Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:This deceleration can be caused by the structural change of the Mn4CaO5 catalyst that occurs upon the S2 → S3 transition and/or by the additional oxidation of the OEC [13,52,53].
Category: