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At least 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Materials Data on Cs(Nb2Cl5)3 by Materials Project

Cs(Nb2Cl5)3 crystallizes in the orthorhombic Pmma space group. The structure is three-dimensional. Cs1+ is bonded in a distorted q6 geometry to ten Cl1- atoms. There are a spread of Cs–Cl bond distances ranging from 3.39–3.56 Å. There are five inequivalent Nb+2.33+ sites. In the first Nb+2.33+ site, Nb+2.33+ is bonded to five Cl1- atoms to form distorted corner-sharing NbCl5 square pyramids. There are a spread of Nb–Cl bond distances ranging from 2.47–2.84 Å. In the second Nb+2.33+ site, Nb+2.33+ is bonded to five Cl1- atoms to form corner-sharing NbCl5 square pyramids. There are a spread of Nb–Cl bond distances ranging from 2.46–2.74 Å. In the third Nb+2.33+ site, Nb+2.33+ is bonded to five Cl1- atoms to form corner-sharing NbCl5 square pyramids. There are a spread of Nb–Cl bond distances ranging from 2.47–2.73 Å. In the fourth Nb+2.33+ site, Nb+2.33+ is bonded to five Cl1- atoms to form corner-sharing NbCl5 square pyramids. There are four shorter (2.49 Å) and one longer (2.74 Å) Nb–Cl bond lengths. In the fifth Nb+2.33+ site, Nb+2.33+ is bonded to five Cl1- atoms to form corner-sharing NbCl5 square pyramids. There are four shorter (2.47 Å) and one longer (2.76 Å) Nb–Cl bond lengths. There are eleven inequivalent Cl1- sites. In the first Cl1- site, Cl1- is bonded in a rectangular see-saw-like geometry to two equivalent Cs1+ and two equivalent Nb+2.33+ atoms. In the second Cl1- site, Cl1- is bonded in a distorted square co-planar geometry to two equivalent Cs1+ and two Nb+2.33+ atoms. In the third Cl1- site, Cl1- is bonded in a 2-coordinate geometry to two equivalent Nb+2.33+ atoms. In the fourth Cl1- site, Cl1- is bonded in a 2-coordinate geometry to one Cs1+ and two Nb+2.33+ atoms. In the fifth Cl1- site, Cl1- is bonded in a 3-coordinate geometry to one Cs1+ and two Nb+2.33+ atoms. In the sixth Cl1- site, Cl1- is bonded in a 3-coordinate geometry to one Cs1+ and two Nb+2.33+ atoms. In the seventh Cl1- site, Cl1- is bonded in a 3-coordinate geometry to one Cs1+ and two Nb+2.33+ atoms. In the eighth Cl1- site, Cl1- is bonded in a 2-coordinate geometry to two equivalent Nb+2.33+ atoms. In the ninth Cl1- site, Cl1- is bonded in a 2-coordinate geometry to two equivalent Nb+2.33+ atoms. In the tenth Cl1- site, Cl1- is bonded in a 2-coordinate geometry to two equivalent Nb+2.33+ atoms. In the eleventh Cl1- site, Cl1- is bonded in a distorted bent 150 degrees geometry to two Nb+2.33+ atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Cs(Bi2Te3)2 by Materials Project

