A Partial Anion Disorder in SrVO 2 H Induced by Biaxial Tensile Strain

: SrVO 2 H, obtained by a topochemical reaction of SrVO 3 perovskite using CaH 2 , is an anion-ordered phase with hydride anions exclusively at the apical site. In this study, we conducted a CaH 2 reduction of SrVO 3 thin films epitaxially grown on KTaO 3 (KTO) substrates. When reacted at 530 °C for 12 h, we observed an intermediate phase characterized by a smaller tetragonality of c / a = 0.96 (vs. c / a = 0.93 for SrVO 2 H), while a longer reaction of 24 h resulted in the known phase of SrVO 2 H. This fact suggests that the intermediate phase is a metastable state stabilized by applying tensile strain from the KTO substrate (1.4%). In addition, secondary ion mass spectrometry (SIMS) revealed that the intermediate phase has a hydrogen content close to that of SrVO 2 H, suggesting a partially disordered anion arrangement. Such kinetic trapping of an intermediate state by biaxial epitaxial strain not only helps to acquire a new state of matter but also advances our understanding of topochemical reaction processes in extended solids.


Introduction
Mixed-anion compounds have been attracting a great deal of attention due to their potential applications such as photocatalysts, anionic conductors, and thermoelectric materials [1]. In terms of structural chemistry, what distinguishes mixed-anion compounds from simple oxides is a heteroleptic coordination environment around a metal centre, which induces new degrees of freedom including cis and trans geometries for the MO4X2 octahedron. For example, the perovskite oxynitrides AMO2N (e.g., SrNbO2N, SrTaO2N [2], and BaTaO2N [3]) are composed of only cis-MO4N2 octahedra, which occurs in order to maximize more covalent M dπ-N pπ bonds [2]. This local constraint gives rise to correlated disorder with various types of zigzag -M-N-M-N-chains assembled within the perovskite framework [4], as originally stated by Pauling for the structure of ice [5]. Other mixedanion oxides such as BaScO2H, prepared under high pressure, and SrFeO2F, prepared by the fluorination of SrFeO2, exhibit a cis preference, but only partially [6,7].
Transformation from cis to trans or vice versa is an important and challenging issue that may allow for tuning chemical and physical properties. As mentioned above, the TaO4N2 octahedra in ATaO2N are exclusively of cis configuration, but the short-range-ordered state with zigzag -M-N-M-N-chains is not polar (or ferroelectric), which is unfortunate given the more polarizable nature of nitride anions compared with oxide anions. However, the recent thin film study of SrTaO2N and its Ca-substituted solid solution has shown that the application of compressive biaxial strain from a SrTiO3 (STO) substrate induces a partial transformation to trans [12,13], leading to ferroelectric behavior [12].
The present study addresses a case where trans-VO4H2 octahedra in SrVO2H are converted partially to cis coordination. It has been previously shown that SrVO2H thin films, under small compressive strain (−0.7%) from SrTiO3 (001) (a = 3.905 Å) substrates have a fully anion-ordered phase (with 100% trans-coordination) [14], as in the bulk case [8]. Here, we applied tensile strain (1.4%) by using a KTaO3

Low-Temperature Reduction of SrVO3 Films
SrVO3 films with a thickness of about 100 nm were grown on KTO substrates and were reacted with CaH2 at 530 °C for 12 and 24 h, respectively. The lower reaction temperature compared to that of the powder samples (610 °C) [8] is due to a short anion diffusion path for thin films, as in the case of SrVO2H/STO [14]. The reduced films were investigated using high-resolution X-ray diffraction (XRD). The out-of-plane θ-2θ XRD scans are shown in Figure 2a, along with the as-deposited film (i.e., before reduction). The 2θ peak at 47.4°, corresponding to c = 3.83 Å, can be assigned to the 002 reflection of SrVO3 (a = 3.841 Å for bulk [15]). Both reduced samples exhibited only 00l peaks, indicating a good orientation of the films along the c axis. For the 24h-reacted SrVO3 film, the 002 peak is significantly shifted to a higher angle of 49.4° corresponding to c = 3.68 Å, which is close to that of bulk SrVO2H (c = 3.6671(3) Å [8]). Most remarkably, the 002 peak for the 12h-reacted film appears between these two peaks. This peak is absent in bulk [8] and thin film (on the STO substrate) [14] samples of reduced SrVO3, suggesting the emergence of an intermediate phase. This intermediate phase could not be assigned simply as SrVO3−δ with random anion vacancies, because the 00l peaks should appear at lower angles than for SrVO3 [8,14]. The FWHM of the θ-2θ peaks for the film reacted for 12 h is broader than the film reacted for 24 h (Figure 2a), implying lower crystallinity and/or a certain compositional distribution. Note that epitaxial SrVO2H films cannot be obtained on an (La0.3Sr0.7)(Al0.65Ta0.35)O3 (LSAT) substrate, which provides a compressive strain of −1.7% [14], and this was reproduced in our study (not shown). It is also reported that even small tensile strain from a DyScO3 substrate (0.3%) destabilizes the SrVO2H phase [14].

