BaTiO structure

Figure 35 (a) Molecular structure of Ba-Ti oxo-alkoxide, reproduced with permission from the Royal Society of Chemistry, (b) Comparison of the core structure of monoclinic barium-titanium oxo-isopropoxide with the BaTiOs structure viewed along the [111] direction, reproduced with permission from Wiley. ... 

In the present composite, since BaTiOs structure extends in the poling direction whereas AI2O3 matrix maintains its crystal structure and orientation, even after poling the residual stress would be generated in AI2O3 matrix. In the pervious work , residual stresses were observed in the same composite as the present study. 

Fig. 3. Crystal structure and lattice distortion of the BaTiO unit ceU showiag the direction of spontaneous polarization, and resultant dielectric constant S vs temperature. The subscripts a and c relate to orientations parallel and perpendicular to the tetragonal axis, respectively. The Curie poiat, T, is also shown.

In sodium nitrite the ferroelectric polarization only occurs in one direction. In BaTiOs it is not restricted to one direction. BaTiOs has the structure of a distorted perovskite between 5 and 120 °C. Due to the size of the Ba2+ ions, which form a closest packing of spheres together with the oxygen atoms, the octahedral interstices are rather too large for... 

Whereas the first microscopic theory of BaTiOs [1,2] was based on order-disorder behavior, later on BaTiOs was considered as a classical example of displacive soft-mode transitions [3,4] which can be described by anharmonic lattice dynamics [5] (Fig. 1). BaTiOs shows three transitions at around 408 K it undergoes a paraelectric to ferroelectric transition from the cubic Pm3m to the tetragonal P4mm structure at 278 K it becomes orthorhombic, C2mm and at 183 K a transition into the rhombohedral low-temperature Rm3 phase occurs. 

Fig. 3 a Structural transitions in BaTiOs according to the displacive scenario involving the freezing of the soft TO lattice mode (Cochran 1960) [3]. b Strnctnral transitions in BaTiOs according to the order-disorder scenario. Different Ti ion colors denote different occupancies (Chaves et al. 1976) [18]... 

Summarizing the features of the hexagonal fluoroperovskites it should be noted, that the structures of the BaTiOs- and BaRuOs-types are but different mixed forms of both, the purely cubic perovskites, e.g. CsCdFs with 3 layers in sequence ABC, and the purely hexagonal perovskites , e.g. CsNiFs with 2 layers in sequence AB. The dimensions of the c-axes are given by the number of layers and are therefore larger in the case of the mixed structures than for the basic types (e.g. CsMnFa 6 layers, CsCoFs 9 layers). 

The incorporation of Cu ions in the perovskite structure is known for only a few examples since this particular structure is normally stabilized by or requires a B atom in a high formal oxidation state such as Ti4+ in BaTiOs, or Rhs+ in LaRhOs. Further, since Cu can not be readily stabilized in its Cu(m) state, and is unknown in the tetravalent state, the simple formation of ternary compounds such as LaCuOg or BaCuOs is not expected. Even in the K2NiF4 structure, the stabilization of Cu4+ as in Ba2Cu04 is not expected, but the formation of a stable Cu(II) state is a distinct possibility, as in La2Cu04. Copper(II), however, has been introduced in the doubled-or tripled-perovskite structure. Examples of these, which include structural distortions from cubic symmetry, are listed ... 

Figure 3 Disproportionation and distortions in the perovskite structure. Arrows represent the spin of antibonding electrons in the Bini-0"n and Cu1- 11 bonds. For BaTiOs, the dots represent bonding electrons, <7 and jt.

Fig. 10.4. The structure of BaTiOs (23759) is composed of an alternation of the layers of BaO and Ti02 shown in Fig. 10.3.

The parent perovskite structure shown in Fig. 10.4 consists of alternating layers of composition AO and BO2, as for example in BaTiOs (23759) and CaTiOs (62149). It is also possible to have several AO layers between each BO2 layer providing each AO layer is sheared by half a unit cell from the adjacent AO layers, as shown for La2Ni04 in Fig. 12.1. This permits a wide range of structures with an even wider range of compositions to be prepared. Which compositions are possible depend on how well the structure can accommodate the bonding requirements of the atoms A and B. 

