Oxychalcogenide BaGeOSe2: Highly Distorted Mixed-Anion Building

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Oxychalcogenide BaGeOSe2: Highly Distorted Mixed-Anion Building Units Leading to A Large Second-Harmonic Generation Response Bin-Wen Liu, Xiao-Ming Jiang, Guan-E Wang, Hui-Yi Zeng, MingJian Zhang, Shu-Fang Li, Wei-Huan Guo, and Guo-Cong Guo Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b03649 • Publication Date (Web): 25 Nov 2015 Downloaded from http://pubs.acs.org on December 1, 2015

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Oxychalcogenide BaGeOSe2: Highly Distorted Mixed-Anion Building Units Leading to A Large Second-Harmonic Generation Response Bin-Wen Liu,a Xiao-Ming Jiang,b Guan-E Wang,a Hui-Yi Zeng,a Ming-Jian Zhang,a Shu-Fang Li,a Wei-Huan Guo,a Guo-Cong Guo*,a a

State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter Chinese Academy of Sciences, Fuzhou 350002 (P. R. China) b

Institute for Quantum Materials, Hubei Polytechnic University, Huangshi 435003 (P. R. China)

ABSTRACT: By a facile synthetic routine, the heavy Se2− anion has been incorporated in oxide to generate a new oxychalcogenide, BaGeOSe2, with acentric mixed-anion basic building units (BBUs) of GeO2Se2 tetrahedron and BaOSe6 polyhedron. Remarkably, these highly distorted BBUs lead to a large nonlinear optical (NLO) response, which would provide a new opportunity to further explore NLO materials.

Second-order nonlinear-optical (NLO) materials are of importance and interest in view of their uses in laser frequency conversion based on the second-harmonic generation (SHG) and optical parametric oscillation (OPO) processes. 1 – 5 Noncentrosymmetric (NCS) crystallographic structure is an essential prerequisite for the NLO materials. However, for NLO materials to be practical availability in laser devices, the requirements of large NLO response, wide transparent window, suitable birefringence for phase-matching and good environmental stability should be satisfied. Among them, how to design large NLO efficiency is a pressing and significant topic, which has prompted the continuous research for finding new NLO materials. The Chen’s Anionic Group Theory, 6 , 7 which attributes the SHG response in a material to the NLO-active basic building units (BBUs), has been very successful in guiding the development of NLO borates.8, 9 To further enhance the SHG efficiency based on the Anionic Group Theory, other specific BBUs have been introduced, such as anionic groups with d0 transition-metal centers (e.g., Ti4+, V5+, Nb5+, Mo6+, etc.) 10, 11 or main group cations with stereoactive lone pairs (e.g., Pb2+, Se4+, Te4+, I5+, etc.),12−14 both of which are susceptible to secondorder Jahn-Teller (SOJT) distortions. These distortions are mainly responsible for the remarkable SHG response, such as the well-known LiNbO3 and KTiOPO4 etc. Their high SHG efficiencies stimulate chemists to rationally design specific NLO-active BBUs with high polarity. The common BBUs above-mentioned is mainly focused on a cationic center coordinated by one type of anion to form coordination polyhedron, denoted as single-anion BBUs; instead, we propose a new strategy for the design and synthesis of new NLO materials, namely mixed-anion BBUs, in which the metal center is coordinated by two or more types of anions orderly. Unlike to single-anion BBUs, in which the distortion of metal coordination poly-

hedron is limited, mixed-anion BBUs possess not only larger distortion of coordination polyhedron to yield higher polarity but also larger anisotropy due to the different size and electronegativity of anions, where the higher polarity may enhance the NLO response, and the larger anisotropy can lead to big birefringence which is beneficial for the phase-matching behavior of NLO materials. These motifs make mixed-anion BBUs to be a favorable strategy for the exploration of new NLO materials. Generally, chalcogenide materials display higher SHG coefficients than the oxide analogues due to the presence of greater polarizability of metal-chalcogen bonds. 15 Therefore, adopting our reported synthetic technique of replacing oxygen from metal oxides with chalcogen Q (Q = S, Se, Te) using elemental boron,16−19 we can choose the known NLO oxides as precursors to partially replace O2− anions with more polarizable Q2− anions to yield new NLO-active mixed-anion BBUs, [MOxQy], with expected stronger NLO response. On the basis of these ideas, we reported a germanium oxychalcogenide BaGeOSe2 (1), which exhibits strong powder SHG response of 1.1 times that of benchmark AgGaS2 at a laser irradiation of 1400 nm. Theoretical calculations demonstrate that the highly polarizable mixedanion BBUs GeO2Se2 tetrahedra and BaOSe6 polyhedra make the predominant contribution to the large SHG response. Comparing to widely explored oxides and chalcogenides, oxychalcogenides are rarely investigated, one of the main reasons is attributed to their synthetic challenge, for example, the synthesis process often come with high temperature, low yield and difficulty to obtain large enough crystals, which seriously limited their further properties studied.20−22 Here, we employed our previously reported synthetic technique 16 that replacing oxygen

