Article pubs.acs.org/IC
Rhombohedral Polytypes of the Layered Honeycomb Delafossites with Optical Brilliance in the Visible John H. Roudebush,* Girija Sahasrabudhe, Susanna L. Bergman, and R. J. Cava Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States S Supporting Information *
ABSTRACT: We report the synthesis of the Delafossite honeycomb compounds Cu3Ni2SbO6 and Cu3Co2SbO6 via a copper topotactic reaction from the layered αNaFeO2-like precursors Na3Ni2SbO6 and Na3Co2SbO6. The low-temperature exchange reaction exclusively produces the rhombahedral 3R polytype subcell, whereas only the hexagonal 2H polytype subcell has been made by conventional synthesis. The thus-synthesized 3R variants are visually striking; they are bright limegreen (Ni variant) and terracotta-orange (Co variant), while both of the conventionally synthesized 2H variants have a burnt-red color. The new structures are characterized by powder X-ray diffraction and Rietveld analysis as well as magnetic susceptibility, Xray photoelectron spectroscopy (XPS), and diffuse-reflectance optical spectroscopy. Using thermogravimetric analysis, we identify a second order 3R → 2H phase transition as well as a first-order structural transition associated with rearrangement of the honeycomb stacking layers. The optical absorbance spectra of the samples show discrete edges that correlate well to their visual colors. Exposing Cu3Ni2SbO6 to O2 and heat causes the sample to change color. XPS confirms the presence of Cu2+ in these samples, which implies that the difference in color between the polytypes is due to oxygen intercalation resulting from their different synthetic routes.
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INTRODUCTION Delafossite materials have the general formula ABO2, where A = Cu, Ag, Pd, and Pt and B = a cation such as Ga, Al, Fe, Y, or La with a 3+ charge. The B atoms form triangular layers of edge-sharing BO6 octahedra. The A atom is found in layers linearly coordinated to the O atoms of the BO6 octahedra. Thin films of CuGaO2, CuAlO2, and many other members of the Delafossite family have been pursued as p-type transparent conducting oxides,1−3 while CuFeO2 and CuRhO2 have been studied as photocathodes.4−6 The related phases NaxCoO2 and CuCrO2 have been investigated for their thermoelectric and ferroelectric properties.7,8 Delafossite, in which the B site is a rare-earth atom, have demonstrated oxygen intercalation. The magnetism of these layered phases has also been intensely studied because their octahedra are arranged in a triangular geometry, a common motif for magnetic frustration.9−12 The Delafossite structure has two common polytypes that are distinguished by the stacking of their oxygen sublattices in either cubic or hexagonal close-packed arrays. This translates to a difference in the relative orientations of the BO6 octahedra across the layers. In the three-layer rhombohedral 3R form (ccp O atoms), all octahedra face the same direction, while in the two-layer hexagonal 2H form (hcp O atoms), the octahedra are rotated by 180° around the c axis for adjacent layers. The energy barrier between the 2H and 3R variants is low, as evidenced by the fact that it is common for samples made by conventional high-temperature methods to contain mixtures of 2H and 3R polytypes. The difficulty in isolating one polytype makes a comparison of the property measurements between © XXXX American Chemical Society
polytypes for the same composition challenging, and consequently there has been relatively little work (or concern) reported for any differences that might be present. The “honeycomb” motif is a variation of the Delafossite and related α-NaFeO2 structure type; it has been synthesized with the compositions AB′2/3B″1/3O2 (B′ = Mg, Cu, Co, Ni, Zn; B″ = Sb, Bi).13−16 When the B′ and B′′ atoms are crystallographically ordered, they form the honeycomb arrangement; the composition in that case can be written as A3B′2B″O6 and is the form used in this work. These ordered variants typically have monoclinic unit cells due to distortions of the oxygen sublattice; however, the subcell can still be described by using either the 3R and 2H notations to designate the polytype. We make use of these notations to distinguish the orientation of the MO6 octahedra; thus, for example, 2H Cu3Ni2SbO6 has a subcell where the octahedra are rotated 180° from each other between layers, while in 3R Cu3Ni2SbO6, all octahedra face the same direction. Copper-containing Delafossites are typically synthesized by the mixing of oxide powders and heating at high temperatures (>700 °C). However, a low-temperature synthesis route by topotactic exchange (TE) can also be employed and is most utilized to synthesize silver-containing Delafossites.12,14,17−21 This method is far less common for the copper-containing Delafossites22,23 presumably because they often can be formed by traditional solid-state methods, whereas for silver-containing Received: November 23, 2014
