Hexagonal Perovskite Research Articles

Can Vacuum Deposition Apply to Bismuth-Doped γ-CsPbI3 Perovskite? Revealing the Role of Bi3+ in the Formation of Black Phase.

The B-site doped CsPbI3 has been demonstrated to be very promising for photovoltaics owing to its low black phase transition temperature. Though B-site doped black-CsPbI3 perovskites have been successfully achieved by solution-processing, it is unclear whether these systems are available by other methods such as vacuum deposition. In this work, heterovalent doped CsPb1-xBixI3 is targeted. To incorporate Bi3+ into the final film via vacuum deposition, the solid solution precursor Pb1-xBixI2 (0.01 ≤ x ≤ 0.04) is developed. However, these coevaporated films not only are dominated by another hexagonal perovskite phase but also fail to decrease the black phase transition temperature. The role of Bi3+ in the formation of the black phase is further studied by solution methods with different types of precursors. It is demonstrated that the key factor in the low-temperature black phase transition is small grain size, as well as the colloid size within the precursor solution, rather than simple substitution of Pb2+ with Bi3+.

Black Cool Pigments for Urban Heat Island (UHI) Control: from Cr-Hematite to Mn-Melilite

Black cool pigments are very interesting for its application in asphalt urban pavements and building floors for moderate the urban heat island effect (UHI) and improving air conditioning energy efficiency. Cool black pigments based on Cr doped hematite Fe2O3 (trigonal, R-3c), hexagonal perovskites YMnO3 (hexagonal, P63cm) and Sr4CuMn2O9 (trigonal, P321) and melilite Sr2(Mg0.5Mn0.5)Ge2O7 (tetragonal, P-421m) with high NIR reflectance synthesized by ceramic and coprecipitation method, are analyzed and compared from color yield in alkyd paint, ceramic glazes and porcelain stoneware, NIR reflectance, bandgap and photocatalytic activity on Orange II substrate. Sr4CuMn2O9 black powders show the nearest hue h to the reference carbon black and the highest NIR reflectance (51%). All pigments show high NIR reflectance in all tested applications. The Fe1.2Cr0.8O3 pigment shows good behavior in the free ZnO glaze and also in porcelain stoneware, YMnO3 and Sr4CuMn2O9 pigments are compatible with low temperature glazes, but Sr2(Mg0.5Mn0.5)Ge2O7 pigment loses the black color even in low temperature glazes. Sr4CuMn2O9 pigment shows moderate photoactivity on Orange II (t1/2=216 min) and the Fe1.2Cr0.8O3 pigment also shows some activity (t1/2=329 min).

Halide-modulated self-assembly of metal-free perovskite single crystals for bio-friendly X-ray detection

Halide-modulated self-assembly of metal-free perovskite single crystals for bio-friendly X-ray detection

Sb-doping effects on twin-shift option and microwave dielectric properties of Ba8CoNb6O24 eight-layer hexagonal perovskite ceramics

B-site deficient eight-layer hexagonal perovskites possess in general high quality factors and large positive temperature coefficients of resonant frequency (τf). Recently we successfully lowered τf of shifted perovskite Ba8CoNb6O24 down to near zero through one-third Ta5+ substitution but raised the cost of the materials. In this work, Ba8CoNb6O24 was doped with cheaper elements Sb5+ on Nb5+ sites using conventional solid-state reactions, and near-zero τf of 0.66(2) ppm/°C, modest εr of 27.99(3), Q × f of 1.016(6) × 104 GHz, and significantly reduced cost of the ceramics were achieved on the composition Ba8CoNb5.4Sb0.6O24. The lower polarizability of Sb5+ than Nb5+ results in smaller εr and therefore τf values. About 15 % Sb5+ substitution for Nb5+ in Ba8CoNb6-xSbxO24 dramatically transforms the shifted structure to twinned structure, in contrast with 50 % Ta5+ substitution in the Ta5+ case. This is ascribed to reduced SOJT effects and ionic size of d10 Sb5+ compared with d° Nb5+ and Ta5+.

