Transformation of the Chromium Coordination Environment in

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Transformation of the Chromium Coordination Environment in LaCrAsO Induced by Hydride Doping: Formation of La2Cr2As2OyHx Sang-Won Park,† Hiroshi Mizoguchi,† Haruhiro Hiraka,‡ Kazutaka Ikeda,‡ Toshiya Otomo,‡ and Hideo Hosono*,†,§ †

Materials and Structures Laboratory and Materials Research Center for Element Strategy, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan ‡ Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305-0801, Japan § ACCEL Program, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan S Supporting Information *

ABSTRACT: We report the synthesis of La2Cr2As2OyHx (0.1 < y < 1.6) oxyhydride solid solutions using a solidstate reaction under high pressure with a solid-state hydrogen source and exhibit an example of how H− doping can also promote structural changes: H− doping in LaCrAsO results in the formation of La2Cr2As2OyHx with the La2Fe2Se2O3-type layered structure. Remarkably, this transformation includes a change of the coordination number of Cr from 4 to 6, with the some of the H− being accommodated in new sites within the CrAs layers. In this way, H− not only serves as a conventional electron dopant by the substitution of O2− but also makes new bonds to the transition metals. n inorganic solids, the hydride ion (H−) has the following characteristics: (a) the charge is changeable; (b) the size is flexible; (c) it is a strong σ donor, comparable to CN−. Recently, potential applications of the oxyhydride family have attracted much attention because of the large mobility and labile character of the H− ion.1 It is also expected to be used as a precursor for the low-temperature synthesis of oxynitride materials.2 However, there have been few reports about oxyhydrides,3,4 which is consistent with our expectation that the presence of both H− and O2− in a solid would lead to OH− formation. For investigation of the behavior of the H− ion in oxides, comparison with the fluoride ion (F−) often provides us chemical implications because of their equal charge and similar ionic size.5 Hosono and co-workers reported electron doping into LaFeAsO with the ZrCuSiAs-type layered structure, by using H−/F−.6 In this tetragonal structure (Figure 1a), commonly referred to as the 1111 type, an antifluorite-type Fe2+2As3−2 layer of an edgesharing FeAs4 tetrahedron is stacked alternately with a fluoritetype La3+2O2−2 blocking layer along the c axis. Electron doping into the Fe2As2 layers by substitution of O2− sites with F− causes a phase transition from an antiferromagnetic (AFM) metallic state to a Pauli paramagnetic metallic one, which leads to a superconducting transition at ∼26 K.6a In the case of LaFeAs(O1−xXx), the substitution limit expands greatly from x = 0.12 for X = F to x = 0.53 for X = H.6 Although large solubility of the H− ion is much unexpected, the oxyhydrides might appear around us commonly, as was recently observed in an amorphous

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Figure 1. Crystal structure of (a) LaCrAsO with the 1111 type (space group: P4/nmm)8b or (b) La2Cr2As2OyHx with the 2223 type (I4/ mmm). In the structural change from 1111-type to 2223-type structure, a slip occurs along the slip plane (001) with the slip direction [110].

In−Ga−Zn−O thin film with the concentration of H− ions to ∼1020 cm−3 for thin-film transistor applications.7 Here, we report the results of our attempts to perform this H− substitution for O2− of LaCrAsO,8b which is an early 3d transition-metal end member of the 1111-type family, LaMAsO (M = Cr−Ni).8 We attempted the synthesis of LaCrAs(O1−xHx) via a solidstate reaction. The powder X-ray diffraction (XRD) patterns for the reaction products are shown in Figure 2. With increasing H content, the XRD peaks arising from the 1111-type parent compound vanished after only 20% substitution of H−, and new peaks emerged that could be indexed with a tetragonal supercell of the 1111 type with an approximately doubled c axis. The crystal structure of the new phase was refined using Rietveld methods based on the La2Fe2Se2O3 structure type with space group I4/mmm (No. 139).9 (Hereafter, we denote this structure type as the 2223 type.) Received: September 8, 2017

A

DOI: 10.1021/acs.inorgchem.7b02316 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

Figure 3. (a) Crystal structures of La2Cr2As2OyHx and local coordination of Cr or La. Selected bond distances for La2Cr2As2O1.6H1.3 (La2Cr2As2O0.1H2.4) are also described. (b) TOF-NPD data collected at 300 K for La2Cr2As2O1.6H1.3 and Rietveld refinement results, showing the observed (black circle), calculated (red line), and difference (black line) patterns. The upper and lower bars represent the positions of the calculated Bragg reflections from the nuclear and magnetic scattering contributions, respectively.

