- Email: [email protected]
Growth of large La2–xSrxNiO4+δ single crystals by the floating-zone technique
Journal of Crystal Growth 237–239 (2002) 815–819
Growth of large La2–xSrxNiO4+d single crystals by the floating-zone technique D. Prabhakarana,*, P. Islab, A.T. Boothroyda b
a Clarendon Laboratory, Department of Physics, University of Oxford, Oxford, OX1 3PU, UK ! CSIC–Universidad de Zaragoza, 50009 Zaragoza, Spain Instituto de Ciencia de Materiales de Aragon,
Abstract We report on an investigation into the growth of large, high-quality single crystals of La2–xSrxNiO4+d (x ¼ 020:5) by the floating zone technique. The results of attempts to grow crystals in different atmospheres are discussed. The oxygen non-stoichiometry (d) of the as-grown crystals is found to decrease with the Sr substitution level. Magnetic and electrical transport measurements on the crystals are reported. r 2002 Elsevier Science B.V. All rights reserved. PACS: 81.10.Fq; 71.45.Lr; 64.70.Kb Keywords: A2. Floating zone technique; A2. Single-crystal growth; B1. Oxides; B2. Magnetic materials
1. Introduction The celebrated discovery of high-Tc cuprate superconductors has encouraged investigations into the structural, magnetic and electrical properties of other doped transition metal oxides possessing the K2NiF4 structure. One such system is that formed by doping La2NiO4+d with alkaline elements. La2NiO4 is an antiferromagnetic insulator in which the Ni2+ ions carry a spin S ¼ 1: Although no superconductivity is found at any doping level, compounds in this nickelate family do exhibit charge stripe ordering as well as other phenomena similar to those found in the cuprates. Charge ordering in nickelates was first reported in measurements on ceramic samples of La2xSrx*Corresponding author. Tel.: +44-1865-272222; fax: +441865-272400. E-mail address: [email protected] (D. Prabhakaran).
NiO4+d [1]. Because of the intrinsic anisotropy of this compound, however, many of the most useful measurements that probe the spatial correlations in the spin and charge ordered states require single-crystal samples. For some techniques, such as neutron scattering, these crystals need to be large (Bcm3). The first investigations into crystal growth of La2xSrxNiO4+d used the skull melting technique [2]. Since then, studies of crystal growth by the floating-zone method have been published on several occasions [3–6], and in addition to these studies, other groups have reported using crystals prepared by the floating-zone method for experiments [7,8]. These papers report the growth of large crystal rods of undoped or lightly doped (xp0:5) La2xSrxNiO4+d under various conditions. A prominent feature of the La2xSrxNiO4+d system is that the magnetic and structural properties are very sensitive to the oxygen non-stoichiometry (d) [9,10], and control over the oxygen
0022-0248/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 1 ) 0 2 0 3 9 - 5
816
D. Prabhakaran et al. / Journal of Crystal Growth 237–239 (2002) 815–819
content is important if reliable results are to be obtained from physical properties measurements. A detailed study of the phase diagram of polycrystalline La2NiO4+d as a function of d (0– 0.18) was reported by Tamura et al. [11]. In this paper we report experiments to optimize the conditions required to grow large single crystals of La2xSrxNiO4+d (0pxp0:5) by the floating-zone method. Our study differs from previous ones in the use of a high-pressure Argon-rich atmosphere for the growth. We also explore how different concentrations of oxygen in the atmosphere affect the crystal growth and oxygen content of the as-grown crystals.
neutron diffraction. Measurements of the electrical resistivity were made by the standard four-probe method, and magnetic data were obtained with a SQUID magnetometer and an AC susceptometer.