Structural CharacterizatIon
To further investigate the crystal structures of the three films on the KTO substrates, XRD reciprocal space mapping (RSM) measurements were performed around the 103 reflection, and the results are shown in Figure 2b-d. The in-plane positions of the 103 reflections of the 12h-and 24hreacted films are found to be near the qx positions of the KTO substrates (shown by the dashed lines), suggesting that both films are grown almost coherently, unlike the as-deposited SrVO3 film that is relatively relaxed. There was no domain formation or orientation change, which was often observed in strained films [16] or films after metal hydride treatments [17,18]. The estimated lattice parameters for the as-deposited SrVO3 film are a = 3.85 Å and c = 3.83 Å, indicating a small tetragonal lattice distortion (c/a = 0.995) compared to the bulk (cubic; a = 3.841 Å [15]), although RSM showed a partial relaxation (Figure 2b). The cell constants (a = 3.95 Å and c = 3.68 Å) for the 24h-reacted film indicate that the reduced film is slightly under tension, compared with bulk SrVO2H (a = 3.9331(4) Å, c = 3.6671(3) Å) [8]. One can therefore presume that tensile strain has been continuously applied to the films via the KTO substrate throughout the whole reaction process from SrVO3 to SrVO2H. Note that even if the KTO substrate is reduced, the corresponding cell expansion and resulting effect on strain should be very small, as XRD data did not show any anomalies. The hydride exchange was examined with secondary ion mass spectrometry (SIMS). We measured two areas (100 × 100 µm 2 ) of each film in order to investigate the hydrogen content and its distribution. Figure 3 shows the SIMS profiles of the ratio of secondary ion intensities 1 H/ 18 O for the films after 12 h and 24 h reduction. For both films, a high hydrogen signal was observed, with clear contrast at the boundary between the substrate and the film. The hydrogen signal in the KTO substrate is much smaller than the target films, though it is 10-100 times higher than the case of LSAT [19,20], which might reflect some reactivity with CaH2. For the film reacted for 24 h (i.e., SrVO2H), homogeneous hydrogen distribution is confirmed, with 1 H/ 18 [21]. Further studies are necessary to clarify the anion arrangement of this intermediate phase, using, e.g., linearly polarized X-ray absorption spectroscopy as utilized to determine cis/trans-TaO4N2 configuration in Ca-doped SrTaO2N [13].

