Isothermal section of the Na 0.3 CoO 2 H 2 O phase diagram at 22 C from 11 to 100% relative humidity

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1 Solid State Sciences 7 (2005) Isothermal section of the Na 0.3 CoO 2 H 2 O phase diagram at 22 C from 11 to 100% relative humidity Viktor V. Poltavets a, Konstantin A. Lokshin b, Martha Greenblatt a, a Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey, 610 Taylor Road, Piscataway, NJ 08854, USA b LANSCE-12, Los Alamos National Laboratory, Los Alamos, NM 87544, USA Received 2 May 2005; accepted 23 May 2005 Available online 1 July 2005 Abstract An isothermal section of the Na 0.3 CoO 2 H 2 O system phase diagram at 22 C from 11 to 100% relative humidity is presented. The superconducting Na 0.3 CoO 2 1.2H 2 O phase is stable at a relative humidity (RH) higher than 30%; Na 0.3 CoO 2 0.6H 2 O is the stable phase at RH below 30%. The unit cell parameters and temperature of superconducting transition of Na 0.3 CoO 2 1.2H 2 O do not depend on relative humidity. The Na 0.3 CoO 2 1.2H 2 OandNa 0.3 CoO 2 0.6H 2 O hydrates are line phases and have a constant water content over the water vapor pressure range of their stability Elsevier SAS. All rights reserved. Keywords: Superconductivity; CoO2-layer superconductor; Phase diagram; Chemical analysis; Magnetic susceptibility; Thermogravimetry 1. Introduction The crystal structure of Na x CoO 2 yh 2 O superconductor (x 0.3, y 1.3) consist of CoO 2 layers separated by sodium and water layers. The sodium water part of the structure is considered to act as a charge reservoir and a spacer [1 4]. The optimum sodium content (x = 0.3) for superconductivity has been established [5]. In spite of the charge neutrality of water molecules their role appears to be more than just the separation of CoO 2 layers. It is known that a critical number of H 2 O layers is required in the structure for superconductivity. For example, superconductivity is completely suppressed in the monolayer hydrate phase (MLH) Na x CoO 2 0.6H 2 O, where the CoO 2 layers are already well separated. Sakurai et al. [6] suggested that in the bilayer hydrate (BLH) Na x CoO 2 1.3H 2 O the water molecules shield the random Coulomb potential of the Na + ions, providing a smoother potential at the CoO 2 layers. * Corresponding author. Tel.: ; Fax: address: martha@rutchem.rutgers.edu (M. Greenblatt). Na x CoO 2 1.3H 2 O transforms rapidly to Na x CoO 2 0.6H 2 O under ambient conditions [7], while the reverse reaction is substantially slower and occurs at higher relative humidity (RH) or in liquid water. Due to this chemical instability of the BLH, differences in sample handling might lead to differences in water content and even in some physical properties. Thus, the very different values for superconducting volume fractions reported [7 9], could possibly indicate differences in the amount of adsorbed water on the grain surfaces or, in some cases, the presence of monolayer hydrate phase admixture. The lack of phase composition RH diagram and absence of a general facile method of water content optimization for Na x CoO 2 yh 2 O phases resulted in the frequent use of relatively uncontrollable procedures such as brief drying under ambient conditions [1,5,7,10 12]. Alternatively, samples have been stored at 100% RH [3,13], equilibrated at definite D 2 O pressure in a vacuum system [3] or freeze dried below the D 2 O melting point until elimination of ice peaks in the neutron powder diffraction pattern [2]. Relative humidity can be controlled conveniently and accurately by the use of saturated aqueous solutions of salts, as relative humidity over such solutions is fixed at a definite /$ see front matter 2005 Elsevier SAS. All rights reserved. doi: /j.solidstatesciences

2 V.V. Poltavets et al. / Solid State Sciences 7 (2005) temperature [14]. Thus using saturated solutions of different salts can be a suitable method for phase/water content optimization in the Na x CoO 2 H 2 Osystem. In the present paper, the isothermal section of the phase diagram as well as the amount of crystal and adsorbed water of the Na 0.3 CoO 2 H 2 Osystemat22 C in the % relative humidity range is reported. 