3. Results and discussion We found it possible, after certain adjustments to the growth conditions, to prepare crack-free single crystals of La2xSrxNiO4+d for all Sr doping levels up to x ¼ 0:5: Fig. 1 illustrates a grown crystal with x ¼ 0:2: Fig. 2 shows a rocking curve of an x ¼ 0:33 crystal measured by neutron diffraction and the actual crystal used for the
2. Experimental procedure For the present study, stoichiometric proportions of La2xSrxNiO4+d (x ¼ 0:020:5) were prepared by mixing the starting materials La2O3 (preheated at 11001C for 12 h), SrCO3 and NiO (preheated at 1501C for 6 h) followed by solid-state reaction at 1200–13001C for 72 h. Reaction cycles were repeated with intermittent regrinding until the starting materials were single phase. Feed rods of 12 mm diameter and 100 mm length were prepared using a hydraulic press. To make the feed rods dense and straight we sintered them in a vertical furnace at 15001C for 6–12 h. Crystal growth was carried out in a floating-zone mirror furnace (Crystal Systems Inc.) with 4 1.5 kW halogen lamps as the heating source. The atmosphere during crystal growth was maintained at 5– 7 atm of Ar mixed with a small concentration of O2. The seed crystal and feed rod were counterrotated at 40 rpm and the melt zone was scanned upwards during crystal growth with a speed of 3– 3.5 mm/h. The phase purity and lattice parameters of the grown crystals were determined at room temperature by powder X-ray diffraction (XRD), and the bulk crystalline quality of some of the crystals was checked by neutron diffraction. The chemical composition of the crystals was analysed by electron probe microanalysis (EPMA) and thermogravimetric analysis (TGA). The orientation of the crystals was determined by Laue XRD and
Fig. 1. Single crystal of La1.8Sr0.2NiO4 grown by the floatingzone technique.
Fig. 2. Neutron diffraction rocking curve on the (0 0 4) reflection for an x ¼ 0:33 crystal. The inset shows the exact crystal used for the measurement.
D. Prabhakaran et al. / Journal of Crystal Growth 237–239 (2002) 815–819
measurement is shown in the inset of Fig. 2. Because neutrons penetrate the whole crystal the width of the rocking curve (0.31 FWHM) demonstrates the high crystalline quality of the entire crystal rod. For x ¼ 0:420:5 the increase in grain size as the melt was scanned was much slower than for lower Sr concentrations, and so only the very end of the x ¼ 0:420:5 rods contain single-crystal grains. Crystals cut from these rods are correspondingly much smaller than those shown in Fig. 1 or 2. All the crystal rods exhibited two facets running parallel to the growth direction on opposite sides of the rod. These facets were found to be parallel to the (0 0 1) planes. The cylindrical growth axis of the crystals was found to be almost parallel to the [1 1 0] crystallographic direction (tetragonal notation). It has previously been found [4] that crystals with xo0:2 contain excess oxygen which occupies vacant positions between the NiO layers. The oxygen content must then be varied by postannealing in a suitable atmosphere. For large crystals, however, post-annealing may not be an efficient way to obtain a homogeneous oxygen content, and one may ask whether it is possible to control the oxygen content by applying a suitable inert atmosphere during crystal growth. We have investigated this possibility by experimenting with crystal growth in vacuum, pure Ar flow, Ar+O2 flow, high-pressure Ar gas and high-pressure Ar+O2 gas. We now discuss the results of these trials. Growth in vacuum caused the melt to evaporate rapidly and deposit over the hot zone of the quartz tube. Further growth was then prevented due to resulting attenuation of the light, or worse, reaction of the deposit with the quartz tube. On the other hand, growth in a flow of Ar gas at 1 atm yielded good crystals with two clear facets. After the growth, however, we observed a thin layer of white powder on the crystal surface, corresponding to La2O3, and a green NiO layer formed near the end where the feed rod had been separated from the crystal. Furthermore, within one day of growth the whole crystal started to disintegrate into small pieces. This occurred even though there were no cracks visible on the outer surface of the as-grown crystal. Thick layers peeled away from
817
