Gohil Singh
Thakur
a,
Hans
Reuter
b,
Helge
Rosner
a,
Gerhard H.
Fecher
a,
Claudia
Felser
a and
Martin
Jansen
*ac
aMax-Planck-Institut für Chemische Physik fester Stoffe, Nöthnitzer Straße 40, 01187 Dresden, Germany
bInstitut für Chemie neuer Materialien, Universität Osnabrück, Barbarastraße 7, 49069 Osnabrück, Germany
cMax-Planck-Institut für Festkörperforschung, Heisenbergstr. 1, 70569 Stuttgart, Germany. E-mail: M.Jansen@fkf.mpg.de
First published on 18th March 2019
We report the synthesis, crystal structure, and basic physical properties of Ag8PtO6, which represents the first silver platinum ternary oxide. The crystalline compound was obtained from appropriate mixtures of the binary constituents under alkaline conditions at high oxygen pressure, while applying relatively mild thermal conditions (573 K). Ag8PtO6 crystallizes in a new crystal structure in the triclinic system (P). The structure consists of slightly distorted, discrete PtO6 octahedra, which are linked via O–Ag–O dumbbells to form a three dimensional framework. It is a diamagnetic semiconductor with a band gap of 0.9 eV. DFT based calculations confirm an electronic ground state that corresponds to a 5d6 6s0 configuration of the Pt atoms, in accordance with the observed diamagnetism.
Most of the phases so far studied represent ternary oxides of PGEs and group I and II elements. Monovalent cations of group XI, Cu1+ and Ag1+, which are known to interrupt or at least to weaken magnetic exchange paths, have attracted much less interest, yet. Aiming at keeping the electronic effects mentioned above as localized as possible by suppressing long range hopping, we started to explore the ternary systems spanned by silver(I) oxides and PGEs.
Here we report on Ag8PtO6, the first silver oxo-platinate, its crystal structure and basic physical properties. In our computational analyses, we place particular emphasis on the nature of the electronic ground state, which one would conventionally associate with a low spin 5d6, t2g6 configuration. However, due to relativistic contraction of the 6s orbitals, states derived from the 6s2, 5d4 (t2g4) configuration might be lower in energy and give rise to a Jeff = 0 scenario, as encountered e.g. with Ir5+ oxides.12 Applying density functional band structure calculations, we can clearly assign a 5d6 6s0 configuration to the Pt atoms, consistent with a non-magnetic state.
Susceptibility of crushed single crystals was measured in applied magnetic fields up to μ0H = 7 T and in the temperature range between 2 and 400 K in a MPMS-XL7 magnetometer (Quantum Design).
Temperature dependent resistivity was measured on a plate-like single crystal of dimension 0.6 × 0.15 × 0.025 mm3 by a two-probe method in a PPMS instrument (Quantum Design).
Crystallographic data, details of data collection and structure refinement are given in Table 1, atomic coordinates, and equivalent isotropic displacement parameters in Table 2, bond lengths in Table S1† and anisotropic displacement parameters Table S2 in the ESI.†
Formula | Ag8O6Pt |
---|---|
Fw | 1154.05 (g mol−1) |
T/K | 296(2) |
λ/Å | 0.71073 |
Space group | P |
a/Å | 6.1911(2) |
b/Å | 6.9665(2) |
c/Å | 10.3901(3) |
α/° | 87.396(1) |
β/° | 85.152(1) |
γ/° | 71.751(1) |
V/Å3 | 423.98(2) Å3 |
Z | 2 |
Calc. d/g cm−3 | 9.04 |
μ(Mo-Kα)/mm−1 | 34.471 |
F(000) | 1004 |
Crystal size | 0.167 × 0.067 × 0.035 mm |
Theta range for data collection | 3.079° to 29.991°. |
Reflections collected/unique | 75196/2467 [R(int) = 0.0495] |
Completeness to theta = 25.242 | 99.9% |
Absorption correction | Semi-empirical from equivalents |
Refinement method | Full-matrix least squares on F2 |
Data/restraints/parameters | 2467/0/140 |
R 1/wR2 [all data] | 0.0135/0.0282 |
R 1/wR2 [I > 2σ(I)] | 0.0137/0.0283 |
Extinction coefficient | 0.00371(6) |
Largest diff. peak/hole | 1.096/−1.067 e/Å3 |
Atom | x | y | z | U(eq) |
---|---|---|---|---|
Ag(1) | 5000 | 0 | 5000 | 4(1) |
Ag(2) | 1528(1) | 3618(1) | 4037(1) | 5(1) |
Ag(3) | 4589(1) | 7214(1) | 388(1) | 6(1) |
