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Crystal Structures, Optoelectronic Properties, and Electronic Structures of Layered Oxychalcogenides MCuOCh (M ) Bi, La; Ch ) S, Se, Te): Effects of Electronic Configurations of M3+ Ions Hidenori Hiramatsu,*,† Hiroshi Yanagi,‡ Toshio Kamiya,†,‡ Kazushige Ueda,†,§ Masahiro Hirano,†,| and Hideo Hosono†,‡,| ERATO-SORST, Japan Science and Technology Agency (JST), in the Frontier Research Center, S2-6F East, Mailbox S2-13, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama 226-8503, Japan, Materials and Structures Laboratory, Mailbox R3-1, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama 226-8503, Japan, Department of Materials Science, Faculty of Engineering, Kyushu Institute of Technology, 1-1 Sensui-cho, Tobata-ku, Kitakyushu 804-8550, Japan, and Frontier Research Center, S2-6F East, Mailbox S2-13, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama 226-8503, Japan ReceiVed August 15, 2007. ReVised Manuscript ReceiVed October 23, 2007
Crystal structures, optoelectronic properties, and electronic structures of layered oxychalcogenides BiCuOCh (Ch ) S, Se, Te) have been compared to those of LaCuOCh, with an emphasis on the electronic configurations of Bi3+ (5d106s2) and La3+ (5d06s0). The BiCuOCh series were expected to exhibit better hole-transport properties than the LaCuOCh series because the pseudo-closed-shell 6s2 electronic configuration of the Bi3+ cation was expected to form valence band maxima (VBM) by admixing with the p orbitals of the Ch anions. However, the two series of compounds exhibited similar electrical properties, suggesting that the contribution of the Bi 6s orbitals to the VBM is small in BiCuOCh. The crystal structures and optical properties showed distinct differences; for example, the band gaps of BiCuOCh were smaller than those of LaCuOCh. These findings can be understood on the basis of the electronic structures obtained by photoelectron spectroscopy and density functional theory calculations. The Bi 6s orbitals form stronger and deeper chemical bonds with the O 2p orbitals than with Ch p orbitals and are located ∼2 eV below the VBM, which is mainly formed from the Cu 3d and Ch p orbitals. Thus, the Bi 6s2 configuration contributes little to the VBM, and BiCuOCh and LaCuOCh have similar hole-transport properties. Also, the smaller band gaps of BiCuOCh result from the deepening of the conduction-band-minima levels, which are composed of unoccupied Bi 6p orbitals.
Introduction Quaternary oxychalcogenides MCuOCh (M ) lanthanide ion; Ch ) chalcogen ion) with the layered crystal structure shown in Figure 11–5 exhibit unique optoelectronic properties. For example, many of these compounds are transparent p-type conductors [i.e., p-type semiconductors having wide band gaps (width larger than ∼3 eV)]6–8 and exhibit intense * To whom correspondence should be addressed. Tel: +81-45-924-5128. Fax: +81-45-924-5127. E-mail:
[email protected]. † ERATO-SORST, Japan Science and Technology Agency. ‡ Materials and Structures Laboratory, Tokyo Institute of Technology. § Department of Materials Science, Kyushu Institute of Technology. | Frontier Research Center, Tokyo Institute of Technology.
(1) Palazzi, M. Acad. Sci., Paris, C. R. 1981, 292, 789. (2) Zhu, W. J.; Huang, Y. Z.; Dong, C.; Zhao, Z. X. Mater. Res. Bull. 1994, 29, 143. (3) Popovkin, B. A.; Kusainova, A. M.; Dolgikh, V. A.; Aksel’rud, L. G. Russ. J. Inorg. Chem. 1998, 43, 1471. (4) Charkin, D. O.; Akopyan, A. V.; Dolgikh, V. A. Russ. J. Inorg. Chem. 1999, 44, 833. (5) Takase, K.; Sato, K.; Shoji, O.; Takahashi, Y.; Takano, Y.; Sekizawa, K.; Kuroiwa, Y.; Goto, M. Appl. Phys. Lett. 2007, 90, 161916. (6) Ueda, K.; Inoue, S.; Hirose, S.; Kawazoe, H.; Hosono, H. Appl. Phys. Lett. 2000, 77, 2701. (7) Ueda, K.; Hosono, H. J. J. Appl. Phys. 2002, 91, 4768. (8) Ueda, K.; Takafuji, K.; Hiramatsu, H.; Ohta, H.; Kamiya, T.; Hirano, M.; Hosono, H. Chem. Mater. 2003, 15, 3692.
Figure 1. Crystal structure of MCuOCh (M ) Bi, La; Ch ) S, Se, Te), which belongs to the tetragonal crystal system and the P4/nmm space group (No. 129). The solid-line box shows a unit cell.
blue to near-ultraviolet light emission9–12 and large thirdorder optical nonlinearity13 arising from room-temperature(9) Ueda, K.; Inoue, S.; Hosono, H.; Sarukura, N.; Hirano, M. Appl. Phys. Lett. 2001, 78, 2333. (10) Takase, K.; Koyano, M.; Shimizu, T.; Makihara, K.; Takahashi, Y.; Takano, Y.; Sekizawa, K. Solid State Commun. 2002, 123, 531. (11) Hiramatsu, H.; Ueda, K.; Takafuji, K.; Ohta, H.; Hirano, M.; Kamiya, T.; Hosono, H. J. Appl. Phys. 2003, 94, 5805.
