A series of novel mercury(I) selenites and tellurites containing SOJT Mo⁶⁺ cations

Abstract

Using Hg₂Cl₂, MoO₃, and TeO₂ (or SeO₂) as starting materials, four new mixed-metal tellurites and selenites have been obtained in the unexplored mercury(I)–Mo⁶⁺–Se⁴⁺/Te⁴⁺–O system, namely, Hg₂MoSeO₆ (1), α-Hg₂MoTeO₆ (2), β-Hg₂MoTeO₆ (3), and Hg₂Mo₂TeO₉ (4). They represent the first mercury(I) tellurites and selenites containing octahedrally coordinated d⁰ transition metal (TM) cations. All four compounds are centrosymmetric. They show three types of 3D structures based on Hg₂²⁺ dumbbells, MoO₆ octahedra, QO₃²⁻ (Q = Se, Te) or TeO₄⁴⁻ anions. Compounds 1–3 feature similar 1D anionic chains of [MoO₃(QO₃)]²⁻ (Q = Se or Te), in which the 1D chains of corner-sharing MoO₆ octahedra are further decorated by bidentate bridging selenite or tellurite anions. Compounds 1 and 2 are isostructural and their 3D network structures are based on 2D mercury tellurite or selenite layers in the ac plane pillared by MoO₆ octahedra, whereas the 3D network of β-Hg₂MoTeO₆ (3) is composed of 1D mercury tellurite chains along the a axis and 1D chains of [MoO₃(TeO₃)]²⁻ anions along the b axis. Compound 4 features an unusual 3D network structure composed of novel [Mo₂O₅(TeO₄)]²⁻ double layers interconnected by Hg₂²⁺ cations. Within the [Mo₂O₅(TeO₄)]²⁻ double layer, the cyclo-Mo₄O₂₀ tetramers are further interconnected via corner sharing into a 2D molybdenum oxide double layer with TeO₄ groups capping the walls of the thus-formed eight-membered rings (8-MRs). Thermal stabilities and optical properties as well as theoretical calculations based on density functional theory (DFT) methods were also performed.

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Introduction

Metal selenites and tellurites can form a diversity of unusual structures due to the presence of the stereochemically active lone-pair in the Se⁴⁺ and Te⁴⁺ cations, which means they can serve as structure-directing agents.¹ In addition, Te(IV) can be coordinated by three, four or five oxygen atoms to form TeO_X (X = 3, 4, 5) polyhedra, which are further condensed into various clusters, 1D chains, 2D layers and 3D networks, whereas only Se₂O₅²⁻ can be formed for the selenite anion.² Furthermore, the combination of a QO₃²⁻ (Q = Se, Te) group and an octahedrally-coordinated d⁰ TM cation such as Ti⁴⁺, Nb⁵⁺, Mo⁶⁺, W⁶⁺, Ta⁵⁺ and V⁵⁺, both of which are susceptible to second-order Jahn–Teller (SOJT) distortion, favors the formation of noncentrosymmetric (NCS) or polar structures with possible second-harmonic generation (SHG).^2b,3–9 For example, BaMo₂TeO₉ displays a very strong SHG response of 600 × α-SiO₂, and its large crystals have also been obtained successfully.^1c,9

So far, counter cations used to balance the negative charges of d⁰ TM selenites and tellurites include alkali,^10a,b alkaline earth,^1c,9 rare earth,^10c transition metal^10d and post-transition metal main group elements.^10e No compounds have been reported in similar systems with Hg⁺/Hg²⁺ as balanced cations, though a few ternary mercury(I) or mercury(II) selenites and tellurites have been structurally characterized,^11–13 including HgSeO₃,^12a Hg₃(HSeO₃)₂(SeO₃)₂,^12b HgSeO₃·0.5H₂O,^12c α-, β- and γ-Hg₂SeO₃,^12d HgTeO₃,^13a Hg₂Te₂O₇,^13b and Hg₂TeO₃.^13c

In this paper, we will focus our research attention on the unexplored mercury(I)–Mo⁶⁺–Se⁴⁺/Te⁴⁺–O system due to the fact that the combination of the Hg⁺ cations, which are unique in bonding character with two types of SOJT building units, may result in a variety of new compounds with novel structures and interesting physical properties. Our research in this aspect has led to the isolation of four new compounds, namely, Hg₂Mo₂TeO₉ (1), α-Hg₂MoTeO₆ (2), β-Hg₂MoTeO₆ (3), and Hg₂MoSeO₆ (4). Herein, we report their syntheses and crystal structures as well as electronic structure calculations.