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DOI: 10.1021/ic502790n Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
A TA Q600 TGA/DSC instrument was used to measure the weight change and heat flux upon heating. Samples were heated in oxygen or argon gases (100 mL/min) at a rate of 10 °C/min. Sample masses were between 30 and 60 mg. Magnetic measurements were made using a Quantum Design physical properties measurement system. Powder of 40−60 mg mass was wrapped in plastic and inserted into drinking straws. Zero-fieldcooled magnetization measurements were made as a function of the temperature with a 0.1 T applied field. Magnetic susceptibility was defined as M/μ0H for the 0.1 T applied field data; M versus μ0H was linear up to these fields for all temperatures studied. XPS characterization was performed with a VG ESCA Lab Mk.II, with all spectra measured using Mg Kα radiation (1284 eV) and a 20 eV pass energy. The sample powder was placed on carbon tape attached to the metal sample holder. To avoid charging effects during XPS, the sample holder was biased using an external bias of +10 V. The external potential acts as an electronic flood gun, attracting stray electrons toward the surface and effectively neutralizing the sample.30,31 Charge-induced broadening and splitting of the peaks were completely compensated for at +10 V. No further change in the peak shape, position, or intensity was observed when a more positive potential was applied. Spectra were obtained for NiO, parent compound, 3R and 2H polytypes. All scans were taken with a 0.05 eV step size and a 0.5 s dwell time. The obtained scans were fit with Casa XPS software using a Shirley background, and the area and positions were constrained using standard values. DR spectra for solid samples were measured in a Hitachi 131-90071 model U3210/U3410 recording spectrophotometer. The wavelength of the incident light in this instrument was varied from 200 to 2700 nm. The sample was placed in a “white” reflecting sphere so that all of the reflected and scattered light from the sample reaches the detector. KBr was used as a standard for calibrating the baseline; during calibration, the KBr pellets were placed in the two 1/2-in.-diameter pellet holders in the reflecting sphere. The reflectance from the samples was measured after calibration of the baseline. Each sample pellet for the reflectance measurement was made from a homogeneously ground mixture of the sample powder and KBr in a ∼1:10 weight ratio. During the measurement, the sample-impregnated KBr pellets were placed in the two pellet holders. The wavelength was scanned with a 150 nm/min rate and a 0.1 nm step size.
phases, Ag2O quickly reduces to silver metal at elevated temperatures. Still, with properties such as magnetism, conductivity, and photocurrent that can be affected by subtle chemical and structural changes, the development of new synthetic routes is desirable. For copper-containing Delafossites, a low-temperature exchange reaction also offers the opportunity to access new polytypes and even sometimes serendipitously reveals new phases or properties not found using traditional solid-state methods. In this work, we demonstrate the viability of a lowtemperature copper TE reaction to exclusively produce the 3R polytypes of Cu3Ni2SbO6 and Cu3Co2SbO6 and compare their properties to the high-temperature 2H polytypes to identify their differences. We find that the polytypes have similar structural and magnetic properties but strikingly different colors. Thermogravimetric analysis (TGA)/differential scanning calorimetry (DSC) is used to identify the 3R → 2H structural phase transition and measure the samples’ sensitivity to oxygen. We use X-ray photoelectron spectroscopy (XPS) and diffuse reflectance (DR) to compare the electronic binding environments and optical properties of the materials, ultimately leading us to propose an explanation for their difference in color.
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EXPERIMENTAL SECTION
To synthesize the 3R polytypes of Cu3Ni2SbO6 and Cu3Co2SbO6, first the starting materials Na3Ni2SbO6 and Na3Co2SbO6 were synthesized, the details of which are reported elsewhere.24−26 In short, they were formed by mixing Na2CO3, NiO, or Co3O4 and Sb2O4 (Alfa Asear, 99.99% metals) in an argon gas flow for 5 days at 800 °C. The obtained single-phase precursor materials were mixed with 3.1 equiv of CuCl (Sigma; or CuI) in an argon-filled glovebox, pelletized, placed in a high-density alumina crucible, and sealed in a quartz jacket under vacuum (
