Infrared study of H+(H2O)nβ″ alumina

Solid State Communications, Vol. 32, pp. 467—471. Pergamon Press Ltd. 1979. Printed in Great Britain.

INFRARED STUDY OF W(FI~O)~ ~3’ALIJMINA Ph. Colomban Laboratoire th Chimie Appliquée de l’état solide. ENSCP, 11, rue Pierre et Marie Curie, 75005 Paris, France and A. Novak Laboratoire de Spectrochimie Infrarouge et Raman. CNRS. 2. rue Henn-Dunant 94320 Thiais, France (Received 28 May 1979 by M. Balkanski) Infrared spectra of H~(l-l

2O)~ ~“ alumina show than the dehydrated samples contain H3O~ions as dominant species while the hydrated ones consists mainly of H3O~and E-l~O~ entities. Oxonium ions can occupy many different positions more or less distant form the ideal prismatic sites. This structural disorder in the conductivity plane is beheved to be the main factor responsible for the high conductivity ot the material. I. INTRODUCTION

(D(D2O),~)were obtained by exchange of the hydrated

SUPERIONIC CONDUCTORS have been widely investigated during the last few years because of their proper ties as solid electrolytes used in energy storage and conversion systems [11. ~ and ~“ alumina are among the best ionic conductors of’ considerable chemicin and thermai stab~tyand we have unde~akena spectroscopic study of these compounds. The results concerning i3 alumina containing hydrated proton [2 41 and ammonium ions

ones with hea~ water at about 500 K in sealed ~ass tubes. Spectra. The infrared spectra between ~000 and 250 cm ‘,of single crystais (thin plates of 6 x 3 x 0.05 mm) with the c axis parallel to the be~mdirection were recorded on a Perkin Elmer 180 spectrophoto meter using a beam condenser. Polycrystallmne samples were studied as muils in Nujol and Fluorolube using

[41have already been published. We are reporting here an infrared study of H~(H,O)~ ~“ alumina with the purpose to identify the entities present in the conducting planes and to correlate the spectroscopic data with the conductivity. Previous work on tins material is connected mainly with its conductivity, NMR spectra [5, 6J and thermal stability [7J.

CaF2 and CsI windows. Single cr~sta1swere examined at 200 and 320 K arid mulls at 77 and 300 K using a conventional cryostate 3. RESULTS AND DISCUSSION 3.1. Crystal Structure Na*~3~ alumina forms rhombohedral err stals of

2 E~ERIMENTAL

R3rn space group with lattice parameters a 5623 and c 33.59 A tor triply primitive hexagonal c”ll The c

Samples. Fully hydrated H*(H,O),. ~“ alumina was obtained by heating single crystals of non-stoichiometric Na~~“ alumina of general formula Al10 33Mg0 ~O17Na .6~ [SI,(ideal formula is A110MgO17Na2), with cnncentrated sulfuric acid; the product was washed with boiling water and dried at 320K. Further heating of hydrated sample caused a con tmnuous loss of water up to about 630 K when more or less dehydrated sample stable to 780 K is obtained. Intermediate spinel phases are observed between 780 and 1500 K and abo~e1500 K the compound yields nM203 and MgAI2O4. Mixed crystals of~”alumina contaming ~ariable proportions of’ monoatomic cations ~ Na~,K~,Ag~)and oxonium ions were prepared by exchange of H~(Fl20)~ ~“ alumina with the corresponding molten nit ate MNO3. Deuterated deri~arives

parameter is M~substitution sensitive and increases to 34.35 and 34.51 A for dehydrated and tuU~hrdrated phase of H~(H2O)~ deri~ative,respectivek . In hexagonal representation the unit cell consists of spin~IbloLLs separated by three conductmg planes perperdirular to the c axis (Fig. 1) Each plane has a hexaaonal tiet~orL of ~2 ions and in between are ditterent kmnJ~of sites. tetrahedral (3 m) and pnsmatic ( m or ~n. shown in Fig. I available to ,~f*mobile cations respcn ible for the hiali conducti~iryof the material. In the ci of \a~and Ag~the canons are small and can o~.cup~ main1~ terrahedral sites [8 101. In the case of H~(H2O,,~‘ alumina there are three ~O and or N3O~sneLles or onit ~elI plane if all prismatiL and only nv~oit~all teri±-edraisites are occupied. Th~analysis of protonie ~ a~c,ana [~. 6] shows that 2.~mu I .~ 5ites per orUt ‘elf :a~ ire 46

INFRARED STUDY OF H~(H20)~ fi” ALUMINA

468

Vol. 32, No.6

• • Conducting

plane

I

••,~ .O..