2. Experimental Na 0.7 CoO 2 was prepared by heating of a mixture of Na 2 CO 3 (Fisher, A.C.S.) and Co 3 O 4 (Aldrich, 99.8%, nanopowder) in oxygen flow at 810 C for 24 hours. A 10% molar excess of Na 2 CO 3 was added to compensate for loss of Na due to volatilization. Na 0.3 CoO 2 yh 2 O was prepared as reported in the literature [2].Na 0.7 CoO 2 was stirred in an acetonitrile solution of Br 2 at room temperature. A Br 2 concentration of 40 times the molar excess relative to the theoretical amount needed to remove all of the sodium from Na 0.7 CoO 2 was employed. After immersion in the solution for 4.5 days, the product was filtered, washed several times with acetonitrile and water and then dried briefly (20 min) under ambient conditions. The washed product was divided into portions, which were kept at 22 ± 1 C over saturated solutions of the following salts: KNO 3 (Aldrich, ACS), Li 2 SO 4 (Fisher, ACS), NaNO 3 (Alfa, 99%), NaBr (Alfa, 99%), Ca(NO 3 ) 2 (Alfa, ACS), Zn(NO 3 ) 2 (Alfa, 99%), FeCl 3 (Riedel de Haen, 98%), LiI (Acros, 99%), LiCl (Fluka, 99%) (Table 1). Samples were equilibrated between 4 to 16 weeks. Sample preparation for X-ray diffraction, magnetic measurements and thermogravimetric analysis was performed in a nitrogen filled polymer glove bag, where a constant RH was maintained by continuous N 2 (g) circulation over the appropriate saturated salt solution (i.e., the same solution where a sample was equilibrated). Table 1 Relative humidity of the saturated salt solutions and amount of crystal structure (y) and adsorbed (z) water for {Na 0.3 CoO 2 yh 2 O + zh 2 O} Salt Relative humidity (%) of the saturated salt solution at 22 C TGA data for {Na 0.3 CoO 2 yh 2 O + zh 2 O} y z (40 days) (H 2 O only) 2.91 (100 days) KNO 3 92 [15] (40 days) 0.40 (100 days) Li 2 SO 4 84 [15] NaNO 3 73 [16] NaBr 59 [16] Ca(NO 3 ) 2 48 [17] Zn(NO 3 ) 2 38 [17] FeCl 3 30 [15] 0.84 a 0.04 LiI 18 [14] LiCl 11 [18] a Two hydrate phase sample (see text). The powder X-ray diffraction (PXD) patterns were recorded at room temperature over an angular range 5 2θ 90 withastepof0.02 (2θ) on a Bruker D8 Advance diffractometer (Bragg Brentano geometry, CuKα radiation). A low background airtight specimen holder with a dome-like plastic cover was used to maintain constant relative humidity over the samples during X-ray measurements. Silicon (NIST 640c, a = (1) Å) was used as an internal calibration standard, and lattice parameters were refined with the Checkcell program [19]. Thermogravimetric analysis (TGA) was performed with a TA Instrument 2050 thermal analyzer. Samples were ramped in an oxygen flow at 2 C/min to a final temperature of 900 C. The DC magnetic susceptibility measurements were carried out on powder samples with a commercial SQUID magnetometer (Quantum Design, MPMS-XL) at 10 Oe. All measurements were performed by warming the samples in the applied field after cooling to 2 K in zero field (ZFC, zero field cooling) and by cooling the samples in the applied measuring field (FC, field cooling). Small temperature increment (0.05 K) and long equilibration times were used between 4 and 5 K for a precise superconducting transition temperature determination. Quantitative elemental analysis of sodium and cobalt was performed on an Agilent 7500 Series inductively coupled plasma mass spectrometer (ICP-MS). 3. Results and discussion The unit cell parameters for the Na 0.7 CoO 2 (a = 2.840(1) Å, c = 10.81(1) Å) are slightly larger than the literature data [20]. Given the near-linear relationship between composition and spacing between adjacent CoO 2 layers in the 0.7 x 1.0 range [20], the actual composition should be near Na 0.75 CoO 2. This estimation is in good agreement with the Na 0.74 Co 1.00 O 2 composition as determined by ICP-MS. Chemical oxidative sodium deintercalation from Na CoO 2 was performed for 4.5 days instead of 5 days reported in the literature [2]. Such procedure modification was necessary since the reaction for 5 days resulted in nonsuperconducting samples after hydration. The difference in product properties is attributed to smaller crystallite size of our Na 0.7 CoO 2 sample that led to an overoxidized product. The stoichiometry of the deintercalation product was Na 0.30 Co 1.00 O 2 as determined by ICP-MS. After washing Na 0.3 CoO 2 with water and drying briefly under ambient conditions, the product consisted of two phases: Na 0.3 CoO 2 and Na 0.3 CoO 2 0.6H 2 O. This mixture was divided into portions, which were kept at 22 C at stable relative humidities (Table 1). It should be noted that the partial water vapor pressure at the same relative humidity is different at different temperatures. Nevertheless, at a given temperature there is a direct relationship between RH and partial water vapor pressure.