Experimental section

Materials and instruments

Hg₂Cl₂ (99+%, AR), MoO₃ (99.5+%, AR), SeO₂ (99+%, AR), TeO₂ (99+%, AR), and BaSO₄ (99.9+%, AR) were used as received. All of the chemicals were supplied by the Shanghai Reagent Factory. The X-ray powder diffraction patterns were collected in continuous mode on a Rigaku MiniFlex II diffractometer at room temperature (Cu-Kα radiation, flat plate geometry). Data were collected in the 2θ range of 5–70° with a step size of 0.02°. Microprobe elemental analyses for the Hg, Mo, Te and Se elements were carried out on a field-emission scanning electron microscope (FESEM, JSM6700F) equipped with an energy dispersive X-ray spectroscope (EDS, Oxford INCA). Infrared spectra were collected on a Magna 750 FT-IR spectrometer as KBr pellets in the spectral range of 4000–400 cm⁻¹ with a resolution of 2 cm⁻¹ at room temperature. The UV-Vis diffuse reflectance spectra were measured with a PE Lambda 900 UV-Vis spectrophotometer in the range of 190–2500 nm at room temperature. A BaSO₄ plate was used as a standard (100% reflectance) for the baseline correction. The absorption spectrum was converted from the reflectance spectra using the Kubelka–Munk function: α/S = (1 − R)²/2R = K/S,¹⁴ where α, R, S and K represent absorption coefficient, reflectance, scattering coefficient and absorption, respectively. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were performed with a NETZCH STA449C unit at a heating rate of 10 °C min⁻¹ under a N₂ atmosphere.

Syntheses

The four compounds were hydrothermally synthesized by reaction of a mixture of Hg₂Cl₂, MoO₃ and TeO₂ (or SeO₂) in 6 mL of deionized water. The loaded compositions are as follows: Hg₂Cl₂ (141.6 mg, 0.3 mmol), MoO₃ (43.2 mg, 0.3 mmol), SeO₂ (88.8 mg, 0.8 mmol) and H₂O (6 mL) for Hg₂MoSeO₆ (1); Hg₂Cl₂ (141.6 mg, 0.3 mmol), MoO₃ (57.6 mg, 0.4 mmol), TeO₂ (127.7 mg, 0.8 mmol), and H₂O (6 mL) for α-Hg₂MoTeO₆ (2); Hg₂Cl₂ (141.6 mg, 0.3 mmol), MoO₃ (57.6 mg, 0.4 mmol), TeO₂ (159.6 mg, 1.0 mmol), and H₂O (6 mL) for β-Hg₂MoTeO₆ (3); Hg₂Cl₂ (141.6 mg, 0.3 mmol), MoO₃ (86.4 mg, 0.6 mmol), TeO₂ (79.8 mg, 0.5 mmol) and H₂O (6 mL) for Hg₂Mo₂TeO₉ (4). The mixtures were sealed in an autoclave equipped with a Teflon liner (23 mL) and heated at 210 °C (for 1) or 220 °C (for 2, 3 and 4) for 4 days, followed by slow cooling to room temperature at a rate of 2 °C h⁻¹. Yellow prism-shaped crystals of 1 were obtained as a single phase in a yield of about 92% based on Mo, which has been confirmed by X-ray diffraction (XRD) studies (ESI, Fig. S1a†). Yellow block-shaped single crystals of 3 and red flake-shaped crystals of 4 were collected in ca. 52% and 45% yields (based on Mo), respectively, as confirmed by X-ray diffraction (XRD) studies (ESI, Fig. S1b and S1c†). Though many attempts have been made to synthesize a pure phase of α-Hg₂MoTeO₆ (2) by changing the molar ratios of the starting materials and the reaction temperatures, products always contain compounds 3 and 4 as impurities, hence α-Hg₂MoTeO₆ was not subject to optical and thermal measurements. For compounds 2, 3 and 4, the main impurities are rodlike crystals of MoTe₂O₇. Results of EDS elemental analyses on the single crystals of 1–4 gave molar ratios of Hg : Mo : Q (Q = Se or Te) of 2.02 : 1.12 : 1, 2.12 : 1.15 : 1, 2.23 : 1.17 : 1, and 2.04 : 2.00 : 1, respectively, which are in good agreement with those determined from single-crystal X-ray structural studies.