the surface first, and afterwards these disintegrated into a powder of black and white particles. Finally, all that remained was a slice of crystal of thickness of 2–3 mm from the central portion of the rod, extending across the diameter of the rod and finishing on the two facets. Thus, our efforts to reduce the excess oxygen in the crystal by using pure Ar flow were unsuccessful. On the other hand, the mixing of 2–5% O2 with the Ar gas was found to allow growth of crystals that were subsequently stable even after long exposure to air. Moreover, the amount of La2O3 surface layer formation in these crystals was considerably less than the crystals grown in pure Ar. In a previous study, it had been reported that incorporation of a few per cent excess NiO in the feed rod helps to stabilize the crystal against disintegration or hydrolysis after growth [5]. In all our studies, however, we used a stoichiometric amount of NiO. Experiments were next performed with a high-pressure (5–7 atm) atmosphere of pure Ar gas. These runs were more promising than those with 1 atm of Ar flow. After the growth there was some break up of the crystal, but large pieces survived without disintegrating totally into powder. Finally, we performed runs with a high-pressure mixture of Ar+O2 gas as the atmosphere. With these conditions we were able to obtain excellent crystals with no disintegration or surface-layer formation even after several months exposure to air. The optimum amount of O2 in the atmosphere was found to increase with the Sr content. For xo0:275 we used 1–3% of O2, while above x ¼ 0:275 the fraction of O2 increased rapidly with x, up to B40% for x=0.5. Polycrystalline samples of La2–xSrxNiO4+d tend to be oxygen-deficient when x > 0:275; and so the use of much higher amounts of O2 in the crystal growth atmosphere helps to counter this tendency. Even with this higher concentration of oxygen we observed a brown colour at the grain boundaries for crystals with x > 0:275; indicating oxygen deficiency in these regions. Attempts to grow crystals in a pure O2 atmosphere (high pressure) were not successful. After several hours growth we observed that the feed rod began to melt several millimetres above
818
D. Prabhakaran et al. / Journal of Crystal Growth 237–239 (2002) 815–819
Fig. 3. Resistance as a function of temperature measured on a crystal of La1.67Sr0.33NiO4. The anomaly at 240 K indicates the charge ordering.
the molten, and eventually this part of the feed rod collapsed and terminated the run. We now describe the basic characterization and physical properties of the crystals grown under optimum conditions of high-pressure Ar+O2. Analysis of the oxygen content of a selection of these crystals by TGA gave d values of 0.10, 0.07, 0.01, 0.015 and 0.02 for x ¼ 0:0; 0.1, 0.2, 0.33 and 0.5, respectively. The experimental uncertainty is 70.01. These data show that the growth conditions used have produced crystals that are approximately stoichiometric in oxygen for 0:2pxp0:5: For xo0:2; however, there is still an excess of oxygen in the as-grown crystals. The crystals were found to be single phase to within the sensitivity of X-ray powder diffraction, and EPMA analysis at different places along the length and across the diameter of the rods gave a La:Ni ratio of 2 with the few percent uncertainty of the technique. This analysis indicates that the crystals are homogeneous and do not contain any significant amount of impurity phase. Fig. 3 shows the resistivity of a crystal with x ¼ 0:33 as a function of temperature. The curve exhibits an anomaly at a temperature of 240 K due to charge ordering, in agreement with previous work [8]. The magnetic susceptibility of an asgrown crystal with x ¼ 0 measured with the field
Fig. 4. Magnetic susceptibility of a crystal of La2NiO4.1. The peak at 14 K represents an as-yet-unidentified magnetic transition.
applied parallel to the [1 1 0] direction is plotted in Fig. 4. This curve illustrates two curious features of the lightly doped nickelates that have been noted previously (see Ref. [4] for example). First, the susceptibility increases with decreasing temperature, and second, there is a prominent peak at 14 K. The nature of the magnetic transition indicated by the peak is not clear, and the temperature of the peak varied a little with x: 14 K (x ¼ 0), 30 K (x ¼ 0:1), 30 K (x ¼ 0:2), 15 K (x ¼ 0:33) and 22 K (x ¼ 0:5). Further studies are needed to identify the type of transition involved.
4. Conclusion We have optimized the growth conditions for preparation of large, high-quality single crystals of La2xSrxNiO4+d (x ¼ 020:5) under a high-pressure Ar+O2 atmosphere. The crystals are homogeneous, single phase and stable over long periods of storage in air. For xo0:2 the as-grown crystals contain excess oxygen (d > 0), and for these crystals we were not able to find conditions that avoided this excess oxygen. At present, therefore, it seems that the only way to achieve stoichiometry in the oxygen content (d ¼ 0) is to post-anneal the
D. Prabhakaran et al. / Journal of Crystal Growth 237–239 (2002) 815–819
crystals in a reducing atmosphere, as described by Hosoya et al. [6]. By increasing the proportion of O2 in the atmosphere we have been able to grow crystals with larger Sr doping levels (x > 0:33), and the oxygen content in these as-grown crystals is close to stoichiometry. The electrical and magnetic measurements made on the crystals are in agreement with previous results, and show that the crystals are suitable for detailed studies of the spin–charge stripe ordering.
