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Photophysical properties of open-framework germanates templated by nickel complexes† M. V. Peskov and U. Schwingenschlo ¨ gl* Open-framework germanates are a group of germanium oxides with a well-defined porous structure, suitable for ion-exchange and gas adsorption applications. Recently, Ni incorporation into the porous structure by establishing Ge–O–Ni bonds with the molecular complexes [Ni(H2N(CH2)2NH2)2] was realized. We investigate the optical and electronic features of these systems (SUT-1 and SUT-2) from first principles.

Received 26th February 2014, Accepted 30th March 2014

To describe the photophysical behavior, we analyze the bonding between the Ni and nearest-neighboring

DOI: 10.1039/c4cp00836g

nanomaterials are expected to be essential components of future optical and electronic devices. We discuss

atoms and simulate the absorption spectra. Because of their optical characteristics, germania-based to what extent molecular transition-metal complexes embedded into porous germanium oxide can modify


the optical response to potentially expand the area of applications.

Introduction Open-framework germanates are a group of germanium oxides with a porous structure that includes a system of large and extralarge cavities interconnected by 12- to 30-ring channels.1 Stable open-framework germanates with permanent porosity are capable of gas adsorption and ion-exchange.2 The atomic structure of open-framework germanates may include heteroatoms on framework sites, for example boron,3 silicon,4 aluminum,5 gallium,5,6 antimony,7 zirconium,8 and niobium.9 Recently, two openframework germanates, SUT-1 and SUT-2, built from Ge10O27 building units10 with Ni atoms introduced into an intricate porous structure of germanium oxide by establishing Ni–O bonds between molecular complexes of bis(ethylenediamine)nickel(II) nitrate [Ni(en)2] (en = ethylenediamine) and the framework atoms, were reported.11 Usually, nickel complexes are found inside the framework pores of a compound where they primarily act as filling agents (templates) responsible for framework interruptions and pore formation.12 Except for the germanates of the SUT series, where Ni connects layer-like structural fragments, only one other germanate with framework-forming Ni is known.12c Molecular transition-metal complexes embedded into the structure of germanium oxide result in a change of the optical properties: SUT-1 and SUT-2 have yellow and pale lilac color in visible light, respectively.11 Although it is obvious that this Division of Physical Sciences and Engineering, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Kingdom of Saudi Arabia. E-mail: [email protected] † Electronic supplementary information (ESI) available: Selected bond angles in the coordination sphere of Ni for SUT-1 and SUT-2, computational details of the SIESTA calculations, and electronic structures of the complexes [Ni(en)3](NO3)2 and [Ni(C4H10N5)2]. See DOI: 10.1039/c4cp00836g

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change is related to the inclusion of the transition-metal atoms (GeO2 polymorphs normally do not exhibit any color), a deeper study of the electronic structure and optical behavior is necessary to establish the origin of the color. In this work we investigate the optical and electronic properties of SUT-1 and SUT-2 from first principles. In order to describe the photophysical properties of the compounds, we analyze the bonding of Ni with its surroundings to enable an interpretation of simulated absorption spectra, which are also compared to available experimental data for the host germanates.

Computational details Structural models To build initial models of the SUT-1 and SUT-2 germanates, we used the crystallographic data published in ref. 11. While SUT-1 does not contain disordered framework-constituting atoms, the structure of SUT-2 includes partially occupied tetrahedrally coordinated Ge sites. The Ge(21) and Ge(22) positions, respectively, are disordered with the nearby Ge(24) and Ge(23) positions. Considering that the latter have a low site occupancy of 0.064, we do not consider them in our model, but represent SUT-2 only by the highly occupied sites Ge(21) and Ge(22). The pores of the SUT-1 germanate contain positively charged complexes [Ni(en)2]2+ that balance the excessive negative charge of the framework. While the centers of the complex ions in general are not disordered, the CH2 groups of the en ligands have two possible conformational orientations. The site occupancy of the water positions in the pores, in particular O(27) in SUT-1 and O(63) and O(64) in SUT-2, is assumed to be 1. Charge neutrality of the crystals is provided by single or double protonated structure-directing agent

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molecules in the pores.11 The resulting structures contain 780 (SUT-1) and 752 atoms (SUT-2). Finally, to build the models of the complexes with 4- and 6-coordinated Ni atoms we used the published crystallographic data for bis(1,1-dimethylbiguanido)nickel(II) [Ni(C4H10N5)2]13 and tris(ethylenediamine)nickel(II) nitrate, [Ni(en)3](NO3)2,14 respectively.

