J. MATER. CHEM., 1994, 4(8), 1313-1317 Examination of the Orientation Dependence of the Quasielastic Scattering of Neutrons by Pellicular Zirconium Phosphate Filmt Robert C. T. Slade,*" Helen A. Pressman: Antonella Peraioavb and Mario CasciolaC a Department of Chemistry, University of Exeter, Stocker Road, Exeter, UK EX4 400 lstituto Superiore di Ricera e Formazione sui Materiali Speciale per le Technologie Avanzate (ISRIM), Loc. Pentima Bassa 21, 05700 Terni, ltaly Dipartimento di Chimica, Laboratorio di Chimica lnorganica, Universita di Perugia, Via Elce di Sotto 8, 06100 Perugia, Italy Variable-temperature quasielastic neutron scattering (QNS) measurements on a time-of-flight spectrometer have been used in investigation of the orientation dependence of quasielastic scattering by a film of pellicular zirconium phosphate [p-ZrP, Zr(HP0,)2.1.3H,0].The partially oriented film, in the plane of which the lamellae of the ZrP structure lie, was produced by intercalation and deintercalation reactions of n-propylamine with microcrystalline a-zirconium phosphate. QNS spectra were recorded with the plane of the film inclined at 135" ('transmission') and 45" ('reflection') to the incident neutron beam. Quasielastic broadenings consistent with hydrogenic species undergoing motions with a timescale s in p-ZrP were observed at T>318 K, and the scattering law was also found to be dependent on sample orientation. The dependence on scattering vector magnitude, Qe,, of the scattering law with the sample in 'transmission' geometry [in which scattering was dominated by motion in the plane of the film (parallel to ZrP lamellae)] indicated detection of both reorientational and translational (diffusion in the plane of the film) motions.Quasielastic scattering with the sample in 'reflection' geometry was less intense and could not be modelled with a simple scattering law. Zirconium phosphates [Zr( HPO,),.nH,O, ZrP] are of con- temporary interest as inorganic ion-exchangers, acid catalysts and proton-conducting solid electrolytes. The structure of crystalline a-Zr(HP04),-H20 (aZrP) is well known. Single- crystal X-ray diffraction was first reported by Clearfield and Smith,' and showed a monoclinic unit cell of space group P2,lc. The structure was shown to consist of zirconium phosphate layers of pseudo-hexagonal symmetry (Fig.1)hE1d together by van der Waals forces (interlayer distance z 7.54 A), with quasi-zeolitic interlayer cavities in which the water molecules reside. The 0-H bonds in the acid phosphate groups point into the interlayer region. Various preparative routes to a-ZrPs of differing crystallinities are kn~wn.~-~ 7 Neutron scattering experiments were carried out at the Institut Laue-Langevin, Grenoble. Fig. 1 The ab layer of the a-ZrP structure, showing the approximate position of the monoclinic unit cell. The hydrogen atoms of the acid phosphate groups are attached to terminal oxygens, the OH bond pointing into the interlayer region. Colloidal suspensions of ZrP lamellae can be obtained by intercalation-deintercalation reactions of n-propylamine and a-ZrP; after re-acidification large thin films (pellicles) are obtained on filtration of, or deposition from, such suspen- sions.8 These films are termed pellicular zirconium phosphate (p-ZrP).In p-ZrP films the lamellae are believed to lie in the plane of the pellicle, this resulting in a marked anisotropy in protonic cond~ctivity~*~~ and the observation of only broad OOn reflections in the X-ray powder diffraction profile (p-ZrP is turbostratic, in common with clays). The water content of p-ZrP varies with relative humidity (RH)," with a maximum of 1.3H20 per formula unit being accommodated, This vari- ability of composition is associated with&ructural defects resulting from some hydrolytic attack during the intercal- ation-deintercalation reactions, the imperfect stacking of ZrP lamellae, and a large extension of the interparticle liquid phase.For the 'H nucleus the cross-section, oinc,for incoherent scattering of neutrons is very high and, as a consequence, the incoherent quasielastic scattering of neutrons (QNS) offers a powerful method for examination of reorientational and trans- lational motions of hydrogenic species," provided that the quasielastic broadening of the elastically scattered peak is observable with an available instrumental resolution (this criterion corresponds to a motional timescale 10-''-1 0-l2 s). The low symmetry of the unit cell for a-ZrP would cause severe experimental difficulties in recording neutron scattering spectra free of coherent Bragg scattering (diffraction) in the elastic component, and consequently would lead to intractable problems in data analysis (modelling both the elastic and the quasielastic components of the scattering).The diffraction profile of p-ZrP is much simpler (eliminating the problems due to Bragg scattering), and p-ZrP is also a suitable novel material for investigation of QNS by a