Sr Vacancies Research Articles

Microphases deriving from Ba2ZrF8 structure type: III: Structural approach of dis-Sr17Zr9F70, Sr23Zr12F94, Pb29Hf15F118. General model of formation of M′2p-1MpF8p-2 microphases

The domain Sr1-xZrxF2+2x studied at 670 °C between the limits 0.340 ≤ x ≤ 0.353 show the presence of a new phase Sr17Zr9F70 and of modulated phases, all deriving from Ba2ZrF8 structure type. In the same domain studied below 600 °C, Sr17Zr9F70 tends to lose its long range ordering between Sr cations and Sr vacancies, tripling the b lattice parameter; moreover a new monoclinic superstructure Sr23Zr12F94 [a = 3sinβ×a (Ba2ZrF8); b = b (Ba2ZrF8); c = c (Ba2ZrF8), β = 105.31°] is also characterized. The homologous Hf phases are isostructural.With Pb, between 555 and 530 °C, another superstructure Pb29Zr15F118 [a = 5 × a (Ba2ZrF8); b = b (Ba2ZrF8); c = c (Ba2ZrF8] and its isotypic Pb29Hf15F118 are evidenced and their structure is solved in great part with a similar cationic disorder between Pb cations and Pb vacancies. Between 530 and 510 °C, a second much more complex superstructure exists but has not been characterized.Moreover, at 630 °C, single crystals of Pb2HfF8 are obtained which allows to confirm that, contrary to Ba2ZrF8, Pb2HfF8 is non-centrosymmetric (Pn21a space group). but the atomic positions deviate only slightly from centrosymmetry.A general structural model for all these M′2p-1MpF8p-2 (p = 6, 9, 12, 15) microphases is developed.

Searching for new two-dimensional spintronic materials: Doping-induced magnetism in graphene-like SrS monolayer

In this work, doping approach is explored to induce feature-rich electronic and magnetic properties in graphene-like SrS monolayer and make it prospective spintronic two-dimensional (2D) candidate. For such goal, 3d transition metals (V, Cr, Mn, and Fe) and halogens (F, Cl, Br, and I) are selected as dopants at Sr and S sublattice, respectively. Pristine SrS single layer shows good dynamical and thermal stability. Its indirect-gap semiconductor nature is also asserted with energy gap of 2.87/3.81 eV obtained by PBE/HSE06 functional, generated by the separation in energy between S-3p and Sr-4d orbitals. The magnetic semiconducting with total magnetic moment of 2.00 μB is obtained by creating a single Sr vacancy, meanwhile S single vacancy preserves the non-magnetic nature. The monolayer is significantly magnetized by doping with transition metals, where large total magnetic moments of 3.00, 4.00, and 5.00 μB are obtained for the V-, Cr/Fe-, and Mn-doped SrS monolayer, respectively. In these cases, impurities play a key role on producing magnetic properties and generating the magnetic semiconductor nature. This feature-rich nature is also induced by doping with F atom, where a total magnetic moment of 1.00 μB is obtained that is originated mainly from Sr atoms closest to the doping site. Besides, Cl doping leads to the emergence of the half-metallicity. Importantly, the magnetization becomes significantly weaker according to increase the atomic number of halogen dopants, such that the non-magnetic nature is preserved by doping with I atom. This feature is attributed to the increase of the electronic hybridization. Results presented herein introduce the doped SrS monolayer as promising 2D spintronic materials, exhibiting novel properties that are not found in the pristine counterpart.

Dielectric Spectroscopy of Non-Stoichiometric SrMnO3 Thin Films

The dielectric properties of non-stoichiometric SrMnO3 (SMO) thin films grown by molecular beam epitaxy were systematically investigated. Especially, the effects of cation stoichiometry-induced diverse types and densities of defects on the dielectric properties of SMO films were revealed. Two anomalous dielectric relaxation behaviors were observed at different temperatures in both Sr-rich and Mn-rich samples. High-temperature dielectric relaxation, resulting from a short-range Mn-related Jahn–Teller (JT) polaron hopping motion, was reinforced by an enhancement of JT polaron density in the Sr-rich film, which contained abundant SrO Ruddlesden–Popper (R-P) stacking faults. However, an excessive number of disordered Sr vacancy clusters in Mn-rich thin film suppressed the hopping path of JT polarons and enormously weakened this dielectric relaxation. Thus, The Sr-rich film demonstrated a higher dielectric constant and dielectric loss than the Mn-rich film. In addition, low-temperature dielectric relaxation may be attributed to the polarization/charge glass state.