10.1021/cm702303r CCC: $40.75 2008 American Chemical Society Published on Web 12/11/2007
Layered Oxychalcogenide Structures and Properties
stable excitons. Furthermore, regardless of the wide band gaps, these materials have p-type conductivities as large as 910 S/cm with hole concentrations larger than 1021 cm-3 and demonstrate degenerate conduction while maintaining hole mobilities as high as 3.5 cm2 V-1 s-1.14 Utilizing these carrier transport and light-emitting properties of LaCuOSe, we have demonstrated room-temperature operation of an excitonic blue light-emitting diode.15 Because it is possible to synthesize numerous materials having similar layered structures but containing different ions, exploring new materials and their electrical, optical, and magnetic properties has been of interest.16–26 These efforts have led to recent findings of new layered superconductors such as LaFeOP27,28 and LaNiOP29 by extending the material system from oxychalcogenides to oxypnictides. The M ions in MCuOCh have trivalent formal charges, and most of the M ions are limited to Y and lanthanide ions. We have reported that MCuOCh compounds with common Ch ions have similar optoelectronic properties,8,30 except for Ce-substituted oxychalcogenides such as CeCuOS, which have narrower band gaps (104 counts) on each measurement]. The measurement temperature was maintained at a constant value ((0.2 °C) during each measurement. The lattice parameters were refined by least-squares fitting using the diffraction angles of the peaks having 2θ > 70° corrected with an internal standard Si powder (NIST #640c). Crystal structures were refined by the Rietveld method with the RIETAN-2000 program,54 using the structure of LaCuOS (space group: P4/nmm, No. 129) as an initial model. It is generally difficult to obtain reliable data for site occupancies (g) and isotropic displacement parameters (B) from powder XRD data because these two values are strongly correlated with each other. Therefore, we carefully examined the refinement processes using different refinement paths in order to confirm that the results were not in a local minimum. Measurement of Optoelectronic Properties. Electrical properties were examined using the polycrystalline disk samples. Electrical conductivities were measured using a four-probe technique in the temperature range 10-300 K (apparatus: Quantum Design Physical Property Measurement System). Sputtered Pt-Pd alloy films (∼30 nm thick) were used as ohmic contacts. Thermoelectric powers (∆Vthermo) were measured near room temperature, with temperature (50) Ohtani, T.; Hirose, M.; Sato, T.; Nagaoka, K.; Iwabe, M. Jpn. J. Appl. Phys. 1993, 32 (Suppl. 32–3), 316. (51) Takano, Y.; Yahagi, K.; Sekizawa, K. Phys. B 1995, 206–207, 764. (52) Sekizawa, K.; Takano, Y.; Mori, K.; Yahagi, K. Czech. J. Phys. 1996, 46 (Suppl. S4), 1943. (53) Hiramatsu, H.; Orita, M.; Hirano, M.; Ueda, K.; Hosono, H. J. Appl. Phys. 2002, 91, 9177. (54) Izumi, F.; Ikeda, T. Mater. Sci. Forum 2000, 321–324, 198.
Hiramatsu et al. differences (∆T) varied from 2 to 10 K at the electrodes to determine carrier polarity at room temperature, and Seebeck coefficients at room temperature were obtained from the slopes of plots of ∆Vthermo versus ∆T. Diffuse reflectance spectra were measured on undoped powder samples in the visible-near-IR region at room temperature using a conventional spectrophotometer (apparatus: Hitachi U-4000). To estimate the optical band gaps, the observed diffuse reflectance spectra were converted by the Kubelka–Munk equation55 to plots of R/S, where R and S denote an absorption coefficient and a scattering factor, respectively. In the IR region, Fourier-transform (FT)-IR transmission and reflectance spectra (apparatus: PerkinElmer Spectrum One) were also measured for BiCuOTe at room temperature in a N2 flow with optical-quality dry KBr as a reference and diluting medium. Photoelectron Spectroscopy. Photoelectron spectroscopy (PES) measurements were conducted at room temperature on high-density samples (>90% in apparent density) sintered at 350-400 °C and 380 MPa using an SPS-515S spark-plasma sintering system (SPS Syntex Inc.) in order to reduce surface/grain boundary contamination and background noise. Ultraviolet photoelectron spectroscopy (UPS) using He II (40.8 eV), He I (21.2 eV), and Ne I (16.7 eV) excitation sources was employed to observe electronic structures in the vicinity of the valence band (VB). The sample surfaces were scraped with a diamond file under vacuum (∼1 × 10-7 Pa) in a preparation chamber that was connected to the measurement chamber. UPS measurements were performed at pressures less than 1 × 10-6 Pa. The Fermi energy of polycrystalline Au was used for calibrating the binding energy. Electronic structures of the conduction band (CB) were examined using inverse photoelectron spectroscopy (IPES) in bremsstrahlung isochromat spectroscopy (BIS) mode, which monitored the emission intensity of 9.5 eV photons with a band-pass photon detector. The PES and IPES spectra were combined by aligning their energy zeros in order to discuss the electronic structures in light of the DFT calculations. Density Functional Theory Calculations. The electronic structures were calculated via the full-potential linearized augmented plane wave/augmented plane wave + local orbitals (LAPW/ APW+lo) method using the WIEN2k program with PBE96 GGA functionals.56 The cutoff energy for the plane waves was 125 eV. It was confirmed that the fundamental electronic structures, especially the types of band-to-band transitions, did not change with the choice of functionals [the local density approximation (LDA) was also examined] or use of the pseudopotential plane wave method [via the CASTEP program (Accelrys KK)57].