Figure 2. (a) Powder XRD patterns measured for the La2Cr2As2OyHx series. (b) Variation of the magnetic moment (μ), unit cell parameters, and cell volume across H−-substituted samples.

In this structure, two anionic sites with the potential for O and H occupancy exist, preventing a definitive determination of the composition for powder XRD data alone. The total amounts of O were measured using electron probe microanalysis (EPMA). The samples with nominal y values of 1.8, 1.6, 1.4, 1.0, 0.6, or 0 were measured as 1.46(2), 1.65(7), 1.42(3), 0.96(1), 0.58(2), or 0.08(2), respectively. As a result, the chemical compositions were close to La2Cr2As2OyHx for y = 0.1−1.6. The vacancy fraction on each O site was estimated by refining the structure against the powder XRD data while fixing the total amounts of O to that measured by EPMA. To determine the H content and site preferences, time-offlight neutron powder diffraction (TOF-NPD) data were collected for La2Cr2As2OyHx and used as a basis for another round of Rietveld refinements. The results of these refinements are given in Figure 3b and Tables S1 and S2 in the Supporting Information. Fitting of the 101 peak at Q ∼ 1.6 Å−1 of the TOFNPD data of La2Cr2As2OyHx was noticeably improved by including a magnetic contribution indexed with the same unit cell with a magnetic propagation vector k = 0 (Table S1 in the Supporting Information) The crystal structure is shown in Figure 3a, while selected interatomic distances for these compounds are listed in Table S3 in the Supporting Information. As can be seen in Figure 3a, the crystal structure of La2Cr2As2OyHx shows a similar alternate stacking of where an La2X2 blocking layer and the Cr2As2-based layer are along the tetragonal c axis (X = O2− or H−), as observed for the original 1111 type structure (Figure 1a). However, while the La2O2 layers remain largely unchanged aside from H−/O2− substitution on the O site [the X(1) site], the Cr−As layers have undergone a substantial transformation through the incorporation of anions at a site that we label as X(2). The coordination number of Cr thus increased from 4 to 6, with the Cr−As distances lengthening correspondingly from 2.49 Å (LaCrAsO) to 2.58 Å (La2Cr2As2O0.1H2.4), as listed in Table S3 in the Supporting Information. The Cr now bears a CrAs4X(2)2 octahedron (shown in Figure 3a), and it shares four faces with adjacent octahedra to form the Cr2As2X(2) layer. Notably, to the best of our knowledge, this is the first observation of a structural change from the 1111 type to the 2223

type by systematic H− doping. Indeed, the 2223 type structure generally appears in 3d transition-metal oxychalcogenides,9 with the previously reported hydrides being La2M2As2Hx (M = Ti− Mn) and NdScSiD1.47.10 The series of La2Cr2As2OyHx also represents the first 2223 type oxyhydrides with a wide solid solution range (y = 0.1−1.6). In addition, this phase is unique in it being a chromium oxyhydride, as the only precedent for such a compound SrCrO2H with perovskite-type structure.4c As summarized in Table S1 in the Supporting Information, O2−/H− site preferences are clearly observed in La2Cr2As2OyHx. O2− or H− prefers the X(1) or X(2) site, respectively. Similar tendencies have been reported in LaSrCoO3H0.7 and Sr2VO3H, where apical and equatorial sites of anions are clearly distinguished.4a,d Figure 2b shows the anisotropic structural change of La2Cr2As2OyHx as a function of y. As the H content is increased, the volume of the unit cell decreases. However, this reduction is almost entirely due to a decrease in the c lattice parameter because a is largely unchanged across the series. These trends have simple structural origins: the rigidity of the Cr2As2X(2) layers leads to a being essentially fixed. By contrast, the ability of the La ions to shift along the z direction into the hollows of the Cr2As2 layers allows for the c parameter to easily contract. This shift gives rise to a drastic decrease of the La−As distance. However, this change of La ions appears to be compensated for by elongation of the La−X(1) distances (Table S3 in the Supporting Information). It is noted that, in La2Cr2As2O0.1H2.4, the La−H(2) distance (2.89 Å) is shorter that of La−As (3.09 Å). The magnetic structure at 300 K was obtained by Rietveld refinement against the NPD patterns (Figure S1 in the Supporting Information). It reveals a checkerboard-type AFM order in the planes of the Cr2As2X(2) layers and AFM coupling between layers along c, resulting in a G-type AFM spin configuration. The magnetic configuration is, in fact, the same as that in its 1111 parent phase LaCrAsO,8b despite differences in the local coordination environments of the Cr atoms. The B

