The periodic arrangement of equimolar two cations in the corundum structure produces the ilmenite-type and LiNbO3-type orderings. The phase (polymorphism) control is one of the important topics in solid-state chemistry. Using a pulsed-laser deposition technique, we examined the formation of MnSnO3 films as a function of oxygen gas pressure. Under the optimal low oxygen gas pressure, c-axis oriented single-crystalline films of the ilmenite-type MnSnO3 were synthesized on Al2O3 (0001). We found that, with an increase of oxygen gas pressure, x-ray diffraction peaks characteristic of the ilmenite-type ordering disappeared and the c-axis length approached that of the LiNbO3-type MnSnO3 while the crystal structure retained basic features of corundum derivatives. The optical bandgap measurement revealed the decrease of bandgap in the LiNbO3-type (or disordered corundum-type) MnSnO3. The thin-film approach can add a new degree of freedom in the control of structural and physical properties in corundum-derivative oxides.
Linked to the recent progress in the physics of complex oxides, there has been a growing interest in corundum-derivative compounds.1–9 In the ternary ABO3 systems, periodic arrangement of two cations (A and B) produces the nonpolar ilmenite (IL)-type structure (space group: ) and the polar LiNbO3 (LN)-type structure (R3c), as shown in Fig. 1.10–12 IL-type oxides containing 3d transition metals have been investigated extensively for their rich magnetic ordering phenomena originating from spin interactions in a triangular lattice geometry.5 In LN-type oxides that lack the inversion symmetry (Fig. 1(b)), ferroelectricity can appear with a large spontaneous polarization,1,7 e.g., 71 μC/cm2 for LiNbO3 and 50 μC/cm2 for LiTaO3 (Ref. 13). By adding 3d transition metals to the polar LN structure, multiferroic functions can also be induced.2–4,7,9 Exploration of such interesting physical properties in the corundum-derivative oxides sharing basic AO6 and BO6 octahedral units relies on the controlled formation of the two ordered structures by appropriate synthetic techniques.
Since the early stage of bulk research on ABO3 oxides, high-pressure techniques have been widely used to study the structural stability of the IL, LN, and also perovskite-type ABO3.11,14–16 Soft chemistry techniques, e.g., ion exchange,6 using precursors are also effective for obtaining metastable target phases by suppressing the formation of thermodynamically stable phases. Bulk samples prepared by those techniques, however, generally have powder or small sintered pellet forms, making it difficult to characterize their optical and electronic properties. With the aim of expanding opportunities for studying their unique physical properties, we attempted the phase control of corundum-derivative oxides in the thin-film form.17,18 We selected MnSnO3 as the target material.3,14,16 Recently, A. Aimi et al., have reported a structural transformation from the IL-type to the LN-type MnSnO3 under the application of 7 GPa.3 Associated with this structural transformation, spontaneous polarization emerges in addition to the antiferromagnetic properties originally present in the IL-type MnSnO3,14 leading to multiferroic functions in the LN-type MnSnO3.3 In this study, we report the successful growth of IL-type MnSnO3 thin films by pulsed-laser deposition and the critical role of oxygen gas pressure PO2 on the occurrence of the IL-type ordering. The PO2 dependence of lattice parameters suggests that the structural transformation to the LN-type (or disordered corundum-type) MnSnO3 can be driven by PO2.
