Volume 232, Issue 1-3 - Special Issue: Final reports of DFG priority program 1415 Crystalline non-equilibrium phases / Guest Editors: Wolfgang Bensch, Josef Breu

New routes to vanadium sesquioxide and tantalum oxide nitride (γ- and δ-phase) are presented. Phase pure V 2 O 3 with bixbyite-type structure, a metastable polymorph, was obtained from vanadium fluoride hydrates at ~750 K. It crystallizes in the cubic crystal system in space group Ia3¯$Ia\bar 3$ with lattice parameter a =939.30(5) pm. The catalytical properties of the corresponding oxide nitride phases and their oxidation and reduction solid-state kinetics were investigated. The preparation of γ-TaON as a phase pure sample can be realized by ammonolysis of X-ray amorphous tantalum oxide precursors at 1073 K. This metastable tantalum oxide nitride crystallizes in the monoclinic VO 2 (B)-type structure in space group C 2/ m . The same precursors can be used to synthesize the δ-modification with an anatase-type structure at 1023 K. It crystallizes in the tetragonal crystal system in space group I 4 1 / amd . A maximum yield of 82 m % could be obtained. The fundamental band gaps of the synthesized and of other metastable TaON polymorphs were calculated from first principles using the GW method. The present results are compared to experimental data and to previous calculations at hybrid DFT level.

Theoretical and experimental studies indicate that crystal nucleation can take more complex pathways than expected on the ground of the classical nucleation theory. Among these pathways are the formation of pre-nucleation clusters and amorphous precursor phases. A direct in situ observation of the different pathways of nucleation from solution is challenging since the paths can be influenced by heterogeneous nucleation sites, such as container walls. Here, we provide insights into the crystallization process using the in situ combination of an acoustic levitator, Raman spectroscopy, and X-ray scattering. The contactless sample holder enables the observation of homogeneous crystallization processes and the detection of intermediates and final crystalline forms. We provide evidence for the existence of multiple pathways of nucleation based on the investigation of the crystallization of organic molecules from different solvents. Starting from a diluted solution, a supersaturation is reached during the experiment due to the evaporation of the solvent. The highly supersaturated solution reveals different pathways of crystallization. Depending on the degree of supersaturation either the thermodynamically stable or the metastable crystal form is observed.

Radiation damage in minerals is caused by the α-decay of incorporated radionuclides, such as U and Th and their decay products. The effect of thermal annealing (400–1000 K) on radiation-damaged pyrochlores has been investigated by Raman scattering, X-ray powder diffraction (XRD), and combined differential scanning calorimetry/thermogravimetry (DSC/TG). The analysis of three natural radiation-damaged pyrochlore samples from Miass/Russia [6.4 wt% Th, 23.1·10 18 α-decay events per gram (dpg)], Panda Hill/Tanzania (1.6 wt% Th, 1.6·10 18 dpg), and Blue River/Canada (10.5 wt% U, 115.4·10 18 dpg), are compared with a crystalline reference pyrochlore from Schelingen (Germany). The type of structural recovery depends on the initial degree of radiation damage (Panda Hill 28%, Blue River 85% and Miass 100% according to XRD), as the recrystallization temperature increases with increasing degree of amorphization. Raman spectra indicate reordering on the local scale during annealing-induced recrystallization. As Raman modes around 800 cm −1 are sensitive to radiation damage (M. T. Vandenborre, E. Husson, Comparison of the force field in various pyrochlore families. I. The A 2 B 2 O 7 oxides. J. Solid State Chem. 1983 , 50 , 362, S. Moll, G. Sattonnay, L. Thomé, J. Jagielski, C. Decorse, P. Simon, I. Monnet, W. J. Weber, Irradiation damage in Gd 2 Ti 2 O 7 single crystals: Ballistic versus ionization processes. Phys. Rev. 2011 , 84 , 64115.), the degree of local order was deduced from the ratio of the integrated intensities of the sum of the Raman bands between 605 and 680 cm −1 divided by the sum of the integrated intensities of the bands between 810 and 860 cm −1 . The most radiation damaged pyrochlore (Miass) shows an abrupt recovery of both, its short- (Raman) and long-range order (X-ray) between 800 and 850 K, while the weakly damaged pyrochlore (Panda Hill) begins to recover at considerably lower temperatures (near 500 K), extending over a temperature range of ca. 300 K, up to 800 K (Raman). The pyrochlore from Blue River shows in its initial state an amorphous X-ray diffraction pattern superimposed by weak Bragg-maxima that indicates the existence of ordered regions in a damaged matrix. In contrast to the other studied pyrochlores, Raman spectra of the Blue River sample show the appearance of local modes above 560 K between 700 and 800 cm −1 resulting from its high content of U and Ta impurities. DSC measurements confirmed the observed structural recovery upon annealing. While the annealing-induced ordering of Panda Hill begins at a lower temperature (ca. 500 K) the recovery of the highly-damaged pyrochlore from Miass occurs at 800 K. The Blue-River pyrochlore shows a multi-step recovery which is similarly seen by XRD. Thermogravimetry showed a continuous mass loss on heating for all radiation-damaged pyrochlores (Panda Hill ca. 1%, Blue River ca. 1.5%, Miass ca. 2.9%).

