8-Layer Shifted Hexagonal Perovskite Ba8MnNb6O24: Long-Range

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8‑Layer Shifted Hexagonal Perovskite Ba8MnNb6O24: Long-Range Ordering of High-Spin d5 Mn2+ Layers and Electronic Structure Fengqiong Tao,† Chaoping Liang,*,‡ Xiaoming Wang,§ Xiaohui Li,† Florence Porcher,∥ Mathieu Allix,⊥ Fengqi Lu,† Haoran Gong,‡ Laijun Liu,† and Xiaojun Kuang*,† †

MOE Key Laboratory of New Processing Technology for Nonferrous Metal and Materials, Guangxi Universities Key Laboratory of Non-ferrous Metal Oxide Electronic Functional Materials and Devices, College of Materials Science and Engineering, Guilin University of Technology, Guilin 541004, P. R. China ‡ State Key Laboratory of Powder Metallurgy, Central South University, Changsha, Hunan 410083, P. R. China § Key Lab Macromolecular Science of Shaanxi Province, College of Chemistry and Chemical Engineering, Shaanxi Normal University, Xian 710062, P. R. China ∥ CEA Saclay, Laboratoire Léon Brillouin, F-91191 Gif Sur Yvette, France ⊥ CNRS, CEMHTI UPR3079, Univ. Orléans, F-45071 Orléans, France ABSTRACT: A new 8-layer shifted hexagonal perovskite Ba8MnNb6O24 has been synthesized in air, featuring unusual long-range B-cation ordering with single octahedral high-spin d5 Mn2+ layers separated by ∼1.9 nm within the corner-sharing octahedral d0 Nb5+ host, analogous to Ba8(Zn/Co)Nb6O24. The large size and charge differences between high-spin Mn2+ and Nb5+, as well as the out-of-center distortion of NbO6 octahedra associated with the bonding covalence and second-order Jahn−Teller effect of Nb5+, drive longrange cationic ordering, thus stabilizing Ba8MnNb6O24. The Ba8MnNb6O24 pellet exhibits a high dielectric permittivity, εr ∼ 38, and a large temperature coefficient of resonant frequency, τf ∼ 20 ppm/K, but a dielectric loss (Qf ∼ 987 GHz) and conductivity (∼10−8−10−3 S/cm within 473−1173 K) much higher than those of Ba8ZnNb6O24. Electronic structures from density functional theory calculations reveal that Ba8MnNb6O24 is a Mott insulator in contrast with the charge-transfer insulator nature of Ba8ZnNb6O24, and they confirm that the off-center distortion of Nb5+ contributes to stabilization of the 8-layer ordered shifted structure. The contrast between conductivity and dielectric loss of Ba8MnNb6O24 and Ba8ZnNb6O24 is understood based on the electronic structure that depends on high-spin d5 Mn2+ and d10 Zn2+ cations. The hopping of 3d valence electrons in high-spin Mn2+ to Nb5+ 4d conduction bands over a small gap (∼2.0 eV) makes Ba8MnNb6O24 more conductive than Ba8ZnNb6O24, where the electrons are conducted via the hopping of a lattice O 2p valence electron to the Nb5+ 4d conduction bands over a larger gap (∼3.9 eV). The high microwave dielectric loss of BMN may be mainly ascribed to the half-filled Mn 3d orbits, which is understood based on the softened infrared modes that increase the lattice vibration anharmonicity as well as the resonant spin excitation of unpaired d electrons.

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INTRODUCTION Hexagonal perovskite oxides containing mixed cubic (c) and hexagonal (h) AO3 layers display structural diversity from the various stacking sequences of the AO3 layers and important physical properties, such as high dielectric permittivity,1−4 oxide ionic mobility,5,6 magnetism,7,8 photocatalytic activity,9−11 etc. Hexagonal perovskite usually forms on B-site deficient compositions owing to the reduced B−B electrostatic repulsion in face-sharing octahedral (FSO) sites.11,12 Two major types of twinned or shifted structures occur in hexagonal perovskites, depending on whether a single hexagonal layer or two consecutive hexagonal layers separate the cubic blocks in the stacking repeating unit of the AO3 layers,12 respectively. The twinned and shifted structures form the AO3 layer stacking sequences of (c...ch)2 and (c...chh)m (m = 1 or 3) in the unit cell, respectively, for which the schematic plots are shown in Figure © XXXX American Chemical Society

1. Among the AnBn−1O3n B-site deficient hexagonal perovskites, the n = 4−7 members exclusively form the shifted structures (Figure 1a) while the n = 8 members have the twin-shift option, with fewer occurrences of the shifted structure compared to the twinned structure (Figure 1b).12 Recently the 8-layer B-site deficient hexagonal perovskite A8B7O24 tantalates and niobates have drawn much attention owing to their low dielectric loss (i.e., high quality factor, Q; the Qf value is usually employed as a figure of merit) and twin-shift phase competition coupled with B-cation and vacancy ordering variation. Ba8MTa6O24 (M = Zn, Ni, and Co) adopts the 8-layer twinned structure containing partially ordered cations and vacancies among the FSO dimer sites (Figure 1b), displaying high Q performance (Qf ∼ 70000− Received: November 30, 2017

A

DOI: 10.1021/acs.inorgchem.7b03023 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

hexagonal perovskite structure can be stabilized by large highspin d5 Mn2+ cations as demonstrated for the B-site deficient Ba8MnNb6O24 (BMN) composition featuring long-range ordering of octahedral high-spin Mn2+ layers in a way similar to that of the BCN and BZN cases. We report the electrical properties and electronic structures of the shifted BMN with BZN as a reference containing d-shell fully filled Zn2+. The electronic structure calculations assist us with obtaining insight into the stabilization and electrical behavior of 8-layer shifted ordered hexagonal perovskites.

