Synthesis, Structure and Phase Separation of a New 12R‐Type Perovskite‐Related Oxide Ba₃NdMn₂O₉.

Abstract

9 . -The new title compound is synthesized by solid state reaction of a stoichiometric mixture of Nd2O3, BaCO3, and MnCO3 (1300°C, 12 h). TEM reveals a chemical phase separation phenomenon leading to the coexistence of the perfect 12R-type structure with an average chemical composition of Ba4NdMn3O12 (space group R3m) and the Nd-rich modulated superstructure. In the 12R-type structure, the Nd cations are located in the corner-sharing octahedra, whereas the Mn cations are located in the face-sharing octahedra, leading to a remarkable cation ordering. -(YANG, H.; TANG, Y. K.; YAO, L. D.; ZHANG, W.; LI, Q. A.; LI, F. Y.; JIN, C. Q.; YU*, R. C.; J.

Microstructural Investigation of Ba(Ti(1−x)Mnx)O3 Ceramics with 6H‐ and 12R‐Polytypes

“…In our study, 12R‐polytype was detected in the Ba(Ti 0.5 Mn 0.5 )O 3 ceramics, and the XRD patterns were indexed by the similar trigonal‐BaFeO 3 structure (JCPDS 74‐0646). The structure of trigonal‐BaFeO 3 is the same as the 12R‐type perovskite‐related oxide Ba 3 NdMn 2 O 9 and Ba(Ti 0.5 Mn 0.5 )O 3 17,28,38 ; they are constructed from 12 BaO 3 layers in the (000 l ) close packing, and the stacking sequence of an unit cell is (BACA)(CBAB)(ACBC) or (cchh) 1 (cchh) 2 (cchh) 3 along the [0001] direction 17,38 . The 12R‐trigonal‐BaFeO 3 structure 38 with the space group is based on the hexagonal system due to the lattice parameters a = b ≠ c , α=β=90°, and γ=120°.…”

“…6(a) and (b)) of 6H‐ and 12R‐polytypes viewed from show a well‐ordered material feature with a left‐inclined 4 and a right‐inclined 4 of the 6H structure and a left‐inclined 2 and a right‐inclined 4 of the 12R structure. The stacking sequences of 6 and 12 BaO 3 layers in a unit cell are (chc) 1 (chc) 2 and (cchh) 1 (cchh) 2 (cchh) 3 along the [0001] direction, separately 15,17,38 . The bright dots correspond to rows of Ba and O atoms.…”

“…Forbidden conditions for the 12R phase are that h ≠3 n for , l ≠3 n for (000 l ) 12R , and − h + k + l ≠3 n for ( hkil ) 12R 17,27 . For the 6H phase, that is l =2 n +1 for 41 .…”

“…The Ba–Mn–O system possesses various polytypes, because different stacking sequences of the BaO 3 layers lead to different perovskite‐related structures 17–26 . The variety of different oxidation states of manganese that replace the Ti 4+ ions makes the Ba(Ti 1− x Mn x )O 3 structure highly susceptible to the formation of new perovskite‐related polytypes 17,23–28 . However, the solid solubility of manganese in 6H‐Ba(Ti 1− x Mn x )O 3−δ at 1400°C is about x =0.34 29 .…”

“…However, the solid solubility of manganese in 6H‐Ba(Ti 1− x Mn x )O 3−δ at 1400°C is about x =0.34 29 . The Ba 12 In 3 Mn 9 O 34.5 , Ba 4 REMn 3 O 12 (RE=Ce and Pr), and Ba 3 NdMn 2 O 9 oxides also show a structure related to the 12R‐polytype ([hhcc] 3 sequence) 17,27 . Keith et al 28 reported that the crystal structure of the 12R‐type perovskite with a composition of Ba(Ti 1/2 Mn 1/2 )O 3–δ is space group , a =0.569 nm, and c =2.792 nm.…”