Optimized thermoelectric performance driven by A‐site deficient in SrxLa0.1TiO3−δ/TiO2−y composites

AbstractFor thermoelectric applications at high temperatures, perovskite SrTiO3 is a promising contender as an n‐type oxide thermoelectric (TE) material. In this work, SrxLa0.1TiO3−δ/TiO2−y composite ceramics were successfully synthesized by solid state reaction, using a combination of A‐site cation vacancy and composite fabrication. Through X‐ray diffractometer and scanning electron microscopy analysis, a complex microstructure composed of two kinds of titanium oxides was formed after reductive sintering and annealing. In addition, controlling the content of Sr vacancies in the SrxLa0.1TiO3−δ matrix increased the thermoelectric power factor (PF) by optimizing the weighted mobility in the composite, resulting in the highest PF of 425.8 μW m−1 K−2 and the maximum ZT of 0.16 for the Sr0.875La0.1TiO3−δ/TiO2−y composite through the synergistic optimization of electrical and thermal transport properties. The strategy proposed in this study of design A‐site‐deficient composites with nano‐sized metal paved a new way to fabricate high performance oxide thermoelectric materials.

High negative voltage activating perovskite oxide with bi-vacancy synergistic regulation for water oxidation

Oxygen vacancy engineering is a strategy to design efficient oxygen evolution reaction (OER) catalysts, but it may lower the band center of O 2p and result in high energy barrier. Here, La0.6Sr0.4Co0.9Fe0.1O3‑δ (LSCF) perovskite catalyst with O and Sr vacancies were prepared by high reduction voltage treatment. The bi-vacancy facilitates the synergistic regulation by providing the inverse movement the d-band center and p-band center to enhance M−O covalency, thus activating lattice oxygen of LSCF. The synergistic effect of Sr and O vacancy reduces the OH adsorption energy on the surface of LSCF, which makes OH adsorption and desorption easier. By optimizing the reduction voltage and time, the overpotential (at 10 mA cm−2) of the high reduction voltage treated LSCF (LSCF- −2V-80 s) is reduced by 130 mV compared to LSCF. The LSCF- −2V-80 s still showed clear lattice fringes after stability test. Moreover, the stronger covalency of LSCF- −2V-80 s to produce O22– intermediates is inclined to be lattice oxygen mediated mechanism (LOM). The bi-vacancy strategy made LSCF- −2V-80 s produce a current density of 174 A/g at 2.50 Vcell in anion exchange membrane water electrolyzer. Bi-vacancy synergistic strategy derived from one-step high reduction voltage provides a new route for OER catalysts.

Multi-scale collaborative optimization of SrTiO3-based energy storage ceramics with high performance and excellent stability

SrTiO3 (ST)-based ceramics are considered as promising candidates for energy storage applications. However, the low polarization intensity in ST-based materials limits their energy storage performance, rendering materials that usually exhibit a low recoverable energy-storage density. In the present study, we have optimized the energy storage performance of ST-based ceramics by using a combined optimization strategy of structural engineering and microstructural regulation. High performance (Sr1-x-y-2φNayBixCaφ□φ)TiO3 (abbreviated as zSNBCT, where □ represents the Sr vacancies) ceramics were thereby designed. During composition optimization, the phase state of zSNBCT was adjusted to a critical point where the relaxor ferroelectric-paraelectric phase transition occurred around room temperature. It thus induced a strong relaxation behavior with the formation of ferroelectric polar nano-regions, yielding a high recoverable energy-storage density (Wrec) of ∼6 J/cm3 and a high energy-storage efficiency (η) of ∼92% under a large breakdown electric field of 440 kV/cm, for z = 0.2 sample. Moreover, the breakdown strength (BDS) of the 0.2SNBCT ceramic was further improved by adopting a two-step sintering and spark plasma sintering approach for the microstructural refinement. Simulation models containing grains and grain boundaries were well established using phase-field simulation and finite element analysis. These simulations came to a similar conclusion for the enhanced BDS. Namely, the fine-grained microstructure significantly hindered the growth of breakdown cracks under an applied electric field. Most importantly, the 0.2SNBCT sample showed excellent frequency stability (1−1000 Hz), thermal stability (20−140 °C), and cycling stability (105 cycles), rendering it a promising candidate for energy storage systems. Our designed strategy of structural engineering and microstructural regulation may provide a new paradigm for the design of high-performance energy storage ceramics for pulse power applications.

