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Miscellaneous Oxide Films
401
Chapter 22 M I S C E L L A N E O U S O X I D E FILMS Electrochromism has been noted, though not studied in detail, in several oxides. They are discussed briefly below. The emphasis is on optical properties, and other aspects are given cursory presentations. It should be pointed out that some oxides, whose optical properties appear not to be significantly changed under ion intercalation/deintercalation, are discussed under ion storage materials in Ch. 27. The demarcation between electrochromic films and ion storage films is not sharp, though. This chapter covers oxides of rhenium (22.1), rhodium (22.2), ruthenium (22.3), iron (22.4), chromium (22.5), tantalum (22.6), copper (22.7), and praseodymium (22.8). Doped strontium titanate is discussed in (22.9).
22.1
Rhenium Oxide
Rhenium oxide is a defect perovskite, i.e., it has the same crystal structure as electrochromic W oxide and 13-type Mo oxide. ReO 3 is not known as an electrochromic material, and it is not expected to have this property to any large degree. Nevertheless it is of interest to pay some attention to Re oxide here since it provides clues to the mechanism underlying the electrochromism in oxides of W, Mo, etc.; these aspects are discussed in Ch. 23. Crystalline ReO3 can serve as an intercalation host for H § and Li § (633, 2442). A number of crystallographic modifications take place as the ionic content is increased, which is in analogy with the case of WO 3 (cf. Fig. 2.3). Optical data have been obtained from reflectance measurements on single crystals (1956) and from calorimetric measurements on polycrystalline samples (3583). The luminous absorption was strong, with broad peaks at photon energies being -0.4 and ~1.0 eV. These results are in good agreement with bandstructure calculations (2232-3).
22.2
Rhodium Oxide
Ruffle-like structures exist among several noble-metal-based oxides. Thus Ir oxide--discussed in Ch. 14--has this structure, and the same is true for bulk crystals of RhO 2, RuO 2, OsO 2, and PtO 2. The interatomic distances are very similar for all of them (576, 2009, 2428, 2441). The bandstructures are analogous as well--at least for RuO 2, OsO 2, and IrO2--although the Fermi levels lie at different positions in the t2g band (808, 2237, 29r cf. Fig. 14.17. Burke and O'Sullivan (547, 552) discovered the electrochromism of Rh oxide in 1978, and more detailed studies were
Chapter 22 M I S C E L L A N E O U S O X I D E FILMS Electrochromism has been noted, though not studied in detail, in several oxides. They are discussed briefly below. The emphasis is on optical properties, and other aspects are given cursory presentations. It should be pointed out that some oxides, whose optical properties appear not to be significantly changed under ion intercalation/deintercalation, are discussed under ion storage materials in Ch. 27. The demarcation between electrochromic films and ion storage films is not sharp, though. This chapter covers oxides of rhenium (22.1), rhodium (22.2), ruthenium (22.3), iron (22.4), chromium (22.5), tantalum (22.6), copper (22.7), and praseodymium (22.8). Doped strontium titanate is discussed in (22.9).
22.1
Rhenium Oxide
Rhenium oxide is a defect perovskite, i.e., it has the same crystal structure as electrochromic W oxide and 13-type Mo oxide. ReO 3 is not known as an electrochromic material, and it is not expected to have this property to any large degree. Nevertheless it is of interest to pay some attention to Re oxide here since it provides clues to the mechanism underlying the electrochromism in oxides of W, Mo, etc.; these aspects are discussed in Ch. 23. Crystalline ReO3 can serve as an intercalation host for H § and Li § (633, 2442). A number of crystallographic modifications take place as the ionic content is increased, which is in analogy with the case of WO 3 (cf. Fig. 2.3). Optical data have been obtained from reflectance measurements on single crystals (1956) and from calorimetric measurements on polycrystalline samples (3583). The luminous absorption was strong, with broad peaks at photon energies being -0.4 and ~1.0 eV. These results are in good agreement with bandstructure calculations (2232-3).
22.2
Rhodium Oxide
Ruffle-like structures exist among several noble-metal-based oxides. Thus Ir oxide--discussed in Ch. 14--has this structure, and the same is true for bulk crystals of RhO 2, RuO 2, OsO 2, and PtO 2. The interatomic distances are very similar for all of them (576, 2009, 2428, 2441). The bandstructures are analogous as well--at least for RuO 2, OsO 2, and IrO2--although the Fermi levels lie at different positions in the t2g band (808, 2237, 29r cf. Fig. 14.17. Burke and O'Sullivan (547, 552) discovered the electrochromism of Rh oxide in 1978, and more detailed studies were