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Increase in magnetoresistivity in Ba2CoS3 via Zn2+/Co2+ substitution Mark R. Harrison,a Vincent Hardy,b Antoine Maignanb and M. Grazia Francesconi*a
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Received (in Cambridge, UK) 3rd October 2008, Accepted 12th February 2009 First published as an Advance Article on the web 5th March 2009 DOI: 10.1039/b817363j We report an increase of negative magnetoresistivity from 1.7% to 9% in the Ba2Co0.5Zn0.5S3 series (0 r x r 0.5), derived from Zn2+/Co2+ substitution in Ba2CoS3; Zn2+ substitution destroys the one-dimensional antiferromagnetic ordering typical of Ba2CoS3, in favour of a paramagnetic behaviour, and considerably increases the value of the resistivity, and to the best of our knowledge Ba2Co0.5Zn0.5S3 shows the most negative magnetoresistivity among the one-dimensional sulfides (MR B 9%). Ba2CoS3 shows a one-dimensional structure, in which cornerlinked CoS4 tetrahedra form chains along the c axis.1 The synthesis and antiferromagnetic properties of Ba2CoS3 were originally reported by Nakayama et al. but no structural details were given.2 In 1972, Hong and Steinfink reported the crystal structures of a number of phases within the Ba–Fe–S(Se) system, among them Ba2FeS3 which is isostructural with Ba2CoS3,3 but again no detailed crystallographic data were given. Our own structural study, based on the Rietveld analysis of X-ray diffraction data collected from a polycrystalline sample of Ba2CoS3, showed that the unit cell is orthorhombic with space group Pnam with parameters a = 12.000(1) A˚, b = 12.470(1) A˚ and c = 4.205(2) A˚.4 We performed ab initio calculations, which suggested that the Co2+ cations are in a high-spin state (S = 3/2). Using a perturbative approach, we predicted the intra-chain coupling between nearest-neighbour Co2+ cations via a Co–S–Co superexchange pathway to be antiferromagnetic.5 This is consistent with the temperature dependence of the magnetic susceptibility, which showed a broad maximum at B135 K, indicative of one-dimensional antiferromagnetic coupling between the Co2+ cations.2 The inverse susceptibility was not a linear function of temperature below 380 K, and therefore could not be modelled using the Curie–Weiss law. Subsequent analyses showed that susceptibility was best described over a wide temperature range using a one-dimensional Wagner–Friedel model5 with an intra-chain exchange constant J = 37 2 K and a Lande´ factor g = 2.36 0.01. However, no real compound is truly one-dimensional, and although the estimated value of the antiferromagnetic inter-chain coupling constant (J 0 E 1 K) certainly permits the description of Ba2CoS3 as a quasi one-dimensional system, the temperature dependence of both the susceptibility and the heat capacity data showed evidence, albeit only a subtle change in gradient, a
Department of Chemistry, University of Hull, Cottingham Road, Hull, UK HU6 7RX. E-mail:
[email protected]; Fax: +44 (0)1482 46610; Tel: +44 (0)1482 465409 b Laboratoire CRISMAT, UMR CNRS 6508, NSICAEN6 Boulevard du Mare´chal Juin, Caen, 14050, Cedex-France. Fax: +33 (0)231951600; Tel: +33 (0)231452634
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for the onset of 3-D long-range antiferromagnetic ordering in Ba2CoS3 at 46 K.4,5 Subsequently, we proved the existence of an antiferromagnetic ground state at 1.5 K by low-temperature neutron diffraction.4 We have also shown that Ba2CoS3 exhibits metallic-like behaviour and negative magnetoresistance (1.7% at 10 K and 7 T), making it the first one-dimensional sulfide containing Co2+ to do so.6 Here, we report a large increase of the negative magnetoresistivity in the samples Ba2Co0.75Zn0.25S3 and Ba2Co0.5Zn0.5S3. The synthesis of the polycrystalline samples Ba2Co0.75Zn0.25S3 and Ba2Co0.5Zn0.5S3 was achieved via a solid–gas reaction between CS2 and a stoichiometric mixture of powdered BaCO3, Co and ZnO. Carbon disulfide is a liquid with a low vapour pressure. If nitrogen gas is bubbled through liquid CS2, the gas acts as a carrier and a vapour of N2–CS2 can be passed through a tubular furnace. The nitrogen gas was first passed through concentrated sulfuric acid in order to remove any moisture and then through a Dreschel bottle containing liquid CS2, which was in turn connected