Perovskite Polymorph Research Articles

First-Principles Thermodynamics of CsSnI3.

CsSnI3 is a promising ecofriendly solution for energy harvesting technologies. It exists at room temperature in either a black perovskite polymorph or a yellow 1D double-chain, which irreversibly deteriorates in the air. In this work, we unveil the relative thermodynamic stability between the two structures with a first-principles sampling of the CsSnI3 finite-temperature phase diagram, discovering how it is driven by anomalously large quantum and anharmonic ionic fluctuations. Thanks to a comprehensive treatment of anharmonicity, the simulations deliver a remarkable agreement with known experimental data for the transition temperatures of the orthorhombic, rhombohedral, and cubic perovskite structures and the thermal expansion coefficient. We disclose how the perovskite polymorphs are the ground state above 270 K and discover an abnormal decrease in heat capacity upon heating in the cubic black perovskite. Our results also significantly downplay the Cs+ rattling modes' contribution to mechanical instability. The remarkable agreement with experiments validates our methodology, which can be systematically applied to all metal halides.

Double Double to Double Perovskite Transformations in Quaternary Manganese Oxides.

Control of cation ordering in ABX3 perovskites is important to structural, physical and chemical properties. Here we show that thermal transformations of AA'BB'O6 double double perovskites, where both A and B sites have 1:1 cation order, to (A0.5 A'0.5 )2 BB'O6 double perovskites with fully disordered A/A' cations can be achieved under pressure in CaMnMnWO6 and SmMnMnTaO6 , enabling both polymorphs of each material to be recovered. This leads to a dramatic switch of magnetic properties from ferrimagnetic order in double double perovskite CaMnMnWO6 to spin glass behaviour in the highly frustrated double perovskite polymorph. Comparison of double double and double perovskite polymorphs of other materials will enable effects of cation order and disorder on other properties such as ferroelectricity and conductivity to be explored.

Double Double to Double Perovskite Transformations in Quaternary Manganese Oxides

AbstractControl of cation ordering in ABX3 perovskites is important to structural, physical and chemical properties. Here we show that thermal transformations of AA′BB′O6 double double perovskites, where both A and B sites have 1:1 cation order, to (A0.5A′0.5)2BB′O6 double perovskites with fully disordered A/A′ cations can be achieved under pressure in CaMnMnWO6 and SmMnMnTaO6, enabling both polymorphs of each material to be recovered. This leads to a dramatic switch of magnetic properties from ferrimagnetic order in double double perovskite CaMnMnWO6 to spin glass behaviour in the highly frustrated double perovskite polymorph. Comparison of double double and double perovskite polymorphs of other materials will enable effects of cation order and disorder on other properties such as ferroelectricity and conductivity to be explored.

(Invited) Crystal Phase Engineering of Cesium Lead Iodide By Nanoconfinement

Nanoscale dimensional reduction leads to perturbed electronic structure, a foundational concept in the study of quantum dots. In this work, we extend this concept to consider how nanoscale dimensionality perturbs the crystal phase diagram of a solid. We apply this to the phase behavior of cesium lead iodide, a material that possesses favorable optoelectronic properties in its perovskite crystal polymorph at high temperature, and exhibits an undesirable crystal phase at room temperature. We develop and test the hypothesis that for solids such as cesium lead iodide that simultaneously exhibit both predominantly covalent bonding (Pb—I) and predominantly ionic bonding (Cs—I), low-density, high-symmetry phases can be favored by restricting crystal dimensions below 100 nm. This is based on a model in which short-range, orbital-orbital repulsive forces, which favor low-density and high-symmetry phases, are in competition with long-range electrostatic forces, which favor high-density. The electrostatic forces, being long-range in nature, are more diminished by truncating the crystal. Experimentally, we show that the phase sequence can be inverted and the equilibrium shifted by as much as 250 ºC through various nanostructuring approaches, including colloidal nanocrystal co-assembly and crystallization within nanoporous scaffolds. Ultimately, we seek to develop a phase diagram in which temperature and size are the independent variables.

