Environment Dependence of Metal or Ligand Oxidation in Copper

From the systematic nature of the reported heats of formation of binary compounds analyzed per ligand X ion, ΔHX, arguments are presented for the exi...
2 downloads 0 Views 601KB Size
Download PDF
J. Phys. Chem. 1996, 100, 5017-5024

5017

Environment Dependence of Metal or Ligand Oxidation in Copper Oxide Systems: Evidence from Heats of Formation and 7Li Solid State NMR Studies† P. Ganguly,*,‡,§ T. N. Venkatraman,§ S. Pradhan,§ P. R. Rajamohanan,§ and S. Ganapathy§ Materials Chemistry DiVision and Physical Chemistry DiVision, National Chemical Laboratory, Pune 411008, India ReceiVed: September 20, 1995; In Final Form: December 18, 1995X

The stability of the Cu3+ ion in an oxide matrix such as La2Li0.5M0.53+O4 (M ) Cu, Ni, or Co) or La2-xSrxCu1-yLiyO4 (x ) 0 or 0.15, y ) 0 or 0.025) is examined. Heats of formation (as measured by solution calorimetry) and 7Li NMR have been used for this purpose. From the systematic nature of the reported heats of formation of binary compounds analyzed per ligand X ion, ∆HX, arguments are presented for the existence of a maximum value of ∆HX corresponding to a maximum ionicity (∼660 kJ/g at X). A scheme is then proposed to extract the heat of formation per oxygen, ∆H(Cu)O, in the CuO1+δ component of ternary copper oxides of metals such as La, Ba, Li, etc. which have ∆HO close to the maximum value. We then find that the value of ∆H(Cu)O in La2Li0.5M0.53+O4 (∼400 kJ/g at O) is very large relative to that in CuO (∼165 kJ/g at O) and suggests an ionic Cu3+-O2- linkage (metal oxidation). In La2-xSrxCuO4 the low value of ∆H(Cu)O is consistent with considerable O f Cu charge transfer and creation of holes on oxygen. These conclusions are supported by 7Li NMR studies which probe the local environment in the CuO matrix. Advantage is taken of the paramagnetic shifts of the resonance frequency in such anisotropic systems and the dual principal axes (quadrupolar and magnetic dipolar) interaction tensors to understand the 7Li NMR. The main results of the NMR study are that in La2Li0.5M0.5O4 systems the quadrupolar splitting of the 7Li is nearly constant for all M ions indicating a similar LiO6 environment. When coupled with the ∆H(M)O data, this implies that there is little O f M charge transfer. In La2-xSrxCu1-yLiyO4 on the other hand the 7Li NMR shows three kinds of environments for the Li ions despite there being only a single crystallographic site. We suggest that at least one of these environments is due to the creation of holes on oxygen.

Introduction issue1

in several problems of An important physicochemical current interest is the question of whether the oxidation (creation of holes) of a heteropolar solid of the general formula Mp+ (Lq-)V (V ) p/q and Mp+ is a metal of valence p and Lq- is a ligand of valence q), accompanied by a decrease in V leads to an increase in the value of p (metal oxidation) or to a decrease in the value of q (ligand oxidation). When the concerned material is rendered metallic by such a doping process (V can be nonintegral), as in the high-temperature cuprate superconductors, the single band description of the system may seem to render the question of the exact location of the hole somewhat irrelevant.2,3 However, there has been some recent debate on whether a two-band description is not more relevant for the description of the NMR and neutron scattering results.4 Moreover the question of the location of the hole is important in understanding the low-energy excitations that could give insight into the mechanism of high-temperature superconductivity in the cuprates,5,6 for example. In the case of the doped holes in copper oxide systems such as La2-xSrxCuO4 (denoting the doped holes as [CuO]+) we may consider the equilibrium

Cu3+ + O2- T [CuO]+ T Cu2+ + O-

(1)

We may associate the left- and right-hand sides of eq 1 with ionic (more charge separation, metal oxidation) and covalent (ligand oxidation) states, respectively. There has been extensive †

National Chemical Laboratory Communication Number 5248. Materials Chemistry Division. § Physical Chemistry Division. X Abstract published in AdVance ACS Abstracts, March 1, 1996. ‡

