Crystallization perovskite

A key issue for perovskite solar cells is the stability of perovskite materials due to moisture effects tmder ambient conditions (134). An improved CHsNHsPbls-xClx perovskite quality with good crystallization and stability by using water has been developed as an additive during the crystal perovskite growth. 

Thus, water additive-based perovskite solar cells present a high-power conversion efficiency of 16% and an improved cell stability under ambient conditions in comparison to previous materials. These findings provide a route to control the growth of crystal perovskites to improve the stability of organic-inorganic halide perovskites (134). 

S. Royer et al reported the catalytic activity for CH4 oxidation and resistance to SO2 poisoning for three LaCoi.xFex03 prepared by reactive grinding. It was found that the catalysts prepared by this method showed well-crystallized perovskite structure and various specific surface areas. The activity in CH4 oxidation correlates well with BET specific surface area, the reaction temperature for 95% methane conversion decreased from 503 °C to 478 °C... 

Certain glass-ceramic materials also exhibit potentially useful electro-optic effects. These include glasses with microcrystaUites of Cd-sulfoselenides, which show a strong nonlinear response to an electric field (9), as well as glass-ceramics based on ferroelectric perovskite crystals such as niobates, titanates, or zkconates (10—12). Such crystals permit electric control of scattering and other optical properties. 

Oxides. Although not widespread commercially, glass-ceramics consisting of various oxide crystals in a matrix of siUceous residual glass offer properties not available with mote common siUcate crystals. In particular, glass-ceramics based on spinels and perovskites can be quite refractory and can yield useful optical and electrical properties. 

Perovskites have the chemical formula ABO, where A is an 8- to 12-coordinated cation such as an alkaU or alkaline earth, and B is a small, octahedraHy coordinated high valence metal such as Ti, Zr, Nb, or Ta. Glass-ceramics based on perovskite crystals ate characteri2ed by their unusual dielectric and electrooptic properties. Examples include highly crystalline niobate glass-ceramics which exhibit nonlinear optical properties (12), as well as titanate and niobate glass-ceramics with very high dielectric constants (11,14). 

Lead zirconate [12060-01 -4] PbZrO, mol wt 346.41, has two colorless crystal stmctures a cubic perovskite form above 230°C (Curie point) and a pseudotetragonal or orthorhombic form below 230°C. It is insoluble in water and aqueous alkaUes, but soluble in strong mineral acids. Lead zirconate is usually prepared by heating together the oxides of lead and zirconium in the proper proportion. It readily forms soHd solutions with other compounds with the ABO stmcture, such as barium zirconate or lead titanate. Mixed lead titanate-zirconates have particularly high piezoelectric properties. They are used in high power acoustic-radiating transducers, hydrophones, and specialty instmments (146). 

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). 

Elemental composition, ionic charge, and oxidation state are the dominant considerations in inorganic nomenclature. Coimectivity, ie, which atoms are linked by bonds to which other atoms, has not generally been considered to be important, and indeed, in some types of compounds, such as cluster compounds, it caimot be appHed unambiguously. However, when it is necessary to indicate coimectivity, itaUcized symbols for the connected atoms are used, as in trioxodinitrate(A/,A/), O2N—NO . The nomenclature that has been presented appHes to isolated molecules (or ions). Eor substances in the soHd state, which may have more than one crystal stmcture, with individual connectivities, two devices are used. The name of a mineral that exemplifies a particular crystal stmcture, eg, mtile or perovskite, may be appended. Alternatively, the crystal stmcture symmetry, eg, rhombic or triclinic, may be cited, or the stmcture may be stated in a phrase, eg, face-centered cubic. 

Certain perovskites with Pb on the A site are particularly important and show pronounced piezoelectric characteristics (PbTiO, PZT, PLZT). Different responses are found in BaTiO and PZT to the addition of donor dopants such as La ". In PZT, lead monoxide [1317-36-8] PbO, lost by volatilization during sintering, can be replaced in the crystal by La202, where the excess positive charge of the La " is balanced by lead vacancies, leading to... 

