Single crystal EPR study of VO2+ in LiCsSO4

J. Phys. Chem. Solida Vol. 54, No. 7, pp. 833-843, 1993 Printed in Great Britain.

SINGLE

CRYSTAL KUM~

cum-3697193 1.00 + 0.00 61993PcrgmonRusLtd

EPR STUDY OF V02+ IN LiCsSO,

Mmni~

ANANTHand P. T. MANOHARAN

Department of Chemistry, Indian Institute of Technology, Madras-600 036, India (Received 11 December 1992; accepted 2 February 1993) Abstrac-A detailed single crystal EPR study of V@+ in LiCsSO, (LCS) has been carried out from room temperature down to 133 K. The room temperature EPR analysis indicates that Vo2+ substitutes for the Cs atom and that there are six magnetically inequivalent Cs+ sites. The charge compensation takes place in two different ways. For two of the sites the charge compensation occurs by removal of a neigbbouring Cs atom at a distance of m4.5 A; whereas for the other four magnetically inequivalent sites the charge compensation is due to removal of a neighbouring Li atom at a distance of 3.5 A from it. The single crystal

analysis yields the following spin-Hamilton&m parameters at room temperature: g,, = 1.987f 0.002, g,, = 1.983f 0.002, g,, = 1.932f 0.002; A, = 7.8 f 0.2 mT, A,, = 7.4 f 0.2 mT, A, = 19.8f 0.2 mT. Around 202K there is a ferroelastic phase transition evidenced by the presence of two chemically inequivalent sites. Keywords: EPR, LiCsSO,, V02+ impurity, room temperature magnetic inequivalence, low temperature ferroelastic.

1. INTRODUCTION

LiCsSO, (LCS) belongs to the same family of crystals as LiKSO, and K,SeO,, which are known to undergo a large number of interesting phase transitions [l, 21. LCS is orthorhombic at room temperature (RT) with space group Pcmn and 2 = 4 [3]. It undergoes a phase transition at 202 K to a ferroelastic phase with space group P2,ln and Z = 4 [4-61. At room temperature the SO, and LiO, tetrahedra are slightly distorted. The coordination polyhedron about the Cs atom is asymmetric and 11-vertexed [3]. The principal structural change in the ferroelastic phase is a rotation of the SO, tetrahedra about the c-axis through roughly 14”. The SO, and LiO, tetrahedra remain practically undistorted; however, the coordination 11 vertex polyhedron of cesium changes considerably [3]. Extensive work has been done on the vanadyl ion in a variety of lattices [7-91, following the pioneering work of Ballhausen and Gray [lo] on its electronic structure. Also, more recently, an exhaustive review of the EPR work on V@+ was made [ll]. The vanadyl ion can exist in a variety of ligand field environments. The V@+ EPR spectrum is additionally very sensitive to the crystalline field environment [12-131. EPR of VO*+ is particularly suited to study the low symmetry effects and is reviewed [12]. It rotates freely in lattices like alkali halides [14, lq, nitrates [16], NH,Cl [17], BaC1,.2H20 and BaBr,.ZH,O [18]. In lattices like alums [19,20], double sulphates [21,22] and (NH4)2S0, [23], however, it is found to have a fixed orientation. It has

also been found to be a good probe for phase transition study [24-271. In this paper we present a detailed single crystal EPR analysis of V@+ in LCS done at RT. Also, our temperature variation study of V02+ in LiCsSO, in the 300-133 K range has shown a phase transition at 202 K. 2. EXPERIMENTAL Single crystals of LCS doped with VO*+ were grown by slow evaporation at room temperature from an aqueous solution containing a large excess of Cs2S04 over Li2S04.H20. to which a small quantity of VOS0,.5H20 (< 1 mole%) was added. The crystals were hexagonal in shape and were characterized by their X-ray powder patterns. The EPR spectra were recorded with the magnetic field (B) in the three othogonal planes ab, bc and cu of the crystal (rotated with respect to B) (axes are as marked in Fig. la) using a Varian E-112 spectrometer operating at X-band and employing a 100 kHz magnetic field modulation. Single crystal spectra were recorded at 5” intervals. The temperature variation (from RT down to 133 K) was carried out using a gas flow type temperature variation accessory. The temperature stability was f 1 K. 3. RESULTS The crystals of LCS doped with VW+ are hexagonal in shape. As shown in Fig. la the crystallographic u-axis is along one edge of the crystal and the b-axis