3 1314 V.V. Poltavets et al. / Solid State Sciences 7 (2005) Thus in this article we use RH as a compositional variable at a fixed temperature. We observed that prolonged treatment was necessary for the BLH formation, especially at RH below 40%. For example, Na 0.3 CoO 2 0.6H 2 O and a small amount of Na 0.3 CoO 2 1.3H 2 O were found by PXD after 2 weeks at 38% RH. An additional two weeks at the same RH resulted in pure Na 0.3 CoO 2 1.3H 2 O. Therefore BLH is the equilibrium phase at such conditions, but 2 weeks are not enough to achieve the equilibrium state. Due to the slow rate of MLHto-BLH transformation, samples were equilibrated for up to 16 weeks. The reverse transformation is much faster and care should be taken to prevent BLH decomposition during handling or characterization. It was found that within 1 2 minutes a substantial amount of MLH formed from BLH at laboratory conditions (RH 25%). It is well known that if water is a structural element of a compound, the partial water vapor pressure (T = constant) is higher for the composition with greater number of water molecules. Therefore, the substance with higher water content (BLH) is stable at higher relative humidity. Indeed, after equilibration at RH 38% only single phase BLH was found by PXD, while, a mixture of MLH and BLH was observed after equilibration at 30% RH. Pure MLH was found in samples prepared at 10% and 18% RH (lower RH values were not investigated) (Fig. 1). The unit cell parameters of BLH (a = 2.824(1) Å, c = 19.69(1) Å) and MLH (a = 2.822(1) Å, c = 13.80(1) Å) do not depend on the equilibration relative humidity. This indicates that both phases should be considered as line phases and not solid solutions with variable water contents. The so called over-hydrated phase Na x CoO 2 1.8H 2 O mentioned in the literature [21] was not observed by PXD even after 100 days at 100% RH and measurement in a water slurry. The water content was analyzed by TGA; loss of water occurs in several steps (Fig. 2). In the first stage, adsorbed Fig. 1. Powder X-ray diffraction patterns for Na 0.3 CoO 2 yh 2 O equilibrated at different relative humidities. Fig. 2. Thermogravimetric curves of Na 0.3 CoO 2 0.6H 2 O (dashed line) and Na 0.3 CoO 2 1.2H 2 O (solid line). Fig. 3. Amount of crystal structure (y) and adsorbed (z) water as a function of relative humidity for {Na 0.3 CoO 2 yh 2 O + zh 2 O}. water on the grain surface is evaporated. Subsequent water loss on heating leads to phase decomposition with some intermediate product formation. It was reported previously that a fully dehydrated phase (y = 0) forms at 320 C [11]. All the TGA measurements in this work were performed with a heating rate of 2 C/min; a much slower heating rate of 0.1 C/min was employed previously [11]. Such difference in the heating rates resulted in a shift of our TGA curves toward higher temperature. Therefore, the water content of the samples studied here was calculated by taking the weight remaining at 360 C as that of the fully dehydrated phase, and assuming no oxygen deficiency in the samples. The total sample composition can be written as {Na CoO 2 yh 2 O + zh 2 O}, where z is amount of adsorbed/ condensed water on the sample surface. Two regions with a different amount of crystal structure water can be clearly distinguished on the y RH plot (Fig. 3(a)). At a lower humidity range y is close to 0.6 (H 2 O/Na 2). At a higher RH, the amount of crystal structure water (y) is about 1.2 (H 2 O/Na 4) and does not depend on the RH (Table 1). The boundary line between RH ranges with y 0.6 and y 1.2

