+++
title = "3D structures"
weight = 20
+++

## 3D structures (volume particles)

Particles distributed throughout a layer's volume. Use `Dilute3D` to describe
a random volume distribution and `fill3D` to add it to a layer:

```python
structure = ba.Dilute3D(volume_density, particle)

layer = ba.Layer(material, thickness)
layer.fill3D(structure)
```

The density is specified as volume number density (particles per nm^3).

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