MnSnO3 films were grown on double-side polished Al2O3 (0001) substrates at a substrate temperature of 820 °C by pulsed-laser deposition using a KrF excimer laser (repetition rate = 10 Hz). A mixed-phase Mn2SnO4-SnO2 target was prepared by a spark-plasma-sintering technique at 1100 °C and 80 MPa in a vacuum using a stoichiometric mixture of MnO2 and SnO2 powders pre-fired at 900 °C in air. Prior to the deposition, Al2O3 substrates were annealed at 900 °C in an oxygen flow with a tube furnace to obtain an atomically smooth surface with a step-and-terrace structure. The films were characterized by x-ray diffraction (XRD) using Cu Kα radiation, x-ray photoelectron spectroscopy (XPS) using Al Kα radiation, and optical transmittance spectroscopy. For energy calibration of the XPS spectra, the C 1s peak at 284.8 eV was used. As a control sample, a c-axis oriented film of the IL-type MnTiO3 was fabricated on Al2O3 (0001) by pulsed-laser deposition at 850 °C and PO2 = 1 × 10−1 Torr.19,20
Figure 2(a) shows an out-of-plane XRD pattern for a Mn-Sn-O film grown at PO2 = 4 × 10−4 Torr, measured with a condition maximizing the film diffraction intensity. In the IL structure, (0003n) diffraction peaks (n: natural number) are allowed while only (0006n) are allowed in the LN structure due to the different symmetry (Fig. 1). All the diffraction peaks from the Mn-Sn-O film are consistent with (0003n) of the IL structure, representing that AO6 and BO6 layers are alternately stacked along the c-axis direction (Fig. 1(a)). From the XRD reciprocal space mapping shown in Fig. 2(b), the lattice parameters of the Mn-Sn-O film are calculated to be a = 5.352 Å and c = 14.40 Å, which are close to a = 5.358 Å and c = 14.505 Å of the IL-type MnSnO3 bulk (JCPDS, PDF No. 00-033-0913). In addition to the lattice parameters, we determined the valence of Mn ions by XPS. Figure 2(c) shows Mn 2p core-level XPS spectra of the Mn-Sn-O and MnTiO3 (Mn2+ reference) films. The overall spectral shape and the satellite peak around 647 eV (Refs. 21 and 22) clearly demonstrate the dominant Mn2+ valence state in the Mn-Sn-O film. On the basis of these results, we conclude the formation of IL-type MnSnO3 films.
The effects of PO2 on the IL-type ordering in MnSnO3 films are summarized in Fig. 3. Out-of-plane XRD patterns, displayed in Fig. 3(a), show that the (0003) diffraction peak (indicated by the red arrows) appears at PO2 ≤ 1 × 10−3 Torr but not at 1 × 10−1 Torr, while basic extinction rules for corundum derivatives are maintained with the appearance of the (0006) diffraction peak. In addition, Mn2SnO4 is segregated in the film grown at a low PO2 of 1 × 10−4 Torr. From the (0003) diffraction intensities in these films, the optimal PO2 for stabilizing the IL-type MnSnO3 is found to be 4 × 10−4 Torr (Fig. 2). In Fig. 3(b), the in-plane orientation relationships between the film and substrate investigated by the in-plane ϕ scans for MnSnO3 and Al2O3 are compared. In view of the three-fold symmetry in the Al2O3 substrate shown in the top panel, the MnSnO3 film fabricated at PO2 = 4 × 10−4 Torr has the identical three-fold symmetry (the middle panel of Fig. 3(b)). This indicates the formation of a single-crystalline MnSnO3 film, whose in-plane orientation aligns to that of the substrate. In contrast, twin domains of MnSnO3, which are rotated by Δϕ = 60° in the (0001) plane, are generated in the non-optimal PO2 conditions as seen in the six-fold symmetry of the diffraction peaks (PO2 = 1 × 10−3 and 1 × 10−4 Torr). Figure 3(c) shows the diffraction intensity ratio of MnSnO3 (0003) to (0006), which reflects the degree of the IL-type ordering of AO6 and BO6 layers along the c-axis direction. For fair comparison, we separately measured the MnSnO3 (0003) and (0006) intensities because they were maximized at slightly different incident ω angles. Figure 3(d) shows the diffraction intensity ratio of MnSnO3 at ϕ = 60° (the in-plane orientation matched domain) to that at ϕ = 120° (the rotated domain). This ratio is defined as the in-plane orientation matching, and the intensities were calculated by averaging the three diffraction intensities corresponding to each domain. Both the out-of-plane IL-type ordering (Fig. 3(c)) and in-plane orientation matching (Fig. 3(d)) are well developed at PO2 = 4 × 10−4 Torr. These PO2 dependences highlight the narrow PO2 window for obtaining single-crystalline IL-type MnSnO3 films. The lower bound of PO2 seems to be limited by the segregation of Mn2SnO4, which likely comes directly from the mixed-phase Mn2SnO4-SnO2 target. Considering that the IL-type ordering disappears in the high PO2 region, we speculate that the oxidation states of A and B cations in the plume affect the layer stacking processes. The ablation of appropriate AO6 and BO6 precursors and moderate (or no additional) oxidation at the substrate surface may be the key to forming the IL-type ordering.
