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LiGaP2O7: A Potential UV Nonlinear-Optical Crystal Yunfei Li,†,‡,§ Fei Liang,†,§ Huimin Song,†,‡,§ Wang Liu,†,‡,§ Zheshuai Lin,† Guochun Zhang,*,†,‡ and Yicheng Wu⊥
Inorg. Chem. Downloaded from pubs.acs.org by UNIV AUTONOMA DE COAHUILA on 05/10/19. For personal use only.
†
Center for Crystal Research and Development, Key Laboratory Functional Crystals and Laser Technology of Chinese Academy of Sciences, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China ‡ Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, P. R. China § University of Chinese Academy of Sciences, Beijing 100049, P. R. China ⊥ Institute of Functional Crystal Materials, Tianjin University of Technology, Tianjin 300384, P. R. China S Supporting Information *
and Y = trivalent metal atom) and X2YP2O7 (X = alkali-metal atom and Y = divalent alkaline-earth metal) type pyrophosphates, have been continuously discovered, including RbNaMgP2O7,13 CsNaMgP2O7,14 LiCsCdP2O7,15 LiCrP2O7,16 LiFeP2O7,17 and so on. Among them, CsNaMgP2O7 is composed of [P2O7] dimers as an NLO functional unit, which shows an augmented SHG response of 1.1KDP, a short absorption edge, and phase-matching ability. LiFeP2O7 and LiCrP2O7 have relatively strong nonlinear response (e.g., 4KDP for LiFeP2O7) because the transition-metal-centered octahedra [FeO6] and [CrO6] were severely distorted. However, Fe and Cr atoms with unfilled 3d orbitals cause the narrowed band gap (2.58 eV for LiFeP2O7 and 1.60 eV for LiCrP2O7), resulting in a red shift of the absorption edge, which hinders their application in the UV region. Recently, a band engineering strategy with isovalent/ aliovalent substitution18,19 has been proposed to enlarge the band gap and maintain the NLO response and phase-matching ability in the exploration for novel NLO materials. For instance, in the Pb2NOxFy(SeO3)2Cl family (N = Ti4+, Nb5+, V5+), Pb2GaF2(SeO3)2Cl19 obtained by the substitution of N cations for Ga3+ exhibits the widest band gap of 4.32 eV in all of the reported NLO selenites that can achieve phase matching. Inspired by the above idea, we substitute M−O polyhedra into the LiMP2O7 (M = Fe, Cr) compounds with [GaO6] functional building blocks, and a new asymmetric pyrophosphate LiGaP2O7 was successfully designed and synthesized. The d− d electronic transitions are absent in LiGaP2O7, and the overall structure features with strong distortion [GaO6] octahedra are preserved. As a result, LiGaP2O7 exhibits an increased band gap of 4.56 eV and a phase-matchable SHG response equivalent to 0.6KDP (under 1064 nm laser radiation). We synthesized the powder specimen of LiGaP2O7 through solid-state reaction technology. At first, we tried to obtain pure phases using raw reagents with a stoichiometric ratio of LiGaP2O7, but the observed X-ray diffraction (XRD) patterns always exhibited the coexistence of trace impurities of Li9Ga3P8O27 (PDF 52-1464). The molar ratios of the raw reagents were adjusted several times in order to improve the purity. Finally, when the molar ratio of Li2CO3/Ga2O3/
ABSTRACT: A new noncentrosymmetric pyrophosphate, LiGaP2O7, is designed and synthesized by a reasonable energy-band regulation engineering strategy with isovalent substitution. The title compound crystallizes in the monoclinic space group P21 (No. 4) with lattice parameters a = 4.7593(10) Å, b = 7.9586(16) Å, c = 6.8940(14) Å, and Z = 2, which is the isomorphic compound of LiMP2O7 (M = Fe, Cr). Compared with LiMP2O7, LiGaP2O7 exhibits a wide band gap of 4.56 eV due to no d−d electronic transitions. Meanwhile, good phase-matching ability and a moderate second-harmonicgeneration (SHG) response in LiGaP2O7 are maintained. First-principles calculations of the band structure and nonlinear-optical performance were also performed in order to gain insight into the role of the Ga−O groups in the band gap and SHG effect source.
