First focus was on the quasi-one-dimensional La3MoO7. To overcome the difficulties in the interpretation of the results on polycrystalline sample containing impurities, the Fellow has introduced a new approach to obtain large single crystals of La3MoO7, which allowed the exploration its intrinsic magnetic properties in depth. Subsequent analyses were conducted on the La3MoO7 single crystals, including single crystal diffraction including both X-ray and neutron sources, temperature-dependent X-ray powder diffraction (PXRD), magnetisation measurements at both low and high temperatures, heat capacity, neutron powder diffraction, among others. These analyses revealed that La3MoO7 undergoes intricate structural and magnetic phase transitions. While XRD analyses offer a comprehensive view of the structural phase transitions, magnetic measurements indicate a magnetic transition above room temperature, not observed in the single crystal neutron diffraction data. Using neutron powder diffraction experiments additional evidence regarding the ordering wave vector in La3MoO7 was obtained. However, several weak anomalies were also observed, indicating that further research is needed to fully understand this intricate system.
The next focus was on strontium copper orthoborate, SrCu2(BO3)2 with the aim to tune the magnetic couplings in this material through chemical substitutions. Prior to conducting the doping experiments, we investigated the synthesis of SrCu2(BO3)2 using conventional solid-state method and unconventional techniques such as hydrothermal and mechanochemical synthesis. This evaluation identified the most efficient method with minimal impurities. High-pressure syntheses were also performed in a multi-anvil press with the Walker-type module at our secondment partner's facility, where pressure and temperature parameters were carefully adjusted.
Next, we developed a new, facile approach to grow mm-sized deep blue plate-like SrCu2(BO3)2 single crystals which requires simple equipment and can be done in a box furnace (Figure 1).
First, the A-site substitutions were investigated, in which Sr2+ is systematically replaced by La3+ or Na+. Synchrotron PXRD and energy dispersive spectroscopy (EDS) analyses have shown that in polycrystalline samples, neither La nor Na are incorporated to an extent detectable by these techniques. However, incorporation of La was possible during the single crystal growth. To study alterations in quantum magnetism of La-doped SrCu2(BO3)2, magnetic susceptibility and X-band EPR at low temperature down to 2 K and high magnetic fields up to 16 T were performed. Subtle changes in the magnetism induced by La-doping were found, e.g. a small decrease in spin gap and direct evidence of dimer-free Cu2+ S = ½ spins, which underline the need of further investigations of higher concentrations of La in doped crystals.
Second, we aimed the B-site doping of SrCu2(BO3)2, i.e. magnetic dilution, through Mg2+ substitutions on the Cu2+ site. We aimed of pushing the doping level to the highest values, targeting 10 mol% nominal doping concentration or x = 0.10 in SrCu2−xMgx(BO3)2 by using a different source of magnesium compared to the literature. Solid-state synthesis was performed and a successful substitution of Cu with Mg in the parent SrCu2(BO3)2 structure was proven by synchrotron PXRD and EDS. Magnetisation and EPR measurements conducted on these samples at both low temperatures and high magnetic fields indicated that Mg-doping results in a softening of the magnetism, evidenced by a systematic decrease in the spin gap, the Curie-Weiss temperature, and the maximum of the magnetic susceptibility. Mg-doping is indeed successful and creates intrinsic Cu2+ defects that couple to the parent dimer lattice and creates an extra magnetic structure stabilised at magnetic fields of about 9 T. [https://doi.org/10.48550/arXiv.2404.15021].
The research results of the QMatCh project have been disseminated in the form of 7 invited contributions, 4 oral and 4 poster contributions at national and international conferences, and summarised in four manuscripts. These results were also shared to the general public through scientific demonstrations at JSI, visits to high schools and participation in major events such as the European Researcher's Night and Znanstival in Slovenia, and the Festival della Scienza in Genova in Italy (Figures 2 and3). The Fellow’s research has also been published in the national newspaper DELO and on several websites and blog posts.