Note here that the appearance/disappearance of the IL-type ordering is discussed for the samples with comparable film qualities; the strong MnSnO3 (0006) peak remains even at a high PO2 of 1 × 10−1 Torr (the top panel in Fig. 3(a)). Mn 2p core-level XPS spectra also keep the overall spectral signatures of Mn2+ against the PO2 variation (see Fig. S1 in supplementary material). Although the satellite peak21,22 becomes weak at high PO2, the valence of Mn ions (i.e., Mn2+) is probably maintained for the sake of the robust Sn4+ character even under a low PO2 of 4 × 10−4 Torr and charge neutrality in the A2+B4+O3 structure. The growth rate is also not changed much in the varied PO2 range (Fig. S2), and thus the kinetic effect is not the primary origin. Therefore, the PO2 dependences presented in Fig. 3 indicate that the modification of the out-of-plane atomic ordering (AO6 and BO6 layer stacking) occurs in the MnSnO3 films.
This hypothesis is supported by the decrease of the c-axis length with increasing PO2, shown in Fig. 4(a). The c-axis length of the films grown under the low PO2 region (≤ 4 × 10−4 Torr) is comparable to that of the IL-type MnSnO3 bulk (the upper dashed line). As PO2 increases, the c-axis length decreases, approaching that of the LN-type MnSnO3 bulk3 (the lower dashed line). Although further characterization is needed to determine the microscopic structure of the LN-like MnSnO3 film, this result agrees with the disappearance of the MnSnO3 (0003) diffraction peak at high PO2 (Fig. 3(a)). The incomplete shrinkage of the c-axis length may mean that both LN-type and IL-type orderings partially coexist as small domains. To examine this structural modification via the associated changes in physical properties, we measured ultraviolet-visible absorbance spectra (the inset of Fig. 4(b)). As shown in Fig. 4(b), the optical bandgap Eg decreases with increasing PO2. The simultaneous deceases in the c-axis length and Eg suggest their strong correlation. In MnSnO3 bulk samples, the application of a high pressure (7 GPa) induces the structural transformation from the IL-type to the LN-type MnSnO3.3 This is reasonable because the LN-type MnSnO3 has a reduced unit-cell volume than that of the IL-type MnSnO3 (Ref. 3 and JCPDS, PDF No. 00-033-0913). Although the decrease of Eg in the LN-like MnSnO3 films with reduced lattice parameters is opposite to Vegard’s law (the relations on the increase of Eg with reducing lattice parameters generally used for isostructural semiconductor alloy systems), a consistent theoretical proposal was reported for a sister compound ZnSnO3 that Eg becomes smaller for the LN-type ZnSnO3 than for the IL-type ZnSnO3.23 We therefore attribute the decreased Eg to the LN-type (disordered corundum-type) MnSnO3. The PO2-induced structural transformation in the thin-film form can be an effective approach to study the ferroelectric and multiferroic properties expected for the high-pressure phase LN-type MnSnO3.3
We have fabricated single-crystalline ilmenite-type MnSnO3 films on Al2O3 (0001) by pulsed-laser deposition. As demonstrated by the optical bandgap measurement, the single-crystalline film is useful for characterizing the physical properties. By constructing multilayer-based device structures on the films, detection of the antiferromagnetic transition24–26 and multiferroic properties may also become possible. Moreover, we demonstrated the possibility of controlling the corundum-derivative structures by PO2. The selective formation of nonpolar IL-type and polar LN-type phases with a simple PO2 tuning may have advantages over conventional high-pressure techniques used in bulk studies. Understanding of the effect of PO2 on the initial growth as well as control experiments using substrates with different symmetry and lattice constants will be important to apply the present finding to the phase control of other corundum-derivative oxides.
See supplementary material for Mn 2p core-level XPS spectra of MnSnO3 films grown at various PO2 (Fig. S1), and PO2 dependence of the growth rate of Mn-Sn-O films (Fig. S2).
The authors thank K. Harata for the target synthesis and K. Omura for the XPS measurement. This work was performed under the Inter-University Cooperative Research Program of the Institute for Materials Research, Tohoku University (Proposals No. 17G0417 and 18G0406). This work was partly supported by JSPS KAKENHI (JP15H02022), JST CREST (JPMJCR18T2), and the Futaba Research Grant Program of the Futaba Foundation.