Lead-halide perovskites increasingly mesmerize researchers because they exhibit a high degree of structural defects and dynamics yet nonetheless offer an outstanding (opto)electronic performance on par with the best examples of structurally stable and defect-free semiconductors. This highly unusual feature necessitates the adoption of an experimental and theoretical mindset and the reexamination of techniques that may be uniquely suited to understand these materials. Surprisingly, the suite of methods for the structural characterization of these materials does not commonly include nuclear magnetic resonance (NMR) spectroscopy. The present study showcases both the utility and versatility of halide NMR and NQR (nuclear quadrupole resonance) for probing the structure and structural dynamics of CsPbX 3 (X = Cl, Br, I), in both bulk and nanocrystalline forms. The strong quadrupole couplings, which originate from the interaction between the large quadrupole moments of, e.g., the 35 Cl, 79 Br, and 127 I nuclei, and the local electric-field gradients, are highly sensitive to subtle structural variations, both static and dynamic. The quadrupole interaction can resolve structural changes with accuracies commensurate with synchrotron X-ray diffraction and scattering. It is shown that space-averaged site-disorder is greatly enhanced in the nanocrystals compared to the bulk, while the dynamics of nuclear spin relaxation indicates enhanced structural dynamics in the nanocrystals. The findings from NMR and NQR were corroborated by ab initio molecular dynamics, which point to the role of the surface in causing the radial strain distribution and disorder. These findings showcase a great synergy between solid-state NMR or NQR and molecular dynamics simulations in shedding light on the structure of soft lead-halide semiconductors.

The RNiO 3 perovskites are known to order antiferromagnetically below a material-dependent Néel temperature T N . We report experimental evidence indicating the existence of a second magnetically-ordered phase in TlNiO 3 above T N = 104 K, obtained using nuclear magnetic resonance and muon spin rotation spectroscopy. The new phase, which persists up to a temperature T * N = 202 K, is suppressed by the application of an external magnetic field of approximately 1 T. It is not yet known if such a phase also exists in other perovskite nickelates.Although first synthesized already in 1970 [1], the rare-earth nickelates RNiO 3 have been the subject of intense research efforts during the last decade due to their peculiar metal-insulator transition at T MI and subsequent antiferromagnetic (AFM) order between Ni spins below a Néel temperature T N [2, 3]. By substituting rare-earth ions R 3+ with different radii, the perovskite lattice can be distorted continuously. This distortion affects the magnetic couplings of Ni spins by modifying the Ni-O-Ni superexchange angle. A phase diagram of RNiO 3 , mostly based on previously published results, is shown in Fig. 1. Despite recent theoretical [4][5][6] and experimental [7] progress, the nature of the paramagnetic (PM) insulating phase is still unclear. The AFM order below T N is characterized by a propagation vector k pc = ( 1 /4, 1 /4, 1 /4) with respect to the (pseudo-)cubic unit cell of the ideal perovskite structure. Thus, a period of the magnetic structure comprises four Ni sites along each pseudocubic crystal axis. Previous experimental reports suggested a collinear "up-up-down-down" structure [8-10], while others argued for a non-collinear spiral spin configuration [11][12][13][14]. The AFM phase is predicted to be ferroelectric [15,16], but this has not yet been confirmed experimentally. Since the magnitude of the ferroelectric polarization is very different for the two candidate spin arrangements [16], investigations of the magnetism of RNiO 3 are crucial for the understanding of their possible multiferroicity. Nuclear magnetic resonance (NMR) is a powerful technique to locally probe magnetic behavior. However, to our knowledge, the only previous NMR work involving a nickelate perovskite was a 139 La-NMR study of LaNiO 3 [17], which is a PM metal at all temperatures.In the present work, we present the first NMR investigation of an insulating member of the RNiO 3 family, in the form of a combined NMR and muon-spin rotation (µSR) study of the magnetic properties of TlNiO 3 . This material is known to have the same qualitative behavior as the analogous rare-earth nickelates [18]. From an NMR perspective, TlNiO 3 is unique among the RNiO 3 compounds because both 203 Tl and 205 Tl nuclei are excellent for NMR. In addition to the previously known AFM phase below T N = 104 K, our measurements reveal a previously unknown magnetically-ordered phase between T N and T * N = 202 K. An applied magnetic fields of 1 T is sufficient to suppress the static magnetic order between T N an...

The simultaneous presence of a Fe-related spin-density wave and antiferromagnetic order of Eu 2+ moments ranks EuFe 2 As 2 among the most interesting parent compounds of iron-based pnictide superconductors. Here we explore the consequences of the dilution of Eu 2+ magnetic lattice through on-site Ca substitution. By employing macro-and microscopic techniques, including electrical transport and magnetometry, as well as muon-spin spectroscopy, we study the evolution of Eu magnetic order in both the weak and strong dilution regimes, achieved for Ca concentration x(Ca) = 0.12 and 0.43, respectively. We demonstrate the localized character of the Eu antiferromagnetism mediated via RKKY interactions, in contrast with the largely itinerant nature of Fe magnetic interactions. Our results suggest a weak coupling between the Fe and Eu magnetic sublattices and a rapid decrease of the Eu magnetic interaction strength upon Ca substitution. The latter is confirmed both by the depression of the ordering temperature of the Eu 2+ moments, T N , and the decrease of magnetic volume fraction with increasing x(Ca). We establish that, similarly to the EuFe 2 As 2 parent compound, the investigated Ca-doped compounds have a twinned structure and undergo a permanent detwinning upon applying an external magnetic field. arXiv:1805.03896v2 [cond-mat.mtrl-sci]

The chemistry encyclopedia Römpp Online and the German universal encyclopedia Wikipedia were assessed by first-year university students on the basis of a set of 30 articles about chemical thermodynamics. Criteria with regard to both content and form were applied in the comparison; 619 ratings (48% participation rate) were returned. While both encyclopedias obtained very good marks and performed nearly equally with regard to their accuracy, the average overall mark for Wikipedia was better than for Römpp Online, which obtained lower marks with regard to completeness and length. Analysis of the results and participants' comments shows that students attach importance to completeness, length and comprehensibility rather than accuracy, and also attribute less value to the availability of sources which validate an encyclopedia article. Both encyclopedias can be promoted as a starting reference to access a topic in chemistry. However, it is recommended that instructors should insist that students do not rely solely on encyclopedia texts, but use and cite primary literature in their reports.

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