Synthesis, microstructure, and electrical properties of the delafossite compound CuGaO2

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Abstract

The electrical properties and microstructural characteristics of solid-state synthesized CuGaO2 ceramics were investigated. Undoped CuGaO2 ceramics exhibited p-type conductivity with a Seebeck coefficient of 780 μV/K and a room temperature conductivity of 0.0033 S/cm. Examination of the microstructure of CuGaO2 ceramics revealed the existence of thin laminar twins oriented along the {0 0 0 1} basal plane with thickness varying from several to several tens of nanometers. Doping with Ni2+ and Mg2+ did not result in a significant increase in conductivity and doping with Sn4+ resulted in a remarkable decrease in conductivity. With evidence from diffraction data on Sn-doped ceramics, it is proposed that the Sn4+ was ionically compensated with Cu vacancies.

Introduction

New consumer electronic devices are developed to be ultra-portable and efficient, combining communication, storage, and multimedia technologies in one package. These so-called smart devices require transparent electrodes and circuitry in order to retain portability; thus, much research has been conducted in the broad field of transparent conducting oxides (TCOs) [1], [2], [3], [4], [5], [6], [7]. The first device application of a TCO material was as a de-icer for WWII bomber windows [6]. In addition to being used in automobile and supermarket freezer display windows, TCOs are now used in a variety of applications that exploit certain aspects of the unique combination of electrical and optical properties they possess. Some applications, such as low emissivity and electrochromic windows, are more passive device applications in which the high IR reflectivity (controlled by the location of the plasma absorption edge) is utilized to reflect heat back into or out of certain spaces. Other applications, such as state-of-the-art flat panel displays, push the envelope of existing TCO properties such as conductivity and transmissivity.

In the past, TCO materials have been predominantly n-type. This has limited their applications since functional semiconducting devices require a pn-junction. It was not until recently that p-type TCO materials have become well known [8], and with their arrival, the possibility of transparent devices has become a reality [9], [10]. However, since their conductivities are several orders of magnitude lower than their n-type counterparts, many groups have sought new materials in hopes of reaching more practical conductivity values.

The delafossite structure is one such material that has the potential to achieve high p-type conductivity (Fig. 1). The delafossite-type compounds have the formula AIIBVIOIV2, where the A site is host to a monovalent cation (Cu, Ag, Pd, and Pt), while the B site is typically host to a trivalent transition metal cation (Al, Co, Cr, Ga, In, etc.) [11], [12], [13]. In this structure layers of A cations that are linearly coordinated to two oxygen atoms, are alternately stacked between layers of edge-sharing B3+O6 octahedra oriented perpendicular to the c-axis. It is important to note that in these B3+O6 octahedra, the oxygen atom is in pseudo-tetrahedral coordination as B3AO. Depending on the stacking, the B3+O6 octahedral layers can lead to either rhombohedral or hexagonal structures, with space groups R3¯m and P63/mmc, respectively.

The layered stacking sequence in the delafossite structure leads to high anisotropy in the structure. In fact, due to their close proximities, the Asingle bondA bonding distances actually approach distances close to their respective metallic counterparts [11]. In the case of Pd, the comparative Pdsingle bondPd bonding lengths in the PdCoO2 delafossite and their metal counterpart are 2.83 and 2.75 Å, respectively. While in the case of Ag, the bonding lengths in the AgCoO2 delafossite and Ag metal are 2.873 and 2.89 Å, respectively.

The structural anisotropy in the delafossite structure transcends into its properties. In the case of CuCoO2, large anisotropies are observed in the materials resistivity and activation energy. The resistivity and activation energy along the c-axis are 5 × 107 Ω cm and 0.7 eV, respectively, while along the a-axis, these values change to 2 × 105 Ω cm and 0.2 eV [11].

Due to the anisotropic crystal structure, the lattice parameters of the delafossite system are strongly influenced by the ionic radii of the A- and B-site cations. As Fig. 2 illustrates, the a-axis is highly dependent upon the ionic radii of the B-site cation, while the c-axis is fixed largely by the Osingle bondAsingle bondO bond length. Due to the fact that the B-site cation is incorporated into the B3+O6 octahedra, which are located in the ab plane of the structure, any changes that occur on the B-site primarily impact the a cell length. Also, due to the repulsive nature of the B3+ cations along the shared octahedral edges, a distortion occurs resulting in a shortened interatomic distance between the oxygen anions. As the B-site cation radii are increased, the Bsingle bondO distance increases while the Osingle bondO contact distance remains relatively unchanged. Therefore, an increase in B-cation size has little impact on the c-axis lattice parameter.

While a considerable amount of work has been achieved in the investigations of CuAlO2, CuYO2, and other delafossites [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], few reports have been obtained for delafossites containing the B-site cation Ga [30], [31], [32], [33]. We have previously investigated the high-temperature phase equilibria of isovalent cation-doped CuGa1−xInxO2 [34]. In this work, we present the synthesis, electrical properties, and microstructure of aliovalent cation-doped CuGaO2.

Section snippets

Experimental

CuGaO2 powders were prepared from stoichiometric mixtures of Cu2O and Ga2O3. Doping of CuGaO2 was achieved with powders of SnO2, NiO, and MgO. All powders were vibratory milled with zirconia media in ethanol for 6 h. To ensure a homogeneous distribution of the dopants, the binary oxides of the B-site cations were pre-reacted at 1100 °C in air for 12 h. The pre-reacted B-site oxides were then added to an appropriate amount of Cu2O and vibratory milled in ethanol again. The powders were then

Results and discussion

As shown in Fig. 3, single phase CuGaO2 delafossite was obtained after sintering at 1100 °C for 24 h. Within the detection limits of XRD, no secondary phases were observed. As a side note, examination of the microstructure with scanning electron microscopy (SEM) images with energy dispersive spectroscopy (EDS) also revealed no evidence of secondary phases. Analysis of the diffraction data yielded the hexagonal unit cell parameters a = 2.976 Å and c = 17.160 Å. These values are comparable to other

Conclusions

The electrical properties of doped and undoped CuGaO2 ceramics prepared by solid-state synthesis are reported. Undoped CuGaO2 ceramics exhibited p-type conductivity with a Seebeck coefficient of 780 μV/K and a room temperature conductivity of 0.0033 S/cm. Microstructural investigations revealed a high density of laminar twins oriented in the {0 0 0 1} basal plane. Doping with divalent ions resulted in only a modest increase in conductivity, while doping with Sn4+ resulted in a decrease in

Acknowledgement

This work was supported by the National Science Foundation under grant DMR-0093616.

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