**1. Introduction**

Ceramic oxides (MxOy) typically have a combination of properties that make them attrac‐ tive for many applications. These oxides exhibit fracture toughness and hardness, and can be used to make low weight composites of high strength. Ceramic oxides can also be used as protection coatings because of their interesting corrosion inhibiting properties [1-3]. These

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structural ceramics can be used to coat specific components exposed to extreme condi‐ tions, such as high temperature, high stress, or high friction. Of the ceramic oxides, rare earth oxides (REOs) are interesting materials and are a type of ceramic oxide that has many promising properties. Rare earth oxides can be used to color glass, for example, Er2O3 adds a light pink color while Sm2O3 produces a yellow color [4, 5]. REOs are used in the making of phosphors or fluorescent lighting [6, 7]. Rare earth oxides such as cerium oxide have also been important in automotive catalytic converters [8]. The most common stoichiome‐ try for rare earth oxides is R2O3; however other compounds containing Ce, Pr, or Tb can exhibit several oxide phases, ROx (1.5 < x < 2) and compounds like CeO2, Pr6O11, and Tb4O7 are common. Applications for rare earth oxide coatings include gas sensors [9, 10], fuel cells [11, 12], catalysis [13, 14], and corrosion protection [1, 15-18].

There is a long list of processing techniques for producing rare earth oxides including spray hydrolysis, pulsed laser deposition, chemical vapor deposition, solid state reactions, sol–gel method, and melt infiltration [19-24]. However, electrochemical synthesis has not been used extensively to deposit rare earth oxide coatings. The electrodeposition mechanism can be complex for many of the reactions and present a formidable challenge. In fact the majority of the electrodeposition work has focused on cerium oxide coatings and powders [25-28]. Typically, the redox potential for the rare earth oxides is not readily accessible in aqueous solutions, making synthesis difficult. But electrochemical deposition offers several advantages including low processing temperature, control of the driving force, and deposition onto various shapes [29-31]. This chapter covers the electrochemical deposition (not electrophoretic or soaking methods) of rare earth oxides as films for various applications.
