6 votes

Why is the base-centered orthorhombic crystal lattice a unique crystal system?

All the centered Bravais lattices are redundant in the sense that it is possible to use a primitive cell of smaller volume instead. However, this often means that crystal symmetries are no longer ...
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5 votes
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If a good choice of a unit cell should be the one of most symmetry ,then why keep body centered tetragonal if face centered cubic exists?

Body-centered tetragonal is face-centered cubic only if $c/a=\sqrt2$. If you try your transformation with a $c/a$ value greater/less than $\sqrt2$, your "cube" will have lateral edges that ...
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Why are there only 14 types of Bravais lattices and not 28 when there are 7 types of unit cells and each can have four variations?

Essentially, certain combinations of the possible point-group symmetries (cubic, tetragonal, hexagonal, trigonal, orthorhombic, monoclinic, triclinic) and possible translational symmetries (simple, ...
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4 votes

Why is the base-centered orthorhombic crystal lattice a unique crystal system?

Monoclinic lattices do not have their two oblique axes equal; or in terms of point group symmetry, the $C_\mathrm{2h}$ symmetry characteristic of monoclinic lattices is promoted to $D_\mathrm{2h}$, ...
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4 votes

Does a molecule need to be placed symmetrically in the unit cell?

Don't think of it so much as putting the center mass of an atom on a vertex but as matching the unit cell symmetry to the crystal symmetry. Take the definition from google below, I've bolded key parts:...
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3 votes
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Is there a standard scholarly reference for lattice constants of crystals of the elements?

The CRC Handbook of Chemistry and Physics contains a dedicated compilation by H. W. King, titled «Crystal Structures and Lattice Parameters of Allotropes of the Elements». In case your research ...
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3 votes

Why are there only 14 types of Bravais lattices and not 28 when there are 7 types of unit cells and each can have four variations?

Whether to use a centered cell or the smaller primitive cell is a question of convention. The conventions are guided by making life easy. You could take a primitive triclinic cell and redefine the ...
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2 votes

Height of FCC arrangement

My first instinct was to say that FCC has no height, because that's a cubic cell and it only has one parameter. Then again, you didn't ask about the cell. The question was about the FCC arrangement, ...
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2 votes

Conventional unit cell for a hexagonal crystal system

A crystal repeats contents of a unit cell in three dimensions. If you choose the unit cell to be a parallelepiped, the translation vectors are simply the edges of it. This is an easy way to describe a ...
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2 votes

Height of CCP Lattice

Face-centered cubic (FCC, or closest-packed cubic, CCP) and hexagonal close-packed spheres have the greatest (and equal) density of packed spheres, occupying 74.048% of the actual volume. Ref 1 ...
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2 votes
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How do the three Miller indices (hkl) denote planes orthogonal to the reciprocal lattice vector?

There are two equivalent ways to define the meaning of the Miller indices: via a point in the reciprocal lattice, or as the inverse intercepts along the lattice vectors. The reflecting plane are ...
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1 vote

Conventional unit cell for a hexagonal crystal system

Interestingly enough, the reason why the cell in bold is the unit-cell for the crystal is actually because the hexagonal cell has this higher symmetry you mention. To understand why, we have to ...
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1 vote

Lattice type from image

The picture is not a lattice, it is a periodic pattern. In a lattice, all lattice points are related by the translational symmetry given by the unit cell vectors. I can take any two lattice points and ...
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1 vote

Do two Miller planes need to have similar arrangement of atoms?

In answer to you first question, no if hkl differ, yes if they do not. I try to explain below. In a crystal the unit cell defines the repeating unit. Inside the unit cell the atoms are arranged as ...
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