When 0.4 < x < 0.53, an orthorhombic phase is observed in the AgxNb02+xFi.x system. This phase undergoes a phase transition at 900°C that leads to the formation of a tetragonal phase, which crystallizes in a tetragonal tungsten bronze-type structure with cell parameters a = 12.343 and c = 3.905 A. When 0.82 < x < 1, solid solutions based on AgNb03 were found, which crystallize in a perovskite-type structure. 

Lithium dioxyfluoroniobate (IV), LiNb02F, also has a LiNb03-type crystal structure, while dioxyfluoroniobates of sodium and potassium, NaNb02F and KNb02F, crystallize in a perovskite-type structure [247]. 

When all of the atomic displacement vectors are parallel to a polar axis of the crystal structure, the compound belongs to the one-dimensional category. In this case, linkage manner of octahedrons, MeX6, is of fundamental significance of spontaneous polarization appearance. Typical examples of compounds that belong to the one-dimensional category include perovskites,... 

In the Lai.,CsxMn03 catalyst, the T decreases with an increase of x value and shows an almost constant value upon substitution of x>0.3. It is thought that the oxygen vacancy sites of perovskite oxide increase with an increase of amount of Cs and the oxidation activity also increases. This result is also verified by the TPR result of these catalysts(Fig. 3). As shown in Fig. 3, the reduction peak appears at low temperature with an increase of x value and no change is shown at more than x=0.3. It can thus be concluded that the catalytic performance of these oxides increases as the amount of Cs in the crystal lattice increases. However, the substitution of Cs to more than x=0.3 leads to excess Cs, which is present on the surface of mixed oxides might have no effect on the catalytic activity... 

Fig. 2 shows the temperature as a function of irradiation time of Cu based material under microwave irradiation. CuO reached 792 K, whereas La2Cu04, CuTa20e and Cu-MOR gave only 325, 299 and 312 K, respectively. The performances of the perovskite type oxides were not very significant compared to the expectation from the paper reported by Will et al. [5]. This is probably because we used a single mode microwave oven whereas Will et al. employed multi-mode one. The multi-mode microwave oven is sometimes not very sensitive to sample s physical properties, such as electronic conductivity, crystal sizes. From the results by electric fixmace heating in Fig. 1, at least 400 K is necessary for NH3 removal. So, CuO was employed in the further experiments although other materials still reserve the possibility as active catalysts when we employ a multi-mode microwave oven. 

Sadakane, M., Asanuma, T., Kubo, J. et al. (2005) Facile procedure to prepare three-dimensionally ordered macroporous (3DOM) perovskite-type mixed metal oxides by colloidal crystal templating method, Chem. Mater. 17, 3546. 

Huang YH, Fjellvag H, Karppinen M, Hauback BC, Yamuchi H, Goodenough JB (2006) Crystal and magnetic structure of the orthorhombic perovskite YbMn03. Chem Mater 18 2130-2134... 

Perovskites are compounds of the ABC3- type where C is often oxygen, but not always. Figure 11.6 shows two versions of the perovskite crystal structure... 

Figure 11.6 Views of perovskite crystal structure. Top—conventional cubic unit cell white circles = oxygen black circle = transition metal gray circles = alkali or alkaline earth metal. Bottom—extended unit cell to show the cage formed by the oxygen octa-hedra. Adapted from Bragg et al. (1965).

The latter indicates that the dominant bonding type is covalent. This was also observed for CaTi03 and BaTi03, both of which have the perovskite crystal structure, but are considerably softer than MgSi03.The Mg perovskite is about twice as hard as crystobalite (quartz). However, hydration converts MgSi03 to talc, which is very soft. 

The perovskite crystal structure is exhibited by a large number of compounds because numerous metals can form the octahedral sub-units, and other metal ions can lie between the sets of eight octahedra. The main constraint is on the sizes of the ions that can be chosen to fit compactly together. 

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