Results and Discussion Crystal Structures. Figure 2 shows the XRD pattern of the BiCuOS powder and the results of the Rietveld analysis. Peaks that would be assigned to another phase were not observed, indicating that the sample was a single phase, and the XRD measurements indicated that all the other samples in this study were also single phases (see Figure S1 in the Supporting Information). Tables 1 and 2 summarize the refined crystal structures, lattice parameters, and reliability factors (with errors reported as standard deviations). It was found that the site occupancies (g) were almost 1.0 for all (55) Kubelka, P.; Munk, F. Z. Tech. Phys. 1931, 12, 593. (56) Blaha, P.; Schwarz, K.; Madsen, G.; Kvasnicka, D.; Luitz, J. WIEN2k, An Augmented Plane WaVe + Local Orbitals Program for Calculating Crystal Properties; Karlheinz Schwarz, Vienna University of Technology: Vienna, 2001 (ISBN 3-9501031-1-2). (57) Segall, M. D.; Lindan, P. J. D.; Probert, M. J.; Pickard, C. J.; Hasnip, P. J.; Clark, S. J.; Payne, M. C. J. Phys.: Condens. Matter 2002, 14, 2717.
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Figure 2. XRD pattern of BiCuOS powder and results of the Rietveld analysis: (top) observed XRD pattern; (middle) positions of Bragg diffraction peaks; (bottom) difference spectrum of the observed and simulated XRD patterns. Analogous data for the other MCuOCh compounds are shown in Figure S1 in the Supporting Information. Table 1. Multiplicities and Wyckoff Notations (WN), Site Occupancies (g), Fractional Coordinates (x, y, z), and Isotropic Displacement Parameters (B), Refined by the Rietveld Method, for Sites in (a) BiCuOCh and (b) LaCuOCha formula and site
WN
g
x
y
z
B (Å2)
(a) BiCuOCh BiCuOS Bi O Cu S BiCuOSe Bi O Cu Se BiCuOTe Bi O Cu Te
2c 2a 2b 2c
1.0 1.0 0.97(2) 1.0
1/4 3/4 3/4 1/4
1/4 1/4 1/4 1/4
0.14829(5) 0 1/2 0.6710(2)
0.37(1) 0.4(1) 0.65(1) 0.39(4)
2c 2a 2b 2c
1.0 1.0 0.97(2) 1.0
1/4 3/4 3/4 1/4
1/4 1/4 1/4 1/4
0.14020(4) 0 1/2 0.6758(1)
0.25(1) 0.5(1) 0.65(4) 0.21(3)
2c 2a 2b 2c
1.0 1.0 1.0 1.0
1/4 3/4 3/4 1/4
1/4 1/4 1/4 1/4
0.12733(9) 0 1/2 0.6810(1)
0.30(5) 0.5(2) 0.50(5) 0.51(8)
(b) LaCuOCh LaCuOS La O Cu S LaCuOSe La O Cu Se LaCuOTe La O Cu Te
2c 2a 2b 2c
1.0 1.0 0.96(3) 1.0
1/4 3/4 3/4 1/4
1/4 1/4 1/4 1/4
0.14763(7) 0 1/2 0.6623(3)
0.17(2) 0.3(1) 0.84(3) 0.46(5)
2c 2a 2b 2c
1.0 1.0 0.94(2) 1.0
1/4 3/4 3/4 1/4
1/4 1/4 1/4 1/4
0.13963(6) 0 1/2 0.6698(1)
0.34(2) 0.4(1) 0.62(1) 0.60(4)
2c 2a 2b 2c
1.0 1.0 0.96(2) 1.0
1/4 3/4 3/4 1/4
1/4 1/4 1/4 1/4
0.1267(1) 0 1/2 0.6754(1)
0.38(5) 0.4(1) 0.75(1) 0.53(5)
a Values in parentheses are standard deviations in the last digit. The g values for which standard deviations are not shown converged to 1.00 with errors less than 0.01 in the Rietveld refinements.
