Thermodynamic Guiding Principles in Selective Synthesis of Strontium

We demonstrate the selective fabrication of Ruddlesden-Popper (RP) type SrIrO3, Sr3Ir2O7, and Sr2IrO4 epitaxial thin films using pulsed laser deposition (PLD). The RP strontium iridate series is an ideal system for studying the concerted effects of electron correlation and spin-orbit interaction. Their unique physical properties susceptible to lattice distortions motivate studies in epitaxial thin film form. However, improvement in the film quality remains a challenge due to the noble metal character of iridium and the existence of the gaseous IrO3 species. Here we fabricated three different RP phases from a single SrIrO3 target by actively controlling the Ir/Sr ratio in the films. Through systematic growth studies, we identified that the growth conditions stabilizing each RP phase directly map onto the phase diagram expected from thermodynamic equilibria. This synthetic approach allows precise control over the cation stoichiometry as evidenced by the stabilization of single phase Sr3Ir2O7 for the first time, overcoming the close thermodynamic stability between neighboring RP phases. Despite the nonequilibrium nature of PLD, these results highlight the importance of thermodynamic guiding principles to strategically synthesize the targeted phase in complex oxide thin films.


THE M ANUSCRIPT
2][3] The perovskite iridates and its extended family of the Ruddlesden-Popper (RP) series Sr n+1 Ir n O 3 n+1 (n = 1 , 2 , ×××, ∞) is a representative system, evolving from a paramagnetic metal SrIrO 3 (n = ∞), 4 to the so-called spin-orbit Mott insulators Sr 3 Ir 2 O 7 (n = 2 ) 5,6 and Sr 2 IrO 4 (n = 1) [7][8][9][10] due to the concerted effect of strong electron correlation and spin-orbit coupling. 11These drastic changes in their physical properties strongly depend on the number of the interleaving perovskite blocks (n) between rock-salt SrO layers.Based on these bulk properties, epitaxial thin films of Sr n+1 Ir n O 3 n +1 have been recognized as an ideal structure for studying the lattice distortions in well-defined crystallographic orientations while tuning the dimensionality of the system as shown in the cases of SrIrO 3  12, 13 and Sr 2 IrO 4 [14][15][16] thin films on various substrates.For such studies, pulsed laser deposition (PLD) is the most widely used technique for fabricating the thin films.One of the biggest challenges in the growth of iridate thin films by PLD is considered to be the control of the iridium oxidation state.The noble metal character of iridium requires high oxygen chemical potential; however, at excessively oxidizing conditions, gaseous IrO 3 is generated resulting in loss of iridium from the films. 172][23] In this study, we systematically reduced the film Ir/Sr ratio by controlling the thermodynamic conditions during growth, and succeeded in stabilizing three phases of Sr n+1 Ir n O 3 n +1 (n = 1, 2, and ∞) epitaxial thin films from a single stoichiometric SrIrO 3 target, including Sr 3 Ir 2 O 7 epitaxial thin films for the first time.
Careful examination of the obtained growth phase diagram reveals close correspondence with bulk phase stability calculations based on chemical equilibria, suggesting the dominance of thermodynamic considerations during the non-equilibrium growth process.We believe these results will be highly relevant for many other RP compounds.