3500

—

~

-.

Tetrahedral site

25C(’

~00

3 00

25~Y

2000

500

Fig. 2. Infrared spectra of sinale crystals of FU(H,0)~ (a. b, c, d) and D~(D20~) ~“ alumina (e, f, g); incident beam is parallel to the c-axis. Mixed crystal: Agt.~H30 (a) and Ag~ D30~(e); isotopkally diluted crystal (b); more or less dehydrated samples (c) and (f); hydrated samples (d) and (g).

-

--

occupied for fully hydrated and dehydrated sample. repectively, i.e. prismatic sites are predominant in the former while both prismatic and tetrahedral sites can be occupied in the latter.

Prisrnat~c site

(A )

3000

3.2. Infrared spectra ,The infrared spectra of \f

(~)

Fig. 1. Hexagonal description of the structure of~3” alumina, spinel blocks are separated by conducting planes containing two types of sites: tetrahedral (block trian~es)and pnsmatic sites (squares).

~“

alumina and of its

H~(H20)~ derivatives, both hydrated and dehydrated, are very similar in the 1300 250 cm region which indicates that the characteristic structure of~”alumina does not change with the substitution of the cation. The

Table I. Infrared OH and OD stretching frequencies and half-widths of H~(H20I~ and D(D20)~3’ alumina uOH (cm 3680 3500 3450 3420 3300

1)

~zi120H (cm 1)

nOD (cm

300

2690 2600

180

25a0

250

400 250

i)

(cm

i)

Species

Compound

H,0 H30~ Ag~mixed crystal H30~cluster ~“

H0~

alumina

-

3050

2300

2560 3508

350 15

1950 2590

200 11

3395

42

2465

30

H50~ H50~ H30~ Ag~BR mixed crystal H30* MO site

I

}~ )

site

alumina

INFRARED STUDY OF H~(H,O)~ 13” ALUMINA

Vol. 32, No. 6

corresponding high frequency spectra between 4000 and 1800 cm on the other hand, are very different (Fig. 2). The observed absorption bands are all due to different OH a ~roups since they shift appropriately on deuteration. Identification of these OH groups is facilitated by using progressive dehydration, isotopic dilution and partial exchange with monoatomic cations such as Na~,K~and Aa~ (mixed crystals). Comparison of infrared spectra of 0 dehydrated and hydrated samples of,8” alumina shows that the former contains principally H 3O’ species while in the latter oxonium (H30~)and dioxonium (H5O~)

J

~

ions are present. Dehydrated samples. The OH stretching region of dehydrated sample consists of a broad asymmetric absorption band centered near 3450 cm ‘(Fig. 2. Table 1). In the spectrum of isotropically diluted sample the main band shifts to 3420 cm’ and two additional weaker features can be distinguished: a high frequency shoulder

ib) ______________________________

-.

~

]~

hydrogen bonded OH group of water molecule (the isotopically diluted sample could not be obtained entirely dehydrated) and a low-frequency shoulder near 3300 cm’ which is likely to correspond to H30~ions as the near 3620 cm’ which must be due to practically nonmain 3450 cm” band. H3O~ions can occupy both prismatic and tetrahedral sites. Occupation of prismatic sites implies that the shortest 0 . . . 0 distance between the

I

J

469

I

oxygen atom of the oxonium ion and the nearest oxygen is about 2.8 A. The corresponding distance appears to be considerably shorter, about 2.65 A, for tetrahedral site. These distance can be estimated from the variation of the c-parameter of 13” alumina with M~cation and by analogy with 13 alumina [3,41The shorter 2.65 A dis-

d)

I

-~

aooo

3~uu .‘,

tance is consistent with a medium strong hydrogen bond and requires an OH stretching frequency near 3000 cm° following the relationship between vOH frequency and

2500

cm

Fig. 3. Infrared spectra of: (a) mixed crystal Ag~ H30~ of 13” alumina; (b) mixed crystal Agt~H3O~ of 13alumina; (c) H~(H20),,13 alumina and (d) H~(H20),, 13 alumina,

0 . . . 0 distance [1lJ. However, the maximum of the vOH absorption band of the dehydrated samples is observed near 3400cm ~, i.e. much higher than required by the 2.65 A distance but close to the position expected for the 2.8 A length. It can thus be concluded that H3O* ions occupy prismatic rather than tetrahedral sites.

