Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userHigh-Resolution In-situ Synchrotron X-ray Studies of Inorganic Perovskite CsPbBr$_3$: New Symmetry Assignments and Structural Phase Transitions
96
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
Perovskite photovoltaic ABX$_3$ systems are being studied due to their high energy-conversion efficiencies with current emphasis placed on pure inorganic systems. In this work, synchrotron single-crystal diffraction measurements combined with second harmonic generation measurements reveal the absence of inversion symmetry below room temperature in CsPbBr$_3$. Local structural analysis by pair distribution function and X-ray absorption fine structure methods are performed to ascertain the local ordering, atomic pair correlations, and phase evolution in a broad range of temperatures. The currently accepted space group assignments for CsPbBr$_3$ are found to be incorrect in a manner that profoundly impacts physical properties. New assignments are obtained for the bulk structure: $Im$$bar{3}$ (above $sim$ 410 K), $P$2$_1$/$m$ (between $sim$ 300 K and $sim$ 410 K), and the polar group $Pm$ (below $sim$ 300 K), respectively. The newly observed structural distortions exist in the bulk structure consistent with the expectation of previous photoluminescence and Raman measurements. High-pressure measurements reveal multiple low-pressure phases, one of which exists as a metastable phase at ambient pressure. This work should help guide research in the perovskite photovoltaic community to better control the structure under operational conditions and further improve transport and optical properties.
rate research
Read More
The transformation between the metallic ($beta$) and semi-conducting ($alpha$) allotropes of tin is still not well understood. The phase transition temperature stated in the literature, 286.2 K, seems to be inconsistent with recent calorimetric measurements. In this paper, this intriguing aspect has been explored in Sn and Sn-Cu (alloyed 0.5% Cu by weight) using temperature resolved synchrotron x-ray diffraction measurements performed at the Indus-2 facility. Additionally, the $alpha rightleftharpoons beta$ Sn transition has been recorded using in-situ heating/cooling experiments in a scanning electron microscope. Based on these measurements, a protocol has been suggested to reduce the formation of $alpha$-Sn in potentially susceptible systems. This will be useful in experiments like TIN.TIN (The INdia-based TIN detector), which proposes to employ ~100 - 1000 kg of superconducting tin-based detectors to search for neutrinoless double beta decay in the isotope $^{124}$Sn.
We have studied structural phase transitions in high quality underdoped La$_{2-x}$Ba$_x$CuO$_4$ single crystals using high resolution x-ray scattering techniques. Critical properties associated with the continuous High Temperature Tetragonal (HTT, $I4/mmm$) to Middle Temperature Orthorhombic (MTO, $Cmca$) phase transition were investigated in single crystal samples with x=0.125, 0.095, and 0.08 and we find that all behavior is consistent with three dimensional XY criticality, as expected from theory. Power law behavior in the orthorhombic strain, 2(a-b)/(a+b), is observed over a remarkably wide temperature range, spanning most of the MTO regime in the phase diagram. Low temperature measurements investigating the Low Temperature Tetragonal (LTT, $P4_{2}/ncm$) phase, below the strongly discontinuous MTO$to$LTT phase transition, in x=0.125 and x=0.095 samples show that the LTT phase is characterized by relatively broad Bragg scattering, compared with that observed at related wavevectors in the HTT phase. This shows that the LTT phase is either an admixture of tetragonal and orthorhombic phases, or that it is orthorhombic with very small orthorhombic strain, consistent with the ``less orthorhombic low temperature structure previously reported in mixed La$_{2-x}$Sr$_{x-y}$Ba$_y$CuO$_4$ single crystals. We compare the complex temperature-composition phase diagram for the location of structural and superconducting phase transitions in underdoped La$_{2-x}$Ba$_x$CuO$_4$ and find good agreement with results obtained on polycrystalline samples.
There is considerable controversy about swift heavy ion (SHI) irradiation induced displacive phase transitions in thermally insulating oxides. We present here unambiguous evidence for tetragonal to monoclinic and rhombohedral to monoclinic phase transitions in BaTiO3 under swift heavy ion irradiation (120MeV 108Ag+9 ions) using in-situ x-ray powder diffraction (XRPD) studies. The anomalous splitting/broadening of 111/222pc, 200pc and 220pc pseudocubic peaks for fluences greater than 3*1012 ions/cm2 reveal structural changes before amorphization at higher fluences. Lebail analysis of XRPD profiles confirm that the monoclinic phase is of MA type in the Cm space group. Shear stress for the structural phase transition is estimated to be ~ 430MPa, which we believe is generated as a result of stopping of the SHI.
Lead-halide perovskite (LHP) semiconductors are emergent optoelectronic materials with outstanding transport properties which are not yet fully understood. We find signatures of large polaron formation in the electronic structure of the inorganic LHP CsPbBr$_3$ by means of angle-resolved photoelectron spectroscopy. The experimental valence band dispersion shows a hole effective mass $0.26pm0.02,,m_e$, 50% heavier than the bare mass $m_0 =0.17 m_e$ predicted by density functional theory. Calculations of electron-phonon coupling indicate that phonon dressing of the carriers mainly occurs via distortions of the Pb-Br bond with a Frohlich coupling parameter $alpha=1.82$. A good agreement with our experimental data is obtained within the Feynmann polaron model, validating a viable theorical method to predict the carrier effective mass of LHPs ab-initio.
The high energy density of electronic excitations due to the impact of swift heavy ions can induce structural modifications in materials. We present a X-ray diffractometer called ALIX, which has been set up at the low-energy IRRSUD beamline of the GANIL facility, to allow the study of structural modification kinetics as a function of the ion fluence. The X-ray setup has been modified and optimized to enable irradiation by swift heavy ions simultaneously to X-ray pattern recording. We present the capability of ALIX to perform simultaneous irradiation - diffraction by using energy discrimination between X-rays from diffraction and from ion-target interaction. To illustrate its potential, results of sequential or simultaneous irradiation - diffraction are presented in this article to show radiation effects on the structural properties of ceramics. Phase transition kinetics have been studied during xenon ion irradiation of polycrystalline MgO and SrTiO3. We have observed that MgO oxide is radiation-resistant to high electronic excitations, contrary to the high sensitivity of SrTiO3, which exhibits transition from the crystalline to the amorphous state during irradiation. By interpreting the amorphization kinetics of SrTiO3, defect overlapping models are discussed as well as latent track characteristics. Together with a transmission electron microscopy study, we conclude that a single impact model describes the phase transition mechanism.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