Solid-phase hetero epitaxial growth of \u03b1-phase formamidinium perovskite

Conventional epitaxy of semiconductor films requires a compatible single crystalline substrate and precisely controlled growth conditions, which limit the price competitiveness and versatility of the process. We demonstrate substrate-tolerant nano-heteroepitaxy (NHE) of high-quality formamidinium-lead-tri-iodide (FAPbI3) perovskite films. The layered perovskite templates the solid-state phase conversion of FAPbI3 from its hexagonal non-perovskite phase to the cubic perovskite polymorph, where the growth kinetics are controlled by a synergistic effect between strain and entropy. The slow heteroepitaxial crystal growth enlarged the perovskite crystals by 10-fold with a reduced defect density and strong preferred orientation. This NHE is readily applicable to various substrates used for devices. The proof-of-concept solar cell and light-emitting diode devices based on the NHE-FAPbI3 showed efficiencies and stabilities superior to those of devices fabricated without NHE.

High pressure structure studies of three SrGeO3 polymorphs – Amorphization under pressure

We report on the synthesis and high pressure behavior of three polymorphs of SrGeO3. At ambient pressure, SrGeO3 crystallizes in the monoclinic structure pseudo-wollastonite. Two high pressure polymorphs, triclinic walstromite, and cubic perovskite were synthesized using a large volume multi-anvil press. The crystal structures of the three polymorphs were investigated with powder X-ray diffraction as a function of pressure using diamond anvil cells. It was found that the pseudo-wollastonite polymorph becomes amorphous at 10 GPa and equation of state fitting of the volume data yielded a bulk modulus of K0 = 47(4) GPa, reported for the first time. Compression of the walstromite structure showed the structure to be very compressible with two distinct phase transitions at around 10–12 GPa and 35–38 GPa. The data suggest that the structure then becomes amorphous although it retains a small degree of long-range order to the highest pressure studied. The perovskite polymorph was very incompressible and equation of state fitting of the volume data yielded a high bulk modulus of K0 = 194(3) GPa. All the experimental data was compared to density functional theory calculations, which were observed to fit well with the experiments.

Enhancing the Phase Stability of Inorganic α-CsPbI3 by the Bication-Conjugated Organic Molecule for Efficient Perovskite Solar Cells.

Inorganic CsPbI3 perovskite has demonstrated promising potentials for photovoltaic applications, whereas the black perovskite polymorph (α phase) of CsPbI3 was easily prone to converting into yellow phase (δ phase) under ambient moist environment, which restrained its practical application and further studies severely. In this study, p-phenylenediammonium iodide (PPDI) was employed to posttreat CsPbI3 films for controlling the phase conversion, strengthening moisture resistance, and improving device performance. The multiple roles of PPDI were as follows: (1) avoiding spontaneous octahedral tilting by ionic bonds between NH3+ of PPD2+ and I- of [PbI6]4-; (2) enhancing the hydrophobicity induced by exactly exposed oil-wet (hydrophobic) benzene rings; and (3) passivating surface defects and filling I vacancies. As a result, after the treatment, mutable a-CsPbI3 could maintain its α phase for at least 30 d in dry air (<20% RH). The perovskite solar cells with PPDI treatment exhibited reproductive photovoltaic performance with a champion power conversion efficiency (PCE) of 10.4, and 91% of the initial PCE was retained after storage for 504 h in a dark dry box without any encapsulation.

Monoclinic SrIrO3—a Dirac semimetal produced by non-symmorphic symmetry and spin–orbit coupling

SrIrO3 crystallizes in a monoclinic structure of distorted hexagonal perovskite at ambient pressure. The transport measurements show that the monoclinic SrIrO3 is a low-carrier density semimetal, as in the orthorhombic perovskite polymorph. The electronic structure calculation indicates a semimetallic band structure with Dirac bands at two high-symmetry points of Brillouin zone only when spin–orbit coupling is incorporated, suggesting that the semimetallic state is produced by the strong spin–orbit coupling. We argue that the Dirac bands are protected by the non-symmorphic symmetry of lattice.

Cubic or Orthorhombic? Revealing the Crystal Structure of Metastable Black-Phase CsPbI3 by Theory and Experiment

Room-temperature films of black-phase cesium lead iodide (CsPbI3) are widely thought to be trapped in a cubic perovskite polymorph. Here, we challenge this assumption. We present structural refinement of room-temperature black-phase CsPbI3 in an orthorhombic polymorph. We demonstrate that this polymorph is adopted by both powders and thin films of black-phase CsPbI3, fabricated either by high- or low-temperature processes. We perform electronic band structure calculations for the orthorhombic polymorph and find agreement with experimental data and close similarities with orthorhombic methylammonium lead iodide. We investigate the structural transitions and thermodynamic stability of the various polymorphs of CsPbI3 and show that the orthorhombic polymorph is the most stable among its other perovskite polymorphs, but it remains less stable than the yellow nonperovskite polymorph.