835

KUMM

836

MURTW ANANTHand P. T. MANOHARAN

perpendicular to it in the crystallographic ab plane. The third axis c is perpendicular to the ub plane and along the principal axis of the crystal. The room temperature (RT) RPR spectra of VW+ in LCS are very complex and show more than 32 lines in any orientation off the axes; however, when the magnetic field is along the axes th9 merge into 16 lines. Figure la, b and c shows spectra recorded along B//a, B//b and B//c, respectively. Figure 2 shows the angular variation plots in the three orthogonal planes

ub, bc and cu. In order to preserve clarity only the outermost lines are shown for sites (I) and (III), whereas for sites (II) the entire EPR spectral variation is shown. The spin-Hamiltonian parameters have been derived by fitting the spectra to the spin-Hamiltonian z = /%!7.g.B+ S.A.J

where the hype&e tensor A is measured in units of (mT) and the other terms have their usual meaning.

IOmT

I I

04

(1)

lom’l:

ig. la and b.

Single cry&l EPR rtudy of V@+ ia LCS (d

a37

D_qPH 1OmT

I 300K

Fig. 1. Vo2+ EPR .spcctrum of LiCsSO, for (a) B//u, (b) B/lb and (c) B//c at room temperature. The field position of the nth hyper6ne line is determined after applying the second order perturbation correction [28,29]

the ma@&

B = Bo - WKIgS)

where

field B g2=a+j3cos20+ysin28,

- [A :(A : + ,4:)/4&g2/12K71(I + 1) - ~47

2a

=g2, +gt,

28

(3)

=
-M2[(A i - A :)‘g$:/2B&/3’Kqsin2e cos2e, (2) 2Y-
where B. = hv/gj3, g and K are as given below and 8 is the angle between a reference axis in the plane and

I

280

300

320

K2g2=a’+B’cos2fl

340

360

B(mt1

380

400

+y’sin28

420

Fig. 2. The angular variation of V@+ in LiCsSO, in the three orthogonal Planes ab, fx aad co at room temperature. Sites I, II and III arc marked on the @WC.For site II all the eight hyPc&nc lines m shown. for sitesI and III only the grst and eighth hy@iac lines are aboiwh.

(4)

838

KUUNMUR~IY-andP.T.MTable 1. NmnericA values of tbc three mat&u I, II and III conqonding I. II and III marka! ill Fig. 2 Situt I II III

g:, 3.784 3.916 3.937

s:z *0.003 *0.067 *0.005

a:, *to.091 f 0.022 f 0.039

l& 3.961 3.794 3.956

to the sites

i& *0.001 *0.067 *0.003

g:, 3.928 3.908 3.734

t(l). (II) and (III) markd here correspond to the same letter@ as in Fig. 2.

where 2a’ =

A:g: + A’_g!.,

2/I?’= (A:g:

- A’_g’_)cos 20+,

2~’ = (A:g:

- A’_g’_)sin 28,.

The quadrupole and Zeeman interactions are ig nored, as these are negligibly small. Following !Ichonland’s procedure [29] for the analysis of EPR spectra the squares of the g and A tensors (as given by eqns (3) and (4)) referred to the a,b and c axes was developed for sites (I), (II) and (III) as shown in Fig. 2. Eight matrices are set up for each pair of (I), (II) and (III), respectively, by permuting all possible signs of the off-diagonal elements. The numerical values of the matrices (I), (II) and (III) for g’ are given in Table 1. The resulting 24 matrices from sites (I), (II) and (III) were diagonalized to give the eigenvahxs and direction cosines. The eight matrices from sites (II) gave consistently two sets of eigenvahks as shown in Table 2. Four of the matrices (II) gave eigenvahks El, and the other four eigenvalues fl,. Matrices I and III each gave only one set of eigenvalues E, and El,, for

all the eight possibk combinations, as given in Tabk 2. The direction cosines indicate that the tensors g and A are coincident for all cases. The analysis of the A tensor has been performed in a similar manner to that of the g tensor. The polycrystalline EPR spectrum was measured by powdering a single crystal of LCS doped with V@’ as is shown in Fig. 3. From the tigure, only one set of parallel and perpendicular lines can he identified ckarly. The g and A values obtained are as follows: g, = 1.935 f 0.002, A, = 19.7 f 0.2 mT,

g1= 1.991 f 0.092 A, = 7.4 f 0.2 mT.

