Paramagnetic resonance study of the phase change in sodium azide

Volume 35, numbPr 2

PARAhlAGNETIC FJ.

CHEMICAL

PHYSICS

LETTERS

1 September

1975

RESQNAiiCE STUDY OF THE PHASE CHANCE 5’4 SODIUM AZIDE

OWENS

Feltnm

Research Lobcratory, Nmtinny

Arsenal, Dover, New Jersey, USA

Received 14 April 1975 Revised manuscript

received 26 May 1975

The EPR ofthe 6S 5,3 ground state of vacvlcy compensate3 divaleut manganese is used as -2probe to study the detailed charges that occur when the crystal changes phase from a rhombohedral to monoclinic unit cell at 19°C. Within the kl”C temperature control the zero-field splitting vties near Tc 2s (Tc-T)“’ consistent with mean field theory and Rarmxn meawreztents of the soft mode. A marked reduction of the intensity of the resonances is observed near Tc as the temperature approaches Tc from above or below. No broadening or change of c&ape of the resonaxes is observed to be correlated with this intensity reduction. Power saturation studies of the spectrum at different temperatures near Tc suggest that the time. reduction in line intendty may be due to 1 len~~ech7g of the spin-izttice retition

troscopy

[3-6].

Softening

modes has been observed

of important ad

vibrational

provide data to which

The spin hamiltonian parameters, such as the 2xial zero-fkld splitting of transition metal ions, provide a sensitive probe of the strength 2nd symmetry of local

the EPR data can be correlated.

crystal fields in the lattice. Often information about the coupling of the icn to lattice vibrations can be ob-

2. Crystal structure and EPR of Mn2+ in NaN,

tained throu,#~ meaurements of the temperature and the pressure dependence of the spin hamiltonian p2rameters. The electron paramagneiic resonance (EPR) spectrum of such ion; can ‘herefore provide a means of study&g the dettiled structural and potentA changes that accompany a phase change in a crystal. Sometimes c&k21 fluctuations near the trvlsition temperature are manifested in the EPR spectrum. In this paper the detailed nature of the rhombohedral to monoclinic phase change of sodium tide (NaN3) as reveded in the EPR spectnrm of vacancy compensated divalent manganese in the iattice is studied. The I&N, : Mn2+ system is particularly suited to such a study. The EPR spectrum of Mn2+ has been c!xxacterized in the rhombohedral phase [1,2]. The large axial zero-field splitting associated with the low cry&l symmetries of NaN, rnake the zero-field splitting sensitive to subtle changes that occur at the phase change. The phase change has been studied by other techniques such as X-r2y diffraction and Raman spec-

The unit cell of sodium 2zide, shown in fig. 1, is rhombohedral at room temperature 2nd contains one linear azide ion per unit cell. The ion lies aiong the [l 111 direction of the unit cell. The sodium ions lie in planes parallel to the (111) plane and the lines joining any three nearest neighbor sodiums form an equilateral triangle in this plane. Below I9’C the unit cell chenges symmetry from a rhombohedral cell to a monoc!inic celI. There are slight vxiations in tha literature of the transition temperature. R2m2R studies indicate 2 temperature of ! 5°C while the temperature from an X-ray diffraction study is reported 2s 13°C [3,5]. The new phase involves a distortion of the normally equilateral triangles of the sodium ions lying in the (Ill) plane of the rhombohedral unit cell to Isosceles triangles with a shearing of the sodium layers. The linear tide ion also tilts from the [l 1I ] direction of the rhombohedral cell. The previous studies indicate that the transition is second order and reversible [3-S 1. The manganese ion dopes itito the lattice in three 269

