Article pubs.acs.org/IC
Persistence of Mixed and Non-intermediate Valence in the HighPressure Structure of Silver(I,III) Oxide, AgO: A Combined Raman, X‑ray Diffraction (XRD), and Density Functional Theory (DFT) Study Adam Grzelak,†,‡ Jakub Gawraczyński,†,‡ Tomasz Jaroń,‡,§ Maddury Somayazulu,§ Mariana Derzsi,*,‡ Viktor Struzhkin,*,§ and Wojciech Grochala*,‡ †
Faculty of Chemistry, University of Warsaw, ul. Pasteura 1, 02-093 Warsaw, Poland Center of New Technologies, University of Warsaw, ul. Banacha 2C, 02-097 Warsaw, Poland § Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road NW, Washington, D.C. 20015, United States ‡
S Supporting Information *
ABSTRACT: The X-ray diffraction data collected up to ca. 56 GPa and the Raman spectra measured up to 74.8 GPa for AgO, or AgIAgIIIO2, which is a prototypical mixed valence (disproportionated) oxide, indicate that two consecutive phase transitions occur: the first-order phase transition occurs between 16.1 GPa and 19.7 GPa, and a second-order phase transition occurs at ca. 40 GPa. All polymorphic forms host the square planar [AgIIIO4] units typical of low-spin AgIII. The disproportionated Imma form persists at least up to 74.8 GPa, as indicated by Raman spectra. Theoretical hybrid density functional theory (DFT) calculations show that the firstorder transition is phonon-driven. AgO stubbornly remains disproportionated up to at least 100 GPain striking contrast to its copper analogue−and the fundamental band gap of AgO is ∼0.3 eV at this pressure and is weakly pressure-dependent. Metallization of AgO is yet to be achieved.
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• In Class III, ions of different oxidation states are indistinguishable, because of rapid exchange of electric charge. A further division of this class is made into subclasses III-A and III-B. In III-A, electron delocalization occurs within polynuclear clusters of limited size and results in electronic transitions in the visible range and insulating properties.1 One of the most thoroughly studied examples is the Creutz-Taube ion, which is a binuclear complex of ruthenium consisting of two Ru atoms coordinated by five NH3 ligands each and bridged by pyrazine.18−20 The two Ru atoms are indistinguishable and have a formal oxidation state of 2.5.18,19 Subclass IIIB is characterized by electron delocalization over the entire cationic sublattice, which gives rise to metallic properties: electric resistivity in the range of 10−2−10−6 Ω cm, an absorption edge in the infrared region, and metallic reflectivity in the visible range.1 The above categorization of the mixed-valence compounds demonstrates that, although rare in nature, they form a family of compounds with remarkably rich electron transport properties. These can be further modified by changing external pressure and temperature ((p,T)) conditions, as exemplified by magnetite (iron(II,III) oxide, Fe3O4), which undergoes a phase
INTRODUCTION The chemistry of mixed-valence compounds primarily involves transition metals and post-transition metals, because their d-, f-, or p-orbitals often allow for multiple oxidation states.1 Relatively few compounds with such characteristics are known to occur in nature. These include magnetite (Fe3O4),2 minium (Pb3O4),3−5 hausmannite (Mn3O4),6 and pitchblende (U3O8).7 Many more have been synthesized, such as Cs2Au2X6 (X = Cl, Br, I), 8 −1 1 Ag 3 (SbF 6 ) 4 , 12 Ag 3 O 4 , 1 3 Tl 4 O 3 (TlI3TlIIIO3),14 Sb2O4,15 Ga2Cl4,16 or the well-known Prussian Blue (Fe4[Fe(CN)6]3), to name just a few. Robin and Day divided such materials into three classes, based on their electron transport properties.1 • Compounds of Class I have, in their structure, two distinct crystallographic sites with very different local coordination; electrons do not move freely between the two sites.1 These compounds are either electrical insulators or semiconductors. An example of such a compound is AgO (AgIAgIIIO2), which is a n-type semiconductor.17 • Class II is characterized by ions at two distinguishable oxidation states, but with nearly identical local environment and a slight charge delocalization, which results in at least one visible-range electronic transition (hence, for example, the deep-blue color of Fe4[Fe(CN)6]3). The barrier for valence interconversion is small but not zero.1 © 2017 American Chemical Society
Received: February 15, 2017 Published: May 2, 2017 5804
DOI: 10.1021/acs.inorgchem.7b00405 Inorg. Chem. 2017, 56, 5804−5812
Article
Inorganic Chemistry
approach from ref 39, but with considerably larger supercell, leads to a new AgO polymorph with a potential to excellently interpret all experimental high-pressure data, in contrast to the previous theoretical results.
