Single crystal electron paramagnetic resonance study of rubidium lead hexanitrocuprate (II) at 295 K

SlNCLECRYSTALELECTRQNPARAMAGNETICRESONANCE STUDY OF RUBIDIUM LEAD HEXANlTROCUPRATE

(II) AT 29.5 K

Melvin D. JOESTEN, Shozo TAKAGI Department Arash vil!e.

of Chemistry,

Vanderbilt

f/niversity.

Tennessee 3 7135, L’SA

and

Rcccivcd 2! July 1975 ‘,

The minimum g tensor principal axis is aligned with the c : At 295 ic. g,, = 2.073 and gl = 2.155 for RbzPbCu(h’Oz)b. axis of the unii cell which is along rhc short 0-N bond in the compressed tctragonal CuNh environment.

3. Discussion

I. Introdrlctidn WC recently

reported

that the CuNs

environment

in RbzFbCu(N02); is compressed tetragonal at 295 K [i]. Alihough the crystallographic site symmetry is .Dz,., (mmm) for C.u(II), two of the three Cu-N distances , 2.063(4),

2.176(S),

and 2.166(S),

nre equal

within experimental error. The present communication describes the single crystal electron pzramagnetic resonance

2.

results

for Rk,PbCu(N0,)6

at 295 K.

_&xperimental

Rubidium lead hexanitrocuprate(I1) was synthesized according to published procedures [2]. Crystals were grown by slow cooling #(lo/2 hours) of a saturated aqueous

solution

from 5.5” to 25” C. The

cays-

selected for EPR work measured 0.6 X 0.4 X 0.4 mm. Unit cell axis were identified from precession photographs and the crystal lkrastransferred to a quartz mount and placed in an EPR tube. EPR spectra were obtained with a Varian R-3 ):-band spectrometer equipped with a.Varian E-229 goniometer for rotating the singlecrystal. tal

534

.’

The unit cell parameters of RbzPbCu(NO& from single crystal X-ray diffraction are II = 10.8296(7), 6=10.8196(7),andc= 10.6113(7),& [l].EPRspectra were obtained for both the UC and the ab planes of the crystal in order IO determine the relationship OF the g tensor principal axes to the unit cell axes and to the symmetry axes of the complex. The minimum g value, g,, = 2.073, was obtained with the magnetic field aligned with c, the compressed axis of the complex (fig. la). Themaximumgvalue,gL = 2.155,was obtained with the magnetic field in the& plane. Fig. I b is a representative spectrum for this plane. The value of gl is constant throu& a 180” rotation in the ab plane as expected from the near equivalence ofa and b unit

cell distances and two of the CU-N bond lengths. Previous EPR studies by Harrowfield and co-workers [3,4] on the low temperature phase transitions of K2PbCu(v0,)6 and T12PkCu(N0,)6 were interpreted in terms of a compressed tetragonal environment around Cu(II). The g values obtained for K,PbCu(NO,), at 275 K (g,, = 2.059, gL = 2.147) and for TI~PbCu(NO&

at 291 K (g,, = 2.072,g, = 2.146) are similar to those for Rb,PbCu(NO& at 295 K.

Volume 36, number 4

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PHYSiCS LETTERS

1975

2ll sites in a domain distort in the sarme direction and that the domain boundaties are { 110). We have verified the { 110) boundary in zero-level precession phctographs of R,MCu(NO& with M = Ca, Ba, Pb 15). Crystrtls of Rb,PbCu(NO,), also contain two lattices with one more prominent thz.n the o*&er. Precession photographs of the crystal used in the present study showed weak reflections due to 2 second lattice. E-Iowever, the single crystal EPR spectra at 295 K (fig. 1) show only one magnetic site no doubt because of the line width of the signal attributed to the promi. nent lattice. When the crystal is cooled to 183 K, the presecce of the second lattice is seen in the EPR spectrum (fig. 2). At 183 K the line width is less (60 gauss versus 65 ~NISS), artd there is 2 farger difference in g values (g,, = 2.065, gL 7 2.150). Additional evidence for the non-random occupa. tion of domains is our observation [6-S J that crystals of ~‘~~~‘~~~0~)~ compounds which are ~rthorh~mbit at 295 K (M = EC,M’ = Ca, BE, Sr; M = Rb, M’ = Pb) vary considerably in the amount of second lattice present. The cooperative J&m-Teller effect appears

(b: 295’K

183OK

i u=Z.O65

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Fig. 1. Single crystiI EPR spectra of RbtPbCu(NOZ16 at 295 K. (a) Magnetic field is parallel to c unit cell axis. (b) Magnetic field is in ub plane.

Harrow~e~d and Pilbrow [3] have shown that me ratio of parallel to perpendicular intensities in EPR spectra df K,PbCu(NO& below 281 K is 3 function of temperature, crystal, and uniaxial stress. These resu!ts were zttrihuted to a ddmain structure caused by a cooperative Jahn-Teller effect. They proposed that

Fig. 2. Shingle &ystiJ EPR spectrum of Rb2PbCu(NQ& 183 K. Magnetic Geld is prudel to c unit cell axis of nent Iattice.

at

promi537

‘Volume 36, nirmber 4 .-

to be itrong&,for

CHEMICAL PHYSKR

these ccmmpounds ihanfor

K2PbCu(NO& or Tl~PbCu(N02 j6 since tie former compounds contain distorted CEN, entirotients .at295K.

‘,. -

.’ Acknbwtedgemen

t

Support of this research by the National Science Foundation

~~P-3~~~2X)

!s gratefully

acknowledged..

1 December

LiXTERS

f2j D. Wrm, C. Fri&el and KP. Reetz, J. Solid State Chem. 4 (1972) 103. [3! B.V. Harrowfie!d and J.R. %Ubrow, J. Fhys. C 6 (1973) 755. IdI B.V. Harrois~eld, A.J. Dempster, T.E. Freeman and J.R. P!brow, I. l’hys. C 6 (1973) 2058. ISI S. Takagi; M.D. Jocs?en 211d P.G. Le&eit, Chem. Phyf I.~t?cr~ 34 (1975) 92 161 S. 73&i, PG. Le.nh~-r; and M.D. Jo&en, J. Am. mcm. sac. 96 (1974) &xE M.D. Jozeten and P.G. Zezlhert, Acta Cryst. .V! 2. la&i, 832 (197.5) 596. r‘s1 S. T?.kn$: M.D. Joecten and P.G. Lenhert, Acta Cry%., sulJ~itted kr pubkation.

References 111 S.Takagi, MI). Joesten sot 97 11975) 444

an3 P.G. Lenttert, I. An. CTlem.

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P

‘.

j38..,,';

:.

.:.,:,

1975