0022-3654/96/20100-5017$12.00/0

early literature using high-energy spectroscopic studies giving evidence for ligand oxidation7-10 in oxides. X-ray spectroscopic studies of the Cu L-III edge by Bianconi and co-workers8 on compounds containing nominally trivalent copper in an oxide matrix, such as La2Li+0.5Cu3+0.5O4 (derived12 from La2CuO4 by replacing half of the copper ions by Li+ ions), led to the prevailing notion that eq 1 is shifted to the right in all copper oxides irrespective of the other chemical species in the environment. This environment independence of the stability of various electronic states of an ion is counterintuitive in terms of chemical experience. For example, it is well-known that basic ions (such as the alkaline earth ions Ca, Sr, Ba, etc. or alkali metals such as Li, Na, K, etc.) in the environment enhance the stability of oxides of nominally tetravalent Pb ions, e.g., PbO2 decomposes at temperatures close to 900 K while compounds such as Ba2PbO4 require higher temperatures (>1200 K) for their syntheses. We have examined the environment dependence of the stability of nominally trivalent Cu ions in oxide matrices with K2NiF4 structure11 such as in La2Li0.5Cu0.5O4 (in which the copper atoms are isolated) and in La2-xSrxCuO4 in which the copper atoms are in an extended Cu-O-Cu square-planar network. For comparison we have also studied La2Li0.5M0.5O4 (M ) Co or Ni) systems. We have used two well-established low-energy experimental methods, although in a novel manner, as we shall discuss later. One of these methods involves the measurement of the heats of formation, which may be taken as a measure of the ionic character.12-14 The state on the lefthand side in eq 1 involving metal oxidation is expected to be more ionic (in terms of the strength of metal-ligand electrostatic interactions than the state involving ligand oxidation on the right-hand side. In the second method we have used an NMR © 1996 American Chemical Society

5018 J. Phys. Chem., Vol. 100, No. 12, 1996 probe such as 7Li. The Li ions substitute at the octahedral Cu sites in La2-xSrxCu1-yLiyO4. Two aspects, the quadrupolar splitting of the 7Li nucleus (I ) 3/2) and the chemical or anisotropic paramagnetic shifts,15-18 now become important for the NMR in these anisotropic low-dimensional perovskite oxide systems with a nonvanishing electric field gradient (EFG) tensor. The position of the central (+1/2 T -1/2) transition relative to the satellite quadrupolar transitions (+3/2 T +1/2 or -3/2 T -1/2) is dependent on the relative orientations of the two interaction tensors.17 Information on the location of the magnetic moments may therefore be obtained. We find, from the calorimetric and 7Li NMR studies reported in this paper, that there is little evidence for the predominant creation of holes on oxygen in La2Li0.5M0.5O4 (M ) Ni, Co, or Cu) compounds even when M ) Cu. In La2-xSrxCuO4 there is clear evidence for ligand oxidation from both theromchemical and 7Li NMR data. The component of the heat of formation per oxygen associated with copper, ∆H(Cu)O, in La2Li0.5Cu0.5O4 is much larger than the corresponding value in La2-xSrxCuO4. The 7Li NMR in La2-xSrxCu1-yLiyO4 show at least three different 7Li NMR resonances despite there being a single crystallographic site. The origin of these different environments has been discussed in terms of possible effects due to spin and charge separation. II. Experimental Section The La2Li0.5M0.5O4 (M ) Co, Ni, or Cu) samples were prepared by firing premixed stoichiometric amounts of La2O3, Li2CO3 (in slight excess), and the corresponding metal oxalates with repeated pelletizing and refiring at 750-800 °C for 48 h in oxygen. The La2-xSrxCu1-yLiyO4 compounds were prepared from La2O3, SrCO3, CuO, and Li2CO3 fired for long times at 950 °C. The oxygen content and single phase nature of the compounds were checked by iodometric titration and X-ray diffraction studies. All the compounds had the required oxygen stoichiometry, and there were no impurity phases. 116.64 MHz 7Li NMR studies were carried out at the indicated temperatures using a Bruker MSL-300 NMR spectrometer. Pulse sequences and other parameters are given in the figure captions. A 1 M LiCl solution was used as an external reference. The temperature was controlled by a Bruker BVT 1000 unit. The heats of dissolution of an accurately weighed quantity of the oxide in 3 M HCl solution were determined by calorimetry using a Setaram C-80 microcalorimeter. The methodology used by us are similar to those reported earlier.24 For each oxide at least 8-10 experiments were carried out. The mean of the best reproducible value of heat of dissolution was used for calculation of heat of formation (∆HO). Heats of dissolution in other acids such as perchloric acid and H2SO4 were also measured wherever possible. The calculated heats of formation were found to be independent of the nature of the acid used. III. Results and Discussion III.1. Calorimetric Studies. We seek to demonstrate first of all through empirical correlation of published data that thermochemical quantities such as the heats of formation yield important information on the ionicity of the system. This in turn gives information on the question of ligand Vs metal oxidation in the compounds chosen. III.1.1. Empirical Correlations on Heats of Formation. Some important correlations obtained from the literature on the heats of formation of binary systems are given below. (i) We show in Figure 1 the heat of formation20 per X anion, ∆HX, for some of the binary MX compounds as a function of the parameter (n2 - 1)/(n2 + 2) that appears for example in the