4 V.V. Poltavets et al. / Solid State Sciences 7 (2005) is in agreement with sample composition observed by PXD. The case of two hydrate phases in the 30% RH sample will be discussed later. The independence of cell parameters and water content (y) on RH for MLH and BLH strongly supports the idea of definite composition for both hydrate phases. This conclusion is in agreement with Jorgensen s group finding [3] based on a crystal structure analysis of neutron diffraction data. The amounts of adsorbed/condensed water (z) aresimilar in the 38 92% RH region and much higher at 100% RH (Table 1). The effect of water condensation is observed by the formation of a sticky substance at 100% RH, whereas the powders were free-flowing at lower RH. A substantial increase of the amount of condensed water was found for a 100% RH sample after 100 days in comparison with that for 40 days, while no difference was found for a 92% RH sample after similar times. The superconducting transition temperature was 4.55 ± 0.05 K for all samples equilibrated at % RH. The value of magnetic susceptibility per gram of substance for the 100% RH sample was smaller in comparison with 38 92% RH samples. A correction for the weight of adsorbed water substantially diminished the difference (Fig. 4). After correction for excess water the superconducting volume fraction was about 40%, indicative of bulk superconductivity. A very small amount (z 0.04) of the adsorbed water was found in the MLH region. Intermediate amount of the adsorbed water, in comparison with those obtained at lower and higher RH, was found for the two-phase 30% RH sample. It is noteworthy that the appearance of BLH in the PXD corresponds well with a sharp increase in the amount of adsorbed water (Fig. 3(b)). It seems reasonable that water adsorption is necessary for BLH formation. The very slow rate of BLH formation at 30% RH can be attributed to relatively small amount of adsorbed water at this condition. According to the Gibbs phase rule, if the higher hydrate, the lower hydrate and water vapor are in equilibrium, then the system will have one degree of freedom (F = 1) at thermodynamic equilibrium (F = = 1, where the number of components is 2 (Na 0.3 CoO 2 and H 2 O) and the number of phases is 3). Thus at a given temperature the vapor pressure must be constant; thus two hydrates can coexist only at a single RH point, not over a region. Two hydrate phases were observed at 30% RH. Nevertheless, it is very unlikely that RH over saturated FeCl 3 solution is exactly equal to the vapor pressure in the 3 phase equilibrium point of the Na 0.3 CoO 2 H 2 Osystemat22 C. As long as BLH is partially formed from the starting Na 0.3 CoO 2 plus Na 0.3 CoO 2 0.6H 2 O mixture, and this reaction is the slowest in the system, we speculate that the 30% RH point at 22 C belongs to the single phase region of BLH; i.e., if it were equilibrated sufficiently, pure BLH would be formed. The equilibrium triple point at 22 C of the BLH, MLH, H 2 O(v) system is between our experimental points of 18 and 30% RH, and seems to be closer to 30% RH, since BLH dehydration at ambient ( 26%, measured by Omega RH-20C) humidity was observed. The isotherm for the Na 0.3 CoO 2 H 2 Osystemat22 C in the % RH range is shown on Fig. 5(a). The horizontal line represents the water vapor pressure at which the BLH, MLH and water vapor are in equilibrium. The water vapor pressure cannot change before one solid phase has disappeared. Each vertical line represents the range of vapor pressure over which a particular hydrate is stable. Since the compositions of the hydrates are fixed, the variation of vapor pressure must be shown by vertical lines. The isotherm diagram is redrawn in a more conventional form of temperature composition (RH) coordinates in Fig. 5(b). The hydration of Na 0.3 CoO 2 and dehydration of Na 0.3 Co- O 2 1.3H 2 O phase diagrams were published very recently. Foo et al. [22] also used various saturated salt solutions to control RH. The isothermal section of the Na 0.3 CoO 2 H 2 O Fig. 4. Zero field cooled d.c. magnetic susceptibility χ vs. temperature for Na 0.3 CoO 2 1.2H 2 O equilibrated at 100% (triangles) and 92% (circles) of relative humidities. Data for magnetic susceptibility before (closed symbols) and after (open symbols) correction for the weight of adsorbed water are shown. Fig. 5. Isothermal section of the Na 0.3 CoO 2 H 2 O system phase diagram at 22 C from 11 to 100% relative humidity (a) composition RH coordinates (H 2 O(v) is not shown on the diagram for simplicity); (b) RH temperature coordinates.

