Room Temperature Orthorhombic Phase Research Articles

Observation of the multiple magnetic phases in double perovskite Pr1.8La0.2CoFeO6

The structural and magnetic properties of the double perovskite Pr1.8La0.2CoFeO6 have been investigated. The X-ray diffraction study shows that the system acquires room-temperature orthorhombic phase with the Pnma space group. The X-ray photoemission spectroscopy (XPS) measurement confirmed that the B-site ions are present in mixed valence states i.e. Co has been found in Co2+ and Co3+ valance states whereas Fe has been found in Fe3+/Fe4+. Magnetic measurements of the system confirm the existence of several fascinating magnetic behaviors, such as long-range canted antiferromagnetism withTN ∼ 271 K, giant exchange bias, and re-entrant cluster glass phase (TG ∼ 33 K). Field-dependent magnetization shows a large spontaneous exchange bias; HSEB∼ 1.8 kOe and giant conventional exchange bias, HCEB∼ 2.4 kOe at 5 K. Antiferromagnetic materials with large exchange bias can be utilized in high-density spintronic devices. Temperature-dependent Raman study demonstrates that the observed phonon mode exhibits anomalous behavior close to the magnetic transition temperature. Analysis of anomalous softening of phonon mode below TN, clarifies the presence of significant spin-phonon coupling while the magnetostriction effect does not play any significant role in the observed phonon anomaly.

Finite-temperature properties of CsPbI3 thin films

${\mathrm{CsPbI}}_{3}$ is an inorganic perovskite solar cell material with a band gap appropriate for solar cell applications and photo conversion efficiency greater than 18%. However, this material is also known to undergo many phase transformations within the perovskite structure, starting from a high-temperature cubic phase to a tetragonal phase, leading to a room-temperature orthorhombic phase, with each phase having a different photovoltaic response. The tetragonal phase has been shown to have the highest photoconversion efficiency (18.4%), followed by the cubic phase (17%) and the orthorhombic phase (12.5%). In this work, we use the newly formulated first-principles--based effective Hamiltonian for ${\mathrm{CsPbI}}_{3}$ to investigate the phase transformation sequence in ${\mathrm{CsPbI}}_{3}$ thin films. We investigate the effect of film thickness and substrate strain on the phase transformations and the room-temperature phase. The effect of screening is also investigated. The three perovskite phases previously found in bulk by experiments and by computations were also found to exist in the thin film. It is shown that thickness and substrate strain can stabilize photoactive perovskite tetragonal and cubic phases at room temperature, both of which can be used for solar cell applications.

Thermal properties of single crystals of the low-positive thermal expansion material Al2W3O12

Investigations of the thermal conductivity and heat capacity of a monocrystalline sample Al2W3O12, along with computational investigations using density functional theory, reveal a complete picture of anisotropic thermoelastic properties of Al2W3O12, a low-positive thermal expansion material. The thermal conductivity at room temperature was essentially isotropic, with a directionally average value of 2.1 ± 0.2 W m−1 K−1, about twice the value reported for polycrystalline samples. The thermal conductivity shows a significant peak at about 20 K, and at 5 K corresponds to a phonon mean free path of about 2000 nm, much shorter than that of a typical high-quality oxide crystal with a more rigid framework. The flexible Al2W3O12 structure also gives rise to low-frequency modes, as evidenced by the high heat capacity in comparison with that of the constituent oxides. The thermal shock resistance figures of merit for moderate and severe thermal shock are both anisotropic, but high, with directionally average values of about 6500 W m−1 and 3300 K, respectively. An additional finding was that the phase transformation from the low-temperature monoclinic phase to the room-temperature orthorhombic phase shows considerable hysteresis and, evidenced by DSC on shards of single crystals, nucleation is a stochastic process.

CeTaN3 and CeNbN3: Prospective Nitride Perovskites with Optimal Photovoltaic Band Gaps

Perovskites constitute an exceptionally tunable materials family with diverse applications in electronics, optoelectronics, energy, and quantum technologies. Out of the thousands of known perovskites, the majority of compounds are oxides, halides, and chalcogenides. In contrast, only two nitride perovskites are currently known. In this work we perform a thorough ab initio computational screening of possible nitride perovskites, and we identify two new compounds, CeNbN3 and CeTaN3, with band gaps in the near-infrared to visible range, depending on temperature. In their room-temperature orthorhombic phase, we predict that both compounds exhibit direct or quasidirect band gaps in the range 1.1–2.0 eV, with the Pnma phases matching the Shockley–Queisser limit for photovoltaic energy conversion efficiency. These compounds are also predicted to be strong light absorbers, with absorption coefficients surpassing those of high-performance semiconductors such as GaAs and CH3NH3PbI3. The present findings reveal a potentially new class of nitride semiconductors with promise for electronics, optoelectronics, and light harvesting and for integration with existing nitride-based lighting technology.