Table 2. conductivit’u’ of non stoichiometric H~(H20) 13 and 13” alumina

Compound

H~/H,O

E4 (eV)

a (l”i°cm’°)

T (K)

Hydrated 13 alumina Hydrated 13” alumina Anhydrous

0.43

0.6

10

300

0.55 1

1.3

lO”2 10 6

300 800

13

alumina

Mechanism

Ref.

H~jump 4’jump H H 3O~jump

[51 [51 [5]

470

INFRARED STUDY OF H~(H

2O)~ 13” ALUMINA

Vol. 32, No. o

The OH stretching band has a half-width of about 400 cm ~. This breadth can be due to different factors such as hydrogen bonding, intermolecular coupling, and structural disorder. Intermolecular coupling can be eliminated or reduced by preparing mixed crystal contaimng a high proportion (50—80%) of Ag* ions the rest being H30” species, The corresponding infrared spectrum shows that the OH stretching band shifts to higher wavenumbers with the absorption maximum near 3500 cm and narrows to about 300cm ~. This “intrinsic” half-width which is still considerable cannot thus be due to intermolecular coupling. There is but little hydrogen bonding since the vOH frequency is close to the trequency of the free H30* ion [12] and the band width must thus be ascribed essentially to structural disorder. In other words, oxonium ions may occupy many different sites more or less distant from the ideal prismatic sites resulting into a distribution of different OFt distances and thus vOH frequencies. The comparison of the OH stretching bands of the mixed Ag H3O* crystals of 13 alumina and 13” alumina is particularly striking (Fig 3). The former shows a narrow band

such as present in fully’ hydrated 13 alumina [2]. This does not seem very surprising since the I-130~/H20ratio is higher tor 3” than for 13 alumina with the respective values of 0.55 and 0.43. The dioxonium ion appears essentially similar for both 13” and 13 alumina. i.e. H20 molecule is asymmetrically bonded to H30~ion the hydrogen bond strength being almost the same as mdicated bs similar nOH frequency near 2550cm of the central 0 H . . 0 hydrogen bridge. The terminal hydrogen bonds, on the other hand, appear to be weakefor 13” alumina as shown by higher nOH frequency of’ the terminal Old group of’ oxonium ion at 3050cm cornpared to 2900cm for 13 alumina [2]. The terminal dioxonium in 13” alumina could not be identified since they are overlapped by the broad 3400 cm ‘ absorption of o’sonmurn ion. All the vOH bands of hydrated FU(H2O)~13” alumina are broader than those ot 3 alumina (Fm ~. 3) indicating a higher dc—tee ot disorder for the form~r.The only exception is a narrow 3680cm water band, absent from time spectra of 13 alumina Flowev’r, the corresponding almost free water immolecules do riot necessarily occupy the commdu~timmg

2 15 urn i) corresponding to H3O~ions on well defined Beavers Ross sites [3,4] while the latter gives