to the silica work tube of the furnace. Nitrogen gas was used to flush the system of air before the reaction was started, and to remove CS2 once the reaction was completed. The downstream end of the silica tube was connected to another Dreschel bottle containing paraffin oil, which sealed the system from the air and acted as a post-reaction scrubber, thus reducing the release of any residual CS2. X-Ray diffraction analysis was performed using a Siemens D5000 diffractometer (Cu Ka, 25 1C) over an angular range 5 r 2y r 1101, step size 0.021. Magnetic measurements were carried out using a SQUID magnetometer (Quantum Design) over the temperature range 5–400 K. Measurements of w(T) were recorded in a magnetic field of 0.1 T. The resistivity measurements were carried out in a Quantum Design PPMS. Resistivity measurements (four-probe technique) were recorded in the temperature range 5–400 K and in magnetic field of strength ranging 0–7 T. Isothermal magnetoresistance was measured by sweeping the magnetic field between 7 T and +7 T. The magnetoresistance was calculated by using the relation MR ¼ 100
jH rH ¼ 0 rH ¼ 0
The structure of Ba2Co0.75Zn0.25S3 and Ba2Co0.5Zn0.5S3 was analysed by powder X-ray diffraction (PXRD), with Rietveld refinement performed using the software package GSAS in conjunction with EXPGUI (Fig. 1).7 The structure of Ba2CoS3 was chosen as the starting model with Zn included on the Co site and the occupancy set to 75% Co and 25% Zn This journal is
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Fig. 1 Rietveld refinement of Ba2Co0.75Zn0.25S3: observed (+), calculated and difference (bottom). The top tick marks correspond to BaS and the bottom tick marks to Ba2CoS3.
for Ba2Co0.75Zn0.25S3 and 50% Co and 50% Zn for Ba2Co0.5Zn0.5S3. A two-phase refinement was necessary in order to include the BaS impurity, which was found to be approximately 2%. Despite the presence of BaS no crystalline Co or Zn containing impurity was found as would be expected from the stoichiometric starting mixtures and there is no evidence for any Ba vacancies in the Ba2Co1xZnxS3 compounds from refinement of the occupancy. The unit cell parameters for Ba2Co0.5Zn0.5S3 are a = 12.0031(4) A˚, b = 12.5433(4) A˚, c = 4.2090(1) A˚ (Z = 4, number of reflections = 795, RMM = 433; a shifted Chebyshev function was used to refine the background. The residual values were Rp = 6.3% and Rwp = 7.5%). The unit cell parameters for Ba2Co0.75Zn0.25S3 are a = 11.9990(1) A˚, b = 12.5056(1) A˚, c = 4.20656(5) A˚ (Z = 4, number of reflections = 971, RMM = 431; a shifted Chebyshev function was used to refine the background with Rp = 2.96% and Rwp = 4.17%). The refinement confirmed that Ba2Co0.75Zn0.25S3 and Ba2Co0.5Zn0.5S3 maintain the same structure as Ba2CoS3. The unit cell parameters of Ba2Co0.75Zn0.25S3 and Ba2Co0.5Zn0.5S3 are very close to or slightly larger than those of Ba2CoS3 (a = 12.000(1) A˚, b = 12.470(1) A˚, c = 4.205(2) A˚), as the ionic radius of Zn2+ is larger than that of Co2+ (r(Co2+)tet = 0.58 A˚; r(Zn2+)tet = 0.60 A˚).8 The susceptibility curve (Fig. 2) for Ba2CoS3 showed a broad peak indicative of one-dimensional antiferromagnetic behaviour, given by antiferromagnetic interaction between Co2+ centres, mediated by S2 anions.5 Substitution of Zn2+ for Co2+ breaks the antiferromagnetic order resulting in a paramagnetic pattern in the w(T) curves of Ba2Co0.75Zn0.25S3 and Ba2Co0.5Zn0.5S3 (Fig. 2). The Curie–Weiss law fitting to the linear part of the w1(T) curves for Ba2Co0.5Zn0.5S3 and Ba2Co0.75Zn0.25S3 (Fig. 2 inset) yields Curie constants of C = 1.45 emu mol1 K and C = 3.07 emu mol1 K, respectively. This yields paramagnetic effective moments of meff = 3.4 mB and meff = 4.9 mB, respectively. The room temperature resistivity of Ba2CoS3 (B101 O cm) is relatively low. The r(T) of Ba2CoS3 shows a slight increase in r as T is decreased, but this variation is too small to be regarded as an indication of intrinsic semi-conducting behaviour. Instead, such an effect is more probably related to the enhanced impact This journal is
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Fig. 2 Susceptibility curves for Ba2CoS3 (marked by open circles), Ba2Co0.75Zn0.25S3 (open squares) and Ba2Co0.5Zn0.5S3 (filled circles). The reciprocal susceptibility curves are shown in the inset.
