def crystal_search(crystals, histogram_type):
    """Creating a dictionary of values list ​​divided into centering
    types for a given histogram type.

    Parameters
    ----------
    crystals : list

        A list of crystal.
    histogram_type :  unicode str (on py3)

        Type of histogram e.g. 'a', 'gamma'.

    Returns
    -------
    crystal_dict : dict

        A dict of values lists
        key- type centering
        value - list
    """
    crystal_dict = {}
    for crystal in crystals:
        crystal_dict.setdefault(crystal['centering'], []).append(
            crystal[histogram_type])
    return crystal_dict