CsBi4Te6 crystallizes in the monoclinic C2/m space group. The structure is three-dimensional. there are two inequivalent Cs1+ sites. In the first Cs1+ site, Cs1+ is bonded in a 10-coordinate geometry to ten Te2- atoms. There are a spread of Cs–Te bond distances ranging from 3.88–4.29 Å. In the second Cs1+ site, Cs1+ is bonded in a 9-coordinate geometry to nine Te2- atoms. There are a spread of Cs–Te bond distances ranging from 3.80–4.21 Å. There are eight inequivalent Bi+2.75+ sites. In the first Bi+2.75+ site, Bi+2.75+ is bonded to six Te2- atoms to form BiTe6 octahedra that share corners with three BiTe6 octahedra, a cornercorner with one BiTe5 square pyramid, and edges with seven BiTe6 octahedra. The corner-sharing octahedral tilt angles are 7°. There are a spread of Bi–Te bond distances ranging from 3.03–3.45 Å. In the second Bi+2.75+ site, Bi+2.75+ is bonded to six Te2- atoms to form a mixture of corner and edge-sharing BiTe6 octahedra. The corner-sharing octahedra tilt angles range from 2–7°. There are a spread of Bi–Te bond distances ranging from 3.14–3.27 Å. In the third Bi+2.75+ site, Bi+2.75+ is bonded to six Te2- atoms to form BiTe6 octahedra that share corners with three BiTe6 octahedra, a cornercorner with one BiTe5 square pyramid, and edges with eleven BiTe6 octahedra. The corner-sharing octahedral tilt angles are 7°. There are a spread of Bi–Te bond distances ranging from 3.09–3.32 Å. In the fourth Bi+2.75+ site, Bi+2.75+ is bonded to six Te2- atoms to form BiTe6 octahedra that share corners with three BiTe6 octahedra, edges with seven BiTe6 octahedra, and edges with four BiTe5 square pyramids. The corner-sharing octahedra tilt angles range from 2–7°. There are a spread of Bi–Te bond distances ranging from 3.04–3.44 Å. In the fifth Bi+2.75+ site, Bi+2.75+ is bonded to six Te2- atoms to form a mixture of corner and edge-sharing BiTe6 octahedra. The corner-sharing octahedra tilt angles range from 5–15°. There are a spread of Bi–Te bond distances ranging from 3.04–3.45 Å. In the sixth Bi+2.75+ site, Bi+2.75+ is bonded to six Te2- atoms to form BiTe6 octahedra that share corners with three BiTe6 octahedra, edges with five BiTe6 octahedra, and edges with two equivalent BiTe5 square pyramids. The corner-sharing octahedra tilt angles range from 7–15°. There are a spread of Bi–Te bond distances ranging from 3.00–3.40 Å. In the seventh Bi+2.75+ site, Bi+2.75+ is bonded to five Te2- atoms to form BiTe5 square pyramids that share a cornercorner with one BiTe6 octahedra, corners with two equivalent BiTe5 square pyramids, edges with two equivalent BiTe6 octahedra, and edges with three BiTe5 square pyramids. The corner-sharing octahedral tilt angles are 10°. There are a spread of Bi–Te bond distances ranging from 3.06–3.45 Å. In the eighth Bi+2.75+ site, Bi+2.75+ is bonded to five Te2- atoms to form BiTe5 square pyramids that share a cornercorner with one BiTe6 octahedra, corners with two equivalent BiTe5 square pyramids, edges with four BiTe6 octahedra, and edges with three BiTe5 square pyramids. The corner-sharing octahedral tilt angles are 6°. There are a spread of Bi–Te bond distances ranging from 3.09–3.55 Å. There are twelve inequivalent Te2- sites. In the first Te2- site, Te2- is bonded to six Bi+2.75+ atoms to form TeBi6 octahedra that share a cornercorner with one TeBi6 octahedra, corners with two equivalent TeCsBi4 trigonal bipyramids, edges with eight TeBi6 octahedra, and edges with three TeCsBi4 trigonal bipyramids. The corner-sharing octahedral tilt angles are 7°. In the second Te2- site, Te2- is bonded to one Cs1+ and four Bi+2.75+ atoms to form distorted TeCsBi4 trigonal bipyramids that share corners with three TeCs2Bi4 octahedra, corners with two equivalent TeCsBi4 trigonal bipyramids, and edges with three TeBi6 octahedra. The corner-sharing octahedra tilt angles range from 7–13°. In the third Te2- site, Te2- is bonded in a 7-coordinate geometry to three Cs1+, two equivalent Bi+2.75+, and two equivalent Te2- atoms. Both Te–Te bond lengths are 3.58 Å. In the fourth Te2- site, Te2- is bonded to two equivalent Cs1+ and four Bi+2.75+ atoms to form TeCs2Bi4 octahedra that share corners with two equivalent TeBi6 octahedra, corners with four TeCsBi4 trigonal bipyramids, and edges with seven TeBi6 octahedra. The corner-sharing octahedral tilt angles are 3°. In the fifth Te2- site, Te2- is bonded to six Bi+2.75+ atoms to form TeBi6 octahedra that share corners with four TeBi6 octahedra, edges with nine TeBi6 octahedra, and edges with two equivalent TeCsBi4 trigonal bipyramids. The corner-sharing octahedra tilt angles range from 1–7°. In the sixth Te2- site, Te2- is bonded to six Bi+2.75+ atoms to form TeBi6 octahedra that share a cornercorner with one TeBi6 octahedra, corners with two equivalent TeCsBi4 trigonal bipyramids, edges with four TeBi6 octahedra, and edges with three TeCsBi4 trigonal bipyramids. The corner-sharing octahedral tilt angles are 1°. In the seventh Te2- site, Te2- is bonded to one Cs1+ and four Bi+2.75+ atoms to form distorted TeCsBi4 trigonal bipyramids that share corners with five TeBi6 octahedra, corners with two equivalent TeCsBi4 trigonal bipyramids, and edges with five TeBi6 octahedra. The corner-sharing octahedra tilt angles range from 5–35°. In the eighth Te2- site, Te2- is bonded in a 7-coordinate geometry to three Cs1+, two equivalent Bi+2.75+, and two equivalent Te2- atoms. In the ninth Te2- site, Te2- is bonded in a 7-coordinate geometry to three equivalent Cs1+, two equivalent Bi+2.75+, and two equivalent Te2- atoms. Both Te–Te bond lengths are 3.67 Å. In the tenth Te2- site, Te2- is bonded in a 5-coordinate geometry to two equivalent Cs1+ and three Bi+2.75+ atoms. In the eleventh Te2- site, Te2- is bonded in a 6-coordinate geometry to four equivalent Cs1+ and two equivalent Bi+2.75+ atoms. In the twelfth Te2- site, Te2- is bonded in a 5-coordinate geometry to five Bi+2.75+ atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Characterization of Cs 3 Sb photocathodes at cryogenic temperatures