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Computational approach The electronic ground states of SUT-1 and SUT-2 are simulated by VASP, using a cutoff energy of 400 eV for the plane wave expansion.15 An increased cutoff energy of 500 eV does not result in differences in the electronic structures. The valence electron density is generated by the Perdew–Burke–Ernzerhof (PBE) functional,16 while the core electrons are described by projector augmented wave pseudopotentials.17 As the weak van der Waals and H-bonds of the framework atoms and the templates (which support the integrity of SUT frameworks11) are not well described on the PBE level, the experimental lattice parameters and atomic positions are maintained during the simulations. Because of the large lattice parameters and the number of atoms it is possible to restrict the sampling of the Brillouin zone to the G-point. A Gaussian smearing with a width of 0.05 eV is used for calculating the density of states (DOS). The same approach is used for simulating the electronic structures of the crystalline complexes [Ni(C4H10N5)2] and [Ni(en)3](NO3)2. Bader’s analysis of the charge density of the complexes is performed using the method of G. Henkelman et al.18 The light absorption is evaluated using the dynamic macroscopic dielectric matrix obtained from the electronic ground state charge density. For computing the imaginary part of the frequency dependent dielectric function the total number of bands is doubled in all cases. The real part is determined by Kramers–Kronig transformation.19 To estimate the strength of the Ni–O interaction between the complexes and the framework, we study the structure using SIESTA20 followed by a Mulliken population analysis. The PBE functional is applied together with norm-conserving pseudopotentials. Basis sets including double-z and polarization orbitals are used together with k-grids of 36 and 27 points for the unit cells of SUT-1 and SUT-2, respectively, generated using the method of Moreno and Soler.21 The fineness of the real-space grid20 for calculating charge densities is defined by an energy cutoff of 600 Ry. Convergence of the total energy with respect to the number of k-points and the grid cutoff energy has been verified. The incorporated Ni atoms have 6-fold coordinations in both germanates (Fig. 1 and 2), where the coordination spheres are formed by four Ni and two O atoms arranged in a slightly distorted octahedral motif with the O atoms in opposite corners. Since the Ni–O distances are larger than the Ni–N distances, the octahedron is stretched in the z-direction (Table 1). The N–Ni–N bond angles deviate from 901 by up to 4.11 (Table S1†). Ni atoms that have not been incorporated into the framework form [Ni(en)2]2+ complexes in the pores of SUT-1. The structure of the germanates contains two crystallographically inequivalent Ni positions with the site-symmetries C2 (SUT-1, space group Pcan) and Ci (SUT-2, space group P21/n), whereas the 4-coordinated Ni positions in the pores of SUT-1 possess the

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Fig. 1 (a) Fragment of the SUT-1 germanate, showing the environment of a 6-coordinated Ni atom incorporated in the framework. (b) [Ni(en)2]2+ complex filling the pores of the structure.

Fig. 2 Fragments of the SUT-2 germanate, showing the environment of 6-coordinated (a) Ni(1) and (b) Ni(2) atoms in the framework.

Table 1 Ni–N and Ni–O bond lengths in the germanates SUT-1 and SUT-2 and the corresponding Mulliken atomic overlap populations


d (Å)

Mulliken overlap population

SUT-1 Ni(1)–N(1) Ni(1)–N(2) Ni(1)–O(18) Ni(2)–N(3) Ni(2)–N(4) Ni(2)–N(5) Ni(2)–N(6)

2.069 2.053 2.233 1.904 1.905 1.916 1.903

0.18 0.19 0.11 0.20 0.20 0.21 0.21

SUT-2 Ni(1)–N(1) Ni(1)–N(2) Ni(1)–O(15) Ni(2)–N(3) Ni(2)–N(4) Ni(2)–O(38)

2.075 2.066 2.251 2.085 2.083 2.215

0.18 0.19 0.11 0.18 0.18 0.11

lower C1 site-symmetry. Furthermore, the 6-coordinated Ni atoms occupy 4c Wyckoff positions in SUT-1 and 2c and 2d Wyckoff positions in monoclinic SUT-2.