Acknowledgements This work was supported by the EPSRC. We thank Ramon Burriel of the University of Zaragoza for help with the TGA and SQUID measurements.
819
References [1] C.H. Chen, S.-W. Cheong, A.S. Cooper, Phys. Rev. Lett. 71 (1993) 2461. [2] D.J. Buttrey, H.R. Harrison, J.M. Honig, R.R. Schartman, J. Solid State Chem. 54 (1984) 407. [3] K. Dembinski, J.M. Bassat, J.P. Coutures, P. Odier, J. Mater. Sci. Lett. 6 (1987) 1365. [4] W.-J. Jang, H. Takei, Jpn. J. Appl. Phys. 30 (1991) 251. [5] W.-J. Jang, K. Imai, M. Hasegawa, H. Takei, J. Crystal Growth 152 (1995) 158. [6] S. Hosoya, T. Omata, K. Nakajima, K. Yamada, Y. Endoh, Physica C 202 (1992) 188. [7] T. Ido, K. Magoshi, H. Eisaki, S. Uchida, Phys. Rev. B 44 (1991) 12094. [8] S.-H. Lee, S-W. Cheong, Phys. Rev. Lett. 79 (1997) 2514. [9] A. Demourgues, P. Dordor, J.-P. Doumerc, J.-C. Grenier, E. Marquestaut, M. Pouchard, A. Villesuzanne, A. Wattiaux, J. Solid State Chem. 124 (1996) 199. [10] N.J. Poirot, P. Odier, P. Simon, J. Alloys and Compounds 262/263 (1997) 147. [11] H. Tamura, A. Hayashi, Y. Ueda, Physica C 258 (1996) 61.
Growth of large La2–xSrxNiO4+d single crystals by the floating-zone technique D. Prabhakarana,*, P. Islab, A.T. Boothroyda b
a Clarendon Laboratory, Department of Physics, University of Oxford, Oxford, OX1 3PU, UK ! CSIC–Universidad de Zaragoza, 50009 Zaragoza, Spain Instituto de Ciencia de Materiales de Aragon,
Abstract We report on an investigation into the growth of large, high-quality single crystals of La2–xSrxNiO4+d (x ¼ 020:5) by the floating zone technique. The results of attempts to grow crystals in different atmospheres are discussed. The oxygen non-stoichiometry (d) of the as-grown crystals is found to decrease with the Sr substitution level. Magnetic and electrical transport measurements on the crystals are reported. r 2002 Elsevier Science B.V. All rights reserved. PACS: 81.10.Fq; 71.45.Lr; 64.70.Kb Keywords: A2. Floating zone technique; A2. Single-crystal growth; B1. Oxides; B2. Magnetic materials
1. Introduction The celebrated discovery of high-Tc cuprate superconductors has encouraged investigations into the structural, magnetic and electrical properties of other doped transition metal oxides possessing the K2NiF4 structure. One such system is that formed by doping La2NiO4+d with alkaline elements. La2NiO4 is an antiferromagnetic insulator in which the Ni2+ ions carry a spin S ¼ 1: Although no superconductivity is found at any doping level, compounds in this nickelate family do exhibit charge stripe ordering as well as other phenomena similar to those found in the cuprates. Charge ordering in nickelates was first reported in measurements on ceramic samples of La2xSrx*Corresponding author. Tel.: +44-1865-272222; fax: +441865-272400. E-mail address: [email protected] (D. Prabhakaran).