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Figure 1. Schematic plots of (a) shifted and (b) twinned hexagonal perovskites with repeating units of (c...ch) and (c...chh) for the AO3 layer stacking. The green and red spheres denote A and O atoms, respectively. The fade octahedra highlight the FSO sites which accommodate the vacancies in the B-site deficient hexagonal perovskites.

EXPERIMENTAL SECTION

The BMN polycrystalline samples were synthesized using BaCO3 (99%+, Aladdin), MnCO3 (99%, Aladdin), and Nb2O5 (99.99%, Aladdin) as starting materials in a conventional high-temperature solid-state reaction process. The starting materials were weighed in 2 g batches according to the correct stoichiometries and mixed in ethanol with an agate mortar and pestle. The dried mixtures were precalcined at 1473 K for 6 h in alumina crucibles. These precalcined powders were then ground, pressed into pellets under a pressure of 330 MPa, and fired at 1573−1723 K for 6−24 h with heating and cooling rates of 5 K/min. Dense BMN pellets for microwave dielectric property measurements were prepared via the following process. Stoichiometric BaCO3, MnCO3, and Nb2O5 were weighed in 6 g batches, mixed, and then calcined at 1473 K for 6 h. The calcined powders were mixed with a 5% poly(vinyl alcohol) (PVA) solution (organic binder) and then pressed into pellets under a 330 MPa pressure, which were fired at 1723 K for 24 h with heating and cooling rates of 5 K/min. The densities of the pellets were calculated according to the geometric sizes and the masses of the pellets. All of the synthetic experiments were carried out in an air atmosphere, and the BMN products appear black. Powder X-ray diffraction (XRD) was performed on a Panalytical X′pert Pro diffractometer with Cu Kα radiation. High-quality XRD data for Rietveld refinement were measured over a 2θ range of 10− 120°. Neutron powder refinement (NPD) data with a constant wavelength (λ = 1.225 Å) were collected at room temperature on the 3T2 powder diffractometer at the Laboratoire Léon Brillouin (Saclay, France) over a 2θ range of 5−120°. Rietveld analysis was carried out using the Topas Academic software.21 Bond valence sums (BVSs) were calculated by Brown and Altermatt′s method.22 Selected area electron diffraction (SAED) patterns and high-resolution transmission electron microscopy (HRTEM) images were recorded using a JEOL JEM-2100F instrument via transmission electron microscopy (TEM) with a point resolution of 1.9 Å and operated at 200 kV. The microstructures of the pellets were examined with a Hitachi (Tokyo, Japan) S4800 instrument via scanning electron microscopy (SEM). Prior to the SEM observations, gold was sprayed on the surface to form a thin conducting layer. Energy dispersive X-ray spectroscopy (EDS) elementary analysis was carried out during the SEM experiments. The microwave dielectric properties were measured by the Hakki− Coleman dielectric resonator method23 with the TE011 mode using an Agilent N5230A network analyzer. The temperature coefficient of resonate frequency τf value was measured from 298 to 358 K. AC impedance data were collected using a Solartron 1260A impedance/ gain-phase analyzer over a temperature range from room temperature to 1073 K within 10−1−107 Hz. Prior to measurements, the pellet was coated with platinum paste and fired at 1073 K for 40 min in order to remove the organic components and to form electrodes. X-ray photoelectron spectroscopy (XPS) measurements were carried out on an ESCALAB 250Xi X-ray photoelectron spectrometer with monochromatized Al Kα radiation (hν = 1486.6 eV) at a power of 75 W under 5 × 10−10 mbar. The corrections of all these peaks are based on the C 1s peak. The peak deconvolutions were carried out using the XPSEAK software based on a combination of Gaussian− Lorentzian peak functions. Magnetization measurements were carried out using a Physical Property Measurement System PPMS-9 QUANTUM. Magnetization versus temperature curves were measured