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Results and discussion

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Electronic structure Because of the octahedral coordination of the Ni atoms in the complexes incorporated in the framework, the ground state of Ni+2 bound to O is a triplet, as predicted by ligand field theory. The unit cells of SUT-1 and SUT-2 contain four such complexes. Employing SIESTA, we investigated two possible variants of the electron spin orientation for the 6-coordinated Ni+2: parallel and antiparallel orientation of the unpaired Ni spins, further referred to as the FM and AFM cases. In the AFM case we split the 6-coordinated atoms in two groups with opposite magnetic moments (Table S2†) so that the total magnetic moment of the unit cells is zero. Our calculations show a small difference in the total energy of about 0.05 eV between the FM and AFM cases, suggesting that there is no magnetic coupling between the d-electrons at the Ni sites. Owing to this fact, the Ni centers in the unit cell can be assumed to be independent, thus allowing us to consider them separately in further simulations to reduce the number of configurations. [Ni(en)3](NO3)2 complexes with 6-coordinated Ni (Fig. S1†) have been studied thoroughly with respect to their structure14,22 and electronic23 and optical properties.24 We compare these findings to our results for the SUT-1 and SUT-2 germanates, with a focus on the electronic configurations of Ni and the atoms in its neighborhood. The electronic structure of the Ni atoms in [Ni(en)3](NO3)2 shows a typical eg–t2g splitting of the d8 configuration by the ligand field (Fig. 3a and Fig. S3†). The s-bonding orbitals from overlapping s (a1g) and p (t1u) states of Ni and N are located energetically below the bonding eg orbitals constructed from the d3z2 r 2 and dx2 y2 states of Ni and p states of N (Fig. S3†). The non-bonding t2g orbitals (dxy, dxz, and dyz states) lie energetically between the eg and eg* orbitals (d3z2 r 2 and dx2 y2 states of Ni and p states of N). The calculated occupancies indicate that the antibonding eg* orbital is half-filled, with states in the spin-up channel only (Fig. 3a and Table S3†). Analysis of the DOS projected onto the Ni d-orbitals shows that the exchange energy for the t2g orbitals (0.87 eV) is very similar to the value (0.85 eV) obtained by Bridgeman and coworkers using the BP86 functional.23 However, the width of the crystal field gap (1.88 eV) is slightly higher than the value (1.40 eV) reported in ref. 23. According to Fig. 3a, the bonding d3z2 r 2 and dx2 y2 states are located at 4.4 eV and 3.5 eV and antibonding spin-up states at 0.1 eV, whereas the empty antibonding spin-down states appear at 2.0 eV. The DOS of the 6-coordinated Ni+2 ions incorporated in the frameworks of the SUT-1 and SUT-2 germanates reveals a similar configuration of the d-levels. In fact, the symmetry reduction of the ligand field from the ideal octahedral Oh symmetry to the D4h symmetry separates the d3z2 r 2 and dx2 y2 energy levels in all cases (Fig. 4 and 5 and Fig. S3†). As compared to the Ni–N bonds, the Ni–O bonds in the SUT germanates are longer (Table 1) so that the energy of the d3z2 r 2 states is reduced. The energy of the empty states in the spin-down channel is reduced even more significantly, thus affecting the crystal field gap. Splitting of the dxy, dyz, and dxz

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Fig. 3 DOS of (a) the six N atoms linked to Ni in the complex [Ni(en)3](NO3)2 and (b) the central Ni atom and the four adjacent N atoms in the complex [Ni(C4H10N5)2]. The Kohn–Sham energies are reported with respect to the HOMO level.

states is less evident because of weaker interactions of these non-bonding states with O and N. For example, only a slight shift of about 0.1 eV of the dxy states with respect to the dxz and dyz states is observed. To analyze the band splitting in a Ni{N4} unit with a square planar arrangement of the N atoms, we compare our SUT-1 results (Fig. 4b) to the crystalline [Ni(C4H10N5)2] complex (Fig. 3b and Fig. S2†). Overall, the Ni energy levels of the [Ni(C4H10N5)2] complex (Fig. 3b and Fig. S6†) are in good agreement with the molecular orbital diagram in Fig. S4.† The filled Ni dx2 y2 states (b1g symmetry) are mixed with N states due to s-bonding, whereas the empty b1g* states are antibonding. The latter are partially occupied by the 6-coordinated Ni atoms incorporated in the framework and, consequently, are probably responsible for the elongation of the Ni–N bonds with the Ni(1) atoms in SUT-1 and the Ni(1,2) atoms in SUT-2 (Table 1). However, the characters of the dxz and dyz states are different. The DOS in Fig. 3b is consistent with the overlap between the N pz lone-electron pairs and the Ni dxz and dyz orbitals in the complex [Ni(C4H10N5)2]. The observed splitting of the eg states (dxz and dyz states) into the bonding eg and antibonding eg* branches does not

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Fig. 4 DOS of the two symmetry inequivalent Ni atoms in the germanate SUT-1, calculated on the PBE level. The Kohn–Sham energies are reported with respect to the HOMO level.

