The research into adamantane-type compounds has gained momentum in recent years, yielding remarkable new applications for this class of materials. In particular, organic adamantane derivatives (AdR4) or inorganic adamantane-type compounds of the general formula [(RT)4E6] (R: organic substituent; T: group 14 atom C, Si, Ge, Sn; E: chalcogenide atom S, Se, Te, or CH2) were shown to exhibit strong nonlinear optical (NLO) properties, either second-harmonic generation (SHG) or an unprecedented type of highly-directed white-light generation (WLG) - depending on their respective crystalline or amorphous nature. The (missing) crystallinity, as well as the maximum wavelengths of the optical transitions, are controlled by the clusters' elemental composition and by the nature of the organic groups R. Very recently, it has been additionally shown that cluster cores with increased inhomogeneity, like the one in compounds [RSi{CH2Sn(E)R '}3], not only affect the chemical properties, such as increased robustness and reversible melting behaviour, but that such 'cluster glasses' form a conceptually new basis for their use in light conversion devices. These findings are likely only the tip of the iceberg, as beside elemental combinations including group 14 and group 16 elements, many more adamantane-type clusters (on the one hand) and related architectures representing extensions of adamantane-type clusters (on the other hand) are known, but have not yet been addressed in terms of their opto-electronic properties. In this review, we therefore present a survey of all known classes of adanmantane-type compounds and their respective synthetic access as well as their optical properties, if reported. Adamantane-type clusters exhibit a huge diversity of chemical compositions, structural details, and resulting properties.

Cluster materials of the composition AdR4 (Ad = adamantane, R = organic substituent) and [(RT)4E6] (R = organic substituent; T = Si, Ge, Sn; and E = S, Se, Te) exhibit directional white light emission or produce second harmonics when irradiated with a continuous wave infrared laser source. The nature of the nonlinear optical properties correlates with the macroscopic structures of the cluster materials. The desired white light emission predominantly occurs in amorphous materials. It is therefore crucial to understand the geometric structures of the materials and the order within the materials. Here, we investigate the geometric structures of 12 different adamantane-like cluster materials by molecular dynamics simulations using a nonperiodic particle approach. The comparison of the calculated structure factors for two cluster materials with the corresponding experimental data obtained from diffraction and EXAFS measurements shows very good agreement. Our computations revealed that, on the one hand, larger, more flexible core structures (Ad < {Si4S6} < {Ge4S6} < {Sn4S6}) tend to lead to amorphous solids. On the other hand, larger substituents (methyl < phenyl < naphthyl) lead to more defined nearest neighbor interactions, with a tendency toward crystalline solids. Overall, our results show that a beginning order in the material results from a combination of the degree of flexibility of the core structure and the variation of the nearest neighbor interaction determined by the substituents.

The microscopic structures of two amorphous molecular solids with extremely nonlinear optical properties have been studied. They consist of organotetrel chalcogenide clusters with the chemical formula [(RSn)4S6]. The basic molecular building blocks are adamantane-like {Sn4S6} cores with organic ligands R attached to the Sn atoms. While the material equipped with R=naphthyl generates frequency doubling upon irradiation with a simple infrared laser diode, the material decorated with R=phenyl responds by emitting brilliant white light. The structural differences were investigated using x-ray scattering and extended x-ray absorption fine structure combined with molecular Reverse Monte Carlo. Transmission electron microscopy and scanning precession electron diffraction were used to examine structural differences from mesoscopic down to microscopic scales. Characteristic differences were found on all scales. While close core-to-core distances between {Sn4S6} cluster cores and molecular distortions are found in the white light emitting material, undistorted molecules and significantly larger core distances characterize the material showing frequency doubling. Here however, results of scanning precession electron diffraction reveal the formation of nanocrystalline structures in the amorphous matrix, which we identify as cause for the suppression of white light emission.

