Ceramic structural deficiency

The amount and positions (atomic locations) of oxygen atoms in the superconductors are highly critical and determine the properties of the superconductor. The oxygen vacancies (or deficiency) can be ordered in these materials. Neutron-diffraction experiments were required to determine the population parameters and the atomic positions of oxygen in these structures. The superconducting transition temperature in these "ceramic" oxides is a critical balance between the oxygen content and a proper mix of Cu2+ and Cus+ ions generated in the anneal or post-heat treatment. 

A significant step in the history of the HTSs was the discovery in 1966 of superconductivity in the oxygen-deficient perovskite SrTi03 5, containing some barium or calcium substituted for strontium. Although the Tc value was very low (0.55 K), in retrospect it can be seen as the first superconducting ceramic. In 1979 a Tc of approximately 13 K was discovered for BaPb075Bi025O3, which also has the perovskite structure. 

Uheda K, Yamamoto H, Yamane H, Inami W, Tsuda K, Yamamoto Y, Hirosaki N (2009) An analysis of crystal structure of Ca-deficient oxonitridoaluminosilicate, Ca0.88Al0.91Si1.09N2.85O0.15. J Ceram Soc Jpn 117 94... 

The principles of light absorption have thus been worked out well. Yet, the details are difficult to predict. That is, what combination of chemical entities with what structure would produce what color cannot be precisely predicted with current theories. Not that the theories are deficient, but the theoretical calculations necessary cannot be done very precisely for relatively complicated systems. Besides, the production of ceramics is in general difficult to control. That is, it is difficult to make a pot precisely the way one wants. [Though, today, industrial production of ceramics is fairly well controlled]. All these conditions make coloring of ceramics a kind of arts, rather than science. In order to obtain an exact tinge of color, one has to follow rigorously a prescribed procedure that has been laboriously developed by trial and error. 

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