We report on microsecond-resolved in-situ SAXS experiments of the early nucleation and growth behavior of both cadmium sulfide (CdS) quantum dots in aqueous solution including the temperature dependence and of gold (Au) nanoparticles. A novel free-jet setup was developped to access reaction times as early as 20 μs. As the signal in particular in the beginning of the reaction is weak the containment-free nature of this sample environment prooved crucial. The SAXS data reveal a two-step pathway with a surprising stability of a structurally relaxed cluster with a diameter of about 2 nm. While these develop rapidly by ionic assembly, a further slower growth is attributed to cluster attachment. WAXS diffraction confirms, that the particles at this early stage are not yet crystalline. This growth mode is confirmed for a temperature range from 25°C to 45°C. An energy barrier for the diffusion of primary clusters in water of 0.60 eV was experimentally observed in agreement with molecular simulations. To access reaction times beyond 100 ms, a stopped-drop setup -again contaiment- free is introduced. SAXS experiments on the growth of Au nanoparticles on an extended time scale provide a much slower growth with one population only. Further, the influence of ionizing X-ray radiation on the Au particle fromation and growth is discussed.

The formation and transformation of crystalline chalcogenido metalates in ionic liquids as solvents is discussed as a new synthetic approach for the generation of novel materials. A comparison to conventional solvothermal reactions, classical high-temperature syntheses and chemical solution based approaches in common is discussed.

The amphiphilic nature of benzoic acid, benzoates and benzamide causes an unexpected rich polymorphism. Featuring rather rigid and small molecular structures these compounds are ideal model systems for gaining a more fundamental understanding of molecular polymorphism by systematic and concerted investigations. The hydrophilic head allows for hydrogen bonding while the phenyl moiety gives rise to various π-stacking modes. Variations of hydrogen bonding versus π-stacking modes give rise to four polymorphs of benzamide. The central synthon in all phases is a dimer where hydrophilic units form double hydrogen bonds. As suggested by MD simulations of the nucleation process, variations of the crystallization conditions trigger whether the first self-assembly occurs via the hydrophilic head or the hydrophophic tail groups. Based on NMR crystallographic investigations for the co-crystallization of benzamide with benzoic acid, we observed yet another variation of the balance of the two dominating intermolecular interactions leading to the formation of a 1:1 co-crystal. The average crystal structure resembles the packing motive of pure benzoic acid with alternating ribbons of homogenous benzamide and benzoic acid dimers. For alkali-benzoate salts a coordination dilemma arises that is of general importance for many active pharmaceutical ingredients (APIs). A 1:1 stoichiometry requires condensation of coordination polyhedra of small inorganic cations which in turn causes steric stress that varies with the relative volumes of cation and anion. Interestingly, one way of resolving the dilemma is microphase separation which is directly related to the amphiphilic character of benzoate.

The sesquioxides of molybdenum and tungsten have been reported as thin films or on surfaces as early as 1971, but the preparation of bulk materials and their crystal structures are still unknown up to the present day. We present a systematic ab initio approach to their possible syntheses and crystal structures applying complementary methods and basis-set types. For both compounds, the corundum structure is the most stable and does not display any imaginary frequencies. Calculations targeted at a high-pressure synthesis starting from the stable oxides and metals predict a reaction pressure of 15 GPa for Mo 2 O 3 and over 60 GPa for W 2 O 3 .