Ganguly et al.

Figure 1. Plots of heat of formation per X anion, ∆Hx Vs (n2 - 1)/(n2 + 2) for various metal M ions in binary compounds, ∆MpX. Open circles correspond to halides, open triangles to oxides, filled circles to sulfides and selenides.

TABLE 1: Heats of Solution ∆Hsol and Heats of Formation ∆Hf of Ternary Oxides and Heats of Formation per Oxygen for Metal (∆HO) and for Ligand (∆HO) compound

∆Hsol (kJ/mol)

∆Hf (kJ/mol)

La2O3 Na2O Na2O2 PbO PbO2 Li2O Li2O2 BaO BaO2 SrO SrO2

A Cation -1810 -417.6 -512.9 -221 -277 -595 -635 -563 -648 -596 -651

CuO MnO NiO

M Cation -158 -389 -242

La2CuO4 La2Li0.5Co0.5O4 La2Li0.5Ni0.5O4 La2Li0.5Cu0.5O4 La1.85Sr0.15CuO4

∆HO (kJ/g) at O

Ternary Compounds -493 -1986 -299 -2431 -323 -2360 -344 -2301 -487 -1961

∆HO (kJ/g) at O

-603 -417.6 -95.3 -221 -56.0 -595 -40.0 -563 -85.0 -596 -55.0 -158 -389 -242 -176 -629 -539 -455 -171

-140

Clausius-Mossotti relation expressed in the form21

R/V ) ( - 1)/( + 2) ) (n2 - 1)/(n2 + 2)

(2)

where R is the gas-phase refractivity, V is the molar volume,  is the dielectric constant, and n is the refractive index. ∆HX saturates at a maximum, ∆HX,max ∼ 600-650 kJ/g at (∼6.506.75 eV) as R/V f 0 or n f 1 and decreases almost monotonically as n increases. R/V ) 0 or n ) 1 characterizes the extremely ionic state; R/V > 1 characterizes the metallic state.21a From such empirical considerations, we associate a value of ∆HX ∼ 6.75 eV with maximum ionic character. (ii) Among the constituents of the ternary Ap+xMq+yO(xp+yq)/2 perovskite oxides,22 all the Ap+Op/2 oxides (such as La2O3, SrO, BaO, CaO, Y2O3, etc., Table 1) as well as some Mq+Oq/2 oxides (such as Li2O) have ∆H(A)O values (Table 1) close to the maximum value of ∆HX,max in Figure 1. In ternary Ap+xMq+yO(xp+yq)/2 compound, the heats of formation of the y(MOq/2) component may be obtained by subtracting out the contribution of x(AOp/2) from the total heat of formation, ∆Hf, of the ternary compound, especially when ∆H(A)O ∼ ∆HX,max.

Oxidation in Copper Oxide Systems

Figure 2. The K2NiF4 structure of La2Li0.5M0.5O4. The axial, OII, and basal, OI, oxygen ions are shown.

