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Justus Just

Publications and source records attributed to Justus Just.

4 recordsLinked to original sources
arXiv2026

Probing local coordination and halide miscibility in single-, double-, and triple-halide perovskites using EXAFS

Lead-halide perovskites are a promising material platform as semiconductors in next-generation solar cells because of their solution processability, defect tolerance, and tunable optoelectronic properties. Mixed iodide-bromide perovskite compositions are attractive as wide bandgap absorbers but suffer from significant operational instabilities. Incorporation of chloride to form triple-halide perovskites has been shown to improve both stability and performance; however, the extent of halide miscibility and chloride incorporation remain poorly understood. While bulk metrics such as diffraction-derived lattice parameters and optical bandgap can confirm single-phase behavior, they do not establish homogeneous mixing on the halide site. Here, we use cryogenic X-ray absorption spectroscopy (XAS) to directly probe local lead-halide coordination across single-, double-, and triple-halide perovskite compositions. We show formation of a single-phase triple-halide perovskite whose miscibility is mediated by bromide content. We identify signatures of halide mixing from the Pb L3-edge EXAFS of mixed double- and triple-halide perovskites using both quantitative fits and Cauchy wavelet transforms. Finally, using wavelet transforms of the Br K-edge EXAFS, we demonstrate halide intermixing on the single PbX6 octahedron level, exploiting forward-scattering amplified third-shell halide-bromide interactions. These results are a step forward in the understanding of local structure that is required to fully describe and optimize halide incorporation for novel perovskite compositions.

cond-mat.mtrl-sci↗
arXiv2022

Multi-Stage Phase-Segregation of Mixed Halide Perovskites under Illumination: A Quantitative Comparison of Experimental Observations and Thermodynamic Models

Photo- and charge-carrier induced ion migration is a major challenge when utilizing metal halide perovskite semiconductors for optoelectronic applications. For mixed iodide/bromide perovskites, the compositional instability due to light- or electrical bias induced phase- segregation restricts the exploitation of the entire bandgap range. Previous experimental and theoretical work suggests that excited states or charge-carriers trigger the process but the exact mechanism is still under debate. To identify the mechanism and cause of light-induced phase-segregation phenomena we investigate the full compositional range of methylammonium lead bromide/iodide samples, MAPb(Br$_x$I$_{1-x}$)$_3$ with $x = 0\ldots 1$, by simultaneous in-situ X-ray diffraction and photoluminescence spectroscopy during illumination. The quantitative comparison of composition-dependent in-situ XRD and PL shows that at excitation densities of 1 sun, only the initial stage of photo-segregation can be rationalized with the previously established thermodynamic models. However, we observe a progression of the phase-segregation that can only be rationalized by considering long-lived accumulative photo-induced material alterations. We suggest that (additional) photo-induced defects, possibly halide vacancies and interstitials, need to be considered to fully rationalize light-induced phase-segregation and anticipate our findings to provide crucial insight for the development of more sophisticated models.

cond-mat.mtrl-sci↗
arXiv2013

Nanoscale order in the frustrated mixed conductor La$_{5.4}WO$_{12-δ}$

We report a comprehensive investigation of the average and local structure of La$_{5.4}$WO$_{12-δ}$, which has excellent mixed proton, electron and oxide ion conduction suitable for device applications. Synchrotron X-ray and neutron powder diffraction show that a cubic fluorite supercell describes the average structure, with highly disordered lanthanum and oxide positions. On average the tungsten sites are six-fold coordinated, and we detect a trace (4.4(2) %) of anti-site disorder. In addition to sharp Bragg reflections, strong diffuse neutron scattering is observed, which hints at short-range order. We consider plausible local configurations, and show that the defect chemistry implies a simple 'chemical exchange' interaction that favours ordered WO6 octahedra. Our local model is confirmed by synchrotron x-ray pair distribution function analysis and EXAFS experiments performed at the La K and W L3-edges. We show that ordered domains of around ca. 3.5 nm are found, implying that mixed conduction in La$_{5.4}$WO$_{12-δ}$? is associated with a defective glassy-like anion sublattice. The origins of this ground state are proposed to lie in the non-bipartite nature of the fcc lattice and the pairwise interactions which link the orientation of neighbouring octahedral WO6 sites. This 'function through frustration' could provide a means of designing new mixed conductors.

cond-mat.mtrl-sci↗