planes hut st’aS rather on time sUrlace

rise to an absorption which is twenty times broader the absorption maxima of the two, howeser, ~ieldni— practically the same frequency. A similar phenomenon Imas been observed for the NH stretching band of py rrole which has similar frequency for crystalline-ordered and amorphous disordered state but widely different bandwidth [1 ]. Another indication of structural disorder in 13” alumina is the fact that no polarisat ion e t’f’ect on the nOll band has been obsersed, i ‘. the reiatise inten~ityis the same for single crystal and polycristalline material unlike in the spectra of nonstoichionietric 1130* 13 alumina [2 I. £ molly . the band breadth is heimesed to be due essentially to a static disorder as no appreciable temperature effect 55 as det ~cted between so mud .300 K. As I ur as t lie occupat iOu 01 tIme condLictin ‘ planes o ‘oncerned there mis he planes contoumnn” I . or 3 H30 ions shire the aserage ii uumiber iii o xouu umim ions per plane is I (sO. The plane with one 1130* on is e\pect ‘d to giv’ rms ‘ to time highest DII it retchumg Ir ~ 3~00cmi ‘) the by dro—emi bonding beun” sen weak. The planes with two or three H ~0 momis, on t ~ otime hand, would base stronger molecular interactions and thus low ‘r vOH I requemicics. We ‘are t lucre fore assi—n mime the 3420 :mnd 3300cm bands (Table I) to tiso amid three Ions per plane. respeetmvels , the tormimer ris being the most I r ~uen I Ht’r!m-urorl samples Corn parison of time spe~tra of liy Orated ~ammsples ol 13 and 13” alumina (Fig. ~) shows limit H~(ll~0)U” alumina contains boOm H30 amid H~o~ OCrICS ot no o~asunahl’ iuammtitmes of II-U~entities

3.3. (‘onductim’iti’ H ~(1120),, 13” alumina is an excellent protonuc con ductor its conductivity being many orders of nmagnitude (uglier than that of corresponding 13 alumina (Table 2) Time higher conductismty of 13” alumnina rannot be ascribed onk to time h~hmerii umber of the cond uctirm~ cal ions per unit cell plane svhmirh increases I row 1 3 to 1 .66 is lien goin” from 13 no 13” domino hut must he due above all to time number of asailable sites in the pl:mne. The infrared spe ‘Ira show that time snoi~hionmetrue H ~0 13 aiunmmn’a contamims o \on iunm ions occupi ing omuli Beavers Roos sites mud that ci ~‘mi non stomcfmionmetrL compound consists of’ well defined Bees e rs Roos and mmd oxs gen I I) end () sites the corr ~spondmn’~OH tret ‘hun ‘ b mud heurmo few ~mndn:mrross [~] TIme h sdrat ‘ I mud lebs rim aied U” ulunumni. on lie o ti ‘r I and is I t horouglmls rhisord ‘red is ~l ‘in ‘usia ‘ mu 0) 1 en. hmoad nOll ah’orpnuon fimere an thus a reat numb ‘r ol possible sites many of tIme nu heiw’ mimore r less U stall I troism tim ‘ udeal’ prmsnmat mc it’ RIcEl’ RL’NCE S I 2. 3 4.

I Jensen & P. \lcCeehirm, %!at Set I ott 13, 909 ~378) (I Ph Coloniban, 0. Lucazeau, R \lercier & A. \osak,.J C/room m’s 67. a 244 (1)77) Ph Cob i mban, P. (‘ham on - (3. Guuiloteau & I P. Bomlot. Bull .Soe fr Cr ram. 119. 3 Il 975). Pu. Colonihaim, J P. Boulot, A. Khum & C’ III 1 / ‘ Iii \oii u’ / (‘hi” ~, 1 1 I

Vol. 32, No. 6 5.

6.

7.

8.

INFRARED STUDY OF H(H~O)~ 13” ALUMINA

CC. Farrington &J.L. Briant,Mat, Res. Bull. 13, 763 (1978). G.C. Farrington, J.L. B~iant,H.S, Story & W, Bailey, Second International Meeting on Solid Electrolytes, St Andrews, Scotland, Extended Abstracts No. 6.6 (1978). Ph. Colomban,J.P. Boilot, A. Kahn& G. Lucazeau, Second International Meeting on Solid Electrolytes, St Andrews, Scotland, Extended Abstracts No. 6.7 (1978). A. Kahn, Ph. Colomban & J,P. Boilot (submitted

9. 10. 11. 12. 13.

471

toActa &vstallogr.). CR. Peters. M, Bettman, J.W. Moore & M.D. Glick,Actu Cr,ystatlog’r. 827B, 1826 (1971). W.L. Roth, W.C. Hamilton & S,J. Laplaca, Am Cryst. Assoc. Abstracts 2, 169 (1973). A. Novak, Structure and Bonding 18, 177 (1974). B. Desbat & P.V. Huong,Spectrochim. Acta 31A. 1100 (1975). A. Lautie & A. Novak,J. Chim, Phys. 56, 2479 (1972).