5 1316 V.V. Poltavets et al. / Solid State Sciences 7 (2005) phase diagram in the present work is in good agreement with their dehydration diagram, with the exception of the 33% RH point, where Foo et al. [22] reported only the presence of single phase Na 0.3 CoO 2 0.6H 2 O. The essential differences between our data and the hydration diagram of Na 0.3 CoO 2 published in Ref. [22] may be attributed to differences in the experimental procedures and equilibration times used. From a practical point of view, it has been shown that BLH is stable at 22 CatRH 30%. However, since BLH formation is faster at higher RH, hydration at % RH range is recommended. Once the BLH phase is formed, the amount of adsorbed water content can be minimized at 75 92% RH. The common practice of the BLH storage at 100% RH leads to higher amount of adsorbed H 2 O, hence uncertain data for weight related properties for such samples. Moreover, due to exchange between H + from liquid H 2 O and Na + [8] an excess of adsorbed water is undesirable. BLH should not be stored at 100% RH unless condensed water is used as a source of humidity to prevent sample decomposition during handling. Nevertheless, we did not observe any T c degradation for as long as 3 months for samples kept at 100% RH. Due to the fast dehydration reaction, the use of a glove bag with constant relative humidity produced by a saturated salt solution is convenient for sample handling. MLH is safely handled in different laboratories at ambient conditions, but storage over saturated LiCl solution will guarantee single phase composition. 4. Conclusions An isothermal section of the Na 0.3 CoO 2 H 2 Osystem phase diagram at 22 C from 11 to 100% relative humidity is established. The border line between the single phase regions of the Na 0.3 CoO 2 0.6H 2 O and Na 0.3 CoO 2 1.2H 2 O is slightly below 30% RH. Both hydrates have a specific composition over the whole RH range of their stability. The composition of the superconducting phase, corrected for the amount of adsorbed water, is Na 0.3 CoO 2 1.2H 2 Owith4water molecules per Na + ion. A slow rate of formation of the superconducting phase at RH lower than 40% was observed. Facile conditions for optimizing the H 2 O content of Na 0.3 CoO 2 yh 2 O over saturated salt solutions are proposed. Acknowledgements The authors are grateful to Dr. G. Popov for helpful discussions. This work was supported by the National Science Foundation Solid State Chemistry Grant DMR References [1] K. Takada, H. Sakurai, E. Takayama-Muromachi, F. Izumi, R.A. Dilanian, T. Sasaki, Nature 422 (2003) 53. [2] J.W. Lynn, Q. Huang, C.M. Brown, V.L. Miller, M.L. Foo, R.E. Schaak, C.Y. Jones, E.A. Mackey, R.J. Cava, Phys. Rev. B: Condens. Matter 68 (2003) [3] J.D. Jorgensen, M. Avdeev, D.G. Hinks, J.C. Burley, S. Short, Phys. Rev. B: Condens. Matter 68 (2003) [4] K. Takada, K. Fukuda, M. Osada, I. Nakai, F. Izumi, R.A. Dilanian, K. Kato, M. Takata, H. Sakurai, E. Takayama-Muromachi, T. Sasaki, J. Mater. Chem. 14 (2004) [5] R.E. Schaak, T. Klimczuk, M.L. Foo, R.J. Cava, Nature 424 (2003) 527. [6] H. Sakurai, K. Takada, F. Izumi, R.A. Dilanian, T. Sasaki, E. Takayama-Muromachi, Physica C (2004) 182. [7] M.L. Foo, R.E. Schaak, V.L. Miller, T. Klimczuk, N.S. Rogado, Y. Wang, G.C. Lau, C. Craley, H.W. Zandbergen, N.P. Ong, R.J. Cava, Solid State Commun. 127 (2003) 33. [8] G. Cao, X. Tang, Y. Xu, M. Zhong, X. Chen, C. Feng, Z. Xu, Solid State Commun. 131 (2004) 125. [9]G.Cao,C.Feng,Y.Xu,W.Lu,J.Shen,M.Fang,Z.Xu,J.Phys.: Condens. Matter 15 (2003) L519. [10] B.G. Ueland, P. Schiffer, R.E. Schaak, M.L. Foo, V.L. Miller, R.J. Cava, Physica C 402 (2004) 27. [11] C.J. Liu, C.Y. Liao, L.C. Huang, C.H. Su, S. Neeleshwar, Y.Y. Chen, C.C. Liu, Physica C 416 (2004) 43. [12] K. Takada, H. Sakurai, E. Takayama-Muromachi, F. Izumi, R.A. Dilanian, T. Sasaki, Physica C (2004) 14. [13] M.M. Maska, M. Mierzejewski, B. Andrzejewski, M.L. Foo, R.J. Cava, T. Klimczuk, Phys. Rev. B: Condens. Matter 70 (2004) [14] L. Greenspan, J. Res. Nat. Bureau Standards Phys. Chem. 81A (1977) 89. [15] A. Apelblat, E. Korin, J. Chem. Thermodyn. 30 (1998) 459. [16] A. Apelblat, E. Korin, J. Chem. Thermodyn. 30 (1998) 59. [17] A. Apelblat, E. Korin, J. Chem. Thermodyn. 34 (2002) [18] A. Apelblat, J. Chem. Thermodyn. 25 (1993) 63. [19] J. Laugier, B. Bochu, LMGP-suite of programs for the interpretation of X-ray experiments, ENSP/Laboratoire des Matériaux et du Génie Physique, BP 46, Saint Martin d Hères, France; and [20] B.L. Cushing, J.B. Wiley, J. Solid State Chem. 141 (1998) 385. [21] R. Jin, B.C. Sales, P. Khalifah, D. Mandrus, Phys. Rev. Lett. 91 (2003) [22] M.L. Foo, T. Klimczuk, R.J. Cava, Mater. Res. Bull. 40 (2005) 665.