NMR study of deuterium molybdenum bronze

CHEMICAL PHYSICS LElTERS

Volume 96. number 3

8 April 1983

NMR STUDY OF DEUTERIUM MOLYBDENUM BRONZE C. MARINOS, S. PLESKO. J. JONAS Utdr*ersity of Illinois, Urbana. Illinois 61801. USA

Jfaterials Research Laboratop.

and D. TINET and J.J. FRIPIAT Centre Xational de Ia Recherche Scientifiqtxz, CROSCI, IB rue de la Ferrolerie. 45045 OrIeans Cede-x, Fmnce Recehed

9 December

1982

The deuterium NBIR lineshape and spin-lattice relaxation time, 2-1. have been measured in deuterium molybdenum bronze. D1_6XIoO3, over the temperature range 166400 fc. The ‘D quadrupole coupling constant is 21 kiiz at room temperature. The temperature dependence of the 2D T1 has been interpreted in terms of two independent motional processes for deurerium. The data suggest that one of the processes corresponds to diffusion of the 2D nuclei whereas the other may arise from a 1 SO0 flipping of the OD2 moieties. This specific interpretation agrees with the results obtained for proton T1 and proton liieshape data reported earlier.

The IH NMR spectra and relaxation in the socalled hydrogen molybdenum trioxide bronze with composition H,_,Mo03, obtained either through hydrogen spillover [ I] or be reacting Moo3 with nascent hydrogen [2] are already well documented 13-61. However, in view of the various contributions to the relaxation mechanism of protons in these materials [3-s], the current understanding of these systems is far from complete_ This fact provided the main motivation for the present study of the deuterium spin-laitice relaxation, T1, in the deuterium analog of the hydrogen molybdenum bronze. D1_6Mo03. Since quadrupolar relaxation provides the dominant relaxation mechanism for the 2D nuclei, the interpretation of the 2D relaxation data is simpler when compared to proton relaxation dara. We emphasize that this is the first study of *D NMR in deuterium molybdenum bronzes because earlier attempts to observe 2D signals failed [3]. We obtained good 2D NMR spectra both at low magnetic fields Cl.4 T) and high magnetic fields (4.2,8.4 T). A detailed study of DI_6h!003 and other bronzes is in progress but the in-

0 009-2614/83/0000-0000/S

03.000

1983 North-Holland

teresting results so far obtained for the ‘D T1 behavior justify a preliminary report. In order to be able to discuss the 2D data in the context of earlier results obtained from the proton NhlR measurements [3-61 one has to summarize the main results obtained for Ht_6MoO,. The lH NMR limeshape is found to be associated with shielding anisotropy in HI.6 Moo3 powder [4] _A monocrystal study of the same bronze has shown that the principal a?ris frame of the shielding tensor is related to the crystal frame of the crystal and that its unique component is perpendicular to the layers of the hlo octahedra [7] _ The dipolar interactions become important in broadening the line below 230 K [3-61. This temperature of 230 K also coincides with the transition between Pauli behavior of the paramagnetic susceptibility (above 230 K) and Weiss-Curie behavior (below 230 K) as well as with a discontinuity in the plot of the heat capacity of inserted hydrogen_ These observations suggest a transition in the electronic state of H1_$ioO~ occurring at ~230 K in spite of the fact that the monoclinic structure and structural param357

Volume 96.

CHEWCAL

number 3

PHYSICS LElTERS

ctcrs are independent of temperature down to ~100 K;. The proIon spin-lattice rela\arion time e.xhibits a :I~IIIIIIILIIII between 320 and 190 K indicating that dipolar inIcractions (either homonuilear or heteronuclear) modulated by motion are responsible for relaxBIIO~. lkm~evcr. the nature of rhe relaxation mechanisms in.i~ charlge \%itli temperature. In the teinpcratuIc region where the pammagnetic susceptibility C~W\S PJllli‘s la\% IS]. the interaction between nuclear hprns and conduction electrons map also contribute In addition. in the temperature IO the p’UroI1 T;‘. 1qo11.

