Perovskite-type semiconductors

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. 

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

Introduction of sodium into empty 12-coordinate sites of the WO3 lattice results in a series of oxide bronzes. These have general formula NaxWOa, where 0.0sodium tungsten bronzes retain the monoclinic structure of the parent WO3 and they are n-type semiconductors [276,277]. With increasing Na content the structure evolves through two distinct tetragonal phases and for x>0.43 the bronzes adopt an essentially cubic perovskite structure [278] closely related to that of ReOs (fig. 18). The Na 3s levels lie about 10 eV above the bottom of the W 5d bands and each added Na atom is therefore ionised to Na, with donation of one electron into the W 5d band of local t2g symmetry [279]. For x values of less than 0.26, the 5d electrons are localised, probably by an interplay between polaronic effects, disorder... 

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

When water molecules are adsorbed on semiconducting oxides, the conductivity increases or decreases according to whether the oxides are n- or p-type. In response to demands for operation at elevated temperatures, semiconductor humidity sensors using metal oxides, such as perovskite-type oxide, have been proposed as shown in Table 20-2. 

In reality the conductivity of proton conducting phases (Section 5.3) is more complicated than described. Successfully doped materials are not electronic insulators, as the aforementioned equations imply, but are generally weak p-type semiconductors. This comes about because at high temperatures a small amount of oxygen can react with the defective perovskite to partially fill the vacancies and generate a population of holes ... 

Perovskite-type rare earth aluminates are also prospective materials for deposition of GaN and AIN layers and manufacturing of blue light semiconductor lasers, as well as epitaxial layers of HTSC materials. LaAlOs and LSAT (Lai j Srj Ali yTay03) are well known in this respect. The application of a material as a substrate requires the knowledge of its physical characteristics and their temperature evolution in addition to the standard condition, which is the minimum mismatch between the cell parameters of the deposited film and the substrate. For details refer to O Bryan et al. (1990). 

Maeda, K. (2014) Rhodium-doped barium titanate perovskite as a stable p-type semiconductor photocatalyst for hydrogen evolution under visible light. 

Humidity sensors which use ionic oxides operate at around room temperature. The output of such a humidity sensor changes gradually with time. However, humidity sensors of the semiconductor type work well at elevated temperatures and maintain a stable output although there is interference from reducing gas, particularly ethanol. When the water vapor content increases, interference from reducing gases becomes smaller. The rare earth lanthanum also plays an important role in the perovskite-type humidity sensor. The rare earth enhances the interaction between the adsorbed species and the oxides at the surface (Shimizu et al. 1985). 

Rare-earth based perovskite oxides are p-type semiconductors. The resistivity of the oxide increases in a reducing gas atmosphere. However, the increase in resistivity is different for each reducing gas. For example, the increase is small for hydrogen and carbon monoxide, but a large increase in resistivity is observed for ethanol. This is why the perovskite oxide can be operative as an alcohol sensor. 

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