Supporting Information

A Joint Theoretical and Experimental Study of Phase Equilibria and Evolution in Pt Doped Under Redox Conditions

Baihai Li 1, 2 , Michael B. Katz2, Yingwen Duan 2, Xianfeng Du 2, Kui Zhang 2, Liang Chen 3, Anton Van der Ven 4, George W. Graham 2, and Xiaoqing Pan 2,*

1School of Energy Science and Engineering, University of Electronic Science and Technology of China, Chengdu 611731, People’s Republic of China 2Department of Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan 48109, USA 3Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, People’s Republic of China 4Materials Department, University of California, Santa Barbara, California 93106, USA

Stoichiometric, cation-defective, and anion-defective crystal structures, including orthorhombic, cubic, and tetragonal CaTiO 3; the Ruddlesden-Popper-type Ca 3Ti 2O7 and Ca 4Ti 3O10 ; CaTi 2O4, the newly characterized structure of CaTi 5O11 (to be presented by us in a forthcoming paper), TiO 2 [TiO 2 (A)], TiO 2 [TiO 2

(R)], brookite TiO 2 [TiO 2 (Br)] and bronze TiO 2 [TiO 2 (B)], TiO, Ti 2O3, Ti 3O5, CaO, etc. , are taken into account to construct the first-principles phase diagram of Ca-Ti-O system. Additional structures involving Pt single atoms and dimers doped at cation and interstitial sites of stoichiometric and reduced CaTiO 3, TiO 2 species, Ti 2O3, as well as Pt-Ti and Pt-Ca binary crystals, Pt oxides, etc. , are included to construct the

Pt-Ca-Ti-O phase diagrams.

1. Ca-Ti-O first-principles phase diagram under redox conditions

The first-principles phase diagrams of Ca-Ti-O under redox environments are shown in Figure S2, where the hcp Ti and fcc Ca are chosen as the references. The partial pressure is fixed at 0.2 atm, matching the experimental conditions, to investigate the phase diagram changes as a function of temperature. The phase diagram in Figure S2(a) shows that TiO 2 (B), orthorhombic CaTiO 3, CaTi 5O11 , and

CaO are stable under the oxidizing conditions (100 °C, 0.2 p0). The relative stability of TiO 2 phases is in the order of TiO 2 (R) < TiO 2 (Br) < TiO 2 (A) < TiO 2 (B), and the total energies of the three less stable structures are 76.1, 38.6 and 1.7 meV/f.u. higher than that of TiO 2 (B), respectively. When the temperature increases to 800 °C, these four phases remain and no additional phases appear – the formation energies of the existing phases simply become less negative (Figure S2(b)). Reduced structures with ordered oxygen vacancies were calculated as well, but are not energetically favorable and do not appear in the calculated phase diagrams. It can be expected that disordered oxygen vacancies may appear at the CaTiO 3 surface in experiments, but, as this is not a bulk phase, it is not considered. Ordered cation defect phases are also energetically unfavorable, thus these structures do not appear in the phase diagrams under our considered environment. TiO 2 (B) gives way to Ti 2O3, and CaTi 5O11 disappears from

-20 the phase diagram under aggressively reducing conditions ( e.g. 800 °C, 10 p0). Further reduction leads to the formation of CaTi 2O4 at the expense of orthorhombic

CaTiO 3, as indicated in Figure S2(d).

Because entropy due to various excitations – vibrational, configurational, and electronic – is not considered in zero Kelvin first-principles calculations, some discrepancies between first-principles and experimental phase diagrams obtained at non-zero temperatures are to be expected. This manifests in the

Ruddlesden-Popper-type Ca 3Ti 2O7 and Ca 4Ti 3O10 phases, which have been found experimentally, 1 not appearing in our first-principles-derived phase diagrams. We do not expect, however, that this discrepancy is critical for the expanded investigations of phase stability and evolution of the Pt-Ca-Ti-O system, as these structures were also absent in our experimental systems.

2. Pt-Ca-Ti-O first-principles phase diagram under redox conditions

The first-principles calculated isothermal phase diagrams of the four-component

Pt-Ca-Ti-O system manifests as a three-dimensional convex hull. In order to conveniently display and interpret the results, we projected the convex hull onto a two-dimensional pseudo-ternary phase diagram, holding the oxygen chemical potential constant. In agreement with the phase diagram of Ca-Ti-O system, TiO 2 (B),

CaTiO 3 (orthorhombic), CaTi 5O11 , CaO also appear in the Pt-Ca-Ti-O diagram.