N
onlinear-optical (NLO) crystals are the critical substance for the all-solid-state laser-frequency transformation technology.1−5 To date, new exploration for the UV NLO materials mainly has been focused on borates, which results in the discovery of many splendid borate NLO crystals, such as KBe2BO3F2, CsLiB6O10, β-BaB2O4, and LiB3O5, etc.6−9 In recent years, phosphates have also attracted much attention because of their rich structural diversity, and a series of new phosphate UV NLO crystals have been developed, such as RbBa2P5O15,10 CsLa(PO3)4,11 LiCs2PO4,12 etc. In view of practical applications, an excellent NLO crystal should have a large second-harmonic-generation (SHG) response, a broad transparency interval, and phase-matching capability. Among these requirements, the phase-matching ability is a crucial factor for phosphate NLO crystals. Generally, because the [PO4]3− tetrahedral groups in the phosphates have much smaller polarizability anisotropy than planar triangular [BO3]3− groups in the borates, it is relatively difficult to realize phase matching in phosphate NLO crystals. In the above-mentioned phosphates, CsLa(PO3)4 has infinitely long chains of [PO3]∞ consisting of PO4 phosphate units, which are non-phase-matchable under 1064 nm laser radiation. Fortunately, it has been found that pyrophosphate exhibits excellent NLO properties. A great deal of pyrophosphates, in particular, XYP2O7 (X = alkali-metal atom © XXXX American Chemical Society
Received: April 3, 2019
A
DOI: 10.1021/acs.inorgchem.9b00970 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry NH4H2PO4 is 0.8:1:2, we have successfully obtained the impurity-free LiGaP2O7 polycrystalline specimen. The powder sample was studied through the powder XRD technique, and the purity of the phase was proven (Figure 1a). The experimental results coincide well with the calculation results according to the single-crystal XRD data.
As depicted in Figure 2a, the whole structure of LiGaP2O7 features two-dimensional (2D) zigzag [GaP2O11] layers stacking
Figure 1. (a) XRD patterns of LiGaP2O7. (b) DSC and TG curves of LiGaP2O7.
Figure 2. (a) Projection of LiGaP2O7 in the [100] direction. Lilac tetrahedra and light-green octahedra represent the PO4 and GaO6 groups, separately. (b) 2D zigzag [GaP2O11] layers in the ab plane.
The thermogravimetric (TG) and differential scanning calorimetry (DSC) curves of polycrystalline LiGaP2O7 are displayed in Figure 1b. In the DSC curve, two endothermic peaks appear that located at 825 and 962 °C, respectively, and there was no weight change until 1000 °C in the TG curve. The above results show that LiGaP2O7 is a thermally incongruent melt compound. For further verification, the powder samples of LiGaP2O7 were heated to 950 °C until they were thoroughly melted, and then the melt was slowly cooled to room temperature using a slow speed of 2 °C/h. The residues after melting were characterized by employing a powder XRD analysis. The results are different from those before melting (Figure S1), and the main component is GaPO4, which confirmed that LiGaP2O7 melts incongruently. On the basis of the above results, we grew single crystals of LiGaP2O7 using a spontaneous crystallization technique from high-temperature melts using a Li2O−P2O5 self-flux (see the Supporting Information). LiGaP2O7 crystallizes in the monoclinic space group P21 (No. 4), which is isostructural with LiCrP2O7 and LiFeP2O7. The cell parameters are a = 4.7593(10) Å, b = 7.9586(16) Å, c = 6.8940(14) Å, and Z = 2. All of the atoms in this space group can only be located in Wyckoff position 2a. In the asymmetric unit, there are seven unique O atoms, two unique P atoms, one unique Ga atom, and one unique Li atom. All P atoms linked with four O atoms to construct a [PO4] basic block and then form [P2O7] dimers by the sharing corner of P(1)O4 and P(2)O4 (Figure S2). Within each [P2O7] dimer, the P−O lengths are within the range of 1.594(9)−1.602(10) Å, which are longer than those varying from 1.502(9) to 1.524(9) Å in the P−O−Ga bonds. The Ga atom coordinates with six O atoms to form [GaO6] octahedra, in which there are three long Ga−O bonds of 1.988(7), 1.997(8), and 2.000(1) Å and three short Ga−O bonds of 1.9179(7), 1.929(6), and 1.949(7) Å. The Li atom is coordinated to four O atoms to constitute a [LiO4] tetrahedron, and the Li−O bond lengths are from 1.937 to 2.110 Å, which are comparable with those of other Li-containing phosphates,20,21 but the O−Li−O intersection angles in the range between 77.7(9) and 169.5(14)° are far from that of the regular [LiO4] tetrahedron. More interestingly, all of the bonded O atoms are on the same side of the Li atom and exhibit a typical “see-saw”-type configuration (as shown in Figure S3); as a result, the LiO4 tetrahedron was seriously distorted.