Here, we report measurements of quantum efficiency (QE) and mean transverse energy (MTE) from Cs 3 Sb photocathodes in a wide range of photon energies at both room and cryogenic temperatures. Our measurements show a strong temperature dependence of MTE even at photon energies well above threshold, indicating the presence of strong inelastic scattering of excited electrons during transport before emission into vacuum. We also demonstrate a cathode cooling method that largely preserves the QE while reducing MTE, allowing us to achieve MTEs as low as 58 meV with 3% QE in green light from Cs 3 Sb photocathodes. Our results are crucial for producing brighter electron beams for various photoinjector applications like ultrafast electron diffraction and microscopy, x-ray free-electron lasers, and particle colliders.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Ion Exchange of Selected Group II Metals and Lead by Crystalline Silicotitanate and Competition for Cs Exchange Sites

A series of batch contact tests were conducted to evaluate the exchange behavior of Ba, Ca, Pb, and Sr onto crystalline silicotitanate (CST) in support of an expedited Cs removal and pretreatment system at the Hanford site. Binary Na/M 2+ and ternary Na/Cs/M 2+ isotherms were generated to understand selectivity, capacity, and competitive impact of each analyte on Cs uptake from a simple 1 M NaOH/4.6 M NaNO 3 simulant. Analyte loading from a 0.1 M NaOH/5.5 M NaNO 3 simulant was assessed to determine the effect of hydroxide concentration on binary Na/M 2+ isotherms. Finally, results from binary and ternary isotherms indicated that group II metals, and Pb do not impact CST performance toward CST at concentrations expected in Hanford tank waste supernate.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Carbon Storage Technical Viability Approach (CS TVA) Data Availability Results Database

The Carbon Storage Technical Viability Approach (CS TVA) Data Availability Results Database v1.0 is the output of a data availability assessment workflow that was developed by NETL to create a tool for spatial data availability assessment. This results database is the output of the first version of the workflow performed on the data with the CS TVA Database v2.0.

carbon storage↗
DOE OSTI2025

Carbon Storage Technical Viability Approach (CS TVA) Matrix

The Carbon Storage Technical Viability Approach (CS TVA) Matrix is a knowledge framework developed to outline the information needed for geologic carbon storage. The CS TVA Matrix contains 5 categories, 14 sub-categories, and 47 components. This framework can be leveraged to assess the availability of data and information needed for a carbon storage project. The information categories of the matrix are tied to a list of required data using weighted mapping, published herein.