Additionally, Ca 4PtO 6 (R-3cH , 167) has been observed in the range of 667 ~ 1127 °C by Jacob et al. in the Ca-Pt-O system. 2 Indeed, this structure is a stable phase in our calculated phase diagram under the considered redox conditions. The overall phase evolution shows that orthorhombic CaTiO 3 with Pt substituting on the Ti site at various concentrations ( e.g. Ca 16 Ti 15 PtO 48 , Ca 4Ti 3PtO 12 , CaPtO 3) appear in the phase diagrams under the oxidizing condition, and disappear gradually in descending order of Pt concentration in the CaTi 1-xPt xO3 as the environment is made more reducing, with the phase less able to contain Pt, as shown in Figure S3. Analogs of the perovskite phase, wherein Pt substitutes for Ca or O or resides in interstitial sites do not appear in the phase diagrams. These phase diagrams are a precursor to those shown and discussed in the main text, which have included free energy results from the cluster expansion effort.

In addition to substituting Pt onto Ti sites in CaTiO 3, an effort was also made to calculate the stability of Pt substitutions on Ti sites in various titanias. Figure S4 shows formation energies for various Pt Ti configurations in TiO 2 (A) and TiO 2 (Br), in which all substituted configurations have higher energies than their end members. Pt therefore cannot, in a stable fashion, exist within TiO 2 (A) and TiO 2 (Br). Figure S5 contains similar information for all calculated Ti 1-xPt xO2 phases. Finally, a list of all phases considered for the calculations in this work is shown in Table S1.

References (1) Jacob, K.; Gupta, S. Bull. Mater. Sci. 2009 , 32, 611. (2) Jacob, K.; Uda, T.; Waseda, Y.; Okabe, T.; Metallkd, Z. 1999 , 90, 491.

Figure S1. HAADF-STEM micrograph showing a typical area of an as-grown CaTi 0.95 Pt 0.05 O3 thin film. The image was acquired with the sample off-axis, so as to minimize the perovskite lattice background and highlight the individually dispersed Pt species, which appear as bright spots in the micrograph.

Figure S2. First-principles calculated Ca-Ti-O phase diagrams under the specific redox condition. The oxygen partial pressure in (a) and (b) is fixed at 0.2 atm, whereas the temperature in (c) and (d) is fixed at 800 °C.

Figure S3. First-principles calculated Pt-CaTiO 3 phase diagrams under the various redox conditions, discounting any oxides of Pt. The oxygen partial pressure in (a) and (b) is fixed at 0.2 atm, the temperature in (c) and (d) is fixed at 800 °C.

Figure S4. First-principles formation energies of Pt doped (a) anatase TiO 2 (A) and (b) brookite

TiO 2 (Br) systems, respectively.

Figure S5. The comparison of first-principles formation energies of Pt doped TiO 2 (B) and TiO 2

(R), as well as TiO 2 (A) and orthorhombic PtO 2 (O), which are denoted as red, blue, cyan and purple dots, respectively.

Figure S6. Calculated CaO x-TiO y-Pt pseudo-ternary phase diagrams calculated for various extremely reducing conditions for 800 °C.

Figure S7. HAADF-STEM micrographs of ordered Pt a-site substitutions in the re-oxidized

Ca 1.1 Ti 0.9 Pt 0.1 O3 film. The bright Pt atoms order into every other a-site sublattice site.

Table S1. List of all phases considered in this study, along with their total energies. Those that actually appear in the phase diagrams are bolded and in red.

Phase Etot (eV) Phase Etot (eV)