From the characteristics of the EPR spectrum of the polycrystalline sample, it is obvious that all the sites are chemically equivalent at RT. Tabk 2. The two sets of cigcn~ahs obtainabk from a set of eight matiice I, II and III, rqcctively gu 1.934 1.932 1.935 1.930

Fig. 3. Tk EPR pow&r spctrum of LiCa!Q d@

gvv 1.990 I .983 I .972 1.985

g..x I.993 I .987 1.995 I .989

with V@+ at room taarperotm.

Single crystal BPR study of V@+ in LCS

As the crystal was cooled the spectrum remained unchanged down to 250 K. At this temperature the line shape changed suddenly. Figure 4 shows spectra recorded as a function of temperature with the magnetic field B along the c-axis of the crystal. Only the outermost (eighth) hyper6ne line on the high field side of the EPR spectrum of VW+ in LCS is shown on an expanded scale. As can be seen from this figure, at around 250 K, the change in line shape occurs only on the high field side of this line as a wing “w”. The line shape does not change with time, thereby indicating a stable phase. On cooling further, the wing “w” becomes broader. From around 202 K the line shape starts changing also on the low field side. Around 197 K there is a drastic reduction in intensity of the low field side of the line, followed by the appearance of yet another line. We shall designate this line as (VO’+ ),i and the original line as (V02+ ), (see Fig. 4). The (V02+ )a line starts picking up in intensity with lowering temperature, as can be seen clearly form the line shape recorded at 143 K (Fig. 4). Of course, the wing “w” continues to get broader with lowering of temperature. Also, the peaks (V@+), and (V02+),, become more prominent and sharp on decreasing temperature. Figure 5a shows the entire EPR spectrum of V02+ in LCS along B II c recorded at 143 K, and Fig. 5b shows the corresponding spectrum at RT. On comparing the two figures it is apparent that the effect of temperature clearly shows up only in the seventh and eighth lines of the spectrum. The magnetic field separation between these two lines of the (V02+),, species equals that of the spacing between the hyperfhie lines seven and eight of the (V02+ ), species. The angular variation measurements were made at this temperature and the g and A values are found to be the same as at RT. Figure 6 shows the polycrys-

839

talhne (obtained by powdering a single crystal) EPR spectrum of VOr+ in LCS measured at 143 K. Two chemically inequivalent sites can be identified clearly on the high field side (parallel lines) of the spectrum. 4. DISCUSSION The vanadium in V@+ is tetravalent and has the electron con&ration [Ar] 38 which thereby leads to paramagnetism in Vo2+. The s’V nucleus (99.g% abundant) has a nuclear spin Z = 7/2 and a large magnetic moment. Thus, the EPR spectrum of a single crystal doped with V@+ would consist of different sets of octets corresponding to different orientations of V02+ sites in the lattice (as can be seen in Fig. la-c). A prominent feature of the angular variation plots shown in Fig. 2 is the equal magnitudes of projections of all the sites (I), (II) and (III) in the ab plane (at least). This feature clearly indicates that all the sites are chemically equivalent. Of course, the powder spectrum shown in Fig. 3 corroborates this inference. A close look into the matrix elements of the sites given in Table 1 indicates that for both sites (I) and (III) the magnitude of at least one of the off-diagonal elements is nearly zero, while for sites (II) all the three off-diagonal elements are quite large. It can be shown by direct calculation that in a real 3 x 3 symmetric matrix, if one of the off-diagonal elements is virtually zero, the principal values are invariant to the signs of the larger off-diagonal elements. Therefore, as was mentioned earlier, the matrices for sites (I) and (III), respectively, give only one set of eigenvalues Z?, and &a. The corresponding eigenvectors c, and cl,, are given below. Diagonalixation of matrices (II) yields two sets of eigenvalues and eigenvectors. For sites (II) four matrices yield eigenvalues E’,, and the remaining

Fig. 4. The temperature variation of the outermost hypedine line (on the high field side) of V@+ in LiCsSO, along B//c.

