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Controlling the shape and size of nanostructured materials has been a topic of interest in the field of material science for decades. In this work, the ferroelectric material Srx Ba1-x Nb2 O6 (x=0.32-0.82, SBN) was prepared by hydrothermal synthesis, and the morphology is controllably changed from cube-shaped to hollow-ended structures based on a fundamental understanding of the precursor chemistry. Synchrotron X-ray total scattering and PDF analysis was used to reveal the structure of the Nb-acid precursor, showing Lindqvist-like motifs. The changing growth mechanism, from layer-by-layer growth forming cubes to hopper-growth giving hollow-ended structures, is attributed to differences in supersaturation. Transmission electron microscopy revealed an inhomogeneous composition along the length of the hollow-ended particles, which is explained by preferential formation of the high entropy composition, SBN33, at the initial stages of particle nucleation and growth.

Controlling the shape and size of nanostructured materials has been a topic of interest in the field of material science for decades. In this work, the ferroelectric material SrxBa1−xNb2O6 (x=0.32–0.82, SBN) was prepared by hydrothermal synthesis, and the morphology is controllably changed from cube‐shaped to hollow‐ended structures based on a fundamental understanding of the precursor chemistry. Synchrotron X‐ray total scattering and PDF analysis was used to reveal the structure of the Nb‐acid precursor, showing Lindqvist‐like motifs. The changing growth mechanism, from layer‐by‐layer growth forming cubes to hopper‐growth giving hollow‐ended structures, is attributed to differences in supersaturation. Transmission electron microscopy revealed an inhomogeneous composition along the length of the hollow‐ended particles, which is explained by preferential formation of the high entropy composition, SBN33, at the initial stages of particle nucleation and growth., Take shape: Controlling the shape and size of nanostructured materials has been a topic of interest in the field of material science for decades. Here SrxBa1−xNb2O6 (x=0.32–0.82, SBN) was prepared by hydrothermal synthesis, and the morphology is controllably changed from cube‐shaped to hollow‐ended structures based on a fundamental understanding of the precursor chemistry.

The growth of hopper crystals is observed for many substances, but the mechanism of their formation remains ill understood. Here we investigate their growth by performing evaporation experiments on small volumes of salt solutions. We show that sodium chloride crystals that grow very fast from a highly supersaturated solution form a peculiar form of hopper crystal consisting of a series of connected miniature versions of the original cubic crystal. The transition between cubic and such hopper growth happens at a well-defined supersaturation where the growth rate of the cubic crystal reaches a maximum (∼6.5 ± 1.8 μm/s). Above this threshold, the growth rate varies as the third power of supersaturation, showing that a new mechanism, controlled by the maximum speed of surface integration of new molecules, induces the hopper growth of cubic crystals in cascade., The Journal of Physical Chemistry Letters, 9 (11), ISSN:1948-7185

Nodular chromite is a characteristic feature of ophiolitic podiform chromitite and there has been much debate about how it forms. Nodular chromite from the Troodos ophiolite in Cyprus is unusual in that it contains skeletal crystals enclosed within the centres of the nodules and interstitial to them. 3D imaging and electron backscatter diffraction have shown that the skeletal crystals within the nodules are single crystals that are surrounded by a rim of polycrystalline chromite. This rim is composed of a patchwork of chromite grains that are truncated on the outer edge of the rim. 3D analysis reveals that the skeletal crystals are partially or completely formed cage or hopper structures elongated along the axis. The skeletal crystals first formed undisturbed in a stagnant or slowly flowing magma supersaturated in Cr and silicates were trapped between the chromite skeletal crystal blades as they grew. The formation of skeletal crystals was followed by a crystallisation event which formed a silicate-poor rim of chromite grains around the skeletal crystals. This was subsequently subjected to corrosion or erosion probably in an environment of rapid magma flow. At contact points where nodules impinge on each other the grains of chromite are deformed.