\\here

CUIIC--UCISS

tllc

pdr,unagnetic

susceptibility

obeys

the proton r<’ should reflect J ~~~tnbution front interaction with paramagnetic icfitfrs. cithcr hli?+ or hl&_ Thus the behavior of Iilc profm 7.;* \\ith tcmpcraturc is difflcult to intcrpm. l‘or instsnse it was also observed that by dilu~rng l~orom with dcurcrims (II/D = 0.X) the pIotoII T, dots not chnge in the “lligll”-temperature t l’rtclllerlcy-lridcpcndcnt ) region wllere.is it ChJlgCS to behavior.

.I coIIsidcr.ibk

e\tcnt

iii lhc

“lO\v”-tCIllpc~tlIre

(frC-

ri”““-..\-d~Per~Jenr) mgion [3_1]_ Iii tlicir first paper 011 proton rclawtion. Cirillo JIIJ l‘np1Jt 1.31 used the Torrcl relatIonship [l)] modIIkd by Kiugcr 1lo] .JSslIIlliJl 6 that tile Illost iiiiporIXII

~u~~trihutioil

is the IIonwuuclear

dipolar

contri-

I~IIIIOII

The p.u.unrtcr Q introduced by Kruger ;IS the I,ItIo (r’)/l :I[‘. wllcrc ($1 is rhc ~wdn squaw jump dI~1afIcc .iIld tf 1s the dIsr.i~lcc of closest approach bct\\erII tno IIIIcrdctiIig spins. was t.dicn equal to infinII\ and the corrcl.IIion times calcuLIted from T, nlca~LIICJ .II three I~~quc~~c~~s gave 3 good linear log T1 \cr\us 1~l‘rcLttIoIIslIip at 1CJst in the tcnipcrature rmgc brr\\ccII .G3 and 300 K. \\hile the data below 200 K sI.irtcd I0 br Inore srattcred. II is c1c.u Ihat iI would lmc been nios~ desirable III Inc.I~urc ‘11 rcla\atiun time in either D,,XloO3 or III p.lrtially dcorcratcd bronms bccausc deutcron rei~urIo11

Iel.i\.itioJl.

~lle~han~sn~ lkir

is dominated

IlieJIlS

by 111e quadrupolc

11131electronic

and

paraniag-

rcld\.IIioII contributions can be safely Ilcglccted. .\Iw spII1 / = I 11~s the smplifyinp property to have I? = I_ w rlrar rlw exprcssiuns of the spectral dcnsitics J~.Iilablc fw spin 1 = l/Z 1x1~) be used [ 1 1 ] _ fhc JePendence of the quadrupole coupling consi.ult (Qc*c) wirh lenlpcrature as obtained from the 1Incsl~ape provides infonmtion on the electric field grlJIcnt tensor ( EFG). Fig. 1 shows the room-muIWIIC

8 April 1983

DI_6Moo3

T=273W

I

I6 ktiz

--

D- SPECTRUIk,

rig. 1. ‘D lineshape at 300 K in potbdered Dr.,hloO3 sample at Bo = 8.4 T. The small peak at the center frequency corresponds to D20 impurity on the surface of the bronze.

persture k lineshape recorded at the magnetic iield rZ, = 8.4 T for the Dte6M003 powder. For randoInly oriented powder srunple. the doublet splitting is &s = ;(l

- +_+Q/k

(1)

where ~‘~Q/II = QCC. in frequency units, and g is the asymmetry parameter of EFG (0 < 9 < 1). The observed QCC at room temperature is 31 kHz and the shape of the ?D signal suggests that p is snlall. The minor peak in the center of the signal is due to DzO impurity on the surface of the bronze. Below room temperature the lineshape changes as it will be shown later in a more detailed study. The variation of the ‘D spin-lattice relaxation time. Tl.atthe deutcriurn frequency pL = 26.7 MHz is shown in fig. 3. These data can be interpreted in terms of two independent tliernially activated processes. It would not be easy otherwise to account for the two very different slopes observed on both sides of the T1 minimum and the “crossover” at around T=330 K. From the asymptotic behavior at high and low temperature a BPP-type contribution [I?]. T;.can be separated as represented by the dashed line in fig. 2. The T; rninirnum occurs at -20s K. Using the equation l/Tl=