transition to a CaTi2O4-type structure at pressures of >25 GPa.21−24 The high-pressure phase is metallic and retains the mixed valence of iron.21 Or take Cs2AuIAuIIICl6; this black compound contains Au atoms at +1 and +3 oxidation states in equal quantities, and it transforms from tetragonal phase to cubic phase at 12.5 GPa.25 Phase transition from Cs2AuIAuIIICl6 to CsAuIICl3 is an example of pressure-induced comproportionation. Electrical conductivity of the high-pressure phase was not measured, but experiments with the analogous Cs2AuIAuIIII6 found that charge ordering at Au sites produces a narrow band gap semiconducting phase with an activation energy for conductivity of merely 40 meV at 5.5 GPa.26 An extremely interesting case of disproportionation achieved by means of compression is H2S.27 After metallization at 90 GPa and upon cooling to 203 K, the H2S sample becomes a superconductor. Theoretical calculations indicate formation of superconducting higher hydride HxS (where x ≈ 3) and elemental sulfur; this has been very recently confirmed by X-ray diffraction (XRD) measurements.28 Disproportionation may seem awkward at first, but, in fact, it is to be expected at elevated pressure: the spheres of different sizes may be packed more effectively than uniform spheres, while the pressurevolume (pV) factor predominates free enthalpy of matter at very high pressures.29,30 The possibility of tuning the electronic properties of the mixed-valence compounds via compression has motivated us to study the high-pressure behavior of AgO. Silver(I,III) oxide (AgO) is a prototypical example of a synthetic mixed-valence compounddespite its apparent similarity to CuIIO, it contains Ag ions at two different oxidation states (AgI and AgIII in 1:1 ratio).17 In the past, it has been incorrectly called “silver(II) oxide”such a description would imply the presence of paramagnetic AgII centers; however, AgO is diamagnetic.31 Crystal structure of AgO was first determined in the 1950s by several independent groups to be a monoclinic type, belonging to the C2/c space group,32,33 but the positions of the O atoms remained dubious. It is now known to crystallize in the P21/c space group, where AgI cations are coordinated linearly by two O atoms (coordination number, CN = 2), while AgIII cations coordinated by four O atoms in a square (CN = 4).31 The mixed-valence nature of AgO has been confirmed by means of X-ray photoelectron spectroscopy (XPS),34 vibrational spectroscopy,35 and theoretical studies.17 Another polymorphic form of silver(I,III) oxide, AgO, can be synthesized electrochemically and crystallizes in a tetragonal I41/a space group, but is less stable.36 In fact, the observed crystal structure of both polymorphs of AgO, as well as structures of other late transition-metal monoxides, can all be traced back to phonondriven distortion of rock salt lattice coupled to electronic structure.37,38 Theoretical work on the high-pressure behavior of AgO has already been done independently by some of us39 and by Hou et al.40 The former paper predicts a transition to a P1̅-type highpressure (HP) structure via phonon instability at ∼45 GPa.39 The HP structure would exhibit small density of states (DOS) at the Fermi level (EF) but, at the same time, preserve the mixed valence of silver. Hou et al., on the other hand, have used a learning algorithm approach and predicted a transition to a highly symmetric R3̅m phase, combined with full comproportionation of silver into a single AgII state at pressures of >75 GPa, resulting in metallic behavior and high DOS at EF.40 The validity of these results have been further discussed by Derzsi and Grochala.41 As we will see below, using the phonon-
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EXPERIMENTAL METHODS
Commercially available AgO purchased from Alfa Aesar (purity 99.9%, black powder, grain size ca. 50 μm) was used in the experiments. For both Raman and XRD measurements, several grains of AgO powder were loaded into a diamond-anvil cell (DAC) (cullet diameter = 250 μm). Neon (loaded at ca. 170 MPa) was used as hydrostatic pressure transfer medium, and rhenium was used as gasket material. Raman spectroscopy measurements were performed with a 532 nm laser. Pressure was determined using the high-edge frequency of firstorder Raman band of diamond from diamond anvil.42 The sample of AgO was compressed up to 75 GPa and decompressed. X-ray diffraction (XRD) measurements were performed using the Advanced Photon Source synchrotron at Argonne Laboratories operating at a wavelength of 0.406626 Å (corresponding to 30.491 keV). The size of the X-ray beam raster was 5.9 μm × 8.0 μm. CeO2 powder at ambient pressure was used for instrument calibration. Sample of AgO was compressed together with a small grain of gold for pressure determination.43 The sample also contained some tungsten contamination from the needle that was used to load the sample onto the diamond cullet. The following software was used: “Jana2006” for Rietveld refinement of the XRD patterns,44 “VESTA” for drawing crystal structures45 and “Fityk” for drawing XRD and Raman spectra.46 In Rietveld refinement of XRD patterns, pseudo-Voigt functions were used for profiles and Legendre polynomials for background. Preferred orientation (March-Dollase parameter) was used for gold and neon and, whenever required, high-pressure AgO phases as well.
