Chapter 2

EXPERIMENTAL TECHNIQUES

A brief discussion on EPR instrumentation, crystal growth and crystal struchve

of host lattices has bccn discussed in this chapter. The evaluation of spln Hamiltonian

parameters such as g and hyperfine coupllng constant tensors from EPR spectra and

procedure for calculating the direction cosines have also been mentioned here The

EPR-NMR program, used to evaluate the spln Hamiltonian parameter and SimFonia

program, for s~mulat~on of EPR spectra have also been dlscussed briefly. The

procedure to calculate spin-lattice relaxation tlmes (TI) from the line width of

paramagnetic Impurity, when a paramagnetlc Impurity is doped In paramagnetlc host

Iamce, has also been dlscussed briefly In thls chapter.

EPR study of magnetrcally concentrated complexes provide the fragmentary

information about the paramagnetic center, due to dipolar and exchange Interactions

between the nelghborlng paramagnetlc species and relaxatlon processes, namely spin-

spin (Tx) and spln-lattlce relaxation (TI), hence rt has been discouraged. The relaxatlon

process, one of the resonance lines broadening mechanism and being temperature

dependent, can be m~nlmized when the temperature of the sample IS reduced from

room temperature The dipolar and exchange interactions, dependent on the d~stance

between two paramagnetic centers, are concentration dependent Hence, these

interactions can be kept under control when paramagnetlc specles are studled under

magnetlcally dlluted conditions. However, in solut~ons, this condition is readily

achieved at reasonably low concentrations In sohds, there are two common methods.

One of the methods is to dope the paramagnetic impurity in a diamagnetic

lattice or psramagnmc lattice, which is EPR inadive at room temperatum. If the

dopant concentration is quite low (of the order of 1 mol % or less), the statistical

disbibution of the dopant centers is sufficiently low enough to achieve the des~red

isolation. This leads to well-resolved EPR spectra, free from dipolar and exchange

effects and the treatment of the system as independcni centers 1s quite justified.

Such dop~ng is quite common with trans~t~on metal complexes In these cases,

the dopant substance does not necessar~ly have the same crystal structure as the host

lattice. Exper~ments show that the environment of the host is invar~ably imposed on

the dopant, thus makag it possible to study dopant complex under circumstances that

does not extst In ~ t s pure form

The other method is to produce paramagnetic centers In a d~amagnetic lattlce.

The most common method of atta~nlng thls is radiation damage uslng UV, X-ray or

y-rays This techn~que may be employed In pure host lattices or doped host latt~ces.

However, we are not deal~ng the paramagnet~c centers of this type produced by t h ~ s

techn~que In the present thesis Hence, no further details are mentioned

Instrumentation:

The deta~ls of instrumentabon and measurement techniques have been

discussed extensively in the literature [I-41 The schematic d~agram of an EPR

spectrometer 1s shown in Flgure 2.1 From the resonance condlt~on hv = gPB, 11

follows that the EPR spectra can be measured by fixed frequency and vanable tield or

vise-versa; it is always convenient to follow the former procedure. Depending upon

t i bues rns

sod i___

; Flcordn , Au&o mrsp

gsncraror

F~gure 2 1 Block d~agram of a typical X-band EPR spectrometer

the irradiation ffWIency, EPR spectrometers are classified as S, X K and Q band

spectromctcrs, the most common ones an X and Q band spectrometers. At X-band,

the hquency is normally around 9 GHz, with free-electron resonance field at -320

mT, while at Q-band the corresponding values are 35 GHz and 1,250 mT. The

approximate hquency ranges and wavelength of these bands are given below:

