Fluorine insertion in inorganic materials

132

Fluorine insertion in inorganic materials Colin Greaves*

and M Grazia Francesconi?

In recent years, research into the chemistry temperature

superconducting

noticeable

shift towards

manipulation

a detailed

optimum

new superconducting

low temperature

investigation

insertion

electronic phases.

of fluorine

properties Reactions have played

and have rekindled interest in alternative for oxide fluoride fullerenes

superconductors.

has also attracted

the synthesis

of high

oxides has witnessed

of the anion (rather than the cation)

not only to achieve create

copper

of new materials

synthetic

Fluorine

attention

reactions involving the insertion formed inorganic structures.

of fluorine atoms into pre-

of chemical sublattice, but also to involving

the

a key role, strategies

insertion

In addition to insertion into oxides, fluorination studies of fullerenes have also attracted interest. Alkali metal insertion, and concomitant reduction of C,, cages to form superconducting materials, has prompted related studies, and here we summarise some recent reports involving fluorine.

into

due to its potential

with unique

a

for

properties.

Fluorine insertion in mixed metal copper oxides Fluorination

Addresses School of Chemistry, University of Birmingham, Birmingham B15 2lT, UK *e-mail: [email protected] te-mail: [email protected]

of oxides to form oxide fluorides can result in:

1. The substitution

of one fluorine

2. The incorporation

of two fluorines

for one oxygen. for each oxygen lost.

Current Opinion in Solid State & Materials Science 1998, 3:132-136

3. The insertion

Electronic identifier: 1359-0286-003-00132

All three possibilities are associated with substantial structural (2 and 3) and electronic (1 and 3) changes. The latter normally correspond to either oxidation, (3), or reduction, (l), of a transition metal ion during fluorination.

0 Current Chemistry ISSN 1359-0286 Abbreviations T Tetragonal structure of K,NiF, (and substituted La&uO,) T’ Tetragonal structure of Nd&uO, Superconducting transition temperature T, X-ray photoelectron spectroscopy XPS XRD X-ray diffraction YBCO YBa,Cu,0,_6

Introduction The structure and physical properties of solids are closely related to chemical composition, and few things are more challenging than tuning the electronic and structural characteristics of compounds via manipulation of their chemical formula. Although the simplest, and consequently most frequently adopted, synthetic strategies for inorganic materials involve aliovalent cation substitutions and control of the redox nature of annealing gases, for example the use of oxygen or nitrogen, fluorine insertion reactions can offer an important and powerful alternative. In recent years, this has been well demonstrated in the field of high T, copper oxide superconductors, and has resulted in a resurgence of interest in the synthesis and characterisation of other oxide fluorides. In this paper we review some of the latest developments relating to inorganic oxide fluorides and give special emphasis to the electronic and structural changes induced by low temperature fluorination of copper containing oxides, which has resulted in the formation of new classes of p-type superconductors. These advances have stimulated additional studies employing higher temperatures and pressures [1,2], which have resulted in superconducting oxide fluorides with a T, up tolllK [l]. New oxide fluorides unrelated to superconductivity have also been reported using more conventional synthesis [3], but we focus only on

of fluorine

into interstitial

sites.

Routes for fluorine insertion The exploitation of ‘chimie deuce’ methods to insert fluorine into inorganic materials has proved essential for the synthesis of new alkaline-earth copper oxide fluorides, because at elevated temperatures (T > 400°C) the target compounds, for example, Sr2,AXCu02F2+s (0 5 x I 2; A = Ca, Ba), decompose with formation of very stable SrF2 (and AF,). The importance of ‘chimie deuce’ lies in the kinetic constraint it imposes on such thermodynamically driven processes, and the restriction of structural transformations at low temperatures. The most important feature of all the fluorination methods discussed here is the use of relatively low temperatures (200~00°C). Sr2Cu02F2+~ was initially prepared by reaction between Sr2Cu03 and F,, and optimization of the key synthesis parameters, temperature and time, led to the production of a single phase product [4]. The synthesis of Sr,CuO,F,+8 and related compounds can also be achieved using alternative fluorinating agents, thereby avoiding the use of toxic, elemental F2. The solid state reaction between Sr&uO, and NH,F performed at a low temperature (240°C) produced superconducting SrECuOLFZ+8 samples [S], but SrF2 was always present as a secondary phase. Such contamination, however, can be avoided by using transition metal fluorides MF, (M = Cu, Zn, Ni, Ag) as fluorinating agents, if the appropriate ratio of MF2 to precursor oxide is used [6]. Of course, using a simple mixture of the precursor and MF, results in contamination with a metal oxide (MO; or M for M = Ag) from the decomposed