We have investigated the local atomic structures of several compositions of the amorphous phase of the system Cu x Ge50-x Te50 (0 ? X ? 33.3), based on extended x-ray absorption fine-structure as well as anomalous x-ray scattering experiments, and discuss the unusual trend regarding their thermal stability as a function of the Cu content. At low concentrations (X ? 15), Cu atoms tend to agglomerate in flat nanoclusters reminiscent of the crystalline phase of metallic Cu, leading to a more and more Ge-deficient Ge-Te host network structure with growing Cu content and an increasing thermal stability. At higher Cu concentrations (X ? 25), Cu is incorporated into the network, leading to an overall weaker bonding situation which is associated with a decreasing thermal stability.

The local structure of four amorphous organotin sulfide compounds is investigated by X-ray absorption spectroscopy. These compounds exhibit a nonlinear optical (NLO) response upon irradiation with a continuous-wave near-infrared laser. Their basic structural motif is a hetero-adamantane cluster with different organic substituent, but depending on the morphology of the compound and the choice of the organic substituent, the nature of the NLO response changes to either a second harmonic generation or the generation of a supercontinuum, potentially appearing as white light. The structural results provide an experimental evidence that the nature of the NLO properties is tied to distortions occurring at the cluster core, with almost ideal clusters in compounds that show second harmonic generation, and strongly distorted clusters in the case of compounds that generate a supercontinuum. These distortions may enable a closer proximity of the cluster cores, altering the intermolecular order, and thereby influencing the NLO response.

The microscopic structure of a new infrared-driven amorphous white-light-generating material was explored by X-ray diffraction, EXAFS and Reverse Monte Carlo simulation. In this material, structural disorder appears to be prerequisite for this nonlinear optical effect. The results are consistent with quantum chemical predictions, but it is also found that the molecular cores are distorted, which is identified as a crystallization inhibitor. Sulfur atoms thereby form a uniform vibrational network, which may be responsible for the high capability of the material to absorb infrared radiation.

To understand the relation of the glass-forming ability (GFA) to the local atomic configurations of a Pd42.5Ni7.5Cu30P20 (PNCP) metallic glass having the best GFA at present, the local structures were investigated by combining data obtained from anomalous X-ray scattering, X-ray and neutron diffraction, and applying reverse Monte Carlo modeling. By comparing the results of PNCP with Pd40Ni40P20 (PNP) and Pd40Cu40P20 (PCP) having a slightly and much worse GFAs, respectively, characteristic features were observed in the hyper-ordered atomic structures. Firstly, the concentration inhomogeneity of Ni/Cu in PNCP is larger than that of Ni in PNP and Cu in PCP. Secondly, a Voronoi tessellation showed that the fraction of pure icosahedral arrangements around the Cu atoms increases significantly in PNCP by adding icosahedral-preferred Ni atoms in PCP. Finally, a persistent homology (PH) analysis reveals the largest intermediate-size Cu PH rings in PNCP among the PH rings in these Pd-based BMGs. The structural heterogeneity for the excellent GFA of PNCP would be considered by an incompatible mixture of specific Pd-P configurations and icosahedral clusters around the secondary Ni and Cu metals.

The Front Cover illustrates the generation of a transmission electron microscopy diffraction pattern from an amorphous adamantane-type cluster compound and the conversion of infrared laser light into directed white light. Cover design by Elisa Monte. More information can be found in the Research Article by Kerstin Volz and co-workers.

The microscopic structure of two new infrared-driven amorphous white light generators, namely [(PhSn)(4)S-6] and [(CpSn)(4)S-6] and a related amorphous material [(NpSn)(4)S-6], showing 2nd harmonics generation instead, were explored by X-ray and neutron diffraction, EXAFS and Reverse Monte Carlo simulations to explore relations between their extreme nonlinear optical behavior and microscopic structural properties. The current state of research is reported and experimental and simulation results are discussed. The prominent observation is that the molecular units are distorted in the WLG materials which seems not to be the case for the 2nd harmonics generator. Associated is the formation of a net of similarly spaced intra- and intermolecular sulfur atoms which is interpreted as a vibrational network that could explain the high receptivity of the material for infrared radiation. It is also found that the molecules arrange in chains with staggered configuration regarding the arrangement of the organic ligands.

