Читать книгу Solid State Chemistry and its Applications - Anthony R. West - Страница 61

There is much recent interest in a family of perovskite‐related phases typified by CaCu₃Ti₄O₁₂ because of the possibility that they exhibit a giant dielectric constant. It is now known that the measured high permittivities are not a property of the bulk crystals but are associated with the grain–grain contacts in ceramics. The high permittivity is a consequence of the geometry of the regions responsible since the capacitance of a region is proportional to its area but inversely proportional to its thickness. CCTO has a most unusual crystal structure since the A sites of perovskite contain intermediate‐size Ca cations whose coordination number is reduced to eight by tilting of TiO₆ octahedra together with square planar CuO₄ units. The crystal structure is shown in Fig. 1.42(a) in which the left‐hand diagram emphasises tilting of the TiO₆ octahedra and the right‐hand diagram shows the square planar Cu‐based units which have three equivalent orientations in the cubic unit cell. In Glazer notation, the tilt system for CCTO is a ⁺ a ⁺ a ⁺ .

Figure 1.42 (a) Crystal structure of CaCu3Ti4O12.

Modified from E. S. Bozin et al., J. Phys. Condens. Matter 16, S5091 (2004).

(b) The brownmillerite structure.

Modified from G. J. Redhammer et al., Amer. Miner. 89, 405 (2004).

(c) The La2Ni2O5 structure.

Modified from J. A. Alonso et al., J. Phys. Cond. Matter 9, 6417 (1997).

(d, e) A 2D section through the perovskite structure showing corner‐linked BO6 octahedra in which different cations B, B′ may be either (d) disordered or (e) ordered: A‐site cations are omitted for clarity; (f) structure‐field map showing the effect of A and B sizes on structure type; (g) crystal structure of BaNiO3 as an [001] projection with some NiO6 octahedra emphasised: space group P63/mmc; a = 5.629, c = 4.811 Å; atomic positions Ba (2d) 1/3, 2/3, 3/4; 2/3, 1/3, 1/4; Ni (2a) 0, 0, 0; 0, 0, 1/2; O 6( h) x, 2x, 1/4: x = 0.1462; (h) one column of face‐sharing octahedra parallel to c. O and Ba positions together form BaO3 layers; (i–l) different polytypes and labelling notations. For each, h and c layers and the number of layers in the unit cell are shown: (i) (ch)2 BaRuO3, (j) (cch)2 BaTiO3, (k) (cchc)2 Ba8Ta4Ti3O24, (l) (cchcc)2 Ba10Ta7.04Ti1.2O30.

A wide range of compositions, AA′₃B₄O₁₂ form the same structure as CCTO. A is an intermediate‐sized alkali, alkaline earth or rare earth cation such as Na⁺, Ca²⁺, La³⁺, Pr³⁺, Nd³⁺ or a lone pair p‐block cation Pb²⁺, Bi³⁺ that occupies the 8‐coordinate sites; A′ is either of the Jahn‐Teller active ions Cu²⁺ or Mn³⁺ that occupies the square planar sites; B is a transition metal such as Ti, V, Mn, Fe, Co, Rh, Ir, or a p‐block element Ga, Ge, Sn, Sb that occupies the octahedral sites. The structures are cubic with a = 2a _p, where a _p is the lattice parameter of the perovskite subcell.

The electrical and magnetic properties of these materials depend on the valence states of the transition metal cations that occupy A′ and B sites. These may change if charge disproportionation phenomena occur, such as disproportionation at low temperatures of Fe⁴⁺ into Fe³⁺ and Fe⁵⁺ in the Fe analogue of CCTO or if temperature‐dependent local electron transfer or redox reaction occurs between cations such as Cu²⁺⁺/Fe⁴⁺ and Cu³⁺/Fe³⁺ in LaCu₃Fe₄O₁₂. Novel phenomena and properties may be anticipated following in‐depth studies of structure–property correlations in such materials.