.2558 mmol), and CsCl flux (200 mg). The reaction mixture was first heated to 773 K at 40 K/h, held at 773 Received: May 16, 2016

A

DOI: 10.1021/acs.inorgchem.6b01202 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry K for 12 h, then heated to 1173 K at 17 K/h, and annealed there for 96 h. The reaction mixture was cooled to 673 K at 10 K/h, followed by rapid cooling to 298 K at 31 K/h. In addition to BaUSe3 (Ba:U:Se ≈ 1:1:3), the reaction product also contained reddish block-shaped crystals of BaSe37 (Ba:Se ≈ 1:1), black blocks of Ba4USe629 (Ba:U:Se ≈ 4:1:6), and black plates of UOSe.38 Anaerobic Synthesis of BaUSe3. Because of the discrepancy between the experimental and theoretical band gaps (see below), the synthesis was repeated in a manner that avoided any possible air oxidation and light sensitivity of the product. A flask was covered with Al foil. In the flask, the product in the halide flux was dispersed in degassed DMF under flowing N2 and dried with ether. The resultant black dry powder was transferred to a glovebox where it was ground to a fine powder. Under N2 gas, an X-ray powder diffraction pattern (Rigaku Miniflex600 powder X-ray diffractometer) and a UV−vis spectrum (Shimadzu UV-3101PC double-beam, double-monochromator spectrophotometer) were obtained. The diffraction pattern confirmed that the target phase was present. The color of the material was black from the very beginning, and there was no apparent color change during the steps just described. The spectrum (Figure 1) confirms that there is no optical band gap above 1 eV.

Table 1. Crystallographic Data and Structure Refinement Details for BaUSe3 BaUSe3 space group a (Å) b (Å) c (Å) V (Å3) Z λ (Å) T (K)a ρ (g cm−3) μ (mm−1) R(F)b Rw(Fo2)c

− Pnma 7.8416(2) 10.7764(3) 7.4016(2) 625.47(3) 4 0.71073 100(2) 6.502 49.422 0.0183 0.0528 D16 2h

a

At 298 K, the cell constants are a = 7.8040(7) Å, b = 10.7914(11) Å, and c = 7.4936(8) Å. bR(F) = ∑||Fo| − |Fc||/∑|Fo| for Fo2 > 2σ(Fo2). c Rw(Fo2) = {∑[w(Fo2 − Fc2)2]/∑wFo4}1/2. For Fo2 < 0, w−1 = σ2(Fo2). For Fo2 ≥ 0, w−1 = σ2(Fo2) + (0.0236Fo2)2 + 5.164Fo2. structural point of view, the data are best refined and described in Pnma. Then, BaUSe3 is isostructural with BaUS3.25 No systematic absence violations occurred in the BaUS3 data set.25 The program STRUCTURE TIDY43 in PLATON42 was used to standardize the atomic positions. Resistivity. Four-probe high temperature dependent resistivity data were collected using a homemade resistivity apparatus equipped with a Keithley 2182 nanovoltmeter, a Keithley 236 source measure unit, and a high-temperature vacuum chamber controlled by a K-20 MMR system. An I−V curve from 1 × 10−8 to −1 × 10−8 A with a step of 4 × 10−9 A was measured for each temperature point from 298 to 500 K, and resistance was calculated from the slope of the I−V plot. Data acquisition was controlled by custom-written software. Graphite paint (PELCO isopropanol based graphite paint) was used for electrical contacts with copper wire of 0.025 mm in thickness (Omega). The DC current was applied along an arbitrary direction on a single crystal of BaUSe3 with dimensions 0.20 mm × 0.18 mm × 0.16 mm. Temperature Dependence of the Structure. Again, because of the discrepancy between the experimental and theoretical band gaps (see below), it was important to establish whether or not BaUSe3 undergoes a phase transition between 100 K, where structural results were obtained on which the DFT calculations were based, and 300 K, where the UV−vis spectrum and the resistivity data were obtained. A second crystal was selected from the original batch of crystals. It was used to determine the crystal structure again at 100 K and also at 298 K. The 298 K and two 100 K data sets led to insignificant differences in the resultant refinements. Theoretical Calculations. The calculations were performed using density functional theory44,45 and the projector augmented wave method,46 as implemented in the Vienna Ab initio Simulation Package (VASP).47,48 For the exchange and correlation potential, the Heyd− Scuseria−Ernzerhof (HSE) functional49−51 with spin polarization was used. The different magnetic orders that are possible within the crystallographic cell were computed, and the one with the lowest total energy was retained as the ground state. The parameters of the cell and the positions of the atoms were taken from the single-crystal structure determination. To reach numerical convergence of the calculations, the Brillouin zone was sampled with a 4 × 2 × 4 mesh, and the plane wave part of the Kohn−Sham wave function was expanded up to the default cutoff.