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from metal oxides with heavy chalcogen Q (Q = S, Se) using element boron (see the supporting information). Needle-like crystals of 1 with high-yield of 95% (based on Ba) and medium-size of a single crystal with 0.8×0.8×3 mm3 (Figure S1) were successfully achieved, suggesting that this synthetic technique can be further applied in the investigation of oxychalcogenides. Its purity was confirmed by X-ray diffraction (XRD) powder diffraction analysis (Figure S2). Energy dispersive spectroscopy (EDS) analysis of the crystal gives the Ba/Ge/Se molar ratio of 1.1:1.0:2.0, which is close to that determined from singlecrystal XRD analysis (Figure S3). Differential scanning calorimetry (DSC) at a rate of 10 °C∙min−1 reveals that compound 1 undergoes melting at 920 °C upon heating and crystallization at 814 °C upon cooling (Figure S4). The powder XRD patterns of the residue from a melting/recrystallization cycle are matching well with that of the initially synthesized products and that simulated from the CIF (Figure S5), indicating the phase is congruently melting.

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a axis. The chains are built from corner sharing GeO2Se2 tetrahedra, in which Ge4+ ion is coordinated by two terminal atoms Se and two bridging atoms O in a tetrahedral geometry (Figure 1b), presenting a novel coordination environment for germanium in the solid state. The large Ba atoms are bonded to six Se atoms and one O atom to form asymmetric polyhedra, and a further combination 1 [GeOSe ]−2 chains results in the with the asymmetric ∞ 2 crystallographic asymmetry of compound. The results of bond valence calculations 24 (Ba, 1.96; Ge, 4.10) indicate that the Ba and Ge atoms are in oxidation states of +2, and +4, respectively. It is interesting to compare the structural features in 1 and BaGeO3 25 to illustrate their structural evolution. The overall structure of 1 is similar to the known BaGeO3 that belongs to pyroxene-type structure type. It also can be described that compound 1 derive from the substitution of Se2− for terminal O2− anion in BaGeO3. Nevertheless, distinct from BaGeO3 structure, except the cell constants and cell volume greatly increase in 1 due to the different size of O2− (ionic radius: 1.21 Å) and Se2− (ionic radius: 1.84 Å) anions, the main differences in the structure are GeO4 units being replaced by GeO2Se2 units, and the BaO7 units replaced by BaOSe6 units (Figure 2). Specially, the Gecentered tetrahedra in 1 are distorted, as evidenced by the Ge−Se bond lengths (2.291(1)–2.305(1) Å) and Ge−O bond distances (1.789(4)–1.790(4) Å) as well as the ∠Se-Ge-Se, ∠O-Ge-Se, and ∠O-Ge-O bond angles (124.77, 99.50– 111.97 and 104.46 °, respectively). And the BaOQ6 polyhedra are also distorted with Ba–Se bond lengths ranging from 3.283(1) to 3.616(1) Å and Ba–O bond length of 2.608(4) Å. The inequilateral coordinations in these mixed-anion BBUs are inclined to result in a NCS space group and are also beneficial for enhancing the dipole moment of individual polyhedra, which may make significant contribution to strong SHG intensity.

Figure 1. (a) Structure of 1 viewed slightly skewed from the 1 [GeOSe ]−2 chain built of corner sharing a-axis. (b) A single ∞ 2 GeO2Se2 tetrahedra.

Figure 2. The BBUs of GeO4 and BaO7, GeO2Se2 and BaOSe6 polyhedron in BaGeO3 and 1, respectively.

Figure 3. Phase-matching curves, i.e., particle size versus SHG response; Insert: SHG signal of compound 1 and AgGaS2.