90000 GHz, permittivity εr ∼ 27−29).2,13−15 The niobate counterparts of Ba8CoNb6O24 (BCN)16 and Ba8ZnNb6O24 (BZN)17 form the 8-layer shifted structure with completely ordered vacancies between two hexagonal BaO3 layers (the central octahedral site in the FSO trimer, Figure 1a) as well as long-range ordered Co/Zn layers separated by ∼1.9 nm in the central corner-sharing octahedral (CSO) sites within the cubic perovskite blocks. These two shifted niobates show a larger εr value (∼31−35) and lower quality factor (Qf 43400−53200 GHz)16,17 compared to those of the twinned tantalates Ba8MTa6O24. The unusual long-range B-cation ordering in BCN and BZN has been attributed to multiple factors, including the B-cationic size and charge differences and covalent bonding of Nb5+, along with the vacancy ordering that leads to single empty octahedral layers.17 Assisted by the second-order Jahn−Teller (SOJT) effect of the highly charged d0 Nb5+, the Nb5+ cations next to the empty octahedral layers are displaced toward the empty octahedral layers, forming three short bond lengths with oxygen anions in the empty octahedral layer, favorable for stabilizing the oxide anions in the empty octahedral layers. The stabilization of the twin-shift configuration for the 8-layered tantalate and niobate hexagonal perovskites described above has been associated with the subtle difference between bonding covalence and SOJT effects of Ta5+ and Nb5+.17 The more ionic nature and weaker SOJT effect of the Ta5+ cation compared to the Nb5+ cation17−19 could make the out-of-center distortion in the oxygen octahedron of Ta5+ insufficient for stabilizing oxide anions in the empty octahedral layer and, therefore, destabilize the shifted structure for the 8-layer hexagonal perovskite tantalates.17 Forming a twinned structure with a shorter periodicity for the ordering of the B-site vacancy and B-cations is energetically more favorable for the 8-layer hexagonal perovskite tantalate compositions. The occurrence of an 8-layer ordered shifted hexagonal perovskite structure is rare. More recently, in order to isolate more 8-layer shifted phases, we successfully synthesized Ba8NiNb6O24 (BNN) but found that it is isostructural with the twinned Ba8NiTa6O24 instead of the cationic ordered shifted BCN and BZN materials, which has been ascribed to the smaller size difference between Ni2+ and Nb5+.20 On the other hand, the larger size difference between Zn2+/Co2+ and Nb5+ cations is favorable for cationic ordering and thus contributes to the stabilization of shifted structures on the BCN and BZN compositions. Here we show that the 8-layer shifted B

DOI: 10.1021/acs.inorgchem.7b03023 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry between 5 and 300 K in field-cooled (FC) and zero-field-cooled (ZFC) modes with applied fields of 500 Oe. Ultraviolet and visible (UV−vis) light absorbance spectra on the dry-pressed disk sample were obtained via measurements of diffuse reflectance using a UV3600 UV−vis spectrometer fitted with BaSO4 as the standard material in the wavelength region of 200−800 nm with a resolution of 0.1 nm.

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COMPUTATIONAL METHODS

The band structure and partial density of states (PDOS) were calculated based on the density functional theory (DFT) with Hubbard U correction (DFT+U)24 using the projector-augmented wave (PAW) method25 as implemented in the Vienna Ab initio simulation package (VASP).26 We used the generalized gradient approximation (GGA) of Perdew and co-workers27 to describe the exchange and correlation interactions. The U values were tested for Mn and Nb in the range of Ueff = 0−6 eV. The optimized U values of 5.0 and 3.0 were adopted for Mn and Nb ions, respectively. Cutoff energies of 520 and 850 eV were adopted for the plane−wave basis and the augmentation charge, respectively. For the k space integration, the gamma-center smearing method and the modified tetrahedron method of Blöchl−Jepsen−Andersen28 were used for dynamical and self-consistent static calculations, respectively. For all the structures studied in this work, an equivalent optimized k-point mesh was used to ensure a convergence of 1 meV per unit cell and to guarantee the same density per unit volume of the reciprocal lattice. In each calculation, periodic boundary conditions were added in the three directions, allowing a full relaxation of the supercell. The energy convergence criteria were 0.01 eV/Å and 1 meV for ionic and electronic relaxations, respectively, while 0.01 meV was used in the self-consistent static calculations. Spin-polarized magnetic configurations were considered in all cases.

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RESULTS Crystal Structure. Figure 2 shows ex situ XRD data of Ba8MnNb6O24 (BMN) powders fired at various temperatures.

Figure 3. (a) SAED pattern projection and (b) HRTEM image along [010] for Ba8MnNb6O24. The inset in (b), enlarging the bottom left part of the pattern, shows the 8-layer shifted stacking sequence (cccccchh) for the BaO3 layers.

Figure 2. XRD patterns of Ba8MnNb6O24 samples fired at different temperatures for 6−12 h. 3C, 5H, and 8H denote the 3-layer Ba 3 MnNb 2 O9 , 5-layer Ba 5Nb 4O15 , and 8-layer BMN phases, respectively.

structure with a stacking sequence cccccchh for the close-packed BaO3 layers in BMN, similar to the cases of shifted BCN16 and BZN.17 The EDS elementary analysis of this 8H-shift phase gave an average cationic composition of Ba8Mn0.85(7)Nb6.75(3), agreeing well with the expected composition. Combined Rietveld refinement of XRD and NPD data for BMN was performed based on the structural model of ordered shifted BCN/BZN in space group P3̅m1.16,17 The refinement confirms that BMN is isostructural with BCN/BZN; Mn cations are ordered in the central CSO sites within the CSO blocks isolated by the empty octahedral layers (Figure 4). The Rietveld plots of XRD and NPD data for BMN are shown in Figure 5. The final refined structural parameters and bond lengths of BMN are listed in Tables 1 and 2, respectively. The

Firing the BMN sample at 1473 K for 6 h resulted in a mixture of 5-layer shifted hexagonal perovskite Ba5Nb4O15 and 3-layer complex perovskite Ba3MnNb2O9 phases. These two phases started to react with each other at 1573 K to form a new phase which became the major phase in the samples fired at 1673− 1723 K. The indexation gave a hexagonal cell of a ∼ 5.806 Å and c ∼ 18.93 Å for this new phase, suggesting the formation of an 8-layer hexagonal perovskite structure. Figure 3a shows a [010] SAED pattern of BMN, which confirms this unit cell and displays no reflection conditions. The HRTEM image (Figure 3b), also recorded along the [010] axis, suggests a shifted C