Hydration and Ionic Conduction Mechanisms of Hexagonal Perovskite Derivatives

Hydration and Ionic Conduction Mechanisms of Hexagonal Perovskite Derivatives

Salts of octabismuth(2+) polycations crystallized from Lewis-acidic ionic liquids

Abstract The reaction of Bi and BiCl3 with RbCl or CsCl in the Lewis-acidic ionic liquid (IL) [BMIm]Cl·4AlCl3 at T = 200 °C yielded air-sensitive, shiny black crystals. X-ray diffraction on single crystals revealed the hexagonal structures of two new salts (Bi8)M[AlCl4]3 (M = Rb, Cs), which are isostructural to the high-temperature form of (Bi8)Tl[AlCl4]3. The known (Bi8)2+ polycation is a square antiprism and can be interpreted as an arachno cluster following modified Wade rules. The crystal structure is a complex variant of the hexagonal perovskite structure type ABX 3 with A = (Bi8)2+, B = M + and X = [AlCl4]–. Chiral strands ∞ { M [ AlCl 4 ] 3 } 2 − ∞ 1 $\infty {}_{\infty }{}^{1}{\left\{M{\left[{\text{AlCl}}_{4}\right]}_{3}\right\}}^{2-}$ (M = Rb, Cs) run along [001]. The (Bi8)2+ polycations are only weakly coordinated inside a cage of 24 chloride ions and show dynamic rotational disorder at room temperature. Upon slow cooling to 170 K, the reorientation of the clusters was frozen, yet no long-range order was established.

YAG thermal barrier coatings deposited by suspension and solution precursor thermal spray

Yttrium aluminium garnet (YAG) is a promising topcoat material for thermal barrier coatings due to its high temperature stability and better CMAS (calcium-magnesium-alumino-silicate) resistance. YAG topcoats were deposited by suspension and solution precursor high-velocity oxy-fuel (HVOF) thermal spray. The relationships between processing, microstructure and final properties were studied through a range of characterization techniques and thermal cycling tests. The microstructure of the as-sprayed YAG topcoat from stoichiometric solution precursor (SP-YAG) had distributed pores and inter-splat boundaries, while the as-sprayed topcoat produced from suspension (S-YAG) had vertical and branched micro cracks, pores, and inter-splat boundaries. Both as-sprayed coatings were composed of amorphous phase, hexagonal yttrium aluminium perovskite (YAP) and cubic YAG. In thermal cycling tests, 20% of SP-YAG failure was reached after the 10th cycle; whereas, S-YAG reached the failure criteria between the 60th and 70th cycle. The failure of both the SP-YAG and the S-YAG topcoats occurred due to thermal stresses during the thermal cycling.

Ba3[Rh(OH)6]2 ⋅ H2O – a Precursor to Barium Oxorhodates with One‐dimensional Hydrogen Bonding Systems

AbstractOrange‐colored single‐crystals of barium hexahydroxo‐rhodate(III) monohydrate, Ba3[Rh(OH)6]2 ⋅ H2O, were synthesized in a PTFE‐lined autoclave at 200 °C starting from Ba(NO3)2, RhI3 and concentrated sodium hydroxide solution (water‐base ratio 4 : 1). Single‐crystal X‐ray diffraction revealed the triclinic space group (no. 2) with lattice parameters a=824.60(6) pm, b=852.78(5) pm, c=885.34(6) pm, α=94.294(3)°, β=92.422(3)°, γ=91.673(3)° at 100(1) K. The crystal structure consists of [Rh(OH)6]3− octahedra surrounded by barium cations and water molecules. Hydrogen bonds with a wide range of donor‐acceptor distances are located between hydroxide groups and water molecules to form one‐dimensional hydrogen bonding networks in c direction. Upon heating in air, Ba3[Rh(OH)6]2 ⋅ H2O decomposes first to “BaRhO3” with a superstructure of the hexagonal perovskite type, which undergoes a phase transition to the 2H hexagonal perovskite Ba9Rh8O24 above 900 °C.

Hexagonal Perovskites: Atomic Structure of the Initial Nucleation Layer in Hexagonal Perovskite BaRuO3 Thin Films (Adv. Mater. Interfaces 7/2021)

A cross-sectional scanning transmission electron microscopy image overlayed with a strain map by geometrical phase analysis reveals how the initial nucleation layer facilitates the epitaxial growth of a hexagonal perovskite BaRuO3 thin film. More details can be found in article number 2100023 by Woo Seok Choi, Ho Nyung Lee, and co-workers.