of the ions except Cu (Table 1). As noted in Experimental and Computational Procedures, it is difficult to obtain reliable and quantitative occupancy factors using powder diffraction analysis, and most of the deviations from 1.0 obtained for the g values were within 2 standard deviations of the mean. However, in previous work58 we systematically observed that the Cu sites in MCuFCh (M ) Sr, Eu) showed occupancy values that were clearly less than 1.0. Therefore, the present
results suggest that LaCuOCh and BiCuOCh contain Cu vacancies, which generate mobile holes and lead to p-type conduction as discussed in the next section. The lengths of both the a and c axes increased as the Ch ions were changed from S to Se to Te in both BiCuOCh and LaCuOCh (Table 2). This increase is attributed to the increase in the ionic radii of the Ch2- ions. Although the ionic radii of Bi3+ and La3+ are almost equal, the c/a axis-length ratios differed between BiCuOCh and LaCuOCh with common Ch ions. The a-axes were shorter and the c-axes longer in BiCuOCh than in LaCuOCh, suggesting that the bond angles differ in the two series of compounds. The Cu-Ch bond lengths were nearly equal in BiCuOCh and LaCuOCh with common Ch ions, but the M-O and M-Ch bond lengths were shorter in BiCuOCh than in LaCuOCh (Table 3). Bond lengths provide information about the nature of the chemical bonds because it is expected that the measured bond length (d) of an ideal ionic bond is close to the estimated bond length (dc + da) given by the sum of the ionic radii of a cation (dc) and a neighboring anion (da). Table 3 shows that the Cu-Ch bonds in both systems were reasonably explained by the ionic radii because the measured bond lengths agreed well with the estimated values. In contrast, the measured Bi-O and La-O bond lengths were much shorter than the estimated values, which suggests that the Bi-O and La-O bonds have greater covalent character than the Cu-Ch bonds. In addition, the La-Ch bond lengths in LaCuOCh were much longer than the estimated values, indicating that the La-Ch bonds are rather weak. This observation supports the view that LaCuOCh is a layered crystal in which the LaO layers are weakly bonded to the CuCh layers. A similar trend was observed for the Bi-Ch bond lengths in BiCuOCh for Ch ) S and Se, but the Bi-Ch lengths were shorter than the La-Ch lengths in LaCuOCh with common Ch ions. Moreover, the differences between the measured and estimated values were smaller in BiCuOCh than in LaCuOCh, suggesting that LaCuOCh has a more-distinct layered nature than BiCuOCh. The difference between the measured and estimated bond lengths diminished almost to zero for the Bi-Te bonds in BiCuOTe, which suggests that the two-dimensional nature of BiCuOTe is much weaker than those of other MCuOCh. As noted above, characteristic structural differences between BiCuOCh and LaCuOCh were found in the chemical bond angles and the shapes of the constituting coordination polyhedra. Figure 3a summarizes the variation of the Ch-Cu-Ch bond angles (R and β) in the distorted CuCh4 tetrahedra as Ch was changed from S to Te through Se. The Ch-Cu-Ch angles in LaCuOS had similar values that were close to that in a regular tetrahedron (109.5°). Changing the Ch ion from S to Te caused R to decrease and β to increase in both BiCuOCh and LaCuOCh, making the distortion larger and the tetrahedral shape more stereoscopic. Similar behavior was seen in the Ch-M-Ch angles in the MO4Ch4 polyhedra (Figure 3b). Although the Cu-Ch bond lengths were nearly equal in BiCuOCh and LaCuOCh with common Ch ions, CuCh4 distortions were larger in BiCuOCh than in LaCuOCh. (58) Motomitsu, E.; Yanagi, H.; Kamiya, T.; Hirano, M.; Hosono, H. J. Solid State Chem. 2006, 179, 1668.
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Table 2. Lattice Parameters and Reliability Factors (R) Obtained from Rietveld Analysis at the Stated Measurement Temperatures (T)a
T (°C) a (Å) c (Å) Rwp (%)b Rpb (%) Re (%)b S ()Rwp/Re) RF (%)b
BiCuOS
BiCuOSe
BiCuOTe
LaCuOS
LaCuOSe
LaCuOTe
25.2(1) 3.8691(1) 8.5602(4) 6.0 4.4 3.4 1.8 1.5
25.1(1) 3.9287(1) 8.9291(2) 7.7 5.9 5.1 1.5 2.8
25.3(2) 4.0411(2) 9.5237(5) 11 7.9 8.3 1.3 2.4
25.1(2) 3.9938(2) 8.5215(4) 6.2 4.8 5.7 1.1 1.2
24.9(2) 4.0670(1) 8.8006(8) 4.9 3.8 4.3 1.1 1.3
24.6(1) 4.1808(2) 9.3441(8) 8.1 6.1 6.4 1.3 2.4
a Values in parentheses are standard deviations in the last digit. b Rwp ) {∑iwi[yi - fi(x)]2/∑iwiyi2}1/2, Rp ) ∑i|yi - fi(x)|/∑iyi, Re ) [(N - P)/∑iwiyi2]1/2, and RF ) ∑K||Fo(hK)| - |F(hK)||/∑K|Fo(hK)|, where wi is a statistical weight, yi is an observed intensity, fi(x) is a theoretical intensity, N is the number of measurement points, P is the number of refinement parameters, Fo(hK) is an estimated structure factor, and F(hK) is a calculated structure factor.