(iii) The heat of formation per anion in Mq+XV-q/V compounds (see CuO, MnO, and NiO in Table 1) ∆HX, for a given metal and anion, is weakly dependent on the valency q (metal oxidation), decreasing slightly with q (∆H(Ti)O in TiO, Ti2O3, and TiO2 is, respectively, 523, 516, and 477 kJ/g at O; for MnO, Mn3O4, Mn2O3, and MnO2, ∆H(Mn)O ) 387, 347, 320, and 263 kJ/g at O, respectively). (iv) The heat of formation ∆HO associated with an oxidation (ligand oxidation) yielding peroxides or other unstable oxides (such as PbO to PbO2, BaO to BaO2, SrO to SrO2, etc., see Table 1) is usually less than 100 kJ/g at O so that ∆HO , ∆HO. Such a reduction in ∆HO perhaps reflects the strong O-O covalency instead of a markedly increased M-O covalency. III.1.2. Heats of Formation for La2Li0.5Mo0.5O4 (M ) Co, Ni, or Cu). In Table 1 we also report the heats of formation of the La2Li0.5M0.5O4 (M ) Cu, Ni, or Co) compounds. ∆H(M)O, of the (MO1.5) component in La2Li0.5M0.5O4 compounds (with isolated MO6 octahedra), is much higher than that of the corresponding binary oxides. The M-O bonding as measured by the heat of formation in La2Li0.5M0.5O4 is thus extremely ionic (see conclusion i of III.1.1). It seems that to maximize the electrostatic basal-plane Li-OI interaction (see Figure 2) the O1 f M charge transfer is reduced thereby rendering the M-OI bond more ionic23 so that ∆H(M)O is increased. Mo¨ssbauer studies24 on La2Li0.5Co0.5O4, which show 100% conversion of low-spin 57CoIII ions to low-spin 57FeIV ions by β-decay is also consistent with a strongly ionic Co-O bonding. The value of ∆H(Cu)O in La2Li0.5Cu0.5O4 is still very large compared to that of CuO (Table 1), despite the higher oxidation state of copper in the former (see conclusions ii and iii in section III.1.1). From such calorimetric evidence we may conclude that the equilibrium of the [CuO]+ state in eq 1 is shifted to the left favoring the low-spin,8 d8L, configuration of nominally trivalent copper in such isolated CuO6 octahedra. III.1.3. La2-xSrxCuO4: Holes on Oxygen. The heats of formation per mole, ∆Hf, of La2CuO4 and La1.85Sr0.15CuO4 (Table 1) agree with that reported by other workers.19 The value of ∆H(Cu)O in La2CuO4 is close to that in CuO itself indicating considerable Cu-O covalency. ∆Hf is found to increase in the

J. Phys. Chem., Vol. 100, No. 12, 1996 5019

Figure 3. Comparison of the 116.64 MHz 7Li static QEP (outer) and SP (inner) spectra of La2M0.5Li0.5O4. (a) M ) Cu and (b) M ) Co. The simulated spectra are shown in c and d. (c) M ) Cu and (d) M ) Co. For QEP spectra, a spectral width of 1 MHz, a refocusing delay of 40 µs, π/2 pulse of 1.9 µs, and a repetition delay of 2 s were employed. The SP spectral data were collected with a dead time delay of 5 µs. The experimental data were processed with a LB of 500 Hz before Fourier transformation for sensitivity enhancement. The spectra were simulated using a computer program (ref 31) with the following parameters: e2Qq/h ) 89.0 kHz, η ) 0.0, and LB ) 5 kHz for M ) Cu (c) and e2Qq/h ) 83.2 kHz, η ) 0.0, σ11 ) 52 ppm, σ22 ) 4.4 ppm, σ33 ) -42 ppm, and LB ) 5 kHz for M ) Co (d).

