Temperature-dependent X-ray Diffraction Research Articles

Cooling-Induced Order-Disorder Phase Transition in CsPbBr3 Nanocrystal Superlattices.

Perovskite nanocrystal superlattices are being actively studied after reports have emerged on collective excitonic properties at cryogenic temperatures, where energetic disorder is minimized due to the frozen lattice vibrations. However, an important issue related to structural disorder of superlattices at low temperatures has received little attention to date. In this work, it is shown that CsPbBr3 nanocrystal superlattices undergo a reversible order-disorder transition upon cooling to 90 K. The transition consists of the loss of structural coherence, that is, increased nanocrystal misalignment, and contraction of the superlattices, as revealed by temperature-dependent X-ray diffraction, and is ascribed to the solidification of ligands (on the basis of Raman spectroscopy). Introducing shorter amines on the nanocrystal surface allows to mitigate these changes, improve order, and shorten interparticle distance. It is demonstrated that the low temperature phase of the short ligand-capped nanocrystal superlattices is characterized by a strong exciton migration observable in the photoluminescence decay, which is due to the shrinkage of the inter-nanocrystal distance.

Hofmann Clathrates: A "Blue Box" Approach to Modulate Spin-Crossover Properties.

Hofmann coordination polymers (CPs) with cationic ligands provide an innovative strategy for recognizing π-electron-rich aromatic molecules - similar to the "little blue box". In this study, we demonstrate that hydroquinone molecules can be incorporated into these coordination polymers when redox-active bipyridinium derivatives are used as axial ligands. The insertion leads to a significant structural modification, resulting in a shift of the spin transition by 150 K and an approximate 23 % increase in volume, caused by the strong donor-acceptor π-π stacking interaction formed between the ligands and the guest molecule. These findings have been confirmed through temperature-dependent single crystal X-ray diffraction, magnetic measurements and optical reflectivity measurements.

An Amorphous-to-Anatase Phase Transition Study of TiO2 Nanotubes by Temperature-Dependent Synchrotron-Based In Situ X-ray Diffraction.

TiO2 nanotubes (NTs) with amorphous and crystalline structures have attracted interest due to their wide range of applications, particularly black TiO2, which addresses the limitations of conventional TiO2 (a wide band gap of 3.0-3.2 eV). Understanding the amorphous-to-anatase phase transition is crucial for phase control of crystalline TiO2. This study investigates the size-dependent phase transition behavior of TiO2 and black TiO2 NTs using temperature-dependent synchrotron-based X-ray diffraction. Thermal treatments reveal that phase transitions occur in the ranges of 210-240 °C for normal NTs and 270-300 °C for black NTs. The onset temperature and crystallization growth are dependent on size, especially NT length, at least in the system under investigation. We observe anisotropic crystallization in quantum confinement, with the unit cell undergoing compression in the a-axis and expansion in the c-axis during crystallization. The longest black NTs (BV50T4) show a significant difference in the c/a ratio. Defects, such as oxygen vacancies, localize on specific NT planes.

Anomalous Behavior in Dark-Bright Splitting Impacts the Biexciton Binding Energy in (BA)2(MA)n-1PbnBr3n+1 (n = 1-3).

Two-dimensional Ruddlesden-Popper series are an excellent system for tuning physical properties of the perovskite by controlling the layer number (n). For instance, bandgap and exciton binding energies of the series gradually increase upon reducing n via enhanced quantum and dielectric confinements. Here, we present findings that challenge the anticipated trend in electron-hole exchange interaction within (BA)2MAn-1PbnBr3n+1 (n = 1-3), which causes spin-dependent exciton level splitting into bright and dark states, where the latter is partially visible near the surface of the Br-based two-dimensional Ruddlesden-Popper series. Contrary to expectations, the smallest gap between bright and dark exciton levels is observed from n = 2 at 10 K. This anomaly results in the strongest biexciton binding between two dark excitons occurring at n = 2, rather than at n = 1 as initially hypothesized. The observed anomaly arises from a phase transition induced by octahedral tilting occurring only for n = 2 near 100 K as confirmed by temperature-dependent optical and X-ray diffraction measurements. Our results show that Coulomb interaction need not vary gradually with n, which can impact the optoelectronic properties of the Ruddlesden-Popper series.

