New Ag₈PtO₆: synthesis, crystal structure, physical properties and theoretical analyses

Abstract

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 (P1̄). 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.

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Introduction

The terrestrial abundance of the platinum group elements (PGEs) is quite low. However, this class of transition metals continues to play a prominent role in science and technology.¹ Alloys based on such constituents are among the thermally and chemically most durable metallic structural materials.² Catalysts, with PGEs as constituents, employed in large scale chemical production,³ for exhaust gas purification⁴ or in fuel cells⁵ are legendary, not to mention cis-platinum complexes in use as cancer drugs.^6,7 In science, e.g., complexes of PGE helped to bring coordination chemistry as a new sub-discipline of inorganic chemistry to birth,⁸ or the so-called Krogmann's salts⁹ attracted considerable attention during at least two decades in the second half of the last century. In current condensed matter research, discovery of exotic electronic phases prevailing in certain PGE oxides has stimulated vibrant activity. Here, as a common thread, Hund's coupling, spin orbit coupling (SOC), crystal field (CF) splitting, hopping integrals, and electron correlations (U_H) are energetically on a comparable scale, thus giving rise to diverse uncommon electronic and/or magnetic ground states.^10,11

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, t_2g⁶ configuration. However, due to relativistic contraction of the 6s orbitals, states derived from the 6s², 5d⁴ (t_2g⁴) configuration might be lower in energy and give rise to a J_eff = 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.

Experimental section

Single crystal synthesis

Single crystals of Ag₈PtO₆ were prepared by reacting Ag₂O and PtO₂, 0.257 g and 0.0631 g, respectively, as a typical batch composition, in an alkaline medium at 573 K and under high oxygen pressure in a steel autoclave. Ag₂O was pre-synthesized by precipitating from an aqueous solution of AgNO₃ with an excess of NaOH. PtO₂ was used as purchased (Sigma Aldrich, 99.9%) without any further purification. Well ground mixtures of the oxides were placed in a gold crucible of ∼4 cm height, welded from one end and mechanically crimped from the other. About 1.5 ml of 3 M KOH was used as a mineralizer to promote crystal growth. The crucible was placed in a 20 ml stainless steel autoclave¹³ and about 11.2 ml of liquid oxygen was condensed in it. The gas-tight sealed autoclave was heated at 573 K in a vertical furnace for 96 hours and cooled slowly to room temperature in about 24 hours. The O₂ pressure generated in the autoclave at this temperature is estimated to be around 250 MPa. Large corrugated plate-like shiny black single crystals of maximum length ∼1.2 mm (see Fig. 1) were obtained along with small amounts of tiny polygonal crystals of Ag₂O. The crystals were easily handpicked and sonicated in ethanol to remove any surface impurities. The crystalline product is insensitive to air and moisture.

Fig. 1 Photographs of few single crystals of Ag₈PtO₆, left: on a millimeter grid.

Instruments for physical property measurements

Thermogravimetric (TGA) analysis of Ag₈PtO₆ was carried out on a Netzsch STA 449 C analyzer. About 30 mg of the sample was placed in a corundum crucible, which was heated and subsequently cooled at a rate of 5 K min⁻¹ in the range of 300–1273 K under dynamic argon flow.

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).

Structure determination and refinement

A black plate-like crystal fragment of appropriate size was obtained by cutting a larger crystal under a microscope. It was fixed on a 50 micron Micromesh MiTeGen Micromount using FROMBLIN Y perflouropolyether (LVAC 16/6, Sigma-Aldrich). Diffraction data were collected at ambient temperature on a Bruker Kappa APEX II CCD-based 4-circle X-ray diffractometer using graphite monochromated MoKα radiation (λ = 0.71073 Å) of a fine focus molybdenum target X-ray tube operating at 50 kV and 30 mA. Collection and reduction of data were carried out with the Bruker Suite software package.¹⁴ Intensities were corrected for absorption effects semi-empirically from equivalents via SADABS.¹⁵ The structure was solved by direct methods and refined by full-matrix least-squares fitting on F² with the SHELXL software package.^16,17 Figures were drawn using DIAMOND.¹⁸

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.†

Table 1 Crystal and structure refinement data for Ag₈PtO₆

Formula	Ag8O6Pt
Fw	1154.05 (g mol^−1)
T/K	296(2)
λ/Å	0.71073
Space group	P1̄
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/Å^3	423.98(2) Å^3
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	75 196/2467 [R(int) = 0.0495]
Completeness to theta = 25.242	99.9%
Absorption correction	Semi-empirical from equivalents
Refinement method	Full-matrix least squares on F^2
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/Å^3

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Table 2 Atomic coordinates (×10⁻⁴) and equivalent isotropic displacement parameters (Ų × 10⁻³) for Ag₈PtO₆. U(eq) is defined as one third of the trace of the orthogonalized U_ij tensor

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)	10 319(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)

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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 20^th degree, 1/x function, crystallite size, micro-strain and cell constants.

Fig. 2 Le Bail fit of the powder X-ray diffraction data of Ag₈PtO₆ collected at 300 K.

Theoretical calculations

Relativistic density functional (DFT) electronic structure calculations were performed using the full-potential FPLO code, version fplo18.00-52.²⁰ For the exchange–correlation potential, within the general gradient approximation (GGA), the parametrization of Perdew–Burke–Ernzerhof was chosen.²¹

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.

Results and discussion

New Ag₈PtO₆ has been obtained from the binary constituent oxides under hydrothermal conditions, applying an elevated oxygen pressure of 250 MPa, however, rather mild thermal conditions of 573 K. It starts decomposing at about 673 K, loosing oxygen in a few steps and being completely reduced upon heating to 873 K, see Fig. S1 in the ESI.† The total weight loss is about 8.5% corresponding to complete loss of oxygen. The remaining product is a mixture of the constituent metals Ag and Pt.

The title compound crystallizes in a triclinic system with space group P1̄, 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].

Fig. 4 Primary PtO₆ structural unit with connected AgO₂ dumbbells. All atoms are drawn as thermal displacement ellipsoid at the 40% probability level. Symmetry code to generate equivalent atoms: (1) 1 + x, −1 + y, z.

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 ų 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 (t_2g⁶), S = 0 and 6s², 5d⁴ (t_2g⁴), J_eff = 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

Conclusions

A new silver-platinate, Ag₈PtO₆, has been synthesized as single crystals. This ternary oxide is inaccessible via solid state synthesis or solution chemistry, unless elevated oxygen pressure is applied, sufficiently high to suppress decomposition of the binary starting oxides. This finding is accentuating again the great potential of oxygen high pressure techniques for the synthesis of multinary oxide materials based on noble metals, be it in an explorative or purposeful approach. Such advanced preparation techniques have opened an entrance to a widely unexplored area of the energy landscape of chemical matter³³ spanned e.g. by the oxides of PGEs and coinage metals. Compounds made of appropriate combinations of noble metals are offering interesting prospects for basic research as well as for applications. The title compound will for sure show activity as a heterogeneous catalyst, yet, its pertinent performance in comparison with catalysts actually in use needs to be checked.

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.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors thank Dr Walter Schnelle and Dr Reinhard Kremer for the magnetic measurements. Open Access funding provided by the Max Planck Society.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8dt05081c

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