Fluorine insertion in inorganic materials Greaves and Francesconi

MF,. For fluorinations using CuF2, two processes may be involved: an F-O exchange reaction between CuFz and atmospheric 0, followed by a similar exchange involving F, and the starting alkaline earth copper oxide [7]. Whereas fluorinations using F, gas or MF, are highly oxidative and result in high values for 6 (typically O&0.8), the use of NH,F yields lower fluorine contents (6-0.3).

133

Figure 1

The differences between the chemical mechanisms of these fluorination routes (as described above) have been highlighted by fluorination studies of Ruddlesden Popper phases [8]. Incorporation of large levels of fluorine can be achieved by all of these methods, but insertion of fluorine into interstitial sites seems to be favoured for direct fluorination by F, gas, whereas substitution of fluorine for oxygen is more prevalent for solid state reactions with NH& A combination of these processes has been reported for solid state reactions with transition metal fluorides MF, [8].

0

F

0

La

Sr2Cu02F2+S and a new fluorinated YBCO-related phase have also been obtained via fluorination of the parent compounds Sr2Cu03 and YBa2Cu,06,i1 by XeFz in the 100~250°C temperature range [9,10”]; for Sr2Cu02F2+h, however, T, was found to be lower than that for samples prepared by the alternative routes outlined above. Electronic control using fluorination

La&uO,F, (T, = 40K; Figure 1) is a p-type superconductor obtained by fluorination of La2Cu04 [ 1 l] which causes intercalation of fluorine into the interstitial sites of the K,NiF, (T) structure. Charge balance involves oxidation of the Cu02 planes and, when optimised, p-type superconductivity is induced. This method of electronic control has recently been applied to La2Cu04 thin films, which have been made superconducting by post-annealing in F, gas [ 121;this indicates a potential method for electronic control in thin film electronic devices. Owing to its different structure (T’), Nd&uO, is not sensitive to insertion reactions, but fluorine can be substituted for oxygen, to form Nd2Cu0,3F6, using high temperature conditions [13]. Electronic charge balance, when optimised, results in reduction of the CuOz planes and hence n-type superconductivity. The crystal site occupied by fluorine has been a matter of debate for a long time, but recent 19F NMR studies [14,15] indicate that fluorine substitutes at the 0 site between the Nd planes rather than in the CuOz sheets. Low temperature fluorination studies of Ln2CuOq-Sr&uOs (Ln = La, Nd, Pr) solid solutions have also produced important electronic modifications in the parent oxides: superconductivity was induced in La0,,Sr,~,Cu(0,F)4+~ (T, = 55K) [16], Nd0,7Sr,,&u(0,F)4_s (T, = 44K) [17] and PrSrCu(O,F),_s (T, = 15K) [18], and in all cases evidence for n-type superconductivity was reported.

