Hexagonal perovskite-related structures

The BaNiOj structure (often called the 2H—BaNiOj or just the 2H structure) is the hexagonal analogue of the ideal cubic perovskite structure (Section 1.2). 

Perovskites Structure-Property Relationships, First Edition. Richard J. D. Tilley. 2016 John Wiley Sons, Ltd. Published 2016 by John Wiley Sons, Ltd. 

2 BaNiOj-Related Phases Containing Trigonal Prisms 

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Perovskite-related Oxides.—The perovskite-related oxides have been studied extensively in recent years because of the large variety of device applications for which these materials are suited. The interaction between structure, properties, and stoicheiometry is significant at all levels, but here we will discuss only the narrow areas where intergrowth is a dominant structural feature. We will not, therefore, consider solid solutions typified by the Pb(Zr Tii )03 ferroelectrics, and neither will we discuss the structurally complex but stoicheiometric phases related to hexagonal BaTiOj, which includes BaNiOj, which has a simple two-layer repeat in the c-direc-tion, the nine layer BaRuOj, the twelve layer Ba4Re2CoOj2, and the twenty-four layer Sr5Re20ig phase. The crystal chemistry of these phases is treated in detail by Muller and Roy. The materials we shall discuss are the two series of phases A B 0 +2 and A + B 02n+, and the bismuth titanates. Some of the anion deficient perovskites, ABO -x, will be considered in Section 5. 

The structures of the compounds AMeFs are closely related to each other and can be derived from the well known perovskite structure. Therefore they may be generalizing referred to as fluoroperovskites, although some deformations of the cubic perovskite t e may occxir orthorhombic, tetragonal and hexagonal structures have been observed in ternary fluorides, in addition to the basic cubic type. 

As in all the perovskites — they might be defined that way — the A-and F-ions in the CsMnFs-structure form common close-packed layers AFs, in which the A-ion (Cs) displays a C. N. of 12 (Cs—F =3.12... 3.22 A in CsMnFs). The sequence ABC of three layers, characteristic of cubic perovskites, has been changed, however, to a hexagonal sequence of six layers ABC—ACB. This explains the relation found between the lattice constants ( hex = V2 eub Chex = 2 ]/3 acuu) from which follows Chex/ hex = ]/2 /3 = 2.45 or a value nearby. 

The structure of BaNiCh is closely related to perovskite (CaTiC>3, 3 2Pi/4 3/4O1/4), but the packing sequence is hep rather than ccp as for perovskite. The hexagonal cell contains two molecules, CB , C63me, a0 = 5.580 and c0 = 4.832 A. As shown in Figure 5.40, each Ni atom is at the center of an octahedron formed by six O atoms. With P layers at B and C positions all Ni atoms in O layers are at A positions, sharing three O atoms with another Ni atom. The Ba atom is at the center of a trigonal prism formed by six Ni atoms. The packing sequence of this structure is shown below ... 

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