partially ordered sample, for which the scattering law is predicted to depend on the relative orientations of the sample and the incident neutron beam." We have previously reported an orientation dependence of the inelastic neutron scattering vibrational spectrum of p-ZrP J.MATER. CHEM., 1994, VOL. 4 film (in the intensities in the region 720-1120 cm-' associated with acid phosphate groups).12 In the same study, quasi- elastic broadening at 343 K was observed on a triple-axis spectrometer, and a fuller investigation using a time-of-flight spectrometer was suggested. We now present the results of that investigation. Experimental Sample Preparation Microcrystals (2-10 pm) of a-ZrP were prepared by refluxing an amorphous ZrP in aqueous phosphoric acid (10 mol dmP3) for ca. 300 h. These was then intercalated with n-propylamine by mixing (using a DK Mettler Automatic Titrimeter operating by the equilibrium-point method) a dispersion of the microcrystals suspended in water with a standard aqueous solution of n-propylamine (0.1 mol dme3).A dispersion of the intercalate was slowly acidified to pH 2 with vigorous stirring and highly hydrated lamellae of ZrP settled out. On removing the electrolyte (by repeated washing with distilled water) a dense suspension of lamellae was obtained. Slow filtration (using a Millipore RAWPOl plastic filter), followed by air-drying, resulted in a compact, nacreous and fairly flexible film (thickness ~0.1-0.2 mm) of p-ZrP, which was easily removable from the filter.8 The X-ray powder diffraction profile (Philip diffractometer, Ni-filtered Cu-Ka radiation, A =1.541 78 A) was in full agreement with previous studies;8 a broad peak occurred at 26 = 12.5" with a second, low-intensity peak at 28 =48.2". Isothermal thermogravimetric dehydration (at 150 "C) of the sample gave a total water loss of 1.3H20 per ZrP formula unit [i.e.in this study p-ZrP =Zr(HPO4),.1.3H,O]. QNS Measurements QNS spectra were collected on the time-of-flight instrument IN6 at the Institut Laue-Langevin (ILL, Grenoble). The p-ZrP film was contained in a sealed slab-shaped aluminium can of circular cross-section. An incident neutron beam with io=5.9 A was used; the resolution function for IN6 is known to be a close wproximation to a gaussian. Any quasi- elastic broadenings would be consequent on motions with a characteristic time of ca. s. Data acquisition times were typically 2 h. Spectra were recorded across an energy range of -2 <E/meV <214 Fnd a range of elastic scattering vector magnitude 0.25 <Qel/A-' <1.75 (at scattering angle 6, Qel= 47r sin 6/10).QNS spectra were recorded for slab orientations of 135" and 45" to the incident beam (corresponding to 'transmission' and 'reflection' of neutrons respectively), and at temperatures T/K=363, 348, 333, 318, 303, 288, 273, 258 (controlled by a standard ILL cryostat with a heated centre- stick). For both orientations the experimental Qel-dependent resolution function (scattering from a similarly mounted vanadium sheet sample) and empty-can scattering were determined at T =300 K. After subtraction of background and empty-can scattering, scattering spectra were corrected for absorption and slab geometry, and converted to the symmetrised scattering law S(Q,o)form (all steps using standard ILL procedures), where Q is the scattering vector.Spectra at Qel values contaminated by Bragg scattering (as determined from the raw dada by using the ILL program CSUM) were ignored, uiz. Qel/A-'= 1.35, 1.46 for the sample in transmissiqn geometry (135" to the incident neutron beam) and Qel/A-' =0.55, 0.66, 1.75 for the sample in reflection geometry (45" to the incident neutron beam). Results QNS spectra recorded for T>318 K showed appreciable quasielastic broadening for both orientations of the sample. Spectra below this temperature were indistinguishable from the instrumental resolution function (at all values of Qel and for both orientations of the sample) and were therefore not used in data analysis.The further discussion of QNS results will consider spectra at 363 K (at which the largest quasielastic broadenings were observed), but the same general features were observed at lower temperatures. Fig. 2 and 3 show the Qel dependence of the scattering law S(Q, o)at 363 K for the sample in transmission (135") and reflection (45") geometries, respectively. Spectra were initially fitted individually to the empirical Qel-dependent resolution function [i.e. assuming no quasielastic broadening; A,( Q)= 1 in eqn. (1) below]. For IN6 that function is known to be approximately gaussian in form; dashed lines in Fig. 2 and 3 correspond to gaussian fits to the eel-dependent resolution function (no broadening), with the corresponding instrumental resolutions being given in Table 1.Quasielastic broadenings are clearly evident at high Qel values. The form of the scattering law will depend on the detail of the dynamic processes that the hydrogenic species present in a given