In this report, we summarize our theoretical and experimental investigations on the zeolitic imidazolate framework (ZIF) system [Zn(dcim) 2 ] (dcim=4,5-dichloroimidazolate) that have been published recently. These comprise: (1) a theoretical study on hypothetical conformational [Zn(dcm) 2 ]-SOD polymorphs with the same underlying sodalite (SOD) topology but distinct dcim linker orientations, (2) a synthetic work that resulted in the experimental realization of the most stable predicted (trigonal) SOD-type framework conformer and improved synthetic protocols for a previously discovered cubic SOD-type material, (3) a detailed structural analysis of the trigonal and cubic SOD-type materials, (4) a comparative characterization of the SOD-type materials by gas physisorption measurements, (5) a synthetic work that resulted in the discovery of a complete series of intermediate frameworks with the trigonal and cubic SOD-type materials as the end members, and (6) time-resolved in-situ light and stopped-flow synchrotron small-angle and wide-angle X-ray scattering experiments on the rapid crystallization of the RHO-type polymorph (ZIF-71). In addition, we report as yet unpublished work, concerning time-resolved in-situ angular-dispersive synchrotron X-ray diffraction experiments on RHO-/SOD-type phase selection via the coordination modulation approach during competitive formation of the RHO-type and SOD-type materials.

The application of the EnPhaSyn (theoretical Energy diagrams, experimental Phase formation, Synthesis and characterisation) concept is reviewed with respect to prediction of structures and stability of element allotropes and compound polymorphs, their phase formation and transition processes, and their directed synthesis, respectively. Therein, the relative energetical stability (En) of target compounds and possible decomposition are determined from quantum chemical DFT calculations. Phase formation and transition (Pha) is probed by a gas balance method, developed as high temperature gas balance concept. It helped to study the synthesis and stability range of several compounds experimentally. Applications of the concept and synthesis principles (Syn) of non-equilibrium phases are presented for allotropes of P, As, P 1-x As x , as well as binary and ternary compounds including the Zintl and Laves like phases IrPTe, NiP 2 , CoSbS, NiBiSe, Li 0.2 CdP 2 , Cu 3 CdCuP 10 , and Cd 4 Cu 7 As.

In many cases, limitations in conventional synthesis routes hamper the accessibility to materials with properties that have been predicted by theory. For instance, metastable compounds with local non-equilibrium structures can hardly be accessed by solid-state preparation techniques often requiring high synthesis temperatures. Also other ways of preparation lead to the thermodynamically stable rather than metastable products. Fortunately, such hurdles can be overcome by mechanochemical synthesis. Mechanical treatment of two or three starting materials in high-energy ball mills enables the synthesis of not only new, metastable compounds but also of nanocrystalline materials with unusual or enhanced properties such as ion transport. In this short review we report about local structures and ion transport of oxides and fluorides mechanochemically prepared by high-energy ball-milling.

Phase pure metastable indium oxyhydroxide (InOOH) with crystallite size in the range ca. 2–7 nm is synthesized by a nonaqueous solvothermal synthesis route in ethanol. The influence of synthesis parameters such as temperature, basicity (pH), synthesis time, and water content is carefully addressed. T-pH maps summarize the impact of synthesis temperature and pH and reveal that phase pure InOOH is obtained in water-free solutions at mild temperatures (150–180°C) in highly basic conditions (pH>12). Subsequent calcination of InOOH at 375–700°C in ambient air atmosphere results in metastable nanoscaled rhombohedral indium oxide (rh-In 2 O 3 ). The synthesis protocol for phase pure nanocrystalline InOOH material was successfully upscaled allowing for obtaining ca. 3 g of phase-pure InOOH with a yield of ca. 78%. The upscaled InOOH and rh-In 2 O 3 batches are now available for a detailed in-situ characterization of the mechanism of decomposition of InOOH to rh-In 2 O 3 to c-In 2 O 3 as well as for the characterization of the functional properties of InOOH and rh-In 2 O 3 materials.

In this contribution we report on the development and application of modern NMR approaches for the in situ characterization of the crystallization of metastable materials. The work was performed within the framework of the DFG priority programme SPP 1415 “Crystalline Non-Equilibrium Phases”. As one of the goals of this project, the development of a NMR methodology which enables an analysis of local structural motifs on short (1–2 Å) and extended (2–6 Å) length scales without the need for fast magic angle spinning (MAS) has been defined, since the enormous centripetal forces which occur during fast sample rotation (up to 10 7 g) may intervene with the chemical or physical process which is being monitored. To achieve this goal, we developed a magic angle turning probe and pulse sequences allowing to trace the isotropic chemical shifts and heteronuclear dipolar couplings and hence the determination of structural motifs on short and intermediate length scales. With the implementation of novel inductive heating approaches the range of accessible rotation frequencies for in situ high temperature NMR measurements has been enlarged, now covering the ν MAS range of 0–10 kHz with an accessible temperature of up to 700°C. Application of NMR methodologies for the characterization of crystallization processes and the structure and dynamics of novel phases, partially in joint collaborations within the priority program, are also reported.