Lattice parameters and electronic bandgap of orthorhombic potassium sodium niobate K_{0.5}Na_{0.5}NbO_{3} from density-functional theory

We perform a theoretical analysis of the structural and electronic properties of sodium potassium niobate K_{1-x}Na_{x}NbO_{3} in the orthorhombic room-temperature phase, based on density-functional theory in combination with the supercell approach. Our results for x=0 and x=0.5 are in very good agreement with experimental measurements and establish that the lattice parameters decrease linearly with increasing Na contents, disproving earlier theoretical studies based on the virtual-crystal approximation that claimed a highly nonlinear behavior with a significant structural distortion and volume reduction in K_{0.5}Na_{0.5}NbO_{3} compared to both end members of the solid solution. Furthermore, we find that the electronic bandgap varies very little between x=0 and x=0.5, reflecting the small changes in the lattice parameters.Graphic abstract

Layered Arrangement of 1D Wavy Chains in the Lead‐Free Hybrid Perovskite (PyrCO2H)2BiI5: Structural Investigations and Properties

AbstractLead free organic‐inorganic hybrid perovskites have attracted increasing attention because of their variety of applications such as photovoltaics or lighting. Here, we reported a new iodobismuthate organic‐inorganic perovskite (PyrCO2H)2BiI5 where PyrCO2H+ is the 4‐carboxy‐1‐methylpyridinium. In the room temperature orthorhombic phase, the one‐dimensional (1D) inorganic framework which consists of cis‐connected corner‐sharing BiI6 octahedra, exhibits a sinusoidal deformation along the chain axis, while a regular chain leading to a sub‐cell is found in the high temperature phase (T>390 K). An original structural feature is the relative position of consecutive 1D chains in planes leading to a layered character of this organic‐inorganic compound. The electrical properties as function frequencies and temperatures have been studied in particular allowing the extraction of Rg and further the calculation of σg=f(T). Interestingly, a change of activation energy is detected at the phase transition temperature (390 K). The higher value of activation energy above 390 K can be attributed to change of the conduction mechanism involving ionic conduction.

Comment on “Phase transitions, screening and dielectric response of CsPbBr3” by Š. Svirskas, S. Balčiūnas, M. Šimėnas, G. Usevičius, M. Kinka, M. Velička, D. Kubicki, M. E. Castillo, A. Karabanov, V. V. Shvartsman, M. R. Soares, V. Šablinskas, A. N. Salak, D. C. Lupascu and J. Banys, J. Mater. Chem. A, 2020, 8, 14015

A recently published report on a new low-temperature phase transition in CsPbBr3 was verified. We provide strong evidence that the room-temperature orthorhombic phase of the crystal is stable at least up to 95 K.

Inkjet Printing All Inorganic Halide Perovskite Inks for Photovoltaic Applications.

A method for synthesizing photoactive inorganicperovskite quantum dot inks and an inkjet printer deposition method, using the synthesized inks, are demonstrated. The ink synthesis is based on a simple wet chemical reaction and the inkjet printing protocol is a facile step by step method. The inkjet printed thin films have been characterized by X-ray diffraction, optical absorption spectroscopy, photoluminescent spectroscopy, and electronic transport measurements. X-ray diffraction of the printed quantum dot films indicates a crystal structure consistent with an orthorhombic room temperature phase with (001) orientation. In conjunction with other characterization methods, the X-ray diffractionmeasurements show high quality films can be obtained through the inkjet printing method.

On the “inverted” phase transitions in ferroic crystals containing propylenediammonium cations

The method of preparation of [NH3(CH2)3NH3]CuCl4 and [NH3(CH2)3NH3]CdBr4 crystals is given, and optical and dilatometric studies are presented. Optical polarized microscopic observation revealed multidomain states at room temperature orthorhombic phase with domain walls in (101) and (10-1) planes. The temperature dependence of the linear birefringence and the thermal expansion coefficient were measured and critical indices α and β were determined for the [NH3(CH2)3NH3]CdBr4 crystal. The linear birefringence and domain structure studies revealed the single first order transition in the [NH3(CH2)3NH3]CuCl4 crystal.

Structural phase transition in a perovskite-type NH3(CH2)3NH3CuCl4 crystal – X-ray and optical studies

ABSTRACTX-ray powder studies and optical studies (polarized microscopic observation and linear birefringence studies) of the crystal NH3(CH2)3NH3CuCl4 are presented. The X-ray powder studies revealed a change of symmetry from orthorhombic room-temperature phase to monoclinic phase above 434 K. A reversible phase transition of the first order at 434 K on heating and 432 K on cooling was observed in birefringence studies. Optical polarized microscopic observation revealed monodomain and multidomain states in the room-temperature orthorhombic phase with domain walls in (110) and (1-10) planes. The hypothetical prototypic phase is expected to be tetragonal. The change of symmetry from orthorhombic to monoclinic and expected domain structure was found above 434 K in the (010) plane.

