JOURNAL OF SOLID Oxygen STATE CHEMISTRY 74, 233-238 (1988) Excess in Layered Lanthanide Nickelates D. J. BUTTREY,* P. GANGULY,? J. M. HONIG,* R. R. SCHARTMAN,* AND G. N. SUBBANNAt C. N. R. RAO,t *Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, and tSolid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560 012, India Received April 29, 1987; in revised form July 19, 1987 Density measurements on large single-crystal specimens of La,NiO 4+sand Pr2Ni04+S show that oxygen nonstoichiometry arises from the presence of excess lattice oxygen. X-ray photoelectron spectra as well as X-ray absorption edge studies provide no evidence for the existence of Ni3+ in these oxygenexcess nickelates under the conditions of the measurements. Transmission electron microscopy has revealed a.novel type of exsolution process of the stoichiometric phase out of nonstoichiometric La2Ni04 during heating in COz at 870 K for 3 h. An interpretation of the results in terms of the existence of peroxide species within the conducting layers is proposed. Q 1988 Academic press, ~nc. Introduction Oxides of the type AzB04 possessing the K2NiF4 structure or closely related structures have long been known to exhibit a significant range of nonstoichiometry in oxygen content (Z-5). The nature of the deviations from the ideal 0/(2A + B) stoichiometry has, however, remained unresolved. In many cases, presence of RuddlesdenPopper phase intergrowths, deviations in the 2/l A/B ratio, or variable valence of the B ion have been cited as being associated with the defect structure (5-8). Recent characterization of single crystals of the lanthanide nickelates, LazNiOd+a, has revealed that large deviations in nonstoichiometry (6 > 0) cannot be attributed to the presence of intergrowth phases or deviations in the metal atom ratio (9). It therefore remains to be established whether the oxygen nonstoichiometry is associated with the presence of vacancies in stoichiometric proportions on the metal atom sublattice [i.e., (A2B)1-,04] or with the presence of interstitial oxygen in the lattice (i.e., A2 B04+*). Ganguly (10) pointed out that sufficient space exists in the lattice to permit excess oxygen to be incorporated as a peroxide species (see also below). The importance of deviations from ideal stoichiometry in characterizing the structural features and physical properties of these layered materials has become apparent in a number of studies over the past few years (1, 12, 22). The very recent intense interest in the high temperature superconductivity of substituted lanthanum cuprates has led to the recognition of the importance of stoichiometry in determining T, (13-15). A detailed investigation of the defect structure of existing materials of this type is clearly necessary for the full understanding of their fascinating and widely varying properties. In the present study measurements of the 233 0022-4596/88 $3.00 Copyright Q 1988 by Academic Press, Inc. All rights of reproduction in any form reserved. 234 BLJTTREY ET AL. density of lanthanide nickelate single crystals are used in conjunction with X-ray lattice parameters, XPS , K-absorption edge, and TEM studies to distinguish between the metal-deficient and oxygen-excess models for the defect structure. The consequences of this study are discussed in terms of the influence on transport and magnetic behavior. Materials Starting materials for the crystal growth of LnzNi04 (Ln = La, Pr) were La203 @x99%), Prz03 (99.9%), and NiO (99.999%). Lg03 was dehydrated by heating at 1000°C in air for 24 h and cooling under vacuum. Iodometric titration and reduction in hydrogen were combined to determine the extent of nonstoichiometry and hydration in Prz03 (9). Stoichiometric proportions of Lnz03 or PrZ03 and NiO were prereacted in air by grinding, followed by sintering in a nickel crucible for 24 h. The material was then air quenched, ground, and fired a second time. With this starting material, single crystals were grown by the radiofrequency skull melting technique (9). One sample of LaZ Ni04 was prepared by ceramic techniques starting with the component oxides (16). Crystal characterization included the determination of the degree of nonstoichiometry by iodometric titration. The Ln/Ni ratio was determined by titrations with EDTA and was found to be 2.00 ? 0.01 (9). Laue back-reflection photographs verified the single-crystal nature of the samples. No evidence of additional phases was detected by polarized light microscopy, X-ray powder and electron diffraction, or high-resolution lattice imaging. Density Measurements The Archimedes method was used to determine the density of LnzNiOd+s crystals by weighing a sample in air and in a liquid of known density to determine the sample volume. The procedure closely followed that of Cawrthorne and Sinclair (17). A Cahn 2000 microbalance was used for all mass determinations. Doubly deionized water was used as the immersion liquid.’ The temperature of the water was measured so that accurate values of its density could be obtained. The single largest source of error in this experiment arises from surface tension forces on the sample support wire at the air-liquid interface. It is crucial that surface tension forces be minimized when determining the difference between the sample support weight in water with and without a sample. To this end, several of Cawrthorne and Sinclair’s (I 7) recommendations were followed. A small amount of a wetting agent was used to reduce the surface tension effects. The latter were minimized by use of very-small-diameter tungsten support wires. The wire was cleaned by dipping in 6 M HCl, in alcoholic KOH, and in ethanol, and was then dried. Because of the surface tension forces, it is difficult to estimate the absolute error in the density measurement. Therefore, the density of a standard (99.99% Ni) was measured to check the accuracy of our results. The agreement between the expected density of 8.90 g/cm3 and our determinations was always better than kO.025 g/cm3. Table I shows the experimentally determined density, along with the theoretical density predicted using lattice parameters for the two defect types. On this basis one concludes that oxygen interstitials are the dominant defect type. ’ As noted by Cawrthome and Sinclair (16), liquids of higher density would be expected to give more accurate results, but this is not the case because of an observed increase in convection currents with the increase in the density of the liquid. OXYGEN TABLE IN LANTHANIDE I DENSITY DATA AND LATTICE PARAMETERS FOR NONSTOICHIOMETRIC La2Ni04 AND Pr2Ni04 Lattice parameter Defect model (La2Ni)l-,O, (x = 0.01325) LazNiOd+a (6 = 0.053) (F’rZNi)tmr04 (x = 0.0228) Pr2NiO,+s (6 = 0.091) 64 a = 5.480 E = 12.67 (I= 5.469 c = 12.31 (g/L) 6.915 7.006 7.156 7.324 Psx (g/cd 7.02 7.30 X-ray Photoelectron Spectra and K-Absorption Edge Studies X-ray photoelectron spectra of nonstoichiometric LazNiOd and Pr2Ni04 (for which 6 fell in the range 0.05-o. 12 as estimated by iodometry) prepared by the ceramic method (16) and by skull melting (1, 9) exhibited two distinct O(ls) signals close to 530 and 532 eV (Fig. 1). These signals remained even after the samples were etched in ultrahigh vacuum. We assign the 530-eV feature to the O(ls) level of the oxide ion. NICKELATES 235 The 532-eV feature may arise from carbonate, O’-, or O’,- species. Since the signal remains after Ar+ etching and heating (i.e., after cleaning in situ) we suggest that the signal may be due to O’- or to O’,-, probably the latter. It should be noted that metal perovskites in general exhibit an O’,- signal near 532 eV. For example, the presence of peroxide species has recently been inferred (18) in Lai.sSr,&uO~ and in YBa$&07. Since XPS is a surface technique, evidence based on O(ls) binding energies alone would be insufficient. We have examined the Ni(2p) spectra of the nickelates as well. The Ni(2p) signal of LazNiOd could not be studied since the La(3d) and Ni(2p) signals could not be separated. The Ni(2p) spectrum of Pr2Ni04 shows a symmetric peak due to Ni2+ with no evidence for a shoulder or a peak due to Ni3+ on the higher binding energy side (Fig. 1). These XPS results thus suggest that peroxide-type species may be present in oxygen-excess La2Ni04 and Pr2Ni04 but provide no evidence for the BElcV FIG. 1. X-ray photoelectron spectra of (a) O(k) in LazNi04 after etching, (b) O(ls) in PrzNi04 after etching, and (c) Ni(2p) in PrzNiOa. 236 BUTTREY presence of Ni3+ ions under the conditions of the experiment. To further confirm whether Ni3+ is present in oxygen-excess LazNi04 and PrzNiOd, we carried out Ni K-absorption edge measurements. The results are summarized in Table II. The parent compound NiO, with Ni only in divalent state, exhibits a chemical shift, AE, of 6.7 eV. In LaNi03 and LaSrNiOd, where only Ni3+ is present, the chemical shift is close to 11 eV. In La3Ni207 with 50% of Ni in the 3+ state, AE is 8.4 eV. In LazNiOA prepared by the skull method as well by the ceramic method, with an apparent oxygen excess of 6 = 0.05-o. 12 as determined by iodometry, the chemical shifts remain in the range 6.3-6.7 eV. Similarly, Pr,NiO, exhibits chemical shifts in the range 6.3-6.8 eV. These results further suggest that Ni3+ is unlikely to be present in significant amounts in oxygenexcess nickelates. The manner of accommodating the peroxide ion in La2Ni04 or Pr2Ni04 is worthy of comment. In LaZNi04, there are two types of oxygen; one of them, 01, bonded to Ni having the coordinates (0.5,0,0) and the other, On, having the coordinates (0, 0, z), z = 0.16. There is a third possible position, OnI, with coordinates (0.5,0,0.25) which is vacant. The Oii and Oiii cannot be occupied simultaneously by oxide ions; however, the TABLE II Ni K-ADSORPTION