carbon storage↗
DOE OSTI2022

Luminescence from Self‐Trapped Excitons and Energy Transfers in Vacancy‐Ordered Hexagonal Halide Perovskite Cs 2 HfF 6 Doped with Rare Earths for Radiation Detection

Abstract Compared to halides Cs 2 HfX 6 (X = Cl, Br, I) with a vacancy‐ordered cubic double perovskite structure, the halide Cs 2 HfF 6 (CHF), with a hexagonal Bravais lattice, possesses a higher mass density and chemical stability for radiation detection. Luminescence properties and energy transfer mechanisms of rare‐earths‐doped CHF materials are studied here. The structure of CHF is identified as a new type of vacancy‐ordered hexagonal perovskite, with the same type of building blocks of the double perovskite but stacked with single layers. Density‐functional theory calculations reveal a large bandgap of CHF. A broad emission is observed from the pristine CHF host, which is suggested to be associated with self‐trapped excitons (STEs). A series of rare‐earths‐doped materials are designed utilizing the STE emissions, and efficient energy transfers from STEs and Tb 3+ to Eu 3+ are achieved for tunable emissions. The codoped material shows stable emission under X‐ray irradiation, with 10.2% reduction from its initial emission intensity, associated with possible structural evolution by radiation‐induced deformation of the soft host. The radiation responses of singly and codoped materials are evaluated, and the codoped material is found to be more sensitive to the radiation energy than the singly doped or pristine CHF for radiation detection.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Improved field-portable system to measure Cs-137 in wildlife

Here, we have developed an improved system to measure Cs-137 in wildlife at the Savannah River Site. This field-portable system consists of a shielded 5 cm by 10 cm by 40 cm NaI detector controlled by an Ametek Ortec Digibase. Measurement of an animal’s radioactivity is made by placing the animal at a predefined location on the detector system for a one minute count-time. The counts, animal type, and animal weight are then used as inputs to an algorithm which calculates the amount of Cs-137 within the whole animal and within the edible meat portion of the animal. The results from these calculations are used to estimate the received dose from eating this animal and is included in the Savannah River Site’s Hunter Dose Tracking System. This system has a detection limit of 0.60 pCi/g (22.20 Bq/kg) with a typical measurement uncertainty of less than 0.32 pCi/g (11.84 Bq/kg).

63 RADIATION, THERMAL, AND OTHER ENVIRON. POLLUTAN↗
DOE OSTI2020

Crystal growth and scintillation properties of pure and Tl-doped Cs 3 Cu 2 I 5

Here, the Bridgman crystal growth and scintillation properties of both undoped and Tl-doped Cs 3 Cu 2 I 5 are presented. This material is very attractive for gamma and X-ray detection applications, with a density of 4.53 g/cm 3 and effective atomic number of 51.9. Undoped Cs 3 Cu 2 I 5 had a light yield of 41,500 photons/MeV, with an energy resolution of 4.4% at 662 keV. Thallium doping at 0.5 mol % resulted in a much-improved scintillation response, in which light yield increased to 98,200 photons/MeV and energy resolution reduced to 3.3% at 662 keV. The X-ray excited emission is centered at 442 nm for the undoped and 500 nm for the Tl-doped crystals. The undoped emission is broad, typical of excitonic emission, while thallium doping results in an even broader band with features of both the undoped and thallium defect-mediated emissions.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Probing the limits of statistical neutron capture for the r process: Experimental constraints on 141 Cs nuclear level densities

The r-process abundance peaks, particularly near mass number A ∼ 130, reflect underlying nuclear structure effects such as closed neutron shells, yet modeling the nucleosynthesis in this region remains hindered by uncertain neutron-capture rates. These rates are especially sensitive to nuclear level densities (NLDs) and γ-ray strength functions of neutron-rich nuclei, where experimental data are scarce. We present the first experimental constraint on the NLD of 141 Cs using the β-Oslo method, extending sensitivity to the neutron-rich regime near the N = 82 closed shell. Our data allow for critical calibration of microscopic NLD models and reveal that 141 Cs lies near the limit of statistical model applicability. Using this experimental input, we evaluate radiative neutron-capture rates across neighboring isotones using both Hauser–Feshbach (HF) and High Fidelity Resonance (HFR) models. Our results show order-of-magnitude rate increases for nuclei along the N = 86 line, signaling a transition to resonance-dominated capture in this region. These findings underscore the importance of constraining NLDs to improve r-process reaction network predictions, particularly in environments where the validity of statistical models breaks down.