NiO4+d [1]. Because of the intrinsic anisotropy of this compound, however, many of the most useful measurements that probe the spatial correlations in the spin and charge ordered states require single-crystal samples. For some techniques, such as neutron scattering, these crystals need to be large (Bcm3). The first investigations into crystal growth of La2xSrxNiO4+d used the skull melting technique [2]. Since then, studies of crystal growth by the floating-zone method have been published on several occasions [3–6], and in addition to these studies, other groups have reported using crystals prepared by the floating-zone method for experiments [7,8]. These papers report the growth of large crystal rods of undoped or lightly doped (xp0:5) La2xSrxNiO4+d under various conditions. A prominent feature of the La2xSrxNiO4+d system is that the magnetic and structural properties are very sensitive to the oxygen non-stoichiometry (d) [9,10], and control over the oxygen
0022-0248/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 1 ) 0 2 0 3 9 - 5
816
D. Prabhakaran et al. / Journal of Crystal Growth 237–239 (2002) 815–819
content is important if reliable results are to be obtained from physical properties measurements. A detailed study of the phase diagram of polycrystalline La2NiO4+d as a function of d (0– 0.18) was reported by Tamura et al. [11]. In this paper we report experiments to optimize the conditions required to grow large single crystals of La2xSrxNiO4+d (0pxp0:5) by the floating-zone method. Our study differs from previous ones in the use of a high-pressure Argon-rich atmosphere for the growth. We also explore how different concentrations of oxygen in the atmosphere affect the crystal growth and oxygen content of the as-grown crystals.
neutron diffraction. Measurements of the electrical resistivity were made by the standard four-probe method, and magnetic data were obtained with a SQUID magnetometer and an AC susceptometer.
3. Results and discussion We found it possible, after certain adjustments to the growth conditions, to prepare crack-free single crystals of La2xSrxNiO4+d for all Sr doping levels up to x ¼ 0:5: Fig. 1 illustrates a grown crystal with x ¼ 0:2: Fig. 2 shows a rocking curve of an x ¼ 0:33 crystal measured by neutron diffraction and the actual crystal used for the
2. Experimental procedure For the present study, stoichiometric proportions of La2xSrxNiO4+d (x ¼ 0:020:5) were prepared by mixing the starting materials La2O3 (preheated at 11001C for 12 h), SrCO3 and NiO (preheated at 1501C for 6 h) followed by solid-state reaction at 1200–13001C for 72 h. Reaction cycles were repeated with intermittent regrinding until the starting materials were single phase. Feed rods of 12 mm diameter and 100 mm length were prepared using a hydraulic press. To make the feed rods dense and straight we sintered them in a vertical furnace at 15001C for 6–12 h. Crystal growth was carried out in a floating-zone mirror furnace (Crystal Systems Inc.) with 4 1.5 kW halogen lamps as the heating source. The atmosphere during crystal growth was maintained at 5– 7 atm of Ar mixed with a small concentration of O2. The seed crystal and feed rod were counterrotated at 40 rpm and the melt zone was scanned upwards during crystal growth with a speed of 3– 3.5 mm/h. The phase purity and lattice parameters of the grown crystals were determined at room temperature by powder X-ray diffraction (XRD), and the bulk crystalline quality of some of the crystals was checked by neutron diffraction. The chemical composition of the crystals was analysed by electron probe microanalysis (EPMA) and thermogravimetric analysis (TGA). The orientation of the crystals was determined by Laue XRD and
Fig. 1. Single crystal of La1.8Sr0.2NiO4 grown by the floatingzone technique.
Fig. 2. Neutron diffraction rocking curve on the (0 0 4) reflection for an x ¼ 0:33 crystal. The inset shows the exact crystal used for the measurement.