Table 3. Measured and Estimated Bond Lengths in (a) BiCuOCh and (b) LaCuOCha bond
d (Å)
dc + da (Å)
Diff. (%)
(a) BiCuOCh BiCuOS Bi-O Bi-Ch Cu-Ch BiCuOSe Bi-O Bi-Ch Cu-Ch BiCuOTe Bi-O Bi-Ch Cu-Ch
2.31 3.14 2.43
2.57 3.01 2.44
-10.0 4.4 -0.6
2.33 3.23 2.51
2.57 3.15 2.58
-9.4 2.5 -2.5
2.36 3.39 2.66
2.57 3.38 2.81
-8.3 0.3 -5.5
(b) LaCuOCh LaCuOS La-O La-Ch Cu-Ch LaCuOSe La-O La-Ch Cu-Ch LaCuOTe La-O La-Ch Cu-Ch
2.36 3.26 2.43
2.56 3.00 2.44
-7.8 8.5 -0.4
2.38 3.33 2.52
2.56 3.14 2.58
-7.2 6.0 -2.2
2.40 3.49 2.66
2.56 3.37 2.81
-6.2 3.5 -5.5
a The column labeled “d” gives measured bond lengths, and the column labeled “dc + da” gives bond lengths estimated as the sum of the ionic radii of a cation (dc) and a neighboring anion (da). “Diff.” denotes the difference between the measured and estimated distances. Standard deviations, which are estimated from those of the lattice parameters and the fractional coordinates, are all smaller than 0.002 Å.
On the other hand, the O-Bi-O angles in BiCuOCh were much smaller than the O-La-O angles in LaCuOCh. The bond angles in the polyhedra show that the bonding character of BiCuOCh differs from that of LaCuOCh, reflecting the different electronic configurations of the M ions. It is noteworthy that the structure of the BiO layer in BiCuOCh was the same as that of the PbO layer in R-PbO (see Figure S2 in the Supporting Information) and that the asymmetric structure of the PbO layer, which accompanies the asymmetric electron density of the Pb ion, can be explained in terms of the Pb 6s2 “lone pair” of electrons in a classical stereochemistry view. This classical view assumes that the Pb 6s orbitals hybridize with Pb p orbitals, leading to the formation of chemically inert but sterically active “lone pair orbitals” that project outward from the side opposite the Pb-O chemical bonds, thereby stabilizing the asymmetric structure.59,60 The more asymmetric structures of the BiO layers in BiCuOCh compared to those of the LaO layers in LaCuOCh could also be explained in a similar manner.
Figure 3. Bond angles in MCuOCh. Closed and open symbols refer to BiCuOCh and LaCuOCh, respectively. The corresponding MCuOCh polyhedra are shown at the right. (a) Angles in the distorted CuCh4 tetrahedra. The angles R (circles) and β (triangles) denote the Ch1-Cu-Ch2 ()Ch3-Cu-Ch4) and Ch1-Cu-Ch3 ()Ch2-Cu-Ch4) angles, respectively. (b) Angles in the MO4Ch4 bicapped trigonal prisms, in which the M ion is coordinated by four O and four Ch ions. Circles and triangles denote O-M-O and Ch-M-Ch angles, respectively.
However, care should be taken in employing this classical view because recent ab initio calculations have indicated that it is inaccurate. In particular, the ns2 electrons in Pb2+ and Sn2+ are not inert but instead form chemical bonds with ligand anions.61,62 Therefore, the ns2 electrons are not regarded as a lone pair in a strict sense. On the other hand, from the viewpoint of electrical properties, the ab initio calculations have also indicated that the Pb 6s and Sn 5s (59) Orgel, J. E. J. Chem. Soc. 1959, 1959, 3815. (60) Cotton, F. A.; Wilkinson, G. AdVanced Inorganic Chemistry: A ComprehensiVe Text, 4th ed.; John Wiley: New York, 1980; p 327. (61) Watson, G. W.; Parker, S. C. J. Phys. Chem. B 1999, 103, 1258. (62) Watson, G. W. J. Chem. Phys. 2001, 114, 758.
