Semiconductors perovskites

Semiconductor- Perovskite-type oxide Conductivity change by 10 -10 ppm 2-3 min. Air conditioner [29[, [31]... 

The potassium salts are the most soluble and other salts usually are precipitated by addition of the appropriate metal chloride to a solution of the corresponding potassium salt. The metaniobates, MNbO, and orthoniobates, MNbO, generally are prepared by fusion of the anhydrous mixed oxides. The metaniobates crystallize with the perovskite stmeture and are ferroelectric (131) (see Ferroelectrics). The orthoniobates are narrow band-gap semiconductors (qv) (132). 

Oxides play many roles in modem electronic technology from insulators which can be used as capacitors, such as the perovskite BaTiOs, to the superconductors, of which the prototype was also a perovskite, Lao.sSro CutT A, where the value of x is a function of the temperature cycle and oxygen pressure which were used in the preparation of the material. Clearly the chemical difference between these two materials is that the capacitor production does not require oxygen partial pressure control as is the case in the superconductor. Intermediate between these extremes of electrical conduction are many semiconducting materials which are used as magnetic ferrites or fuel cell electrodes. The electrical properties of the semiconductors depend on the presence of transition metal ions which can be in two valence states, and the conduction mechanism involves the transfer of electrons or positive holes from one ion to another of the same species. The production problem associated with this behaviour arises from the fact that the relative concentration of each valence state depends on both the temperature and the oxygen partial pressure of the atmosphere. 

Acceptor doping in perovskite oxides gives materials with a vacancy population that can act as proton conductors in moist atmospheres (Section 6.9). In addition, the doped materials are generally p-type semiconductors. This means that in moist atmospheres there is the possibility of mixed conductivity involving three charge carriers (H+, O2-, and h ) or four if electrons, e, are included. 

A natural question to ask is whether this two-regime theory is consistent with the known properties of LSM. As recently reviewed by Poulsen, the defect structure of LSM has some similarities with other more reducible perovskites such as LSG and LSF. Like these other perovskites, LSM has electrical properties on the border between that of a p-type semiconductor and a metaP and becomes oxygen substoichiometric at high temperature and low as shown in Figure 35. However, unlike its more reducible cousins (which may have significant vacancy concentration at atmospheric Pq ), LSM maintains a nearly full perovskite stoichiometry above atm and in fact becomes superstoichio-... 

The favoured material is modified lanthanum manganite (e.g. La0 8Sr0 2Mn03+x) which has the perovskite structure. It is a p-type semiconductor the electron transport occurring by electron-hopping (see Section 2.6.2) between the +3 and +4 states of the Mn ion. The strontium-doping enhances the conductivity. 

When these compositionally modified perovskites, which are p-type semiconductors, are heated to high temperatures in a water vapour atmosphere protons are incorporated into the structure, most probably is the form of hydroxyl groups, with the proton able to hop from oxygen to oxygen. The... 

The extensive variety of properties that these compounds show is derived from the fact that around 90% of the metallic natural elements of the periodic table are known to be stable in a perovskite-type oxide structure [74], Besides, the possibility of synthesizing multicomponent perovskites by partial substitution of cations in positions A and B gives rise to substituted compounds with a formula A, A B,. B 03 ft. The resulting materials can be catalysts, insulators, semiconductors, superconductors, or ionic conductors. 

In this case, we have introduced more hydrogen than those incorporated in the previously referred paper [66], Therefore, more electrons were introduced during the H2 incorporation into the perovskite. Consequently, we can conclude that the studied material at 1023-1273 K behaves as a small band gap semiconductor, because of the increase of electrons with temperature in the conduction band, due to the shifting to the conduction band of the Fermi level and the hydrogen-induced level. Then, there will be a sufficient number of electrons in the conduction band to screen the proton and allow it to have a high coordination number, and be interstitially located in the tetrahedral and octahedral sites. 

LuRh03 2.2 Distorted perovskite structure with p-type semiconductor behavior. 429... 

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