Fig. 5 DOS of the two symmetry inequivalent Ni atoms in the germanate SUT-2, calculated on the PBE level. The Kohn–Sham energies are reported with respect to the HOMO level.

contribute to the Ni–N bond, because the eg and eg* states are equally occupied. The DOS obtained for the central Ni atom in the complex [Ni(C4H10N5)2] is similar to that of the 4-coordinated Ni(2) atom in SUT-1. However, a tetrahedral arrangement of sp3 hybridized atomic-like states of N in [Ni(en)2]2+ allows minimization of the p-type interactions in SUT-1, resulting in less splitting of the eg states (dxz and dyz states), see Fig. 4b. Apparently, no overlap between the dxz and dyz orbitals and the N orbitals occurs in SUT-1. The low energy of the dxy states points to p-like interactions, i.e., not within the xy-plane, between the Ni dxz and dyz orbitals and the N orbitals, in agreement with studies conducted on [1,3,5-triazapentadienato]nickel(II) complexes, as reported in ref. 25. A charge transfer from ligand to metal results in a redistribution of the electron density in the Ni complexes. Calculations of Bader charges for Ni and the atoms of the environment yields +1.1 for the 6-coordinated Ni centers (Table 2). In the 4-coordinated state, Ni draws even more electron density reducing its own

positive charge to +0.8, as evidenced by the 4-coordinated Ni(2) atom in SUT-1. In part this may be a result of the tight Ni–N interaction in the square-planar coordinated Ni complexes. Note that the effect is not limited specifically to SUT-1 and SUT-2, but can be seen in the [Ni(en)3](NO3)2 and [Ni(C4H10N5)2] complexes as well, where a similar ligand-to-metal charge transfer takes place (Table 2). Analysis of the charge density projected on the Ni d states shows a rather low occupancy for the bonding part of the dx2 y2 states on 4-coordinated Ni centers and d3z2 r 2 and dx2 y2 states on 6-coordinated Ni centers (Table S3†). However, our Mulliken population analysis indicates a relatively strong Ni–N bonding, where the overlaps are 0.20 to 0.21 and 0.18 to 0.19 for the 4- and 6-coordinated Ni atoms, respectively. Because Mulliken populations depend on the size of the basis set used, the obtained values are not absolute. However, they can describe trends. Ni–O interactions are almost twice weaker, totaling to 0.11 electrons in the Mulliken overlap populations. On the other hand, an average Ge–O bond of the GeO6 octahedra has an overlap of 0.23 to 0.37.