Sr-substituted sample as seen from Table 1. However, ∆H(Cu)O decreases slightly on Sr substitution, which is consistent with an increase in the formal oxidation state (see section III.1.1 conclusions ii and iii). The contribution, ∆H(Cu)O, to the heat of oxidation of divalent copper in La2CuO4 to trivalent copper in La1.85Sr0.15CuO4 is now ∼140 kJ/g at O. This value is just between the values typical of metal oxidation and those for ligand oxidation (see section III.1.1). The calorimetric studies therefore show that the CuO bonding in the superconducting La1.85Sr0.15CuO4 system is consistent with a state with both metal and ligand oxidation. Such a conclusion is supported by 7Li NMR studies. III.2. 7Li NMR Studies. In systems such as La2-xSrxCu1-yLiyO4, the choice of 7Li nuclei (y > 0) instead of 63,65Cu or 17O (y ) 0) as an NMR probe of the valence states of oxygen as well as copper is advantageous because of the extremely ionic character of the Li-O linkage. This is useful especially in eliminating effects due to large chemical shielding interactions arising because of increasing covalency with the ligands.25-27 III.2.1. La2Li0.5M0.5O4 Compounds. III.2.1.1. Quadrupolar Splitting. We compare in Figure 3 the static 7Li NMR spectra when M ) Cu or Co. For M ) Cu, perfect axial symmetry of the EFG tensor is noticed, in agreement with the spectral features reported by Villeneuve et al.28 The expected29,30 powder pattern of an I ) 3/2 7Li nucleus in the presence of a nonvanishing EFG tensor is more faithfully reproduced in the quadrupolar echo pulse sequence (QEP) than in the single pulse (SP) sequence as seen from the comparison with the calculated pattern.31 The results are shown in Table 2. There is no significant change in the quadrupolar coupling constant, νq, as M is changed as compared to the large changes in the calculated EFG32 (Table 2) from the lattice parameters (see footnote to Table 2). This shows that the nature of the charge distribution on the oxygens in the LiO6 octahedra remains nearly the same irrespective of the nature of the M ion.33 The basal-plane O-M bonding when M ) Cu is thus nearly as ionic

5020 J. Phys. Chem., Vol. 100, No. 12, 1996

Ganguly et al.

TABLE 2: NMR and Structural Parameters of La2Li0.5M0.5O4 compound La2Li0.5Co0.5O4 La2Li0.5Ni0.5O4 La2Li0.5Cu0.5O4

lattice parameter ((0.005) (Å) a ) 3.786 c ) 12.753 a ) 3.746 c ) 12.848 a ) 3.727 c ) 13.185

(Li,M)-O distancea (Å) (Li,M)-OI (Li,M)-OII

average (Li,M)-O distance obsd calcdb

EFG calcd (au)

νq (kHz)

1.98

1.89

1.92

2.05

-0.0712

83.2

2.18

1.87

1.98

2.052

-0.2249

80.0

2.24

1.86

1.99

2.06

-0.2613

89.0

a The distances were calculated using I4/mmm symmetry in which the basal distance is half the lattice parameter. The distance was calculated from the refinement of the z parameter of OII ion. The z coordinates are M ) Co, z ) 0.155, M ) Ni, z ) 0.17; M ) Cu, z ) 0.17. b From coordination number-dependent ionic radii as tabulated by Shannon.33

as when M ) Co. Equation 1 is therefore shifted mainly to the left with little O f Cu charge transfer. When M ) Co, the observed 7Li NMR spectrum exhibits a small asymmetry of the satellite transition. The observed pattern has been simulated by considering both quadrupole and paramagnetic dipolar interactions. In the Co case the paramagnetism arises due to the presence of paramagnetic high-spin Co3+ in the system. Considering the unique axis of the EFG tensor to be along the c axis, and that the dipolar interaction between Li and Co essentially acts in the basal plane, the observed 7Li NMR spectrum can be simulated using parameters shown in Figure 3. There is a marginal change in the quadrupolar coupling constant (νq) calculated for M ) Cu and Co (see Table 2). III.2.1.2. Magnetic Dipolar Effects. The following features of magnetic dipolar effects are important: (i) The line width is directly proportional34 to the magnetic susceptibility since it affects the spin-spin relaxation time (T2). The dominant contribution is from the local susceptibility due to the nearest neighbors. (ii) The magnitude of the anisotropic paramagnetic shift is given by the local magnetic susceptibility as well as the local anisotropy. The magnetic susceptibility of the La2Cu0.975Li0.025O4 composition35 is much smaller than that of La2Li0.5Ni0.5O4, for example, and is nearly temperature independent. The observed large paramagnetic shifts as well as large line widths (see section III.2.2) must therefore be attributed to the local magnetic environment of Li in the host matrix. The NMR data presented have been obtained using mainly the QEP36 and SP pulse sequences, though other pulse sequences37,38 can also be employed. The static 7Li NMR spectrum of La2Li0.5Ni0.5O4 taken using QEP (a) and SP (b) modes of acquisition is shown in Figure 4. The characteristic powder pattern features are more faithfully reproduced in the QEP mode. The central transition is shifted to higher frequency when M ) Ni compared to that when M ) Cu or Co (Figure 3). In terms of the analysis of France,17 the asymmetry 7Li NMR of La2Li0.5Ni0.5O4 is consistent with contributions from Ni ions to a paramagnetic dipolar field in the ab plane, which is perpendicular to the principal axis (c axis) of the EFG tensor. In La2Li0.5Cu0.5O4 the satellite transitions are symmetrically positioned with respect to the central transition (Figure 3a) as there is no magnetic dipolar fields due to paramagnetic species in the basal plane. The equilibrium in eq 1 is shifted markedly to the left in La2Li0.5Cu0.5O4. The results are consistent with a diamagnetic low-spin ionic configuration39 of the Cu3+ state. Such a state could be the precursor to other states such as the charge-transfer Zhang-Rice singlet.3 III.2.2. La2-xSrxCu1-yLiyO4 Systems. Small amounts of Li doping does not significantly enhance the weak magnetic susceptibility or dramatically alter the magnetic ordering temperature.35 The crystal structure admits to only one site. We find evidence for different kinds of Li environments as discussed below. In what follows we shall refer to the various compositions in the La2-xSrxCu1-yLiyO4 series as 100y:100x).

