Crystals and crystal growth

Internal structure and unit cellsMicrographs of virus crystals Crystals consist of ordered arrays of objects(molecules, ions .....) The description of crystal structures can besimplified using the translational symmetryof the solid and breaking it up intotranslationally equivalent unit cellsDefects in crystals Real crystals never contain perfectlyordered arrays of atoms - there are alwaysdefectsThe unit cell Always use a right handed axis systemThe seven crystal systemsCrystal system Unit cellUnit cell Lattice symmetryedge lenghs anglesTriclinic a ° b ° c ° ° ° 90 o -12/mMonoclinic a ° b ° c = = 90 o > 90 oOrthorhombic a ° b ° c = = = 90 o mmmTetragonal a = b ° c = = = 90 o 4/mmmTrigonalrhombohedral latticea = b = c = = ° 90 o -3mHexagonal a = b ° c = = 90 o6/mmm = 120 oCubic a = b = c = = = 90 o m3mCrystal growth -theory Initially form a nucleus– this is highly dependent upon the presence ofimpurities and vessel surfaces Growth of nucleus– growth depends upon the presence of lowenergy surface sites such as steps– low energy faces grow fastest– presence of additives in solution may alterrelative rates for different faces

Crystal growth - practice Many techniques are available– choice of method depends upon the material Grow from molten solid Grow from solution– this can include solvents as diverse as water andmolten PbO! Grow from vapor phaseGrowth from the melt Very important, but not possible for mostmaterials Material must be stable above melting point Phase diagram must allow directcrystallizationLi 2 O - Nb 2 O 5 phase diagramGrowth from the vapor phase Not very common Used for some sublimable compounds Used where a material can be transported bythe addition of an appropriate reagent–ZnS (s) + I 2(g) --- ZnI 2(g) + 1/8S 8(g) (exothermic)Growth from solutionVapor and solute diffusion The most common method Based on precipitation from a saturatedsolution Saturation can be achieved in many ways– cool solution– evaporate solvent off– add things to solvent to reduce solubility

Crystal habit The habit of a crystal is its external shapeMeasurement of interfacial angles The angles between the external faces of acrystal are characteristic of the material Measured using a goniometer External faces are the slowest growing faces Habit depends upon growth conditionsDescribing crystal facesMiller Indices Crystal faces can be assigned labels calledMiller Indices– derived from how the face cuts the cell axesStereographic projectionsDifferent habits for cubic crystals

Translational symmetryThe lattice Real crystals consist of repeating patterns ofmolecules or ions or clusters ... It is often convenient to separate therepeating pattern from the object that isbeing repeated