Molten Salt Synthesis, Polymorphism, and Microwave Dielectric Properties of Ba8NiTa6O24 Perovskite

Molten salt synthesis (MSS) of the eight‐layer hexagonal perovskite Ba8NiTa6O24 was performed using mixed KCl–NaCl salts in comparison with solid‐state synthesis (SSS). In the SSS, the hexagonal Ba8NiTa6O24 formed at 1300°C via a reaction between cubic Ba3NiTa2O9 and hexagonal Ba5Ta4O15. While the MSS did not lower the synthesis temperature of the hexagonal Ba8NiTa6O24 but stabilized an unusual A‐ and B‐site‐deficient cubic perovskite polymorph of Ba8NiTa6O24 below 1350°C as an intermediate phase prior to transforming into the hexagonal phase. This cubic polymorph contains ~3% A‐site and ~9.5% B‐site vacancies plus ~3% Ba cations in the B sites and demonstrated remarkable stability below 1350°C when without presence of the molten salt. The cubic polymorph displayed larger ε ~ 36 and τf ~ 110 ppm/°C than the hexagonal polymorph from the MSS (ε ~ 29 and τf ~ 67 ppm/°C). The hexagonal SSS‐processed ceramics showed advantageous dielectric properties (Qf ~ 52 000 GHz, τf ~ 30 ppm/°C) over both cubic and hexagonal MSS‐processed ones (Qf ~ 18 600–20 000 GHz), while displaying anisotropic grain growth. The anisotropic grain growth was suppressed significantly by the MSS processing.

Hole doping and structural transformation in CsTl1-xHgxCl3.

CsTlCl(3) and CsTlF(3) perovskites have been theoretically predicted to be superconductors when properly hole-doped. Both compounds have been previously prepared as pure compounds: CsTlCl(3) in a tetragonal (I4/m) and a cubic (Fm3̅m) perovskite polymorph and CsTlF(3) as a cubic perovskite (Fm3̅m). In this work, substitution of Tl in CsTlCl(3) with Hg is reported, in an attempt to hole-dope the system and induce superconductivity. The whole series CsTl(1-x)HgxCl(3) (x = 0.0, 0.1, 0.2, 0.4, 0.6, and 0.8) was prepared. CsTl(0.9)Hg(0.1)Cl(3) is tetragonal as the more stable phase of CsTlCl(3). However, CsTl(0.8)Hg(0.2)Cl(3) is already cubic with the space group Fm3̅m and with two different positions for Tl(+) and Tl(3+). For x = 0.4 and 0.5, solid solutions could not be formed. For x ≥ 0.6, the samples are primitive cubic perovskites with one crystallographic position for Tl(+), Tl(3+), and Hg(2+). All of the samples formed are insulating, and there is no signature of superconductivity. X-ray absorption spectroscopy indicates that all of the samples have a mixed-valence state of Tl(+) and Tl(3+). Raman spectroscopy shows the presence of the active Tl-Cl-Tl stretching mode over the whole series and the intensity of the Tl-Cl-Hg mode increases with increasing Hg content. First-principle calculations confirmed that the phases are insulators in their ground state and that Hg is not a good dopant in the search for superconductivity in this system.

Formamidinium-Containing Metal-Halide: An Alternative Material for Near-IR Absorption Perovskite Solar Cells

Solid-state, solution processed solar-cells based on organic–inorganic methyl ammonium lead halide absorbers have achieved efficiencies in excess of 15%, which has superseded liquid dye sensitized cells, as well as various thin film-based photovoltaics. This report introduces a new metal-halide perovskite, based on the formamidinium cation (HC(NH2)2+), that displays a favorable band gap (1.47 eV) and represents a broader absorption compared to previously reported absorbers that contained the methylammonium cation (CH3NH3+). The high open-circuit voltage (Voc = 0.97 V) and promising fill-factor (FF = 68.7%) yield an efficiency of 4.3%, which make this material an excellent candidate for this new class of perovskite solar cell. This report also investigates the formation of a black trigonal (P3m1) perovskite polymorph and a yellow hexagonal nonperovskite (P63mc) polymorph. Further solar cell development would entail the stabilization of the black trigonal (P3m1) perovskite polymorph over the yellow hexagonal nonperovskite (P63mc) polymorph.

