Immediately after deposition, CuP 2+x films were cut into smaller pieces and annealed in a lamp-based rapid thermal annealing (RTA) furnace in a N 2 atmosphere. Because of the sputtering target geometry and differences in their applied power, small gradients in P/Cu ratio and film thickness were obtained across the substrate. These gradients enabled us to characterize several data points ("samples") for each annealing run, each with a distinct composition and thickness. More details on film deposition and annealing are available in the Supporting Information.