DOI: 10.1021/acs.inorgchem.7b03023 Inorg. Chem. XXXX, XXX, XXX−XXX

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field-cooled (ZFC) and field-cooled (FC) runs display no difference over the measured temperature regions (Figure 6a). The inverse magnetic susceptibility displays essentially linear variation with the temperature, and the Curie−Weiss law fit gave an effective moment of 5.59(1) μB per Mn and an antiferromagnetic Weiss constant of −13.6(6) K, as shown in the inset of Figure 6a. This result is close to the spin-only effective moment 5.9 μB of high-spin Mn2+ (S = 5/2) based on the isolated MnO6 octahedra without Mn−O−Mn superexchange interaction, consistent with ordered single MnO6 layers in the shifted BMN. The oxidation state of Mn in the shifted BMN is further confirmed by XPS measurements. In BMN, the main 2p3/2 peak of Mn is located at 641.8 eV (Figure 6b), close to those of Mn2+ in the MnCO3 (641.3 eV) ref 29 as measured here and MnO (640.9 eV) data from the literature,30 further confirming the +2 oxidation state for Mn in BMN. Electrical Properties. AC impedance data of the BMN pellet comprise bulk and grain boundary responses. Figure 7a shows typical complex impedance plots at 698 K, consisting of two semicircular arcs displaying one capacitance plateau ∼4 pF/cm in the high frequency range and another capacitance plateau ∼4 × 10−9 F/cm in the low frequency range, which are ascribed to the bulk and grain boundary responses, respectively. Figure 7b shows the Arrhenius plot for bulk conductivity of the BMN pellet in comparison with that of BZN. The bulk conductivity varied from 10−8 to 10−3 S/cm in the temperature region of 473−1173 K (Figure 7b), which is 3−4 orders of magnitude higher than that for BZN. The temperature dependencies of conductivities of BMN and BZN show an increase of activation energy (Ea) in the high-temperature region above 923 K to ∼1.2 eV (BMN) and 1.5 eV (BZN) from ∼0.62 eV (BMN) and 0.77 eV (BZN) in the lowtemperature region. Figure 7c shows the UV−vis light absorption spectrum for BMN in comparison with that of BZN. BMN displays an absorbance higher than that of BZN over the entire measured wavelength range, consistent with the black and white colors of BMN and BZN, respectively. As the DFT calculations below indicate that BMN and BZN have indirect and direct band gaps, respectively, (ahv)1/2 and (ahv)2 values (a and v denote the absorbance and phonon frequency, respectively) were calculated for BMN and BZN, , respectively. The estimated band gaps from the plots of (ahv)1/2 (BMN) and (ahv)2 (BZN) versus energy, hv (inset in Figure 7c), are ∼2.0 and ∼3.9 eV for BMN and BZN, respectively. The high temperature Ea values for BMN (∼1.2 eV) and BZN (∼1.5 eV) from the conductivity data are close to half of their band gaps (∼2.0 eV for BMN and ∼3.9 eV for BZN) determined by the UV−vis light absorption spectra (Figure 7c). This suggests that the high-temperature conduction is owing to intrinsic electronic conduction from excitation across the band gap. The lowtemperature electronic conduction is thus ascribed to the defect ionization. Figure 8 displays the surface morphology of the BMN ceramic, which shows flattened grains with their side surfaces exposed, leading to a column-like shape (widths ∼3−15 μm and lengths ∼10−60 μm) along the pellet surface. Such an anisotropic feature on the grain growth has been frequently observed in other 8-layer hexagonal perovskite materials,13,31−33 consistent with the anisotropic nature of the hexagonal perovskite. The microwave dielectric measurement showed that the BMN pellet (∼93% of theoretical density) possesses a high dielectric permittivity εr ∼ 38 but an extremely low Qf value ∼ 978 GHz with a positive temperature coefficient

Figure 4. [010] projection for the 8-layer shifted structure of Ba8MnNb6O24. The green, red, pink, and yellow spheres denote Ba, O, Mn, and Nb atoms, respectively.

Figure 5. Rietveld plots of (a) XRD and (b) NPD data for Ba8MnNb6O24. The reliability factors for the combined Rietveld analysis of XRD and NPD data are Rwp ∼ 5.63%, Rp ∼ 4.04%, and RB ∼ 4.43% for the XRD data and Rwp ∼ 2.76%, Rp ∼ 2.11%, and RB ∼ 1.26% for the NPD data. The insets in (a) enlarge the most misfits in the 2θ range of 25−35° and the plot within the high-2θ range of 80− 120°.