The stability against crystallization of the amorphous phase‐change materials (CGT) exhibits an unusual behavior at low Cu concentrations with being the most stable composition. Three CGT compositions with , 15, and 25 are analyzed using extended X‐ray absorption fine structure (EXAFS) at the Ge‐K, Cu‐K, and Te‐K edge at 10 K. EXAFS analysis shows that the average coordination number (CN) increases with the Cu concentration from 2.8 at to 4.0 at . Cu‐EXAFS spectra indicate the formation of Cu clusters for low Cu concentrations ( and ). The existence of these clusters is further analyzed by X‐ray absorption near‐edge structure calculations, supporting small polyhedral and/or disk‐like Cu clusters which are linked to the surrounding amorphous GeTe matrix via CuTe bonds. Hence, the GeTe network is Te enriched, leading to a maximum of GeTe bonds at . As a result, this establishes an amorphous network which is stabilized against crystallization.

The local structure of four amorphous organotin sulfide compounds exhibiting nonlinear optical (NLO) properties is investigated by low‐energy X‐ray absorption spectroscopy (XANES and EXAFS). The basic structural motif of a heteroadamantane cluster with different organic substituents is confirmed by the experiments and by comparison with computer simulations of the near‐edge structure. Essential information is obtained on a special role of the nonaromatic but electron‐rich cyclopentadienyl substituents, which are able to affect the structure of the heteroadamantane cluster core. The EXAFS fits also indicate a structural distinction between two groups within these compounds, which exhibit fundamentally different NLO responses: compounds that show a second harmonic generation can be described well using the single‐molecule approach, whereas the compounds exhibiting the generation of a supercontinuum manifest additional structural contributions.

Amorphous materials are an integral part of today's technology-they commonly are performant and versatile in integration. Consequently, future applications increasingly aim to harvest the potential of the amorphous state. Establishing its structure-property relationship, however, is inherently challenging using diffraction-based techniques yet is extremely desirable for developing advanced functionalities. In this article, we introduce a set of transmission electron microscopy-based techniques to locally quantify the structure of a material. This unique approach allows to clearly identify the spatial distribution of amorphous and crystalline regions and to quantify atomic arrangements of amorphous regions of a representative model system. We study an ensemble of well-defined, functionalized adamantane-type cluster molecules exhibiting exceptionally promising nonlinear optical properties of unclear origin. The nanoscopic structure for three model compounds ([(PhSn)(4)S-6], [(NpSn)(4)S-6], [(CpSn)(4)S-6]) correlates with their characteristic optical responses. These results highlight the advantageous properties of amorphous molecular materials when understanding the microscopic origin.

To understand the relation of the glass-forming ability (GFA) to the local atomic configurations of a Pd 42.5 Ni 7.5 Cu 30 P 20 (PNCP) metallic glass having the best GFA at present, the local structures were investigated by combining data obtained from anomalous X-ray scattering, X-ray and neutron diffraction, and applying reverse Monte Carlo modeling.By comparing the results of PNCP with Pd 40 Ni 40 P 20 (PNP) and Pd 40 Cu 40 P 20 (PCP) having a slightly and much worse GFAs, respectively, characteristic features were observed in the hyper-ordered atomic structures.Firstly, the concentration inhomogeneity of Ni/Cu in PNCP is larger than that of Ni in PNP and Cu in PCP.Secondly, a Voronoi tessellation showed that the fraction of pure icosahedral arrangements around the Cu atoms increases significantly in PNCP by adding icosahedral-preferred Ni atoms in PCP.Finally, a persistent homology (PH) analysis reveals the largest intermediate-size Cu PH rings in PNCP among the PH rings in these Pd-based BMGs.The structural heterogeneity for the excellent GFA of PNCP would be considered by an incompatible

To clarify the relationship between the glass-forming ability (GFA) and local atomic structure of Pd-based metallic glasses, the structures of an amorphous Pd40Cu40P20 (PCP) were investigated using the same methods as for Pd40Cu40P20 (PNP) metallic glass, i.e., by a combination of anomalous X-ray scattering, X-ray and neutron diffraction, and reverse Monte Carlo modeling. From the comparison between the results of PNP with an excellent GFA and the PCP with a worse GFA, important features characteristic to the GFA are found in the hyper-ordered structures. Firstly, the compositional inhomogeneity of Cu in PCP is larger than Ni in PNP. Secondly, a Voronoi tessellation reveals that icosahedral arrangements are found with considerable probability around the Ni atoms in PNP, while a less existence around the Cu atoms in PCP. Thirdly, a persistent homology (PH) analysis shows smaller intermediate-size Cu PH rings in PCP than Ni PH rings in PNP.