Ag(4) | 3812(1) | 6666(1) | 4016(1) | 6(1) |
Ag(5) | 1585(1) | 4657(1) | 1343(1) | 5(1) |
Ag(6) | −1482(1) | 7639(1) | 3046(1) | 6(1) |
Ag(7) | −1378(1) | 8746(1) | 311(1) | 5(1) |
Ag(8) | 0 | 0 | 5000 | 5(1) |
Ag(9) | 3052(1) | 10319(1) | 2278(1) | 5(1) |
Pt(1) | 7132(1) | 2898(1) | 2458(1) | 3(1) |
O(1) | 5185(5) | 2617(4) | 4074(3) | 5(1) |
O(2) | 8931(5) | 3463(4) | 870(3) | 6(1) |
O(3) | 4480(5) | 5361(4) | 2052(3) | 6(1) |
O(4) | 8087(5) | 4724(4) | 3578(3) | 5(1) |
O(5) | 6038(5) | 1025(4) | 1418(3) | 6(1) |
O(6) | 9615(5) | 356(4) | 2952(3) | 6(1) |
Further details of the crystal structure investigation may be obtained from FIZ Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (fax: (+49)7247-808-666; email: crysdata(at)fiz-karlsruhe(dot)de, on quoting the deposition number CSD 434624. During all final calculations, the lattice parameters as obtained from X-ray powder pattern refinement were used.
Laboratory powder X-ray diffraction (PXRD) data (Fig. 2) were collected at room temperature on a HUBER G670 imaging plate Guinier camera with Cu-Kα1 radiation (λ = 1.5406 Å), in a 2θ range of 10–80 degrees. Rietveld refinement and Le Bail fitting were carried out with the TOPAS-4.2.0.2 (AXS) program.19 The refined parameters were the scale factor, zero point of θ, sample displacement (mm), background as a Chebyshev polynomial of 20th degree, 1/x function, crystallite size, micro-strain and cell constants.
The spin–orbit (SO) coupling was treated non-perturbatively solving the four component Kohn–Sham–Dirac equation.22 To obtain precise band structure information, the calculations were carried out on a well converged mesh of 600 k-points (10 × 10 × 6).
To test the reliability of the calculations, the internal positions of the atoms were optimized by minimizing forces with remaining forces smaller than 0.05 eV Å−1. For the shown electronic density of states, (DOS) the experimental crystal structure has been used.
The title compound crystallizes in a triclinic system with space group P, cf.Tables 1 and 2. The unique crystal structure consists of discrete, virtually ideal PtO6 octahedra and linear, or slightly bent, respectively, AgO2 dumbbells (Fig. 3). The primary PtO6 structural units are linked by silver atoms in twofold coordination to form a three-dimensional framework structure. Oxygen is found in two different structural functions, μ3 and μ4, connecting either one platinum atom with two (O2 and O4) or three (O1, O3, O5, and O6) silver atoms, respectively. As a result, each PtO6 octahedron is connected to nine other surrounding octahedra through sixteen O–Ag–O linkages (Fig. 4).
Fig. 3 (a) Crystal structure of Ag8PtO6 viewed along the a-axis, (b) 1D stack of PtO6 octahedra extending along [100]. |
The Ag–O bond lengths range from 2.04 to 2.26 Å (Table S1†) and agree well with the values reported in the literature for Ag–O bonds.23–25 The Pt–O bond lengths vary in the narrow range of 1.99 to 2.03 Å (Table S1†) which are also in excellent agreement with other reported oxides with Pt in +4 oxidation.26–28 The shortest Ag–Ag distance is 2.85 Å (Ag3–Ag5) which is slightly shorter than that of the nearest neighbour distances in Ag metal (2.89 Å); this separation, and in addition those extending up to 2.95 Å, are indicative of the presence of weakly bonding d10–d10 interactions.29 At a first glance, the structure appears to be rather open; however, the calculated free volume amounts to only 11.8 Å3 or 8%.30
Interestingly, in spite of the underlying overall three dimensional connectivity pattern some eye catching one dimensional structural anisotropy is becoming apparent when viewing the crystal structure in the [100] direction, see Fig. 3a. In this perspective the octahedra appear to be stacked in a staggered fashion, like a column excised from a hcp packing of oxygen atoms where every second octahedral void is centred by platinum, while the empty ones are bridged by silver atoms, see Fig. 3b. This particular microscopic feature seems to reflect the fibrous appearance of the crystals’ surface.