BVSs (Table 1) of Mn and Nb sites agree well with the cationic ordering. Magnetization and XPS Data. The oxidation state of Mn in Ba8MnNb6O24 was investigated by magnetism susceptibility measurements. Magnetization data of BMN measured on zeroD

DOI: 10.1021/acs.inorgchem.7b03023 Inorg. Chem. XXXX, XXX, XXX−XXX

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Table 1. Final Refined Structural Parameters for Ba8MnNb6O24a from the Combined Rietveld Analysis of XRD and NPD Data

a

atom

site

x

y

z

occupancy

Biso (Å2)

BVS

Ba1 Ba2 Ba3 Ba4 Mn1 Nb1 Nb2 Nb3 O1 O2 O3 O4

2c 2d 2d 2d 1a 2c 2d 2d 6i 6i 6i 6i

0 1/3 1/3 1/3 0 0 1/3 1/3 0.1678(2) 0.1638(1) 0.1708(2) 0.4993(2)

0 2/3 2/3 2/3 0 0 2/3 2/3 0.8322(2) 0.8362(1) 0.8292(2) 0.5007(2)

0.1877(1) 0.0616(2) 0.4543(1) 0.6814(1) 0 0.3868(1) 0.2556(1) 0.8760(1) 0.3080(1) 0.5703(1) 0.9337(1) 0.1884(1)

1 1 1 1 1 1 1 1 1 1 1 1

0.46(4) 0.61(3) 0.84(4) 0.30(5) 1.15(5) 0.52(2) 0.31(2) 0.24(2) 0.58(2) 0.89(2) 0.75(1) 0.78(2)

2.43 2.20 2.04 2.52 2.41 4.85 4.60 4.49 1.93 2.14 2.02 1.94

a = 5.80574(5) Å, c = 18.9349(2) Å, V = 552.73(1) Å3, and space group is P3̅m1, z = 1.

Table 2. Bond Lengths and the Octahedral Distortion Parameters (Δd) for Ba8MnNb6O24 bond

length (Å)

bond

length (Å)

Ba1−O1(×3) Ba1−O3(×3) Ba1−O4(×6) Ba2−O3(×6) Ba2−O3(×3) Ba2−O4(×3) Ba3−O1(×3) Ba3−O2(×3) Ba3−O2(×6) Ba4−O1(×6) Ba4−O2(×3) Ba4−O4(×3)

2.834(2) 2.870(2) 2.9029(1) 2.9045(1) 2.922(3) 2.923(3) 3.233(3) 2.779(2) 2.9402(5) 2.9099(1) 2.708(2) 2.986(2)

Mn1−O3(×6) Nb1−O1(×3) Nb1−O2(×3) ΔdNb1 (10−3)a

2.127(2) 2.253(2) 1.837(2) 10.3

Nb2−O1(×3) Nb2−O4(×3) ΔdNb2 (10−3)a

1.937(2) 2.099(2) 1.6

Nb3−O3(×3) Nb3−O4(×3) ΔdNb3 (10−3)a

1.967(2) 2.078(2) 0.75

⎤2 ⎦ , where ⟨d⟩ is the average B−O bond length and dn are the individual B−O bond lengths. a

Δd =

1 6

⎡d − ∑n = 1 − 6 ⎣ n d

d

of resonant frequency τf ∼ 20 ppm/K. Therefore, the shifted BMN compound represents a high dielectric-loss material compared with BZN (εr ∼ 35, Qf value ∼ 43400 GHz, and τf ∼ 38 ppm/K).17 Electronic Structure. In order to understand how the nanoscale ordering within the octahedral high-spin Mn2+ layers affects the electrical properties and UV−vis light absorption of BMN, the electronic structure of BMN was investigated by DFT calculations. In the BMN structure, the electronic structure of the relaxed system within spin-polarized GGA shows a weak metallic behavior. The calculated band gap (0.94 eV) is much lower than the experimental values (∼2.0−2.4 eV) obtained from the UV−vis light absorption and conductivity data. This indicates the inadequacy of GGA on investigation of the electronic structure of BMN. So, the band structure and density of states (DOS) were calculated by the GGA+U method. To include the on-site Coulomb interaction of localized d states, we considered two cases: the first case includes only Mn 3d orbits, and the second case includes both Mn 3d and Nb 4d states. For the former type, the range of Ueff in our calculation was 0−6 eV and the interval was 1 eV. The results of the calculation show that the Ueff of 5 eV gave the best band gap for Mn 3d orbits compared to the experimental value. This Ueff value is consistent with those previously employed in some other Mn-containing oxides.34,35 On the basis of the calculation results of the former type, we then

Figure 6. (a) Temperature-dependent ZFC and FC magnetic susceptibilities for Ba8MnNb6O24. Inset shows inverse magnetic susceptibility and Curie−Weiss fit (solid line in red). (b) Mn 2p XPS spectra of Ba8MnNb6O24 with MnCO3 as a reference. The black crosses and the pink solid line denote the experimental and calculated spectra, respectively. The solid lines in color (red, green, and blue) denote the deconvolution of spectra. The arrows mark the Mn2+ 2p3/2 satellite peaks.

considered the latter type and calculated Uxy, where x and y denote values of Ueff in the Mn 3d and Nb 4d orbits, respectively. Here, for simplicity, we show only the result of U53 (Figure 9a), for which the calculated band gap (2.48 eV) is in good agreement with the experimental values. The valence electron distribution and density of states in BMN show that the valence bands are mainly made up of O 2p and few Mn 3d while the conduction bands are mainly made up of a Nb 4d E

DOI: 10.1021/acs.inorgchem.7b03023 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 8. Typical SEM image of the surface morphology of the Ba8MnNb6O24 ceramic.

experimental value (∼3.9 eV). The valence electrons in the BZN distribution are mainly confined at O and Nb atoms (Figure 9e). The DOS plot (Figure 9f) indicates that the valence and conduction bands in BZN are composed mainly of O 2p and Nb 4d orbits, respectively. The VBM is composed of O 2p orbits, which makes BZN a charge-transfer insulator. There are almost no Zn orbits on the VBM (Zn loses all 4s electrons in BZN, while the Zn 3d electrons lie in the inner shell), leading to a weak adsorption in the visible light range (Figure 7c). The position of valence and conduction band edges indicates that BZN has a direct band gap at the gamma point (Figure 9d).