D. Prabhakaran et al. / Journal of Crystal Growth 237–239 (2002) 815–819
measurement is shown in the inset of Fig. 2. Because neutrons penetrate the whole crystal the width of the rocking curve (0.31 FWHM) demonstrates the high crystalline quality of the entire crystal rod. For x ¼ 0:420:5 the increase in grain size as the melt was scanned was much slower than for lower Sr concentrations, and so only the very end of the x ¼ 0:420:5 rods contain single-crystal grains. Crystals cut from these rods are correspondingly much smaller than those shown in Fig. 1 or 2. All the crystal rods exhibited two facets running parallel to the growth direction on opposite sides of the rod. These facets were found to be parallel to the (0 0 1) planes. The cylindrical growth axis of the crystals was found to be almost parallel to the [1 1 0] crystallographic direction (tetragonal notation). It has previously been found [4] that crystals with xo0:2 contain excess oxygen which occupies vacant positions between the NiO layers. The oxygen content must then be varied by postannealing in a suitable atmosphere. For large crystals, however, post-annealing may not be an efficient way to obtain a homogeneous oxygen content, and one may ask whether it is possible to control the oxygen content by applying a suitable inert atmosphere during crystal growth. We have investigated this possibility by experimenting with crystal growth in vacuum, pure Ar flow, Ar+O2 flow, high-pressure Ar gas and high-pressure Ar+O2 gas. We now discuss the results of these trials. Growth in vacuum caused the melt to evaporate rapidly and deposit over the hot zone of the quartz tube. Further growth was then prevented due to resulting attenuation of the light, or worse, reaction of the deposit with the quartz tube. On the other hand, growth in a flow of Ar gas at 1 atm yielded good crystals with two clear facets. After the growth, however, we observed a thin layer of white powder on the crystal surface, corresponding to La2O3, and a green NiO layer formed near the end where the feed rod had been separated from the crystal. Furthermore, within one day of growth the whole crystal started to disintegrate into small pieces. This occurred even though there were no cracks visible on the outer surface of the as-grown crystal. Thick layers peeled away from
817
the surface first, and afterwards these disintegrated into a powder of black and white particles. Finally, all that remained was a slice of crystal of thickness of 2–3 mm from the central portion of the rod, extending across the diameter of the rod and finishing on the two facets. Thus, our efforts to reduce the excess oxygen in the crystal by using pure Ar flow were unsuccessful. On the other hand, the mixing of 2–5% O2 with the Ar gas was found to allow growth of crystals that were subsequently stable even after long exposure to air. Moreover, the amount of La2O3 surface layer formation in these crystals was considerably less than the crystals grown in pure Ar. In a previous study, it had been reported that incorporation of a few per cent excess NiO in the feed rod helps to stabilize the crystal against disintegration or hydrolysis after growth [5]. In all our studies, however, we used a stoichiometric amount of NiO. Experiments were next performed with a high-pressure (5–7 atm) atmosphere of pure Ar gas. These runs were more promising than those with 1 atm of Ar flow. After the growth there was some break up of the crystal, but large pieces survived without disintegrating totally into powder. Finally, we performed runs with a high-pressure mixture of Ar+O2 gas as the atmosphere. With these conditions we were able to obtain excellent crystals with no disintegration or surface-layer formation even after several months exposure to air. The optimum amount of O2 in the atmosphere was found to increase with the Sr content. For xo0:275 we used 1–3% of O2, while above x ¼ 0:275 the fraction of O2 increased rapidly with x, up to B40% for x=0.5. Polycrystalline samples of La2–xSrxNiO4+d tend to be oxygen-deficient when x > 0:275; and so the use of much higher amounts of O2 in the crystal growth atmosphere helps to counter this tendency. Even with this higher concentration of oxygen we observed a brown colour at the grain boundaries for crystals with x > 0:275; indicating oxygen deficiency in these regions. Attempts to grow crystals in a pure O2 atmosphere (high pressure) were not successful. After several hours growth we observed that the feed rod began to melt several millimetres above
818
D. Prabhakaran et al. / Journal of Crystal Growth 237–239 (2002) 815–819
Fig. 3. Resistance as a function of temperature measured on a crystal of La1.67Sr0.33NiO4. The anomaly at 240 K indicates the charge ordering.