O2 -9.077 Pt 5Ti 3 -60.938

Ca bulk -1.909 Ca 0.9375 Pt 0.0625 TiO 3 (Pbnm) -40.146

Ti bulk -7.744 Ca 0.75 Pt 0.25 TiO 3 (Pbnm) -39.349

Pt bulk -6.053 Ca 0.9375 Pt 0.0625 TiO 2.9375 (Pbnm) -39.625

CaO -12.995 CaTi 0.9375 Pt 0.0625 O2.9375 (Pbnm) -39.317

TiO -17.375 CaTi 0.9375 Pt 0.0625 O3 (Pbnm) -39.775

Ti 2O3 -44.827 CaTi 0.75 Pt 0.25 O3 (Pbnm) -37.992

Ti 3O5 -71.406 CaTi 0.75 Pt 0.25 O2.75 (Pbnm) -36.145

TiO 2 (A) -26.617 CaTi 0.875 Pt 0.125 O3 (Pbnm) -39.180

TiO 2 (Br) -26.580 CaTi 0.25 Pt 0.75 O3 (Pbnm) -33.203

TiO 2 (R) -26.542 CaTi 0.5 Pt 0.5 O3 (Pbnm) -35.598

TiO 2 (B) -26.618 CaTi 0.375 Pt 0.625 O3 (Pbnm) -34.397

CaTi 2O4 -58.177 CaTi 0.125 Pt 0.875 O3 (Pbnm) -31.996

Ca 3Ti 2O7 -93.690 CaPtO 3 (Pbnm) -30.794

Ca 4Ti 3O10 -134.062 CaTi 1.875 Pt 0.125 O3.875 -56.291

CaTiO 3 (Pm-3m) -40.012 Ca 0.75 Pt 0.25 Ti 2O4 -57.703

CaTiO 3 (Cmmm) -40.012 Ca 0.75 Pt 0.25 Ti 2O3.75 -56.813

CaTiO 3 (Pbnm) -40.368 Ti 1.917 Pt 0.083 O3 -44.240

CaTi 5O11 -146.904 Ti 1.917 Pt 0.083 O2.917 -43.630

CaPt 5O11 -98.349 Ti 0.875 Pt 0.125 O2 (B) -25.353

CaTi 4PtO 11 -136.971 Ti 0.96875 Pt 0.03125 O2 (B) -26.302

PtTi 5O11 -141.939 Ti 0.9375 Pt 0.0625 O2 (B) -25.988

TiO 1.5 (R) -21.653 Ti 0.9375 Pt 0.0625 O1.9375 (B) -25.561

TiO 1.9375 (R) -25.946 Ti 0.9375 Pt 0.0625 O2 (A) -25.975

TiO 1.875 (B) -25.062 Ti 0.9375 Pt 0.0625 O1.9375 (A) -25.554

Ca 0.9375 TiO 3 (Pbnm) -39.669 Ti 0.96875 Pt 0.03125 O2 (A) -26.294

CaTi 0.9375 O3 (Pbnm) -38.958 Pt 0.03125 +TiO 2 (B) -26.707

CaTiO 2.9375 (Pbnm)-Ov II -39.7540 Pt 0.125 +TiO 2 (B) -26.990

CaTiO 2.9375 (Pbnm)-Ov I -39.734 Pt 0.25 +TiO 2 (B) -27.431

CaTiO 2.96875 (Pbnm)-Ov II -40.069 Pt 0.5 +TiO 2 (B) -28.270

CaTiO 2.96875 (Pbnm)-Ov I -40.067 Ca 0.25 +TiO 2 (B) -27.946

Ca 0.75 TiO 3 (Pbnm) -37.592 Ca 0.5 +TiO 2 (B) -28.630

CaTi 0.75 O3 (Pbnm) -34.828 Ca 0.25 Pt 0.25 +TiO 2 (B) -29.763

Ca 0.9375 TiO 2.9375 (Pbnm)-Ov II -39.397 Pt 0.25 +TiO 2 (A) -27.152

Ca 0.9375 TiO 2.9375 (Pbnm)-Ov I -39.385 Pt 0.5 +TiO 2 (A) -28.481

CaTi 0.9375 O2.875 (Pbnm)-Ov I,II -38.474 Pt 0.75 +TiO 2 (A) -29.278

Ca 0.9375 Ti 1.0625 O3 (Pbnm) -40.589 Pt 1+TiO 2 (A) -30.289

Ca 1.0625 Ti 0.9375 O3 (Pbnm) -39.622 PtO 2 (Pnnm) -17.382

PtTi -15.340 PtO 2 (P-3m1) -17.310 PtTi 3 -31.921 PtO 2 (Pn-3mS) -14.843

Pt 8Ti -59.946 PtO 2 (B) -16.485

CaPt 2 -17.196 PtO 2 (R) -17.264

CaPt 5 -35.378 Pt 3O4 (Pm-3n) -41.069

Ca 3Ti 1.75 Pt 0.25 O7 -91.320 Pt 3O4 (Im-3m) -33.758

Ca 4Ti 2.75 Pt 0.25 O10 -131.692 PtO (P42-mmc) -11.524

CaTi 1.875 Pt 0.125 O4 -57.220 PtO (Fm-3m) -9.900

Ca 2Pt 3O8 -79.962 Ca 4PtO 6 -69.970