population undergo. If different populations of 'H are involved in different motions, the observed spectra will result from summation of the various contributions. For a popu- lation 'static' on the instrumental timescale the scattering law (before convolution with the instrumental resolution function) is a 6 function, for a reorienting population it is the sum of a 6 function and one or more Lorentzian terms (with one Lorentzian being dominant), and for a diffusing population it is Lorentzian in form." p-ZrP film offers a complicated set of proton environments, the resulting total scattering law then corresponding to the population-weighted sum of the laws for the individual environments.Thus, Hs in acid phosphate groups would contribute a 6 function, interlamellar H20s could contribute a 6 function and a Lorentzian of Qe,-independent width, and Hs involved in diffusion (either as H+ or on a carrier) could contribute a Lorentzian of Qel-dependent width. Further, the defective nature of the film leads to incorporation of non-ideal H20 environments, and possibly also H30+.Full interpretation of the experimental scattering laws is, consequently, not possible, but a qualitative discussion is possible. Transmission Geometry Spectra obtained in this geometry could be fitted individually and fully satisfactorily [with no systematic deviation of the model S(Q,co) from the experimental data] to a simple analytical form, consisting of a simple scattering law S(Q, ~)=Bo(Q>6(~)+F(Q,0) (1) convoluted with the instrumental resolution function.The quasielastic component F(Q, co) was taken to be adequately represented by a single Lorentzian, L (see above). The empiri- cal elastic incoherent structure factor [EISF, Ao(Q)] is the ratio of the elastic to the total (elastic +quasielastic) intensity in the incoherent scattering spectrum. Ao(Q>=Bo (Q)/CBo(Q)+ SF(Q, a)dml =Bo(Q) for normalised S(Q,co) (2) The empirical EISF as a function of Qel is shown in Fig. 4. The variation of the halfwidth, r, of the Lorentzian compo- nent, L, with Qel is shown in Fig.5. In the presence of a single reorientational motion of a hydrogenic species r would be essentially independent of Qel;" this not the case in this study. J. MATER. CHEM., 1994, VOL. 4 '.+it, I ArA-CC+i .ti" +i cc* . i 4 *I Fig. 2 The scattering law S(Q, w)obtained on IN6 in transmission geometry (slab inclined a! 135"to the incident neutron beam) for pellicular zirconium phosphate film at 363 K and (left-to right) Qel=0.252, 0.748, 1.159 and 1.558A-I. Dashed lines correspond to fits to the Qe,-dependent gaussian instrumental resolution function (i.e.assuming no broadening). #,I 1 1 1 0,:-0.4 4.2 0 0.2 0.4 0.6 I, Fig. 3 The scattering law S(Q, w) obtained on IN6 in reflection geometry (slab inclined at- 45" to the incident neutron beam) for pcllicular zirconium phosphate film at 363 K and (left-to right) Qe1=0.252, 0.748, 1.159 and 1.558 A-I.Dashed lines correspond to fits to the Qel-dependent gaussian instrumental resolution function (i.e.assuming no broadening). In the presence of only diffusive motion of hydrogenic species Table 1 Empirical resolutions (FWHM) obtained by fitting the exper- it is common to assume that rmQ:l;" this law also is not imental resolution function to a gaussian form obeyed in this study. The observed variation in r with Qel (Fig. 5) could be interpreted as observation of a combination -QA resolution/peV R of these motional types for different populations of hydrogenic transmission geometry species.The individual contributions to the scattering law 0.252 43.2 0.997 could not be resolved. 0.748 45.8 0.967 1.250 54.2 0.980 1.558 66.3 0.995 Reflection Geometry 40.60.252 reflection geometry 0.982 Spectra obtained in this geometry could be fitte< adequately 0.758 38.5 0.970 to a law of the form of eqn (1) only at Q,,< 1.0 A-'. Adding 1.250 48.4 0.992 a second Lorentzian component to eqn. (1) failed to bring 1.558 51.1 0.943 the model law and the empirical data into coincidence at high Qe, values, suggesting either that several components were J. MATER. CHEM., 1994, VOL. 4 It I Fig. 4 Variation of the elastic incoherent structure factor, EISF, with scattering vector magnitude, Qel, for a p-ZrP film at 363 K in ‘transmission’ geometry.Vertical lines indicate associated error bars. 100) 1 + 4 pure diffusion pure rotation V.0.0 1.o 2:o 3.0 Q,,~A-~ Fig. 5Variation with scattering vector magnitude, Qel, of the half- width, r,of the Lorentzian component in the scattering law [eqn (l)] for a p-ZrP film at 363 K in ‘transmission’ geometry. Vertical lines indicate associated error bars. Dashed lines illustrate the forms of the variations anticipated for pure rotation (horizontal) and pure trans- lation (sloping); the height of the horizontal line and the gradient of the sloping line have been given arbitrary values for the purpose of illustration. present adding to the quasielastic intensity at higher Qel or that the quasielastic broadening (<10% of the total intensity) is insufficient for reliable modelling with the available data.In view of these difficulties in fitting, a meaningful presentation of variations in EISF and I? with Q,, could not be given for this geometry. It is, however, evident in Fig. 2 and 3 that the quaisielastic broadening at high Qel does not extend to such high and low energies as in transmission geometry. Influence of Sample Orientation The observations above indicate that the form of Qel-depen- dent QNS spectra is a function of the orientation of the p-ZrP film with respect to the incident beam. As already suggested for conductivity this anisotropy can be related to the partial ordering of the ZrP lamellae in p-ZrP, which results in lamellae tending to lie in the plane qf the film.Two special cases exist at Qel= 1.51 A-l (8=45”): in trans- mission geometry Q then lies parallel in the plane of the film (parallel to the ZrP lamellae if stacking were perfect, Q= Q,,), while in reflection geometry Q is perpendicular to the film (normal to the ZrP lamellae if stacking were perfect, Q= el).The QNS spectrum in transmission geometry thus depends on motion parallel to the plane of the film, while that in reflection geometry depends on motion perpendicular to the film. It follows that the observed quasielastic broadenings at high Qel values (Fig. 2 and 3) in transmission (Q=Q,,)and reflection (QzQl)geometries result primarily (stacking of lamellae will not be perfect) from components of motions in and perpendicular to the plane of the film, respectively.In Fig.2 and 3 it can be seen that the broadened component (observed at high Q,,) is more intense in transmission geometry; at Qel= 1.558 A-l, for instance, the fit to the resolution function accounts for ca. 90% of the total intensity in reflection geometry but for only ca. 65% in transmission geometry (using model-independent intensities estimated using the trapezium rule). This anisotropy may be related to more facile diffusion between lamellae and across particle surfaces than perpendicular to them. The latter process would rely on convoluted pathways and accessing a range of defect types, and this could account for the difficulty in fitting the scattering law at high Qel (the widths of quasielastic contributions arising from diffusion are proportional QZ, 11) in reflection geometry.The greater overall width of quasielastic broadening at high Qel in transmission geometry may indicate that diffusion is predominantly in the plane of the film. Contributions to quasielastic broadening arising from reorientation of interlamellar water can also be expected to differ in the two geometries. p-ZrP film is a deceptively attractive system for investi- gation of the orientation dependence of quasielastic scattering. The system is indeed partially oriented but is, in fact, massively defective (as exemplified by the variable water content) with imperfect stacking of ZrP lamellae.The observation of a diffusive motion [of water, or H+ (by a Grotthus mechanism of hopping between H,O carriers), or H30+, or all three species] in this study, and of enhanced protonic conductivity in low-crystallinity ZrPs in general,13 is consequent on this non-ideality. In contrast, NMR relaxation studies of highly crystalline a-ZrP detect only reorientation of interlamellar water molecules.14 Conclusions Quasielastic broadening consistent with hydrogenic species undergoing motions with a timescale of ca. s in p-ZrP was observed at T>318 K in this work. The form of the scattering law was found to be dependent on the relative orientation of the sample film and the incident neutron beam. The Qel dependence of the scattering law with the sample in ‘transmission’ geometry [in which the origin of scattering is predominantly components of motion in the plane of the film (parallel to ZrP lamellae)] is consistent with detection of both reorientational and translational (diffusion parallel to the plane of the film) motions.QNS spectra with the sample in ‘reflection’ geometry could not be modelled with a simple scattering law, but quasielastic scattering was less intense overall than in ‘transmission’ geometry. We thank the Institut Laue-Langevin for access to the neutron spectrometer IN6. We thank SERC for supporting the Exeter neutron scattering programme and for a studentship for H.A.P. This work was supported in part under the Science programme of the Commission of the European Communities.A.P. thanks the Istituto Superiore Ricerca Materiali (Terni) for a Fellowship. References 1 A. Clearfield and G. D. Smith, Inorg. Chem., 1969,8,431. 2 A. Clearfield and J. A. 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Alberti, J. Muter. Chem., 1992,2, 583. G. Alberti, M. Casciola, U. Costantino, G. Levi and G. Ricciardi, J. Inorg. Nucl. Chem., 1978,40, 533. R. C. T. Slade, C. R. M. Forano, A. Peraio and G. Alherti, Solid State Ionics, 1993,61,23. Paper 4/02386B; Received 22nd April, 1994