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DISCUSSION The Ba8MnNb6O24 phase presented here is the third case of the long-range cationic ordered 8H shifted structure, in addition to the two previously reported BCN and BZN compounds.16,17 As shown by the magnetism susceptibility and XPS measurements, Mn2+ in BMN is in the high-spin state, with an ionic radius (0.83 Å) larger than those of Zn2+ (0.74 Å) and Co2+ (0.745 Å in high-spin).36 Therefore, compared with the B-cationic size differences in BZN and BCN, the size contrast between Mn2+ and Nb5+ (0.64 Å) is more favorable to drive the long-range cation ordering. Similar to the BCN and BZN cases,16,17 in BMN the Mn2+ cations are in centrosymmetric octahedral environments and Nb5+ cations show out-of-center displacement along the c axis (Figure 10). The displacements for the three crystallographically distinct Nb sites have different directions and magnitudes (Figure 10 and Table 2): Nb1 atoms in the perovskite edge next to the empty octahedral layers have the largest displacement; the displacements of Nb3 atoms next to the central Mn2+ layer and Nb2 atoms between the Nb1 and Nb3 layers are much smaller than that for the Nb1 atom. Both Nb1 and Nb2 atoms are displaced toward the empty octahedral layers while Nb3 atoms are moving toward Mn 2+ layers (Figure 10), the same as that in 3C Ba3MnNb2O9.37 Such cooperative out-of-center displacements of highly charged Nb5+ are necessary for satisfying the complex oxygen−anion coordination environments arising from the long-range ordered distribution of Mn2+ and B-site vacancy. Therefore, the combination of cationic ordering and cooperatively out-of-center distortion of Nb5+ plays a key role in the stabilization of 8H shifted BMN. This is unlike the case of complex perovskites, where the cationic ordering is thermally activated from the disordered phase.38

Figure 7. (a) Complex impedance plot of a BMN pellet at 698 K. Rb and Rgb denote bulk and grain boundary resistivities, respectively. The numbers denote the logarithms of selected frequencies marked by filled symbols. (b) Arrhenius plot of bulk conductivity and (c) UV−vis light absorption spectrum for BMN in comparison with that of BZN. The inset in (c) shows the plots of (ahv)1/2 (BMN) and (ahv)2 (BZN) versus energy hv.

orbit (Figure 9 b,c). The conduction band maximum (VBM) is composed of Mn 3d orbits (Figure 9a), which makes BMN a typical Mott insulator, and the positions of the VBM and conduction band minimum (CBM) indicate that BMN has an indirect band gap (Figure 9a). BMN displays a broad absorption within the visible light range (Figure 7c), which indicates that electron hopping from Mn 3d to the empty Nb 4d in the valence band of BMN takes place. The electronic structure of the shifted BZN analogue with dshell-full Zn2+ was also calculated as a reference for BMN. The calculated band gap for BZN is 2.92 eV (Figure 9d), consistent with the insulator behavior of BZN although lower than the F

DOI: 10.1021/acs.inorgchem.7b03023 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 9. Band structures, charge distribution of resolved valence electron orbital (the shadow area in the band structure), and DOS of (a−c) BMN and (d−f) BZN. The atomic spin-polarized DOS of Mn 3d (BMN), Zn 3d (BZN), O 2p, and Nb 4d states are shown in (c) and (f). For the DOS of the Mn 3d state (c), five d electrons take the spin-up state and the spin-down state is empty.

eV/layer) from the out-of-center distortion is more pronounced than that (∼0.39 eV/layer) for the 3C Ba3MnNb2O9 phase. The Nb1 atoms in 8H BMN with larger off-center distortions have d bands much narrower than those in 3C Ba3MnNb2O9. For example, the t2g* of BMN is more localized with a bandwidth of ∼1 eV (Figure 11b), while the counterpart of Ba3MnNb2O9 has a bandwidth close to 2 eV (Figure 11a); besides, the center of t2g* of 8H BMN shifts to a higher energy compared with that of 3C Ba3MnNb2O9, opening further the Nb 4d band gap by ∼1 to 3.06 eV for 8H BMN (Figure 11b). The energy separation between t2g* and eg*, i.e., the splitting energy of Nb 4d orbits, which is understood by the octahedral ligand field effect and modified by the off-center distortion, for the Nb1 atoms in 8H BMN is 1.88 eV, slightly larger than that (1.83 eV) of the 3C phase. The split of d orbits could remove the degeneracy of d bands of Nb5+ and reduce the van Hove

The contribution of the out-of-center distortion of NbO6 to the stabilization of the 8H shifted structure is further understood by energy and electronic structure calculations of Ba8MnNb6O24 and 3C Ba3MnNb2O9 as a reference, which contains one crystallographic Nb site showing off-center distortion comparable to the Nb2 site next to the most offcentered Nb1 site in 8H BMN (Figure 10). Figure 11 illustrates the orbital resolved DOS on the Nb1 4d in 8H shifted Ba8MnNb6O24 in comparison with 3C Ba3MnNb2O9. For the 3C Ba3MnNb2O9 phase without out-of-center distortion, the calculated total energy and band gap are −111.49 eV/cell and 0.90 eV, respectively, while the out-of-center distortion lowers the total energy of 3C Ba3MnNb2O9 to −112.67 eV/cell and enlarges the Nb 4d band gap to 2.46 eV (Figure 11a), indicating that the out-of-center distortion can substantially stabilize the 2:1 ordered 3C structure. This is also applicable to the 8H BMN phase, in which the energy stability gain (∼0.88 G