Bands S X K Q

Approximate Freguency (GHz) 3 9 24 35

Approximate Wavelength @m) 90 30 12 8

Approx~matc Field (mT) for g = 2 110 320 850 1250

lngram [I] and Poole [2] have given detailed descriptton of the basic principles

lnvolved In EPR instrumentation In order to observe a well resolved EPR spectrum,

one has to operate the spectrometer under optimum cond~ttons of mtcrowave power,

modulatton arnpl~tude, spectrometer gain, filter ttme constant, scan range and scan

tlme. The work described in thts them has been carr~ed out on a JEOL JES-TE100

ESR spectrometer operating at X-band frequencies, havtng a 100 kHz field modulation

to obta~n a first derivative EPR spectrum. DPPH, with a g value of 2 0036, has been

used for g-factor calculat~ons. The variable temperature spectra are recorded using ES-

DVT3 variable temperature controller and 77 K spectra are recorded ustng ES-UCD3X

insertion type Dewar

Crystal growth:

The full information about a complex can be a q u ~ r e d from the single crystal

EPR measurements rather than from powder and solution measurements Due to thls

W O , o m prefers single crystals rather than powders and solut~ons. Hence, a brief

discussion about crystal growth is menhoned. The main techn~que mvolved in crystal

growth is slow evsp~at ion method. Grow~ng crystals by allowing a saturated solution

of a material to lose solvent by evaporatlon a one of the stmplest methods [5, 61

Many interesting crystals can be grown w ~ t h linle knowledge of fine deta~ls simply by

evaporation of the solvent or temperature change. The evaporatton of the solvent

makes the solution supenahlratcd so that, it attempts to ach~eve the equ~librium

saturated state by rejecting the seed crystals to solut~on However, care must be taken

to prevent the solution bewm~ng too much supersaturated, for, crystals would then

appear spontaneously throughout the solut~on The factors that control the growth

process are

a) character of the solution

b) effect of add~t~ves and

C) operating variables such as the degree of super saturation and the temperature

range.

The cho~ce of solvent is an Important factor that determtnes the growth of a

crystal from solut~on Growth of a large crystal 1s v~rtually ~mpossible unless a solvent

IS found In which the solute a appreciably soluble In the present work, water IS used

as the solvent for all the crystal growth. No add~ttves are added lo the solution except

the dopant The rate of growth depends on the temperature at which the solution 1s

ma~ntained. At h~gher temperatures, the growth rate will be h~gh. All the crystals are

grown at room temperatures (- 298 K)

Interpretation of EPR spectra:

As can meaPure EPR s-a from solution, powder and single crystal

sampla, the p d u r e to obtain spin-Hamiltoninn parameters fium theses spectra must

be known. A brief d~scussion IS mentioned below. In order to calculate the g and A

values, the following expression h a been used to calculate g factor.

g = ( ~ W P H BDPPHYB - 42 . 11

where B is the magnetic field pos~tion at the EPR peek, B ~ p p " is the field posihon

comspondrng to DPPH and ~ D P P H is the g-value of DPPH whlch is equal to 2.0036.

One can as well calculate the g-value d~rectly using the spectrometer frequency (u) at

which resonance occurs The expression is as follows

g = 71 44836 X u (GHz) I B (mT) 21

The hyperfine coupling constant 'A' 1s glven by the field separation between

the hyperfine components If the spaclng 1s unequal, an average of them is taken to be

the value of A. For n number of hyperfine Ilnes, the average hyperfine value 1s

A = (B.-BI) l(n-1) -----[2 31

where B, 1s the field position for then" hyperfine line and 6, 1s the first hyperfine line

field pos~tion

In the present thew, EPR spectra are recorded for var~ous paramagnetic

samples, both In single crystal and poly-crystalline forms and g, A and D values have

been evaluated; the uncertainty in g is f0.004, in A and D f 0 4 mT. A bnef outline of

the interpretation of the data is given below

Powder and ghmm:

In powdcn and glasses, the observed spectrum is a result of superposition of all

possible orientations of single aystals glving rise to stahstically weighted average.

