LatticeFunction.rst

LatticeFunction

.. index:: LatticeFunction

Description

After a list of Bragg reflections has been indexed, it is often necessary to refine the unit cell parameters that have been used to assign indices. LatticeFunction can be used to achieve that task with :ref:`algm-Fit`. The function can work with a PeaksWorkspace or with a TableWorkspace that contains two columns named HKL and d (see :ref:`algm-PawleyFit` for specification of the table).

After setting the LatticeSystem attribute to one of the seven lattice systems, the function exposes the corresponding lattice parameters, as well as a ZeroShift. In most cases it's recommended to fix this additional parameter as this is often taken care of in earlier steps of the data reduction.

Usage

The following script demonstrates how the function can be used. The algorithm :ref:`algm-PoldiCreatePeaksFromCell` is used to generate Bragg reflections that are expected for the crystal structure of Silicon.

.. testcode:: ExSiliconTheoretical

    import numpy as np

    # Create Silicon peaks for indexing
    peaks_Si = PoldiCreatePeaksFromCell(
                SpaceGroup="F d -3 m",
                a=5.4311946,
                Atoms="Si 0 0 0",
                LatticeSpacingMin=0.7)

    # Fit a cubic cell, starting parameter is 5
    Fit(Function="name=LatticeFunction,LatticeSystem=Cubic,a=5",
        InputWorkspace=peaks_Si,
        Ties="ZeroShift=0.0",
        CreateOutput=True,
        Output="Si",
        CostFunction="Unweighted least squares")

    # Print the refined lattice parameter with error estimate
    parameters = AnalysisDataService.retrieve("Si_Parameters")

    a_true = 5.4311946
    a = np.round(parameters.cell(0, 1), 7)
    a_err = np.round(parameters.cell(0, 2), 7)

    print("Refined lattice parameter: a = {0} +/- {1}".format(a, a_err))
    print("Difference from expected value: a_observed - a_expected = {0}".format(np.round(a - a_true, 7)))

Executing the script produces some output with information about the fit:

.. testoutput:: ExSiliconTheoretical
   :options: +ELLIPSIS +NORMALIZE_WHITESPACE

    Refined lattice parameter: a = 5.4311946 +/- ...
    Difference from expected value: a_observed - a_expected = 0.0

In addition there is also an output workspace, which contains information about the peaks used for the fit and how well the peak positions calculated from the fitted parameters match the observed positions.

.. testcleanup:: ExSiliconTheoretical

    DeleteWorkspace("peaks_Si")
    DeleteWorkspace("Si_Parameters")
    DeleteWorkspace("Si_Workspace")
    DeleteWorkspace("Si_NormalisedCovarianceMatrix")

.. attributes::

.. properties::

.. categories::

.. sourcelink::