of defects in the case of one-dimensional (1D) conduction. The order of magnitude of the resistivity in Ba2CoS3 is found to be quite high (101 O cm) for metallic conduction, but it can still be considered within the range of values characteristic of the so-called ‘‘bad metals.’’ The Zn2+/Co2+ substitution in Ba2Co0.75Zn0.25S3 and Ba2Co0.5Zn0.5S3 increases the magnitude of the resistivity at room temperature and down to 4 K and confers a clear semi-conducting trend to the r(T) curves (Fig. 3). Ba2CoS3 showed a small negative magnetoresistivity of 1.7% at 10 K and 7 T. Here, we highlight that Zn substitution up to 50% greatly improves the negative magnetoresistivity. In fact the isothermal rH/rH = 0 curves for Ba2Co0.75Zn0.25S3 (Fig. 4) show a magnetoresistivity reaching B6% and the isothermal rH/rH = 0 curves for Ba2Co0.5Zn0.5S3 (Fig. 5) show a magnetoresistivity reaching B9% at 5 K and 7 T; this value decreases with temperature but is still around 2% at 100 K. The lowest value of negative magnetoresistivity is reached for Ba2Co0.5Zn0.5S3, i.e. a 50% substitution of diamagnetic Zn2+ for Co2+, and corresponds to a 5-fold increase compared to the undoped Ba2CoS3. Moreover, no anomalies can be detected on the w(T) curve, indicating the onset of a long-range magnetic order at low temperature. To the best of our knowledge this effect has never been observed and its origin is still unclear. There are only a few
Fig. 3 Resistivity of Ba2Co0.75Zn0.25S3 Ba2Co0.5Zn0.5S3 (open squares) at 0 T.
(filled
squares)
and
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Fig. 4 Isothermal resistivity curves for Ba2Co0.75Zn0.25S3. The curves are obtained by sweeping the magnetic field as follows: 0 - 7 T 7 T - 0 T.
Mn–O octahedra which alternate between Jahn–Teller distorted and non-distorted, i.e. the alternation of Mn3+–O and Mn4+–O, which is induced by the Zn2+/Mn3+ substitution.9 In this case the Zn2+/Mn3+ substitution induces ferromagnetism and an increase in the magnetoresistance. In Ba2Co0.75Zn0.25S3 and Ba2Co0.5Zn0.5S3, the w(T) curves do not show any onset for ferromagnetism and, moreover, the formation of Jahn–Teller distorted coordination polyhedra has to be excluded. Zn2+ was substituted for Fe in Sr2FeMoO6, which shows large magnetoresistance at room temperature. Analysis of the magnetisation curves indicated that substitution induces an enhancement in the low-field magnetoresistance and a reduction in the high-field magnetoresistance. Zn2+ locates on the Fe site, disrupting the Fe–O–Fe connections which are responsible for the high-field MR.10 In this case, the Zn2+ substitution decreases the magnetoresistance. In conclusion, we have increased negative magnetoresistance in Ba2CoS3 from 1.7% to 6% and 9% via substitution of 25% and 50% of the Co2+ with the diamagnetic, isovalent Zn2+. To the best of our knowledge this is the first time that such a phenomenon has been observed and further studies will be carried out into its origin and its possible explanation to increase negative magnetoresistivity in other one-dimensional sulfides. In particular, more samples belonging to the Ba2Co1xZnxS3 series with 0 o x o 1 will be investigated to determine a pattern of the decreasing of the magnetoresistance as a function of the doping with Zn2+.
Notes and references
Fig. 5 Isothermal resistivity curves for Ba2Co0.5Zn0.5S3. The curves are obtained by sweeping the magnetic field as follows: 0 - 7 T 7 T - 0 T.
reports on the effect of Zn substitution for magnetic cations in ternary compounds on the magnetoresistance. This effect was investigated in Zn2+ substituted LaMnO3 oxides, LaMn1xZnxO3. It was found that for x = 0.05 the negative magnetoresistance decreases to 270% around Tc, a value that is higher than those obtained with other cationic substitutions, such as Cu, Fe and Cr. The explanation was found in the
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