DFT and k · p modelling of the phase transitions of lead and tin halide perovskites for photovoltaic cells

Abstract3D hybrid organic perovskites, CH3NH3PbX3 (X = halogen), have recently been used to strongly improve the efficiency of dye sensitized solar cells (DSSC) leading to a new class of low‐cost mesoscopic solar cells. CsSnI3 perovskite can also be used for hole conduction in DSSC. Density functional theory and GW corrections are used to compare lead and tin hybrid and all‐inorganic perovskites. The room temperature optical absorption is associated to electronic transitions between the spin–orbit split‐off band in the conduction band and the valence band. Spin–orbit coupling is about three times smaller for tin. Moreover, the effective mass of relevant band edge hole states is small (0.17). The high temperature phase sequence of CsSnI3 leading to the room temperature orthorhombic phase and the recently reported phases of CH3NH3MI3 (where M = Pb, Sn) close to the room temperature, are also studied. Tetragonal distortions from the ideal cubic phase are analysed by a k · p perturbation, including spin–orbit effect. In addition, the non‐centrosymmetric phases of CH3NH3MI3 exhibit a splitting of the electronic bands away from the critical point. The present work shows that their physical properties are more similar to conventional semiconductors than to the absorbers used in DSSC. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The magnetocaloric performance in pure and mixed magnetic phase CoMnSi

Here we study the influence of sample preparation on the magnetocaloric properties of CoMnSi. Slow cooling from the high temperature hexagonal phase of the melt to the room temperature orthorhombic phase encourages the formation of a homogeneous material with large entropy changes when the system undergoes a coincident first order structural and (meta)magnetic transition. Samples that were quenched directly after annealing show a compressed a axis lattice parameter. Hall probe imaging indicates that the quenched sample has spatially inhomogeneous magnetic properties, which we attribute to strain because within error neither x-ray diffraction nor energy dispersive x-ray analysis indicates a second compositional phase. Calorimetric methods and global magnetization are used to examine the entropy changes of the pure and mixed magnetic phase compounds and we make a direct comparison of these materials in terms of their refrigerant capacity.

Lattice dynamics of room temperature orthorhombic SrHfO 3

The lattice dynamics of room temperature orthorhombic phase of SrHfO 3 has been computed, using density functional perturbation theory. The full set of zone center phonon frequencies and infrared reflectivity spectra were reported, providing benchmark theoretical results for the interpretation of future experimental phonon measurements. It is found that the static dielectric constant is relatively anisotropic. This anisotropy has been discussed in relationship to the oscillator strengths and contributions of the lowest IR active modes.

The phase diagram [formula omitted]– [formula omitted

The phase diagram gadolinium fluoride–lutetium fluoride was determined by differential scanning calorimetry (DSC) and X-ray powder diffraction analysis. Both pure components undergo a reversible first order transformation to a high temperature phase. The mutual solubility of both components is unlimited in the orthorhombic room temperature phase. The maximum solubility of Lu in the high temperature phase of GdF 3 (tysonite type) is about 20% and the maximum solubility of Gd in LuF 3 ( α - YF 3 type) is about 40%. Intermediate compositions of the low temperature phase decompose upon heating in a peritectoid reaction to a mixture of both high temperature phases.

Raman spectroscopic study of [formula omitted

The dielectric constant of polycrystalline LiH 2PO 4 has been measured between 297 and 17 K. No marked changes were observed over this range, indicating that the room-temperature orthorhombic phase persisted up to 17 K. Raman spectra of polycrystalline LiH 2PO 4 were also measured at 297, 200, and 70 K in the frequency shift region of 15–4000 cm −1 with Raman-active vibrational modes naively assigned to low-frequency (0–300 cm −1) external and high-frequency (300–4000 cm −1) internal modes. In addition to the internal modes of the PO 4 tetrahedra, the internal modes of the LiO 4 tetrahedra spectroscopically manifested themselves between 390–500 cm −1. This frequency range overlaps those of ν 2 (PO 4) and ν 4 (PO 4). The LiH 2PO 4O–H vibrational frequencies were in good agreement with crystallographic reports that there are two types of hydrogen bonds: intermediate (long bonds) and strong (short bonds).

The phase diagram YF 3–GdF 3

The binary phase diagram YF 3–GdF 3 was studied by differential scanning calorimetry (DSC). Yttrium fluoride and gadolinium fluoride show complete miscibility in all three phases (orthorhombic room temperature phase, trigonal or hexagonal high temperature phase, liquid). The transformations between room temperature and high temperature phases are of first order and occur at 1338.6 K (YF 3) or 1174.8 K (GdF 3). Melting points are 1403.1 K (YF 3) or 1525.7 K (GdF 3), respectively. The c p ( T ) curve of GdF 3 shows a λ shaped local maximum at 1333 K that might be related to a further solid phase transformation of second order.