the molten, and eventually this part of the feed rod collapsed and terminated the run. We now describe the basic characterization and physical properties of the crystals grown under optimum conditions of high-pressure Ar+O2. Analysis of the oxygen content of a selection of these crystals by TGA gave d values of 0.10, 0.07, 0.01, 0.015 and 0.02 for x ¼ 0:0; 0.1, 0.2, 0.33 and 0.5, respectively. The experimental uncertainty is 70.01. These data show that the growth conditions used have produced crystals that are approximately stoichiometric in oxygen for 0:2pxp0:5: For xo0:2; however, there is still an excess of oxygen in the as-grown crystals. The crystals were found to be single phase to within the sensitivity of X-ray powder diffraction, and EPMA analysis at different places along the length and across the diameter of the rods gave a La:Ni ratio of 2 with the few percent uncertainty of the technique. This analysis indicates that the crystals are homogeneous and do not contain any significant amount of impurity phase. Fig. 3 shows the resistivity of a crystal with x ¼ 0:33 as a function of temperature. The curve exhibits an anomaly at a temperature of 240 K due to charge ordering, in agreement with previous work [8]. The magnetic susceptibility of an asgrown crystal with x ¼ 0 measured with the field
Fig. 4. Magnetic susceptibility of a crystal of La2NiO4.1. The peak at 14 K represents an as-yet-unidentified magnetic transition.
applied parallel to the [1 1 0] direction is plotted in Fig. 4. This curve illustrates two curious features of the lightly doped nickelates that have been noted previously (see Ref. [4] for example). First, the susceptibility increases with decreasing temperature, and second, there is a prominent peak at 14 K. The nature of the magnetic transition indicated by the peak is not clear, and the temperature of the peak varied a little with x: 14 K (x ¼ 0), 30 K (x ¼ 0:1), 30 K (x ¼ 0:2), 15 K (x ¼ 0:33) and 22 K (x ¼ 0:5). Further studies are needed to identify the type of transition involved.
4. Conclusion We have optimized the growth conditions for preparation of large, high-quality single crystals of La2xSrxNiO4+d (x ¼ 020:5) under a high-pressure Ar+O2 atmosphere. The crystals are homogeneous, single phase and stable over long periods of storage in air. For xo0:2 the as-grown crystals contain excess oxygen (d > 0), and for these crystals we were not able to find conditions that avoided this excess oxygen. At present, therefore, it seems that the only way to achieve stoichiometry in the oxygen content (d ¼ 0) is to post-anneal the
D. Prabhakaran et al. / Journal of Crystal Growth 237–239 (2002) 815–819
crystals in a reducing atmosphere, as described by Hosoya et al. [6]. By increasing the proportion of O2 in the atmosphere we have been able to grow crystals with larger Sr doping levels (x > 0:33), and the oxygen content in these as-grown crystals is close to stoichiometry. The electrical and magnetic measurements made on the crystals are in agreement with previous results, and show that the crystals are suitable for detailed studies of the spin–charge stripe ordering.
Acknowledgements This work was supported by the EPSRC. We thank Ramon Burriel of the University of Zaragoza for help with the TGA and SQUID measurements.
819
References [1] C.H. Chen, S.-W. Cheong, A.S. Cooper, Phys. Rev. Lett. 71 (1993) 2461. [2] D.J. Buttrey, H.R. Harrison, J.M. Honig, R.R. Schartman, J. Solid State Chem. 54 (1984) 407. [3] K. Dembinski, J.M. Bassat, J.P. Coutures, P. Odier, J. Mater. Sci. Lett. 6 (1987) 1365. [4] W.-J. Jang, H. Takei, Jpn. J. Appl. Phys. 30 (1991) 251. [5] W.-J. Jang, K. Imai, M. Hasegawa, H. Takei, J. Crystal Growth 152 (1995) 158. [6] S. Hosoya, T. Omata, K. Nakajima, K. Yamada, Y. Endoh, Physica C 202 (1992) 188. [7] T. Ido, K. Magoshi, H. Eisaki, S. Uchida, Phys. Rev. B 44 (1991) 12094. [8] S.-H. Lee, S-W. Cheong, Phys. Rev. Lett. 79 (1997) 2514. [9] A. Demourgues, P. Dordor, J.-P. Doumerc, J.-C. Grenier, E. Marquestaut, M. Pouchard, A. Villesuzanne, A. Wattiaux, J. Solid State Chem. 124 (1996) 199. [10] N.J. Poirot, P. Odier, P. Simon, J. Alloys and Compounds 262/263 (1997) 147. [11] H. Tamura, A. Hayashi, Y. Ueda, Physica C 258 (1996) 61.