Kubr~ MuaTIPYANANT~and P. T. MA~~HXRAN

84

6)

DPPH

143K

DTPI

O-9 1OmT

(V02’)

’

(VO2’),

-!i -_- _

Fig. 5. Single crystal EPR spectrum of Vu+

four Ef,. The corresponding c$ are given below.

in LiCsSO, along B//c at (a) 143 K and (b) at RT.

direction cosines ci, and

kO.8995

20.0103

f0.4362

cl

f0.3627

+0.8518

kO.3777

c’;,

kO.3097

kO.8926

iO.3274

c$

f0.1824

kO.039

f 0.9832

cIll.

Next, we try to correlate these direction cosines with atomic directions in the crystal lattice. It is found that the principal direction cosines ci and ci, make a small angle with the Cs-Cs and Cs-Li directions in

the lattice, whereas cul makes a small angle with only the Cs-Cs directions. The direction cosine cl makes larger angles as compared to these (ride infru for details). We therefore take c’;, and the corresponding eigenvalues Eh to belong to the correct set. Also, for sites (I) and (III) we already have four correct matrices each with eigenvalues and direction cosines (corresponding to the principal value gzz) as (E,, c,) and (E,,,, cm). Now, the four correct matrices of each set reduce to two in any of the planes ab, bc and ca because there is a twofold symmetry of the crystal along the czysta.Ilographic axes, so that we obtain six (2 + 2 + 2) magnetically inequivalent Cs+ sites at RT.

Single crystal EPR study of VW+ in LCS

841

Fig. 6. The EPR powder spectrum of LiCsSO, doped with V@+ at 143 K. Tabk 3. The spin-Hamiltonian parameters g.and A at RT Principal

VdWS

Principal values

P

A

g,, = 1.987 g,, = 1.983 g,, = 1.932

Direction cosines n

A, = 7.8 f 0.2 mT A,, = 7.4 f 0.2 mT A, =’ 19.8 f 0.2 mT

0.7452 -0.5595 0.3627

b

c

-0.5085 -0.1250 0.8518

0.4313 0.8193 0.3777

Site I g, = 1.935, g, = 1.991; A, = 19.68 mT, Al = 7.41 mT.

The spin-Hamiltonian parameters g and A are given in Table 3. As can be seen from Table 3 both g and A obtained from the single crystal analysis are axial; this is also indicated by the powder spectrum taken at RT. A comparison of the spin-Hamiltonian parameters for VO*+ in different host lattices is given in Table 4. Our spin-Hamiltonian parameters agree well with these. ,In all these crystals Vo2+ has an octahedral symmetry. Hence, it can be inferred that the symmetry of VO*+ in LiCsSO, is also octahedral. At this stage an attempt was made to correlate the direction cosines of the g and A tensors of the various sites with the RT crystal structure [3]. The number of molecules per unit cell is four. Figure 7 shows a projection of four unit cells in the ac plane. All Table 4. A comparison of the spin;Hamiltonian parameters for V@+ in d&rent single crystal lattices Principalg Matrix LiCsSO, ZnSO,.‘IH,O M?iWH+MfQh GO, 3CdS0,.8H20

VdUCS

1.935 1.991 1.940 1.977 1.914 1.979 1.925 1.973 1.912 1.975

FGlcipalA values

(mT)

19.68 7.41 20.3 8.0 20.21 7.93 19.72 7.41 21.08 8.29

Reference Resent PI PI VI

171

possible Cs-Cs and Cs-Li directions within eight adjacent unit cells are considered as probable V = 0 directions. The shortest Cs-Cs distances e4.457 A areC$-Cs~, C&Cs,B; these makes an angle of *3” with the c-axis of the crystal. These Cs-Cs directions match well with the direction cosines C,, obtained from EPR, and the two sites (III) are indicated in Fig. 7. The other Cs-Cs pairs are broadly classified into one set at k: 7 A and another at % 10 A. The Cs-Cs pairs at x 10 A are connected through Li at 3.6 A and oxygen at % 5 A (see Fig. 7), while the pairs at a7 A have no atoms in between. The orientation of VC?+ site depends on the ease of removal of a cation. The closest cation is Li+ at * 3.5 A and hence the orientation of the V = 0 sites is more probable along these directions. Using eqn (2), the angular variation of the hype&e lines was generated for the direction cosines of various Cs-Cs pairs considered above. A good match between the calculated and experimental points was found for the Cs-Li-O-Cs direction while other directions of Cs-Cs at a7 A and G-0-G at k 10 A did not give any match. This, we feel, indicates that the site occupation of vanadyl is along the Cs-Li-OCs direction rather than other Cs pairs separated by longer distances (above 5A). Within this set the Cs-Li direction is about 3” off from the Cs-Cs direction. The simulation also makes it clear

