(1) that complex rutile solid solutions can be formed in these high-order composition spaces, and (2) that integrating Mn with some combination of Sn, Ti, and Sb in a rutile structure will enable higher OER activity at lower Mn concentration than observed in the Mn-Sb-O system. Each composition library was synthesized via reactive cosputtering from elemental metal targets, as detailed in the ESI. The orientation of the deposition sources for each continuous composition spread is shown in Fig. , where a continuous composition spread within each of the 4 compositions systems was obtained by simultaneously operating the respective deposition sources with thin films collected on a 10 cmdiameter Si wafer with thermal oxide, Ti adhesion layer, and Pt conducting layer. Deposition in an atmosphere of 4.8 mTorr Ar and 1.2 mTorr O2 followed by annealing in air at 700 C resulted in metal oxide libraries that were characterized by 3 methods: i) X-ray fluorescence (XRF) to determine the spatial map of composition, which detects all elements except oxygen and is the source of all composition plots in the present work; ii) X-ray diffraction (XRD) to characterize the phase behaviour; and iii) scanning droplet cell electrochemistry to characterize OER activity. The phase behaviour of the composition libraries is central to evaluating hypothesis (1). XRD measurements were acquired on 203 samples from the 4 composition libraries, including 8 Mn-Sb-O, 63 Mn-Sn-Sb-O, 66 Mn-Ti-Sb-O, and 66 Mn-Sn-Ti-Sb-O compositions. Using ICDD phases as prototypes, the phase constitution as well as the lattice parameters of each phase were determined via a novel XRD analysis tool, CrystalShift. The results are summarized in Fig. and demonstrate fulfilment of hypothesis (1) with 176 phase-pure rutile samples corresponding to formula units of the type (Mn1-x-y-zSbxSnyTiz)O2 where x can vary from 0.06 to 0.81, y from 0 to 0.33, and z from 0 to 0.38. In each of the 4 composition libraries, the solubility limit of Mn in this type of rutile structure is observed via identification of phase fields containing a Mn2O3-type phase (see Table ). The solubility limits of Sb, Sn, and Ti are not characterized by the present data as no phase boundary beyond phase-pure rutile is observed at compositions enriched in these elements. Given the present goal of studying catalysts with low Mn concentration, the XRD results demonstrate that all compositions with Mn concentration below 0.6 are amenable to evaluation of hypothesis (2).