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Direct Synthesis of Chromium Perovskite Oxyhydride with a High Magnetictransition Temperature

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Direct Synthesis of Chromium Perovskite Oxyhydride with a High Magnetictransition Temperature Angewandte Chemie DOI: 10.1002/anie.201405453 Chromium Oxyhydride Direct Synthesis of Chromium Perovskite Oxyhydride with a High Magnetic-Transition Temperature** Cdric Tassel, Yoshihiro Goto, Yoshinori Kuno, James Hester, Mark Green, Yoji Kobayashi, and Hiroshi Kageyama* [7] Abstract: We report a novel oxyhydride SrCrO2H directly and cathode material. Their properties arise from the synthesized by a high-pressure high-temperature method. substitution of oxygen with other anions as well as the fine Powder neutron and synchrotron X-ray diffraction revealed tuning of the anionic and interacting d bands of the octahe- ÀÁthat this compound adopts the ideal cubic perovskite structure dral center B. Pm3m with O2À/HÀ disorder. Surprisingly, despite the non- Unlike the 2p based anions O2À,FÀ,S2À,N3À, the hydride À 2 bonding nature between Cr 3d t2g orbitals and the H 1s orbital, H anion is unique and only has a filled 1 s shell. Its it exhibits G-type spin ordering at TN 380 K, which is higher combination with a 2p anion as well as its interactions with the than that of RCrO3 (R = rare earth) and any chromium oxides. B element could be very interesting towards developing novel The enhanced TN in SrCrO2H with four Cr-O-Cr bonds in magnetic and transport properties. However, preparation of 3+ comparison with RCr O3 with six Cr-O-Cr bonds is reasonably explained by the tolerance factor. The present result offers an effective strategy to tune octahedral tilting in perovskites and to improve physical and chemical properties through mixed anion chemistry. Oxide perovskites ABO3 are the most encountered compounds in solid-state chemis- try. Their compositional flexibility allows numerous properties often establishing ground- breaking performances in applied devices, such as colossal magneto-resistivity,[1] dielectrics,[2] and high-temperature superconductivity.[3] However, increasing attention has been paid Figure 1. a) Crystal structure of SrCrO2H with cubic perovskite structure ðÞPm3m . Gray strontium, blue chromium, and red/gold anion (oxide/hydride) sites. b) Mass to mixed-anion perovskites, such as oxyfluor- spectrometry of H2 desorption (red) during oxidation of SrCrO2H in flowing oxygen. A ides ABO2F and oxynitrides ABO2N as promis- small peak derived from H2O (green) was also observed. The dotted line plots the [4] ing alternative materials in pigments, water- temperature at the sample, where the rapid increase indicates the exothermic reaction [5] [6] splitting photocatalysis, dielectric properties, upon H2 release/O2 uptake. oxyhydrides has been a significant challenge, mainly because [*] Dr. C. Tassel, Y. Goto, Y. Kuno, Dr. Y. Kobayashi, Prof. H. Kageyama hydride itself is a strong reducing agent. LaHO and Graduate School of Engineering, Kyoto University Ba21Ge2O5H24 were prepared by solid-state reaction in Kyoto 615-8510 (Japan) [8] E-mail: [email protected] vacuum, while FeAsLa(O,H) superconductors were pre- [9] Dr. C. Tassel pared under high pressure. Topochemical reactions were The Hakubi Center for Advanced Research, Kyoto University employed to yield layered perovskite oxyhydrides LaSr- [10] Kyoto (Japan) CoO3H0.7 and Sr3Co2O4.33H0.84 using CaH2 as a reductant. Dr. J. Hester There has been only one oxyhydride perovskite reported Bragg Institute, Australian Nuclear Science and Technology to date, BaTiO3ÀxHx that can be prepared through the low- [11] Organization (ANSTO) temperature hydride reduction of BaTiO3. BaTiO3ÀxHx is Locked Bag 2001, Kirrawee DC NSW 2232 (Australia) non-stoichiometric, with hydride substitutable up to x = 0.6. Prof. M. Green Similar reactions with iron or nickel perovskite oxides did not NIST Center for Neutron Research, National Institute of Standards afford the insertion of hydride, although the products are and Technology highly oxygen deficient (SrFeO , LaNiO ).[12] Herein, we 100 Bureau Drive, MS 6100, Gaithersburg (USA) 2 2 show a stoichiometric oxyhydride perovskite SrCrO H [**] This work was supported by the Hakubi Project funding, MEXT 2 through Grant-in-Aid for Scientific Research A (No. 24248016) and (Figure 1) synthesized using a high-pressure high-temper- the Young Scientist Grant B (No. 25810040) and JSPS through ature technique. This material is the first chromium oxy- 3+ FIRST Program“ and CREST. hydride. We also found that neighboring Cr spins order Supporting information for this article is available on the WWW antiferromagnetically at high temperature (TN 380 K), under http://dx.doi.org/10.1002/anie.201405453. which is rather surprising given the non-bonding nature of Angew. Chem. Int. Ed. 2014, 53, 10377 –10380 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 10377 Angewandte. Communications CrÀH bonds between Cr 3d t2g and H 1s orbitals. The observed properties are discussed in comparison with the 3+ related RCr O3 family (R = rare earth) that exhibits a spin order at much lower temperatures. In recent years, high-pressure synthesis has been widely used in the synthesis of transition-metal hydrides: binary systems (e.g., FeH) as well as ternary LiNiH3 and CaPdH3 adopting the perovskite structure.