Widely spaced planes of magnetic dimers in theBa6Y2Rh2Ti2O17−δhexagonal perovskite

We report the synthesis and initial characterization of Ba6Y2Rh2Ti2O17-{\delta}, a previously unreported material with a hexagonal symmetry structure. Face-sharing RhO6 octahedra form triangular planes of Rh2O9 dimers that are widely separated in the perpendicular direction. The material displays a small effective magnetic moment, due to the Rh ions present, and a negative Curie-Weiss temperature. The charge transport and optical band gaps are very similar, near 0.16 eV. A large upturn in the heat capacity at temperatures below 1 K, suppressed by applied magnetic fields larger than {{\mu}0H = 2 Tesla, is observed. A large T-linear term in the specific heat ({\gamma}=166 mJ/mol f.u-K2) is seen, although the material is insulating at low temperatures. These results suggest the possibility of a spin liquid ground state in this material.

Crystal Growth of a New 8H Perovskite Sr8Os6.3O24 Exhibiting High TC Ferromagnetism

Single crystals of a new twinned hexagonal perovskite compound Sr8Os6.3O24 have been synthesized, and structural and magnetic properties have been determined. The compound crystallizes in a hexagonal cell with lattice parameters a = 9.6988(3) Å and c = 18.1657(5) Å. The structure is an eight-layered hexagonal B-site deficient perovskite with the layer sequence (ccch)2 and represents the first example of a hexagonal structure among 5d oxides adopting a twin option. The sample shows spontaneous ferromagnetic magnetization below 430 K with a small saturation moment of 0.11 μB/Os ion. This is the highest Curie temperature (TC) reported for any bulk perovskite containing only 5d ions at the B site.

Two enantiomeric perovskite ferroelectrics with a high Tc raised by inserting intermolecular hydrogen bonds

Molecular ferroelectrics have promising potential as next-generation flexible electronic materials by the advantage of flexibility, structural tunability, and easy processability. However, an obstacle in expanding their promising applications is effectively raising the ferroelectric transition temperature (Tc) necessary for practical applications, especially under high-temperature operating conditions. Herein, taking the advantage of a hydroxyl group that could form stronger hydrogen bonds to insert/tune host–guest and guest–guest interactions, we employed the 3-hydroxypyrrolidine cation to construct two new enantiomeric hexagonal perovskite ferroelectrics, (R)-3-OH-(C4H9N)[CdCl3] and (S)-3-OH-(C4H9N)[CdCl3]. Both of them undergo a ferroelectric phase transition from C2221 to P21 with a high Tc of 350 K, which is 110 K and 47 K higher than that of their parent compound (C4H10N)[CdCl3] (240 K) and F-substituted analogues (R/S)-3-F-(C4H9N)[CdCl3] (303 K), respectively. These findings well demonstrate that, besides the F-substitution strategy, an OH-substitution strategy provides an important and practical way in designing high-Tc ferroelectrics.

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

AbstractHexagonal perovskites are an attractive family of materials due to their various polymorphs and rich structure–property relationships. BaRuO3 (BRO) is a prototypical hexagonal perovskite, in which the electromagnetic properties are significantly modified depending on its lattice structure. Whereas thin‐film synthesis would allow the study of the diverse properties of hexagonal perovskites by epitaxially stabilizing various metastable polymorphs, the epitaxial growth of hexagonal perovskites, especially at the initial growth stage, is yet to be addressed. In this study, it is shown that an intriguing nucleation behavior takes place during the initial stabilization of a hexagonal 9R BRO thin film deposited on a (111) SrTiO3 (STO) substrate. It is found that a nanoscale strained layer composed of different RuO6 octahedral stacking is initially formed at the interface followed by a relaxed single crystal 9R BRO thin film. Within the interface layer, hexagonal BRO is nucleated on the (111) STO substrate by both face‐ and corner‐sharing octahedra. More interestingly, it is found that boundaries between differently stacked nucleation layers (heterostructural boundaries) facilitate strain relaxation, in addition to the formation of conventional misfit dislocations evolved from homostructural boundaries. The observations reveal an important underlying mechanism to understand the thin‐film epitaxy and strain accommodation in hexagonal perovskites.