CHE!MCAL PHYSICS LETTERS

Volume 35, rmmber 2

1 September 1975

ganese acetate (l&s than 0.2% by weight). The temperature dependence of the EPR spectrum arisi?g from ffie vacancy compensated manganese is used to probe the phase transition because the larger magnitude of tSe D value makes it possible to rnoktor the highest ~tnd lowest field resonances without complications arising from resonance lines of the other manganese centers. The temperature control in the experiment ~2s k 1 .O degrees. When the dc magnetic field is parallel to [I 11) and thus perpendicular to the $&es of the sodium ‘&iangtes, the magnetic field makes the same angle of 90 degrees with the different manganese-vacancy axes, and thus Fig. 1. The rhombohedml unit rcli of Ntis is shown Also tilustr;ited is the sffect of the phase change on the triangks in the (1 I.1) plane of the ;hombohe&ral unit eelI made up of lines joining the .xztngiinesr:ion, “tie compensating vacancy end the sodium ion_

different forms: each having a distinctive EPR spectrum [I}. lnititiy xfter doping the EPR spectrum _ ccnsists of one broad resonance centered around the free-electron

g valne and which

is believed

tc arise

from colloidal manganese. When thz crystal is heated to 200°C the broad resonance changes to the characteristic EPR of the 65 5,2 ground state of MI?+_ The Mn ion is substitutional for the Na ion and two different EPR spectra are observed. A spectrum having an axial zero-field splitting of 257 G at room tempera. ture is due to Mn2* iking from MnZC compensated by 2 nearest neighbor so&urn vacancy. The z axis of the spin hamiltonian of the center lies parallel to the

vacancy--MrF axis ti the (Ill) plane of the rhombohe&al unit cell. In the (111) plane there are six possible directions for the z axis related to each other by a 60 degree rotation about the [ 11 l] direction. The EPR distinguishes only three spectra when the magnetic Geld is in the (Ill) plane of the rhombohedral unit cell. A spectrurr, wiih a smaller axial zero-field splitting of 240.0 G at room temperature is due to Mn2+ not eomper.sated by an nearest neighbor so.dium YSCailCY.

3. Expezimensal results The sodium azide crystis are grown from saturated aqueoiis solutions containing tnce amounts of man-

270 :

only one manganese of five sextets.

When

spectrum

is observed

the temperature

consisting

is decreased

below 19°C with the mapetis field parallel to [I! 1 ] each resonance of the spectrum splits into two resonances. This can be understood in terms of the distortion of the triangles of sodiums from equilateral to isoscefes, illustrated in fig. 1. The splitting occurs because in the new phase there are two different Mnvacancy separations in the planes of the sodium ions, and these different types will have slightly different axial zero field splittings. Referring to fig. 1 illustrating the triangles of the sodium ions, in the rhombohedral phase AB = BC = AC while in the monoclinic phase AB’ = AC’ < BC where AB > AB’ and BC’ > CB. Thus for a manganese at A compensated for by a vacancy at C or B the zero-field splitting increases in the monoc~ic phase effectively because the Coulomb perturbation is increased because the vacancy has moved closer to the manganese while for Mn at C compensated far by a vacancy B the vacancy moves further away and the zero-field splitting decreases En the lower symfnetry phase. Clearly the change in the potential due COthe change in the symmetry of the rest of the lattice is also important.

In fig. 2 the temperature dependence of the axial zero-field splitting near the transition temperature is plotted. Tine plot is r”or the Mn vacancy center in which D imxesses il the monoclinic phase. An unusGa1 decrease in the intensity of the resonances is observed as the temperature approaches the transition temperature from either above or below. A plot of the relative intensity of the hrghest magnetic field resonance tising fiOI3 the MS = 312 to 5/2 and mr = 512 transition versus temperature near the &msition temperature is given in t7g. 3. Although the data has been extrapolated

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35,

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CHEMICAL PHYSICS LETTERS

Fig. 4. A plot of the relative intensity

1 September 1975

of the high fieki

reso-

rwrrcevcxus curves

F&. 2. ‘TXe curve kbelled P is a plot of the temperature dcpendence of the magnitude of the axial zero-field splitting on temperature Ned T,. The line labelled b is a plot of D versus (T,-T)“2.