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THEORETICAL CALCULATIONS Hybrid density functional theory (hybrid DFT) calculations were performed using the VASP package.47−51 The structural models were fully optimized, using the HSE06 hybrid functional49,50 (with a HF/DFT mixing parameter of α = 1/4 (0.25)), a plane-wave cutoff of 520 eV, and a k-spacing of 0.3 Å−1. A denser k-spacing of ∼0.2 Å−1 was used for electronic structure and total energy (enthalpy) calculations. The hybrid functional is indispensable to correctly describe the structure and electronic properties of this compound.17 Calculations of lattice dynamics (phonon dispersions and Raman frequencies) were performed using the direct method of Parlinski, Li, and Kawazoe,52 implemented in PHONOPY.53 The phonon dispersion curves for the low-pressure (LP) monoclinic P21/c structure were calculated by taking into account symmetries of the fully relaxed 2 × 4 × 2 supercell containing 128 atoms. The Raman frequencies were calculated considering the conventional unit cells of the respective crystal structures. Because of the large super cell chosen for the phonon dispersion curves calculations, a somewhat-smaller fast Fourier transform (FFT) grid was used for the exact exchange (Hartree−Fock) routines (PRECFOCK = fast) while obtaining the Hellmann−Feynman forces used to calculate the dynamical matrices. This greatly accelerated the calculations and made them manageable within our computational resources.
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RESULTS AND DISCUSSION X-ray Diffraction. XRD patterns were collected from a sample of AgO compressed up to 56 GPa (Figure 1). The sample also contained gold, which was used for pressure determination,43 and neon as a hydrostatic pressure transferring medium. There were also trace amounts of tungsten from the 5805
DOI: 10.1021/acs.inorgchem.7b00405 Inorg. Chem. 2017, 56, 5804−5812
Article
Inorganic Chemistry
where K0 is the bulk modulus, K′0 is the pressure derivative of bulk modulus, and η = (V/V0)1/3 (where V0 is the volume of AgO unit cell under ambient pressure (106.67 Å3) and V is the unit-cell volume under a given pressure). Thus, the values of the bulk modulus and its derivative were determined to be K0 = 82.8 GPa and K′0 = 6.5. These values are somewhat different than those recently reported for CuO (103.4 GPa and 5.3, respectively).58 However, this is not surprising: CuO and AgO have different electronic structures (CuII ions in CuO and AgI/ AgIII ions in AgO) and different crystal structure (CuO crystallizes in the C2/c space group); in addition, Ag is a larger (and, therefore, softer) atom than Cu, hence, the bulk modulus of the LP form of AgO is ∼80% of that for CuO. At 19.7 GPa, the XRD pattern becomes markedly different from that observed at lower pressures. This transition can be seen in Figure 1 and is further illustrated by the pressure dependence of interplanar distances (d) taken from the 7.5°− 10° 2θ range (see Figure S2 in the ESI). We interpret the observed change as a first-order structural transition occurring between 16.1 GPa and 19.7 GPa. The diffraction pattern of the new high-pressure (HP-I) phase can be very well described using a new monoclinic cell (Figure 2) that was predicted to be most stable at pressures above 20 GPa by our new theoretical calculations (details of these calculations are discussed below in a separate section of this paper). Figure 1. X-ray diffraction (XRD) patterns of AgO obtained with synchrotron radiation (λ = 0.406626 Å) (the pressure is increasing from bottom to top).
needle used to load AgO and Au onto the cullet, and since it is one of the heaviest elements in the sample, even such trace amounts are clearly visible in our XRD data. However, the diffraction pattern of tungsten is simple, with no phase transition occurring within the studied pressure range, and it was be easily taken into account during refinement of the data. Incidentally, tungsten has served as another internal pressure standard43,54 besides neon and gold, with all three consistently leading to very similar pressure values. Below, we report pressure values based on a gold scale, with an average uncertainty of 1.5% and within a range of 1.0%−2.3%. The lowest-pressure XRD pattern was collected at 4.9 GPa. Hazen et al. reported a similar value (4.7 GPa) as freezing point of neon at 293 K.55 Therefore, it is not surprising that no reflections of face-centered cubic neon (fcc-Ne) phase are visible at this pressure, since it is probably just below that of the freezing point. The lattice constants of neon and tungsten are in good agreement with previously reported equations of state of these elements (see the Supporting Information (ESI)). At higher pressures, there is some discrepancy between the expected relative intensities of the (111) and (200) reflections of solid neon. However, such phenomenon has been observed in previous works and is attributed to the occurrence of preferred orientation, because of partial recrystallization of neon microcrystals with increasing pressure.56 In the range from 4.9 GPa to 16.1 GPa, the diffraction pattern can be well-described using the LP monoclinic structure of AgO. At 16.1 GPa, the unit cell is compressed to 87.9% of the ambient pressure volume. The obtained data were fitted using the Vinet equation of state (EOS):57
Figure 2. Rietveld refinement of the characteristic XRD patterns of three AgO phases. From top to bottom: low-pressure monoclinic structure, high-pressure monoclinic structure, high-pressure orthorhombic structure. Differential curves are shown at the bottom of each panel. Phase assignments are shown on the left of each panel.