Most functional materials are thermodynamic equilibrium phases representing minima in the thermodynamic phase space. However, it is expected that many metastable phases with highly interesting properties also exist. Here, we report on a systematic approach to prepare thin-films of such non-equilibrium phases based on the gas phase deposition methods sputtering and pulsed laser deposition (PLD). Our synthetic strategy is to deposit a “precursor phase” which is amorphous or already a crystalline non-equilibrium phase. Subsequent heat treatment leads to the nucleation of crystalline phases which again may be metastable or stable compounds. In the present paper we focus on the binary systems Ti–O and Zr–O, both systems being widely applied and technologically relevant. Highly oxygen-deficient titanium oxide (TiO 1.6 ) and zirconium oxide (ZrO) films prepared by pulsed laser deposition at room temperature are optically absorbing and possess electronic conductivities in the range of 10 S/cm. Both materials are metastable in respect to both composition and structure. For TiO 1.6 we find an amorphous matrix with embedded grains of cubic titanium monoxide (γ-TiO) directly after deposition. Upon annealing nanocrystalline grains of metallic Ti are formed in the amorphous matrix due to an internal solid-state disproportionation whereas the electrical conductivity of the films increases and comes close to metal-like conductivity (1000 S/cm) at about 450 °C. Congruently, room temperature deposited ZrO films with an average composition of Zr:O= 1:1 contain small ZrO nanocrystals within an amorphous matrix. Heat treatment again leads to an internal disproportionation reaction whereas small crystals of Zr 2 O and ZrO 2 precipitate at temperatures as low as 75 °C. Increasing the temperature then results in the crystallization of metastable tetragonal ZrO 2 at about 400 °C. Sputter deposition allows a subtler control of the oxygen partial pressure. Slightly non-stoichiometric TiO 2− x films form a degenerate semiconductor with room temperature conductivities as high as 170 S/cm. Moreover, controlling both, the doping level and the vacancy concentration of these films allows to control the phase formation and the transition temperature between the rutile and anatase TiO 2 polymorphs. Niobium doping of sputter deposited TiO 2 can lead to films with very high electrical conductivities while maintaining a high optical transmittance demonstrating the potential of the material as an alternative transparent conducting oxide (TCO) with extraordinary properties.

Investigations on bismuth oxido clusters are focused on the nucleation and growth processes towards large cluster motifs and their stability in the gas phase, which has been studied by electrospray ionization mass spectrometry (ESI-MS), molecular dynamics (MD) simulations and X-ray scattering experiments evaluated by pair distribution function (PDF) analysis. The formation of metastable bismuth(III) oxides was obtained by hydrolysis of polynuclear bismuth oxido clusters and subsequent thermal treatment under non-equilibrium conditions. Temperature dependent PXRD and Raman spectroscopic experiments gave insight into the formation process of metastable β-Bi 2 O 3 starting from the amorphous hydrolysis products as-obtained from polynuclear bismuth oxido clusters. Furthermore, PXRD as well as energy-dispersive X-ray (EDX) spectroscopy confirmed the formation of several new ternary bismuth(III) rich oxides such as Bi 14 O 20 (MO 4 ) (M=S, Se) as-obtained by hydrolysis of bismuth oxido clusters in the presence of diverse additives.

Divalent metal phosphonates are promising hybrid materials with a broad field of application. The rich coordination chemistry of the phosphonate linkers enables the formation of structures with different dimensionalities ranging from isolated complexes and layered structures to porous frameworks incorporating various functionalities through the choice of the building blocks. In brief, metal phosphonates offer an interesting opportunity for the design of multifunctional materials. Here, we provide a short review on the class of divalent metal phosphonates discussing their syntheses, structures, and applications. We present the advantages of the recently introduced mechanochemical pathway for the synthesis of divalent phosphonates as a possibility to generate new, in certain cases metastable compounds. The benefits of in situ investigation of synthesis mechanisms as well as the implementation of sophisticated methods for the structure analysis of the resulting compounds are discussed.

The intermetallic phase [Li x Ba 8− x ][Li y Si 46− y ] is the first example of a clathrate-I silicide containing lithium with the peculiarity that the Li atoms occupy both cage and framework positions. The phase was obtained by ambient-pressure solid-state reactions at 400°C between Ba 4 Li 2 Si 6 and the binary clathrate phase Ba 8− x Si 46 applying different experimental approaches. In this reaction, Li atoms substitute Si atoms of the framework at site 6 c (space group Pm 3̅ n ) and fill up the Ba-deficient dodecahedral Si 20 cages at site 2 a . The presence of Li atoms in the clathrate phase was proven by combined X-ray powder diffraction, 7 Li and 29 Si solid-state NMR analyses. Incorporation of lithium markedly increased the lattice parameter of the clathrate phase, e.g. from a =10.3200(2) Å for Ba 7.48(2) Si 46 to a =10.3715(3) Å for [Li 0.59(2) Ba 7.41(2) ][Li 2.3(1) Si 43.7(1) ]. The critical temperature T c for the transition to the superconducting state decreased from 7.7 K to 3.2 K upon Li incorporation.