Automated real-space lattice extraction for atomic force microscopy images

Analyzing atomically resolved images is a time-consuming process requiring solid experience and substantial human intervention. In addition, the acquired images contain a large amount of information such as crystal structure, presence and distribution of defects, and formation of domains, which need to be resolved to understand a material’s surface structure. Therefore, machine learning techniques have been applied in scanning probe and electron microscopies during the last years, aiming for automatized and efficient image analysis. This work introduces a free and open source tool (AiSurf: Automated Identification of Surface Images) developed to inspect atomically resolved images via scale-invariant feature transform and clustering algorithms. AiSurf extracts primitive lattice vectors, unit cells, and structural distortions from the original image, with no pre-assumption on the lattice and minimal user intervention. The method is applied to various atomically resolved non-contact atomic force microscopy images of selected surfaces with different levels of complexity: anatase TiO2(101), oxygen deficient rutile TiO2(110) with and without CO adsorbates, SrTiO3(001) with Sr vacancies and graphene with C vacancies. The code delivers excellent results and is tested against atom misclassification and artifacts, thereby facilitating the interpretation of scanning probe microscopy images.

Electrochemically Induced Crystalline‐to‐Amorphous Transition of Dinuclear Polyoxovanadate for High‐Rate Lithium‐Ion Batteries

AbstractPolyoxometalates are intriguing high‐capacity anode materials for alkali‐metal‐ion storage due to their multi‐electron redox capabilities and flexible structure. However, their poor electrical conductivity and high working voltage severely restrict their practical application. Herein, the dinuclear polyoxovanadate Sr2V2O7·H2O with unusually high electrical conductivity is reported as a promising anode material for lithium‐ion batteries. During the initial lithiation process, the Sr2V2O7·H2O anode experiences an electrochemically induced crystalline‐to‐amorphous transition. The resulting amorphous structure provides high redox activity and fast reaction kinetics via reversible V4.9+/V2.8+ redox couple through the intercalation mechanism. Furthermore, when coupled with the LiFePO4 cathode, the strong VO bonds of the amorphous anode provide excellent structural stability, with the full‐cell capable of performing >12 000 cycles with a capacity retention of 72%. Another advantage of Sr2xV2O7‐δ·yH2O (0.5 ≤ x ≤ 1.0) is its composition adjustability, which enables delicately regulating the Sr vacancy content without destroying the structure. The defect Sr2xV2O7‐δ·yH2O (x = 0.5) electrodes show significantly improved specific capacity and rate capability without sacrificing other key properties, delivering a high specific capacity of 479 mAh g‐1 at 0.1 mA cm‐2 and 41.9% of its capacity in 2 min. Overall, the preliminary study points the way forward for the facile preparation of high‐quality polyoxometalates for advanced energy storage applications and beyond.

Achievement of full-visible-spectrum lighting in Bi3+-activated strontium gallates via lattice site occupancy engineering toward WLEDs applications

Achieving full-visible-spectrum lighting in a single component is highly desired but still challenging for high-quality phosphor-converted white light emitting diodes (pc-WLEDs). Herein, a broadband emission of Sr3Ga4O9: Bi3+, Yb3+ phosphor covering full-visible-spectrum from 400 to 800 nm is explored via the lattice site occupancy engineering strategy. The Bi3+ singly doped Sr3Ga4O9 phosphor exhibits warm yellow emission for the 3P1→1S0 transition of Bi3+ at Sr1 and Sr3 sites. While the introduction of Yb3+ contributes to the emerge of an extra blue emission band at 445 nm accompanied with a significant enhancement of the integral photoluminescence intensity. It is demonstrated that the chemical substitution of Yb3+ into Sr sites induces the lattice disorder and the increased concentration of Sr vacancies, and the local chemical environment change dominates the occupation of Bi3+ into Sr2 sites, resulting in the remarkable blue emission. Moreover, the WLED device as-fabricated by combining the broadband cyan-emitting Sr3Ga4O9: Bi3+, Yb3+ and the red-emitting CaAlSiN3: Eu2+ phosphors exhibits a high color-rendering index (CRI) of 89.6 and a correlated color temperature (CCT) of 5382 K. This work provides a feasible strategy for regulating the Bi3+occupation in Sr3Ga4O9 matrix and boosts the photoluminescence performance of the corresponding phosphor with broadband emission covering the full visible spectrum.