Ag8PtO6 is semiconducting with an activated type behavior in the temperature range 400 K to 280 K with room temperature resistivity of the order of 50 kΩ cm (Fig. 5a). The Arrhenius fit in the high temperature region yields a band gap of 0.89 eV, which falls in line with the observed semiconducting nature of the sample.
Fig. 5 (a) Arrhenius plot of resistivity of Ag8PtO6 with variable temperature resistivity in the inset (b) variable temperature susceptibility of Ag8PtO6 at various applied magnetic fields. |
The compound is found to be diamagnetic in the entire temperature region studied from 360 K to 2 K, (Fig. 5b). The susceptibility is negative and temperature independent in the high temperature region from 360 to 50 K, below which a weak field dependent Curie–Weiss like behavior is observed, which is due to the paramagnetic contribution of trace molecular oxygen impurity, which corresponds to a total of 0.7% of a spin ½ species. The value of χ∞ estimated from the extrapolation of Honda–Owen plots to infinite field comes out to be −224 × 10−6 emu mol−1. The diamagnetic behavior is consistent with +4 oxidation state of Pt with all d-electrons paired as implied from the empirical formula.
In order to discriminate between the two possible diamagnetic ground states, as can be derived from the configurations 5d6 (t2g6), S = 0 and 6s2, 5d4 (t2g4), Jeff = 0, density functional band structure calculations have been performed. In a first step, we optimized the internal coordinates of the atomic positions with respect to the total energy, assuming the experimentally determined lattice parameters. As a result, we find a surprisingly good agreement of the experimentally refined and the calculated atomic positions, and changes of the Ag–O and Pt–O bond lengths are about 0.01 Å, only. The corresponding difference in total energy between both parameter sets yields 2.5 meV per atom, corresponding to a temperature of about 30 K. These findings provide independent strong support to the structural parameters from the XRD refinement.
Our calculations reveal that Ag8PtO6 is a narrow band gap semiconductor. The calculated band gap has a size of about 150 meV. No instability towards magnetism was found. These findings are consistent with the experimental observations. The experimentally observed gap of 0.9 eV is, however, significantly larger than the calculated one. This is in line with the usual underestimation of the electronic gap size applying standard DFT functionals.
The calculated electronic density of states is shown in Fig. 6 (upper panel). The valence band has a width of about 8 eV, typical of this type of oxide system. The density of states is dominated by the silver 4d bands that are concentrated in the energy range from −6 to −2 eV. The oxygen states are distributed rather uniformly across the complete valence bands. A more detailed analysis shows that these states are of rather different character. The states below −6 eV are bonding states derived from the Pt–O octahedra. Their counterpart is separated by a large covalency split of about 5 eV between −4 eV and the Fermi level at zero energy. The Pt states of the valence band are almost exclusively of 5d character (see Fig. 6, lower panel). They are found in three groups: at the bottom of the valence band below −6 eV, in the middle of the valence band, and close to the Fermi energy above −1.5 eV. The low and high lying states are hybridized with the oxygen states, originating from the covalent PtO6 octahedra, whereas the states between −3.5 and −2 eV fall in the energy range where the majority of Ag 4d-states are located.
Fig. 6 Total and partial electronic density (DOS) of states of Ag8PtO6 (upper panel). The lower panel shows the orbital decomposed DOS of the Pt atom. |
A small amount of Pt 6s (and 6p) states is found at the bottom of the valence band (see Fig. 6, lower panel). A population analysis shows that 6s states are occupied by about 0.15 electrons (on-site contribution). The majority of (unoccupied) Pt 6s states appear about 10 eV above the Fermi energy (not shown here).
The electronic ground state of Pt in Ag8PtO6 is 5d6, 6s0 in very good approximation. Thus, the findings are clearing the suspicion that the 6s might have submerged below the 5d states, as was found earlier for Cs2Pt31 and other platinides.32
The title compound crystalizes in a triclinic unit cell, and represents a new structure type. The structure consists of isolated PtO6 octahedra connected by linear and/or slightly bent Ag–O linkages. It is found to be a diamagnetic semiconductor with an experimental band gap of ∼0.9 eV. Band structure calculations clearly assign a 5d6 6s0 state to the Pt atom, consistent with its non-magnetic state.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8dt05081c |
This journal is © The Royal Society of Chemistry 2019 |