Figure 1. UV−vis spectrum of the powder produced in a synthesis of BaUSe3 in which light and air were rigorously excluded. Structure Determination. Single-crystal X-ray diffraction data for BaUSe3 were collected with the use of a monochromatized (QuazarTM optics) Mo Kα microsource radiation (λ = 0.71073 Å) at 100(2) K on a Bruker APEX2 diffractometer.39 The algorithm COSMO implemented in the program APEX2 was used to establish the data collection strategy with a series of 0.5° scans in ω and φ. The exposure time was 60 s/frame, and the crystal-to-detector distance was 40 mm. The collection of intensity data as well as cell refinement and data reduction were carried out with the use of the program APEX2.39 Face-indexed absorption, incident beam, and decay corrections were performed with the use of the program SADABS.40 The crystal structure of BaUSe3 was solved and refined with the use of the SHELX-14 algorithms of the SHELXL program package.40,41 Details are presented in Table 1 and in the Supporting Information. Because there were 40 systematic-absence violations in the refinement of the structure in space group Pnma, a refinement in lower symmetry, namely, space group P21/m, with the twin condition [−1 0 0 0 −1 0 0 0 1] led to the value of the BASF parameter of 0.5 (within machine round-off error) and fewer systematic-absence violations. However, in the program ADDSYM in PLATON,40,42 the structure reverted to Pnma. It is conceivable that the crystal was indeed nearly but not quite perfectly twinned (BASF ≠ 0.5), but from a

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RESULTS Syntheses. Black single crystals of BaUSe3 were obtained from the stoichiometric reaction of elements in molten CsCl flux in a yield of about 50 wt %. In an attempt to synthesize larger crystals of BaUSe3 for magnetic studies, the same B

DOI: 10.1021/acs.inorgchem.6b01202 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry reaction conditions were employed except for a longer annealing time of 150 h, but this did not result in larger crystals. Attempts to synthesize BaUTe3 by using the same reaction conditions also failed, but further exploratory studies of the Ba/ U/Te system led to the discoveries of BaUTe428 and BaUTe6.28 Crystal Structure. BaUSe3 crystallizes in the well-known GdFeO352 structure type with four formula units in the space group D16 2h − Pnma of the orthorhombic crystal system in a cell with constants of a = 7.8416(2) Å, b = 10.7764(3) Å, and c = 7.4016(2) Å (Table 1). BaUSe3 is isostructural with BaUS325 and is related to the perovskite (ABX3) structure. The unit cell of BaUSe3 is shown in Figure 2, and important interatomic

Figure 3. Local coordination environment of U atoms in BaUSe3. U and Se atoms are shown in black and orange, respectively.

Se distance in BaUSe3 is 3.935(1) Å, too long for any Se−Se interaction. Accordingly, BaUSe3 can be charge balanced as Ba2+U4+(Se2−)3. Table 3. U−Se Interatomic Distances in Some U4+ Compoundsa Figure 2. A general view of crystal structure of BaUSe3. Ba, U, and Se atoms are shown in blue (largest spheres), black, and orange (smallest spheres), respectively.

Table 2. Selected Interatomic Lengths (Å) for BaUSe3 BaUSe3 U1−Se1 U1−Se2 Ba1−Se1

Ba1−Se2 U1···U1

2.8013(4) 2.8192(4) 2.7873(2) 3.2895(5) 3.5175(5) 3.7139(5) 3.2605(8) 3.2911(7) 5.3915(1)

× × × × × ×

2 2 2 2 2 2

compound

structure

U−Se distances (Å)b

reference

BaUSe3 Cs2U3Se7 Ba4USe6 Ba3MnUSe6 Ba3FeUSe6 Tl2Ag2USe4 CsCuUSe3 Cs2Hg2USe5

3D 3D 0D 1D 1D layered layered layered

2.787(1)−2.819(1) 2.685(1)−2.896(1) 2.825(1) 2.842(1) 2.836(1) 2.853(2)−2.881(1) 2.826(1)−2.861(1) 2.872(1)−2.902(1)

this report ref 54 ref 29 ref 29 ref 29 ref 55 ref 56 ref 21

a

In all compounds, the coordination about U4+ is octahedral. bSome interatomic distances have been rounded to facilitate comparisons.

The USe6 octahedra in BaUSe3 are tilted in contrast to those of the ideal perovskite structure of general formula ABX3. Each Ba atom in the structure of BaUSe3 is surrounded by eight Se atoms in a bicapped trigonal prism of m symmetry. In the ideal ABX3 perovskite structure, the coordination polyhedron around the A cation is a 12-fold cuboctahedron. Resistivity. The resistivity of BaUSe3 as a function of temperature (Figure 4) indicates a semiconducting behavior. The resistivity at 298 K is around 7.5 kΩ·cm, which decreases to 1.5 kΩ·cm on heating the crystal to 500 K. The corresponding activation energy estimated from the Arrhenius plot is 0.12(1) eV. This small activation energy suggests a narrow band gap for BaUSe3, consistent with its black color. Electronic Structure. BaUSe 3 is calculated to be antiferromagnetic in its ground state, as seen from the symmetric total density of states with respect to the spin (upper plot of Figure 5). Also, BaUSe3 is found to be a relatively large band-gap semiconductor; a value of 2.5 eV is calculated with the HSE functional. From the partial density of states (PDOS) plots (lower plots of Figure 5), we see that the magnetic moment on the U atoms induces a small symmetry

