Crystal growth and crystal structures of the layered ionic conductors–sodium lithium titanium oxides

Crystal structure, conductivity and reversible water uptake of new layered potassium antimonates KxL(1+x)/3Sb(2−x)/3O2 (L=Ni2+, Mg2+, Co2+)

“…Only phases with prismatic coordination of K + were found, although the prisms in ionic compounds are less stable than octahedra due to shorter O-O distances. This fact confirms the hypothesis [12] that the preference to form P2 or P3 structures rather than O3 is due to the repulsion between the alkali cations. Small size of higher-valence cations dictates short a axis lengths, i.e., 2.94-3.00 Å for titanates [9][10][11][12][13][14][15][16], 3.05-3.08 Å for antimonates ([18] and Table 1), and 3.11-3.25 for stannates [6,7].…”

“…Of these, P2-type K 0.72 (In 0.72 Sn 0.28 )O 2 exhibits maximum conductivity values, higher than those of potassium beta alumina. Later, a number of cationconducting P2-type titanates Na x (L y Ti 1Ày )O 2 (L ¼ Ni, Co, Cr, Li) were described [8][9][10][11][12][13][14][15][16]. Alkali cation transport in these compounds is favored by wide rectangular bottlenecks between prismatic cation sites and by low ionicity of (L,M)-O bonds leading to high ionicity of the O-K (or O-Na) bonds [6][7]17].…”

Alkali Metal Cation and Proton Conductors: Relationships between Composition, Crystal Structure, and Properties

“…When x diminishes, this parameter usually becomes even shorter due to the smaller radii of the higher-valence cations listed above. The P2 and P3 structures may then be stabilized because the number of prisms is twice the number of octahedra in O3; distributing A cations over these prisms makes some A-A distances greater than parameter a, a feature impossible at x ¼ 1 and in the O3 structure [154]. The A-A repulsion is illustrated by the neutron diffraction data for P2-type titanates at 10-300 K indicating off-center location of Na þ ions within their prisms of two types [153].…”

“…Some other high-conductivity materials are represented in Table 7.2 and Figure 7.17, but descriptions of their structures are omitted here. Only a very specific class of oxide structures with a 3 A translation (types I-XIV in Figure 7.19 [48,49,53,[168][169][170][171][172][173][174][175][176][177][178][179][180][181][182][183][184][185][186][187]) will be reviewed briefly. This short distance precludes the location of any atom or ion between two large alkali ions, guaranteeing their free motion in that direction [62].…”

“…The ceramics conductivity is significantly lower: (2-6) Â 10 À4 S cm À1 in types VII-IX [177], 10 À5 -10 À4 S cm À1 in types III and VI [49,177,180], and 10 À6 -10 À4 S cm À1 in type I [168][169][170], all at 300 C. In contrast to all phases discussed in Sections 7.4.1-7.4.5, the type I K x Al x Ti 4Àx O 8 [168,170] (and, probably, K x Zn x/2 Ti 4Àx/2 O 8 [170]) and type III Na x Mg x/2 Ti 2Àx/2 O 4 [49] showed the highest conductivity, with maximum filling of the alkali channels (x ¼ 1 and x ¼ 0.95, respectively). Obviously, ion conduction in the 1-D channels is a cooperative process (possibly, soliton-like) and not due to vacancy hopping.…”