X-Ray crystallography

A yellow brick crystal of Hg₂MoSeO₆ (0.43 × 0.24 × 0.17 mm³), a yellow needle-like crystal of α-Hg₂MoTeO₆ (0.23 × 0.09 × 0.08 mm³), and a yellow brick crystal of β-Hg₂MoTeO₆ (0.36 × 0.33 × 0.27 mm³), as well as a red plate-shaped crystal of Hg₂Mo₂TeO₉ (0.26 × 0.22 × 0.05 mm³) were used for single-crystal X-ray data collections. Diffraction data were measured on a Mercury CCD diffractometer (for Hg₂MoSeO₆, β-Hg₂MoTeO₆, and Hg₂Mo₂TeO₉) and a Saturn724 CCD diffractometer (for α-Hg₂MoTeO₆), with Mo-Kα radiation (λ = 0.71073 Å) at room temperature. All four data sets were corrected for Lorentz and polarization factors as well as absorption by the multi-scan method.^15a All four structures were solved by direct methods and refined by full-matrix least-squares fitting on F² using SHELXL-97.^15b All four structures were checked for possible missing symmetry elements with PLATON,^15c but none were found. All of the atoms were refined with anisotropic thermal parameters. Crystallographic data, structural refinements and important bond distances are summarized in Tables 1 and 2. More details of the crystallographic studies are given as ESI.† Further details of the crystal structure studies can be obtained from the FIZ Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (Fax: (49)7247808666; E-mail: crysdata@fiz-karlsruhe.de), on quoting the depository numbers CSD 428420–428423.

Table 1 Crystal data and structural refinements for the four compounds

a R 1 = ∑||Fo| − |Fc||/∑|Fo|, wR2 = {∑w[(Fo)^2 − (Fc)^2]^2/∑w[(Fo)^2]^2}^1/2.
Formula	Hg2MoSeO6	α-Hg2MoTeO6	β-Hg2MoTeO6	Hg2Mo2TeO9
fw	672.08	720.72	720.72	864.66
Crystal system	Orthorhombic	Orthorhombic	Orthorhombic	Monoclinic
Space group	Pbcm	Pbcm	Pbca	P2(1)/c
a/Å	6.4578(7)	6.429(4)	11.592(8)	9.5533(9)
b/Å	13.3556(19)	13.452(9)	7.343(5)	7.6391(7)
c/Å	7.2072(10)	7.277(5)	14.763(10)	11.3555(10)
α (°)	90	90	90	90
β (°)	90	90	90	92.675(6)
γ (°)	90	90	90	90
V/Å^3	621.61(14)	629.3(7)	1256.7(14)	827.81(13)
Z	4	4	8	4
D c (g cm^−3)	7.182	7.606	7.618	6.938
μ(Mo-Ka) (mm^−1)	57.081	55.139	55.226	43.417
GOF on F^2	1.113	1.248	1.174	1.116
R 1, wR2 [I > 2σ(I)]^a	0.0261, 0.0554	0.0242, 0.0470	0.0284, 0.0496	0.0371, 0.0837
R 1, wR2 (all data)	0.0277, 0.0563	0.0256, 0.0475	0.0301, 0.0504	0.0394, 0.0850

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Table 2 Selected bond distances (Å) for the four compounds

Hg2MoSeO6
Hg(1)–O(1)	2.182(8)	Mo(1)–O(4)	1.971(3)
Hg(1)–Hg(2)	2.528(7)	Mo(1)–O(4)	1.971(3)
Hg(2)–O(1)	2.343(8)	Mo(1)–O(2)	2.214(6)
Hg(2)–O(4)	2.477(7)	Mo(1)–O(2)	2.214(6)
Hg(2)–O(2)	2.551(5)	Se(1)–O(2)	1.699(6)
Hg(2)–O(2)	2.551(5)	Se(1)–O(2)	1.699(6)
Mo(1)–O(3)	1.717(6)	Se(1)–O(1)	1.720(7)
Mo(1)–O(3)	1.717(6)
α-Hg2MoTeO6
Hg(1)–O(1)	2.155(7)	Mo(1)–O(4)	1.957(3)
Hg(1)–Hg(2)	2.532(1)	Mo(1)–O(4)	1.957(3)
Hg(2)–O(1)	2.286(7)	Mo(1)–O(2)	2.185(5)
Hg(2)–O(2)	2.523(6)	Mo(1)–O(2)	2.185(5)
Hg(2)–O(2)	2.523(6)	Te(1)–O(2)	1.868(5)
Hg(2)–O(4)	2.577(7)	Te(1)–O(2)	1.868(5)
Mo(1)–O(3)	1.732(5)	Te(1)–O(1)	1.895(7)
Mo(1)–O(3)	1.732(5)
β-Hg2MoTeO6
Hg(1)–O(2)	2.152(6)	Mo(1)–O(3)	2.088(6)
Hg(1)–Hg(2)	2.519(1)	Mo(1)–O(6)	2.115(6)
Hg(2)–O(1)	2.123(6)	Mo(1)–O(2)	2.305(6)
Hg(2)–O(4)	2.597(6)	Te(1)–O(1)	1.863(6)
Mo(1)–O(5)	1.714(6)	Te(1)–O(3)	1.881(6)
Mo(1)–O(4)	1.769(6)	Te(1)–O(2)	1.922(6)
Mo(1)–O(6)	1.804(6)
Hg2Mo2TeO9
Hg(1)–O(8)	2.198(7)	Mo(2)–O(9)	1.699(8)
Hg(1)–O(1)	2.419(7)	Mo(2)–O(8)	1.745(7)
Hg(1)–Hg(2)	2.5324(6)	Mo(2)–O(5)	1.902(7)
Hg(2)–O(4)	2.159(7)	Mo(2)–O(1)	1.937(6)
Mo(1)–O(6)	1.700(8)	Mo(2)–(3)	2.273(7)
Mo(1)–O(7)	1.705(7)	Mo(2)–O(2)	2.274(7)
Mo(1)–O(5)	1.930(7)	Te(1)–O(4)	1.886(7)
Mo(1)–O(3)	2.011(7)	Te(1)–O(2)	1.907(6)
Mo(1)–O(2)	2.141(6)	Te(1)–O(3)	2.080(7)
Mo(1)–O(4)	2.380(7)	Te(1)–O(1)	2.113(7)

