We have studied the possibility of growing epitaxial films of silver clathrates on single-crystal oxide substrates by photoelectrochemical deposition. The clathrates chosen for this study were Ag₇O₈NO₃ and Ag₇O₈HSO₄. Both materials consist of a cubic lattice of Ag₆O₈ cages that enclose either NO₃ or HSO₄ anions, as illustrated in Fig. 1(a). Silver clathrates have various interesting properties; Ag₇O₈NO₃, for example, becomes superconducting at 1 K. Our purpose was to develop a technique for fabricating epitaxial clathrates with well-defined orientation on a single-crystal substrate. Silver clathrates contain Ag ions in several valence states, including Ag⁺, Ag²⁺, and Ag³⁺, which means that strongly oxidizing synthesis conditions are needed. One possibility is to use photoelectrochemical deposition on a metallic substrate under a suitable substrate bias in either AgNO₃ or Ag₂SO₄ solution. Growth experiments on 0.5 wt% Nb-doped TiO₂ substrates showed that the best selectivity for clathrate formation can be obtained at a bias of +0.2V vs. Ag. The necessary photoexcitation was provided by a high-pressure mercury lamp at a flux of 28 mW/cm². X-ray diffraction analysis showed that the best orientation control could be achieved on TiO₂ (110) substrates, yielding micrometer-scale epitaxial platelets, shown in a scanning electron microscope image in Fig. 1(b). It is clear from the image that the platelets tend to assume a hexagonal shape, characteristic of (111)-oriented growth. The growth habit and epitaxial relationship were further studied by x-ray diffraction and epitaxial relationships were found.

Fig. 1. (a) Structure of Ag₇O₈NO₃ silver clathrate crystals that consist of a cubic lattice of Ag₆O₈ cages that enclose NO₃ anions. (b) SEM image of epitaxial (111)-oriented Ag₇O₈NO₃ platelets photodeposited on a TiO₂(110) substrate surface in a AgNO₃ solution at an electrode potential of +0.3V vs Ag.

The reason for the preferential formation of epitaxial clathrate crystals on TiO₂ (110) surfaces was analyzed in terms of lattice matching between the clathrate Ag₆O₈ cage pseudo lattice and the TiO₂ surface. The (110)-oriented TiO₂ surface has a tetragonal structure where the spacing between adjacent bridging oxygen rows is 6.495 Å. This distance matches reasonably well the clathrate cage spacings of 6.056 Å, when the clathrate lattice is viewed along the (111) direction. It appears that despite the 6% mismatch, an epitaxial relationship is formed during platelet growth. The growth mechanism is thus somewhat similar to the formation of C₆₀ films on crystalline substrates, although the growth of a hexagonal (111)-oriented lattice on a tetragonal surface is unusual.

Various different anions can be integrated in the clathrate lattice. We compared the growth habit and growth rates of silver clathrates containing the HNO₃ and HSO₄ anions and found that while the growth rate of Ag₇O₈HSO₄ was much higher than for Ag₇O₈NO₃, the epitaxial relationship was weaker. Since the lattice parameter of the clathrates is predominantly set by the size of the Ag₆O₈ cages, it appears that the formation of large epitaxial platelets is limited by other factors, with variable anion incorporation kinetics in the Ag₆O₈ cages being the most likely reason. This work showed that epitaxial growth of inorganic cage compounds is possible when commensurate matching between the cage pseudo lattice and a single-crystal substrate occurs.

References

• [1] R. Tanaka, R. Takahashi, S. Takata, M. Lippmaa, and Y. Matsumoto, CrystEngComm 17, 3701 (2015).

Authors

• R. Tanaka^a, R. Takahashi, S. Takata^a, M. Lippmaa, and Y. Matsumoto^a,b
• ^aTokyo Institute of Technology
• ^bTohoku University