DOI: 10.1021/acs.inorgchem.7b02316 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

attained by the use of H−. A plausible reason is the H solubility in solids, which is significantly enhanced by high pressure (>1 GPa), as described by Sugimoto and Fukai.13 Only a few oxyhydrides have been reported to date. The compounds, including Zr3V3ODx, Zr(O1−xHx)0.5, LaOH, and LaFeAs(O1−xHx), are often stabilized by cations with highly electropositive character.3,6b The present study reveals that the chemical potential of H under high pressure works well for the synthesis of oxyhydride containing a moderately positive cation, such as La2Cr2As2OyHx. In conclusion, a range of new oxyhydrides La2Cr2As2OyHx (0.1 < y < 1.6) with a 2223 type layered structure were synthesized by hydride doping of the 1111 type LaCrAsO compound. The structural change from LaCrAsO to La2Cr2As2OyHx involves the following features: (1) H−/O2− anions are incorporated into the Cr2As2 layers, changing the coordination number of Cr from 4 to 6. (2) Extra H−/O2− ions are inserted into the Cr2As2 layers, suppressing reduction of the Cr ion. In this way, we see how hydride substitution in the 1111 type can do more than simply provide conventional electron doping; it can also drive changes in the local coordination structure around the transition-metal sites.

ordered spin moments on the Cr atoms in La2Cr2As2O1.6H1.3 and La2Cr2As2O0.1H2.4 along the c direction at 300 K were refined as 1.96(2) and 1.40(2) μB, respectively (Figure 2b), similar to that of LaCrAsO.8b The temperature dependencies of La2Cr2As2OyHx’s electrical resistivity and magnetic susceptibility over the temperature range 2−300 K are given in Figure S2a and S2b in the Supporting Information, respectively. The electrical resistivity in each sample decreased with decreasing temperature, consistent with metallictype conduction. No peaks related to AFM were confirmed in the χ−T curves of the series of oxyhydrides over the range 2−300 K, suggesting that the magnetic ordering temperature (Néel temperature) is higher than 300 K. There are several broader connections and implications that can be drawn from these results. The first regard is the nature of the transformation driven by H− doping in the 1111-type phases. Upon the incorporation of H−, the tetragonal P (primitive) cell of LaCrAsO is converted to the tetragonal I (body-centered) cell via a [110] slip along the slip plane (001), as shown in Figure 1. The view of the sliding of stacking suggests that H− doping weakens the Cr−As bonds, while the slip generates the X(2) sites to accommodate extra anions. Notably, the observed slip is reminiscent to us of crystallographic shearing commonly observed in reduced d0 oxides of V, Nb, or W.11 Next, we discuss the driving force for this structural change. As we mentioned earlier, H− substitution of the O site in LaCrAsO increases the number of valence electrons on the Cr ions. Given the structural role that electron counts play in intermetallic systems, as described by the Hume−Rothery rules,12 one might suspect that this change in the electron count plays a role in the transformation. However, LaMAsO, in fact, maintains the 1111 type structure for M = Cr−Ni, suggesting that this electron count is not the primary factor. Also, although the LaFeAs(O1−xHx) solid solution maintains 1111 type structure up to x = 0.53,6b only 20% H− substitution is needed to induce the observed structural change of LaCrAsO. This suggests that LaCrAsO has low structural stability relative to other members of the family, which is consistent with its position on the end member of the series LaMAsO (M = Cr−Ni). One possible explanation for these observations is in the way reduction of the Cr2+ ion by H− substitution would increase of the size of the Cr ion. In the 1111 type to 2223 type transition, extra anions are incorporated in order to suppress abrupt reduction of the Cr2+ ion, with the Cr coordination number increasing from 4 to 6. Some precedent for this hypothesis is offered by reports of the 1111 type LaFeAsO phase and its 2223 type derivative La2Fe2Se2O3.8a,9a Electron doping through the replacement of As with Se in LaFeAsO would reduce Fe from 2+ to 1+ if no structural change were to take place. However, the move to the 2223 type introduces extra O atoms into the Fe2As2 layers to suppress reduction of the Fe ion. The Fe ions in the resulting 2223 type phase remain 2+. Finally, we discuss the difference between H− and F− as substitution agents. Despite the well-known resemblance between H− and F− in terms of factors including size and charge,5 in LaCrAsO, F− doping can reach 20% without any structural change,8b while H− substitution was possible until almost 100%. This greater ease of H−/O2− substitution is perhaps unexpected given that O2− and F− have the same electronic configuration (2s22p6), which differs substantially from that of H− (1s2). Also, the formation energies of the solidstate hydrides are generally much smaller than those of fluorides because of the small electron affinity of H−.5 Nonetheless, much heavier substitution into the O site in LaCrAsO or LaFeAsO is