The thtory of powder line shapes of EPR has been given in detail by Kneubuhl 171,

Sand. [81 and Ibers and Swallen [91 A brief p~ctorlal summary of the evaluation of

principal magnetic tensors such as g and A, for a few representative examples is given

in Figure 2.2. In favorable cases, therefore, the principal values can be calculated from

powder data. However powder line shapes become complicated when more than one

type of species is present andfor when hyperfine lines overlap, and especially so when

the tensors do not colnc~de The first two compl~cat~ons can be circumvented to some

extent by measuring the spectra at two different frequenc~es, say X and Q bands, then

sort~ng out the field-dependent and field-independent terms in the Hamiltonian

Sometimes, power saturation techniques [ l , 21 and temperature varlatlon will also help

In this respect

Single crystals:

Many authors, for example, Schonland [lo], Well and Anderson [I I], Pryce

[12], Geuslc and Brown [13], Lund and Vanngard [I41 and Waller and Rogers [IS]

have discussed in detail the procedure for the evaluation of the prlnc~pal values of

magnetlc tensors from single crystal measurements The method consists of measuring

the variation of g2(0)for rotations about three mutually perpendicular axes in the

crystal, whlch may wlncide w~th the crystallograph~c axes or are related to the

crystallographic axes by a s~mple transformat~on. From the maxlma and minlma

Sll'&'LL

T* Ail *n -AL A n - Ac f' I n = %;= 1

Aair An <A,%

611 ,' i3z .. 681

I , *I$

I bc, An en, A,,

A , , .4l . Al3

t l > g m - b1

%x .4t;

ha AX

All 2 A n < An

F~gure 2 2. Schematic d~agrams summarlzlng tlie evaluat~on of pr~nc~ple values of

magnetic tensors from powder data based on the delta funct~on line shape

obtained in the three olthogonal planes, the matrix elements of the $ tensors can be

derived easily [lo]. Jacobi diagonalizatlon of this mstnx glves nse to the eigen values

oomspanLng to the principal values of the g tensor and the transformation mahix,

which diagonalizes the experimental g2 matrix, provides the dtrect~on cosines of these

tensors with respect to the three orthogonal rotat~ons. However, one can encounter

compl~catlons, when more than one magnetically dislinct slte per unlt cell IS present,

because no ap rlorr predictability of the relations between sltes and spectra m the three

planes T h ~ s leads to several possible permutations leadlng to many $ tensors. For

example, if a system has n sltes, then one can end up w~th 2n3 tensors, including for not

performing a proper rotation, i e , clockwise or ant~clockw~se A careful exammation,

however, lnvariabiy leads to the proper combinations and the corresponding dlrect~on

wslnes In the case of hypefine tensor, when g 1s not h~ghly an~sotrop~c, the same

procedure as above can be adopted. When thls 1s not the case, Schonland has

suggested that ~t IS necessary to follow the varlatlon of g2~(fl) In the three principal

planes The reason for this is as follows.

The Hamlltonlan for a paramagnetic system, lncludlng only the electron

Zeeman and hyperfine terms can be expressed as

3t'= p ( ~ I I B I S I + g~zBzSz + D~BISI) + (AIISIII + AzzSz12 + AIISIII) ---[2. 41

Let (nl, nz, n3) be the direct~on cosines of the magnetlc field B with reference to the

axes of the g tensor and hyperfine tensor Here, ~t is assumed that g and hyperfine

tensors are coincident. If M and m are the electron and nuclear spln quantum numbers,

then the energy levels are given by,

Eum =gPBM + PKMm -----[2. 51

Here g and K arc glven by the equations

g = ( ~ I I ' ~ I ' + &21m2 + &32m2)Ln -42.61

and

K = l/g [ ~ I I ~ A I I ' ~ ~ ~ + &22~11zn22 + g 3 3 ' ~ ~ 3 ~ n ? ] ~ ~ 71

The magnetic field 8, where the transition IM, m> ct IM+1, m> occurs 1s given by

hu = gPBm + P K , - - 4 2 . 81

In other words,

B, = hu l gp - (Wg)m -----[2. 91

If A IS hyperfine splitting and the lines are centered around (hulgp), then

A=Wg - 4 2 101

In order to evaluate the matnx elements of the hyperfine tensor, one has to conslder the

angular vmation of @k)', slnce (gk) has a linear angular dependence on g Therefore,

