Communication pubs.acs.org/IC
An Alternative Route to Pentavalent Postperovskite Wilson A. Crichton,*,† Kirill V. Yusenko,‡ Sephira Riva,‡ Francesco Mazzali,‡ and Serena Margadonna*,‡ †
ESRF - The European Synchrotron, 71 avenue des Martyrs, Grenoble 38000, France College of Engineering, Swansea University, Bay Campus, SA1 8EN Swansea, U.K.
‡
S Supporting Information *
related pentavalent KIrO3 and KBiO3 share the KSbO3-type14,15 structure, while NaBiO3 is an ilmenite16 and NaSbO3 is known as both an ilmenite and a pv.17 The synthesis of sodium osmates should cope with the requirement of increasing the oxidation state from OsIV to OsV,2−12 avoiding production of toxic OsO4 while being held in a reducing high-pressure assembly. Sodium peroxide, Na2O2, is used to promote high oxygen fugacities during the reaction inside noble-metal capsules. This combination of reaction conditions hampers in situ experimentation by X-rays because the sample and capsule absorb all but a small percentage of X-rays, rendering any investigation considerably challenging. We have sought to overcome these by first presynthesizing two precursors offline, in platinum capsules, at 6 GPa. Following previously reported conditions,6,8,11 we used two near-identical schemes that incorporate the same nominal mixtures of 1:2 Na2O2/OsO2 with excess peroxide. In the first case, the temperature was kept below 1100 K and the synthesis produced a mixture of KSbO3-type NaOsO3 and hexavalent Na2OsO4 (Figure S1). The scenario was quite different when the reaction temperature was increased above 1100 K (at 6 GPa). In this case, the final product contained pv-NaOsO3. Shi et al. reported similar results.6,8 In using these products for in situ investigation, we effectively fix the initial Os oxidation state at pentavalent or higher, sufficient for the production of OsV ppv. In this way the use of metal capsules is avoided during our in situ exploration of pressure−temperature space. A single-phase sample of pv-NaOsO3 was pressurized up to 16.35 GPa, and the temperature increased in a manner similar to that of a previous pv−ppv study.18 As 1135 K was approached, the diffraction profiles rapidly changed (Figure S2). No further heating was applied. Inspection demonstrated the formation of CaIrO3-type ppv-NaOsO3. This is unexpected because structural indicators are against pv-NaOsO3 forming ppv; its lattice parameters at ambient conditions [Pnma, Z = 4, a = 5.3830(3) Å, b = 7.5763(4) Å, c = 5.3245(3) Å] are closer to cubic, ϕ = 10.55°,19,20 than is normally considered sufficient to lead to the crystallization of ppv at high pressure.21 The tolerance factor is also higher than t = 0.9 (at t = 0.924). Furthermore, the ratio of polyhedral volumes, VA/VB = 4.82, is well above the normal cutoffs for ppv transitions at 4.036.22 However, contrary to Tateno et al.’s observations of limiting tilt, increasing pressure does increase distortion in pv-NaOsO3, in agreement with Fujino et al.’s assessment of the suitability.23 Although the sample was left at 1135 K for 2.5 h, the pv component did not completely
ABSTRACT: Two different high-pressure and -temperature synthetic routes have been used to produce only the second-known pentavalent CaIrO3-type oxide. Postperovskite NaOsO3 has been prepared from GdFeO3-type perovskite NaOsO3 at 16 GPa and 1135 K. Furthermore, it has also been synthesized at the considerably lower pressure of 6 GPa and 1100 K from a precursor of hexavalent Na2OsO4 and nominally pentavalent KSbO3like phases. The latter synthetic pathway offers a new lower-pressure route to the postperovskite form, one that completely foregoes any perovskite precursor or intermediate. This work suggests that postperovskite can be obtained in other compounds and chemistries where generalized rules based on the perovskite structure may not apply or where no perovskite is known. One more obvious consequence of our second route is that perovskite formation may even mask and hinder other less extreme chemical pathways to postperovskite phases.
