Physica C 460–462 (2007) 471–472 www.elsevier.com/locate/physc
Growth and characterization of high quality single crystals of NaxCoO2 C.T. Lin a
, D.P. Chen b, J.B. Peng
, P.X. Zhang
Max-Planck-Institut fu¨r Festko¨rperforschung, Heisenbergstrasse 1, D-70569 Stuttgart, Federal Republic of Germany b ISEM, University of Wollongong, Wollongong, NSW 2522, Australia c Kunming University of Science and Technology, Yunnan, 650093, PR China Available online 28 March 2007
Abstract A systematic study of growing NaxCoO2 (x = 0.32–1.00) single crystals has been performed to determine phase stability and the eﬀect of structure and magnetic property on the sodium content in this system. We give experimental evidence that high quality a-phase NaxCoO2 crystals with x = 0.90–1.00 could be obtained using optical ﬂoating zone method, while other phases with lower sodium content of x < 0.90 are observed to contain Na2O, Co3O4 and Na-poor compound. Furthermore, we demonstrate that the magnetic transition temperature depends strongly on the Na content in the single crystals. Implications of the Na eﬀect for understanding of the structure and magnetization are discussed. 2007 Elsevier B.V. All rights reserved. Keywords: NaxCoO2 single crystals; Structure; Magnetic property
2. Crystal growth
Since the discovery of superconducting NaxCoO2 compound with water molecules intercalated between CoO2 layers , a lot of eﬀorts have been made to study the mechanism of superconductivity, magnetic susceptibility, and to investigate basic properties, as speciﬁc heat, electronic anisotropy [2–4], but even these are far from being understood. Moreover, most studies were carried out using polycrystalline or poor quality samples due to the diﬃculty of obtaining high quality crystals. Therefore, the growth of high quality single crystals is crucial to allow an accurate characterization of the compound with respect to its physical, chemical, magnetic, electrical and thermal properties. In this work, we investigate the growth of high quality NaxCoO2 crystals with a verity of Na content using ﬂoating zone method. Study of the crystal structures and magnetic properties is demonstrated.
Single crystals were grown in an optical ﬂoating zone furnace with 4 · 300 W halogen lamps installed as infrared radiation sources. Starting feed and seed materials were prepared from Na2CO3 and Co3O4 of 99.99% purity. The initial Na content of x = 0.60–1.15 was used to grow a series of NaxCoO2 crystals. The growth was carried out in ﬂowing oxygen atmosphere and a growth rate of 2 mm/ h was applied. More details are described in . Among the growth of all compounds, we observed that the molten zone was stable and easy to form when the initial sodium content of x = 1.05–1.15 was used. As a result, large single crystals up to /6 · 80 mm were obtained. Other compounds using initial sodium content of x = 0.60–1.00 were formed with impurity phases for the ﬁrst 80 mm length of the grown ingot and pure phase was observed to form only at the rest of the rod. 3. Results and discussion
Corresponding author. Tel.: +49 711 6891458; fax: +49 711 6891093. E-mail address: [email protected]
0921-4534/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2007.03.176
By the analysis of EDX (energy dispersive X-ray analysis) and XRD (X-ray diﬀraction), the crystals of
C.T. Lin et al. / Physica C 460–462 (2007) 471–472
Fig. 1. The a- and c-axis lattice parameters measured by XRD are plotted against the Na content x ﬁxed by EDX in crystal samples. The inset is a schematic representation of the O3 and P2 structures, respectively.
NaxCoO2 with x 0.90–1.00 were determined to be a pure a-phase throughout the entire grown ingot. The crystal quality is examined to be high with mosaic structure of FWHM <0.1. The crystals of sodium content between x = 0.60–0.89 consist of multi phases like, a (x = 0.90– 1.00), a 0 (x = 0.75), b (x = 0.55–0.60), c (x 6 0.74) or impurities of Na2O, Co3O4 and Na-poor phases. The compounds at lower sodium content of x < 0.50 could be formed by electrochemical de-intercalation of additional sodium from the single crystal form of Na0.75CoO2. The lattice parameters and cell volumes for all single crystals were determined by least squares reﬁnement of XRD data. The as-grown crystals are identiﬁed to be hexagonal structure with two sheets (prismatic, P2) and trigonal with three sheets (octahedral, O3) of edge-shared CoO6 octahedra in a unit cell for the x 0.32–0.89 and a-phase of x 0.90– 1.00, respectively. Fig. 1 is the plot of the Na content dependence of the NaxCoO2 structure. The c parameters decreasing with the increasing of Na content of the compound while the a is vice versa. The broken lines are drawn to guide eyes. The change of the parameters in the compounds indicates that the sodium intercalant layer expanded with decreasing Na content since the Na removal results in Co oxidation and thus the CoO2 layers are expected to shrink. The magnetic property of the single crystals of NaxCoO2 with x 0.32–1.00 was systematically characterized by ac susceptibility using a SQUID (superconducting quantum interference device) magnetometer, as shown in Fig. 2. The antiferromagnetic (AF) long-range order with Ne`el temperature transition is formed due to SDW (Spin density wave) . This Ne`el phase shows an approximately compositional independence of the critical temperatures TN 22, 20, 18, 19 K at x 0.76, 0.83, 0.90 and 1.00, respectively. There is no evidence of the ‘‘dome’’ shaped behavior 
Fig. 2. Magnetic susceptibilities of NaxCoO2 single crystals measured under zero-ﬁeld cold, 1 T, H//c.
observed in our crystal samples. The susceptibility shows featureless at x 0.60 and 0.72. Obviously they belong to the Curie–Weiss-like regime. Previous work  shows that the metal–insulator transition (MIT) temperatures of Tc 53 and 88 K were observed at x = 0.5 while no features at x < 0.5. We ﬁnd the similar transitions taken place at the Na content of x 0.42, 0.46 and 0.50, which feature is pronounced. The results remain independent of the magnetic ﬁeld of 100 Oe applied. The Tc 13 K at lower Na content of x 0.32 is likely the emergence of the new low-temperature phase SG (spin glass) , while its broad maximum at Tm 32 K is due to the SDW ﬂuctuation, namely intra layer ordering. Nevertheless, the magnetic properties of this system depend strongly on the sodium content in our crystal samples. 4. Conclusions The investigation of growing the series NaxCoO2 single crystals was performed using a variety of Na contents. The high quality compound was formed between x = 0.90–1.00, i.e. a-phase, while other phases were observed to be coexistence and pure one was formed in the tail part of the ingot. The characterization of single crystals indicates the change of the structure from O3 (a-phase) to P2 (all other phases) and the pronounced metal-insulator transitions occurred at x 0.42, 0.47 and 0.50. References       
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