NMR Study of Cation Motion in Guanidinium Iodoplumbates

Size: px

Start display at page:

Download "NMR Study of Cation Motion in Guanidinium Iodoplumbates"

Lawrence Peters
6 years ago
Views:

1 NMR Study f Catin Mtin in Guanidinium Idplumbates M. Grttel ᵃ, M. Szafrański, and Z. Pająk Institute f Physics, A. Mickiewicz University, 6-64 Pznan, Pland Department f Physics, Academy f Agriculture, Pznan, Pland a Z. Naturfrsch. 52 a, (997); received Octber 6, 997 Prtn NMR secnd mments and spin-lattice relaxatin times f plycrystalline C(NH 2 ) 3 PbI 3 and [C(NH2)3]2PbI4 were studied in a wide temperature range. The rder-disrder character f the existing slid-slid phase transitins was cnfirmed and fr tetraidplumbate a new phase transitin at 70 K was discvered. Fr bth cmpunds a dynamical inequivalence and unusual high mbility f the guanidinium catins were revealed. Fr the rerientatins discvered (NH 2 grup flipping, C 3 in-plane rerientatin and a tumbling) the activatin parameters were determined. Key wrds: NMR, Mlecular Mtins, Phase Transitins. Intrductin Crystals f the general frmulas A M X 3 and A 2 M X 4 where A is a mlecular catin, M a divalent metal and X a halgen, have been the subject f extensive studies fr many years [], The interest in these cmpunds arises mainly f r m the variety f phase transitins they underg, but nt nly. It has been reprted that layered tetraidplumbates f r m the A t M X 4 family exhibit specific electrnic prperties [2, 3]. In these crystal structures the inrganic semicnducting Pbl 4 layers are sandwiched between the rganic layers. Such an arrangement leads t the s-called natural-quantum-wells prviding the pssibility fr mdel studies f size-related quantum effects. A tendency t frmatin f the lw-dimensinal structures is characteristic f bth A M X 3 and A 2 M X 4 crystals, hwever, the type f aninic sublattice depends n many factrs, amng thers, n the kind f mlecular catin [4], Recently, tw new idplumbates, C ( N H 2 ) 3 P b I 3 and [C(NH 2 ) 3 ] 2 PbI 4 cntaining guanidinium catins were synthesized [5]. Bth cmpunds underg structural phase transitins. Calrimetric studies f C ( N H 2 ) 3 P b I 3 [5] revealed tw first-rder phase transitins at 255 and 432 K. The transitin entrpies indicate their rder-disrder character. The crystals in phase I, existing abve 432 K, Reprint requests t Prf. Z. Paj^k, Fax: (486) / 97 / exhibit metastable behaviur. The crystal structure f phase II has been recently determined by X-rays [6]. In this phase the crystal has rthrhmbic symmetry f KCdCl 3 type [7], space grup Pnma. The Pb 2 + ins are ctahedrally crdinated and the Pbl 6 ctahedra are arranged in duble chains with chain axis parallel t the 6-axis. The inrganic chains are separated by guanidinium catins. The crystals f [C(NH 2 ) 3 ] 2 PbI 4 underg a cntinuus phase transitin at 307 K. Calrimetric studies suggest disrdering f the high-temperature phase [5 ]. A very recent X-ray investigatin [8] has shwed that the rm temperature phase has mnclinic s y m m e try, space grup Pc, while the high-temperature phase has rthrhmbic symmetry, space grup P m c 2,. In bth phases the aninic sublattice cnsists f puckered sheets f linked Pbl 6 ctahedra with the layer planes perpendicular t the b-axis. The guanidinium catins are lcated in tw different cavities. The abve results f the calrimetric and structural studies induced us t undertake N M R investigatins f bth idplumbates: guanidinium triidplumbate, C ( N H 2 ) 3 P b I 3, and bis-guanidinium tetraidplumbate, [ C ( N H 2 ) 3 ] 2 P b I 4, (hereafter dented as GuPbI 3 and G u 2 P b I 4 respectively). It was interesting t examine rerientatins f the guanidinium catin in all phases f the cmpunds and prvide an insight int the mechanism f the phase transitins. Fr this purpse measurements f the prtn N M R secnd m m e n t s and spin-lattice relaxatin times were perfrmed in a wide range f temperature. $ Verlag der Z e i t s c h r i f t f ü r N a t u r f r s c h u n g, D Dieses Werk wurde im Jahr 203 vm Verlag Zeitschrift für Naturfrschung in Zusammenarbeit mit der Max-Planck-Gesellschaft zur Förderung der Wissenschaften e.v. digitalisiert und unter flgender Lizenz veröffentlicht: Creative Cmmns Namensnennung 4.0 Lizenz. Tübingen This wrk has been digitalized and published in 203 by Verlag Zeitschrift für Naturfrschung in cperatin with the Max Planck Sciety fr the Advancement f Science under a Creative Cmmns Attributin 4.0 Internatinal License.

