First 14-Layer Twinned Hexagonal Perovskite Ba₁₄Mn_1.75Ta_10.5O₄₂: Atomic-Scale Imaging of Cation Ordering

Abstract

Formation of hexagonal perovskite with mixed cubic and hexagonal stacking of AO 3 layers becomes more and more difficult when the number of layers in the stacking repeating unit increases. So far, the highest number of layers reported for twinned hexagonal perovskite is 12, with alternative 5 consecutive cubic layers and one hexagonal layer in the (ccccch) 2 sequence. Here, we present the unexpected formation of a 14-layer twinned hexagonal perovskite with a stacking sequence (cccccch) 2 for the BaO 3 layers on the Ba 14 Mn 1.75 Ta 10.5 O 42 (Ba 8 MnTa 6 O 24 ) composition, the first example of twinned hexagonal perovskite with a periodicity exceeding 12-layers. The B-cation and vacancy distributions are characterized by multiple efficient and complementary techniques including neutron and synchrotron powder diffraction, scanning transmission electron microscopyhigh angle annular dark field (STEM-HAADF) imaging, and electron energy loss spectroscopy (EELS) and X-ray energy dispersive spectroscopy (EDS) elemental mapping. Atomic-resolution STEM-HAADF imaging and EELS/EDS elemental mapping enables direct observation of high-spin d 5 Mn 2+ cation ordering in the d 0 Ta 5+ host, thus demonstrating the great potential of this technique for probing cation ordering and performing structure determination. Moreover, atomic mapping allows for the observation of local defect structure variants, which can be a powerful tool for future new material design. The large high-spin Mn 2+ cation and Ta-vacancy pair formation in face-sharing octahedral sites play key roles on both the stabilization of this 14-layer twinned hexagonal perovskite structure and the Mn 2+ ordering in the central corner-sharing octahedral (CSO) positions within the five-consecutive CSO layers. Compared with the 8-layer twinned Ba 8 ZnTa 6 O 24 material, the low quality factor in microwave frequency and enhanced ultraviolet and visible light absorption of Ba 14 Mn 1.75 Ta 10.5 O 42 as well as the photocatalytic activity on water splitting are discussed in terms of the presence of high-spin Mn 2+ cations in the structure. ■ INTRODUCTIONMetal oxide materials based on the ABO 3 perovskite family are of great importance in current solid state chemistry and physics given their structural diversity and their technologically relevant physical properties, 1,2 e.g., superconductivity, 2,3 ferromagnetism, 2 ionic motion, 4,5 low dielectric loss, 6,7 etc. The cubic ABO 3 perovskite structure is built up from cubic-close-packed AO 3 layers with B cations occupying the octahedral sites between the AO 3 layers. Similarly, hexagonal perovskite oxides contain the same building units of close-packed AO 3 layers but arranged with hexagonal or mixed cubic-hexagonal close packing. These structures are interesting prototypes to be exploited for structural chemistry and physical properties in order to provide a deeper understanding of the fundamental chemistry and physics of materials in the cubic perovskite family and possibly to further discover potential new materials.The mos...

Shift–Twin Option in Eight-Layer Hexagonal Perovskite Niobates Ba8MNb6O24

“…Ba 8 MTa 6 O 24 exclusively adopts the twinned structures containing partially ordered M and Ta cations and vacancies among the FSO dimer sites except for the M = Mn composition unexpectedly adopting a 14-layer twinned structure containing ordered Mn in the central site of the corner-sharing octahedral (CSO) blocks. 11 Among their niobate counterparts, Ba 8 CoNb 6 O 24 (BCoN), 12 Ba 8 ZnNb 6 O 24 (BZnN), 13 and Ba 8 MnNb 6 O 24 (BMnN) 14 form the eight-layer vacancy-ordered shifted structure with ordered Co/Zn/Mn layers separated by ∼1.9 nm in the central CSO sites within the cubic perovskite blocks (Figure 1a). Unlike the BZnN, BCoN, and BMnN compounds, Ba 8 NiNb 6 O 24 (BNiN) adopts the partially ordered twinned structure.…”

Structure, microwave dielectric performance, and infrared reflectivity spectrum of olivine‐type Mg2Ge0.98O4 ceramic

“…For dielectric ceramics, the α l is usually about +10 ppm/°C; herein, the τ ε play a decisive role in the Mg 2 Ge 0.98 O 4 ceramics . Figure shows the temperature dependence of relative permittivity ( ε r ) at 100 Hz, 1k Hz, 10 kHz, 100 kHz, and 1 MHz.…”

“…28 The relationship among the τ f , τ ε , and the α l can be written as the following equation: 29 For dielectric ceramics, the α l is usually about +10 ppm/°C; herein, the τ ε play a decisive role in the Mg 2 Ge 0.98 O 4 ceramics. 30,31 Figure 6 shows the temperature dependence of relative permittivity (ε r ) at 100 Hz, 1k Hz, 10 kHz, 100 kHz, and 1 MHz. The Debye relaxation peak at 0°C-100°C is the dielectric relaxation behavior due to the absorption of moisture by the sample.…”

Atomic Structure of the Initial Nucleation Layer in Hexagonal Perovskite BaRuO3 Thin Films

“…These hexagonal perovskites can possess other varieties of structures depending on their stacking sequence (either by face‐ or corner‐sharing of the structural units). [ 2 ] In this regard, hexagonal perovskites provide an excellent platform for the investigation of a myriad of structure‐property relationships. [ 3–6 ]…”