Nuclear level density↗
DOE OSTI2022

ACuZrQ 3 (A = Rb, Cs; Q = S, Se, Te): Direct Bandgap Semiconductors and Metals with Ultralow Thermal Conductivity

ACuZrQ 3 (A = Rb, Cs; Q = S, Se, Te) were synthesized as black platelet crystals. RbCuZrS 3 , RbCuZrSe 3 , and CsCuZrS 3 crystallize in the KCuZrSe 3 structure type with space group Cmcm, and RbCuZrTe 3 and CsCuZrTe 3 crystallize in the lower symmetry space group Pnma. The tellurides exhibit a second order Jahn-Teller distortion with off-centering of Zr in its octahedral environment. The magnitude of the distortion is larger in RbCuZrTe 3 than in CsCuZrTe 3 . The structures of beta-CsCuS 4 and Rb 2 Cu 5 Te 5 were also determined. CsCuZrS 3 melts at 910 C-circle and exhibits partial decomposition upon heating at 275 C-circle, while CsCuZrTe 3 melts incongruently. Our DFT calculations of RbCuZrQ 3 (Q = S, Se) and CsCuZrS 3 indicate direct gap semiconductors in agreement with experiments. ACuZrTe 3 (A = Rb, Cs) were calculated to be metals which was confirmed for RbCuZrTe 3 with variable temperature conductivity measurements and consistent with heat capacity measurements. Spectroscopic measurements found a bandgap and work function of 1.44(5) eV and 4.89(5) eV for RbCuZrS 3 and 0.95(5) eV and 4.67(5) eV for RbCuZrSe 3 , respectively. Finally, RbCuZrTe 3 did not exhibit an optical bandgap and has a work function of 4.64(5) eV. RbCuZrTe 3 exhibits a low thermal conductivity under 0.5 W m -1 K -1 at room temperature.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Structure and Stability of the Iodide Elpasolite, Cs 2 AgBiI 6

Iodide elpasolites (or double perovskites, A 2 B'B"I 6 , B' = M + , B" = M 3+ ) are predicted to be promising alternatives to lead-based perovskite semiconductors for photovoltaic and optoelectronic applications, but no iodide elpasolite has ever been definitively prepared or structurally characterized. Iodide elpasolites are widely predicted to be unstable due to favorable decomposition to the competing A 3 B 2 I 9 (B = M 3+ ) phase. Here, we report the results of synchrotron XRD and X-ray total scattering measurements on putative Cs 2 AgBiI 6 nanocrystals made via anion exchange from parent Cs 2 AgBiBr 6 nanocrystals. Rietveld refinement of XRD and PDF data shows that these nanocrystals indeed exhibit a tetragonal ($I$$\overline{4}$$m$) elpasolite structure, making them the first example of a structurally characterized iodide elpasolite. In conclusion, a series of experiments probing structural relaxation and the effects of surface ligation or grain size all point to the critical role of surface free energy in stabilizing the iodide elpasolite phase in these nanocrystals.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Cs X Si 15 P 21 ( X = Sn or Pb): Polar Noncentrosymmetric Si–P Frameworks Stabilized by Covalent X –P Bonding

Metal silicon phosphides composed of earth-abundant Si and P tend to exhibit semiconducting properties and adopt diverse crystal structures with relatively small additions of structure-directing elements. The potential of silicon phosphide materials in nonlinear optical applications has been hindered by the inability to systematically produce noncentrosymmetric structures with such a flexible framework. Here, in this work, two isostructural compounds with a novel noncentrosymmetric structure were made possible by the inclusion of elements with stereochemically active lone pairs (Sn 2+ and Pb 2+ ). The structures were determined through single-crystal and synchrotron powder X-ray diffraction. Analysis of chemical bonding in real space through the electron localization function revealed stereochemically active Pb 2+ and Sn 2+ species in a trigonal pyramidal coordination with {Pb/Sn}–P bonds. Such covalent bonding between Pb and P is quite uncommon in extended solids and has been reported in a few rare instances. Band structure calculations and linear optical measurements confirm the semiconducting nature of Cs X Si 15 P 21 ( X = Sn or Pb). The synthesis was optimized to yield high-purity polycrystalline samples. The nonlinear optical properties show promising second-harmonic generation (SHG) coefficients from the Kurtz–Perry method. First-principles calculations of the nonlinear optical properties support the experimentally determined SHG values and provide moderate values of birefringence, suggesting Cs X Si 15 P 21 could be phase-matchable and practical nonlinear optical materials in the mid-IR region.