Layered Oxychalcogenide Structures and Properties
Figure 4. Temperature dependence of electrical conductivities (σ) in (a) BiCuOCh and (b) LaCuOCh: (solid lines) undoped BiCuOCh and LaCuOCh; (dotted lines) Pb-doped BiCuOCh and Sr-doped LaCuOCh.
orbitals hybridize with the VBM in PbO, PbS, and SnO, suggesting that these orbitals contribute to the formation of hole-transport properties in known p-type semiconductors such as PbS and SnO.42–44 However, as shown in the next section, very similar hole-transport properties were observed in BiCuOCh and LaCuOCh, in contrast to the cases of PbS and SnO. Therefore, we examined further the electronic structures of BiCuOCh using DFT calculations, as described in a later section. Electrical Properties. Thermoelectric power measurements were performed near room temperature for undoped BiCuOCh, undoped LaCuOCh, 5 atom % Pb-doped BiCuOCh, and 5 atom % Sr-doped LaCuOCh. All of the samples had positive Seebeck coefficients (see Figure S3 in the Supporting Information), indicating that they were p-type semiconductors. Figure 4a,b shows the temperature dependence of the electrical conductivities. The conductivities increased as Ch was changed from S to Te for both BiCuOCh and LaCuOCh. Pb and Sr doping further increased the conductivities, suggesting that substituting trivalent M3+ ions with divalent Pb2+ and Sr2+ ions generates mobile holes. The room-temperature conductivities in BiCuOCh were an order of magnitude larger than those in LaCuOCh for Ch ) S and Se, revealing that BiCuOCh (Ch ) S, Se) have higher hole concentrations and/or larger mobilities than LaCuOCh. On the other hand, BiCuOTe and LaCuOTe both exhibited metallic conduction and had comparable room-temperature conductivities, but the slope of the conductivity with respect to temperature was larger for BiCuOTe, suggesting that a larger contribution of phonon scattering causes greater deterioration of hole transport. The conductivity of undoped BiCuOS decreased linearly as the temperature decreased in the Arrhenius plot, demonstrating that hole conduction is controlled by a thermally activated process having an activation energy of ∼100 meV. When BiCuOS was doped with Pb2+, the temperature dependence became smaller and showed a large deviation from linearity in the Arrhenius plot, which gave a smaller apparent activation energy of ∼50 meV near room temper-
Chem. Mater., Vol. 20, No. 1, 2008 331
ature. For LaCuOS, the activation energy for hole conduction in undoped LaCuOS was ∼200 meV, while Sr-doped LaCuOS showed degenerate conduction. These observations indicate that the acceptor level is shallower in undoped BiCuOS than in undoped LaCuOS but that carrier generation efficiency is lower for Pb2+ doping of BiCuOS than for Sr2+ doping of LaCuOS. The temperature dependence of the conductivity of undoped BiCuOSe showed a peak at ∼130 K. Similar behavior was reported for BiCu1-xOSe (x ) 0, 0.005) and assigned to a first-order phase transition.48 These results suggest that a phase transition occurs at this temperature in our BiCuOSe samples, but further investigation is needed to clarify its origin. Pb2+ doping of BiCuOSe diminished the peak and increased the conductivity to ∼10 S/cm at room temperature, near which the temperature dependence corresponded to a thermally activated process with an apparent activation energy of ∼34 meV that would reflect an acceptor level. BiCuOTe showed a metallic temperature dependence irrespective of doping, probably because the VBM of the telluride is much shallower than those of the selenide and sulfide, in which case the acceptor level would be in the VB, generating a large number of holes and causing degenerate conduction even in the nominally undoped samples. Similar behavior was also observed in LaCuOTe. From these results, which verify similar hole-transport properties in BiCuOCh and LaCuOCh with common Ch ions, we speculate that the electronic structures of the VBM of BiCuOCh are similar to those of LaCuOCh and that contribution of the Bi 6s orbitals is not large in the vicinity of the VBM. The primary origin of the hole carriers may be the Cu+ vacancies in the nominally undoped samples, as also suggested by the Rietveld analyses. The conductivities increased as Ch was changed from S to Te, which in the case of LaCuOCh can be explained in terms of increased hole mobilities resulting from greater hybridization of the VBM with spatially diffuse Ch p orbitals.12,34–38 Similar behavior was observed in BiCuOCh, suggesting that the Ch p orbitals hybridize with the VBM to a great extent in BiCuOCh. Optical Properties. In contrast to the case of the electrical properties, large differences were observed between the optical properties of BiCuOCh and LaCuOCh. Figure 5a shows R/S spectra of BiCuOCh in the visible-near-IR region. These spectra indicate that the band gaps were in the near-IR region for BiCuOS and BiCuOSe, which was consistent with the black color of the sintered and powder samples. The estimated band gaps were ∼1.1 eV for BiCuOS and ∼0.8 eV for BiCuOSe. These band gaps were much smaller than those of LaCuOS (∼3.1 eV), LaCuOSe (∼2.8 eV), and LaCuOTe (∼2.4 eV), which are shown in the inset of Figure 5a. A clear band-edge structure was not observed for BiCuOTe in the visible-near-IR spectrum. The FT-IR transmission and reflection spectra (Figure 5b) began to rise at 0.4-0.5 eV, suggesting that the band gap of BiCuOTe is 0.4-0.5 eV. These results indicate that the band-gap electronic structures of BiCuOCh obviously differ from those of LaCuOCh. The similar hole-transport properties in Figure
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Figure 5. (a) Optical absorption (R/S) spectra in the visible-near-IR region for BiCuOCh powders, measured at room temperature and converted using the Kubelka–Munk equation. The inset shows spectra of LaCuOCh for comparison. Optical band gaps were estimated from the intersections of the dotted lines. (b) FT-IR transmission spectrum of BiCuOTe powder measured at room temperature. The inset shows the reflection spectrum of BiCuOTe along with that of KBr powder used as a reference. Weak absorption bands at ∼0.4 eV (λ ≈ 2.9 µm, κ ≈ 3420 cm-1), indicated by vertical bars, originate from the O-H stretching vibration of H2O molecules adsorbed on the powder surfaces.