Boosting the lithium-ion storage performance of perovskite SrxVO3−δ via Sr cation and O anion deficient engineering

Perovskite SrVO3 has been investigated as a promising lithium storage anode where the V cation plays the role of the redox center, combining excellent cycle stability and safe operating potential versus Li metal plating, with limited capacity. Here, we demonstrate the possibility to boost the lithium storage properties, by reducing the non-redox active Sr cation content and fine-tuning the O anion vacancies while maintaining a non-stoichiometric SrxVO3−δ perovskite structure. Theoretical investigations suggest that Sr vacancy can work as favorable Li+ storage sites and preferential transport channels for guest Li+ ions, contributing to the increased specific capacity and rate performance. In contrast, inducing O anion vacancy in SrxVO3−δ can improve rate performance while compromising the specific capacity. Our experimental results confirm the enhancement of specific capacities by fine adjusting the Sr and O vacancies, with a maximum capacity of 444 mAh g−1 achieved with Sr0.63VO3−δ, which is a 37% increase versus stoichiometric SrVO3. Although rich defects have been induced, SrxVO3−δ electrodes maintain a stable perovskite structure during cycling versus a LiFePO4 cathode, and the full-cell could achieve more than 6000 discharge/charge cycles with 80% capacity retention. This result highlights the possibility to use the cation defective-based engineering approach to design high-capacity perovskite oxide anode materials.

Synthesis and Dielectric Characterization of Nd Doped SrTiO<sub>3</sub> Ceramics for Energy Storage Applications

A detailed study on the structural and dielectric characterization of the Nd doped SrTiO3 ceramics at radio frequencies were conducted in this paper. Sr1-xNdxTiO3 (x from 0 to 0.13) ceramics were synthesized using the conventional solid state ceramic route and the phase purity was confirmed through XRD analysis. XRD patterns of the ceramics showed almost similar peaks with no additional phases. A reduction in the lattice parameter can be observed with increase in the Nd content, which can be attributed to the possible lattice shrinkage due to the substitution by a smaller sized ion. The SEM images also showed a reduction in grain size which can be due the fact that Nd doping induces Sr and Ti vacancies in the system which inhibits the grain growth. The dielectric characterization at 1 MHz was done and optimal values of εr=5496 and tanδ=0.0925 were observed at x=0.1. Thus the synthesized ceramics can be used for energy storage applications.

Role of Sr doping and external strain on relieving bottleneck of oxygen diffusion in La2−xSrxCuO4−δ

In many complex oxides, the oxygen vacancy formation is a promising route to modify the material properties such as a superconductivity and an oxygen diffusivity. Cation substitutions and external strain have been utilized to control the concentration and diffusion of oxygen vacancies, but the mechanisms behind the controls are not fully understood. Using first-principles calculations, we find how Sr doping and external strain greatly enhances the diffusivity of oxygen vacancies in La2−xSrxCuO4−δ (LSCO) in the atomic level. In hole-doped case (2x > δ), the formation energy of an apical vacancy in the LaO layer is larger than its equatorial counterpart by 0.2 eV that the bottleneck of diffusion process is for oxygen vacancies to escape equatorial sites. Such an energy difference can be reduced and even reversed by either small strain (< 1.5%) or short-range attraction between Sr and oxygen vacancy, and in turn, the oxygen diffusivity is greatly enhanced. For fully compensated hole case (2x ≦ δ), the formation energy of an apical vacancy becomes too high that most oxygen vacancies cannot move but would be trapped at equatorial sites. From our electronic structure analysis, we found that the contrasting change in the formation energy by Sr doping and external strain is originated from the different localization natures of electron carrier from both types of oxygen vacancies.

