Crystallography- Classification Schemes

Question 1

What are the types of classifications?

Answer 1

Crystal systems, bravais lattices, crystal classes/point groups, space groups

Question 2

How to label the edges and angles of the unit cell

Answer 2

Label the 3 different length edges a, b and c. The angle α is between edge b and c and so on. The face A is bound by b and c edges

Question 3

7 crystal systems

Answer 3

Triclinic: all edges and angles different
Monoclinic: edges different, α=γ=90, β variable
Orthorhombic: edges different, angles 90
Tetragonal: a=b, c variable, angles 90
Cubic: edges same, angles 90
Hexagonal: a=b, c variable, α and β 90, γ 120
Trigonal: sides same, angles same not 90

Question 4

Advantage of hexagonal over trigonal

Answer 4

Has two rectangles and one fixed 120° angle. Has one main edge along symmetry (c axis). The advantage of 3 equal a axes for trigonal can be reintroduced for hexagonal by inventing a fourth axis a3.