Current Opinion in SolId State&Materials

The structure coordination

of La,Cu0,F8

showing

and the interstitial

Fluorination

the CuOs

Scxnce

i

octahedral

F positions.

resulting in structural reorganisation

The synthesis of Sr,CuO,F,+8 is based on a combination of substitution and insertion of fluorine in Sr2Cu03 [4]. Unlike La&uO,FS [ 111, fluorine not only provides holedoping but also plays an essential structural role in transforming the anion sublattice of the ‘precursor’ Sr2Cu03, which is structurally related to La2Cu04. Las+ is completely replaced by Srz+ and one oxygen in the ab plane is removed to transform the CuOh octahedra into isolated chains of corner-linked Cu04. Fluorine insertion completes the anion sublattice to form an oxide fluoride with the T structure, but in addition substitutes one of the original three oxygen ions; interstitial sites (similar to those occupied in La2Cu04Fh; Figure 1) are also partially occupied by the extra fluorine, 6, which provides the necessary hole-doping for superconductivity. Bond distances and Madelung energy calculations [4] strongly indicated that the F- ions on normal anion sites reside only in the apical sites. The apical oxide ions in SrZCu03 therefore migrate to the unoccupied equatorial sites in the Cu plane during fluorination and are replaced by fluoride ions (Figure 2). In this way, the CuOz layers, which are

134

Synthesis and reactivity of solids

Figure 2

Figure 3

0

Ba

l

Y

l F QF 00 l

cu Current Opinion in Soled State & Materials Science

The structures Current Opinion in Solid State & Materials Science

The structural rearrangement which occurs during fluorination of Sr&uOs to form Sr2Cu02F2+c The linear chains of CuO, squares become transformed positions.

essential

into layers of Cu0,F2

for superconductivity,

octahedra

with F in apical

are created

by the

fluo-

rine insertion and are separated by SrF blocks. Postsynthesis reduction resulted in a maximum T, of 46K for d - 0.3 [4]. The apical position of the substituting fluorine, the position of the interstitial fluorine and the hole nature of the carriers have been confirmed by XPS measurements [19], and atomistic simulations [20’,21’]. The latter calculations supported the high solubility reported for Ba at the Sr site [S], and suggested that Na and K substitutions may also be feasible energetically. Structural and electronic effects of substituting Ca and Ba for Sr in Sr2Cu02F2+h to produce Sr2_xAxCu02F2+6 (0 I x 22; A = Ca, Ba) were also examined: T, is increased by Ba substitution, with T,max = 64K for x = 0.6 [S], the highest T, for a material with the La2CuOq structure, whereas it is decreased by Ca substitution [6]. In contrast to Ca2Cu02F2+6 is characterised by a T’ strucSr&u$Fz+h, ture and shows

no evidence

of superconductivity

[Z!].

In view of the small superconducting volume fraction (typically 5%) determined for SrZCu02F2+6 samples, it has been suggested [23,24] that this phase may not, in fact, be responsible for the observed superconductivity. However, strong support for bulk superconductivity in SrZCu02FZ+~ is provided by the dramatic changes in T, induced by Ba or Ca substitutions, because minor phase contaminants are unlikely to be affected in this way. Moreover, Wani et al. [24] draw their conclusions from results obtained on multiphasic samples. Thin been

films of superconducting reported using sol-gel

Sr2Cu(0,F)4+s have routes [ZS]. Fluorine

now was

of (a) YBa,CusO,

and (b) YBa2Cu30sF2,

highlighting

the Cu polyhedra.

introduced either in situ (using CuF2 and trifluoroacetic acid sources) or as a post-anneal (ZZO’C) of Sr2Cu03 films after treatment with a solution of NH,F in methanol. Only using the latter route, which closely resembles the bulk synthesis observed.

methodology,

was

superconductivity

The stoichiometric oxide fluoride Sr2Cu02F2 has now been synthesised [26’). Although this was expected to be the structural parent of Sr2Cu02F2+6, this is not the case because structural rearrangement to the T’ structure occurs when the excess fluorine is removed. The remaining F- ions occupy sites between the Sr layers, and this structure appears suitable for supporting n-type superconductivity, as in doped Nd,CuO, phases. In contrast to the structure of SrZ,A,Cu02F2+8, fluorine has been located on equatorial positions in the Cul layer of a new YBCO-related phase obtained by fluorination of the nonsuperconducting YBa2Cu306,11 [lo”]. Complete fluorination yields a material with approximate composition YBa&u,O,F,, which is the first superconducting phase (T, = 94K) in which the superconducting regions - double Cu02 sheets - are connected by octahedrally coordinated Cu (Figure 3) Ii@**]. The equatorial location of the F- ions will make the electronic properties of this octahedral bridge highly anisoptropic and nonsuperconducting.