along the b axis. In each [GaP2O11] layer (see Figure 2b), [GaO6] octahedra are linked to four neighboring [P2O7] dimers via corner-sharing. Further, two adjacent layers are linked by O3 sites to construct the three-dimensional framework. Li atoms filled the interstitial voids formed by the 2D [GaP2O11] layers. The void space is much larger than that of the normal [LiO4] tetrahedron, which may be the reason for the formation of the unique “see-saw”-type Li−O configuration. The IR spectrum of crystalline LiGaP2O7 is depicted in Figure 3a. The peaks near 967 and 1080 cm−1 should be attributed to the asymmetric and symmetric stretching vibrations of the [PO3] terminal groups in the [P2O7] groups, separately. The absorption bands near 620 and 769 cm−1 should be attributed to the P−O−P bridge stretching. The above designations show no difference from previous reports.22−25 Figure 3b shows the UV− vis−near-IR (NIR) diffuse-reflectance spectrum for polycrystal-
Figure 3. (a) IR spectrum and (b) UV−vis−NIR diffuse-reflectance spectra of LiGaP2O7. (c) Powder SHG intensity curves at 1064 nm. (d) Oscilloscope-recorded traces of the SHG signals in the dimensional range of 151−200 μm. B
DOI: 10.1021/acs.inorgchem.9b00970 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry
deep valence region around −15 eV but not close to the forbidden gap. The upper-valence-band and bottom-conduction-band regions mainly include the P 3p and O 2p orbitals, thus suggesting a certain hybridization effect between the P and O atoms and strong P−O covalent bonds. Generally, the optical response of a compound derived from electron transitions near the Fermi level, accordingly, [PO4] groups, would play a dominant role in controlling the SHG effect of LiGaP2O7. This also conforms to the anionic group theory raised by Chen et al.28 In addition, Mulliken bond analysis exhibits a Ga−O bond having a certain covalent population of 0.31−0.39, thus suggesting that [GaO6] octahedra are also an effective anionic group for an enhanced SHG effect. Moreover, we obtained the SHG coefficients of LiGaP2O7 according to the formula developed by Lin et al.29 Its largest SHG component d14 is −0.43 pm/V, which is a little stronger than the experimental value (0.6KDP). This value is close to that of the other reported phosphate NLO materials, such as d13 = −0.59 pm/V for RbBa2(PO3)5,10 d15 = 0.46 pm/V for LiRb2PO4,30 d31 = 0.368 pm/V for CsNaMgP2O7,14 d14 = −0.365 pm/V for LiCs2Y2(PO4)3,31 etc. When its wide UV transparency, moderate SHG effect, and simple chemical composition are combined, LiGaP2O7 would be a potential NLO crystal for frequency conversion, e.g., the Nd:YAG SHG process. In summary, we synthesized a new asymmetric pyrophosphate LiGaP2O7 by the rational isovalent substitution of M3+ in LiMP2O7 (M = Fe, Cr) with Ga3+. LiGaP2O7 features 2D zigzag [GaP2O11] layers stacked along the b axis. Its band gap was increased to 4.56 eV owing to the lack of d−d transition. LiGaP2O7 has a moderate SHG response of about 0.6KDP and exhibits phase-matchable behavior. These results show that LiGaP2O7 may have a potential use in the field of frequency conversion. Large crystal growth and the search for related physical properties are in progress. This work inspires us to design new crystals with excellent NLO properties by the band engineering strategy with isovalent substitution.
