Q-spacing, Bragg-spacing, D-spacing & Repeat Distances Explained

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SUMMARY

Q-spacing, Bragg-spacing, D-spacing, and repeat distance are distinct concepts in crystallography. D-spacing refers to the interplanar distance in a crystal, while Q-spacing is the length of a reciprocal lattice vector, which corresponds to the conditions for observing a Bragg peak. Bragg-spacing represents any distance over which a system exhibits translational invariance, leading to Bragg peaks. In superalloys, additional Bragg peaks can appear, indicating larger Bragg spacings, and similar phenomena can occur in polymers and liquid crystals.

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  • Understanding of crystallography principles
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  • Knowledge of Bravais lattices
  • Basic concepts of Bragg's law
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thearny
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Could anyone tell me what the difference between Q-spacing Bragg-spacing D-spacing and repeat distance is? I've always thought that they were same, but seemingly not? What is the realtionship, what is Q?

Its keeping me up at night!
 
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d-spacing is simply the interplanar distance in a crystal. In this context, the Q-spacing is the length of a reciprocal lattice vector (you have a Bragg peak when the wave vector of the radiation used matches a reciprocal lattice vector). The Bragg-spacing is any distance over which a system is translationally invariant in some direction, and will hence produce a Bragg peak. So, in a crystal, the bragg spacing will be the d-spacing itself. In an superalloy, you will see an adiitional larger (but weaker) Bragg peak giving rise to an additional Bragg spacing. Likewise, you can have a Bragg spacing for a polymer and even a liquid crystal.

The relationship between reciprocal lattice vectors and real lattice vectors depends on the type of Bravais lattice involved.
 
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