Pis'ma v ZhETF, vol.91, iss.8, pp.457-460
© 2010 April 25
Phase segregation in Na^CoC^ for large Na contents
T. A. Platova+*1\ I. R. Mukhamedshin+*, A. V. Dooglav+, H. Alloul* + Physics Department, Kazan State University, 420008 Kazan, Russia * Laboratoire de Physique des Solides, UMR 8502, Université Paris-Sud, 91405 Orsay, France
Submitted 12 March 2010
We have investigated a set of sodium cobaltates (Na^CoCh) samples with various sodium content (0.67 < x < 0.75) using Nuclear Quadrupole Resonance (NQR). The four different stable phases and an intermediate one have been recognized. The NQR spectra of 59Co allowed us to clearly differentiate the pure phase samples which could be easily distinguished from multi-phase samples. Moreover, we have found that keeping samples at room temperature in contact with humid air leads to destruction of the phase purity and loss of sodium content. The high sodium content sample evolves progressively into a mixture of the detected stable phases until it reaches the x = 2/3 composition which appears to be the most stable phase in this part of phase diagram.
Introduction. The family of sodium layered cobaltates Na^CoOa (0 < x < 1) has a rich phase diagram [1], which includes most interesting scientific phenomena present in condensed matter physics, such as superconductivity [2], spin density wave [3], magnetic frustration in a triangular lattice, coexistence of metallic and magnetic properties, both Curie-Weiss and 2D metal, etc [4]. Moreover, high ionic mobility and high Seebeck coefficient [5, 6] allow to consider this compound for potential thermoelectric applications [7, 8].
The concentration x of sodium ions and their order/disorder in the Na plane play a fundamental role in the physical properties of cobaltates. The Co ions are in the large crystal field induced by their oxygen octahedral environment, so the 3d levels are split and the difference in energy between the lower triplet and upper es doublet is « 2 eV, thus only the t2S triplet states are filled [1]. Therefore the electronic structure of the Co ions is expected to correspond to low spin configurations with total electron spin S = 0orS = l/2 with charge states Co3+/Co4+, respectively.
In the present work we have studied the cobaltates Na^CoOa at large sodium content x range (0.67 < x < < 0.75). This concentration range bears our attention due to the occurrence of an A-type magnetic ordering at x ~ 0.75 and its absence at lower sodium contents x < 0.75 [1, 9]. In Ref. [10] the existence of four stable phases in this Na concentration range has been established. These phases display a similar nearly ferromagnetic in-plane behavior above 100 K but exhibit significantly different ground states. The structure of one of these phases has been proposed recently using
e-mail: tanya.platova8gmail.com
NMR/NQR data and confirmed by x-ray Rietveld analysis [11, 12].
In NQR, nuclei with an electric quadrupole moment have their nuclear spin energies split by the electric field gradient (EFG) created by the electronic bonds in the local environment. So this technique is very sensitive to the nature of the bonding around the nucleus.
Samples. The reproducible preparation of singlephase samples with precise stoichiometries is not straightforward in cobaltates. The high ionic mobility of sodium and its chemical activity (for example, Na ions easily react with molecules present in the ambient atmosphere to form NaOH or sodium carbonates) make the control of Na content even more difficult.
However, the methods for reproducible synthesis of single-phase powder samples in the 0.67 < x < 0.75 sodium range have been reported in Ref. [10, 12]. To protect the powders from the influence of water they were packed into protecting materials. Several protecting materials have been used in our experiments: the epoxy resin (Stycast 1266) and paraffin, which display distinct advantages and disadvantages. The Stycast perfectly protects the powder from water influence, however we have found that for samples packed in Stycast, the NQR spectra displayed resonance lines «1.9 times broader than pure powder or samples packed in paraffin (see Fig.l). Thus, the paraffin packed powder have been used for most further investigations. To eliminate diffusion processes in the Na layers the samples were kept in liquid nitrogen.
Experimental details. The NQR measurements were carried out with a home-built coherent pulsed NMR/NQR spectrometer. The NQR spectra of 59Co were taken "point by point"with a 7r/2 — r — ir radio frequency pulse sequence by varying the spectrometer fre-
IlHCbMa b ?K3T<J> tom 91 Bbin.7-8 2010
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Т. A. Platova, I. R. Mukhamedshin, A. V. Dooglav, H. Alloul
Frequency (MHz)
Fig.l. (Color online) Cobalt NQR lines of pure powder and powder packed in Stycast and paraffin: solid (red) line, dotted (black) line and dash-and-dot (blue) line, respectively
quency. These sweeps were done with equal frequency steps at 4.2 K. A Fourier mapping algorithm [13, 14] have been used for constructing the detailed NQR spectra.