4 suggest similar hole-transport paths. Therefore, it is reasonable to speculate that the VBM structures of BiCuOCh and LaCuOCh are similar. If this is true, then the difference in the electronic structures of the energy gaps should be attributed mainly to variation of the electronic structures of the CBM. It should also be noted that the R/S spectra for BiCuOS and BiCuOSe showed sharp band-edge structures, which may be assigned to direct transitions. However, as will be shown in the next section, DFT calculations indicate that the band-gap structures of BiCuOCh are of an indirect type. This apparent contradiction may be due to the fact that the indirect and direct band-gap energies were very close, preventing broadband edge features often observed in typical indirect-transition-type materials from being distinguished in the diffuse reflectance spectra. Electronic Structures. The aforementioned structural and optoelectronic properties are consistently explained by the electronic structures. Figure 6 shows UPS and IPES spectra along with densities of states (DOSs) calculated using DFT for BiCuOS and LaCuOS (similar plots for BiCuOSe and
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Figure 6. Experimental UPS and IPES spectra along with calculated total and partial densities of states (DOSs) for (a) BiCuOS and (b) LaCuOS. Energies are measured from the valence band maxima (VBM). Onsets of the IPES spectra determine the conduction band minima (CBM). It should be noted that the DFT calculations do not consider correlation effects of localized electrons, which causes the La 4f energy levels, which appear near the CBM in this result, to be underestimated. More accurate calculations (employing the LDA with an on-site Coulomb repulsion parameter U) are reported elsewhere.37
BiCuOTe are found in Figure S4 in the Supporting Information). These spectra indicate that the VBM for both BiCuOS and LaCuOS were mainly composed of Cu 3d and S 3p orbitals. In order to determine the origin of the UPS bands, the UPS spectra were measured using three excitation sources: He II, He I, and Ne I (see Figure S4 in the Supporting Information; the caption of this figure describes the assignments, which are based on refs 38 and 63). The results confirm that BiCuOCh and LaCuOCh had a similar dependence on the excitation source, substantiating that they had similar VBM electronic structures. As described in the Introduction, we hypothesized that Bi 6s orbitals appear in the vicinity of the VBM in BiCuOCh and form more dispersed VBs, which enhance hole conduction. However, the calculated DOSs implied that the Bi 6s orbitals in BiCuOCh are located at 11-12 eV relative to the VBM for
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Figure 8. Schematic band structures of (a) BiCuOS and (b) LaCuOS built on the basis of UPS and optical measurements and DFT calculations. AB, NB, and B denote antibonding, nonbonding, and bonding states, respectively. Relative positions of all the states are based on the vacuum level. Ionization potentials (i.e., the VBM energies) are obtained from the UPS measurements, while the band gaps are obtained from the optical measurements.
Figure 7. Band structures of (a) BiCuOS and (b) LaCuOS. Characters on the abscissas denote special k points in the reciprocal space of a tetragonal lattice. Their coordinates in the Brillouin zone are (0, 0, 0) for Γ, (1/2, 1/2, 0) for M, (1/2, 0, 0) for X, (0, 0, 1/4) for Z, and (1/2, 1/2, 1/4) for A.
