Mixed-valent selenium compounds: Noncentrosymmetric Cd3(SeO3)2(SeO4) and Hg3(SeO3)2(SeO4) and centrosymmetric Pb2(SeO3) (SeO4)

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Abstract

Three mixed-valent selenium compounds: Cd3(SeO3)2(SeO4), Hg3(SeO3)2(SeO4) and Pb2(SeO3) (SeO4) were successfully synthesized by hydrothermal methods. Among these three compounds, Cd3(SeO3)2(SeO4) and Hg3(SeO3)2(SeO4) are isostructural and crystalize in the noncentrosymmetric (NCS) polar space group P21. Powder second-harmonic generation (SHG) measurements using 1064 ​nm radiation indicate that Cd3(SeO3)2(SeO4) and Hg3(SeO3)2(SeO4) exhibit SHG intensities of approximately 0.6 and 0.8 times of KH2PO4 (KDP), respectively. Pb2(SeO3) (SeO4) is centrosymmetric (CS) and crystalizes in the space group Pnma. Pb2(SeO3) (SeO4) exhibits a 3D framework composed of chains of edge-sharing [PbO6] octahedra linked to [SeO3] and [SeO4] groups. M3(SeO3)2(SeO4) (M ​= ​Cd, Hg) also exhibits a 3D structure consisting of layers of [MO8], [MO6] and [SeO3] polyhedra connected by [SeO4] groups. For all of the reported compounds, infrared spectroscopy, UV–vis diffuse reflectance spectroscopy and thermogravimetric measurements were performed.

Graphical abstract

Three mixed-valent selenium compounds: Cd3(SeO3)2(SeO4), Hg3(SeO3)2(SeO4) and Pb2(SeO3) (SeO4) with three-dimensional (3D) structures, have been successfully synthesized and characterized. Pb2(SeO3) (SeO4) is centrosymmetric (CS), whereas Cd3(SeO3)2(SeO4) and Hg3(SeO3)2(SeO4) are isostructural and noncentrosymmetric (NCS) and exhibit SHG intensities of approximately 0.6 and 0.8 times of KH2PO4 (KDP), respectively.

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Introduction

Second-order nonlinear optical (NLO) materials are widely used in semiconductor, manufacturing, photolithography, laser frequency conversion and advanced instrument development [[1], [2], [3], [4], [5]]. From a structural perspective, second-order NLO materials must crystallize in noncentrosymmetric (NCS) space groups [[6], [7]]. Often, NCS structures are designed by introducing NLO-active acentric structural groups [8] such as π-conjugated planar groups ([BO3], [CO3], and [NO3]) [[9], [10], [11], [12], [13]], second-order Jahn-Teller (SOJT) distorted d0 transition metal cations (Ti4+, V5+, Nb5+, Mo6+, W6+, and Ta5+) [[14], [15], [16]], polar displacement of d10 cation-centered (Cd2+, Zn2+, Hg2+, Cu+, Ag+, and Au+) polyhedra [[17], [18]], and cations (I5+, Se4+, Te4+, Sb3+, Bi3+, and Pb2+) with stereochemically active lone-pair electrons [[19], [20], [21]]. In addition, NCS tetrahedral units ([BO4], [GaO4], [SeO4] and [SO4]) have also been widely used for the design of NLO materials [[22], [23], [24], [25]]. Mixed-valent selenium compounds in which the polar triangular pyramidal Se4+O3 and NCS tetrahedral Se6+O4 exist together have attracted our attention. The combination of Se4+ O3 and Se6+O4 groups in one compound could not only increase the possibilities of forming NCS structure, but also result in a rich structural chemistry [[26], [27], [28]]. Several mixed-valent selenium compounds with diverse structures have been reported [22,[26], [27], [28], [29], [30], [31], [32], [33], [34], [35]]. However, only a few of them were found in NCS space groups - Ag4Hg(SeO3)2(SeO4) [22], Au2(SeO3)2(SeO4) [27] Er(SeO3) (SeO4)1/2 · H2O [29], Na2SeO4·H2SeO3·H2O [31], Cd3Se3O10 [34], Hg3Se3O10 [35] With this in mind, we investigated compounds in the d10 cations/lone-pair cations – mixed-valent selenium system. Three compounds, Pb2(SeO3) (SeO4), Cd3(SeO3)2(SeO4) and Hg3(SeO3)2(SeO4) have been successfully synthesized. Cd3(SeO3)2(SeO4) and Hg3(SeO3)2(SeO4) were first reported by Weil, M. et al., in 2002, however, only crystal structures were published [[34], [35]]. Among these three compounds, Pb2(SeO3) (SeO4) is centrosymmetric (CS), whereas Cd3(SeO3)2(SeO4) and Hg3(SeO3)2(SeO4) are NCS and exhibit SHG intensities of approximately 0.6 and 0.8 times of KH2PO4 (KDP), respectively. In this paper, we report on the syntheses, structures, and characterization of the Pb2(SeO3) (SeO4), Cd3(SeO3)2(SeO4) and Hg3(SeO3)2(SeO4).

Section snippets

Reagents

SeO2 (Alfa Aesar, 99.80%), HgO (Alfa Aesar, 99.00%), CdO(Alfa Aesar, 99.95%), PbO (Alfa Aesar, 99.90%) and H2SeO4 (Alfa Aesar, 40.00%) were used as received.

Syntheses

Crystals of Pb2(SeO3) (SeO4), Cd3(SeO3)2(SeO4) and Hg3(SeO3)2(SeO4) were grown by hydrothermal methods. Crystals of Pb2(SeO3) (SeO4) were grown by combining PbO (0.447 ​g, 2 ​mmol), SeO2 (0.222 ​g, 2 ​mmol) with H2SeO4 (2 ​ml) and H2O (8 ​ml). Crystals of Cd3(SeO3)2(SeO4) were grown by combining CdO (0.128 ​g, 1 ​mmol), SeO2 (0.111 ​g,

Pb2(SeO3) (SeO4)

Pb2(SeO3) (SeO4) crystallizes in the centrosymmetric space group Pnma (No. 62), and features a 3D structure consisting of [PbO6], [SeO3] and [SeO4] polyhedra. The asymmetric unit of Pb2(SeO3) (SeO4) contains two Pb and Se atoms and five O atoms. Both Pb(1) and Pb(2) atoms are coordinated to six oxygen atoms to form [PbO6] octahedra. The Pb–O bond lengths range from 2.412(4)-2.818(5) Å (Table S4). The [PbO6] octahedra are connected by sharing corners and edges through oxygen atoms into a

Conclusions

Three mixed-valent selenium compounds, Cd3(SeO3)2(SeO4), Hg3(SeO3)2(SeO4) and Pb2(SeO3) (SeO4) have been successfully synthesized and characterized. Powder SHG measurements indicate that Cd3(SeO3)2(SeO4) and Hg3(SeO3)2(SeO4) exhibit SHG intensity of about 0.6 and 0.8 times of KDP, respectively. Dipole moment calculations indicate that the [SeO3] groups are responsible for the majority SHG response. The moderate SHG efficiency can be attributed to the opposite alignment of [Se(2)O3] and [Se(3)O3

CRediT authorship contribution statement

Meng Shang: Conceptualization, Methodology, Writing - original draft. P. Shiv Halasyamani: Funding acquisition, Writing - review & editing, Project administration.

Declaration of competing interest

There are no conflicts of interest.

Acknowledgements

MS and PSH thank the Welch Foundation (Grant E-1457) for support.

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