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shifted AnBn−1O3n-type niobates and tantalates of n = 4−5 members free of additional SOJT-active cations (Table 3) for a better understanding of the twin preference of tantalates in the 8-layer structures. The B-site vacancy ordering in the shifted structure avoids FSO B−B repulsion, which benefits the formation of the shifted structure on the n = 4−7 members regardless of their compositions.12 However, in the n = 8 members, the long-range ordering of B-site vacancies could cost more energy and therefore reduce the phase stability of the shifted phase, which competes with the twinned phase featuring the FSO B−B repulsion, but with an energy-favorable shorter periodicity of vacancy ordering.12,17 The off-center distortion is commonly observed in shifted structures, emphasizing the wide applicability of its role in the stability of various shifted phases.17 In general, there are electronic and structural components affecting the distortion in the bond network. Nb5+ and Ta5+ cations have a close chemical bonding nature; both d0 5-period Nb5+ and 6-period Ta5+ cations have the same sizes36 owing to the lanthanide contraction, excluding the cationic size or tolerance factor effect on the shift-twin option of the 8-layer niobates and their Ta counterparts, and may form covalent bonds with oxygen anions through overlapping of lowlying empty d orbits with the filled O 2p orbits. The SOJT effect of these two d0 cations may remove degeneracy of the d orbits and also result in the distortion to the octahedra showing multiple bond lengths.19,39 It is well-known that the statistics on the structures and physical properties of tantalates and niobates, as well as the theoretical calculations, indicate that Ta5+ is generally more ionic and shows less SOJT distortion than Nb5+.19,40,41 In the n = 4−5 shifted niobate and tantalate compounds listed in Table 3, the Nb5+ and Ta5+ cations close to empty octahedral layers display essentially the same extents,16,17,42,43 close to those in long-range ordered 8H shifted niobates, irrespective of the more ionic bonding nature for Ta5+ than Nb5+. This suggests that connectivity of the bond network could structurally drive the off-center distortion for stabilizing the oxygen anions in the empty octahedral layers in these n = 4−5 members.19 However, unlike both niobate and tantalate compositions for the n = 4−7 members that form shifted phases, the 8-layer tantalate compositions display a strong twin preference, which may be understood based on the difference between the bonding covalence and the SOJT effect of d0 Ta5+ and Nb5+.17 The frequent formation of twinned structures on the 8-layer B-site deficient tantalate compositions indicates that, compared with the more covalent Nb5+ host, the Ta5+ host could not be ideal for the long-range B-site vacancy ordering in the 8-layer shifted structure. The structural imposition that works well for the off-center distortion in the shorter-periodicity shifted phases could be taken over by the electronic SOJT effect of d0 cations to drive the off-center distortion in the 8-layer B-site deficient shifted structures. The longer-range ordered structure in the 8-layer shifted phases requires further assistance from the SOJT effect for lowering the total energies, on which the more ionic Ta5+ host with a reduced degree of overlap of 5d with O 2p orbits and therefore decreased SOJT distortion, could not be favorable compared with Nb5+. Therefore, the 8-layer shifted BMN phase presented here emphasizes the key roles of coupling of bonding covalence and SOJT effect of Nb5+, as well as the cationic size/charge difference on the stabilization of long-range ordered 8-layer shifted structures. The 8-layer shifted BMN ceramics present much higher conductivity and a lower quality factor compared with BZN.

Figure 10. Schematic plot of the cooperative out-of-center distortion of NbO6 octahedra in the shifted Ba8MnNb6O24 with the bond lengths (Å) of Nb−O and Mn−O labeled. The black arrows denote the outof-center distortion directions of Nb5+, and their lengths roughly mark the relative distortion parameters (Δd) listed in Table 2.

Figure 11. Orbital resolved DOS of the Nb 4d in (a) 3C Ba3MnNb2O9 and (b) 8-layer shifted Ba8MnNb6O24 on the Nb1 site.

singularity, thus lowering the total energy and therefore stabilizing the structures. Similar to the 8H shift-twin phase option of BCN/BZN and their tantalate counterparts, the tantalate counterpart (Ba8MnTa6O24) of BMN also adopts a twinned structure but with 14 layers (Ba14Mn1.75Ta10.5O24)11 instead of 8 layers. Here the 8-layer B-site deficient hexagonal perovskite Ba8M(Nb/ Ta)6O24 pairs reported so far are compared with some close H

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Table 3. Typical AnBn−1O3n-Type Shifted and Twinned Hexagonal Perovskite Niobates and Tantalates Free of Additional SOJTActive Cations

a

composition

structure

orderinga

composition

structure

orderinga

Ba8MnNb6O24 Ba8ZnNb6O2417 Ba8CoNb6O2416 Ba8NiNb6O2420 Ba5Nb4O1543 Ba3LaNb3O1245

shifted shifted shifted twinned shifted shifted

CO CO CO PO NA NA

Ba8MnTa6O2411 Ba8ZnTa6O242 Ba8CoTa6O2414 Ba8NiTa6O2444 Ba5Ta4O1542 Ba3LaTa3O1246

twinned twinned twinned twinned shifted shifted

PO PO PO PO NA NA

CO and PO denote completely and partially ordered B-site vacancies and B-cations, respectively. NA denotes not applicable.