Site Disorder Drives Cyanide Dynamics and Fast Ion Transport in Li6PS5CN.

Halide argyrodite solid-state electrolytes of the general formula Li6PS5 X exhibit complex static and dynamic disorder that plays a crucial role in ion transport processes. Here, we unravel the rich interplay between site disorder and dynamics in the plastic crystal argyrodite Li6PS5CN and the impact on ion diffusion processes through a suite of experimental and computational methodologies, including temperature-dependent synchrotron powder X-ray diffraction, AC electrochemical impedance spectroscopy, 7Li solid-state NMR, and machine learning-assisted molecular dynamics simulations. Sulfide and (pseudo)halide site disorder between the two anion sublattices unilaterally improves long-range lithium diffusion irrespective of the (pseudo)halide identity, which demonstrates the importance of site disorder in dictating bulk ionic conductivity in the argyrodite family. Furthermore, we find that anion site disorder modulates the presence and time scales of cyanide rotational dynamics. Ordered configurations of anions enable fast, quasi-free rotations of cyanides that occur on time scales of 1011 Hz at T = 300 K. In contrast, we find that cyanide dynamics are slow or frozen in Li6PS5CN when site disorder between the cyanide and sulfide sublattices is present at T = 300 K. We rationalize the observed differences in cyanide dynamics in the context of elastic dipole interactions between neighboring cyanide anions and local strain induced by the configurations of site disorder that may impact the energetic landscape for cyanide rotational dynamics. Through this study, we find that anion disorder plays a decisive role in dictating the extent and time scales of both lithium ion and cyanide dynamics in Li6PS5CN.

In-situ cryogenic X-ray diffraction analysis of Poly(dimethylsiloxane) oil and film

Understanding the phase transitions and crystallographic characteristics of polysiloxane materials poses a significant challenge. To address this issue, a custom cryogenic chamber has been developed and integrated with a laboratory X-ray diffraction apparatus, enabling the execution of in-situ temperature-dependent X-ray diffraction (XRD) analyses across the cooling and heating cycles. Through the utilization of diverse sample holders, the real-time monitoring of crystallization and melting phenomena in both linear poly(dimethylsiloxane) (PDMS) oil and crosslinked PDMS film has been made feasible. Notably, high-quality X-ray diffraction data, encompassing 13 diffraction lines, are successfully acquired for the first time for linear PDMS oil at 153K. Subsequent crystallographic examination has revealed a tetragonal unit cell characterized by lattice parameters a = b = 8.3379 Å, c = 11.9284 Å, and space group I 41, suggesting the adoption of a fourfold helical conformation by two polymer chains, each comprising four monomer units, within the lattice structure. This improved and versatile X-ray instrumentation presents clear advantages over conventional commercial powder X-ray diffractometers, which is anticipated to provide a promising in-situ characterization approach for investigating the low-temperature behaviors of liquid or oil materials.

Regulating thermometric properties of single-band ratiometric luminescent thermometers based on Eu3+-activated Y2W3O12·3H2O nanoparticles via controlling doping content and excitation model

Series of Eu3+-activated Y2W3O12·3H2O nanoparticles were designed to realize high-sensitive optical thermometry. The resultant nanoparticles can emit bright visible emission from Eu3+ upon the excitation of 296 nm (charge-transfer state (CTS) of O2- → W6+), 394 nm (ground state absorption (GSA)) and 536 nm (excitation state absorption (ESA)), of which the fluorescence intensities were influenced by dopant content. The negative thermal expansion properties of studied nanoparticles were clarified via temperature-dependent X-ray diffraction patterns. Moreover, the thermal quenching behaviors of final products were also investigated through temperature-related emission spectra. Specifically, upon the CTS and GSA processes, the developed nanoparticles underwent significant thermal quenching effect, whereas the thermally enhanced luminescence was observed under ESA process. Herein, through using the single-band ratiometric (SBR) method to investigate the fluorescence intensity ratio of the red emission from 5D0 → 7F2 transition of Eu3+ at high temperature, the thermometric properties of prepared nanoparticles were studied. It was found that maximum absolute and relative sensitivities of Eu3+-activated Y2W3O12·3H2O nanoparticles reached up to 0.061 and 4.944 % K−1, respectively, and they can be regulated by changing Eu3+ dopant and SBR models (i.e., ESA+CTS and ESA+GSA combinations), realizing high-sensitive optical thermometry. Our findings may provide some guidelines for manipulating the thermometric properties of rare-earth ions activated materials and developing new SBR luminescence thermometers.