T
he study of electronic and magnetic responses of 5d compounds is at the center of a considerable research effort because of their fascinating properties, with much of the research concentrated on Os-containing systems.1−5 High-pressure synthetic procedures have been used recently to isolate new compounds in the sodium−osmium oxide system; these include the production of Na2OsVIO4,6,7 perovskite NaOsVO3,8 and compounds Na 3 Os VIIO 5 9 and Na 5 Os VIIO 6 .10 Previously, NaOsO3 had only been briefly described as a stoichiometric KSbO3-type compound (with a primitive cubic lattice and a = 9.17 Å) as part of a survey of related chemistries produced at 3 GPa and 900 °C.11 KSbO3 structures are, more generally, a family of tunnel-containing compounds that exhibit flexibility in their chemistries and a plethora of attractive physical properties. Some of these have been prepared only at high-pressure and -temperature conditions, e.g., Ba2Ir3O912 or NaOsO3 itself.11 Osmate KSbO3 family compounds are therefore well-suited precursors for targeting new high-pressure protocols that can vary the oxidation state, stoichiometry, and bulk chemistry to produce novel phases. The recent identification of a perovskite (pv) phase in NaOsO3 has generated quite a bit of interest because it displays a continuous metal insulator transition driven by 3D antiferromagnetic order.1−5,8 Similarly, a very significant body of work now exists for the synthesis of postperovskite (ppv) phases in fluorides and oxides; the most pertinent to this work is NaIrO3, which is not known as a pv, or any other pentavalent form, but remains the sole pentavalent ppv oxide reported to date.13 The © XXXX American Chemical Society
Received: March 29, 2016
A
DOI: 10.1021/acs.inorgchem.6b00780 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry disappear, in spite of rapid onset, suggesting that the pressure and temperature conditions had evolved toward the equilibrium phase boundary. After 1 h of heating, the lattice of pv had a = 5.28 Å, b = 7.428 Å, and c = 5.1480 Å, and V/Z = 50.47 Å3/fu and that of ppv had a = 2.769 Å, b = 10.034 Å, c = 7.218 Å, and V/Z = 50.14 Å3/fu. Therefore, there is less than a 1% volume change upon transition. The postheating pressure was 13.26 GPa. This initial pv sample appears to behave as expected and, following the same reasoning as that in other pv−ppv transformations, can be interpreted as the growth of stable ppv from metastable pv due to the positive Clapeyron slope for the equilibrium transition. Nonetheless, the pseudocubic tilt of the pv phase continues to be distinctly different from that which we expect in the region of a pv−ppv transformation,21 by at least 10°. A dP/dT slope of 10−17 MPa/K19,21,23,24 for this transformation might assume a room temperature transition at near ambient pressure. The lower-temperature synthetic product resembles a close to 1:1 mixture of the KSbO3 type [a ≈ 9.1415(4) Å; c-NaOsVO3, Z = 12, V/Z = 63.7 Å3/fu] and hexagonal Na2OsVIO4 [a = 9.6242(6) Å, c = 3.1619(3) Å, P6̅2m, following refs 6 and 7; Figure S1]. The pseudocubic lattice of KSbO3 is close to that reported by Sleight,11 although the diffraction signal of this phase does not appear to be only cubic at ambient conditions. Reflections of type hk0 are doubled, and the cubic 312 equivalent is at least a triplet, while 200, 400, and h00 are not, indicating a rhombohedral distortion (Figure S3). The cubic phase was introduced along with hexagonal Na2OsO4, and the remaining peaks were indexed by distorting the Pn3̅ KSbO3-type model to its R3̅R subgroup, while adjusting a = 9.146 Å and α = 91.5°; R3̅R, f 9dcb; equivalent to the refined values of R3̅H, a = 13.1042(7) Å and c = 15.4203(12) Å. Various rhombohedral KSbO3-like chemistries are described as ilmenite and related types, with a ≈ 16,17 However, these lattices 5.4 Å and c ≈ 16.0−18.25 Å; R3H ̅ , fc2. do not account for peak splitting in the same manner. Upon pressurization, the proposed rR form is evidently preferred because the peak splitting becomes more prominent with the load Figure 1A (Figures S4 and S5). During heating, at 6.05 GPa, Na2OsO4 is consumed in the rhombohedral KSbO3like single phase (α rapidly increases to 92.5°) at 750 K (Figure 1B). Because there is no concomitant visible appearance of OsO2, we assume that the rhombohedral form is flexible in its chemistry (like related KSbO3 types11,12). Immediately after this, a continuous transformation from rR- to c-NaOsO3 is observed (830 K; Figures 1C and S6). The (111) peak, characteristic of Pn3̅ KSbO3-type structures, is weak but present as per Sleight’s observation.11 The c-NaOsO3 sample was cooled (Figure 1D) without changes and reheated (now at 5.75 GPa; Figure S7), whereupon two almost coincident events were observed: (i) at 750 K, a sharp S-shaped kink in the c-NaOsO3 peaks (Figure 1E) occurs with the growth of rutile-type OsO2; (ii) at 825 K (Figure 1F), the growth of ppv-NaOsO3 occurs. The rapid peak shift to lower d spacing of all cubic peaks and concomitant growth of OsO2 are indicative of a reduction of the lattice size due to the loss of osmium to its rutile-structured oxide. At conversion (Figure 1F), the lattice of the ppv phase is a = 2.8192(17) Å, b = 10.4047(21) Å, and c = 7.3179(18) Å (V/Z = 53.66 Å3/fu); with c-NaOsO3, it is a = 9.0709(7) Å (V/Z = 61.20 Å3/fu), and their relative volume difference is ΔV = −12.3%. The pressure after this heating cycle is 4.96 GPa. A further run of the same ppv transformation (Figure S8) shows fine-grained texture for recovered ppv, with a minor spotty KSbO3-like component. The ppv lattice has dimensions a =
Figure 1. Details of a series of semicontinuous X-ray diffraction patterns collected upon pressurization and during two heating−cooling cycles. (A) Increased splitting of the (110/1−10) pair during pressurization to 6.05 GPa. (B) Loss of Na2OsO4 at 750 K. (C) rR- to c-NaOsO3 transformation. (D) Temperature quenching and the end of heating cycle 1. Reheating over the same range (now 5.95 GPa) gives part E. (F) The S-shaped-kink at 750 K and the onset of OsO2 and crystallization of ppv at 825 K. This is followed by part G. Temperature quenching and the end of heating cycle 2. Simulated diffraction patterns for c-NaOsO3, OsO2, and ppv-NaOsO3 are shown as keys. Other peaks are due to h-BN (the fast-moving peak at ∼4° is 002) and the 111 peak of MgO, at about 5.4°.
2.8321(3) Å, b = 10.6928(17) Å, c = 7.3342(10) Å, V = 222.10(5) Å3, V/Z = 55.7 Å3/fu, and ρcalc = 7.811 g/cm3 (Table 1 and the Supporting Information). ppv-NaOsO3 is considerably more distorted than NaIrO3 [a = 3.03968(3) Å, b = 10.3576(12) Table 1. Atomic Positions and Bond Lengths for ppvNaOsO3, Refined in the Space Group Cmcm, in Situ, and in Assembly, upon Return to Ambient Conditionsa x/a
atom
site
Os Na O1 O2 Os−O Na−O
4a 0 4c 0 4c 0 8f 0 4 × 1.835(15)Å 2 × 2.34(3) Å
y/b 0 0.249(2) 0.924(3) 0.391(2) 2 × 2.007(9) Å 2 × 2.45(7) Å
z/c 0 0.25 0.25 0.512(10) 4 × 2.70(6) Å
a Lattice a = 2.8321(3) Å, b = 10.6928(17) Å, and c = 7.3342(10) Å. R = 5.59%, Rw = 6.25, and R(all) = 6.70%.