ordering and off-center distortion associated with the bonding covalence and SOJT effect of Nb5+ are important factors for the stabilization of 8-layer shifted hexagonal perovskite. The Ba8MnNb6O24 pellet exhibits a dielectric loss (Qf ∼ 987 at 5.31 GHz) and conductivity (∼10−8−10−3 S/cm within 473− 1173 K) that are much higher than those of the Ba8ZnNb6O24 analogue. Electronic structure calculations reveal Mott and charge-transfer insulator natures for Ba8MnNb6O24 and Ba8ZnNb6O24, respectively, that depend on the 3d electron configurations of the high-spin d5 Mn2+ and fully filled d10 Zn2+ and confirm that the out-of-center distortion of NbO 6 contributes to stabilization of the 8-layer shifted ordered structure. The hopping of 3d valence electrons in high-spin Mn2+ to Nb5+ 4d conduction bands over a small gap (∼2.0 eV) makes Ba8MnNb6O24 more conductive than Ba8ZnNb6O24, where the electrons are conducted via hopping of an O 2p valence electron to Nb5+ 4d conduction bands over a larger gap (∼3.9 eV). The high dielectric loss of Ba8MnNb6O24 is mainly ascribed to the half-filled Mn 3d orbits, which could soften infrared modes, therefore increasing the lattice vibration inharmonicity, and could introduce additional loss from the resonant spin excitation of unpaired d electrons.

Such a phenomenon is in accordance with the observation that the high-spin Mn2+-containing compounds, such as columbite MnNb2O647 and complex perovskite Ba3MnNb2O9,48 possess higher conductivity and microwave dielectric loss compared with the Zn2+ analogues, which may be explained using their electronic structures depending on the 3d electron configurations of the high-spin d5 Mn2+ and fully filled d10 Zn2+. As for the electronic conduction over the extrinsic band gap, in the Mott insulator BMN (similar to the 3C ordered Ba3MnNb2O949), the electrons mainly hop from VBM Mn 3d orbits to CBM Nb 4d orbits (Figure 9 a−c) over a ∼2.0 eV gap. This is much smaller than that (∼3.9 eV) for the chargetransfer insulator BZN, where the electrons mainly hop from the VBM O 2p orbits to CBM Nb 4d orbits (Figure 9 d−f). Therefore, the ease of the electron conduction path and the five unpaired 3d electrons of high-spin Mn2+ cation induce a much higher conductivity across the intrinsic band gap in the Mott insulator BMN compared with the charge-transfer insulator BZN material. The coincidence of much higher conductivities and larger microwave dielectric loss of BMN compared with those of BZN indicated that similar factors take effect on both conductivity and microwave dielectric loss of BMN. Although the dielectric loss is generally highly sensitive to the processing which significantly impacts the extrinsic loss associated with the porosity, defects, cationic order, and secondary phase formation,31,50 the large microwave dielectric loss of BMN could be mainly related to the existence of the high-spin d5 Mn2+ cations in BMN. The half-filled Mn 3d orbits in BMN could soften infrared modes, which increases the lattice vibration anharmonicity and therefore the intrinsic dielectric loss.49 On the other hand, resonant spin excitation of unpaired transition-metal d electrons has been recently recognized as a noticeable origin of the microwave dielectric loss of transitionalmetal-doped complex perovskite Ba3ZnTa2O9 resonators at cryogenic temperatures.51 This mechanism could be also applicable here to BMN, which contains large amounts of unpaired d electrons in the high-spin Mn2+. The contrast between the low-temperature conductivities from the defect ionization for BMN and BZN indicates that the concentration of defects (e.g., point defects as impurity atoms and anionic and cationic vacancies) in BMN is larger than that in BZN, which could also contribute to the higher microwave dielectric loss of BMN.

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ASSOCIATED CONTENT

Accession Codes

CCDC 1588215 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Xiaoming Wang: 0000-0002-1527-3496 Mathieu Allix: 0000-0001-9317-1316 Xiaojun Kuang: 0000-0003-2975-9355 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The National Natural Science Foundation of China (21361008, 21511130134), NSFC−CNRS bilateral TransLight project (PICS07091), Guangxi Program for Hundred Talents for Returned Scholars, and Guangxi Key Laboratory for Advanced Materials and New Preparation Technology (12AA-11) are acknowledged for their financial support. The authors thank

CONCLUSIONS Large high-spin d 5 Mn 2+ is shown to stabilize the Ba8MnNb6O24 8-layer shifted hexagonal perovskite, featuring unusual long-range ordering of high-spin d5 Mn2+ layers on the nanometer scale in the purely CSO d0 Nb5+-oxygen octahedral host that displays out-of-center distortion. Both the B-cationic I

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Prof. Xiping Jing (Peking University) and Prof. Tao Yang (Chongqing University) for their valuable discussions on the electronic structure calculations and the band gap calculations from UV−vis light absorption data, Prof. Jing Wang for access to the physical property measurement system at the Sun YatSen University, and Prof. Liang Fang for access to the microwave dielectric measurement system at the Guilin University of Technology.

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K

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