842

KUMAR~ Mv~nnr

ANANTU

and P. T. MANOHARAN

Thus, the EPR analysis clearly indicates that the VW ion substitutes the Cs+ ion in the lattice and charge compensation occurs by the removal of a neighbouring Cs+ ion at a distance of 4.5 A for two of the sites, whereas for the other four magnetically inequivalent sites the charge compensation is due to the removal of a neighbouring Li+ ion located at a distance of B 3.6 A. At room temperature the Cs+ ion is surrounded by 11 oxygens whose bond lengths vary from 3.2 to 3.7 A [3]. Therefore, as such, the symmetry around Cs+ is quite low. The EPR studies using vanadyl ion as a probe in this lattice have clearly established that the g and A tensors are axial. Also the analysis given above has shown that VO*+ substitutes for a Cs+ ion A IO n w w and charge compensation takes place by removal of OCwium 0 Oxygen 0 Lithium a Li+ or another Cs+ ion. It, therefore, means that Fig. 7. Projection over four unit cells of the crystal of after introduction of V02+ ion into the lattice of LiCsSO, at RT in the uc plane. Large circles indicate Cs, LCS, the environment around the V02+ ion resmall circles indicate ti, medium size for oxygen. The unit cell numbers 14 Correspond to translations (O,O,O), arranges itself to provide axial symmetry about the (-a,O,O), (-a, -b,O), (a, 40); S-10 to (O,O, -c). molecular axis at a Cs+ site. The minor deviations (-GO, -c), (-a, -b, -c), (a, -b, -c), (O,O, -2c), (-a, 0, - 2c) referred to in the text. The possible V@+ site between crystal and EPR direction cosines is also in orientations are shown by the joining of atoms. agreement with this possibility. It is a well known fact that in vanadyl containing complexes inclusive of solutions, the V’+ is surrounded by a highly distorted that the V = 0 site orientation is along the Cs-0 octahedron of oxygen ions [30]. One of these is much direction rather than the Cs-Li direction. This is closer than the others defining the axial direction for probable because of the correlation of the remaining the V02+ ion; the remaining ions and oxygens charges (after removal of Li+) towards the Cs-O provide the C;, point symmetry. An attempt was present in the lattice. Summarizing, there are six made to locate a plane of four oxygens perpendicular possible sites: two along Cs-Cs at 4.5 A and four to one of the V = 0 directions in the LCS lattice. The along the Cs-Li-O-Cs directions. The principal four oxygens o;-O’~~ form a plane with large directions of these sites are given below. For sites (I) deviations, and hence cannot form a perfect square to the only possible Cs-0 directions that generate the provide C,, symmetry, unless rearrangement of experimental field positions in the three planes are oxygen atoms around Cs+ takes place upon introduction of V02+ into the lattice 191. Although it is -0.8942 0.0 0.4477 Cs$-LiFa;-Cg difIicult to correlate equatorial oxygens, it is possible -0.8942 0.0 -0.4477. to compare axial oxygen directions with the EPR g,, Cs:*-Li~-Cs: directions. The g,, directions for sites (I) and (II) Similarly for sites (II) the only possible Cs-0 direcrespectively, make an angle of l.OP and 3.7” with the tions that generate the spectra in the three planes are: Cs-0 directions, whereas for sites (III) the g,, direction makes 2.2” with the corresponding Cs-Cs directions. 0.4087 -0.8173 -0.4063 Cs:-O&Li~Cs~ EPR spectra of V@+ in LCS at 143 K shown in -0.4087 -0.8173 -0.4063 Cs$-Li!*-Csp Fig. 5a and b clearly indicate that the lines marked - 0.4987 -0.8173 0.4063 (V02+)i correspond to the RT phase, whereas the cs$-o-Lil+zs,’ lines marked (V02+),, correspond to a chemically 0.4087 -0.8173 0.4063. Cs$-Lif*i-Cs: inequivalent site with almost the same hype&e splitting. There is negligible change in the hyperflne For sites (III) the only possible Cs-Cs directions that splitting with temperature. The g and A tensors for generate the experimental spectra in the three planes these two chemically inequivalent sites are equal are: within experimental error. It was not possible, however, to obtain any quantitative information on -0.1456 0.0 0.9893 Cs:_cs~ the orientation of the g and A tensors below 202 K. The change in the spectrum at 202 K definitely -0.1456 0.0 -0.9893. cs:-Cs,”