2 784 M. Grttel et al. NMR Study f Catin Mtin in Guanidinium Idplumbates 00 I II III 9 i 0 <r Fig.. The temperature dependence f the prtn secnd mment fr GuPbI 3. Experimental The measurements f the prtn NMR secnd mments Mi were carried ut with a wide-line spectrmeter perating at 28 MHz. The secnd mments were calculated by numerical integratin f the first derivative f an absrptin line and crrected fr finite mdulatin amplitude. The measurements f the prtn spin-lattice relaxatin times T\ were perfrmed with a 60 MHz pulse NMR spectrmeter by a saturatin recvery methd. The temperature f the sample was cntrlled t an accuracy f K. Results The temperature dependence f the prtn secnd mment fr GuPbI 3 is shwn in Figure. Its value f abut 7 G 2 registered at the lwest temperatures decreases t the plateau value f 4.8 G 2 bserved between 50 and 20 K. Next it starts t decrease t 2.8 G 2 bserved belw 255 K. At this temperature an abrupt jump f the secnd mment t 2.3 G 2 is bserved, and this value remains cnstant up t 400 K. At 40 K it falls dwn t 0.3 G 2 bserved at higher temperatures. The temperature dependence f the prtn spinlattice relaxatin time fr GuPbI 3 is presented in Figure 2. At lw temperatures up t 255 K a nnexpnential magnetizatin recvery is bserved. The decay curves, decmpsed int tw expnential terms f cmparable amplitudes, yield tw T, values. At 255 K a discntinuity in T, plts is fund and the relaxatin behaviur becmes almst neexpnential, revealing a well defined T, minimum V I. I /T [K ] Fig. 2. The temperature dependence f the prtn spinlattice relaxatin time fr GuPbI 3 ; and : main and weak cmpnents f T,, respectively. Slid line is a theretical fit. 5 Ü 0 5 III T [K] Fig. 3. The temperature dependence f the prtn secnd mment fr Gu 2 PbI 4. f 62 ms at 333 K. The next discntinuity is bserved at 425 K, the temperature abve which magnetizatin recvery becmes again evidently twexpnential. Figure 3 shws the temperature dependence f the prtn secnd mment fr Gu^PbI 4. A fast decrease f the secnd mment frm 6 G 2, registered at 00 K, is bserved up t 70 K. Then a slwer decrease leads t a nearly cnstant value f abut 3.2 G 2 fund abve 250 K. At 307 K the secnd mment falls dwn t I

$N M R Study f Catin Mtin in Guanidinium Idplumbates 37 J f t M 2 00 I 0 II \ / O OO 2 r Jk jk, () where r 0 k and r j m are the distances between respective nuclei, N is a number f nuclei ver which$