1/T;

+ l/T;’

the Ti’ contribution

(7-I

to the ‘D relaxation time is calculated as represented by the crosses in fig. 2. Applying Kruger’s treatment 1 lo] (which can be shown to be also valid for the quadrupolar relasation) to T; and using (y = (r2)/13& = ‘)2 L * the fitting of the ex-

CHEMICAL

Volume 96, number 3

8 April 1983

PHYSICS LETTERS

100

T, = f (PI

80 60

2.5 I 3

I 4

I 5 $

x lo3

1 6

(K-l)

Fig 1. Temperature dependence of the ‘D spin-Iattice relaxation time, Tt, in potxdered Dr_$loOa_ The solid line reprosents the best tit to the experimental Tr values. The dashed line (Ti ) and the dotted line (7-i;) are the contributions due to the fIippin_e motion and diffusion of the deuteriums. respectively.

perimental T;'is good and variation oil” with temperature is shown in fig. 3. The correlation time 7” is in good agreement with the correlation times reported by Cirillo and Fripiat ]3]. and it differs appreciably from 7’ corresponding to the BPP contribution. The relaxation process corresponding to T;'might be thus associated with diffusion of the deuterium nuclei. The BPP-type behavior is typical for quadrupolar relaxation of nuclei jumping between two sites with EFG tensors which are different in magnitude and/or in orientation_ The motional process responsible for T;

may be tentatively interpreted in terms of a 180” flipping of -ODZ moieties_ Such a flipping does not change the dipolar coupling within the -OD2 pairs and it should not contribute considerably to the IH relaxation [ 1 l] _One has also to mention that an estimate of Tl based on deuterium jumps between chemically equivalent sites with different orientations

3.0

3.5

4.0

1000/T

4.5

5.0

5.5

60

[K-‘1

Fiig. 3. Comparison between the correlation times I’ and 7” for the deuterium fIipp@ and diffusion, respectively, in Dr_&oOa (solid lines); the correlation time T in 11 t_saSioOs calculated for Q = - (dash-dotted line); the correlation times T in H 1_64%1003 (dotted line and dashed line) calculated for different values of the parameter Q = (r’)/lM’. The proton data are taken from fis. 5 and S ofref. is].

of the EFC tensors gives the order of magnitude for the deuterium T1 minimum. Clearly, this is only a gross estimate valid for both motional processes discussed above_ If both relaxation processes are associated with nuclear jumping between physically and/or chemically inequivalent sites it is clear that averaging processes are expected for quadrupolar. dipolar and chemical shift tensors. Assuming the splitting of the rigid lattice spectra to be in the order of ~100 kHz for the quadrupolar interaction as estimated from low-temperature measurements. =40 kHz for the dipolar 24-6. 131 and ~2 kHz for the chemical shift anisotropy (lH-0) [3-6], we obtain temperatures in table 1 where the change from the “rigid-lattice” spectra to the motionally narrowed ones is expected. As can be seen, changes in the chemical shift tensor are expected below 140 K. Unfortunately. no reliable measurements of the chemical shift tensor have 359

\‘olumc

96.

number 3

Table 1 Predicted temperatures

-

CHEMICAL PHYSICS LETTERS

for onset of morional narrowing

~. Relaxarion

-__

8 April 1963

-_ Prefactor 50 W

__--

__ .Activation energy (U/mole) ___-

_~. 1.2 x lo-” 1.’ u 10-1s

r; 7-1

-Transition

temperature

‘D quad.

‘H dip.

‘H-o a)

170 90

165 85

140 75

cl;)

-

10 33

‘) Chem~r~1 shift anisotropy.