×2

distances are given in Table 2. The asymmetric unit of the structure contains one U atom (site symmetry 1̅ ), one Ba atom (.m.), and two Se atoms (Se1(1) and Se2(.m.)). U atoms in this structure are octahedrally connected to six Se atoms (Figure 3), and these octahedra of symmetry 1̅ are distorted. Each octahedral unit shares all six corners with neighboring octahedra, forming a three-dimensional network. This arrangement of USe6 units creates large channels along the b axis that are filled by Ba2+ cations (Figure 2). U atoms are connected to each other by bridging through Se atoms with U···U distances of 5.388(1) Å. The U−Se distances (2.7873(2), 2.8013(4), and 2.8192(4) Å) in BaUSe3 are normal and in good agreement with those in the structures of related compounds with U4+ in similar coordination environments (Table 3). The shortest Se− C

DOI: 10.1021/acs.inorgchem.6b01202 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

U atoms in this structure are octahedrally connected to six Se atoms, and these octahedra of symmetry 1̅ are distorted. Each octahedral unit shares all six corners with neighboring octahedra, forming a three-dimensional network. There are no short Se−Se distances in the structure, so BaUSe3 can be charge balanced as Ba2+U4+(Se2−)3. Electronic structure calculations were performed using density functional theory and the projector augmented wave method. For the exchange and correlation potential, the Heyd− Scuseria−Ernzerhof (HSE) functional with spin polarization was used. BaUSe3 is calculated to be antiferromagnetic in its ground state and to be a semiconductor with a band gap of 2.5 eV. Because this band gap is inconsistent with the black color of the material and with the small activation energy of 0.12(1) eV obtained from resistivity measurements, further experiments were necessary. An anaerobic synthesis in which light was excluded afforded only black material. A UV−vis spectrum indicated that there was no band gap above 1 eV. Structure determinations at both 100 and 298 K showed insignificant differences. Hence, there is genuine discrepancy between the experimental band gap of less than 1 eV and the theoretical band gap of 2.5 eV for BaUSe3. One can speculate that, for BaUSe3, intrinsic and extrinsic impurities from the flux create midgap states that lead to the experimentally measured narrow optical gap. But why should this occur in BaUSe3 and not in a host of previously examined compounds? More likely, BaUSe3 presents a challenge to DFT calculations as applied to 5f materials.

Figure 4. Resistivity vs temperature (left) and Arrhenius plot (right) for BaUSe3.

Figure 5. Computed total density of states (upper plot) and partial density of states (lower plots) for BaUSe3.

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break in the PDOS of Se and Ba. Moreover, the top of the valence states and the bottom of the conduction states are dominated by U f states, with small contributions from Se p and Ba d states.

ASSOCIATED CONTENT

S Supporting Information *

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01202. Crystallographic file for BaUSe3 (CIF)

DISCUSSION The DFT calculations rely on the structural results obtained at 100 K. The resistivity data and a UV−vis spectrum were obtained at 298 K. There is no phase transition between 100 and 298 K. Hence, there is genuine discrepancy between the experimental band gap of less than 1 eV and the theoretical band gap of 2.5 eV for BaUSe3. This is surprising because DFT calculations using the same HSE functional and procedures previously have provided band gaps in reasonable agreement with experiment.23,24,33,34,53 As examples: experimentally, the optical band gaps of Ba2Th(S2)2S2 and Ba3Th(Se2)2Se2 are 2.46(5) and 1.96(2) eV, respectively,23,24 to be compared with the calculated electronic band gaps of 2.2 and 1.5 eV. Similarly, the optical band gaps of Ba2Cu2ThS534 and Ba2Cu2ThSe533 were found to be 1.86(2) and 1.75(2) eV compared with the calculated electronic band gaps of 1.7 and 1.2 eV, respectively. One can speculate that, for BaUSe3, intrinsic and extrinsic impurities from the flux create midgap states that lead to the experimentally measured narrow optical gap. But why should this occur in BaUSe3 and not in some previously examined compounds?29,33,34,23,24 More likely, BaUSe3 presents a challenge to DFT calculations as applied to 5f materials.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Use was made of the IMSERC X-ray Facility (NSF DMR1121262) at Northwestern University, supported by the International Institute of Nanotechnology (IIN). S.L. acknowledges HPC resources from GENCI-CCRT/CINES (Grant x2014-085106). C.D.M. was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.

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CONCLUSIONS The black-colored compound BaUSe3 has been synthesized in 50 wt % yield at 1173 K by stoichiometric reaction of the elements in a CsCl flux. BaUSe3 is isostructural with BaUS3 and crystallizes in the GdFeO3 structure type with four formula units in space group D16 2h − Pnma of the orthorhombic system. D

DOI: 10.1021/acs.inorgchem.6b01202 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.6b01202 Inorg. Chem. XXXX, XXX, XXX−XXX