the bonding states with O 2p and at 2-4 eV for the antibonding states. Hence, the Bi 6s orbitals are not virtually located at the VBM (Figure 6a). This result indicates that, as in the case of PbO and SnO,61,62 a lone pair does not exist in the Bi3+ ions in BiCuOCh in an accurate sense. The calculated DOSs also showed that the CBM of BiCuOCh mainly consist of Bi 6p orbitals. This fact indicates that the CBM electronic structure is entirely different from that in LaCuOCh, where the CBM is mainly composed of Cu 4s orbitals with smaller contributions from the La 5d orbitals (or, in the case of LaCuOTe, the Cu 4s and La 5d orbitals make similar contributions to the CBM).34,36 The observed differences in the onset (i.e., CBM) energies and the shapes of the IPES spectra of BiCuOS and LaCuOS can be attributed to the differences in the CBM structures. The deeper CBM in BiCuOCh is the primary cause of the smaller band gaps of BiCuOCh relative to those of LaCuOCh. Figure 7 shows the energy band structures of BiCuOS and LaCuOS. BiCuOS is a semiconductor of the indirect-
transition type (the CBM is at the Z point, but the VBM is on the M-Γ line), while LaCuOS is of the direct-transition type.34,36,37 The calculated band-gap energies were 0.67 eV for BiCuOS, 0.47 eV for BiCuOSe, and 0.21 eV for BiCuOTe (see Figure S5 in the Supporting Information), which are much smaller than those observed in the R/S spectra (Figure 5) because DFT significantly underestimates the band-gap values. However, when the calculated and observed band-gap values were compared, differences between them were similar, indicating that the calculated values appropriately reflect the order of the observed band-gap values. Comparison of BiCuOCh Electronic Structures to Those of Other ns2-Based Compounds. Figure 8 schematically illustrates the electronic structures obtained above. Both the bonding and antibonding states in BiCuOCh formed from Bi 6s orbitals are deep states. Therefore, the Bi 6s2 configuration makes a negligible contribution to the VBM, and the electronic structures of BiCuOCh should be compared to those of other ns2-based compounds. Similar deep bonding states were also formed in the Pb-O bonds in R-PbO (see Figure S6a in the Supporting Information), the Pb-S bonds in PbS (see Figure S6b in the Supporting Information), and the Sn-O bonds in SnO.62 However, in these compounds the antibonding states arising from the ns orbitals formed the VBM, which is the reason why we hypothesized that the Bi 6s orbitals would form the VBM in BiCuOCh. Unexpectedly, the above results on the crystal and electronic structures reveal that the Bi 6s orbitals do not form the VBM in BiCuOCh because they form stronger chemical bonds with O 2p orbitals than with Ch p orbitals. As a result, the Bi 6s-O 2p antibonding states are more
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than 2 eV deeper than the VBM, which is mainly composed of Cu 3d and Ch p orbitals. Thus, the primary reason why the Bi 6s orbitals do not appear at the VBM in BiCuOCh is that although Bi ions are coordinated by both oxygen and chalcogen ions, the Bi-Ch bonds are longer than the Bi-O bonds, as elucidated by the Rietveld analyses. These longer bonds result in a smaller extent of hybridization between Bi 6s and Ch p than between Bi 6s and O 2p. Consequently, the Bi 6s levels are located rather deep in the VB. These comparisons lead us to the conclusion that ns2-based cations should be coordinated only by chalcogen ions in oxychalcogenides (and other mixed-anion compounds) if we design an enhanced hole transport that benefits from the ns orbitals. If such a structure is found in a layered crystal, we expect that it will be a new wide-gap or transparent p-type conductor because of the band-gap widening effect induced by reducing the structural dimension, as observed in La2CdO2Se2 (band gap ∼3.3 eV) compared with CdSe (∼1.8 eV),64 Sr2TiO4 (∼4 eV)65,66 compared with SrTiO3 (∼3.2 eV),67 and so on. Conclusion Single-phase BiCuOCh and LaCuOCh (Ch ) S, Se, Te) were successfully synthesized, and their structures and (63) Yeh, J.-J. Atomic Calculation of Photoionization Cross-Sections and Asymmetry Parameters; Gordon and Breach: Langhorne, PA, 1993. (64) Kamiya, T.; Hiramatsu, H.; Nomura, K.; Hosono, H. J. Electroceram. 2006, 17, 267. (65) Matsuno, J.; Okimoto, Y.; Kawasaki, M.; Tokura, Y. Phys. ReV. Lett. 2005, 95, 176404. (66) Lee, K. H.; Ishizaki, A.; Kim, S. W.; Ohta, H.; Koumoto, K. J. Appl. Phys. 2007, 102, 033702. (67) Noland, J. A. Phys. ReV. 1954, 94, 724.
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optoelectronic properties were examined. It was found that all of these compounds are p-type semiconductors and that hole doping is possible by addition of Pb2+ and Sr2+ to BiCuOCh and LaCuOCh, respectively. The hole-transport properties were similar for BiCuOCh and LaCuOCh with common Ch ions, suggesting that their valence band maxima (VBM) structures are similar. On the other hand, the band gaps of BiCuOCh were much narrower than those of LaCuOCh, indicating that the conduction band minima (CBM) structures differ significantly. A consistent explanation for the VBM and CBM structures was obtained from DFT calculations along with photoelectron spectroscopy. It was also found that the shapes of the coordination polyhedra around the trivalent metal ion are more stereoscopic in BiCuOCh than in LaCuOCh. The Bi 6s orbitals formed only deep levels and did not hybridize with the VBM because Bi 6s forms stronger and deeper chemical bonds with O 2p than with the Ch p orbitals that form the VBM. These results provide a new direction in the search for a new transparent (wide-gap) p-type conductor. Acknowledgment. The crystal structure and the wave function in the TOC figure were drawn using VESTA.68 Supporting Information Available: Crystallographic information data for MCuOCh (M ) Bi, La; Ch ) S, Se, Te) (CIF). XRD patterns, crystal structures, Seebeck coefficients, UPS spectra, band structures, and densities of states (PDF). This material is available free of charge via the Internet at http://pubs.acs.org. CM702303R (68) Momma, K.; Izumi, F. Int. Union Crystallogr. Newsl. 2006, 7, 106.
