Journal of the Less-Common Metals, 175 (1991) 119-129 LCM 1241

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The structure of LiCu202 with mixed-valence copper from twin-crystal data

Rolf Berger*, Auke Meetsma and Sander van Smaalen Materials Science Centre, Laboratory of Inorganic Chemistry, University of Groningen, Nijenborgh 16, NL9747 AG Graningen (The Netherlands)

Margareta Sundberg Department of Iwganic Chemistry, An-he&us Laboratory, Stockholm University, S-106 91 Stockholm (Swede)

(Received February 12, 1991)

Abstract

The structure of LiCuzO, was solved using two sets of X-ray diffraction twin-crystal data. Extended twinning creates virtual tetragonal symmetry. The compound crystallizes in &ma (62) with unit-cell parameters a = 5.72 d;, b = 2.86 A and c = 12.4 A with a certain homogeneity range. The structure consists of LiCu”Oz layers interleaved by layers of Cu’ connected to oxygen in an almost linear coordination. Lithium and copper(B) have five oxygen neighbours in pyramidal arrangements that run as parallel bands through edge connections. Electron diffraction was used for characterizing the twinning.

1. Introduction

In a recent note on the Li-Cu-0 system [ 11, X-ray powder diffraction evidence was presented on two new ternary phases together with an attempt to index the powder pattern of a phase known for more than thirty years, Li3Cu204 [2]. The present paper contains the results of a structure deter- mination of LiCuBOs using electron and X-ray single-crystal diffraction, con- firming the suggested composition and the Cu’-Cu” mixed valence.

2. Experimental details

2.1. Sample preparation LiCu202 was obtained in mixtures with other phases when heating Li&Oa

with CuO in air above 1100 K [ 11. Plate-like crystals could be dissected from the surface of a pellet heated with a torch flame, starting with an Li:Cu ratio of 1: 1. The dark crystals have a metallic lustre, but interference colours and increasing dullness with time indicate that the surface composition

*Present address: Institute of Chemistry, Uppsala University, Box 531, S-751 21 Uppsala, Sweden.

0022-5088/91/$3.50 0 1991 - Elsevier Sequoia, Lausanne

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gradually changes. The effects may be due to oxygen uptake to form copper compounds or may be ascribed to lithium migration to the surface where initial Liz0 may react with humid air to form LiOH followed by Li2C03. As indicated by thermogravimetry [ 11, LiCuZO, is only stable at high temperatures and thus seems to decompose slowly even at room temperature. In samples taken from the pellet surface, a few electron diffraction patterns indicated an unidentified phase not detected by bulk X-ray powder diffraction. It might be a decomposition product.

2.2. Dimaction equipment and experimental procedure Powder diffraction for phase analysis and determination of accurate cell

parameters was performed with a Guinier-Hagg focusing camera using Cu Ka, radiation (h = 1.540598 A) and silicon as the internal calibration standard.

LiCuzOa was always obtained in mixtures with other phases. Its powder pattern was at first indexed on a tetragonal cell (a= 5.72 A, c = 12.4 A), and later revised to an orthorhombic model (a = b) owing to line splittings seen at higher angles on Guinier films of better quality samples.

Single-crystal X-ray diffraction experiments were performed at room temperature using an Enraf-Nonius CAD4F diffractometer equipped with a graphite monochromator for the MO Ka radiation (A= 0.71073 A). Two crystals of LiCuZOZ were selected for data collection. The first crystal (Dataset I) was of dimensions 10 pm X 5.2 pm X 1 pm. Intensities were measured in one hemisphere with 1.6” < 0 < 32”. A total of 32 16 reflections were collected and corrected for absorption (~=207 cm-‘; transmission limits 0.31-0.91), and for Lorentz and polarization effects. The second crystal (Dataset II) was considerably larger (80 pm X 80 pm X 12 pm). Intensities were recorded for 1.6” < 0 < 42”, yielding 6050 reflections. The same measuring strategy and correction procedures (transmission limits O-28-0.77) were applied.

For the electron diffraction (ED) work we used a JEOL 200CX electron microscope equipped with a double-tilt, top-entry goniometer stage. The samples were prepared by crushing some crystals in an agate mortar, dispersing the powder in n-butanol and transferring it to a holey carbon film supported by a copper grid.