B
DOI: 10.1021/acs.inorgchem.6b00780 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry Å, and c = 7.1766(3) Å13], with slightly shorter a and c lengths and a longer b length, a consequence of similar edge-sharing and longer apical distances of Os−O octahedra. The O−Os−O angle is 132° in NaOsO3 compared to the more corrugated 140° in NaIrO3, with average Na−O distances (at 2.55 and 2.57 Å13). The latter route illustrates that ppv can be formed from phases other than pv (even in systems where pv occurs) and that the growth of pv as a precursor is counterproductive, requiring almost 3 times higher pressure to transform, reducing the potential yield. The >10% difference in volume from KSbO3-type to ppv compared to 1−2% from pv to ppv, especially with ppv being less dense than pv at ambient conditions, is crucial in the lower-pressure stabilization of ppv via KSbO3-like NaOsO3. Our results also show that ppv-NaOsO3 can be produced even though all criteria commonly used to predict such transitions are not fulfilled. It appears then that the tilt, polyhedral ratio, degree of covalency of the metal−oxygen bonding, etc., should be reassessed for yet more pentavalent ppv chemistries.13,17,24
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(7) Matar, S. F.; Demazeau, G.; Ouaini, N. Solid State Sci. 2011, 13, 1396. (8) Shi, Y. G.; Guo, Y. F.; Yu, S.; Arai, M.; Belik, A. A.; Sato, A.; Yamaura, K.; Takayama-Muromachi, E.; Tian, H. F.; Yang, H. X.; Li, J. Q.; Varga, T.; Mitchell, J. F.; Okamoto, S. Phys. Rev. B: Condens. Matter Mater. Phys. 2009, 80, 1661104. (9) Mogare, K. M.; Klein, W.; Schilder, H.; Lueken, H.; Jansen, M. Z. Anorg. Allg. Chem. 2006, 632, 2389. (10) Betz, T.; Hoppe, R. Z. Anorg. Allg. Chem. 1985, 524, 17. (11) Sleight, A. W. Mater. Res. Bull. 1974, 9, 1177. (12) Kawamura, Y.; Sato, H. J. Alloys Compd. 2004, 383, 209. (13) Bremholm, M.; Dutton, S. E.; Stephens, P. W.; Cava, R. J. J. Solid State Chem. 2011, 184, 601. (14) Hoppe, R.; Claes, K. J. Less-Common Met. 1975, 43, 129. (15) Nguyen, T. N.; Giaquinta, D. M.; Davis, W. M.; zur Loye, H. C. Chem. Mater. 1993, 5, 1273. (16) Kumada, N.; Kinomura, N.; Muto, F. Nippon Seramikkusu Kyokai Gakujutsu Ronbunshi 1990, 98, 384. (17) Mizoguchi, H.; Woodward, P. M.; Byeon, S.-H.; Parise, J. B. J. Am. Chem. Soc. 2004, 126, 3175. (18) Bernal, F. L.; Yusenko, K. V.; Sottmann, J.; Drathen, C.; Guignard, J.; Løvvik, O. M.; Crichton, W. A.; Margadonna, S. Inorg. Chem. 2014, 53, 12205. (19) Kojitani, H.; Shirako, Y.; Akaogi, M. Phys. Earth Planet. Inter. 2007, 165, 127. (20) O'Keeffe, M.; Hyde, B. G. Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. 1977, 33, 3802. (21) Tateno, S.; Hirose, K.; Sata, N.; Ohishi, Y. Phys. Earth Planet. Inter. 2010, 181, 54. (22) Martin, C. D.; Parise, J. B. Earth Planet. Sci. Lett. 2008, 265, 630. (23) Fujino, K.; Nishio-Hamane, D.; Suzuki, K.; Izumi, H.; Seto, Y.; Nagai, T. Phys. Earth Planet. Inter. 2009, 177, 147. (24) Lindsay-Scott, A. Thermoelastic properties of post-perovskite analogue phases. Ph.D. Thesis, Department of Earth Sciences, University College London, London, U.K., 2011.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00780. Experimental details, illustrated diffraction data, and structure descriptions (PDF) X-ray crystallographic data in CIF format (CIF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (W.A.C.). *E-mail:
[email protected] (S.M.). Author Contributions
All authors have approved the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The ESRF is thanked for allocation of beamtime at ID06LVP through Proposal CH-4530. S.M. gratefully acknowledges the financial support provided by the Welsh Government and Higher Education Funding Council for Wales through the Sêr Cymru National Research Network in Advanced Engineering and Materials (Grant NRN140).
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REFERENCES
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DOI: 10.1021/acs.inorgchem.6b00780 Inorg. Chem. XXXX, XXX, XXX−XXX
