Nanometer-scale separation of d¹⁰Zn²⁺-layers and twin–shift competition in Ba₈ZnNb₆O₂₄-based 8-layered hexagonal perovskites

Abstract

The 8-layered shifted hexagonal perovskite compound Ba8ZnNb6O24 was isolated via controlling the ZnO volatilization, which features long-range B-cation ordering with nanometer-scale separation by ∼1.9 nm of octahedral d(10) cationic (Zn(2+)) layers within the purely corner-sharing octahedral d(0) cationic (Nb(5+)) host. The long-range ordering of the B-site vacancy and out-of-center distortion of the highly-charged d(0) Nb(5+) that is assisted by the second-order Jahn-Teller effect contribute to this unusual B-cation ordering in Ba8ZnNb6O24. A small amount (∼15%) of d(10) Sb(5+) substitution for Nb(5+) in Ba8ZnNb6-xSbxO24 dramatically transformed the shifted structure to a twinned structure, in contrast with the Ba8ZnNb6-xTaxO24 case requiring 50% d(0) Ta(5+) substitution for Nb(5+) for such a shift-to-twin transformation. Multiple factors including B-cationic sizes, electrostatic repulsion forces, long-range ordering of B-site vacancies, and bonding preferences arising from a covalent contribution to the B-O bonding that includes out-of-center octahedral distortion and the B-O-B bonding angle could subtly contribute to the twin-shift phase competition of B-site deficient 8-layered hexagonal perovskites Ba8B7O24. The ceramics of new shifted Ba8ZnNb6O24 and twinned Ba8ZnNb5.1Sb0.9O24 compounds exhibited good microwave dielectric properties (εr ∼ 35, Qf ∼ 36 200-43 400 GHz and τf ∼ 38-44 ppm/°C).

New eight‐layer twinned hexagonal perovskite microwave dielectric ceramic Ba8NiNb6O24

“…These materials exhibit ε ~ 29, high Qf values ~70 000‐90 000 GHz, although the relatively high τ f values within ~30‐45 ppm/°C . The search for cheaper niobate counterparts of the Ba 8 M Ta 6 O 24 led to the discovery of the eight‐layer shifted Ba 8 CoNb 6 O 24 and Ba 8 ZnNb 6 O 24 compounds. These two shifted niobates show larger ε r ~ 31‐35, but lower Qf values ~35 000‐50 000 GHz than the twinned tantalates, and positive τ f values within ~16‐44 ppm/°C similar to the twinned tantalates.…”

“…These empty octahedral layers isolate the perovskite blocks consisting of seven consecutive corner‐sharing octahedral (CSO) layers, where Co/Zn cations are confined in the central octahedral sites of the CSO blocks, leading to single Co/Zn layers apart from each other by ~1.9 nm. The out of center displacement toward empty octahedral layers for the highly charged d 0 Nb 5+ next to the empty octahedral layers owing to the second‐order Jahn‐Teller (SOJT) effect is necessary to stabilize the oxide anions in the empty octahedral layers, thus contributing to the unusual nanometer scale B‐cation ordering in Ba 8 CoNb 6 O 24 and Ba 8 ZnNb 6 O 24 …”

“…The stabilization of the twin‐shift configuration for the eight‐layered tantalate and niobate hexagonal perovskites described above has been associated with the subtle difference between SOJT effects of Ta 5+ and Nb 5+ . The Ta 5+ ‐O 2− bonding is more ionic than Nb 5+ ‐O 2−4,17 ; thus compared with Nb 5+ , the out of center distortion in the oxygen octahedron of Ta 5+ is not sufficient for stabilizing oxide anions in the empty octahedral layer and therefore destabilizing the shifted structure . Forming a twinned structure with shorter periodicity for the B‐site vacancy and B‐cations is more energy favorable for the tantalate compositions.…”

“…These materials exhibit e~29, high Qf values~70 000-90 000 GHz, although the relatively high s f values withiñ 30-45 ppm/°C. 11,12,15,16 17 The stabilization of the twin-shift configuration for the eight-layered tantalate and niobate hexagonal perovskites described above has been associated with the subtle difference between SOJT effects of Ta 5+ and Nb 5+ . The Ta 5+ -O 2À bonding is more ionic than Nb 5+ -O 2À4,17 ; thus compared with Nb 5+ , the out of center distortion in the oxygen octahedron of Ta 5+ is not sufficient for stabilizing oxide anions in the empty octahedral layer and therefore destabilizing the shifted structure.…”

“…The Ta 5+ -O 2À bonding is more ionic than Nb 5+ -O 2À4,17 ; thus compared with Nb 5+ , the out of center distortion in the oxygen octahedron of Ta 5+ is not sufficient for stabilizing oxide anions in the empty octahedral layer and therefore destabilizing the shifted structure. 17 Forming a twinned structure with shorter periodicity for the B-site vacancy and Bcations is more energy favorable for the tantalate compositions.…”