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Computational descriptions

Single-crystal structural data of the four compounds were used for theoretical calculations. All calculations (including band structures and density of states (DOS)) were carried out using the total-energy code of CASTEP.¹⁶ The exchange and correlation effects were calculated using Perdew–Burke–Ernzerhof (PBE) in the generalized gradient approximation (GGA).¹⁷ The interactions between the ionic cores and the electrons were described by the norm-conserving pseudopotential.¹⁸ The following valence-electron configurations were considered in the computation: Hg-5d¹⁰6s², Mo-4d⁵5s¹, Te-5s²5p⁴, Se-4s²4p⁴, and O-2s²2p⁴. The numbers of plane waves included in the basis sets were determined by a cutoff energy of 750 eV for all four compounds. The numerical integration of the Brillouin zone was performed using Monkhorst–Pack k-point samplings of 4 × 2 × 3, 4 × 2 × 3, 2 × 3 × 2, and 3 × 3 × 2, respectively. The other parameters and convergent criteria were the default values of the CASTEP code.

Results and discussion

Syntheses

Four new mercury(I) molybdenum(VI) mixed-metal tellurites and selenites, namely, Hg₂MoSeO₆ (1), α-Hg₂MoTeO₆ (2), β-Hg₂MoTeO₆ (3), and Hg₂Mo₂TeO₉ (4), have been successfully synthesized by hydrothermal reactions. Compounds 2, 3, and 4 were prepared by slightly altering the ratios of the same reaction materials. Using the same starting materials Hg₂Cl₂, MoO₃, and TeO₂, the molar ratios of Hg : Mo : Te were 3 : 2 : 4, 3 : 2 : 5, and 3 : 3 : 2.5 for α-Hg₂MoTeO₆, β-Hg₂MoTeO₆, and Hg₂Mo₂TeO₉, respectively. Obviously, the Mo : Te ratio plays an important role in the chemical composition of the final products. A relatively large Mo : Te ratio favors the formation of a molybdenum oxide layer composed of corner-sharing MoO₆ octahedra as in Hg₂Mo₂TeO₉ (4), whereas for the other three compounds with Mo : Q molar ratios of 1 : 1, the MoO₆ octahedra form a 1D chain via corner sharing.

Structures of Hg₂MoSeO₆ and α-Hg₂MoTeO₆

Hg₂MoSeO₆ and α-Hg₂MoTeO₆ are isostructural and crystallize in the centrosymmetric space group Pbcm (no. 57). In this case, only the structural details of α-Hg₂MoTeO₆ will be described. The structure of α-Hg₂MoTeO₆ features a new 3D network based on 1D chains of [MoO₃(TeO₃)]²⁻ along the c axis, which are interconnected by dumbbell Hg₂²⁺ cations (Fig. 1). Its asymmetric unit contains two Hg⁺, one Mo⁶⁺, and one Te(IV) atom. Both the Mo⁶⁺ and Te⁴⁺ cations are in asymmetric coordination environments attributed to SOJT effect. The Mo⁶⁺ cation is in a distorted octahedral geometry composed of two tellurite oxygen atoms from two tellurite groups and four oxo anions. The Mo–O bond distances are two long [2.185(5) Å], two normal [1.957(3) Å], and two short [1.732(5) Å], hence the MoO₆ octahedron is distorted toward an edge (local C₂ direction). The magnitude of the out-of-center distortion (Δd) was calculated to be 0.92.^2b The Te⁴⁺ cation is in a ψ-TeO₃ trigonal pyramidal environment with the pyramidal site occupied by the lone pair of electrons. The Te–O bond lengths range from 1.868(5) to 1.895(7) Å. Neighbouring MoO₆ octahedra are interconnected via corner sharing (O4) into a 1D chain along the c axis, and Te(1)O₃ groups are grafted on the 1D chain in a bidentate bridging fashion, leading to the formation of the 1D [MoO₃(TeO₃)]²⁻ chain along the c axis (see Fig. 1a).