20 SEGREES

30 (‘2)

Fti. 3. The vxridion of tie reiative intensity of the resonanc;e on temperature near T,.

hgh

cident microwave power was investigated at a number of different temperatures. It was never possible to saturate the resonances with the power available from the klystron of the Varian E-3 spectrometer used in this experiment. At all power levels from 0 to 200 III%’ the resonance intensity increased with increasing power, and thus it was not possible to determine whether the lines were homo’geneously or inhomogeneously broadened. In fig. 4 is plotted the resonance intensity versus microwave power for the highest field resonance at i24’C and +50°C. The increased scatter of the data at t24”C nearer the transition temperature where the line -Sensity is strongly dependent on temperature is probably associated with the 21°C temperature control.

I

1c

microwave power a: c24’C ad c50°C. For both the g-k setting of the spectzometer was the same.

field

4. Discussion and analysis to Zero intensity at T, it is not clear from the measurement that the intensity becomes zero at T,. The line width of-the resonancts measured from peak to peak on the derivative of the absorption was 2 1 .O 2 1.0 G. A detailed study of -Jle line width near the critical temperature indicated that no broadening af the resonances is associated with the intensity reduction. The shape of the resonances is gauss&-Qand no change of shape is observed as the tempersture approaches I”,. In order to obtain further information about the nature of the intensity reduction near T, and to asses ifit is a result of a change in *he spin--lattice rel5ZXatioiE time, rl, tie intensity of the resonances versus in-

Normally ii1 the absence of a &se change, the zero-field splitting in the room temperature range would be linearly dependent on temperature. This is confirmed, for example, by studies of the temperature dependence of the zero-field splitting of Mn2+ in RbN3 and TIN, where there is no phase transition in the vicinity of room temperaWe [7,5]. In these systems the temperature dependence of the zero-field splitting is primarily due to the influence of lattice vibrations and to a lesser extent lattice expansion efFects. In RbNg : M$+, for example, it was estimated that 83 .CE of the linear temperature dependence of D was due to

271

vibraticn effects [7]. The temperature dependen= observed here is clearly anomalow and associaicd with the phase transition. The line labelled b in fig. 2 is a plot 0fD vtrsue (T,- T)“* near the transition temperature indicating that within ‘Jle accuracy of the measurement that the variation 0fD on tem-

lattice

perature near T, is consistent with mean field tleory. Houiever it should be pointed out that the -1°C tem-

perature control is not sufficient to observe smail deviations from the l/2 exponent dependence. Raman mexurements of the frequency of the librational mode (essentially an oscillation of the tilt angle of the linear ion) in the monoclinic phase do show a softening of the mode with a (T,- T)"* dependence [5,6 j. X-ray diffraction studies also show splittings of the diffraction components that depend on (T,- T)“’ [3]. Broadening and reduction of the intensity of resonances near T, have been observed in other materials

such as the titanates [9,10]. Broadening of the resonances of the Fe3+-vacancy

1 September 1975

CHEMICAL PHYSICS LETTERS

Volume 35, number 2

complex

in SrTiCI3 has

enough to be comparable to the EPR splitting. The Rzman data shows that the librational mode of the azide ion, w_tich is believed to be cqupled to the shearing mode, decreases in the _monocknic phase to 2 value of 122 cm-l at T,, orders of magnitude above the EPR splitting [4,5]. There is of course the possibility that other modes which do not involve an oscillation of the Mn2+-,cancy’3xis.soften

completely

to

zero and have not yet been observed. In some systems, such as Gd3’ in EaTi03, a marked reduction of the intensity of the resonances was observed without an apparent broadening of the line width but no adequate explanation of the effect was presented 191. The study of the intensity dependence of the rescnances on power as a function of temperature in this experiment given in fig. 4 can be subjected to 2 s5miquantitative analysis that is suggestive. The line intensity of the resonances is related to the incident power by an expression of the form [I 11 I = &‘*/(l