crystal structure↗
DOE OSTI2023

Rare Earth Nitrate Hybrid Double Perovskites [Me 4 N] 2 [MLn(NO 3 ) 6 ] (M = Na–Cs; Ln = La–Gd, ex. Pm)

An exploration of the synthetic and structural phase space of rare earth hybrid double perovskites A 2 B'BX 6 (A = organocation, B' = M + , B = M 3+ , X = molecular bridging anion) that include X = NO 3 – and B' = alkali metal is reported, complementing earlier studies of the [Me 4 N] 2 [KB(NO 3 ) 6 ] (B = Am, Cm, La–Nd, Sm–Lu, Y) (Me 4 N = (CH 3 ) 4 N + ) compounds. In the present efforts, the synthetic phase space of these systems is explored by varying the identity of the alkali metal ion at the B'-site. Herein, we report three new series of the form [Me 4 N] 2 [B'B(NO 3 ) 6 ] (B = La–Nd, Sm–Gd; B' = Na, Rb, Cs). The early members of the Na-series crystallize in the trigonal space group R3̅ from La to Nd where a phase transition occurs in the phase between 273 and 300 K, going from R3̅ to the high-symmetry, cubic space group Fm3̅m. The preceding trigonal members of the Na-series also undergo phase transitions to cubic symmetry at temperatures above 300 K, establishing a decreasing trend in the phase-transition temperature. The remainder of the Na-series, as well as the Rb- and Cs-series, all crystallize in Fm3̅m at 300 K. The temperature-dependent phase behavior of the synthesized phases is studied via variable-temperature spectroscopic methods and high-resolution powder X-ray diffractometry. All phases were characterized via single-crystal and powder X-ray diffraction and Fourier transform infrared (FT-IR) and Raman spectroscopic methods. These results demonstrate the versatility of the perovskite structure type to include rare earth ions, nitrate ions, and a suite of alkali metal ions and serve as a foundation for the design of functional rare earth hybrid double perovskite materials such as those possessing useful multiferroic, optical, and magnetic properties.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Characterization of Two Positional Isomers of the Cs + Gly Complex Using Two-Color, IR–IR Photobleaching of the Cryogenically Cooled Ions

Metal ion binding to amino acid residues is an important interaction motif that controls the tertiary structures of oligopeptides. Analyses of the vibrational band patterns displayed by the amino acid scaffolds are commonly used to characterize the local docking motifs. Here we carry out two-color, IR-IR photobleaching measurements to obtain isomer-selective vibrational spectra of the Cs + Gly ion-molecule complex isolated in a cryogenically cooled, radiofrequency ion trap. The distinct band patterns of two non-interconverting isomers are observed and traced to different bidentate binding motifs between Cs + and the glycine scaffold. In one isomer, the ion attaches to the oxygen atoms of the carboxyl group whereas in the other it docks to the amino nitrogen and the carbonyl oxygen. Attachment to the acid head group yields a very diffuse absorption associated the OH group engaged in a strong intramolecular H-bond that closes a 5 membered ring. Furthermore, the band assignments, rearrangement pathways and electrostatic distortion of the electron density distributions in the glycine scaffold by the proximal ion are explored with electronic structure calculations and anharmonic theory.

