.. _fcc-paracrystal:
fcc_paracrystal
=======================================================
Face-centred cubic lattic with paracrystalline distortion
=========== ================================== ============ =============
Parameter Description Units Default value
=========== ================================== ============ =============
scale Scale factor or Volume fraction None 1
background Source background |cm^-1| 0.001
dnn Nearest neighbour distance |Ang| 220
d_factor Paracrystal distortion factor None 0.06
radius Particle radius |Ang| 40
sld Particle scattering length density |1e-6Ang^-2| 4
sld_solvent Solvent scattering length density |1e-6Ang^-2| 1
theta c axis to beam angle degree 60
phi rotation about beam degree 60
psi rotation about c axis degree 60
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The returned value is scaled to units of |cm^-1| |sr^-1|, absolute scale.
.. warning:: This model and this model description are under review following
concerns raised by SasView users. If you need to use this model,
please email help@sasview.org for the latest situation. *The
SasView Developers. September 2018.*
**Definition**
Calculates the scattering from a **face-centered cubic lattice** with
paracrystalline distortion. Thermal vibrations are considered to be
negligible, and the size of the paracrystal is infinitely large.
Paracrystalline distortion is assumed to be isotropic and characterized by
a Gaussian distribution.
The scattering intensity $I(q)$ is calculated as
.. math::
I(q) = \frac{\text{scale}}{V_p} V_\text{lattice} P(q) Z(q)
where *scale* is the volume fraction of spheres, $V_p$ is the volume of
the primary particle, $V_\text{lattice}$ is a volume correction for the crystal
structure, $P(q)$ is the form factor of the sphere (normalized), and $Z(q)$
is the paracrystalline structure factor for a face-centered cubic structure.
Equation (1) of the 1990 reference\ [#Matsuoka1990]_ is used to calculate
$Z(q)$, using equations (23)-(25) from the 1987 paper\ [#Matsuoka1987]_ for
$Z1$, $Z2$, and $Z3$.
The lattice correction (the occupied volume of the lattice) for a
face-centered cubic structure of particles of radius $R$ and nearest
neighbor separation $D$ is
.. math::
V_\text{lattice} = \frac{16\pi}{3}\frac{R^3}{\left(D\sqrt{2}\right)^3}
The distortion factor (one standard deviation) of the paracrystal is
included in the calculation of $Z(q)$
.. math::
\Delta a = gD
where $g$ is a fractional distortion based on the nearest neighbor distance.
.. figure:: img/fcc_geometry.jpg
Face-centered cubic lattice.
For a crystal, diffraction peaks appear at reduced q-values given by
.. math::
\frac{qD}{2\pi} = \sqrt{h^2 + k^2 + l^2}
where for a face-centered cubic lattice $h, k , l$ all odd or all
even are allowed and reflections where $h, k, l$ are mixed odd/even
are forbidden. Thus the peak positions correspond to (just the first 5)
.. math::
\begin{array}{cccccc}
q/q_0 & 1 & \sqrt{4/3} & \sqrt{8/3} & \sqrt{11/3} & \sqrt{4} \\
\text{Indices} & (111) & (200) & (220) & (311) & (222)
\end{array}
.. note::
The calculation of $Z(q)$ is a double numerical integral that must be
carried out with a high density of points to properly capture the sharp
peaks of the paracrystalline scattering. So be warned that the calculation
is slow. Fitting of any experimental data must be resolution smeared for
any meaningful fit. This makes a triple integral which may be very slow.
The 2D (Anisotropic model) is based on the reference below where $I(q)$ is
approximated for 1d scattering. Thus the scattering pattern for 2D may not
be accurate particularly at low $q$. For general details of the calculation
and angular dispersions for oriented particles see :ref:`orientation`.
Note that we are not responsible for any incorrectness of the
2D model computation.
.. figure:: img/parallelepiped_angle_definition.png
Orientation of the crystal with respect to the scattering plane, when
$\theta = \phi = 0$ the $c$ axis is along the beam direction (the $z$ axis).
.. figure:: img/fcc_paracrystal_autogenfig.png
1D and 2D plots corresponding to the default parameters of the model.
**Source**
:download:`fcc_paracrystal.py `
$\ \star\ $ :download:`fcc_paracrystal.c `
$\ \star\ $ :download:`sphere_form.c `
$\ \star\ $ :download:`gauss150.c `
$\ \star\ $ :download:`sas_3j1x_x.c `
**References**
.. [#Matsuoka1987] Hideki Matsuoka et. al. *Physical Review B*, 36 (1987)
1754-1765 (Original Paper)
.. [#Matsuoka1990] Hideki Matsuoka et. al. *Physical Review B*, 41 (1990)
3854-3856 (Corrections to FCC and BCC lattice structure calculation)
**Authorship and Verification**
* **Author:** NIST IGOR/DANSE **Date:** pre 2010
* **Last Modified by:** Paul Butler **Date:** September 29, 2016
* **Last Reviewed by:** Richard Heenan **Date:** March 21, 2016