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02316. Experimental section (synthesis, composition and structure analysis, physical properties), supplementary magnetic structure, figures of electrical and magnetic properties, structural parameters, and selected bond lengths (PDF) Accession Codes

CCDC 1574689−1574690 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Sang-Won Park: 0000-0002-2843-9803 Hiroshi Mizoguchi: 0000-0002-0992-7449 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by the Element Strategy Initiative to form a research core. H.H. was supported by the JSPS through a Grant-in-Aid for Scientific Research (S) (Grant 17H06153). The neutron scattering experiment was approved by the Neutron Scattering Program Advisory Committee of IMSS, KEK (Proposal No. 2014S06). We thank Prof. D. C. Fredrickson (University of WisconsinMadison) and Drs. S. Matsuishi, S. Iimura, and J.Bang (Tokyo Institute of Technology) for constructive discussions. C

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oxyselenide La2O3Mn2Se2. Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 82, 214419. (10) (a) Mizoguchi, H.; Park, S.; Hiraka, H.; Ikeda, K.; Otomo, T.; Hosono, H. An Anti CuO2-type Metal Hydride Square Net Structure in Ln2M2As2Hx (Ln=La or Sm, M=Ti, V, Cr, or Mn). Angew. Chem., Int. Ed. 2015, 54, 2932. (b) Tencé, S.; Mahon, T.; Gaudin, E.; Chevalier, B.; Bobet, J.-L.; Flacau, R.; Heying, B.; Rodewald, U. C.; Pöttgen, R. Hydrogenation studies on NdScSi and NdScGe. J. Solid State Chem. 2016, 242, 168. (11) Andersson, S.; Wadsley, A. D. Crystallographic Shear and Diffusion Paths in Certain Higher Oxides of Niobium, Tungsten, Molybdenum and Titanium. Nature 1966, 211, 581. (12) Hume-Rothery, W.; Smallman, R. E.; Haworth, C. W. The Structure of Metals and Alloys; Metals and Metallurgy Trust: London, 1969. (13) Sugimoto, H.; Fukai, Y. Solubility of hydrogen in metals under high hydrogen pressures: Thermodynamical calculations. Acta Metall. Mater. 1992, 40, 2327.

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DOI: 10.1021/acs.inorgchem.7b02316 Inorg. Chem. XXXX, XXX, XXX−XXX