Crystal growth of a hindered phenol compound, tetrakis [methylene-3-(3-5-ditert-butyl-4-hydroxy phenyl) propionyloxy] methane (trade name AO-60), was successfully recorded by optical microscopy (OM) equipped with a hot stage. The morphology of AO-60 crystals, grown at 100 °C from amorphous state, appeared in the form of tetragonal-sloped step growth. Further study using scanning electron microscopy (SEM) and atomic force microscopy (AFM) experimentally demonstrated that AO-60 crystals had a hopper-like morphology, which had occurred rarely in the condition of organic compound crystals but predominantly rather to inorganic compound crystals in the reported literatures. The morphological features observed on the crystal surfaces suggested step growth and hopper growth mechanism. Besides, a raising around the edge of the AO-60 hopper crystal was also experimentally characterized for the first time.

Colorless transparent prismatic crystals (0.5-2.0 mm long) and hopper crystals (1.0-2.5 mm long) of GaN were prepared by heating a Ga melt at 800°C in Na vapor under N2 pressures of 7.0 MPa for 300 h. The photoluminescence (PL) spectrum of a prismatic crystal at 4 K showed the emission peaks of neutral donor-bound exciton (D0-X) and free exciton (XA) at 3.472 eV and 3.478 eV, respectively, in the near band edge region. The full-width at half-maximum (FWHM) of (D0-X) peak was 1.9 meV. The emission peaks of a donor–acceptor pair transition (D0-A0) and its phonon replicas were observed in a lower energy range (2.9-3.3 eV). The emission peaks of the D0-A0 and phonon replicas were also observed in the cathodoluminescence (CL) spectrum at 20 K. The (D0-X) PL peak of a hopper crystal at 4 K was at 3.474 eV (2.1 meV higher), having a FWHM of 6.1 meV which was over 3 times larger than that of the prismatic crystal. A strong broad band with a maximum intensity around 1.96 eV was observed for the hopper crystals in the CL spectrum at room temperature. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We performed dynamic crystallization experiments in the CMAS system at 1 atm to investigate the evolution of the morphology of forsterite crystals as a function of cooling rate and degree of undercooling. In sections parallel to the (010) plane, we observed the evolution of the forsterite morphology from tablets to hopper (skeletal) crystals, and then to swallowtail shapes (dendritic morphology) for increased degree of undercooling. The other shapes described in the literature can be interpreted as particular sections of those three shapes. The onset of dendritic growth is due to a competition between the growth of the faces of the initial hopper crystal and dendrite overgrowths. The forsterite dendrites are formed by a succession of units which look like hopper shapes. This result has been tested by an additional set of experiments.

Calcium carbonate was grown by diffusion of gaseous carbon dioxide into aqueous calcium chloride–alcohol solutions and the relationship between the alcohol additive and the growing crystal was investigated. There was no evidence that crystal growth was controlled through any specific molecular interaction between the growing calcium carbonate facets and the alcohol. Under control conditions, both calcite and vaterite were grown in a ratio of 1∶3, respectively. However, the overall trend for all the alcohols investigated was preferential precipitation of calcite, as compared to the control. In addition to this primary alcohol effect, a morphological variant of perfect calcitic rhombohedra, a hopper crystal, was observed. This morphological form was present in all growth solutions, but made up an increasing proportion of the calcite fraction for solutions with viscosities above approximately 1.6 mPa s, irrespective of the absolute alcohol concentration. The different morphological and polymorphic forms of the calcium carbonate crystals were examined by scanning electron microscopy and powder X-ray diffraction, respectively.

We study the spontaneous nucleation and growth of sodium chloride crystals induced by controlled evaporation in confined geometries (microcapillaries) spanning several orders of magnitude in volume. In all experiments, the nucleation happens reproducibly at a very high supersaturation S~1.6 and is independent of the size, shape and surface properties of the microcapillary. We show from classical nucleation theory that this is expected: S~1.6 corresponds to the point where nucleation first becomes observable on experimental time scales. A consequence of the high supersaturations reached at the onset of nucleation is the very rapid growth of a single skeletal (Hopper) crystal. Experiments on porous media reveal also the formation of Hopper crystals in the entrapped liquid pockets in the porous network and consequently underline the fact that sodium chloride can easily reach high supersaturations, in spite of what is commonly assumed for this salt., 16 pages, 6 figures

Instability conditions in growth are suggested for a crystal developing hopper morphology. We show that these conditions, when applied to the crystal growing as a eutectic phase, determine its development as a spiral.