Fig. 1 1D [MoO₃(TeO₃)]²⁻ down the c axis (a), a Hg₂TeO₃ layer in the ac plane (b) and a view of the structure of α-Hg₂MoTeO₆ down the c axis (c). Hg, Mo, Te and O atoms are drawn as green, cyan, yellow and red circles, respectively. The MoO₆ octahedra are shaded in cyan.

As we all know, Hg⁺ is present in a coupled fashion as Hg₂²⁺, with a Hg(1)–Hg(2) bond length of 2.5322(17) Å, which is close to those reported in other mercury(I) oxide compounds.^11,12 In α-Hg₂MoTeO₆, the Hg(1) atom is bonded to one unidentate TeO₃ group, whereas the Hg(2) atom is connected by three unidentate TeO₃ groups and one oxo anion (ESI, Fig. S2a†). The Hg–O bond lengths are in the range of 2.155(7)–2.577(7) Å, which are close to those reported in other mercury(I) tellurites.¹³ Each Te(1)O₃ group is hexadentate and bridges two Mo⁶⁺ cations, one Hg(1) and three Hg(2) atoms (ESI, Fig. S2b†). Bond valence calculations on α-Hg₂MoTeO₆ gave values of 5.90 and 3.93 for the Mo and Te atoms, respectively, indicating that they are in oxidation states of 6+ and 4+, respectively.²⁰

It is interesting to note that neighbouring dumbbell Hg₂²⁺ cations are linked by bridging TeO₃ groups, leading to a 2D Hg₂TeO₃ layer parallel to the ac plane (Fig. 1b). The above 1D molybdenum tellurite chains and 2D mercury(I) tellurite layers are further interconnected by Hg–O–Mo and Hg–O–Te bridges into a pillared layered 3D network (Fig. 1c).

The overall structure of Hg₂MoSeO₆ is the same as that of α-Hg₂MoTeO₆, the main difference is the TeO₃ groups being replaced by SeO₃ groups. The magnitude of the out-of-center distortion (Δd) for the MoO₆ octahedron in Hg₂MoSeO₆ was calculated to be 1.01, which is slightly larger than that in α-Hg₂MoTeO₆. Furthermore, the b and c axial lengths of Hg₂MoSeO₆ are slightly smaller than those of α-Hg₂MoTeO₆ due to the smaller size of Se⁴⁺ compared with Te⁴⁺.

Structure of β-Hg₂MoTeO₆

β-Hg₂MoTeO₆ crystallizes in the centrosymmetric space group Pbca (no. 61). Its structure can be described as a different 3D network composed of 1D chains of [MoO₃(TeO₃)]²⁻ along the b axis, interconnected by dumbbell Hg₂²⁺ cations (Fig. 2). The network can also be viewed to be formed by the interconnection of 1D [MoO₃(TeO₃)]²⁻ chains along the b axis and 1D mercury(I) tellurite chains along the a axis (Fig. 2a and 2b). Its asymmetric unit contains two Hg⁺, one Mo⁶⁺, and one Te⁴⁺ cation. Both the Mo⁶⁺ and Te⁴⁺ cations are in asymmetric coordination environments, attributed to SOJT effects. The Mo⁶⁺ cation is in a distorted octahedral geometry composed of two tellurite oxygen atoms and four oxo anions. The Mo–O bond distances are three long [2.088(6)–2.305(6) Å] and three short [1.714(6) Å–1.804(6) Å], hence the MoO₆ octahedron is distorted toward a face (local C₃ direction). The magnitude of the out-of-center distortion (Δd) was calculated to be 1.29,^2b which is slightly larger than that for MoO₆ in α-Hg₂MoTeO₆. The unique Te⁴⁺ cation is in a ψ-TeO₃ trigonal pyramidal environment with the pyramidal site occupied by the lone pair of electrons. The Te–O bond lengths range from 1.863(6) to 1.922(6) Å. Neighbouring MoO₆ octahedra are interconnected via corner sharing (O6) into a 1D chain along the b axis, which is further decorated by the TeO₃ groups in a bidentate bridging fashion, so as to form a 1D [MoO₃(TeO₃)]²⁻ anionic chain (Fig. 2a). This chain is slightly different from that in α-Hg₂MoTeO₆, the main differences being the different axes that the chains are extended along and the different distortion directions associated with the MoO₆ octahedra.

