Conference Agenda

Kesterite (Cu₂ZnSnS₄) is not only a mineral, but also a compound semiconductor and currently the most promising material for a fully inorganic thin film photovoltaic (PV) technology that is free of critical raw-materials and thus provides sustainable solutions.

Thin film solar cells are the most ubiquitous and reliable energy source in space. For these applications harsh conditions in space, like radiation and low temperatures, must be considered. The temperature in space can go down to 5K, and extreme temperature swings occur. Thus the low temperature behaviour of photovoltaic materials potentially used in space applications are of extreme importance.

The crystal structure of Kesterite is characterized by a network of corner-sharing tetrahedra. On of the most fascinating properties of tetrahedrally coordinated solids is their negative thermal expansion. This behaviour is a consequence of low-energy lattice vibrational modes with negative Grueneisen parameters, which also explain high-pressure induced structural phase transitions.

We studied the low-temperature behavior of Kesterite-type Cu₂ZnSnSe₄ and Ag₂ZnSnSe₄, sharing the structure type with the mineral pirquitasite (Ag₂ZnSnS₄) by in situ neutron diffraction. We found a negative thermal expansion coefficient α_a below a certain temperature between 50 and 30K for both materials. The thermal expansion coefficient α_c shows a different behavior: in Cu₂ZnSnSe₄ the T-dependence is similar as for α_a, whereas in Ag₂ZnSnSe₄, this coefficient is negative in the whole temperature range studied. This different behavior highlights the role of the monovalent cation in the low-temperature behaviour of these important materials potentially used in space-base PV.

Are there candidates among refractory zirconium-silicate crystal structures with thermal properties potentially useful in future functional ceramics? We present the linear negative thermal expansion of triclinic sheet silicate dalyite, measured by single-crystal X-ray diffraction over the temperature interval from 100 to 375 K. The thermal contraction is with -1*10E-5 K-1 considerable and seems to extend to higher temperatures beyond our measurement range. Its eigenvector lies oblique to all crystallographic axes and also to the sheets of SiO4-tetrahedra. We show low-wavenumber and high-temperature Raman spectra and discuss the vibrational origin of the thermal contraction and how to use this knowledge in the search for other candidate crystal structures.

The pyrochlore structure type A₂B₂O₇, derived from the mineral pyrochlore with the ideal composition Ca₂Nb₂O_7, can accommodate a broad range of cation combinations, mixed site populations, and vacancies and is therefore widely adopted by synthetic multicomponent oxides. Consequentially a large variety of potentially useful physical and chemical properties e.g. exotic magnetic states like spin-glass, spin-liquid, spin-ice can be realized if magnetic cations are occupying the A-, the B- or both sites [1].

The synthetic pyrochlore Tb₂Ti₂O₇ has been intensely studied in regards to its unusual magnetic behavior at low temperatures [2,3]. In this work, large single-crystals of Tb₂Ti₂O₇, grown with the edge-defined film-fed growth technique, were investigated for its thermoelastic properties and thermal expansion in the temperature range between 4.2 K and 1673 K. The coefficients of thermal expansion were obtained by high-resolution dilatometry and the elastic stiffnesses by resonant ultrasound spectroscopy.

The samples show an elastic softening below ambient temperatures indicating precursor effects of a low temperature phase transition that is likely not realized above 0 K. Notably is a shift in the orientation of the maximum longitudinal elastic stiffness from (100) to (111) as the temperature decreases. Additionally, irreversible effects in elastic properties around 900 K were identified, that are attributed to a change in oxygen defects within the material.

[1] Appl. Phys. Rev. 11, 021337 (2024); doi: 10.1063/5.0192415

[2] Phys. Rev. B 68, 180401(R) (2003); doi: 10.1103/PhysRevB.68.180401

[3] Phys. Rev. Lett. 99, 237202 (2007); doi: 10.1103/PhysRevLett.99.237202

Due to their intrinsic microporosity and good biocompatibility, zeolites are interesting materials for the storage and controlled release of pharmaceutically active species. Computational modelling methods can provide insights into zeolite-drug interactions that are not accessible experimentally. In this contribution, I will summarise findings from three recent computational studies addressing the adsorption of the anticancer drug 5-fluorouracil (5-FU) in synthetic zeolites and link these results to experimental observations. The first study looked at the interaction of 5-FU with protonic faujasite-type (FAU) zeolites using dispersion-corrected density functional theory (DFT) calculations. While the interaction of an adsorbed molecule with several protons significantly stabilises the adsorption complexes under anhydrous conditions, the bonds between framework protons and 5-FU are unstable in the presence of water. Since only partial release of 5-FU from loaded FAU samples was observed in experiments, further DFT calculations were used to elucidate whether the presence of defects in the form of framework-associated octahedral aluminium atoms could be responsible for the retention of 5-FU. Indeed, the calculations showed that 5-FU binds essentially irreversibly to these defect sites. A third computational study looked at the diffusion of 5-FU through zeolite pores, employing molecular dynamics simulations with classical force fields. The simulations predicted a fast decrease of the diffusivity with increasing 5-FU loading due to a propensity of 5-FU molecules to cluster together, reducing their mobility. Diffusion limitations might thus be responsible for the relatively modest 5-FU adsorption capacity of FAU-type zeolites.

Funding by the German Research Foundation (project ID 455871835) is gratefully acknowledged.

Aluminophosphate-based framework materials exhibit pronounced structural flexibility that is closely linked to their physical properties and potential applications. In this study, we investigate the temperature-dependent diffuse scattering of AlPO₄-5, and its silicon (SAPO-5), and magnesium substituted (MgAPO-5) counterparts using single-crystal X-ray diffraction experiments performed at the European Synchrotron Radiation Facility (ESRF). While Bragg diffraction provides information on the average crystal structure, diffuse scattering gives access to correlated disorder and dynamic processes within the framework.

High-resolution reciprocal space datasets were collected over a wide temperature range, enabling a systematic analysis of the evolution of diffuse scattering features.

After calcination, AlPO-5 shows well-defined satellite reflections of at least second order, indicating the presence of a long-range ordered modulation within the framework. These features systematically decrease in intensity with increasing temperature, pointing to a gradual loss of structural ordering. In contrast, SAPO-5 and MgAPO-5 predominantly exhibit diffuse streaks instead of distinct satellite reflections, indicating a disturbance of the long-range order by heteroatom substitution. These diffuse features likewise decrease with increasing temperature, suggesting a reduction of correlated distortions at elevated temperatures.

Overall, the results demonstrate that increasing temperature reduces diffuse scattering intensity and associated structural correlations, emphasizing the role of both thermal effects and chemical substitution in governing framework dynamics. In the future, this improved understanding may help to interpret how guest molecules contribute to the structural dynamics of these frameworks and how this potentially influences their catalytic properties.