3 785 M. Grttel et al. N M R Study f Catin Mtin in Guanidinium Idplumbates 37 J f t M 2 00 I 0 II \ / O OO 2 r Jk jk, () where r 0 k and r j m are the distances between respective nuclei, N is a number f nuclei ver which the sum is taken. Equatin () crrespnds t the rigid lattice, while in the case f mlecular rerientatin the intramlecular part f the secnd m m e n t is reduced by the factr p = (3 cs 2 a l ) 2 / 4, /T,, 8 [K] Fig. 4. The temperature dependence f the prtn spinlattice relaxatin time fr Gu 2 PbI 4 ; and : main and weak cmpnents f T,, respectively. Slid line is a theretical fit..7 G 2, the value which is bserved almst cnstant t 440 K. Figure 4 shws the temperature dependence f the spin-lattice relaxatin time T x fr Gu->PbI 4. A nnexpnential magnetizatin recvery, bserved in the temperature range K, was d e c m p s e d int tw expnential terms yielding tw T, values. At lw temperatures a shrt, weaker T, reveals a m i n i m u m f 240 ms at 38 K, while at higher temperatures tw minima fr the main T l value f 0 and 95 ms can be bserved at 90 and 267 K, respectively. Bth T, cmpnents shw evident discntinuities at abut 70 K. Discussin a) Guanidinium / ( / III r I =! F Triidplumbate T describe mlecular dynamics f the guanidinium catin, the experimental value f the secnd mment must be cmpared with the theretical ne. The latter is calculated by using the Van Vleck frmula [9], which fr a plycrystalline sample cntaining tw kinds f nuclei with spins I and S is given by (2) when rtatin takes place abut the n - f l d (n > 3) symmetry axis ( a is the angle between the internuclear vectr and the rerientatin axis) r by p - 3 sin 2 c cs 2 a, (3) when a mtin arund the tw-fld axis is cnsidered. The intramlecular part f the secnd m m e n t fr the rigid structure f the c m p u n d was calculated fr the standard gemetry f the guanidinium catin [0] with C-N =.32 A, N-H =.02 A, H-H =.77 A, since the X-ray analysis has nt yielded precise hydrgen atm psitins. Cnsidering all prtn-prtn ( / - / ) and prtn-nitrgen ( I - S ) spin-interactins, we btained the intramlecular part f the secnd m m e n t equal t 6.5 G 2. The intermlecular part, calculated fr the rm temperature phase II, gave the very lw value f 0.7 G 2, resulting f r m the lng catin-catin distances in the lattice. Thus the calculated ttal secnd mment value f 7.2 G 2 crrespnds very well t the experimental 6.9 G 2 registered at the lwest temperatures studied. This means that the catins are rigid n the N M R time scale. At higher temperatures a reductin in the secnd m m e n t reflects an nset f rerientatins. Experimental plateau values f 4.8 G 2 and then 2.8 G 2 can be well ascribed t the cnsecutive rerientatins f tw amin grups arund their tw-fld symmetry axes (flipping). Theretical calculatins f the reduced secnd m m e n t fr a mdel f nly ne N H 2 grup flipping give 5 G 2, while a cnsecutive nset f anther amin grup flipping reduces the value f M 2 t 3 G 2. Bth values cmpare well with the experimental plateau values bserved in the lw-temperature phase. The activatin

786 M. Grttel et al. N M R Study f Catin M t i n in G u a n i d i n i u m I d p l u m b a t e s energies fr tw cnsecutive NH-, grup mtins, estimated frm the Waugh-Fiedin frmula [ ], are 8.

4 786 M. Grttel et al. N M R Study f Catin M t i n in G u a n i d i n i u m I d p l u m b a t e s energies fr tw cnsecutive NH-, grup mtins, estimated frm the Waugh-Fiedin frmula [ ], are 8.9 kj/ml and 32.7 kj/ml. An abrupt j u m p f the secnd mment, registered at 255 K, reflects the phase transitin invlving a drastic change in dynamical behaviur f the catins in phase II. T interpret the value f 2.3 G 2 bserved in the phase II we had t cnsider different mdels f the guanidinium catin rerientatins. First we cnsidered C 3 rerientatin f the catin, which was revealed in all the guanidinium salts previusly studied. Such a mtin shuld yield a value f abut 6 G 2. W h e n the C 3 rerientatin is preceded by the N H 2 grups flipping, the M 2 value is reduced t 3.5 G 2, ften bserved by us fr a number f guanidinium salts [0, 2-8]. T get the bserved value f 2.3 G 2 we had t assume a dynamical inequivalence f the guanidinium catins: half f them perfrm the C 3 rerientatin preceded by the N H 2 grups flipping and the rest underg a tumbling prcess, that is an istrpic rerientatin arund their centers f gravity. This statement crrbrates the rder-disrder character f the III-+II phase transitin. At the II ^I phase transitin the secnd mment diminishes t 0.3 G 2. Thugh such a value is very ften ascribed t a self diffussin prcess, in ur case the bserved nn-zer linewidth f abut 0.7 G and very weak internuclear interactins pint rather t the tumbling f all the catins. The activatin energy fr the tumbling f the less mbile part f catins was estimated t be abut 63 kj/ml. Our T experiment, revealing at mst temperatures tw-expnential magnetizatin recvery cnfirms the dynamical inequivalence f the guanidinium catins. Prbably tw types f the catins f different mbility, well separated by P b l 3 anin chains, relax independently t the lattice prducing tw different cmpnents f spin-lattice relaxatin time T v The minimum f 62 ms bserved in the intermediate phase II crrespnds t the A M 2 reductin value f 9. G 2. A similar value we btained theretically by assuming C 3 rerientatin f the catins preceded by the cnsecutive tw NH-, grups flipping. Such a mtin is the mst effective mechanism fr the relaxatin in phase II prducing the T { m i n i m u m bserved. The slwer tumbling mtin f the rest part f catins revealed in the wide-line experiment des nt affect the relaxatin in the phase cnsidered. The activatin parameters fr the C 3 rerientatin culd be extracted frm a least-squares fit t the experimental T, data, described by the expressin T,- =? 7 2 A M 2 [ T / ( +U;2T2) + 4T/( +4u;2r2)]. (4) The fitting prcedure yielded the surprisingly lw activatin energy f 6.8 kj/ml and the preexpnential factr r 0 = _ 2 s. Fr mst f the investigated guanidinium salts the C 3 rerientatin f the catin was hindered by the energy barrier f abut 4 0 kj/ml [ 2-8 ]. A similar value culd be expected fr the cmpund studied if the phase III had existed up t sufficiently high temperatures. The III ^ phase transitin evidently lwers the energy barrier prducing much greater dynamical f r e e d m fr the catin rerientatin in phase II. The drastic change in the dynamical behaviur f the guanidinium catin as well as evident discntinuities in the M 2 and T x values at the III > phase transitin suggest a significant change in the crystal structure f the c m p u n d. b) Bis-Guanidinium Tetraidplumbate The crystal structure determined at 285 and 309 K allwed us t calculate, (), the theretical secnd mment values fr the rigid structure f the cmpund in phase I and II. The ttal values btained are the same in bth phases and equal t 9.7 G 2. Hwever, the calculatins revealed strnger inequivalence f the catins in phase I than in phase II. The intermlecular parts fr the tw inequivalent catins are.76 G 2 and 4.50 G 2 in phase I, while in phase II they d nt differ significantly, being equal t 2.98 G 2 and 3.33 G 2. The value f 9.7 G 2 crrespnding t the rigid structure f the c m p u n d was nt reached in ur experiment. The value f 6 G 2 registered at 00 K prves that a rerientatin has already set in, reducing the secnd m m e n t value. Tw cmpnents f the N M R spectrum bserved at lw temperatures pint t the existence f tw dynamically inequivalent guanidinium catins. A plateau value f 3.2 G 2, bserved abve 220 K, can be well interpreted in terms f C 3 rerientatins f all the guanidinium catins, preceded by the N H 2 grups flipping mtin. At the phase transitin II >I the secnd m m e n t is reduced t.7 G 2. Such a value can be theretically btained by assuming that ne f the catins still underges C 3 rerientatin preceded by the N H 2 grups flipping while anther ne is already submitted t a