+bP) ,

0)

been observed near the phase transition and attributed to local fluctuations of the Fe-vacancy axis with respect.to the dc magnetic field, ark-hg from a softening of vibrationai modes involving motions of the @and oxygens about the defect [l!I]. Such a broadenin,0 effect may occur rvhen the oscillation frequency of the defect axis is o!‘the order of magnitude of the EPR splittings. In this experiment the resonance line

where b is proportional to the product tl t2 and the constant LIis only proportional to r2. When the data of fig. 4 is fit to eq. (1) it is found that b is much larger for the data at +24”C than for the data at 60°C. Since there is no variation in the line width or line shape as Tc is approached the increase in b must be due to an increase in tl . Because the resonances were never saturated the above analysis cannot be con-

width was investigatecl as a function of temperature with the dc magnetic !ield along one of the edges of the equilateral triangies of the sodiuin ions in the (I 11) plane of the rhombohzdral unit cell which shorten on going to the monoclinic phase and slightly change direction in the (111) plane with respect to the applied

sidered deftitive. For the direct relaxation process in the the temperature of the phase transition in wiLl be proportional to C/w4T where w is frequency 1121. If this phonon frequency

magnetic field. Referr3g to fig. 1, if the dc magnetic field were along AC in the rhombohedrai phase, after the phase.transi:ion AC would no longer be along the direction of the applied mametic field. Thus if a softening of a mode involving a shearing motion of the sodiurns occurred one m&It expect to observe motiona! broadening of the EPR line near Tc due to the fluctuations of the Mn2+-vacancy axis with respect to the dc magnetic field. Suck an effect was not observed. The absence of this broadetig in the line width near T, may suggest that the normal mode that softens does not involvk 2 shearing motion of the sodium ions or if it does, it does neat so&n to 2 frequency low 272

range of NaN,, rl the phonon

is softening (approaching zero) near T, than an increase in the spin-lattice relaxation time could occur provided all other factors contained in the constvlt C, such as the velocity of sound,.and the strain constants are invarient. In anclusion the EPR study of +he phase trar?sition of NaNg usbg the Km2’ ion 2s a probe supports the second order nature of the phase transition. Although not conc!usiv+,

the unusual reduction

in *&e intensity

of the resonan~s near T, cam be shown from an analysis of the line intensity versus power data at different temperatures to be due to an increase in the sph-Iattice relaxation time. Although such an effect is consistent with a softer@ of phonon frequencies under the assumption of the direct relaxation process, further

Volume 35, number 2

CHEMICAL PHYSICS LETTERS

i September 19’75

work must be done before a definitive explanation

13J G.E. EYisgle ;md 3.E. No&es, Actrr Cryst. I324 (1968)

cm be offered.

262. t41 2. Iqbzi ;md C!.W,Ckistoz, Solid State Commun., to be

The author would like to acknowiedge ~~ions~~i~ Dr. 2. lqbal of this hboratory nature of the phase change in NaN, _

fhitful d.ison the

Reference-s [l] J.G. King and B.S. MiUes, J. Chem. Phys. 41 (1964) 28. [2] B.S. MiJler ;md G.J. King, J_ Ckm. Phys. 39 (1963) 2779.

puSliskpA* 2. Iqbal, J. Chem. Fnys. 59 (1973) 1769. ‘G.J. Simonis and C.E. H+fhway, Phys. Rev. Bf0 (19’74) 4QLo. 171 F.J. Otvem, Pbys. Rev. Bl(l974) 76. VI F.J. Owem, 5. Chem. Fhys. S? (1972) 2349. 191 L. Rimti and G,A. de Mars, Phys. Rev. 122 (1962) 702. rw Tfi. van W&kk.k. KA. MuUer md W. Bertinger, Fhys.

Rev. B7 (1973) 10.52. T.G. Chtnt?;, Phys. Rev. 11.5 (L959) 1506. R. OrSach rendH.J. Stapleton, k: Etectron parma.+ netic resonmcq ed. S. Geschwind (Ptcnum Press, Nevv York, 1972). A. Lzubenv and R. Zwek. 2. Naturforsc&. 25A (1570) 391.

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