lo

I

843

Single crystal EPR study of V@+ in Lcs cxu-responds to a phase transition

in which there are two chemically inequivalent VW+ sites, also corroborated by the powder spectrum recorded at 143 K. Two chemically inequivalent sites are clearly seen on the high field parallel lines shown in Fig. 6. Such chemically inequivalent species for V@+ have been observed in the monoclinic A2/a phase of triammonium hydrogen disulphate [24,26]. LCS is known to undergo a transition to a phase that is monoclinic, in which there are two ferroelastic domains. Therefore our EPR results below 202 K clearly indicate the presence of two ferroelastic domains in this phase. As has been indicated by Fujimoto and Sinha [24] these two domains arise because of the induced strain in the ferroelastic phase of LCS. The change in line shape at 250 K does not correspond to any known phase transition of LiCsSO, [4-61. The origin or the wing “w” is not clearly understood. Gesi and Osaka [31], Gesi et al. [32] and Suzuki et al. (331have suggested that it is due to an incommensurate modulation of the crystal structure. We are studying the temperature dependence of Mn2+ in LCS, as the crystal field terms D and E are sensitive functions of temperature [34], and so may provide conclusive evidence for the origin of the wing “w” seen below 250 K.

REFERENCES 1. Murthy K. and Bhat S. V., J. Pl?ys.C.: SolidSturePhys. 21,597 (1988) md reftherein. 2. Aiki K., J. Phys. Sot. /@I 29, 379 (1970). 3. Krunlik A. I.. Samonov M. A., Zhckzin E. P. and Bclov N. q.., Sov. ghys. Dakl. 24, ti (1979). L. I., Iskornev I. M., 4. Alcksandrov K. S., Zhcrebtsova

Kruglik A. I., Rosanov 0. V. and Flerov I. N., SGV. Phys. Solid Stute 22, 2150 (1980).

i P. E. and Lukaszcwia K.. 5. Pietraszko A.. ToPhase Transi&ns 2, 141 (1981). 6. Gzcki H. and Sawada A., J. Phys. Sot. Jpn. 51,2047 (1982). 7. Kasi Viswanath A., J. Chem. Phys. 67, 3744 (1977). 8. Jain A. K., J. Phys. Sot. Jp. 46, 1250 (1979) and

references therein.

9. Misra S. K. and Char&u

10. 11. 12. 13. 14. 1s. 16, 17. 18.

5. CONCLUSION The single crystal EPR study of V02+ in LCS has shown that VCY+ substitutes for Cs+, and that there are six magnetically inequivalent Cs+ sites at RT. For two of the sites the charge compensation is along the Cs-Cs direction, and for the remaining four along the Cs-0 direction. The very strong axial field from the oxygen of (V = 0)2+ manages to impart an overall axial symmetry despite the low symmetry of the surrounding oxygens which are bonding by weak ionic forces and the VO*+ moiety manages to orient itself along the near neighbour, next near neighbour charge compensating vacancy. The temperature variation study clearly indicates that there is a phase transition a202 K to a phase with two fmoelastic domains.

Acktwwkdgements-KMA

would like to thank the CSIR

for previding Bnancial support. Thanks are due to Dr G. V. R. Chandrameuli and Pref. Submmiinian of R.S.I.C. for their many helpful discussions. KMA would also like to acknowledge the asseeiation of Dr A. Kasi Wsawanath during the early stages of this work.

19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34.

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