Unravelling the Origin of Ultra‐Low Conductivity in SrTiO3 Thin Films: Sr Vacancies and Ti on A‐Sites Cause Fermi Level Pinning

AbstractDifferent SrTiO3 thin films are investigated to unravel the nature of ultra‐low conductivities recently found in SrTiO3 films prepared by pulsed laser deposition. Impedance spectroscopy reveals electronically pseudo‐intrinsic conductivities for a broad range of different dopants (Fe, Al, Ni) and partly high dopant concentrations up to several percent. Using inductively‐coupled plasma optical emission spectroscopy and reciprocal space mapping, a severe Sr deficiency is found and positron annihilation lifetime spectroscopy revealed Sr vacancies as predominant point defects. From synchrotron‐based X‐ray standing wave and X‐ray absorption spectroscopy measurements, a change in site occupation is deduced for Fe‐doped SrTiO3 films, accompanied by a change in the dopant type. Based on these experiments, a model is deduced, which explains the almost ubiquitous pseudo‐intrinsic conductivity of these films. Sr deficiency is suggested as key driver by introducing Sr vacancies and causing site changes (FeSr and TiSr) to accommodate nonstoichiometry. Sr vacancies act as mid‐gap acceptor states, pinning the Fermi level, provided that additional donor states (most probably ) are present. Defect chemical modeling revealed that such a Fermi level pinning also causes a self‐limitation of the Ti site change and leads to a very robust pseudo‐intrinsic situation, irrespective of Sr/Ti ratios and doping.

Effect of laser fluence and charge compensation defects on photoreduction of Eu ions in KSrPO4 crystal using a UV laser

In our previous study, Eu ions in KSrPO4 were reduced from trivalent to divalent by irradiation with a Nd:YVO4 nanosecond laser at a wavelength of 266 nm. This reduction was caused by laser light, so this phenomenon was termed photoreduction. Here, the effect of the laser fluence and charge compensation defects on photoreduction were investigated to model the process. The dependence of the amount of Eu ions reduced on the photoluminescence intensity suggests that a two-photon process is involved. Considering the energy of the laser wavelength at which photoreduction occurs, the two photons were attributed to the excitation of [Eu3+-O2-] charge transfer and charge transfer from a Sr vacancy to the conduction band, respectively. KSrPO4:Eu phosphors co-doped with various metal ions were synthesized, and photoreduction was conducted on these phosphors. In the case of co-doping trivalent metal ions, the amount of reduced Eu ions was increased, while the amount was decreased when co-doping monovalent ions. We consider that this difference is due to the change in the defect state of KSrPO4:Eu by co-doping metal ions and suggest that the presence of charge compensation defects, such as Sr vacancies, are important in the photoreduction process. Based on these results, we model the photoreduction process and discuss the details of the mechanism.

Key roles of surface Fe sites and Sr vacancies in the perovskite for an efficient oxygen evolution reactionvialattice oxygen oxidation

Highly efficient La/Sr-based perovskites with surface Fe sites and Sr vacancies were developed for the active lattice oxygen mechanism (LOM) of the oxygen evolution reaction (OER), and a relationship between the LOM and dynamic surface structure was established.

Rapidly reconstructing the active surface of cobalt-based perovskites for alkaline seawater splitting.

As a potential oxygen evolution reaction (OER) catalyst, Co-based perovskites have received intensive attention. However, Sr readily accumulates on their surface, and makes them inert toward the OER. Herein, we propose a simple but versatile electrochemical reduction method to reconstruct the active surface of Co-based perovskites within a few seconds. By this method, Sr rapidly precipitates from Co-based perovskites, accompanied by the introduction of Sr and oxygen vacancies. After reconstruction, the electrochemical active surface areas of Co-based perovskites greatly increase, and the OER overpotential of the optimized SrNb0.1Co0.7Fe0.2O3-δ (ER-SNCF-20s) reaches 278 mV at 10 mA cm-2. This can be explained by the decrease of overpotentials at the rate-determining step. Using ER-SNCF-20s, the splitting voltage of alkaline natural seawater can reach 1.56 V at 10 mA cm-2, and remains steady for 300 h. This effort offers a feasible method for reconstructing the active surface of Co-based perovskites.

Achieving high energy storage performance and ultrafast discharge speed in SrTiO3-based ceramics via a synergistic effect of chemical modification and defect chemistry