The

expesrcd

chm~e

the dipolx interaction SS and 165 Ii is indeed

of

be-

t\\ccn protons ax around observed III rhc Jars published b> Cirillo et al. 1-11and Tinct 113 1. I lw c~-awe~~cc of ;1 broad proton line NIIII UIII~ hrrlc s~rumrc below SO K displa-s the rigId IJitice ltncsl~~pc. whcrc all motions are frozen Abwe

7‘~ SO k rhc lS0”

flipping

motion

of the

p.ml\ rbe &polar interactions bcI\\cr’n prorow iii difti-&I pairs. howeWr. the intrapm ultcractlou \\111 not be affected by this flipping. Tbcrcforc .I I’.~kc doublet cm be observed in rbc tempc1~1uic rmgc SO < 7‘< 160 K. At reiiiper~tures .xbove 7‘ = 160 IQ t ius doubler disappears and the proton lmc narrows due IO the diffusion of the protons bctwcen differcut sites. Front the above discussion it f0110ws tliat tlic conwpt of two motional processes as ob taned ironi I tttz 'I> d.u ;I IS in full agreement wirh rhc rcsulrs of tbc proton T, and proton lineshape exprrments in hydrogen mol> bdenum bronzes. AS a m1Iler of fxr 0111y by e\aluaring barb the ?D and ‘H results 561101x 3rriw dI this conclusion_ The u~rcrprctatiun of the ‘D rtlasation in Dt 6hloO; in term of Iwo inotiondl processes of the deuteron mclci appears IO be in agreement with the 11 I

pms

.~\cr.~pes

preliminary resulrs of rhe study of l~1_6hio03 using quasi-elastic ueutrm scattering [ IS]_ The demled results of the ID rclaxstion snd lineshape measurements in various miscd D and 11 bronzes (IiD> MOO,) will

be reported

in a forthcoming

study.

This research V.LLS partially supported by the Department of Enxgy. Diwion of hllirerials Sciences

360

under contract DE-ACOI-EROl19S and the National Science Foundation under grant NSF DhlR 50-20250. Our rbanks are due to Dr. E. Oldfield for his help wirb the ‘D linesbape measurements. One of us (JJF) wants to acknowledge the support of the GA. Miller Visiting Professorship awarded to him by the University of Illinois at Champaign-Urbana.

References 111 I~.:\. Sermon md G.C. Bond, J. Chem. Sot. Farada! Trans. 72 (1976) 730.

121 0. Clernser and G. Lutz, 2. Anog. AlIs. Chem. 264 (1951) 17. [31 A. Cirillo and J.J. I-ripkt. J. Phys. (Paris) 39 (1978) 39. 131 A.C. Cirillo. L. Ryan. B.C. Gerstein and J.J. Fripiat, J. Chem. Phys. 73 (19SO) 3060. 151 R.C.T. Slade, TX. Ilalsstead and P-G. Dickens. J. Sohd Srate Chem. 34 (1980) 183. I61 R.1:. Taylor, L-51. Ryan, P. TindalI and B.C. Gersrein, I. C-hem. Phys. 73 (19SO) 5500.

171 AT_ Xicol. D. Tinet and J-J_ Fripiar. J. Phys. (Paris) 41 (19SO) 423. is1 D. Tmet. P. Canesson. 11. Estnde and J.J. Fripiat. J. Phys. Chem. Solids41 (1979) 583. I91 1I.C. Torrey. Phys. Rev. 92 (1953) 962. [lOI C.J_ Kruger. Z_ Naturforsch. 24a (1969) 560. Ill1 A. Xbragm, The principles of nuclear magnetism (Oxford Univ. Press. London, 1961). 11’1 N. Bloembegen, E.M. Purcell and R.V. Pound, Phys. Rev. 73 (1946) 679. [I31 D. Tinet, Thesis, University of Orleans, France (1962). 1141 U_ Haeberlen, in: Adlances in magnetic resonance, Suppl. 1, ed. J.S. Laugh (Academic Press. New York, 1976). 1151 11. fsuade and D. Tinet. unpublished results.