[13] The formation of com- plex metal hydrides usually requires synthesis temperatures that are far beyond the decomposition temperatures of the hydride precursors. While high temperature might favor decomposition of hydrides in the equilibrium of “metal + H2Ðmetal hydride”, high external and high hydrogen pres- sure will stabilize a solid hydride with a dense structure. The high-pressure and high-temperature reaction tech- nique was therefore chosen as a pathway to yield an oxy- hydride perovskite SrCrO2H with a dense structure while preventing its decomposition during heating. This composi- tion with Cr in a trivalent state was selected because of its Figure 2. Structural characterization of SrCrO2H by Rietveld refinement stability against reduction; no binary intermediate phase of SPXRD at 290 K (a) and NPD at 300 K (b). The inset in (a) shows an electron diffraction pattern of SrCrO H taken along [001]. Red exists between Cr and Cr O .[14] The best quality was achieved 2 2 3 crosses =observed, green solid line = calculated, and blue solid line = when the stoichiometric mixture of SrH2, SrO, and Cr2O3 was difference intensities. Black dashes =calculated Bragg reflections and heated at 5 GPa and 10008C, yielding a cubic phase with a red dashes =magnetic reflections. A few angle ranges contained tiny 3.83441(2) . Lower pressures and temperatures resulted peaks from an unknown impurity and were excluded from the fit. in low-purity samples containing residual precursors. Higher temperature reactions led to the reaction of Cr2O3 with the to nuclear peaks, magnetic peaks that are indexed with NaCl sleeve to give NaCr2O4. Note that attempts to top- a propagation vector (p, p, p; see below). Rietveld analysis on otactically obtain the target oxyhydride from SrCrO3 were the basis of the O/H disordered model confirmed the unsuccessful; as previously reported, the CaH2 reaction of presence of hydrogen located at the 3d Wyckoff position SrCrO3 gave oxygen-deficient phases of SrCrO2.75 and with the refined occupancy of gO = 0.337(2), along with good [15] SrCrO2.8. agreement parameters of Rwp = 3.85% and GOF = 0.96. Note To confirm the presence of hydrogen in our sample, we that the model without hydrogen resulted in an unrealistic 0.7+ monitored, using a mass spectrometer, desorbed gases upon stoichiometry of SrCrO1.338(9) having Cr . No off-stoichiom- heating under oxygen flow (108CminÀ1). Figure 1b shows etry was again found in both cationic sites. It is highly likely a sharp H2 peak (n = 2) at around 1708C together with that the sample has the stoichiometric composition of a smaller peak associated with H2O, indicative of a significant SrCrO2H. The refined results are summarized in Table 1. amount of hydrogen in the specimen. After this experiment, We calculated the Goldschmidt tolerance factor of this the sample was studied by X-ray diffraction (XRD), revealing perovskitepffiffiffi oxyhydride, which is defined by t = (rA + rX)/ a mixture of oxidized SrCrO4 and SrCrO3. [ 2(rB + rX)], where rA, rB, and rX denote the ionic radii of the Synchrotron powder X-ray diffraction (SPXRD) profile A, B, and X site. Using rSr2+ = 132 pm, rCr3+ = 62 pm, and rX = (Figure 2a) was indexed with a primitive cubic structure. The 140 pm as employed in many oxides and pure hydrides,[17] we obtained lattice parameter of a = 3.83441(2) is slightly obtained t = 1.00 for SrCrO2H, in agreement with the [16] longer than that of SrCrO3 (a = 3.818 ). A few unknown observed cubic structure. The failure to synthesize peaks not visible in the laboratory XRD were observed. CaCrO2H (not shown) implies that the perfect size matching Rietveld structuralÀÁ analysis was performed using the cubic stabilizes the SrCrO2H phase. We point out that the aliovalent perovskite model Pm3m , with Sr placed on 1b (0.5, 0.5, 0.5), HÀ for O2À substitution allows the use of larger divalent Sr2+ Cr on 1a (0, 0, 0) and O2À on 3d (0.5, 0, 0). Hydrogen is too light to be observed by XRD and thus is not considered here. The refinement readily converged to reasonable agreement parameters of Rwp = 8.45% and GOF = 3.85. The oxygen site Table 1: Structural parameters of SrCrO2H from Rietveld refinement of SPXRD (left) and NPD (right) at 300 K.[a] occupancy (gO) of 0.6699(4) indicates a nearly stoichiometric 2 composition of SrCrO2.009(1). No off-stoichiometry was found Atom Site gxyzUiso (100 ) in both cationic sites. Transmission electron microscopy + Sr2 1b 1 0.5 0.5 0.5 1.66(4)/1.95(7) experiments at room temperature demonstrate diffraction Cr3+ 1a 1 0 0 0 0.58(4)/0.86(9) patterns compatible with a simple cubic perovskite (see the O2À 3d 0.6699(4)/0.663(2) 0.5 0 0 1.56(9)/1.38(7) inset of Figure 2a).
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