Fig. 2 A 1D [MoO₃(TeO₃)]²⁻ chain along the b axis (a), a 1D Hg₂TeO₃ chain along the a axis (b), and a view of the structure of β-Hg₂MoTeO₆ down the b axis (c). Hg, Te and O atoms are drawn as green, yellow and red circles, respectively. The MoO₆ octahedra are shaded in cyan.

As with α-Hg₂MoTeO₆, Hg⁺ also exists as dumbbell Hg₂²⁺. The Hg(1) atom is bonded to one unidentate TeO₃ group, whereas the Hg(2) atom is connected by one unidentate TeO₃ group, and one oxo anion (ESI, Fig. S3a†). The Hg–O bond lengths are in the range of 2.123(6)–2.597(6) Å. The Te(1)O₃ connects with two Mo⁶⁺ and two Hg⁺ cations (ESI, Fig. S3b†). Bond valence calculations on β-Hg₂MoTeO₆ gave values of 5.98 and 3.82 for Mo and Te atoms, respectively, indicating that they are in oxidation states of 6+ and 4+, respectively.²⁰

It is interesting to note that the interconnection of the Hg₂²⁺ ions by bridging TeO₃ groups leads to a zigzag 1D Hg₂TeO₃ chain parallel to the a axis (Fig. 2b). The above two types of 1D chains are further interconnected by Hg–O–Mo and Hg–O–Te bridges into a complicated 3D network (Fig. 2c).

The main differences between the α- and β-forms of Hg₂MoTeO₆ lie in the different axes along which the 1D [MoO₃(TeO₃)]²⁻ anionic chains are extended and the different substructures for mercury(I) tellurite (layered in α-Hg₂MoTeO₆ and 1D in β-Hg₂MoTeO₆).

Structure of Hg₂Mo₂TeO₉

Hg₂Mo₂TeO₉ shows a novel 3D framework structure in which 2D [Mo₂O₅(TeO₄)]²⁻ double layers are bridged by Hg₂²⁺ dumbbells (Fig. 3). There are two Hg⁺, two Mo⁶⁺, and one Te(IV) atom in the asymmetric unit. Both the Mo⁶⁺ and Te⁴⁺ cations are in asymmetric coordination environments attributed to SOJT effect. Both the Mo(1) and Mo(2) atoms are in distorted octahedral geometries composed of six oxygen atoms.^19e The Mo–O bond distances are two long [2.141(6)–2.380(7) Å], two short [1.699(8) Å–1.745(7) Å], and two normal [1.902(7)–1.937(6) Å], hence both MoO₆ octahedra are distorted toward an edge (local C₂ direction). The magnitudes of the out-of-center distortions (Δd) were calculated to be 1.26 and 1.18 for Mo(1)O₆ and Mo(2)O₆, respectively,^2b,12 which are comparable to those of β-Hg₂MoTeO₆ and A₄Mo₅O₁₅(SeO₃)₂(H₂O)₂ (A = K, Rb).^19e Different from those in the other three phases, the Te⁴⁺ cation in Hg₂Mo₂TeO₉ is in a ψ-TeO₄ square pyramidal environment with the pyramidal site occupied by the lone pair of electrons (the seesaw geometry). The Te–O bond lengths range from 1.886(7) to 2.113(7) Å (Table 2). Two pairs of Mo(1)O₆ and Mo(2)O₆ octahedra are corner-sharing to form a cyclic [Mo₄O₂₀]¹⁶⁻ unit (Fig. 3a). These clusters are further interconnected via corner sharing into a novel 2D [Mo₂O₉]⁶⁻ layer with eight-membered rings (8-MRs) (Fig. 3a). TOPOS analysis reveals that the above 2D layer is a 4-connected 2D network with the Schläfli symbol of {4⁴; 6²}.²¹ TeO₄ groups are capped on the walls of the 8-MRs of the [Mo₂O₉]⁶⁻ layer, forming a unique [Mo₂O₅(TeO₄)]²⁻ double layer in the bc plane (Fig. 3b).

Fig. 3 A cyclic [Mo₄O₂₀] cluster unit (a), a 2D [Mo₂O₅(TeO₄)]²⁻ layer in the bc plane (b), a 1D Hg₂TeO₄ chain along the b axis (c), and a view of the structure of Hg₂Mo₂TeO₉ down the b axis (d). Hg, Mo, Te and O atoms are drawn as green, cyan, yellow and red circles, respectively. The MoO₆ octahedra are shaded in cyan.