Environmentally friendly energy storage materials with high energy storage performance and excellent stability for applications in pulse power systems are urgently needed. SrTiO3-based ceramics have a relatively high dielectric constant and a high breakdown strength (BDS). However, a low polarization strength in this system often yields a low energy storage density. In this paper, high energy storage performance of SrTiO3-based ceramics with the composition of (Sr1-x-y-φNayBix□φ)TiO3 (abbreviated as zSNBT, where x = 0.2 + 0.3z, y = 0.5z, φ = 0.1–0.1z, and z = 0.8–0.1; □ represents Sr vacancies) was achieved using a synergistic effect of chemical modification and defect chemistry. With the decrease of z, the ferroelectric macro-domain of zSNBT ceramics vanishes due to the original effect of paraelectric SrTiO3, significantly improving the BDS and energy storage performance. When z = 0.2, the material exhibited a single cubic phase structure with a nearly hysteresis-free P-E loop. The maximum BDS reached 390 kV/cm, and under such electric field, a large recoverable energy storage density (Wrec) of 3.94 J/cm3 and ultrahigh energy efficiency (η) of 94.71% were achieved simultaneously. Meanwhile, the 0.2SNBT ceramic showed good thermal stability and satisfying cycling stability in the temperature range of 20–160 °C. In addition, the 0.3SNBT ceramic demonstrated outstanding thermal stability with an ultrafast discharge speed (t0.9 ≤ 26 ns) in the temperature range of 20–180 °C. Furthermore, a simulation model containing grains and grain boundaries was established to explain the enhanced BDS and the energy storage performance. Fine and uniform grains with many grain boundaries significantly hindered the propagation of breakdown cracks, yielding a higher BDS value. These results indicate that zSNBT ceramics are promising environmentally friendly materials for pulse power capacitor applications.

X-ray absorption spectroscopy and neutron-diffraction study of persistent luminescent Sr2MgSi2O7 glass-ceramics

Glass-ceramics based on the Eu2O3/Dy2O3-doped Sr2MgSi2O7 phosphor have been obtained from sintering and crystallization of glass powders. The doped parent glasses show red emission under excitation of UV light. After thermal treatment, the corresponding glass-ceramics show persistent blue emission which emanates from Eu2+ incorporated in the crystals; red emission also occurs from Eu3+ in the residual vitreous phase.The aim of this work is to investigate the structure of defects such as oxygen and strontium vacancies in the Sr2MgSi2O7 crystals and the environment, oxidation state and coordination of the Eu and Dy ions in the glass-ceramics, their partitioning between crystalline and glassy phase and their influence on luminescence properties, employing X-ray absorption spectroscopy (XAS) and neutron diffraction (ND) as investigative structural tools. We suggest that the luminescence mechanism involves the presence of shallow electron traps that are abruptly emptied for temperatures above 100 K. The most persistent luminescence is likely due to an effective concentration of Eu2+ in the dendritic-shaped crystals in one of the particular compositions, and with a suitable level of Sr vacancies, even though Sr vacancies are most likely formed in all the investigated samples.

Specific cation stoichiometry control of SrMnO3-δ thin films via RHEED oscillations

Cubic heteroepitaxial (001) SrMnO3-δ (SMO) films were grown on SrTiO3 substrates by atomic alternating layer molecular beam epitaxy. Precise control of cation stoichiometry was achieved by in situ reflection high-energy electron diffraction (RHEED) oscillation. During SMO film growth, a correlation between RHEED oscillation features and the cation stoichiometry/monolayer dose was established. In non-stoichiometric films, there were out-of-plane lattice expansions due to off-stoichiometry-dependent defects, including Ruddlesden–Popper SrO planar faults in Sr-rich films and Sr vacancy defect clusters in Mn-rich films.

Stabilization of 6H-Hexagonal SrMnO3 Polymorph by Al2O3 insertion

We demonstrate the structural evolution of polymorphic phases in Al2O3-inserted SrMnO3 ceramics synthesized by solid state reaction. While the 4H-hexagonal phase is predominant in pure SrMnO3 ceramics, a small amount of 6H-hexagonal polymorph is identified in addition to the primary 4H-hexagonal SrMnO3 and the secondary hexagonal SrAl2O4 phases in the as-sintered ceramics, evidenced by x-ray diffraction and subsequent Rietveld refinement analyses. The existence of the 6H-hexagonal SrMnO3 phase is corroborated using Raman spectroscopy. The chemical compositions and electronic structures of the Al2O3-inserted SrMnO3 compounds are also examined using energy dispersive spectroscopy and x-ray photoelectron spectroscopy, respectively. The first-principles calculations reveal that there is no clear difference between the total energies of 4H- and 6H-hexagonal polymorphs regardless of the presence/absence of Sr and oxygen vacancies. Possible origins are discussed with the estimation of actual strain based on the refined lattice parameter of 6H SrMnO3.