3. Structure solution

Dataset I showed Laue symmetry 4/mmm in contradiction to the in- dications from powder diffraction. Averaging symmetry equivalent reflexions within 4/mmm yielded an internal consistency R,=Z(I-I,,,)/YU= 0.058. A solution of the structure was attempted using these data and led to a model using P4a /n that showed reasonable structure and chemical features. However, the variance with powder data warranted the search for another crystal. Dataset II showed orthorhombic symmetry (R, = 0.048 within mmm) with a = b = 5.72 A. The model from the previous preliminary solution could not be satisfactorily modified to the new data set and revised symmetry.

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Electron diffraction was now applied to the phase in order to receive microscopic information. From high-resolution electron microscopy (HREM) it was obvious that the material suffered from extensive disorder. Figure 1 illustrates three different types of ED patterns recorded on thin crystal fragments of LiCu202, all along the c-axis. Figures l(a) and l(b) are similar except for the additional weak reflections in Fig. l(b) which indicate orthorhombic symmetry with an axis ratio very near to two. For comparison, X-ray diffraction using the Weissenberg technique only gave the kind of (I&O) pattern of Fig. l(a). The weak reflections are probably of too low intensity to be recorded by that technique. The pattern of Fig. l(c) may be interpreted as a result of rotational twinning of the pattern given in Fig. l(b). The new

Fig. 1. Electron diffractograms of LX&O, along [OOl 1. The strongest reflections correspond to (hk0) for overall &ma symmetry: (a) normal (WcO) pattern; (b) pattern obtained with weak extra reflections giving the correct axis ratio, their occurrence explained by oxygen disorder; (c) pattern obtained for a rotational twin with two equal domain contributions as in (b) with 90” misorientation.

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model was tested on the X-ray powder diffraction data which were successfully indexed on this smaller orthorhombic cell (a = 5.72 A, b = 2.86 A, c = 12.4 A), where for different compositions the short axis did not change significantly while the axis ratio a/b varied by 1.998-2.003 and the c-axis changed by 0.1 H, [I].

A reinterpretation of the X-ray single-crystal data was now called for. The axis ratio very near two would promote intergrowth. Domains in two orientations, rotated 90” with respect to one another around the common c-axis, would yield a resulting diffraction pattern interpretable on a reciprocal lattice with two axes almost alike (a= b = 5.72 A). For equal domain con- tributions, a tetragonal pattern (4lmmm) is produced, otherwise an ortho- rhombic pattern (mmm). In Fig. 2 this is illustrated schematically where the letters denote intensities. The intensities a A, b B, etc. correspond to pairwise identical structure factors Fi so that, for example, a= k,pF,’ and A = k,( 1 -p)Fa2 with k, being identical proportionality factors and p a measure of the relative volume contribution of a certain orientation of a twin domain. Thus, the intensity ratios u/A, b/B, etc. are constant, equal to p/(1 -p). In the bottom part of Fig. 2 an artifact pattern is produced by superposition, taking care of the different contributions. For p = l/2 the above ratio is near unity, corresponding to a pseudo-tetragonal situation.

The false pattern has the following features. The intensities Iwl and IBul (where u is an odd number and g an even number of indices h, k) reflect the individual contributions of the two orientations, the reflections with hk = gg contain contributions from both individuals, while those with hk = uu are all extinct. A scrutiny of the reflection material of the orthorhombic crystal indeed disclosed all these characteristics. The intensity ratio IwI/Isd was virtually constant (0.27) and all (uul) reflections were found to be extinct; this phenomenon is inexplicable by any ordinary extinction rules.

In order to arrive at a structure model for the domains from this analysis, the b-axis was halved. The new indices h’k’l’ are such that h’ = h, k’= k/2 and 1’ = 1. The former reflections k = u are then all disregarded; they belong to the other orientation and carry no further information. With the revised orthorhombic indices the systematic extinctions were analysed. The reflections seemed to obey the restrictions (Ok’l’) k’+Z’ = 2n, and (h’k’0) h’=2n, conforming to Puma as a possible space-group.

Although hampered by twinning, the tetragonal reflection material (with a twin ratio near to 1: 1) must still contain a great deal of truth, and so also must the proposed structure model refined in P42 In (RF = 4% for cut material). This model showed layers of composition LiCuOz with lithium and copper having a pyramidal coordination of oxygen. These layers were interleaved by layers of monovalent copper having two oxygen neighbours in a slightly bent configuration. In the correct model the constraints of the enforced tetragonal model had to be removed and substituted, keeping the local environments. Application of the new symmetry operators to the old structure model showed that some or all of the atoms were found at approximately the same positions with the former cell halved. Therefore, the new orthorhombic

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structure model might be derived using the previous model as a starting point.