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Table 2

Paper Bader atomic charges in the core of the Ni complexes




SUT-1 SUT-2 [Ni(C4H10N5)2] [Ni(en)3](NO3)2

+1.10 +1.09 +0.87 +1.05

+0.80 +1.11

N(1) 1.15 1.17 1.25 1.02

N(2) 1.15 1.13 1.21

N(3) 1.28 1.15

N(4) 1.14 1.14

N(5) 1.17

N(6) 1.15







These results indicate that the Ni–N and Ni–O interactions in SUT-1 and SUT-2 share the typical characteristics of a dipolar (coordination) bond due to electron donation from ligand orbitals to the metal center in a complex. Absorption spectra The band gaps of solid SUT-1 and SUT-2 are due to the crystal field gaps between the split Ni d states. When Ni is in a 6-fold coordination then the highest occupied band is formed by the dxy states, which are slightly higher in energy than the dxz and dyz states (Fig. 4a, 5a and b). The dx2 y2 states of the antibonding character are vacant at the Ni(2) position (coordination number, CN(Ni) = 4) in SUT-1 and the highest occupied band consists of the d3z2 r 2 states (Fig. 4b). The lowest unoccupied band is formed by the d3z2 r 2 states (CN(Ni) = 6) or dx2 y2 states (CN(Ni) = 4). According to Fig. 4 and 5, a higher Ni coordination number corresponds to a lower band gap: 2.2 eV (d3z2 r 2–dx2 y2) for the 4-coordinated and 1.7 to 1.9 eV (dxy–d3z2 r 2) for the 6-coordinated Ni atoms. The calculated crystal field gaps in SUT-1 and SUT-2 are very similar to those found in the molecular Ni complexes. The real crystal field gaps are probably slightly larger because of the known problems of density functional theory in describing localized d-electrons, resulting in wider d-bands and smaller band gaps. The size of the d-state splitting indicates the possibility of transitions between the occupied and empty d-bands in the SUT germanates. In Fig. 6, absorption spectra of the two SUT germanates are compared to that of the germanate LMN-126 with a pharmacosiderite-type framework27 and a chemical composition of GeO2. Note that the three germanates display similar absorption features in the far ultra-violet range. The spectrum of SUT-1 has two characteristic peaks located at 5.1 eV and 5.7 eV that SUT-2 does not share. These two bands are likely due to charge-transfer N - Ni in the unit Ni{N4} of the [Ni(en)2] complex (compare Fig. 3b and 4b and Fig. S4†). Similar charge-transfer bands in the spectrum of the [Ni(C4H10N5)2] complex are located at 6.3 eV and 6.6 eV (Fig. 6b). The absorption spectrum of the [Ni(en)3](NO3)2 complex also displays a sharp charge-transfer peak at 5.6 eV. Furthermore, the peaks in the energy range from 6.6 eV to 7.8 eV are not related to transitions involving Ni d states, because they appear in the absorption spectra of all three germanates in Fig. 6. To validate the simulated absorption spectra, we have correlated the SUT plots using the optical absorption spectrum of the germanate Sb4Ge3O12,7b which shares the framework of the ASU-14 germanate and is built from the Ge9O22(O,OH)4 building unit, where the Ge atom of the central GeO6 polyhedron is replaced with Sb. Comparison of the calculated absorption spectra of the open-framework germanates SUT and Sb4Ge3O12

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Fig. 6 Absorption coefficients of (a) the open-framework germanates SUT-1, SUT-2, and LMN-1b and (b) the complexes [Ni(en)3](NO3)2 and [Ni(C4H10N5)2].

(ref. 7b) reveals overall similar features, although the peak positions in Fig. 6a are blue-shifted. Transitions between d-states are forbidden by Laporte’s rule if the inversion center is included in the point group of the complex. SUT-2 (SUT-1) has a (nearly) centersymmetric environment of the 6-coordinated Ni positions. The more intensive absorption in SUT-1 can be explained by a higher number of low-symmetry Ni(2) positions.

Conclusions Our first principles electronic structure calculations indicate that the reduced band gaps in germanates of the SUT series are due to the Ni d-states of the [Ni(en)2]2+ complex ions.

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The integration of the complexes into the framework has not much influence on the Ni d DOS though it results in an elongation of the Ni–N bonds in the complexes. The incorporated complexes do not interact with each other, as evidenced by a small energy difference between parallel and antiparallel alignments of the spins on the Ni centers. Analysis of the bond properties in the cores Ni{N4} and Ni{N4O2} allows us to conclude that the Ni–O interactions in SUT-1 and SUT-2 are of the same nature as the Ni–N bonds, i.e., coordination bonds formed by electrons of the ligand donated to molecular orbitals of the complex. The bond lengths as well as the Mulliken overlap populations suggest that the Ni–O bonds in the framework are approximately two times weaker than the Ni–N bonds in the complex. Simulation of absorption spectra of molecular transitionmetal complexes is known to be challenging. The positions of the absorption peaks predicted for the SUT-1 and SUT-2 germanates are reminiscent of the experimental optical spectrum of a germanate without transition-metal atoms, except for a blue-shift. The absorption spectrum of SUT-1 shows clear signatures of ligand-to-metal charge-transfer transitions between Ni states and the sp3-hybridized N states of the [Ni(en)2]2+ complex. The splitting of the Ni electronic states in the molecular complexes clearly resembles that found in the SUT-1 and SUT-2 germanates. Thus the observed color of the germanates may be related to d–d transitions at the transitionmetal centers of the Ni complexes. The more intense color of SUT-1 is explained by the number and lower symmetry of Ni(2) positions of [Ni(en)2]2+.

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Photophysical properties of open-framework germanates templated by nickel complexes.

Open-framework germanates are a group of germanium oxides with a well-defined porous structure, suitable for ion-exchange and gas adsorption applicati...
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