Extended B-Site Vacancy Content Range and Cation Ordering in Twinned Hexagonal Perovskites Ba8Cr4-xTa4+0.6xO24.

The new 8-layer twinned hexagonal solid solution Ba8Cr4-xTa4+0.6xO24 (x = 0.0-3.0) was isolated through the aliovalent substitution of Ta5+ for Cr3+ in Ba2CrTaO6, showing the widest B-site vacancy content range among the 8-layer twinned hexagonal perovskites. Ba8Cr4-xTa4+0.6xO24 forms a simple 8-layer hexagonal perovskite structure within 0.0 ≤ x < 2.4 and a tripled 8-layer hexagonal perovskite superstructure within 2.4 ≤ x ≤ 3.0. The latter shows expanded a and b axes by times in comparison to the simple 8-layer hexagonal perovskite structure owing to the partial face-sharing octahedral (FSO) B cation ordering along the ab plane. The B-cation and vacancy distributions in the tripled superstructure were characterized by neutron and X-ray powder diffraction and further confirmed by a scanning transmission electron microscopy-high angle annular dark field imaging and intensity profile analysis. The formation of 8-layer twinned hexagonal perovskites Ba8Cr4-xTa4+0.6xO24 in an extended solid solution range can be attributed to the presence of both covalent B-B and B-O-B bonding and B-site vacancies in the FSO sites. This work provides an effective way of combining covalent B-B and B-O-B bonding and vacancy creation as well as the cationic ordering in the FSO sites to reduce electrostatic repulsion, which could further enable the stabilization of new hexagonal perovskite compounds.

High oxide-ion conductivity through the interstitial oxygen site in Ba7Nb4MoO20-based hexagonal perovskite related oxides

Oxide-ion conductors are important in various applications such as solid-oxide fuel cells. Although zirconia-based materials are widely utilized, there remains a strong motivation to discover electrolyte materials with higher conductivity that lowers the working temperature of fuel cells, reducing cost. Oxide-ion conductors with hexagonal perovskite related structures are rare. Herein, we report oxide-ion conductors based on a hexagonal perovskite-related oxide Ba7Nb4MoO20. Ba7Nb3.9Mo1.1O20.05 shows a wide stability range and predominantly oxide-ion conduction in an oxygen partial pressure range from 2 × 10−26 to 1 atm at 600 °C. Surprisingly, bulk conductivity of Ba7Nb3.9Mo1.1O20.05, 5.8 × 10−4 S cm−1, is remarkably high at 310 °C, and higher than Bi2O3- and zirconia-based materials. The high conductivity of Ba7Nb3.9Mo1.1O20.05 is attributable to the interstitial-O5 oxygen site, providing two-dimensional oxide-ion O1−O5 interstitialcy diffusion through lattice-O1 and interstitial-O5 sites in the oxygen-deficient layer, and low activation energy for oxide-ion conductivity. Present findings demonstrate the ability of hexagonal perovskite related oxides as superior oxide-ion conductors.

Molecule-like cluster magnetism and cationic order in the new hexagonal perovskite Ba4Sn1.1Mn2.9O12.

The new hexagonal perovskite phase of composition Ba4Sn1.1Mn2.9O12 has been synthesized by solid-state reactions at 1673 K. The crystal structure has been investigated using X-ray and neutron diffraction. The hexagonal perovskite structure has an ordered arrangement of Sn and Mn ions on the corner-sharing octahedral centers and the face-sharing octahedral centers respectively. Short Mn–Mn distances have been evidenced in the face-sharing trimer of MnO6 octahedra. The magnetic susceptibility shows magnetic cluster behavior, with cluster formation temperature ∼220 K. Antiferromagnetic order has been observed at TN ∼ 6 K. Ba4Sn1.1Mn2.9O12 is a semiconductor with a transport activation energy of 0.61 eV.

Pauli-paramagnetic and metallic properties of high pressure polymorphs of BaRhO3 oxides containing Rh2O9 dimers.