Cubic perovskite polymorph of strontium metasilicate at high pressures

By using a diamond-anvil cell (DAC) with laser heating technology, a cubic perovskite polymorph of SrSiO 3 has been synthesized at - 38 GPa and 1500-2000 K for the first time. The P-V data of this new phase give ambient temperate elastic constants of V 0 = 49.18(5) Å 3 , K0 = 211(3) GPa, respectively, when they are fitted against the Birch-Murnaghan equation of state with a fixed K0′ at 4. On decompression, the SrSiO 3 cubic perovskite phase becomes unstable at - 6.2 GPa and disappears completely at - 4.7 GPa. The transformed product can be considered as an amorphous phase with a minor amount of small sized crystals in the amorphous matrix. First principle calculations predicted structural properties of both the cubic and the six-layer-repeated hexagonal perovskite polymorphs of SrSiO 3 in good agreement with experimental results. The experimental and theoretical results indicate that the larger Sr 2+ cation can substitute the Ca 2+ cation and enter into the lattice of the cubic perovskite phase of CaSiO 3 at lower mantle conditions with only a small lattice strain. These results indicate that Sr can be hosted in cubic perovskite CaSiO 3 found as inclusions in diamonds originating from the lower mantle

Room temperature “one-pot” solution synthesis of nanoscale CsSnI3 orthorhombic perovskite thin films and particles

The orthorhombic perovskite polymorph of cesium tin iodide (B-γ-CsSnI3) has been shown to possess an interesting combination of electrical and optical properties. Here we show that a simple “one-pot” method can be used for the solution synthesis and processing of nanoscale B-γ-CsSnI3 at room temperature. This method entails the dissolution of SnI2 and CsI powders in a specific combination of polar solvents, which results in a clear solution. Evaporation of the solvents results in the formation of B-γ-CsSnI3 on substrates, either in the form of nanocrystalline thin films or nanoparticles depending on the concentration of the solution. Thus, it is not necessary to synthesize B-γ-CsSnI3 phase a priori by vacuum melting or solid-state reaction methods before it can be solution processed, as has been implied in other studies. This has important implications for the solution synthesis and processing of nanoscale B-γ-CsSnI3 phase which may find potential new uses in solid-state dye-sensitized solar cells and optoelectronic devices of the future.

Crystal structure, chemical expansion and phase stability of HoMnO3 at high temperature

Anisotropic thermal and chemical expansion of hexagonal HoMnO3 was investigated by high temperature X-ray diffraction in inert (N2) and oxidizing (air) atmospheres up to 1623K. A second order structural phase transition directly from P63cm to P63/mmc was found at 1298±4K in N2 atmosphere, and 1318±4K in air. For the low temperature polymorph P63cm the contraction of the c-axis was more rapid in inert than in oxidizing atmosphere. The c-axis of the P63/mmc polymorph of HoMnO3 displayed anomalously high expansion above 1400K, which is discussed in relation to chemical expansion caused by point defects. The a-axis expanded stronger in inert than oxidizing atmosphere. Anisotropic chemical and thermal expansion of the P63cm phase of YMnO3 in N2, air and O2 atmospheres was found to be qualitatively similar to that of HoMnO3. Decomposition of hexagonal HoMnO3 by two different processes occurs in oxidizing atmosphere above ∼1200K followed by nucleation and growth of the perovskite polymorph of HoMnO3. A rapid, reconstructive transition from the perovskite back to the hexagonal polymorph was observed in situ at 1623K upon reduction of the partial pressure of oxygen. A phase stability diagram of the hexagonal and orthorhombic polymorphs is proposed. Finally, distinctly non-linear electrical conductivity was observed for both HoMnO3 and YMnO3 in oxidizing atmosphere between 555 and 630K, and shown to be associated with excess oxygen.