The y-coordinates from the former model were now doubled (since the b-axis was halved). By this procedure a doubled set of coordinates was obtained for each former site, and equivalent positions were sought within the set. Owing to the cell volume and the short axis, only a four-fold position could be accepted, with all atoms in 4c of JY&m. Reduction of the coordinate set according to the demands of the symmetry operators, and including a change of origin, led to a structure model with features found locally in the former proposal.

The new coordinate set (all atoms with x = l/S) and an overall temperature factor of 0.5 AZ were now fed into a computer program (REMOS f 31) capable of refining the positional and thermal parameters of the structure as well as the ratio of the two twin domain orientations. Scattering factors for Lit, Cu+ and Cu2+ were taken from International Tables [4], including dispersion corrections, while that of 02- was taken from ref. 5.

For dataset II a domain volume ratio of 0.212:0.788 (or 0.269:1, in agreement with the intensity ratios) was obtained, yielding& = Z]]Pcalc] - l_FO& X/-F,+,] = 0.050 for individual isotropic temperature factors and RF= 0.045 after allowing for anisotropic temperature movement of copper. From a total of 1617 unique reflections, 730 with I> a, were used in the refinement, giving all reflections equal weight. ~thou~ extinction correction was performed, the observed structure factor of the strongest reflection, (006), remained 8% too small but was still included in the refinement.

Dataset I was now averaged in Laue symmetry rnmm (R,,=O.O43), resulting in 886 unique reflections from which 412 with I> 2.5~~ were used. The refinement yielded a 0.526:0.474 domain ratio (p= l/2) and, within the error limits, the same structural parameters as for dataset II. However, the standard deviations of the parameters were larger despite a slightly lower

TABLE 1

C~~log~p~c data for LiCu&& as obtained from twin data showing apparent o~horh~mb~c symmetry (a = b)

Atom z .z VI, fJz2 v33 VI3 &,

Li 0.130(3) 0.5719(9) 0.48(S)

CU, 0.1194(3) 0.25490(7) O.OOSS(4) 0.0096(4) 0.0071(2) -0.0007(27) 0.665(15)

Ck! 0.6244(2) 0.90548(6) 0.0045(3) 0.0045(3) 0.0081(3) 0.0004(32) 0.450(13)

0, 0.1365(U) 0.4052(4) 0.49(3)

02 0.1146(12) 0.1049(4) 0.61(3)

Space group prSma(62) for the cell (from powder ditlkaction) a= 5.730(l) A, b =2.8606(4) & c = 12.417(2) h (Z= 4). AlI atoms in 4c with y = l/4. The figures within parentheses denote the estimated stidard deviations. The thermal parameters are given in Sngstrijms squared, refined isotropically or anisotropically (copper) where the equivalent isotropic parameter is also given, as calculated from B, = S&U,, + U,, + &&3. For symmetry reasons V,, = U,, = 0.

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TABLE 2

Interatomic distances in %ngstrBms (up to 3 A) for LiCuaOa as calculated from the orthorhombic twin data; cell dimensions a=5.730 %i, b=2.860 A, c=l2.417 .&

f3.h-02

cu,-0,

CIA,-2cue

cu,-2cu7. cu,-zcu, cu,-2Cu, Cu,-2Li

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G-Qz

1.863(5) 1.869(5) 2.774(2)

2.822(2) 2.860(1) 2.868(3) 2.952(12)

2.74219) 2.860(l) 2.993(9)

cuz-20, cua-202 cur-o, cu7,--2cu* cue-2cu* Cur-Li

CUT2Cua Cua-2Li Cua-Li

1.980(4) 1.984(5) 2.477(5) 2.774(2) 2.822(2) 2.847(17) 2.860(l) 2.880(12) 2.911(17)

Li-0, Li-2Oa Li-20, Li-2Li

Li-Cua Li-2Li Li-2Cua Li-Cua Li-ZCu,

2.070(12)

2.087(13) 2.111(13) 2.730(17) 2.847(17) 2.860(l) 2.880(12) 2.911(17)

2.952112)

discrepancy index (RF= 0.039) which is probably an effect of the smaller number of reflections.