5 787 M. Grttel et al. N M R Study f Catin Mtin in Guanidinium Idplumbates tumbling r diffusive mtin. A fine structure f the N M R spectrum in phase I well supprts such a mdel f a different dynamical behaviur f tw crystallgraphically inequivalent catins. T h e nn-expnential behaviur f the relaxatin times bserved at the lw temperatures reflects a dynamical inequivalence f the catins. A decmpsitin f the magnetizatin recveries int tw expnential terms yields tw different T, values. A theretical fitting f (4) t the experimental T x plt with a m i n i m u m bserved fr the shrter cmpnent yielded the fllwing parameters: E a = 0.5 kj/ml, r D = s and A M 2 =.8 G 2. The latter value indicates that the minimum bserved may result frm the flipping f the N H 2 grup in ne type f the catins. The lw energy value shws that such a mtin culd nt be seen in the wide-line experiment perfrmed, since it wuld reduce the secnd m m e n t already at abut 70 K. The main T, cmpnent, decreasing with temperature, yielded the activatin energy f 3.0 kj/ml which culd crrespnd t an nset f C 3 rerientatin f the ther guanidinium catin. At abut 70 K the bserved discntinuities f bth T, c m p n e n t s strngly suggest the existence f a secnd phase transitin III > nt yet revealed by ther methds. The temperature behaviur f the secnd m m e n t crrbrates this cnclusin. In the phase II tw T, minima bserved culd be well described by the fllwing fitted parameters: E & = 3.0 kj/ml, r 0 = 4.8-0~ 3 s and A M 2 = 4.3 G 2 (the lwer minimum), a = 8.4 kj/ml, r 0 = ' 3 s and A M, = 4.9 G 2 (the higher minimum). Bth minima, characterized by such similar values f r 0 and A M 2, can be well ascribed t the cnsecutive nsets f C 3 rerientatins f tw types f catins rerienting with different activatin energies which are lw and cmparable with the value determined fr the triidplumbate. In phase I, T x increasing with temperature yields the activatin energy f 3.4 kj/ml which culd be ascribed t the tumbling f the catins. A cmparisn f the peculiar catin dynamics revealed in bth cmpunds under study with the results previusly btained by us fr a number f guanidinium salts [0, 2-8] shws the rle f the catinanin interactin in determining the hindering barriers. The unusually high mbility f the tumbling catins and the lw hindering barriers fr their C 3 rerientatin appear t result frm the very weak guanidinium catin-idplumbate anin interactin. The mean charge density n the idide ins must be much lwer than n the ther investigated anins. The catin-anin interactin in bth idplumbates must be weaker than in a simple guanidinium idide [2], The cntributin t the hindering ptential frm interinic hydrgen bnding in bth salts under study must als be much smaller than in the salts previusly studied. Cnclusins GuPbI3: a) NMR study has cnfirmed the existence and rder-disrder character f tw phase transitins and revealed a dynamical inequivalence f tw types f highly mbile guanidinium catins. b) Different dynamical behaviur f guanidinium catins was fund in all three phases: phase III - cnsecutive flipping f tw amin grups (Ea= 8.9 and 32.7 kj/ml), phase II - C 3 rerientatin f half f the catins (E. d = 6.8 kj/ml) and tumbling f the rest, phase I - tumbling mtin f all catins. c) III > phase transitin evidently lwers the energy barrier fr the catin rerientatins, triggering much greater dynamical freedm f the ins in phase II. Gu2Pbl4: a) NMR study has cnfirmed the existence f an rder-disrder phase transitin at 307 K and disclsed a new phase transitin at 70 K. b) Dynamical inequivalence f tw crystallgraphically inequivalent highly mbile guanidinium catins was fund. c) Different dynamical behaviur f guanidinium catins was fund in all three phases: phase III - flipping f N H 2 grup with E & = 0.5 kj/ml, phase II - cnsecutive nset f C 3 rerientatins f tw types f catins rerienting with different activatin energies (3.0 and 8.4 kj/ml), phase I - C 3 rerientatin and the tumbling mtin (Ea = 3.4 kj/ml).