From our material exploration study in wide pressure and temperature conditions, we found a new 6H polymorph of BaRhO3 was stabilised under high pressure conditions from 14 to 22 GPa. The material crystallised in the monoclinic 6H hexagonal perovskite structure in space group C2/c. The 4H BaRhO3 polymorph was stabilised at lower pressures, but the 3C cubic BaRhO3 likely requires pressures greater than 22 GPa. Both 6H and 4H polymorphs contain Rh2O9 dimers and the large 4d Rh orbital spatial diffusivity in these dimers leads to Pauli paramagnetic and metallic ground states, which are also supported by first-principles electronic structure calculations. High Wilson ratios of approximately 2 for either compound indicate strong electron correlation.

Soft and anisotropic local moments in 4d and 5d mixed-valence M2O9 dimers

We investigate via exact diagonalization of finite clusters the electronic structure and magnetism of M$_2$O$_9$ dimers in the mixed-valence hexagonal perovskites A$_3$B'M$_2$O$_9$ for various different fillings of 4$d$ and 5$d$ transition-metal M ions. We find that the magnetic moments of such dimers are determined by a subtle interplay of spin-orbit coupling, Hund's coupling, and Coulomb repulsion, as well as the electron filling of the M ions. Most importantly, the magnetic moments are anisotropic and temperature-dependent. This behavior is a result of spin-orbit coupling, magnetic field effects, and the existence of several nearly-degenerate electronic configurations whose proximity allows occupation of excited states already at room temperature. This analysis is consistent with experimental susceptibility measurements for a variety of dimer-based materials. Furthermore, we perform a survey of A$_3$B'M$_2$O$_9$ materials and propose ground-state phase diagrams for the experimentally relevant M fillings of $d^{4.5}$, $d^{3.5}$ and $d^{2.5}$. Finally, our results show that the usually applied Curie-Weiss law with a constant magnetic moment cannot be used in these spin-orbit-coupled materials.

High frequency atomic tunneling yields ultralow and glass-like thermal conductivity in chalcogenide single crystals

Crystalline solids exhibiting glass-like thermal conductivity have attracted substantial attention both for fundamental interest and applications such as thermoelectrics. In most crystals, the competition of phonon scattering by anharmonic interactions and crystalline imperfections leads to a non-monotonic trend of thermal conductivity with temperature. Defect-free crystals that exhibit the glassy trend of low thermal conductivity with a monotonic increase with temperature are desirable because they are intrinsically thermally insulating while retaining useful properties of perfect crystals. However, this behavior is rare, and its microscopic origin remains unclear. Here, we report the observation of ultralow and glass-like thermal conductivity in a hexagonal perovskite chalcogenide single crystal, BaTiS3, despite its highly symmetric and simple primitive cell. Elastic and inelastic scattering measurements reveal the quantum mechanical origin of this unusual trend. A two-level atomic tunneling system exists in a shallow double-well potential of the Ti atom and is of sufficiently high frequency to scatter heat-carrying phonons up to room temperature. While atomic tunneling has been invoked to explain the low-temperature thermal conductivity of solids for decades, our study establishes the presence of sub-THz frequency tunneling systems even in high-quality, electrically insulating single crystals, leading to anomalous transport properties well above cryogenic temperatures.

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

Two new eight-layer hexagonal perovskites with the composition Ba8MNb6O24 (M = Fe and Cu) are synthesized by solid-state reaction at 1350-1400 °C. Their crystal structures have been investigated using X-ray and electron diffractions as well as high-resolution transmission electron microscopy. Although both compounds have similar M2+ size, Ba8FeNb6O24 and Ba8CuNb6O24 adopt shifted and twinned structures, respectively. Through comparison with the reported shifted Ba8MNb6O24 (M = Mn, Co, and Zn) and twinned Ba8NiNb6O24 as well as inexistent Ba8Mg(Nb/Ta)6O24, we elucidate that the twin-shift competition of Ba8MNb6O24 family could be related with multiple chemical factors including tolerance factors, B-cationic size difference, entropy variation with B-cation and vacancy disorder, Jahn-Teller distortion, and FSO B-B d orbit interactions.