Combining electron transfer, spin crossover, and redox properties in metal-organic frameworks

Hofmann coordination polymers (CPs) that couple the well-studied spin transition of the FeII central ion with electron-responsive ligands provide an innovative strategy toward multifunctional metal-organic frameworks (MOFs). Here, we developed a 2D planar network consisting of metal-cyanide-metal sheets in an unusual coordination mode, brought about by infinitely π-stacked redox-active bipyridinium derivatives as axial ligands. The obtained family of materials show vivid thermochromism attributed to electron transfer and/or electronic spin state change processes that can occur either independently or concomitantly. Importantly, the redox activity of the ligands within the structure leads to the quasi-reversible electrochemical reduction reaction on a spin-crossover complex at solid state. These observations have been confirmed via temperature-dependent single-crystal X-ray diffraction, magnetic measurements, Mössbauer, EPR, optical and vibrational spectroscopies as well as quantum chemical calculations.

NASICON-type medium entropy Li1.5Sn1.0Al0.5Zr0.5(PO4)3 electrolyte for solid state Li metal batteries

Developing solid electrolytes for all-solid-state lithium batteries with superior performance is crucial for portable energy storage. This study uses a traditional solid-state reaction technique to fabricate a NASICON-type medium entropy Li1.5Sn1.0Al0.5Zr0.5(PO4)3 (LSAZP) ceramic electrolyte. The Rietveld refinement of room temperature X-ray diffraction (XRD) data confirms a pure rhombohedral phase (R3‾c) for LSAZP ceramic sintered at 1050 °C. Temperature-dependent synchrotron XRD data demonstrates an increase in lattice parameter c with a positive coefficient of thermal expansion (+2.40 × 10−5 K−1) and a negative coefficient of thermal expansion (−1.26 × 10−6 K−1) for the lattice parameter a with increasing temperature. Interestingly, despite the anisotropic thermal expansion, no intergranular cracks, typically observed in rhombohedral NASICON-type phases, are noticeable in the scanning electron micrographs of the LSAZP samples. The sample sintered at 1050 °C (relative density ∼90 %) exhibits an excellent room temperature conductivity of ∼2.95 × 10−4 S cm−1 and activation energy ∼0.39 ± 0.02 eV. The Li-ion transference number is ∼0.99, suggesting that Li-ion is the dominant charge carrier in the sample. During galvanostatic lithium plating-stripping tests, the symmetric Li|LSAZP|Li cell demonstrates excellent lithium plating-stripping stability over 50 h at a current density of 4 μA cm−2.

Temperature-Dependent Structural and Optoelectronic Properties of the Layered Perovskite 2-Thiophenemethylammonium Lead Iodide.

Improved knowledge of the influence of temperature upon layered perovskites is essential to enable perovskite-based devices to operate over a broad temperature range and to elucidate the impact of structural changes upon the optoelectronic properties. We examined the Ruddlesden-Popper layered perovskite 2-thiophenemethylammonium lead iodide (ThMA2PbI4) and observed a structural phase transition between a high- and a low-temperature phase at 220 K using temperature-dependent X-ray diffraction, UV-visible absorption, and photoluminescence (PL) spectroscopy. The structural phase transition altered the tilt pattern of the inorganic octahedra layer, modifying the absorption and PL spectra. Further, we found a narrow and intense additional PL peak in the low-temperature phase, which we assigned to radiative emission from a defect-bound exciton state. In both phases we determined the thermal expansion coefficient and found values similar to those of cubic 3D perovskites, i.e., larger than those of typical substrates such as glass. These results demonstrate that the organic spacer plays a critical role in controlling the temperature-dependent structural and optoelectronic properties of layered perovskites and suggests more widely that strain management strategies may be needed to fully utilize layered perovskites in device applications.