We emphasize that the full datasets, including the twin superpositions, were used in both refinements. Because of better data, we choose to present the result from dataset II. The positional and individual thermal parameters are presented in Table 1. Interatomic distances are given in Table 2 where we used accurate cell parameters determined from powder difkaction, un- affected by twinning. The structure is illustrated in Fig. 3.

4. Discussion

The ideal composition of the phase is LiCu202. In an ionic picture there is an equal mixture of Cu' + and Cu2 + , the species being distinguished by their oxygen coordination and interatomic distances. The structure may be described by a layer sequence @%Rla along the c-axis where A denotes a slightly buckled layer of composition LiCuOz while B consists of copper. The subscrifit indicates that the sequence has to be repeated to reach the translation distance.

Within the double A-layers Cu” and lithium both coordinate five oxygen atoms in pyramidal arrangements. Lithium lies well within its oxygen pyramid with even Li-0 distances. Cu’ takes four even distances (in a plane) and one significantly longer, obviously as a result of the Jahn-Teller effect of a dQ system. These interatomic distances are quite in line with what has been found in other oxide phases. Cu’ must be seen as monovalent copper with two oxygen neighbours although these do not have perfectly linear coor- dination; the angle 01-Cu1-02 is 1’77.8”. Deviations of the same order of magnitude from the ideal value of 180” were reported for some alkali oxocuprates(I), such as CsCuO 161, K&uO, (71 and Rb&u,O, 181. The Cur-0 distances of 1.863 A and 1.869 A are similar to the bond length 1.867 b

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found in the mineral paramelaconite, Cu,Oa [9], also containing a mixture of Cu’ and Cu”. The Cu-0 distance in Cu,O is slightly shorter.

The insulating superconductor precursor YBa,Cu30, also contains such a Cu’-Cu” mixture [lo]. There, monovalent copper connects Cu”05 pyramids, apex to apex, while in LiCunOp the coupling occurs in a similar manner between pyramids of two kinds, Cu”0, and Li05. Both compounds contain pure copper layers. In Y13azCu306, extra oxygen may be accommodated there.

It is also interesting to make a comparison with LiMnOa [ 111. Its stacking sequence may be described in a similar manner by /AAl in space-group Pmmn (59). Here too, the ratio of the unit-cell axes describing the LiMeO, layers is near to two. Since the sequence is different, the local symmetry is different, so that the lithium and manganese atoms have six near oxygen neighbours. Four of the Mn-0 distances are shorter and two are longer (by 14%), which is ascribed to the Jahn-Teller effect expected for Mn3+. LiCuzO, may thus be seen as a further modulation of the NaCl theme found for compounds of the LiMe02 stoichiometry [ 121, except that mixing in of Cu’ changes the stoichiometry. Efforts to prepare LiCuO, have failed, but changes in the synthesis conditions might lead to success.

The syntheses of LiCuaOs and LiCu303 [ 1 ] in air have created compounds with a localized mixed valence of copper as judged from the compositions and physical properties. Neither of these oxides is metallic; rather they show hopping conduction. Still, they form an interesting structural link between CuO and the new high critical temperature cuprate superconductors. As already alluded to, in some of these structures, copper takes five oxygen neighbours with distances similar to those found for LiCu202.

For some samples, the X-ray powder reflections with h odd were diffuse, the half-widths being 3-4 times larger than for the other sharp reflections. If only the latter are considered, a subcell with a= b = 2.86 %, is obtained which is then only a proper description along the b-axis. The phase coherence is thus better along that axis, where pyramid bands containing either lithium or copper are present, than along the u-axis, where the type of metal alternates.

If we only consider the subcell indices (h * k * I!*) where h * = h/2, k * = k and 1* = 1, we arrive at pseudo-extinctions for h * + k * + I* # 2n. This body- centring condition would hold exactly if all x = 1 /S, and the actual deviations (Table 1) are very minute. In practice, this means that the intensity is very low for those Pnmu reflections with h = 2n that do not fulfill h/2 + k + I= 2n. This effect is striking in the (hk0) layer where, by the demands of the a- glide, only even h-indices occur. Now, every other reflection in these h-rows of reciprocal space is almost extinct, leaving very few reflections (as in Fig. l(a), or schematically as filled circles or large crosses in Fig. 2) from which the true symmetry cannot be read!