The Hg(1) atom is coordinated by one unidentate TeO₄ group and one oxo anion, whereas the Hg(2) atom is bonded to one unidentate TeO₄ group (ESI, Fig. S4a†). The Hg–O bond lengths are in the range of 2.159(7)–2.419(7) Å. The interconnection of the Hg₂²⁺ dumbbells by TeO₄ groups results in a zigzag chain of [Hg₂TeO₄]²⁻ along the b axis (Fig. 3c). The above 2D [Mo₂O₅(TeO₄)]²⁻ double layers are interconnected by mercury(I) ions via Hg–O–Te bridges into a complicated 3D framework (Fig. 3d). Overall, each Te(1)O₄ group is hexadentate, bridging two Hg⁺ and four Mo⁶⁺ cations (Fig. S4b†). Bond valence calculations on Hg₂Mo₂TeO₉ gave values of 5.98 and 3.93 for the Mo and Te atoms, respectively, indicating that they are in oxidation states of 6+ and 4+, respectively.²⁰

It is interesting to compare the [Mo₂O₅(TeO₄)]²⁻ double layer in Hg₂Mo₂TeO₉ with those reported in monoclinic and orthorhombic forms of BaTeMo₂O₉.^1c,22 In the monoclinic form of BaMo₂TeO₉ in a polar space group P2₁,^1c dimeric Mo₂O₁₁ units are corner-sharing with each other to form a Mo–O double layer with six-membered rings capped by TeO₃ groups. In the orthorhombic form of BaMo₂TeO₉ in a polar space group Pca2₁,²² dimeric Mo₂O₁₁ units are corner-sharing with each other to form Mo–O double chains, which are further bridged by pairs of TeO₃ groups to form a double layer with Mo₆Te₂ 8-MRs. The monoclinic form displays strong SHG response of 600 × α-SiO₂, whereas the orthorhombic form only exhibits a weak SHG response of 0.2 × KDP. Due to the much larger size of the Hg₂²⁺ cation, the interlayer distance of about 9.55 Å for Hg₂Mo₂TeO₉ is significantly larger than those of both forms of BaTeMo₂O₉ (both close to 8.8 Å).

Regrettably, although two building units favouring the formation of NCS or polar structures were employed, no NCS or polar structures have been obtained owing to the fact that the asymmetric groups are arranged in an antiparallel manner. Furthermore, the sizes of the constituent cations, hydrogen-bonding and flexible coordination may affect the materials’ symmetries, according to previous reports. However, to design new functional asymmetric materials rationally is still a formidable mission.^2f,4f,23

TGA and DSC

Results of the TGA and DSC analyses indicate that Hg₂MoSeO₆, β-Hg₂MoTeO₆, and Hg₂Mo₂TeO₉ are all stable up to 325 °C (Fig. S5†). Hg₂MoSeO₆ displays two main steps of weight loss in the temperature range of 337–710 °C and exhibits two endothermic peaks at 417 and 695 °C, during which 1 mol of SeO₂ and 2 mol of Hg per formula unit are released.^12d The observed weight loss of 76.1% for Hg₂MoSeO₆ is close to the calculated one (76.2%). In the temperature range of 325–664 °C, β-Hg₂MoTeO₆ shows three steps of weight loss whereas Hg₂Mo₂TeO₉ reveals two steps of weight loss. The observed weight losses of 55.3% and 46.5% respectively for β-Hg₂MoTeO₆ and Hg₂Mo₂TeO₉ correspond to the release of 2 mol Hg per formula unit, and are very close to their calculated values (55.7% and 46.4%).^12d The final residues were not characterized due to their melting with the TGA buckets (made of Al₂O₃) under high temperatures.

Optical properties

The IR spectra of Hg₂MoSeO₆, β-Hg₂MoTeO₆, and Hg₂Mo₂TeO₉ have been measured in the wavelength range of 4000–400 cm⁻¹ at room temperature. All three compounds are transparent in the range of 4000–1000 cm⁻¹. The bands associated with the Te(Se)–O, Mo–O and O–Mo–O vibrations appeared in the wave number range of 400–1000 cm⁻¹ (Fig. S6, Table S1†). The Mo–O stretching vibrations were observed at 799–926 cm⁻¹, whereas Te(Se)–O stretching vibrations appeared at 651–772 cm⁻¹. The absorption bands occurring below 651 cm⁻¹ can be assigned to Mo–O–Te(Se) bending vibrations. These assignments are in good agreement with the literature.^24–27 The IR spectra and assignments are shown in the ESI.†

UV absorption spectra of Hg₂MoSeO₆, β-Hg₂MoTeO₆, and Hg₂Mo₂TeO₉ show no absorption in the range of 760–2500 nm (0.76–2.5 μm) (Fig. S7†). Hence, all three compounds are transparent in the range of 0.76–2.5 μm, corresponding to the visible, near-IR and middle-IR regions. The UV-Vis diffuse reflectance spectrum measurements indicate that the band gaps are approximately 2.71 eV, 2.72 eV and 2.25 eV for Hg₂MoSeO₆, β-Hg₂MoTeO₆, and Hg₂Mo₂TeO₉, respectively, hence all three compounds are wide band gap semiconductors.