O Keeffe (1991Z)) has used bond valences to model the coherent interface that occurs between the semiconductors Si and MSi2 with M = Ni or Co (27139). Although these systems contain Si-Si bonds and therefore do not obey the assumptions of the bond valence model (condition 3.2), the mathematical formalism of the model still works because of the high symmetry. As both Si-Si and Si-Ni bonds are found in NiSi2, the cubic structure is strained (cf. BaTiOs in Section 13.3.2) and this strain affects the structure of the interface. Of the six possible interfacial structures examined, the two with the lowest BSI eqn (12.1) are those that are believed to occur in NiSi2 and CoSi2 respectively, and in both cases the strain introduced at the interface is correctly predicted. 

A wide array of ferroelectric, piezoelectric and pyroelectric materials have titanium, zirconium and zinc metal cations as part of their elemental composition Many electrical materials based on titanium oxide (titanates) and zirconium oxide (zirconates) are known to have structures based on perovskite-type oxide lattices Barium titanate, BaTiOs and a diverse compositional range of PZT materials (lead zirconate titanates, Pb Zr Tij-yOs) and PLZT materials (lead lanthanum zirconate titanates, PbxLai-xZryTii-yOs) are among these perovskite-type electrical materials. 

BaTiOs crystallizes in the perovskite structure. This structure may be described as a barium-oxygen face-centered cubic lattice, with barium ions occupying the corners of the unit cell, oxide ions occupying the face-centers, and titanium ions occupying the centers of the unit cells, (a) If titanium is described as occupying holes in the Ba-O lattice, what type of hole does it occupy (b) What fraction of the holes of this type does it occupy (c) Suggest a reason why it occupies those holes of this type but not the other holes of the same type ... 

Figure 1.1 Unit cell of cubic BaTiOs. The arrow schematically indicates one of the possible displacement of the central Ti4+ ion at the transition to the tetragonal ferroelectric structure that leads to a spontaneous polarization, in reality all ions are displaced against each other.

Fig. 2.2 MOg octahedra arrangements in (a) perovskite-type structures, (b) Ti02 and (c) hexagonal BaTiOs.

The value of of the BaTiO, ceramics is lower than that reported for BaTiOj single crystal [9] (along [100] =4 000). This may be due to the structural and compositional variances. Meanwhile, the size of crystalline particle may affect the dielectric constant , that is, when the particle size is lower than certain value, the constant will decrease with the decrease of the size. In addition to those mentioned above, porosity and the existence of low dielectric constant affect the non-ferroelectric layers at the metal-ferroelectric interface and the grain boundaries. 

Several members of the MM O3 class of ternary metal oxides adopt the perovskite-type (CaTiOs) structure and are sought as worthy target materials possessing ferroelectric properties see Ferroelectricity) Among the more widely investigated members of this class are BaTiOs and SrTiOs. Clearly, use of these materials as potential memory device... 

Hydrolysis and condensation rates depend on the molecular structure of metal alkoxides and alkoxide precursors have to be chosen as a function of the desired material final product. In the case of Ti02, for instance, monomeric precursors such as Ti(OPF)4, in which Ti is fourfold coordinated, react very quickly with water leading to the uncontrolled precipitation of polydispersed Ti02. The reaction is much slower with oligomeric precursors such as [Ti(OEt)4] in which Ti has a higher coordination number. Spherical monodispersed Ti02 powders can be produced via the controlled hydrolysis of diluted solutions of Ti(OEt)4 in EtOH. On the contrary, monomeric precursors are more convenient for the sol-gel synthesis of multicomponent oxides. The perovskite phase BaTiOs is formed upon heating around 800 °C when [Ti(OEt)4] is used as a precursor. This temperature decreases down to 600 °C with the monomeric precursor Ti(OPT)4 which favors the formation of Ti-O-Ba bonds. ... 

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