Compound 1 crystallizes in the NCS space group of orthorhombic P212121.23 As shown in Figure 1a, it features 1 [GeOSe ]−2 anion along the parallel chains of the infinite ∞ 2

The UV−vis diffuse reflectance spectrum of polycrystalline 1 gives the band gap of 3.2 eV, which is consistent with its colorless in nature (Figure S6). Further absorp-

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tion spectrum in Figure S7 shows that the absorption peaks appearing near 12.5 µm, which are correspond to Ge-O stretching mode. The Raman spectrum shows that there are absorption peaks at 325, 316, 251 and 191 cm−1, which are according with that 329, 310, 251 and 196 cm−1 found in GeSe2.26

eV, whereas shallower atomic level of Se 4p states are centered on −1 eV in the VB. Therefore, the electronic structure around the band edges is thus mainly derived from the GeO2Se2 and BaOSe6 groups, which provide the dominant states in the optical matrix elements describing the virtual excitations responsible for the NLO effect in 1.

Since the NCS structure feature of 1, the powder SHG measurements were carried out by the laser of 1400 nm, by a modified Kurtz powder method.27 The crystalline sample of NLO material AgGaS2 with similar particle sizes was served as the standard. The SHG intensities of 1 increase with the increasing of particle size, and then reach a plateau at the maximum value after a certain particle size (Figure 3), suggesting a type I phase-matching behavior, according to the rule proposed by Kurtz and Perry. Remarkably, compound 1 exhibits a large SHG response of 1.1 times that of benchmark AgGaS2 (see the inset in Figure 3). It is worth noting that compound 1 is the first example of oxyselenide that being applied as NLO material. It is well-accepted that the property of the material is determined by its crystal structural futures. In the case of the SHG response, it mainly based on the distortion of the structure when the compound is NCS. To investigate the contribution of the different building units to the SHG response, we calculated the local dipole moments of the BBUs in BaGeO3 and 1 using a bond-valence method described earlier by Poeppelmeier and Halasyamani and their co-workers.28, 29 As listed in Table 1, the dipole moments of the GeO4 and BaO7 units in BaGeO3 are calculated to be 4.65 and 4.70 D, respectively, whereas, the dipole moments of GeO2Se2 and BaOSe6 units in 1 are significantly increasing with the value of 11.25 and 20.47 D, respectively. These results indicate that the mixed-anion BBUs GeO2Se2 and BaOSe6 units have higher polarity in the structure, which are responsible for the observed NLO responses. Table 1. The direction and magnitude (in Debye) of the polyhedral dipole moments for BBUs in BaGeO3 and 1. Species BaGeO3 BaGeOSe2

x(a)

y(b)

z(c)

Magnitude

GeO4

-4.50

0.013

1.16

4.65

BaO7

0.42

-3.81

2.72

4.70

GeO2Se2

-1.74

-5.70

-9.54

11.25

BaOSe6

-9.38

11.50

-14.09

20.47

To better understand the optical properties, electronic structure calculations based on DFT methods were performed. The band structure calculations indicate that compound 1 is a direct band-gap material with the band gap value of 2.70 eV (Figure S9, from G to G). The partial densities of states (DOS) are presented in Figure 4. The conductive band (CB) is mostly composed of Ba 6s, Ge 4s and Se 4p states, mixing with small amounts of Se 4s and Ge 4p states, while the valence band (VB) from −5.0 eV to the Fermi level originates predominately from Se 4p and O 2p states, mixing with small amounts of Ba 6s, and Ge 4p states. Particularly, O 2p states are centered around −2

Figure 4. Partial density of states (DOS) of 1.

To gain further insights into the NLO properties, the calculations of second-order NLO susceptibility were also performed to explain their SHG efficiencies. Under the restriction of Kleinman’s symmetry, compound 1 has only one nonzero independent SHG coefficient because of its P212121 space group. As shown in Figure S14, the calculated d36 coefficient is 2.0 pm/V at the wavelength of 1400 nm (0.886 eV). In addition, the large birefringence value of 1 range from 0.153 to 0.178 with the energy from 0 to 2 eV, indicating compound 1 may favorably achieve the phasematching condition in SHG process. In summary, by a facile synthetic routine, the large Se2− anion has been introduced in the oxide to produce a new oxychalcogenides, BaGeOSe2. Interestingly, it contains an acentric mixed-anion BBUs of GeO2Se2 tetrahedron and BaOSe6 polyhedron, which are responsible for the large NLO response. These results indicate that mixed-anion BBUs can sever as a blue print for the design and synthesis of new NLO active materials as well as other NCS materials. Similar substitution attempts in related oxides are in progress.

ASSOCIATED CONTENT Supporting Information CIF data, additional tables and figures. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author ∗

E-mail address: [email protected] (G.-C. Guo)

Notes The authors declare no competing financial interest.

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ACKNOWLEDGMENT This work was financially supported by the NSF of China (91222204, 21403231, 21101152, and 91021004) and the NSF of Fujian Province (2014J05025 and 2014J05034)

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