The binary compound V 3 Sb (V 2.64 Sb, V 3 Sb and V 3.24 Sb) was synthesized as thin multilayered films with varying V:Sb ratios. The V-content determines the crystallization temperature and it is highest for the film with the lowest amount of V. Ternary chromium–vanadium–antimony (Cr–V–Sb) films were prepared containing Cr from 10 to 51 at-% with the Sb content fixed to yield M 3 Sb (M=Cr, V). In the as-deposited state the layers are already interdiffused which is most likely caused by the very low repeating unit thickness between 0.29 and 0.68 nm investigated by X-ray diffraction experiments. All ternary compounds crystallized from the amorphous state with crystallization temperatures depending more on the repeating unit thickness than on chemical composition. For most samples the simultaneous crystallization of the two phases M 3 Sb (A15 structure type) and MSb is observed. The crystalline A15 compounds are only stable in a limited temperature range and decompose at elevated temperatures. Compared to the binary Cr–Sb system crystallization of the hexagonal phase MSb (M=Cr, V) occurs at remarkably higher temperatures, i.e. in the ternary system nucleation and crystallization of this phase is hindered. The chemical composition requires short-range composition fluctuations to nucleate the binary phase. The first principles total energy calculations using the spin-polarized relativistic Korringa–Kohn–Rostoker (SPR-KKR) method confirm the experimental observations concerning the concentration-dependent stability of different phases of the Cr–V–Sb system. For the ratio M:Sb=3:1 the system is preferably stabilized in the A15 crystal structure for all possible Cr and V concentrations, while an increase of Sb content up to M:Sb=2:1 results in the stabilization of the Ni 2 In structure for almost all Cr concentrations. Only in the V-rich regime of the system the Heusler Ni 2 MnAl-type structure was found to be energetically more preferable.

Three new bismuth arylsulfonatocarboxylates [Bi(OH)(SB)] ( 1 ), [Bi 4 (ST) 2 (HST)O 2 (H 2 O) 2 ]·H 2 O ( 2 ) and [Bi 4 (ST) 2 O 3 (H 2 O) 2 ] ( 3 ) were synthesized under solvothermal reaction conditions at 180°C using the potassium or sodium salt of 4-sulfobenzoic acid (H 2 SB) and 2-sulfoterephthalic acid (H 3 ST), respectively. The compounds were characterized in detail and the crystal structures were determined from single crystal X-ray diffraction data. Phase purity was confirmed by powder X-ray diffraction and elemental analysis. Structural comparisons to the only three other known bismuth sulfonatocarboxylates are presented. Due to the higher reaction temperatures employed for the synthesis of the title compounds a higher degree of condensation of the BiO x polyhedra (X=7 or 8) to tetrameric units, 1D chains or a 2D layer is observed. Connection through the organic linker molecules leads to the formation of 3D coordination polymers in all three title compounds.

Recent advances in our understanding of CaCO 3 nucleation from solution have provoked new and challenging questions. We have studied CaCO 3 formation using precipitation by carbonate ester hydrolysis which ensures precipitation from a strictly homogeneous solution state and allows “titrating” carbonate to a solution with a given Ca 2+ concentration on a timescale suited for kinetic studies. Nucleation and crystallization were traced by combining dynamic light scattering (DLS) and transmission electron microscopy (TEM). DLS served as in situ technique to identify the nucleation time, to monitor particle size evolution, to discriminate different precipitation mechanisms and to validate reproducibility. TEM snapshots taken during different stages of the precipitation process identified different phases and morphologies. At a high level of supersaturation homogeneous nucleation in solution led to the formation of amorphous CaCO 3 particles (Ø≈30 nm), which transformed via vaterite to calcite. Nucleation occurred uniformly in solution which appears to be unique for the CaCO 3 system. In the presence of Na-polymethacrylate (Na-PMA), heterogeneous nucleation was suppressed and Ca-polymer aggregates were formed in the prenucleation stage. Beyond a critical threshold supersaturation CaCO 3 particles formed in solution outside of these aggregates. The nucleation process resembled that without additive, indicating that Na-PMA exerts only a minor effect on the CaCO 3 nucleation. In the postnucleation stage, the polymer led to the formation of extended liquid-like networks, which served as a precursor phase for solid ACC particles that formed alongside the network.