Infrared spectroscopy↗
DOE OSTI2022

Systematic Characterization on Optical and Phonon Properties of Cs[Pb x Sn 1– x ]I 3 Alloyed Perovskites via First-Principles Modeling

Here, understanding the optical properties and phonon thermodynamic features of materials enables the determination of their viability for optoelectronic applications including optical-based gas sensors. Metal halide perovskites ABX 3 as well as their mixed alloys can be attractive in applications of this kind. In particular, the formation of alloyed perovskites A[B x B’ 1–x ]X 3 modulates the functional characteristics that can overtake the single-halide ABX 3 . To this end, we use the first-principles simulations to model the effect of B-site alloying on the optical and phonon properties of cubic Cs[Pb x Sn 1–x ]I 3 (x = 0.75, 0.50, and 0.25). We identified the systematic shifts of optical properties as a function of alloying concentration, such as dielectric constants, refraction indices, and absorption coefficients. It was also found that a linear correlation exists between the optical constants and the band gaps of these materials, which satisfies the Penn model. Modeling of phonons in Cs[Pb x Sn 1–x ]I 3 showed that dissipating the dynamically unstable phonons can be feasible at a carefully chosen alloying concentration, which are originally present in their single-halide Pb- and Sn-ends. Our findings pave a way to optimize the alloying conditions of the alloyed perovskite systems of interest for tuning their optical and phonon stability characteristics.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Octahedral Distortion and Excitonic Behavior of Cs 3 Bi 2 Br 9 Halide Perovskite at Low Temperature

The metal halide ionic octahedron, represented as [MX 6 ] $n$- (M = metal cation, X = halide anion), serves as the basic structural unit in halide perovskites and plays a crucial role in determining their optoelectronic and chemical properties. Thus, it is possible to correlate the responses of metal halide perovskites to various environmental stimuli with the dynamic behaviors of the [MX 6 ] $n$- octahedra. In this study, with the temperature-dependent single-crystal X ray diffraction (SCXRD) measurements on Cs 3 Bi 2 Br 9 2D halide perovskites, we can identify two classes of distortions through the lowering of temperature: intraoctahedral distortion, which is the off-centering of Bi 3+ cation within a [BiBr 6 ] 3– octahedron due to the Bi 3+ 6s 2 lone pair electrons, and interoctahedral distortion, which is the collective misalignments among the [BiBr 6 ] 3– building blocks. Free exciton (FE) and self-trapped exciton (STE) models are used to study the relationship between the distortion of octahedra in Cs 3 Bi 2 Br 9 and the corresponding changes in its optoelectronic properties, which transform from dominating blue emission above 100 K to red emission at 4 K. In conclusion, this work provides new insights into the excitonic behaviors of perovskites and suggests a possibility that we can design and rationalize the optical properties of halide perovskites by regulating the environmental stimuli based on the knowledge of behaviors of the individual [MX 6 ] $n$- building blocks.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

A Ba 6 Cu 31 Te 22 ( A = K, Rb, Cs) Featuring Polyanionic Copper–Telluride Frameworks with Ultralow Thermal Conductivity

In this study, hree polyanionic tellurides, ABa 6 Cu 31 Te 22 (A = K, Rb, Cs), were synthesized in salt flux. The isostructural tellurides crystallize in a new structure type, in the cubic Pa$\overline{3}$ space group with a Wyckoff sequence of d 10 c 2 b 1 and large unit cell volumes of over 5500 Å 3 . The structures feature a framework of [CuTe 4 ] tetrahedra and [CuTe 3 ] trigonal pyramids with disorder in the Cu sites. The polyanionic frameworks have large square antiprism and cuboctahedral voids where Ba and alkali metal cations are situated, forming [BaTe 8 ] and [ATe 12 ], respectively. The overall compositions are close to being charge balanced. The large [ATe 12 ] cuboctahedra allowed for significant anisotropic displacement of the A cations, as observed from both single crystal X-ray diffraction and heat capacity studies. Alkali cations rattling together with Cu atom displacement and disorder leads to the dispersion of phonons, thus softening the lattice and subsequently reducing the thermal conductivity. Evaluations of the electronic band structure revealed the occurrence of a narrow bandgap together with the presence of a flat band near the valence band maximum, giving rise to the high thermopower. The Cs and Rb analogues show a slope change in the temperature dependence of electrical resistivity around room temperature, which is typical for semimetals or degenerate semiconductors. For the as-synthesized and unoptimized materials, high values of the thermoelectric figure-of-merit of ~0.2 were observed at 623 K.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