NQR characterization of the phases. We have studied a series of samples with various sodium contents (0.67 < x < 0.75). The NQR spectra of 50Co nuclei allowed us to clearly differentiate four stable phases (see Fig.2) marked as H67, 071, H72 and H75. This nota-
H67 1 . , . < __
O71 . _ul > * к
H72 лЛа « . * : и : 4
Mix -Л А\», л . л а ji Ul
H75 у-
6.0 6.5 7.0 7.5 8.0 8.5 Frequency (MHz)
Fig.2. (Color online) NQR spectra for the four single phase samples studied in Ref. [10]. The difference in spectra is obvious - each spectrum has at least one line which does not appear in the other phases as shown by the coloured arrows and vertical lines (H67-solid (black) line, 071 -dotted line (red) and H72 - dash-dotted (blue) lines. The sample denoted as Mix, with a Na composition in between the 071 and H72 phases, is the mixture of those phases as its NQR signal is a weighted composition of the NQR signals of these two phases. H75 which is antiferromag-netically ordered at 4.2 K displays a broad ZFNMR signal due to the internal magnetic field
tion is the same as used in Ref. [10]. The number in the phase label is an approximate sodium content (x = 0.67, 0.71, 0.72 and 0.75, respectively) and the letter is a type of unit cell (H-hexagonal, O-orthorhombic). The part of the 50 Co NQR spectra which correspond only to the (±7/2 - ±5/2) transitions of 50Co nuclei [10, 12] are shown in Fig.2. It is clearly seen that every phase has its own unique 50 Co NQR spectrum. The H67 phase has the simplest spectrum which consists of two lines at и 6.5 MHz and и 7.5 MHz in this frequency range. In the 5.5 4- 8.5 MHz range the 071 has 8 and H72 has 6 resonance peaks. The doublet of lines at и 7.8 MHz and the two resonance lines at и 7.75 and и 8.2 MHz are characteristic features of the 071 and H72 phases, respectively. These pairs of lines are labeled in Fig.2 by dotted lines and by dash-dotted lines. The spectrum of H75 phase differs considerably (Fig.2) and it will be discussed below.
Thus, the NQR spectrum is unique and characteristic for each single phase. This allows easily to distinguish samples which are a mixture of two or more phases. We show as an example in Fig. 2 the 50 Co NQR spectrum of the sample (labeled as Mix) with a sodium content intermediate between 071 and H72 which contains both signals from these two phases. So the NQR is a sensitive method to distinguish single phase samples from a mixture of phases. This finding becomes very important as it will allow to characterize better freshly synthesized samples and clarify the phase diagram of the sodium cobaltates.
The spectral lines of H67, 071 and H72 phases are rather narrow (linewidths «30-50 kHz), which points out that the spread of EFG on the nuclear site positions is rather small. These phases have a finite number of cobalt non-equivalent sites indicating the existence of well defined local ordering in the Co and Na planes. The structural model of the H67 phase ^а2/зСоОг compound) have been proposed recently in Ref. [11,12]. One unit cell of this models contains four Co and three Na non equivalent sites which have been detected by NQR [11, 12] and by NMR [15, 16]. From the comparison of the NQR spectra it is obvious, that the H67 phase has the simplest structure in the 0.67 < x < 0.75 sodium concentration range. The structural organization in the sodium planes is still an open question for the 071 and H72 phases.
The existence of the antiferromagnetic (AF) order with Тдг = 22 К is the characteristic feature of the x ~ 0.75 compound. Such AF order was detected by /tSR in our samples [17], as well as by low field bulk susceptibility measurements. Neutron scattering study of the same phase established the A-type AF ordering
Письма в ЖЭТФ том 91 вып. 7-8 2010
Phase segregation in Nax C0O2
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(ferromagnetic in plane and AF between planes) [18,19]. As our studies have been carried out at 4.2 K, the H75 phase sample was magnetically ordered at this temperature. Therefore the observed signal in the H75 phase corresponds to the so-called zero field NMR (ZFNMR). In this case, the nuclear energy levels are split by the internal magnetic field. Thus, the observed spectrum consists of seven lines, which correspond to the typical NMR spectrum for nuclear spin 7/2 (one central line and 6 satellites), but only five of them are shown on Fig.2. We have failed in detecting the NQR spectrum of the H75 phase above TV, due to significant shortening of transverse relaxation time (T2).
During the experiments we have found that the phase content of the samples packed in paraffin were changed. It should be noted that the paraffin packed samples between experiments were kept in liquid nitrogen. To perform the measurements we had to take out samples from liquid nitrogen, warm them to the room temperature and after that insert them in the probe of our spectrometer. In Fig.3 th
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