Solid State

Solid State – Basic concept

The topic of collage is solid state crystal which is shown on fore head of horse

On the neck of horse are shown the objectives of collage:to identify

a)unit cell in a symmetry pattern,

b)to identify a difference in unit cell and lattice

Crystalline solids: highly regular arrangement of atoms, ions, molecules - periodic (repeating)

Amorphous solids: no repeating pattern, only short range order, extensively disordered - non crystalline (e.g. glasses)

We will focus on Crystalline solids :

Let us start with previous knowledge with which we are already familiar 1.crystals are solid but opposite of it is not true ie solids are not necessarily crystalline.

2.It was Kepler who found out that crystals have symmetry and long range order

3.It was Hooks and Hauy who stated that small shapes and spheres can be packed to produce regular shapes of crystals. Haüy argued that the structures of crystals involved regular lattices of repeating units

.Scientist Johannes Kepler always wondered why snow flakes which you see in winter have 6 sides not 5 or seven. packing of polygon , pentagon and heptagon is not possible in two dimension

Crystalline solids :have repeating unit called unit cell.The figure below shows Lattice + motif(wagon) forming symmetry pattern.

Any periodic pattern can be described by placing lattice points at equivalent positions within each unit of the pattern.

To recover original pattern we add the motif(wagon) to each lattice point.

1-D lattices. The regular pattern of wagons below can be described by placing a lattice point at the same place in each wagon. The arrangement of dots is the lattice, which has a given repeat distance. The motif is the wagon. The pattern is recovered by stamping the motif on each lattice point.

back of horse gives the definition of unit cell-A smallest repeat unit of crystal structure,in 3D which shows the full symmetry of the structure. Below is shown the unit cell box having three sides a,b,c with three angles alpha, beta, gamma in portion of lattice.

Seven Unit Cell Shapes

Figure above shows cubic, tetragonal, orthorhombic, monoclinic, triclinic, hexagonal, Rhombohedral seven unit cell shapes with relation between sides a,b and c; angles alpha, beta and gamma are shown on lining of horse.

To define repeating unit we use concept of a lattice

A lattice is "an infinite ,2, or 3-D regular arrangement of points, each of which has identical surroundings". Figure below shows 2-D square lattice.

Below the tommy of horse is shown lattice points : points with identical environment, it is shown in 2D example of Rock salt or Sodium chloride ie NaCl. Space lattice : the points together within the crystal that have identical environments. A space lattice for NaCl, there are infinite ways to construct its space lattice. You have a 3-D cube, so one possible way is:1. points with identical environments could be located at the center of the sodium ions. On the other hand, points with identical environments could be located at the center of the chloride ions. What ever we choose, the resulting space lattice is the same, but the location of the points of the lattice are different.Figure below shows face centered 3-D and 2-D cube unit cell of NaCl.

The figure below shows a front view and three dimensional space lattice of a sodium chloride. You can see that each chlorine atom is surrounded by 4 sodium atoms. We have rows and columns of cations (Na+)(Grey/purple) and anions (Cl-)(Green).

If empty places are present between identical units constituted with atoms, then it is not a unit cell it is shown below the back leg of horse

Below the neck of horse it is shown that choice of origin is arbitrary, lattice points need not be atoms, but unit cell should always be the same

Relation between unit cell and lattice

According to Hauy (1784) a crystal is built up by a number of small crystals having the shape of the original crystal as a whole. This led to the concept of space lattice or crystal lattice.

“A space lattice is an array of points showing how particles (atoms, ions or molecules) are arranged at different sites in three dimensional spaces.”

The unit cell may be defined as, “the smallest repeating unit in space lattice which, when repeated over again, results in a crystal of the given substance”.

The unit cell is the part of the space lattice that will reproduce the whole lattice if repeated in 3 dimensions.

The structure of a crystal is defined by the discription of the shape and size of the unit cell,. Its like a building block, the crystal can be built up by repeating the unit cell in 3D.

A lattice is a (idealized) structure of crystals where each lattice point represent an identical group of atoms.

crystal lattice means atoms of the lattice are arranged in a regular manner and every atom or ion in the structure have same property with the remaining atoms or ions in the structure

The unit cell is therefore the subunit of the crystal lattice.

3-D Space Lattices.

Crystal structures repeat in 3 dimensions. The motif can be single atoms or groups of atoms. Again we assign lattice points to the atomic structure and produce a Space Lattice.

Space lattice + motif = Crystal Structure

Lattices which fill space without gaps are called Bravais lattices. There are 14 in three dimensions. There are 7 unique unit-cell shapes that can fill all 3-D space. These are the 7

Crystal systems.

Although there are only 7 crystal systems or shapes, there are 14 different crystal lattices, called Bravais Lattices. (3 different cubic types, 2 different tetragonal types, 4 different orthorhombic types, 2 different monoclinic types, 1 rhombohedral, 1 hexagonal, 1 triclinic).

Main point covered in collage are-crystal structure basics,difference between unit cell and lattice,7 possible unit cell shapes, Bravais Lattices

Message :Present collage shows solid state-which is shown as horse. Horse has special significance as it has solid body, and in India it was used as means of transportation which has special reference to present day transportation means, which are not eco fiendly ,but horse when used for transportation- mode, it is definetly eco friendly, and the collage indirectly also shows students love/concern for animals also.

References

1.:http://www.sciencehq.com/chemistry/space-lattice-and-unit-cell.html

2.http://www.tutorhelpdesk.com/homeworkhelp/Chemistry-/Space-Lattice-And-Unit-Cell-Assignment-Help.html