Fluorine insertion in fullerenes The fluorination of fullerenes, especially [60]-fullerene, has been the subject of many investigations owing to possible applications as lubricants or unique synthetic reagents. With respect to [60]-fullerene, the products obtained are strongly dependent on the fluorinating agent used, but the main, most stable, products resulting from fluorination have compositions close to C60FJ, or C&,F,s [27-291. Fluorination occurs at low temperatures (typically 300-360K) via the insertion of fluorine into the CbO lattice

Fluorine insertion in inorganic

materials

Greaves

and Francesconi

135

approximately 17.2 w for the stable C&F, compositions [33]. A single crystal X-ray structure determination on C,,F,, with x - 46, has been performed using spherical symmetry for the C and F cages [34’]. Tyo separate shells for C were implied - at 3.13 A and 3.83 A from the cage centre - and a single shell at 5.12 A for E The C atoms at 3.13 k were assigned to those linked by residual double bonds and those at 3.83 A were attributed to s@ C atoms bonded to F as indicated in Figure 4. Calculations suggested that the 7c-electron cloud is concentrated within, rather than external to, the cage (Figure 4) and appears related to the reluctance of C,,F,, cages to form additional C-F bonds without disrupting the C-C network [34’].

Conclusions

(b)

x-electron clouds

Structure

around the C=C

double bond in Cs,F,s:

(a) from above and

(b) from the side. The C atoms bonded to F are marked (a-h),

and the

concentration of rc-electron density within the cage is shown in (b). The C=C bond is shown in bold whilst C and F atoms are represented as open and hatched circles, respectively. peproduced, from 134’1.

with permission,

Although widespread use has been made of aliovalent cation substitutions for the control of carrier density in high T, phases and the search for new materials, only recently has attention switched significantly towards equivalent manipulations using anions. The high reactivity of fluorine gas and other fluorinating agents at low temperatures has resulted in several important new materials and prompted the search for other oxide fluorides using more extreme conditions, for example, high temperature and especially high pressure. The reactivity of fluorine is also apparent in fluorination studies of [60]- and [70]-fullerenes. Simple charge exchange between the cages and the fluorine guest does not occur, but distinct C-F bonds form. There is little doubt that the experience gained from fluorination studies of mixed copper oxides and fullerenes will eventually lead to similar advances in other areas of solid state chemistry and materials science.

Acknowledgement the C,, cages, followed by addition of pairs of F atoms across double bonds in the cages. [70]-fullerene undergoes fluorination much more rapidly as a result of less efficient packing of the nonspherical molecules [29]. It is interesting to note that oxygenated species, believed to involve epoxide linkages, are often produced by direct syntheses using fluorine gas [29,30].

We thank EPSRC superconductors.

for financial

support

for our work

on oxide

fluoride

between

At low fluorine contents, an important driving force for fluorine incorporation appears to be the relief of strain within the C,, cages which results from the formation of C-F bonds [31]. C,,F, materials with x > 48 have reduced stability, which may relate to steric congestion [32], and are thought to involve disruption of the cages via C-C bond breaking [Z&32]. C,, XPS suggests that the C-F bond in C,,-,F,, 30 < x < 47, is intermediate in character between the semi-covalent bonds in fluorine-graphite intercalation compounds and the pure covalent bonds in the graphite fluorides (C,F), and (CF), [33]. Powder XRD has revealed that fluorine insertion in C,, occurs with retention of the face-centred cubic lattice, but the unit cell expands dramatically from 14.2 w in C,, to

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