The use of ED data was crucial for determining the true unit cell. Extensive twinning seems to occur down to unit-cell level, and all crystals selected for X-ray dif3kaction were more or less tiected. The very weak reflections observed in some ED patterns (Fig. l(b)) led to the &ma model

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that successfully explains the X-ray diffraction data. However these reflections do not belong to the (MO) zone for Pnmu symmetry. Rather, their presence indicates that a small deviation from the overall &mu symmetry might occur locally in the crystals.

In order to estimate the effects of local disorder, theoretical ED patterns were calculated. The atomic positions were kept in a triclinic cell of the same dimensions as those obtained from the &ma refinement. On altering the lithium positions or occupancies the Q&O) ED pattern was hardly influenced. Therefore, the oxygen sites were investigated, and it was seen that weak reflections of the correct magnitude would result if either a pyramid apex atom were removed or extra oxygen were inserted in the voids of the layer built up by Cu’. The first operation would yield a decrease in coordination number, from CN5 to 4 for Cu” and Li, and from 2 to 1 for Cu’. This seems crystal-chemically unreasonable. However, oxygen insertion into the Cu’ layers, corresponding to oxidation of copper, would give a CN increase from 5 to 6 for Cu” and Li, and from 2 to 4(6) for the copper formerly seen as monovalent. Such coordinations are well within reason, and the oxygen insertion into the copper layer would then very much resemble the situation in YBa2Cu306 which can be oxidized to YB~&u~O~-~. A homogeneity range does exist, as indicated predominantly by the magnitude of the c-axis being 0.1 8, longer in a specimen prepared in a closed silver tube [ 11.

Since lithium scatters X-rays very weakly we cannot be quite sure that we have arrived at the best model for LiCuaOa. Still, the positions of the light atoms are fairly well determined and the temperature factors carry reasonable values. However, other local defects may very well occur. The coordinations of Li and Cu” are rather similar and make site disorder possible. Because of the limitations with regard to both technique and material, we see no point in trying to refine the model further. We intend to investigate powder material by neutron diffraction in order to shed light on the small deviations from the otherwise satisfactory Puma model.

At the completing stages of this manuscript we received a preprint from Hibble et al. [ 131 who had also characterized LiCuzO,. They noted the same two symmetries as us for their crystals but interpreted this as being due to two modifications. They relined their tetragonal dataset in the space-group P4,/nmc, arriving at essentially the same structure as we did in P4Jn - which we rejected. They found no satisfactory model for their orthorhombic data and were unable to account for the “pseudo-extinctions” (IUti = 0 in a model with a = b). Of paramount importance is the fact that our &mu model explains the diffraction intensities from both kinds of crystals, on the basis of domain formation.

The Cu’ layers are almost planar and quadratic so that, locally, an adjacent LiCui’Oz layer could take a 90” misorientation and yield apparent tetragonal symmetry without introducing any considerable increase in surface energy. Layer misorient&ions of 180” would also be possible, resulting in local Pnmm symmetry. Our twin model carries credence also from a thermodynamic point of view. Hibble et al. obtained their “orthorhombic” crystals on cooling the

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melt more slowly than when obtaining the “tetragonal” crystals. Yet they argue that slow cooling yields more disorder on the Li/Cu” sites. We expect increasing order, which is accounted for by our domain model. The observed tetragonal symmetry is purely an artifact from a high-temperature situation where both domain orientations are equally plausible, entropy stabilized.

In principle, powder diffraction data (which are not affected by twinning) could be used for discriminating between different structure models. Inter- estingly, unresolved powder data (“tetragonal appearance”) are well explained by both their tetragonal and our orthorhombic model. However, their model cannot explain data with orthorhombic appearance, neither from powder nor from crystals.

Acknowledgments

We are indebted to Dr. J. de Boer for collecting the diffractometer data. R.B. gratefully acknowledges the leave of absence granted by the University of Uppsala, Sweden, to permit a temporary position in Groningen, financed by The Netherlands Foundation for Pure Research (NWO/SON) within the Dutch national framework for research on superconductors (NOP- high T,s). The research of S.v.S was made possible by tiancial support from The Royal Netherlands Academy of Sciences and Arts (KNAW). The electron microscopy work (M.S.) was financed by The Swedish Natural Science Research Council

(NI=).

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