⎛1 − η ⎞ ⎡3 ⎤ p = 3K 0⎜ 2 ⎟ exp⎢ (K 0′ − 1)(1 − η)⎥ ⎣ ⎦ 2 ⎝ η ⎠ 5806
DOI: 10.1021/acs.inorgchem.7b00405 Inorg. Chem. 2017, 56, 5804−5812
Article
Inorganic Chemistry The HP-I phase adopts the same P21/c monoclinic space group, but the structure is different from the LP structure in several ways (see Figure 3). As in the low-pressure phase, there
Figure 3. LP P21/c structure at 16.1 GPa (left), HP-I P21/c structure at 19.7 GPa (right), HP-II Imma structure at 55.7 GPa (bottom). [Color legend: gray balls, AgI; blue balls, AgIII; and red balls, O.]
are two distinct crystallographic sites of Ag atoms, which suggest that the mixed-valent nature of the compound is conserved. The main difference lies in the positions and local environment of AgI atoms. Following the phase transition, AgI cations align themselves into chains along the c-direction. At the same time, orientation of corrugated sheets consisting of [AgIIIO4] squares also changes−every other layer shifts by 1/2b. This requires a unit cell that is twice as long in the a-direction. Consequently, O atoms also align themselves into chains along the c-direction and the coordination number of AgI atoms increases from 2 to 8. This change testifies a transition from a more-covalent AgI−O bonding (CN = 2) to a more-ionic AgI− O bonding (CN = 8) at the phase transition. The pressure dependence of the lattice constants and the beta angle (β) of the HP-I phase is shown in Figures 4A and 4B, respectively. Error bars are not shown in the graphs, because they were too small to be visible at this scale. Error values for lattice constants, angles, and volumes at each pressure are provided in the ESI (Tables S2a and S2b). The HP-I phase is more compressible along the shortest crystallographic b-direction (11% reduction) than along the aor c-directions (4% and 2% reduction, respectively) in the studied pressure range. Simultaneously, β decreases with pressure from 101.6° at 19.7 GPa down to 93.2° at 37.0 GPa and becomes equal to 90° at 41.4 GPa. As a consequence, the symmetry of the unit cell increases, relative to the orthorhombic Imma one (HP-II; see Figure 3). This symmetry increase is not accompanied by any further structural changes, apart from the slight decrease of β, which becomes equal to 90°. Optimization of the HP-I structure on the hybrid DFT level leads to the monoclinic β angle being equal to 92.68° at 40 GPa and 90.06° at 80 GPa. Further increases of the monoclinic symmetry to the orthorhombic Imma (β = 0) do not lead to additional changes, neither in the enthalpy nor in the unit-cell volume or the lattice vectors (Figure S10 in the ESI). Thus, we interpret this change from a monoclinic crystal system to an orthorhombic crystal system as a quasi-second-order phase transition occurring below 41.4 GPa. Indeed, upon decompression, an inverse phase transition (HP-II to HP-I) is observed at p < 44.3 GPa. Further decompression to 20.8 GPa did not yield the ambient-pressure monoclinic LP structure,
Figure 4. Pressure dependence of (A) the lattice constants and (B) the beta angle (β) of the HP phases. Solid markers represent compression; open markers represent decompression. Exact values, including errors, are listed in the ESI.
and decompression was discontinued here. Crystallographic data for the three AgO phases at different pressures are listed in Tables 1−3. Atomic coordinates were not refined, because it Table 1. Crystallographic Data for AgO (LP) at 16.1(3) GPa parameter space group unit-cell parameters a b c β Z volume, V Rp Rwp goodness of fit, GOF Ag(I) Ag(III) O bond lengths Ag(III)−O Ag(I)−O
value P21/c 5.674(4) Å 3.223(2) Å 5.352(4) Å 106.56(4)° 4 93.81(3) Å3 1.49% 2.21% 0.54 0, 0, 0 0.5, 0, 0.5 0.2959, 0.3453, 0.2221 1.952(2), 1.9462(15) Å 2.081(2) Å
resulted in only a minimal (
