Journal of the Less-Common Metals, 164 & 165 (1990) 920-925
M.P. SRIDHAR KUMAR, S.M. DOYLE and D. McK. PAUL Physics Department, University of Warwick, Coventry CV4 7AL, U.K.
We have measured some of the physical properties of the Ndg.xSrxNi04, family below room temperature and find a complicated system in which orthorhombic lattice distortion, oxygen stoichiometry, resistivity and magnetic susceptibility change with strontium doping. As the strontium concentration is increased the materials become stoichiometric and tetragonal without reduction and the conductivity of the samples increases although remaining semiconducting. Some compounds demonstrate an unusual low temperature susceptibility which is either temperature independent or falls slightly below -15 K
INTRODUCTION LagCuO4, La2NiO4 and NdgNi04 have similar, distorted K2NiF4 structures but
when some of the rare-earth ions are replaced with strontium the cuprate exhibits superconductivity
while the nickelates
recently found small traces of what may be minority
Some groups1 have
Lap.xSrxNiOq, a discovery which would have great importance for proposed models of high-temperature to understand
the superconductivity the properties
family of compounds
so with this in mind we
An added attraction
is that the magnetic moment of the neodymium
We have made measurements
resistivity and susceptibility
of the two systems may help
site that is not available
ion provides lanthanum
of the structure, oxygen stoichiometry,
for different strontium dopings.
STRUCTURE AND STOICHIOMETRY Powder samples of the solid solution series Nd2.,Sr,NiO4+,
(0 c x < 1.2) were
prepared by the standard ceramic method from Nd203, SrCO3 and NiO at 1200” C. Characterisation
of the samples by x-ray powder diffraction showed that, as in the
case of the LaaNiO4 system2, the parent tetragonal distorted.
structure is orthorhombically
The nickelate samples with lower doping of Sr contained excess oxygen,
which was determined by iodometric titration. 0022-5088190/$3.50
0 Elsevier Sequoia, Printed in The Netherlands
921 Formation of stoichiometric 310’
C and it was observed
was achieved by reduction in hydrogen at
that the reduced compound
has a higher degree of The
were similarly reduced and the X-ray diffraction spectra of the reduced
samples and the results of the iodometric titrations indicated that
for x > 0.4, the materials
and did not require
reduction. The crystal
oxygen present. greatest
for the undoped
of the compounds
by the excess
titration data the excess oxygen was seen to be
and to decrease The
more or less linearly
strain so reduction of the sample in Hz increases the distortion in the An increase
in Sr doping
samples with x 2 0.3 are found to be tetragonal
samples with x 2 0.3 only a small distortion is induced. there is no change in the tetragonal measurements
the as prepared
in structure and for the reduced For samples with x > 0.4,
structure on reduction.
indicate that these samples are stoichiometric
Figure 1 shows how the lattice parameters of the unreduced samples vary with increasing Sr content. tetragonal.
From the figure it is clear that the samples with x 10.3
In this solid solution series, with an increase in x the a and b values
decrease and c increases until x - 0.50 after which there is a reversal of this trend. With x > 0.6, a increases slightly and the value of c drops steeply.
5.50 ..... . .. ..,.. .,.. .%I qb : 5.45 2 “,__ p’ 5.40 0 I ;
5.300.0 0.2 0.4 0.6 0.8 1.0
.a** 3 12.4~...........................t..... 0 re . . 12.3 12.2t 1 " 0.0 0.2 0.4 0.6 0.8 1.0
Figure 1) Lattice parameters of Nd&?lrxNi04+, x, determined from X-ray diffraction spectra.
RESISTANCE Four-terminal measurements of resistance have been made on sintered pellets.
All of the samples show semiconductor-like behavior with an increase in activation energy as temperature decreases (Figure 2).
There is a general decrease in
resistivity and activation energy for samples with increasing strontium This non-metallic behavior and increase in conductivity are concentration. observed for LagNiO4,y on varying y.10 x=0.30
0.01 ’ * 0
loo l!iO Temperature (Kl
Figure 21 Resistance vs temperature for some Ndg.,SrxNiOr+v compounds. 4.
SUSCEPTIBILITY Neutron diffraction data for stoichiometric NdzNi04 show three-dimensional
AFM ordering of the Niz+ ions above room temperature and of the Nds+ ions at about 8 K 3. The magnetic structure of NdgNiOr+y is complicated by the sensitivity of magnetic order to the presence of excess oxygen. Nickel and neodymium AFM order occurs in the stoichiometric material but is destroyed in even partially oxygenated samples. Significant partial ordering or correlation of the Nds+ ions is observed below 80 K which is due not to the intraplanar Nd-Nd interactions but to interplanar JNd_Ni. Similar strong Nd-Cu interactions are observed in NdgCu047. In both the neodymium cuprate and nickelate systems the relative strengths of the different interactions are believed to be JN&Nd< JN&M c JM_M.
We have measured the ac susceptibility of the strontium-doped materials for the range 4 to 40 K (Figure 3). We believe that the signal we detect is that due to Ndg+ ions, which have a moment approximately twice the size of that of the Ni2+ ions 3. It is probable that the Ni ions are already ordered antiferromagnetically and contribute little to susceptibility at these temperatures. The data for reduced samples with low strontium concentration (x
. ,. 3.
., .. ,
. , . .,
. ., . . . . , .