The Role of Tensile Strain
The previous bulk and thin film studies showed that SrVO2H with a fully anion-ordered (thus all trans) structure is a stoichiometric phase (or a line phase) [8,14]. A recent study showed that a small amount of Ti substitution (5%) of V is able to change anionic composition and crystal symmetry, leading to the cubic perovskite SrV0.95Ti0.05O1.5H1.5 (a = 3.888(1) Å) [22]. The present study demonstrates that an intermediate tetragonal phase appears when the SrVO3 film on the KTO substrate reacts with CaH2 in a short period of time (12 h), and its composition is roughly given by SrVO2H. The structure is most likely a partially anion-disordered SrVO2H (PAD-SrVO2H), thus having partial cis-geometry.
Given the absence of such an intermediate phase in the case of the STO substrate with compressive strain [14], the application of tensile strain by the KTO substrate potentially plays a key role in stabilizing PAD-SrVO2H (Figure 4a).
Recall that a longer time treatment with CaH2 (e.g., 24 h) resulted in the anion-ordered SrVO2H (Figure 1b). This fact strongly suggests that the partially disordered phase is a kinetically trapped transient state, shown in the reaction coordinate diagram in Figure 4b. In the bulk form, the energy of PAD-SrVO2H's state is rather high and the potential well of this state is shallow (meaning a low activation energy to the final phase of SrVO2H) so that hydride reduction of SrVO3 leads rapidly to the anion-ordered SrVO2H (Figure 4b, left). On the other hand, when tensile strain is applied, the PAD-SrVO2H state may be kinetically trapped if the strain can stabilize this state (Figure 4b, right), but the increase in the activation energy is not high enough to stop the further reaction from occurring at 530 °C. The origin is not clear, but the stabilization of PAD-SrVO2H by tensile strain might be rationalized from examples with other perovskites, where tensile strain tends to induce oxygen deficiencies (or hydride anions, in our case) at equatorial sites [23].
It is worth considering the reaction route from SrVO3 to SrVO2H, which involves H − /O 2− exchange at the surface and hydride/oxide diffusion in the film [24]. In perovskites, oxide anion diffusion occurs along the octahedral edge (the nearest-neighbor pathway). The involvement of the equatorial oxide anion can be easily understood by the low-temperature conversion from SrFeO3 to SrFeO2 via a SrFeO2.5 intermediate [25]. Hopping of oxide anions (or oxygen vacancies) involving the equatorial site is also suggested in the layered perovskite oxide Sr3Fe2O7−x (0 ≤ x ≤ 1) [26]. The importance of nearest-neighbor anion hopping via octahedral edges has been pointed out for BaTi(O,H)3 [27]. Accordingly, in our case H − /O 2− anion migration predominantly takes place (via anion vacancies) along the V(O,H)6 octahedral edge, rather than the next-nearest-neighbor hopping between the apical sites. If this is the case, when the H − /O 2− exchange proceeds and the hydride content reaches (close to) 1, a partially anion-disordered phase with a composition close to SrVO2H will be formed, which is subsequently transformed into the fully anion-ordered SrVO2H.

Materials and Methods
Epitaxial films of the SrVO3 precursor with a thickness of ~100 nm were grown by using pulsed laser deposition on (001)-oriented KTO substrates. A target of the Sr2V2O7 pellet, prepared at 800 °C in air, was ablated by a KrF excimer laser (λ = 248 nm) with an energy density of 0.6 J/cm 2 and 1 Hz repetition rate. The films were deposited at a substrate temperature of 700-750 °C under vacuum (~1 × 10 −6 Pa). The SrVO3 film obtained was embedded in 0.2 g of CaH2 powder in a quartz tube, sealed under vacuum (<4 × 10 −2 Pa), and then heated at 530 °C for 12 or 24 h. After these reactions, the residual CaH2 and the resulting CaO byproducts on the surface of the films were removed by washing with anhydrous acetone. XRD measurements were performed at room temperature with a RigakuSmartLab diffractometer and a Cu Kα radiation (Rigaku, Akishima, Japan). SIMS measurements were performed for detecting hydrogen in the films using a Cameca ims-4f-E7 SIMS (Cameca, Gennevilliers France) at Kyoto University. The primary ion was 133 Cs + ion, accelerated at 14.5 keV to obtain depth profiles at 100 × 100 µm 2 areas for each film.

Conclusions
We found the intermediate tetragonal phase between SrVO3 perovskite and anion-ordered SrVO2H when the SrVO3 thin film (epitaxially grown on the KTO substrate) was treated with CaH2 for 12 h at 530 °C. XRD and SIMS results show that this intermediate phase is less anisotropic with c/a = 0.96 (vs. 0.93 in SrVO2H) and similar hydrogen content, implying the formation of a partially anion-disordered phase of roughly anion stoichiometric SrVO2H. Since a longer reaction time resulted in the known anion-ordered phase of SrVO2H, this phase is considered an intermediate state, possibly stabilized by applying tensile strain from the KTO substrate. This phase, with partial cisstructure, is also rationalized when considering the reaction process from SrVO3 to SrVO2H, which should involve anion migration through the octahedral edge. Stabilization of such an intermediate state has been demonstrated in many systems tuned by various parameters. For example, high pressure and temperature can kinetically trap a metastable state of dichalcogenides, MnS2, CuS2, and ZnS2 [28][29][30]. The metastable ordered state of a ternary alloy is discussed in terms of the kinetics of atomic species in crystallization [31]. In the present study, we suggest that strain engineering offers a useful strategy to kinetically trap metastable states. Despite numerous examples, topochemical reactions in extended solids have remained poorly understood. The present study sheds light on the underlying reaction mechanism.