A PLD system equipped with the capability to monitor the laser intensity during growth was used in this study. 24The KrF excimer laser (wavelength 248 nm, pulse duration ~ 20 ns) beam operated at 3 Hz was imaged at an angle of 45° to the stoichiometric polycrystalline SrIrO 3 target.The target-substrate distance was set to 5 0 mm.Sr n +1 Ir n O 3 n +1 (n = 1 , 2 , and ∞) thin films were depo sited o n SrTiO 3 (001) substrates ( 5´ 5 mm 2 and 0.5 mm in thickness) by varying the laser fluence F, partial oxygen pressure P O2 , and substrate temperature T sub from 0.75 to 3.78 J×cm -2 , from 10 to 300 mTorr, and from 600 to 1000 °C, respectively.Phase identification of the films was performed by out-of-plane X-ray diffraction (XRD) and reciprocal space mapping using a diffractometer equipped with a four-bounce Ge (220) monochromator and a Cu target (wavelength 1.5406 Å).The film resistivity was measured using the four-probe method inside a temperature controlled liquid helium cryostat.1a.By analyzing the X-ray diffraction (XRD) patterns, we were able to unambiguously identify the phase and epitaxial relationship between the films and SrTiO 3 (001) substrates.As evidenced by the 00h and the 3 1 1 diffractions in Figure 1c and 1d, the orthorhombic SrIrO 3 thin films are stabilized along the (001)-orientation, 25 not in the ambient pressure stable monoclinic form. 26Here we denote this in the pseudo-cubic notation.6][7][8][9][10] Insight into the mechanism of selective phase evolution can be obtained from a systematic growth study varying the oxygen partial pressure P O2 and the substrate temperature T sub .Figure S1a-e  A summary of the growth phase diagram derived from these results is presented in Figure 3a.In order to assess the role of the suggested reaction intermediate, IrO 3 , we focus on the thermodynamic aspects in the selective formation of these RP phases.First, we consider the following chemical equilibrium: The phase boundary curve for Equation 1 was calculated by using the experimentally obtained standard Gibbs energies of formation for the specific binary and ternary oxides. 17,31 he experimental phase boundary between SrIrO 3 and Sr 2 IrO 4 as well as the appearance of (001)-oriented iridium metal at the most reducing conditions (P O2 = 10 mTorr, T sub ≥ 950 °C) is in excellent agreement with the prediction from Equation 1; reducing conditions shift the equilibrium toward the left following Le Chatelier's Principle.It is noteworthy that the Sr 3 Ir 2 O 7 phase is stabilized in between the stability regions for SrIrO 3 and Sr 2 IrO 4 ; co-existing with SrIrO 3 at T sub = 700 -800 °C, and eventually forming single-phase Sr 3 Ir 2 O 7 at higher T sub and P O2 .An exact phase boundary calculation is limited due to the lack of thermochemical data for Sr 3 Ir 2 O 7 , but instead we estimate its stability window based on two characteristic trends commonly observed in other RP series containing different cations (See Supporting Information).Firstly, oxidizing (reducing) conditions are favored for RP phases with higher (lower) n, due to the larger electronegativity of transition metal ions compared to the alkaline earths requiring more oxygen as the transition metal content increases (Figure S2).Secondly, the stability window shrinks for higher n phases, due to the decreasing difference in Gibbs free energy of formation between the neighboring RP phases.The experimentally obtained Sr 3 Ir 2 O 7 stability region as well as the high sensitivity of the c-axis to the growth conditions follow these two thermodynamically trends expected for RP phase stability.
At higher temperatures (T sub > 850 o C), the role of IrO 3 becomes important as can be seen from the following two equilibria: Equation 2 is the equilibrium responsible for the loss of Ir from a SrIrO 3 film converting into Sr 2 IrO 4 at higher P O2 , and Equation 3 is the formation of gaseous IrO 3 from solid iridium metal.We note that the phase boundaries from Equation 2 and 3 drawn as dotted lines in Figure 3 require an additional parameter, the partial pressure of IrO 3 (P IrO3 ), which was set to 4 ´ 10 -6 Torr in the calculations shown in Figure 3a.Although we cannot experimentally quantify P IrO3 , the Sr 2 IrO 4 phase was stabilized at significantly lower temperatures for a series of films grown at P O2 = 100 mTorr under the reduced laser fluence of 0.78 J×cm -2 suggesting a strong correlation between F and P IrO3 as shown in Figure 3b and It is quite remarkable that close correspondence is found between the growth experiments and thermodynamic calculations, considering the non-equilibrium nature of PLD growth such as: the pulsed supply of precursors, high kinetic energy of the ablated species, etc. 32 The consistency with chemical equilibria in this study is due in part to the diffusive regime employed in the deposition, where the laserablated species experience multiple scattering before adsorbing on the growth surface, producing a growth environment much closer to equilibrium than the ballistic regime.Signatures attributable to kinetic effects indeed become notable as we enter increasingly non-equilibrium environments, such as the absence of a phase-pure Sr 3 Ir 2 O 7 , and obscured phase boundaries between the three RP phases around T sub = 700 -750 o C and P O2 = 10 -20 mTorr.These conditions correspond to lower temperatures where adatom migration is slow, and low P O2 .