Temperature /K Figure 31 Low-temperature ac susceptibility of Ndg.,SrxNiO4+, compounds. A broad peak in susceptibility is observed in compounds with 0.40 5 x 5 0.80.
There are considerable differences between the cuprates and nickelates in the ranges of temperature and strontium concentration for which the broad peak occurs. The very wide peak in Lap.xSr,CuOq occurs at about 200 - 500 K for a doping range of 0.10 < x -z 0.24 5 while in Nd2_xSrxNi04+, Tvaries between 5 and 17 K over a much larger range of strontium concentration 0.40 < x < 0.80.
A possible interpretation of the low-temperature downturn in susceptibility would be that part of the sample has become superconducting in a similar fashion to the La2.xSrxNi04 compoundsf. However, the size of our observed dip would imply such a large superconducting fraction that we would expect to see an accompanying transition in resistivity. We do not see this and none of our samples are even metallic. Another possible influence on the low-temperature susceptibility may be crystal field splitting of the ground state J-multiplet.
Calculations from preliminary
inelastic neutron measurements of the crystal field scheme6 show that no downturn at low temperatures is expected and that this is unlikely to be the cause of the observed broad peak. The most likely explanation then is that the broad susceptibility peak is due to incomplete, possibly two-dimensional, AFM ordering of the Nds+ ions. ‘Ihis behavior is very similar to that seen recently in isostructural La2_&&CuO4 485and attributed by the authors to two-dimensional Cu2+ spin fluctuations. An important distinction is that in the cuprate system only the copper ion has a moment while in Nd~,Sr,NiO4+,. both Ndg+ and Ni2+ have moments. It is expected that the intraplanar interactions will be stronger than the interplanar, leading to the type of 2D correlations which have been observed in La2CuO4 518. For the ~~-moment NdsCuO4 Matsuda et ai suggest that it may be necessary for the Cu2+ ions to order before the Nd3+ ions are able to, since the Cu2+-Nd3+ interaction is stronger than the NdB+-Nds*correlation. The Nd ordering that we see at low temperatures may then reflect ordering occurring in the NiO2 plane. A further important question is why the doping range for which the broad peak is present should be so different for the cuprate and nickelate systems. Thurston et al.9 discuss this problem in comparing the values of x at which 3D AFM ordering is destroyed in Nd2.xCexCu04 and La2.xSrxCu04 . The 3D ordering disappears in the lanthanum cuprate when ~~0.02 but persists in the neodymium cuprate up to at least x=0.14. ‘l’he authors suggest that this is because of the physical location of the electronic carriers which are 0-2~ holes in lanthanum cuprate and Sd-band electrons in neodymium cuprate. The oxygen holes frustrate magnetic exchange and destroy ordering more quickly than the formation of Cul+ ions which merely dilutes the magnetism.
CONCLUSIONS The physical properties of the Ndg.,SrxNiO4+, compounds are similar in many
ways to those of strontium-doped La&u04 and LasNiOq. Structural and magnetic properties are sensitive to the presence of excess oxygen, which is found in compounds with ~~0.4. Resistivity is somewhat x-dependent, although none of the samples becomes metallic. The magnetic susceptibility at low temperatures for materials with 0.4 5; x < 0.8 displays a broad peak which seem most likely to be caused by twodimensional AFM correlations. REFERENCES 1) 2. Kakol, J. Spalek and J.M. Honig, Solid State Comm. 71(1989), 283. J. Spalek, Z. Kakol and J.M. Honig, Solid State Comm. 71 (19891, 511. K.S. Nanjundaswamy, A. Lewicki, Z. Kakol, P. Gopalan, P. Metcalf, J.M. Honig, C.N.R.Rao and J. Spalek, Physica C 166 (19901,361. 2) J.M. Honig and D.J. Buttrey, Localization and Metal-Insulator Transitions , eds. H. Fritzche and D. Adler (Plenum, 1985). 3) J. Rodriguez-Carvajal, M.T. FernBndez-Diaz, J.L. Martinez, F. Fernandez and R. Saez-Puche, Europhys. Lett 11 (lQ96), 261. 4) R. Yoshizaki, N. Ishikawa, H. Sawada, E. Kita and A. Tasaki, Physica C 166 WJ99),417. 5) M. Oda, T. Ohguro, H. Matsuki, N. Yamada and M. Ido, Phys. Rev. B 41(19QQl, 26%. 6) A.T. Boothroyd, SM. Doyle, M.P. Sridhar Kumar and D. MCK Paul, Crystal Fields in Nd2_zSrxNi04+,., this volume. 71 M. Matsuda, K. Yamada, KKakurai, Endoh, in print.
H. Kadowaki, T.R. Thurston and Y.
8) R.J. Birgeneau, D.R. Gabbe, H.P. Jensson, M.A. Kastner, P.J. Picone, T.R. Thurston, G. Shirane, Y. Endoh, M. Sato, K. Yamada, Y. Hidaka, M. Oda, Y. and T. Murakami, Phys. Rev. B (1988),6614. Enomoto, M. Suzuki 9) T.R. Thurston, M. Matsuda, K.Kakurai, K. Yamada, Y. Endoh, R.J. Birgeneau, P.M. Gehring, Y. Hidaka, M.A. Kastner, T. Murakami and G. Shirane, in print. 10) M. Sayer and P. Odier, J. Solid State Chem. 67 W87), 26.