A new cubic perovskite in PbGeO3 at high pressures

A new cubic perovskite polymorph of PbGeO3 (Phase II) was synthesized by laser heating in the diamond-anvil cell (DAC) at the pressure of 36 GPa. Fitting the Birch-Murnaghan equation of state against its observed P-V data yields a bulk modulus K0 of 196(6) GPa and the volume V0 of 56.70(13) Å3 when K0′ is assumed being 4. After the pressure is released, the PbGeO3 Phase II changes gradually into an amorphous phase, which contains mainly fourfold-coordinated germanium. It indicates that the PbGeO3 Phase II with a GeO6 octahedron framework transforms to a GeO4 tetrahedron network during the amorphization. The existence of PbGeO3 cubic perovskite Phase II at high pressures indicates that the polarized character of the Pb2+ ion induced by its 6s2 lone pair electrons would be totally reduced in the environment of major silicate perovskites inside the lower mantle, and thus the Pb atom would substitute the Ca atom to enter the CaSiO3 perovskite.

Doping influence on the stability of YMnO3 orthorhombic perovskite obtained by mechanosynthesis

Orthorhombic perovskite solid solutions with YMn1−xNixO3 and YMn1−xCoxO3 compositions (x=0, 0.05, 0.10, 0.15) have been prepared by mechanosynthesis from a mixture of oxides in a planetary mill. This method allows the use of high-pressure and high-temperature route to be avoided, which is the habitual method to prepare rare-earth manganites with small ionic radius. The high energy supplied by the milling induces the synthesis of the perovskite oxides with a high distortion due to the Jahn–Teller effect. The stability of the structure with temperature is increased with small amounts of Ni proving that the influence of the Jahn–Teller effect is more important than that of the tolerance factor on the stability of the perovskite polymorph. Such stability is not achieved by Co substitution, as this doping is not so efficient in the formation of Mn4+, non-Jahn–Teller cations. Magnetic studies were carried out to determine the properties of the materials obtained by this route.

An investigation of the polytypical structure of Sr0.2Ba0.8CoO3–δ by neutron powder diffraction

Abstract The preparation and characterization of two polymorphs of the title composition are described. One hexagonal perovskite, labeled as “H”, was synthesized by thermal treatment of reactive citrate precursor at 900 °C in high oxygen pressure (20 MPa) followed by slow cooling (10 °C/min) to room temperature. This 1D-structure displays a P63/mmc hexagonal space group with a thermal expansion of 13 × 10–6 K–1 between 100 and 900 °C. On the other hand, for the first time in such a Ba-rich composition, a highly oxygen-defective perovskite oxide cubic polymorph, labeled as “C”, was obtained from the citrate precursor heated in air and quenched in liquid N2 from 1000 °C and completely characterized. Neutron powder diffraction evidence that the obtained “C” phase posses a cubic Pm-3m space group and antiferromagnetic order at room temperature. The material can be formulated as Sr0.2Ba0.8CoO2.27(1) where the high number of oxygen vacancies are random distributed with large thermal factors of 3.6 Å2 suggesting a considerable mobility. The evolution of the crystal structure of the 3C phase has been explored in situ by XRD thermo-diffractometry in complement with thermal analysis and thermal expansion. At approx.500 °C this phase irreversibly transforms to a hexagonal “H” phase and at 925 °C a new cubic perovskite “C” is identified, which is transformed again, upon cooling, into the “H” phase at 840 °C.

Thin Film Synthesis and Structural Characterization of a New Kinetically Preferred Polymorph in the RE2Ti2O7 (RE = La−Y) Family

A new polymorph was found for all RE2Ti2O7 (RE = La−Y) compounds during their growth as epitaxial films on SrTiO3(110) substrates. Detailed structural characterization using X-ray and electron diffraction revealed that the new polymorph adopts an orthorhombic crystal structure with the space group Ammm and unit cell parameters (for La) of a = 7.89, b = 5.54, c = 13.45 A; the refined lattice parameters decreased monotonically with decreasing size of the RE cation. Though these parameters are similar to those of the known monoclinic (110)-layered perovskite polymorph, the structures are not related by simple distortive transformations. While both of these polymorphs can be formed as epitaxial films on SrTiO3(110) substrates at T ∼ 900 °C, the new polymorph is favored in conditions where diffusion is more limited between pulses. This implies that the new polymorph is kinetically favored during nucleation over the (110)-layered perovskite phase, in agreement with the smaller electroneutral unit of the new pol...

Control of polymorphism in NaNbO3by hydrothermal synthesis

The metastable ilmenite polymorph of NaNbO(3), instead of the expected perovskite polymorph, may be prepared directly in one step under mild hydrothermal conditions by lowering pH and using close to stoichiometric amounts of metal precursors; in situ energy-dispersive X-ray diffraction shows that crystallisation occurs rapidly via a sequence of intermediate crystalline phases.