The selective fabrication of three pure phases of Sr n+1 Ir n O 3n +1 (n = 1, 2, and ∞) epitaxial thin films by PLD, from a single SrIrO 3 target, shows that thermodynamic considerations provide a powerful guideline even for non-equilibrium thin film growth.Furthermore, given the increasing interest in layered complex oxide thin films to explore various atomic degrees of freedom, 33,34 the growth strategy employed in this study should be effective in fabricating a wide range of RP phases containing cations with accessible volatile oxides.

Figure 1
Figure 1 presents three ca.100 nm thick RP phases selectively fabricated in single-phase form.The crystal structures of SrIrO 3 , Sr 3 Ir 2 O 7 , and Sr 2 IrO 4 on (001)-oriented SrTiO 3 substrates are schematically illustrated in Figure 1a.By analyzing the X-ray diffraction (XRD) patterns, we were able to show the out-of-plane XRD patterns for the films grown in the range from T sub = 600 -1000 °C and P O2 = 10 -300 mTorr while fixing the laser fluence F at 3.75 J×cm -2 .Under all P O2 studied, increase in T sub led to a transformation of the SrIrO 3 to the Sr 2 IrO 4 phase with a narrow stability window for Sr 3 Ir 2 O 7 at T sub = 800 -850 °C and P O2 > 50 mTorr.The out-of-plane lattice constants were calculated for each growth condition.Within the stability window for SrIrO 3 , at T sub < 800 o C, its pseudo-cubic lattice constant decreases and converges with increase in T sub for all P O2 .The converged value approaches the bulk value with reducing P O2 , as illustrated by the dashed line in Figure2a.Above T sub = 800 o C, however, the formation of SrIrO 3 begins to compete with Sr 3 Ir 2 O 7 and Sr 2 IrO 4 formation, crossing over from Sr 2 IrO 4 to Sr 3 Ir 2 O 7 at P O2 = 50 mTorr with increase in P O2 .In the region of single phase Sr 3 Ir 2 O 7 films, the caxis lattice constant was sensitive to T sub , changing from 20.81 Å to 20.96 Å spanning across the bulk value of 20.879 Å 27 even by 50 °C increment in T sub , as shown in Figure 2b.Under the conditions stabilizing Sr 2 IrO 4 , the c-axis lattice constant approached its bulk value (Figure 2c) with increasing T sub similar to the case o f SrIrO 3 .

Figure S1f .
Figure S1f.The calculated phase boundary in Figure 3b corresponds to a P IrO3 of 1 × 10 -7 Torr illustrating that lower P IrO3 corresponds to a SrIrO 3 /Sr 2 IrO 4 phase boundary shift towards lower temperatures, shrinking the stability window for SrIrO 3 .To confirm the direct correlation between F and P IrO3 , we further studied the evolution of the RP phases by systematically reducing F at P O2 = 100 mTorr and T sub = 850 °C, resulting in the phase evolution from Sr 3 Ir 2 O 7 to Sr 2 IrO 4 as detected by XRD in Figure 4a.Moreover, the reduction in F concomitantly reduced the ex-situ determined deposition rate, a
30M) in Figure1c and 1e.We note that Sr 3 Ir 2 O 7 has recently been refined to have the orthorhombic lattice with space group Bbcb 28 and Bbeb,29but the lattice constant differences from the tetragonal lattice is indistinguishable from our XRD measurements.Similarly, c-axis orientation was observed for the K 2 NiF 4 -type (space group I4 1 /acd30) Sr 2 IrO 4 film fro m the 0 0 l diffraction in the out-of-plane XRD and 18 20 reflection in RMS shown in Figure 1c and 1f.The temperature dependent resistivity of these films shows close correspondence with those reported for bulk SrIrO 3 , Sr 3 Ir 2 O 7 , and Sr 2 IrO 4 as shown in Figure