6 788 M. Grttel et al. NMR Study f Catin Mtin in Guanidinium Idplumbates Acknwledgements This wrk was partially supprted by the Scientific Research Cmmittee under Grant N. 2P03B [ N. G. Persnage and L. A. K. Staveley, Disrder in Crystals, Clarendn Press, Oxfrd 978. [2] T. Ishihara, J. Takahashi, and T. Gt, Slid State Cmmun. 69, 933 (989). [3] X. Hng, T. Ishihara, and A. V. Nurmikk, Phys. Rev. B45, 696 (992). [4] B. V. Beznsikv and K. S. Aleksandrv, Kristallgrafiya 39, 05 (994). [5] M. Szafranski, Thermchimica Acta, submitted. [6] A. Katrusiak, private cmmunicatin. [7] W. G. Wyckff, Crystal Structures, vl. II, Wiley- Interscience, New Yrk 964. [8] M. Szafranski and A. Katrusiak, in preparatin. [9] J. H. Van Vleck, Phys. Rev. 74, 68 (948). [0] Z. Pajijk, M. Grttel, and A. Kzil, J. Chem. Sc., Faraday Trans. 2, 78, 529 (982). [] J. S. Waugh and E. I. Fiedin, Fiz. Tverd. Tela, 4, 2233 (962). [2] M. Grttel and Z. Paj^tk, J. Chem. Sc., Faraday Trans. 2, 80, 553 (984). [3] A. Kzak, M. Grttel, A. E. Kzit, and Z. Paj^k, J. Phys. C: Slid State Phys. 20, 5433 (987). [4] M. Grttel, A. Kzak, A. E. Kzil, and Z. Paj^tk, J. Phys.: Cndens. Matter, 7069 (989). [5] M. Grttel, A. Kzak, H. Maluszyriska, and Z. Paj^k, J. Phys.: Cndens. Matter 4, 837 (992). [6] J. W^sicki, M. Grttel, A. Kzak, and Z. Paj^k, J. Phys.: Cndens. Matter 6, 249 (994). [7] M. Grttel, A. Kzak, and Z. Paj^k, Z. Naturfrsch. 50a, 742 (995). [8] M. Grttel, A. Kzak, and Z. Paj^k, Z. Naturfrsch. 5a, 99 (996).

Effect of Paramagnetic Ions in Aqueous Solution for Precision Measurement of Proton Gyromagnetic Ratio

Effect of Paramagnetic Ions in Aqueous Solution for Precision Measurement of Proton Gyromagnetic Ratio 240 Bulletin f Magnetic Resnance Effect f Paramagnetic Ins in Aqueus Slutin fr Precisin Measurement f Prtn Gyrmagnetic Rati Ae Ran Lim, Chang Suk Kim Krea Research Institute f Standards and Science, Taejn