def perovskite_order_param(Rs_order_param=None):
    """ Define an order parameter for peroskite.
    """

    if Rs_order_param is None:
        Rs_order_param = [(1, 0, 1), (1, 0, -1), (1, 1, 1), (1, 1, -1), (1, 2, 1), (1, 2, -1)]
    Rs_in = [(3, 0, 1)] + Rs_order_param

    return Rs_in