line LiGaP2O7. There are no evident absorption peaks between 300 and 1600 nm, which indicates that LiGaP2O7 is transparent in the UV spectral interval. According to the diffuse-reflectance spectrum, we used the Kubelka−Munk equation to assess the band gap of LiGaP2O7.26 The experimental optical band gap was ascertained to be 4.56 eV, which is obviously greater than those of LiFeP2O7 (2.58 eV) and LiCrP2O7 (1.6 eV). We tested the powder SHG property of LiGaP2O7 through the Kurtz−Perry method.27 The measurement experiment was performed on a Q-switched 1064 nm pulsed laser. The experimental platform used the most common Nd:YAG laser. Commercial KDP single crystals were used as the standard. As illustrated in Figure 3c, the SHG magnitude of LiGaP2O7 enhanced gradually with increasing particle sizes and finally reached a maximum SHG intensity. As illustrated in Figure 3d, compared with the SHG intensity of KDP at a similar particle size range of 151−200 μm, LiGaP2O7 has a moderate SHG effect of 0.6KDP, which is almost 2 times that of Rb2Ba3(P2O7)2 (0.3KDP)10 composed of [P2O7] dimers as the NLO-active units. It is worth noting that Kurtz and Perry divided the phasematching behavior for new materials into five categories in their famous work, i.e., classes A−E. Classification is based on not only the phase-matching behavior but also the magnitude of the SHG effect. Among them, the “class B” phase-matching behavior means that the crystal is phase-matchable in the frequencydoubling process and has a small SHG effect (having the same order of magnitude as that of the quartz crystal effect). Therefore, this kind of curve implies that LiGaP2O7 belongs to a typical class B phase-matching behavior. In order to elucidate the correlation among the crystal structure and optical response, we performed first-principles calculations for LiGaP2O7. It is an indirect-band-gap semiconductor. In the electronic structure, the conduction band minimum and valence band maximum are located at the G and Z points, respectively (Figure 4a). The calculated band gap is 3.82
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b00970. Experimental section, crystallographic data (including atomic coordinates, equivalent isotropic displacement parameters, bond valence sums of LiGaP2O7, and selected bond lengths and angles), XRD patterns after melting of the LiGaP2 O 7 powder samples, [P2O 7] dimer in LiGaP2O7, coordination environment of a Li atom, and a comparison of the band structures for LiGaP2O7, LiFeP2O7, and LiCrP2O7 (PDF) Accession Codes
CCDC 1907644 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 4. (a) Calculated band structure of LiGaP2O7. (b) Density of states of LiGaP2O7.
eV, which is a little smaller than the measured band gap (4.56 eV) because of the shortcomings of the PBE functional. A scissor operator of 0.74 eV is adopted to translate the conduction band to the correct position. In contrast with a transition metal (Fe 3d6 and Cr 3d5) with unoccupied 3d orbitals, fully occupied Ga (3d10 electron configuration) atoms do not narrow the band gap of LiGaP2O7 (Figures S4 and 4b).19 Its 3d orbitals locate at the
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. C
DOI: 10.1021/acs.inorgchem.9b00970 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry ORCID
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Fei Liang: 0000-0002-4932-1329 Zheshuai Lin: 0000-0002-9829-9893 Guochun Zhang: 0000-0002-8795-6130 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grants 51572267 and 51890865) and Fujian Institute of Innovation, Chinese Academy of Sciences (Grant FJCXY18010201).
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DOI: 10.1021/acs.inorgchem.9b00970 Inorg. Chem. XXXX, XXX, XXX−XXX