Theoretical calculations

Theoretical calculations based on DFT methods were made.¹⁶ The calculated band structures of Hg₂MoSeO₆, α-Hg₂MoTeO₆, β-Hg₂MoTeO₆, and Hg₂Mo₂TeO₉ along high symmetry points of the first Brillouin zone are plotted in Fig. 4. The state energies (eV) of the lowest conduction bands (L-CB) and the highest valence bands (H-VB) of the four compounds are listed in the ESI, Table S2.† The calculated band structures (Fig. 4, Table S2†) indicate that all four compounds are indirect band gap semiconductors with band gaps of 2.42, 2.34, 2.46, and 2.00 eV for Hg₂MoSeO₆, α-Hg₂MoTeO₆, β-Hg₂MoTeO₆, and Hg₂Mo₂TeO₉, respectively, which are slightly smaller than their experimental values (2.71, 2.72, and 2.25 eV, respectively, for Hg₂MoSeO₆, β-Hg₂MoTeO₆, and Hg₂Mo₂TeO₉). This is not surprising as it is well-known that the GGA does not accurately describe the eigenvalues of the electronic states, which usually causes quantitative underestimation of band gaps for semiconductors and insulators.²⁸

Fig. 4 The scissor-added band structures of Hg₂MoSeO₆ (a), α-Hg₂MoTeO₆ (b), β-Hg₂MoTeO₆ (c) and Hg₂Mo₂TeO₉ (d).

The bands can be assigned according to the total and partial densities of states (DOS) as plotted in Fig. 5. DOS plots of the four compounds are very similar, hence Hg₂Mo₂TeO₉ is used as a representative. For Hg₂Mo₂TeO₉ (Fig. 5c), the bottom-most VBs from −19.8 to −16.3 eV originate from the O 2s mixing with a small amount of the Te 5s, Te 5p and Mo 4d states. The bands between −10.0 and −8.8 eV are composed of the Te 5s and O 2p states. We will focus on the VB and the CB in the vicinity of the Fermi level (between −8.0 and −7.8 eV), which counts for most of the bonding characteristic in a compound. In the region of −8.0–0 eV, the Hg 5d state overlaps fully with O 2p, and in the region of −8.0–7.8 eV, the Te 5p and Mo 4d states overlap fully with O 2p, indicative of the Hg–O, Mo–O and Te–O bonding interactions.

Fig. 5 The total and partial density of states of Hg₂MoSeO₆ (a), α-Hg₂MoTeO₆ (b), β-Hg₂MoTeO₆ (c) and Hg₂Mo₂TeO₉ (d).

Population analyses give more information about quantitative bond analysis. The calculated bond orders of Hg–Hg, Hg–O, Mo–O, and Te(Se)–O bonds are 0.33–0.71, 0.02–0.35, 0.14–1.05, and 0.35–0.53 e, respectively, which indicate that the Hg–O bonds have more ionic character whereas the Hg–Hg, Mo–O and Te(Se)–O bonds are much more covalent in nature.

Conclusions

In summary, we have successfully synthesized four new mercury(I) molybdenum(VI) selenites and tellurites, namely, Hg₂MoSeO₆ (1), α-Hg₂MoTeO₆ (2), β-Hg₂MoTeO₆ (3), and Hg₂Mo₂TeO₉ (4). They represent the first mercury(I) selenites and tellurites containing octahedrally coordinated d⁰ transition metal (TM) ions. Compounds 1–3 feature 1D anionic chains of [MoO₃(QO₃)]²⁻ (Q = Se, Te) whereas Hg₂Mo₂TeO₉ (4) features a novel [Mo₂O₅(TeO₄)]²⁻ anionic double layer. The results of our studies indicate that small changes in reaction conditions in the same system can lead to different chemical compositions and structures of the final products obtained. Our future research efforts will be focused on the introduction of other octahedrally-coordinated d⁰ transition metal cations such as V⁵⁺, Ti⁴⁺, W⁶⁺, etc. into the mercury selenite or tellurite systems.

Acknowledgements

This work was supported by National Natural Science Foundation of China (grants no. 21231006, 21403232 and 91222108), NSF of Fujian Province (grant no. 2011J05037), and Key Laboratory of Optoelectronic Materials Chemistry and Physics.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: X-ray crystallographic files in CIF format, simulated and experimental XRD powder patterns, the calculated state energies of the L-CB and H-VB, IR spectra, UV spectra, TGA and DSC curves, coordination environments around the Hg and Te atoms for three compounds. See DOI: 10.1039/c4qi00132j

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