**3. Methods**

#### *3.1. Microscopy*

Scanning Electron Microscopy images were acquired using a Tescan Mira LMU FE-SEM (ISTe, University of Lausanne (UniL), Lausanne, Switzerland) operated at an acceleration voltage of 20 kV and a probe current of 20 nA. We used cold cathodoluminescence (CL) microscopy (model 8200 MkII (OPEATM), ISTe, UniL, Lausanne, Switzerland) to qualitatively estimate the variation in Mn content [41]. Light orange corresponds to a high Mn concentration whereas dark red is associated with low Mn concentrations. Electron microprobe analyses were conducted on a JEOL JKA-8530F (UniL, Lausanne, Switzerland), under beam of 15 nA and current of 10 nA as proposed by Lane and Dalton [42].