The origins of dual-peak emission and anomalous exciton decay in 2D Sn-based perovskites.

Two-dimensional (2D) Sn-based perovskites exhibit significant potential in diverse optoelectronic applications, such as on-chip lasers and photodetectors. Yet, the underlying mechanism behind the frequently observed dual-peak emission in 2D Sn-based perovskites remains a subject of intense debate, and there is a lack of research on the carrier dynamics in these materials. In this study, we investigate these issues in a representative 2D Sn-based perovskite, namely, PEA2SnI4, through temperature-, excitation intensity-, angle-, and time-dependent photoluminescence studies. The results indicate that the high- and low-energy peaks originate from in-face and out-of-face dipole transitions, respectively. In addition, we observe an anomalous increase in the non-radiative recombination rate as temperature decreases. After ruling out enhanced electron-phonon coupling and Auger recombination as potential causes of the anomalous carrier dynamics, we propose that the significantly increased exciton binding energy (Eb) plays a decisive role. The increased Eb arises from enhanced electronic localization, a consequence of weakened lattice distortion at low temperatures, as confirmed by first-principles calculations and temperature-dependent x-ray diffraction measurements. These findings offer valuable insights into the electronic processes in the unique 2D Sn-based perovskites.

Thermal Stability of Chalcogenide Perovskites.

Chalcogenide perovskites (CPs) have recently attracted interest as a class of materials with practical potential in optoelectronics and have been suggested as a more thermally stable alternative to intensely studied halide perovskites (HPs). Here we report a comparative study of the thermal stability of representative HPs, MAPbI3 (MA = CH3NH3+, methylammonium) and CsPbI3, and a series of CPs with compositions BaZrS3, β-SrZrS3, BaHfS3, SrHfS3. Changes in the crystal structure, chemical composition, and optical properties upon heating in air up to 800 °C were studied using thermogravimetric analysis, temperature-dependent X-ray diffraction, energy-dispersive X-ray spectroscopy, and diffuse reflectance spectroscopy. While HPs undergo phase transitions and thermally decompose at temperatures below 300 °C, the CPs show no changes in crystal phase or composition when heated up to at least 450 °C. At 500 °C CPs oxidize on time scales of several hours, forming oxides and sulfates. The structural origins of the higher thermal and phase stability of the CPs are discussed.

Mixed Tin Valence in the Tin(II/IV)-Nitridophosphate Sn3P8N16.

Sn3P8N16 combines the structural versatility of nitridophosphates and Sn within one compound. It was synthesized as dark gray powder in a high-pressure high-temperature reaction at 800 °C and 6 GPa from Sn3N4 and P3N5. The crystal structure was elucidated from single-crystal diffraction data (space group C2/m (no. 12), a=12.9664(4), b=10.7886(4), c=4.8238(2) Å, β=109.624(1)°) and shows a 3D-network of PN4 tetrahedra, incorporating Sn in oxidation states +II and +IV. The Sn cations are located within eight-membered rings of vertex-sharing PN4 tetrahedra, stacked along the [001] direction. A combination of solid-state nuclear magnetic resonance spectroscopy, 119Sn Mössbauer spectroscopy and density functional theory calculations was used to confirm the mixed oxidation of Sn. Temperature-dependent powder X-ray diffraction measurements reveal a low thermal expansion of 3.6 ppm/K up to 750 °C, beyond which Sn3P8N16 starts to decompose.

Negative Thermal Expansion in ABC(MoO4)3 Compounds.

Negative thermal expansion (NTE) compounds provide a solution for the mismatch of coefficients of thermal expansion in highly integrated device design. However, the current NTE compounds are rare, and how to effectively design new NTE compounds is still challenging. Here, a new concept is proposed to design NTE compounds, that is, to increase the flexibility of framework structure by expanding the space in framework structure compounds. Taking the parent compound NaZr2(PO4)3 as a case, a new NTE system AIBIICIII(MoO4)3 (A = Li, Na, K, and Rb; B = Mg and Mn; C = Sc, In, and Lu) is designed. In these compounds, the large volume of MoO4 tetrahedron is used to replace the small volume of PO4 tetrahedron in NaZr2(PO4)3 to enhance structural space and NTE performance. Simultaneously, a joint study of temperature-dependent X-ray diffraction, Raman spectroscopy, and the first principles calculation reveals that the NTE in AIBIICIII(MoO4)3 series compounds arise from the coupled oscillation of polyhedral. Large-radius ions are conducive to enhancing the space and softening the framework structure to achieve the enhancement of NTE. The current strategy for designing NTE compounds is expected to be adopted in other compounds to obtain more NTE compounds.

Infrared spectroscopic study on Nb-ion-irradiated MgB2 thin films

Magnesium diboride (MgB2) is a two-band superconductor with a high superconducting critical temperature (Tc) of approximately 39 K. Owing to the lack of vortex pinning centers, MgB2 exhibits an abrupt decline in the critical current density (Jc) in an applied magnetic field. Here, we prepared 1 MeV Nb ion-irradiated MgB2 thin-film samples with doses of 3×1013, 7×1013, and 9×1013 ions/cm2. Temperature-dependent magnetization and x-ray diffraction (XRD) measurements were performed to determine the Tc and c-axis lattice constant of each sample. Furthermore, a Fourier transform infrared (FTIR) spectroscopy was performed to obtain the infrared properties of the Nb-ion-irradiated MgB2 thin-film samples. The optical conductivity of each sample in the low-energy region was fitted with two (narrow and broad) Drude modes. We found that the spectral weight redistribution from the low-to high-frequency regions and the broadening of the narrow Drude mode caused by irradiation are closely related to the reduction in Tc.

Noncentrosymmetric, transverse structural modulation in SrAl4 , and elucidation of its origin in the BaAl4 family of compounds

At ambient conditions SrAl4 adopts the BaAl4 structure type with space group I4/mmm. It undergoes a charge-density-wave (CDW) transition at TCDW=243 K, followed by a structural transition at TS=87 K. Temperature-dependent single-crystal x-ray diffraction (SXRD) leads to the observation of incommensurate superlattice reflections at q=σc* with σ=0.1116 at 200 K. The CDW has orthorhombic symmetry with the noncentrosymmetric superspace group F222(00σ)00s, where F222 is a subgroup of Fmmm as well as of I4/mmm. Atomic displacements mainly represent a transverse wave, with displacements that are 90 deg out of phase between the two diagonal directions of the I-centered unit cell, resulting in a helical wave. Small longitudinal displacements are provided by the second harmonic modulation. The orthorhombic phase realized in SrAl4 is similar to that found in EuAl4, except that no second harmonic could be determined for the latter compound. Electronic structure calculations and phonon calculations by density functional theory (DFT) have failed to reveal the mechanism of CDW formation. No clear Fermi surface nesting, electron-phonon coupling, or involvement of Dirac points could be established. However, DFT reveals that Al atoms dominate the density of states near the Fermi level, thus corroborating the SXRD measurements. SrAl4 remains incommensurately modulated at the structural transition, where the symmetry lowers from orthorhombic to b-unique monoclinic. The present work draws a comparison on the modulated structures of nonmagnetic SrAl4 and magnetic EuAl4 elucidating their similarities and differences, and firmly establishing that although substitution of Eu to Sr plays little to no role in the structure, the transition temperatures are affected by the atomic sizes. We have identified a simple criterion that correlates the presence of a phase transition with the interatomic distances. Only those compounds XAl4−xGax (X=Ba, Eu, Sr, Ca; 0<x<4) undergo phase transitions, for which the ratio c/a falls within the narrow range 2.51<c/a<2.54. Published by the American Physical Society 2024