Figure 4. 116.64 MHz static 7Li NMR spectra of La2Li0.5Ni0.5O4. (a) Quadrupolar echo pulse and (b) single pulse sequence. All experimental parameters are the same as those given in Figure 3.

III.2.2.1. Basic Spectral Features. The static 7Li NMR spectra of La2-xSrxCu1-yLiyO4 for several 100y;0 compositions are compared for QEP and SP modes of acquisition in Figures 5 and 6, respectively. There are three main spectral features which we shall term as central resonance lines A, B, and C. These features are not likely to be due to crystallographic inhomogeneities as their relative intensities change reversibly as a function of temperature. Line A has chemical shift δA ∼ 0 (as in the 50:0 sample, La2Li0.5Cu0.5O4) with no measurable intensity for the satellite transitions due to quadrupolar splitting for 0.01 < y < 0.20. Line A is seen for all pulse sequences (SP, QEP, and spinecho (SE) sequences). Line C (δC > 350 ppm) has the largest shift, with its intensity increasing with the level of hole doping either by increasing x or y. It is also seen in all the pulse sequences employed. Line B (δB ∼ 250 ppm) is seen for low values of y and is not seen in the SP sequence since its large line width (∼70 kHz) is incompatible with the dead times (4 µs) employed. In La2Li0.05Al0.05Cu0.9O4 the nominally divalent Cu ions of La2CuO4 are substituted by univalent Li and charge-compensating trivalent Al ions simultaneously. Holes are not created in this process. The 7Li NMR shows only line A in the SP mode of acquisition of data with no trace of line C. The paramagnetically shifted broad line seen in the QEP sequence in Figure 7 is likely to have the same origin as that of line B in La2Cu1-yLiyO4 systems. For comparison the 7Li QEP spectra of La2Cu0.95Li0.05O4 and La2Cu0.975Li0.025O4 are also presented in Figure 7. Line C in La2-xSrxCu1-yLiyO4 is to be associated with Li ions in the environment of doped holes.40

Oxidation in Copper Oxide Systems

Figure 5. 116.64 MHz 7Li NMR spectra of La2Cu1-yLiyO4 compositions using the quadrupolar echo sequence. The compositions are (a) 50:0, (b) 20:0, (c) 10:0, (d) 2.5:0, and (e) 1:0. The positions of lines A, B, C are indicated. The experimental parameters are the same as those used in the legend of Figure 3.