EDGE DATA ON LANTHANIDE NICKELATES AND MODEL Ni COMPOUNDS Compound Chemical shift, AE (eV, *0.7 eV) NiO LaNiOx LaSrNi04 LqN120, La2Ni04 (ceramics) La2Ni04 (skull) Pr2Ni04 (ceramic) Pr2Ni04 (skull) 6.7 11.0 10.8 8.4 6.8 6.3 6.4 6.7 ET AL. space available is more than sufficient to accommodate peroxide ions. The composition La2Ni04.125 seems to be unique. This phase is stable when heated in air up to relatively high temperatures. The composition should be specified by La2Ni2+ 0:.875(0~-)0.125 (rather than by La2Ni& Nii&O&), wherein 1/16th of the On sites is occupied by O’,- ions. Transmission Electron Microscopy We have carried out transmission electron microscopic studies on La2Ni04 containing apparent oxygen excess. When the samples were heated close to 870 K in a CO2 atmosphere for 3 h, the bright-field images showed the emergence of dark, oriented regions (Fig. 2a) possibly due to exsolution of the stoichiometric phase. Since the lattice dimensions of the stoichiometric as well as the nonstoichiometric phases are similar, we believe this phenomenon, wherein the stoichiometric phase emerges in this manner from the nonstoichiometric phase, to be novel. High-resolution images (Fig. 2b) show that there is no major transformation in the solid, the lattice spacings being essentially the same throughout the entire crystal. Discussion The above data have forced us to suggest that nonstoichiometry in lanthanum and praseodymium nickelates results from the presence of excess oxygen in the lattice, most likely in the form of peroxide species. The existence of (0,))’ species as defects in the Ni-0 basal planes is contrary to the generally held view that nickel exists in a mixed valence state in the lanthanide nickelates. Furthermore, carbonate ions are unlikely to remain under the conditions of the experiments, quite aside from the fact that it would be difficult to accommodate such bulky units in the lattice. The suppression OXYGEN IN LANTHANIDE NICKELATES 237 FIG. 2. (a) Bright-field image of LazNiOd (samples heated at 870 K for 3 h in CO2 atmosphere). (b) HREM image indicating that the lattice spacings are the same in the entire crystal (beam direction m$). of magnetic ordering (II, 12) and the incre :ase in resistivity within the basal plane with increasing 6 may be understood, in ter ms of the present model, as due to the dis ruption of superexchange interactions and to the increased localization of carril ers. This model is also consistent with an observed increase in the c/u ratio with an increase in 6 (19, 20). In light of the above discussion, the in- 238 BUTTREY fluence of oxygen nonstoichiometry on the physical properties of the lanthanide nickelates is not surprising. While the generalization of this defect structure to other oxides in this family has yet to be demonstrated, it has direct relevance to the interpretation of their interesting properties, such as the recently reported hightemperature superconducting behavior in the alkaline earth-doped lanthanum cuprate systems. ET AL. M. LEBLANC, AND J. CHOISNET, Muter. Res. Bull. 21, 787 (1986). 6. J. FONTCUBERTA, G. LONGWORTH, AND .I. B. GWDENOUGH, Phys. Rev. B 30, 6320 (1984). 7. J. T. LEWANDOWSKI, R. A. BEYERLEIN, J. M. LONGO, AND R. A. MCCAULEY, .I. Amer. Ceram. 5. P. ODIER, Sot. 407 (1984). 10. P. GANGULY, istry” tional Acknowledgment This research 83-12855. was supported by NSF Grant INT 69, 699 (1986). J. DRENNAN, C. P. TAVARES, AND B. C. H. STEELE, Mater. Res. Bull. 17, 621 (1982). 9. D. J. BUTTREY, H. R. HARRISON, J. M. HONIG, AND R. R. SCHARTMAN, .I. Solid State Chem. 54, 8. in “Advances in Solid (C. N. R. Rao, Ed.), p. 13X, Science Academy, New Delhi BUTTREY, J. M. HONIG, AND C. State ChemIndian Na(1986). N. R. RAO, II. D. J. J. Solid State Chem. 64, 287 (1986). 12. D. J. BUTTREY AND J. M. HONIG, J. Solid State Chem. 72, 38 (1988). 13. C. W. CHU, P. H. HOR, R. L. MENG, L. GAO, Z. J. HUANG, AND Y. Q. WANG, Phys. Rev. Lett. 58, 405 (1987). 14. R. J. CAVA, R. B. VAN DOVER, B. BATLOGG, AND E. A. RIETMAN, Phys. Rev. Lett. 58,408 (1987). 15. C. N. R. RAO AND P. GANGULY, Curr. Sci. 56,47 References 16. 1. C. N. R. RAO, GANGULY, AND J. M. D. J. BUTTREY, N. OTSUKA, P. H. R. HARRISON, C. J. SANDBERG, HONIG, J. Solid State Chem. 51, 266 (1984). 2. P. GANGULY AND S. RAMASESHA, Magn. Lett. 1, 131 (1980). 3. N. NGUYEN, J. CHOISNET, M. HERVIEU, AND B. RAVEAU, J. Solid State Chem. 39, 120 (1981). 4. J. B. GOODENOUGH AND S. RAMASESHA, Mater. Res. Bull. 17, 383 (1982). 17. 18. 19. 20. (1987). P. GANGULY AND C. N. R. RAO, Mater. Res. Bull. 8, 405 (1973). C. CAWRTHORNE AND W. D. J. SINCLAIR, J. Phys. E 5, 531 (1972). D. D. SARMA, K. SREEDHAR, P. GANGULY, AND C. N. R. RAO, Phys. Rev., in press; D. D. SARMA AND C. N. R. RAO, J. Phys. C, in press. P. ODIER, Y. NIGARA, AND J. COUTURES, J. Solid State Chem. 56, 32 (1985). D. J. BUTTREY, Ph.D. thesis, Purdue University (1984).