Negative magnetization and magnetic ordering of rare earth and transition metal sublattices in NdFe0.5Cr0.5O3

We investigate the effect of alloying at the 3d transition metal site of a rare-earth-transition metal oxide, by considering NdFe0.5Cr0.5O3 mixed perovskite with two equal and random distribution of 3d ions, Cr and Fe, interacting with an early 4f rare earth ion, Nd. Employing temperature- and field- dependent magnetization measurements, temperature-dependent x-ray diffraction, neutron powder diffraction, and Raman spectroscopy, we characterize its structural and magnetic properties. Our study reveals bipolar magnetic switching (arising from negative magnetization) and magnetocaloric effect which underline the potential of the studied mixed perovskite in device application. The neutron diffraction study shows the absence of spin reorientation transition over the entire temperature range of 1.5–320 K, although both parent compounds exhibit spin orientation transition. We discuss the microscopic origin of this curious behavior. The neutron diffraction results also reveal the ordering of Nd spins at an unusually high temperature of about 40 K, which is corroborated by Raman measurements.

Excitonic emission and phonon anharmonicity in Cs3Sb2Br9

We investigate emission characteristics, phonon–phonon, and electron–phonon interactions in a lead-free halide perovskite Cs3Sb2Br9 through temperature-dependent photoluminescence, Raman scattering, and x-ray diffraction measurements. The exciton–optical phonon coupling leads to below bandgap broad emissions, arising from self-trapped excitons recombination. The anomalous temperature dependence of the lowest frequency Raman mode is attributed to the phonon–phonon and electron–phonon interactions. The temperature-dependent x-ray diffraction measurement reveals a minimum in the volume thermal expansion coefficient at around 120 K. We also quantify the quasiharmonic contributions to the phonon frequency shift for all Raman modes.

Temperature-driven spin reorientation transition in van der Waals Cr1.7Te2 ferromagnet

The phenomenon of spin reorientation transition in magnetic materials is truly captivating, as it involves a fascinating change in the direction of magnetic moments. However, the research on spin reorientation transition in two-dimensional (2D) magnetic materials has received limited attention, thus hindering its immense potential for significant advancements in various device applications. In this study, we present a discovery of a spin reorientation transition from an in-plane to an out-of-plane direction in the van der Waals ferromagnet Cr1.7Te2 (Tc = 300 K). This transition occurs at 70 K when the temperature ranges from 3 to 300 K, which is evidenced by the temperature-dependent Hall effect and magnetic anisotropy energy measurements. Notably, the anisotropic evolution observed reveals that the shape anisotropy effect surpasses the magnetocrystalline anisotropy in van der Waals ferromagnet at low temperatures, which is distinct from reported ferromagnetic materials. Furthermore, temperature-dependent x-ray diffraction characterizations confirm that no structural phase transition occurs during this intriguing spin reorientation transition process. These findings establish a strong and solid foundation, offering a promising platform for the design and development of cutting-edge 2D spintronic devices.

Study of the amorphous and crystalline states in Ge(Te1-xSex) thick films (x = 0, 0.50) by in situ temperature-dependent structural analyses

As-deposited and unencapsulated GeTe1-xSex (x = 0, 0.5) 3-μm-thick amorphous films on Si(001) were obtained via the thermal co-evaporation technique. The crystallization and the structure of these phase change materials were examined using in situ temperature-dependent X-ray diffraction (XRD) and grazing-incidence fluorescence X-ray Absorption Near Edge Structure (XANES) in isochronal annealing conditions under nitrogen flow. The results show that the onset temperature of crystallization is highly sensitive to the substitution of the Te atoms by the Se ones and that the rhombohedral structure, with the space group R3m, is the one that crystallized for both samples without Ge or Te rejection. Moreover, the local order around the Ge and Se atoms, probed by Ge and Se K-edge XANES analyses, presents clear modifications from the amorphous state to the crystalline one expecting a Ge four-fold and a Se two-fold coordination in the amorphous state, with an increase of the local disorder with the Se substitution (x = 0.5).