超イオン導電体Ｃｕ８ＧｅＳ６およびＡｇ７ＴａＳ６の低温相の構造とイオン伝導パス：六次元解析法による多重双晶の解析

The structures of the orthorhombic room-temperature phase of Cu8GeS6 (phase II) and the monoclinic low-temperature phase of Ag7TaS6 (phase II) have been successfully refined based on X-ray diffraction data from 12-fold twinned (Cu8GeS6II) and 24-fold twinned (Ag7TaS6II) crystals. Respectively among 6 major and 6 minor twin domains of Cu8GeS6II, or among 12 major and 12 minor twin domains of Ag7TaS6II, the argyrodite-type frameworks, GeS6or TaS6, can be superposed to each other in principle, and only Cu-Cu or Ag-Ag network directions differ. At higher temperature, the crystals were considered to be 2-fold twinned crystals of superionic-conductor phase I with a space groupF43m. On cooling, each domain transforms into 6 domains of orthorhombic Cu8GeS6II or 12 domains of monoclinic Ag7TaS6II. Superposed projections along 6 directions of the structure of Cu8GeS6II and along 12 directions of the structure of Ag7TaS6II seem to show approximate expressions for Cu-ion and Ag-ion conduction paths in superionic-conductor phases, Cu8GeS6I and Ag7TaS6I.

PHASE TRANSITIONS IN TERNARY CAESIUM LEAD BROMIDE

Phase transitions in ternary caesium lead bromide (CsPbBr3) were studied by means of DSC, TMA and high temperature X-ray diffraction. The samples were prepared from the solution by water evaporation and from the melt. on the DSC curves as well as on the temperature dependence of the lattice constants of CsPbBr3 only two effects were found belonging to the earlier published phase transitions at 88 and 130°C and no further effects. Linear thermal expansion coefficient α of individual CsPbBr3 modifications were calculated from both TMA and high temperature X-ray diffraction. The structural parameters of the room temperature orthorhombic phase were refined and the results are presented. CsPbBr3 prepared from the solution contained about 10% of CsPb2Br5 and so the DSC curve of pure CsPb2Br5 was also measured and an effect at a temperature of 68.5°C was found.

Multiple-twinned crystal of the room-temperature phase of Cu 8GeS 6

The structure of the orthorhombic room-temperature phase of Cu 8GeS 6 (phase II) has been refined successfully on the basis of X-ray diffraction data from a 12-fold twinned crystal. The symmetry operations and the unit cell of the pseudo-cubic expression are (0,0,0; 1/2,1/2,0)+ x, y, z; y, x, z; 1/4− x,3/4− y,1/2+ z; 3/4− y,1/4− x,1/2+ z; a= b=9.907 Å, c=9.870 Å, α= β=90°, γ=90.64°, whose standard settings are Pmn2 1; A=7.045, B=6.966, C=9.870 Å. Among the twin domains 1–6, the argyrodite-type GeS 6 frameworks can be superposed to each other in principle, and only Cu–Cu network directions differ. The minor twin domains 7–12 are respectively related to the major domains 1–6 by a two-fold rotation on [111]. In the high-temperature phase (phase I) of Cu 8GeS 6, Cu ions are expected to distribute statistically over many sites in the framework. On cooling through the phase-transition temperature, the crystal can be converted into a multiple twin of phase II. Cu ions are transferred to five kinds of independent sites from the statistical distribution state.

Structure refinement of Cu8GeS6 using X-ray diffraction data from a multiple-twinned crystal.

The structure of the orthorhombic room-temperature phase of Cu(8)GeS(6) (copper germanium sulfide), M(r) = 773.27, has been refined on the basis of X-ray diffraction data from a 12-fold twinned crystal applying a six-dimensional twin refinement technique. For 1804 unique reflections measured using Mo Kalpha radiation, R(F) was 0.083 with 77 structure parameters and 12 scale factors. The symmetry operations, the unit cell and other crystal data are (0, 0, 0; (1/2), (1/2), 0) + x, y, z; y, x, z; (1/4) - x, (3/4) - y, (1/2) + z; (3/4) - y, (1/4) - x, (1/2) + z; a = b = 9.9073 (3) Å, c = 9.8703 (4) Å, alpha = beta = 90 degrees, gamma = 90.642 (4) degrees; V = 968.7 (1) Å(3), Z = 4, D(x) = 5.358 Mg m(-3), µ = 21.70 mm(-1). The standard setting of the space group and the reduced unit cell are Pmn2(1); a = 7.0445 (3), b = 6.9661 (3), c = 9.8699 (5) Å; Z = 2.