Molecular compounds of the general formula [(RT)(4)E-6] (R= organic or organometallic substituent; T= C, Si, Ge, Sn; E= CH2, S, Se), hence adamantane derivatives and inorganic-organic hybrid compounds based on a heteroadamantane structure exhibit a non-linear optical response upon radiation with a continuous-wave near-infrared laser. The effect depends on the compounds' habitus, which itself depends on the elemental composition of the cluster core, and on the nature of the organic substituents. A combination of these parameters that cause the material to be intrinsically amorphous leads to supercontinuum generation and thus to the emission of a broad spectrum, potentially appearing as white light. Notably, the emission essentially retains the driving laser's directionality. For crystalline samples, second harmonic generation is observed instead, which points to a close relationship of the optical properties and the intermolecular order. Variation of R, T, and E allows further fine-tuning of the emitted spectra. We present all studies made in regards to these effects and our overarching conclusions derived from them.

We present a model equation of states for expanded metals, which contains a pressure term due to a screened-Coulomb potential with a screening parameter reflecting the Mott-Anderson metal-to-nonmetal transition. As anticipated almost 80 years ago by Zel'dovich and Landau, this term gives rise to a second coexistence line in the phase diagram, indicating a phase separation between a metallic and a nonmetallic liquid.

An inelastic x-ray scattering experiment has been performed on molten NaCl over wide wave vector and energy transfer ranges. Data of high statistical quality are analyzed using a memory function approach within a generalized Langevin equation. The approach with two relaxation times for the memory function provides a very good data description over the whole wave vector range beyond the hydrodynamic regime. A slow thermal and a fast structural relaxation process in the memory function completely define the density fluctuations in molten NaCl and evidences the thermal-viscoelastic model as the minimal description for collective particle dynamics in molten alkali halides. The obtained excitation frequencies demonstrate a large positive dispersion effect, which can be related to the viscoelastic reaction of the molten salt. A transition from the viscoelastic to a hydrodynamic response of the molten salt at small wave vectors is observed. In the hydrodynamic regime the resulting thermal diffusivity agrees well with values obtained through light scattering. The modeling indicates some deficiencies at small wave vectors and large energy transfers and the spectra of the current correlation function evidences additional intensity at high frequency. The frequency of these additional modes approach a non-zero value at zero wave vector and indicates a non-acoustic character of these excitations. The frequency center of this additional inelastic intensity coincides with optic-type modes in molten NaCl predicted by simulations.

Abstract We describe structural features of ternary Ge-Sb-Te glasses based on anomalous X-ray scattering experiments for four different compositions along the pseudo-binary line (GeTe)1–x(Sb2Te3)x for 0≤x≤23$0\leq x\leq\frac{2}{3}$, which comprises technologically important phase-change materials. The data are analyzed with reverse Monte Carlo modeling. This way, detailed information on the local environments around all constituent elements are obtained and their evolution with the chemical composition is described. This approach gives a consistent view of the atomic scale structure of the amorphous networks in these compounds. Thereby, it is possible to derive the relationship between the relevant material properties and the determining structural features. In particular, it is shown that the optical contrast related to the phase change is associated with the evolution of the Ge-Te core network.

We report on the recent advances regarding the source code optimization of Reverse Monte Carlo modelling used in scattering data analysis of an amorphous molecular solid which has recently attracted attention as a new brilliant white light emitter if irradiated by a simple infrared laser diode. The algorithm used for generating random molecular starting configurations without overlapping molecules in a box with periodic boundary conditions has been accelerated by a factor of roughly 400 in a 54k atom case. The resulting bigger independent starting configurations are used to gain further insight into previously presented x-ray scattering data. New improved scattering data have been obtained, revealing new structural features in the lower Q range.