The presence of different Li environments is also shown by magic angle sample spinning (MASS) experiments. We show in Figure 8 the 7Li MASS NMR spectrum taken at 4 kHz for the 20:0 composition. Since the SP mode of acquisition is employed, only line A and line C are observed. Of these line A has been effectively narrowed by MASS whereas line C remains broad with no spinning sidebands (SSB). The absence of narrowing of line C in MASS experiments is thus due to the rapid dephasing brought about by a paramagnetic environment. Fluctuations in the dipolar field, proportional to the magnetic susceptibility, broadens the width of the spinning sidebands41 until for large widths there is no narrowing at all. In MASS experiments, the SSBs are generated by repeated refocusing of the magnetization leading to the rotational echos within the time domain given by the spinning speed. Such a refocusing of the Li magnetization under MASS is inhibited by the dephasing due to the interaction with paramagnetic atoms. In the case of La2Li0.5Ni0.5O4, where the powder pattern observed is quite different from that of the M ) Co or Cu case, the MASS at a speed of 3-5 kHz was sufficient to break the observed powder pattern as spinning sidebands. The data obtained at three different spinning speeds are presented in Figure 9. It is observed that the base-to-peak height of the SSBs are reduced as the spinning speeds are reduced.

J. Phys. Chem., Vol. 100, No. 12, 1996 5021

Figure 6. 116.64 MHz 7Li NMR spectra of La2Cu1-yLiyO4 compositions using the conventional single pulse sequence. The compositions are (a) 50:0, (b) 20:0, (c) 10:0, (d) 2.5:0, and (e) 1:0. The positions of lines A, B, and C are indicated. The experimental parameters are the same as those used in the legend of Figure 3.

Figure 7. 116.64 MHz 7Li static NMR spectra of La2Li0.05Cu0.95O4, La2Li0.05Al0.05Cu0.9O4, and La2Li0.025O4 using the quadrupolar echo pulse sequence under the experimental conditions indicated in Figure 3.

The different origins of lines A, B, and C are also seen by the different temperature dependencies of the paramagnetic shifts as well as their line widths. Such a temperature dependence of

5022 J. Phys. Chem., Vol. 100, No. 12, 1996

Figure 8. 116.64 MHz 7Li NMR spectra of the 20:0 composition: (a) static spectrum and (b) magic angle sample spinning spectrum at 4 kHz acquired using the conventional single pulse sequence.

Ganguly et al. varies in a Curie-Weiss fashion43 with the paramagnetic Curie temperatures, θB and θC being ∼100 and ∼0 K, respectively. The relative intensities of lines A and C in the 2.5:15 compound changes reversibly with temperatures (Figure 10d). Such changes cannot be due to changes in the ordering of the Li ions in the various crystallographic sites. There is considerable temperature dependence of δC of the 2.5:15 composition being Curie-like (Figure 10c,d, δC ∼360 ppm at 300 K and ∼530 ppm at 170 K) a characteristic of non-interacting magnetic moments. The Curie-like behavior of the susceptibility associated with both the line width ∆wC and paramagnetic shift δC, indicates that the anisotropy associated with the dipolar field does not vary with temperature. On the other hand, the chemical shift δB of line B decreases with decreasing temperature and is close to zero at temperatures below 200 K as shown in Figure 10a although the line width increases continuously in a CurieWeiss like behavior. The reduction in δB is to be associated with the reduction in magnetic dipolar field anisotropy of the sites responsible for line B. In the absence of any structural change, the only mechanism by which the magnetic anisotropy may be reduced seems to be that involving three-dimensional magnetic ordering. Small amounts of substitution of Cu by Li does not dramatically alter the magnetic ordering temperature,35 which is reported44,45 to be close to 240 K. III.2.2.2. Chemical Considerations. The several (at least three) distinct environments of the Li ions are not likely to be simply due to a broad distribution of the geometry of Li sites due to disorder. The different temperature dependencies of the line widths and chemical shifts clearly show that the different resonances are to be associated with different magnetism and hence chemical environments. We may write the long-range antiferromagnetic order46 in the parent undoped La2CuO4 as

- Cu s O s Cu s O s Cu s O s Cu s < >< > < >< > O s Cu s O - (3) < >< with < and > representing up and down spins, respectively. We associate one unpaired electron per Cu2+ ion (e.g., >) and a singlet pair of electrons on the O2- ion (e.g., < >). The location of a hole in doped systems is indicated by underlining the atom (eqs 4a-4c) we may write:

- Cu s O s Cu s O s Li s O s Cu s < >< > e >< > O s Cu s O - (4a) < >< - Cu s O - -Cus O - +-Lis O s >< < >< Cu s O s Cu s O - (4b) > < >
> e ] >< > +

O s Cu s O - (4c) < >