@k)' = $A' --[2. 111

From this equation, the matrix elements of the hyperfine tensor matrix are evaluated

using the same procedure used to get g tensor matrlx

Schonland has ~ndicated the probable errors in the method descr~bed above to

get the prlnclpal values of g and hypertine tensors But, the errors are very small

compared with the expermental errors ~nvolved, such as mountlng the crystal along the

specific axis, measurement of magnetic field etc

Crystal struetare of the bolt lattices:

A bnef tntroduction of the various host lanioes employed m thls thesis have

been presented here. The details of crystal shuctures ere discussed In the nspcctlve

c h e p m .

The PrWmtlve method of hexaimidazole cobalt(11) drchlor~de tetrahydrate

(HCDT) and hexaimidazole nickel(l1) d~chlor~de tetrahydrate (HNDT) essentially

consists of slow evaporaUon of an aqueous solut~on of lm~dazole and MCI,

{M = CNll) and Nl(I1)) in the ratlo of 6:l after adjusting the pH to 6.9 with very dilute

HCI to get the single crystals of HCDT and HNDT These hosl latt~ces are analogous

to hexaimidazole zlnc(I1) dichlor~de tetrahydrate (HZDT) and belongs to triclinic

crystal group w ~ t h unlt cell dimensions: a = 1.07, b = 0 94 and c = O 84 nm, a = 120,

f3 = 97, y = 98' and Z=l 1161. The crystal structure of HZDT shows that the six-

~m~dazoles are situated around the zlnc ion in a distorted octahedron w ~ t h none of the

Zn-N bonds belng equal In length; they are m the range 0 26 - 0.23 nm and none of the

bond angles being 90 or 180' Crystal rotations have been done along the three

orthogonal axes, namely b, a* and c*. Here, b is the crystallographic b-ax~s, a* IS

perpend~cular to b In ab plane and c* 1s perpendicular to both b and a*

Slngle crystals of cobalt(I1) potasslum phosphate hexahydrate (CoPPH),

zlnc(l1) sodium phosphate hexahydrate (ZSPH) are grown by slow evaporation from

the saturated solution contatn~ng cobaltous sulphate and potassium d~hydrogen

phosphate in equlmolar quantities (for CoPPH), zinc sulphate and sodium d~hydrogen

sulphate (for ZSPH) These crystals are analogues to the mineral (or biomineral)

struvlte variety, M M ' P O I 6HzO w ~ t h M" = Mg, Cd, Co, Zn etc, M' = Na, K, TI, NH4

or Rb [17]. The major importance of t h ~ s mmeral 1s that ~t is related to its occumnce

in human urinary sediments and veslcal and renal calculi [I81 Struvite has a high

dew of rtcumnce and about 39% of stone suffering patients expcnence struvlte

stones [19]. Struivite IS also formed 1n soils as a reaction product from phosphate

fertilizers [18]. It crystallizes In the orthorhombic system with space group Pmn.21

The unit cell parameters are a = 0.6941(2), b = 0 6137(2) and c = 1 1199(2) nm The

structure contains magnesium ions surrounded by six oxygen atoms of water of

hydration The six Mg-0 bond distances are 0.2095, 0.2103, 0 2071, 0 2042, 0.2071

and 2.042 nm.

Single crystals of zinc(l1) ammonlum tr~hydrogen b~s(orthophosphate)

monohydrate, ZnNbH1(P04)2 Hz0 [20] 1s obtalned by mlxlng the aqueous solut~ons

of N ~ H I P O , and Zn(N01)1 It belongs to trlclinlc crystal group, havlng unlt cell

parameter a = 0 769, b = 0 805, c = 8 06 nm and a = 116.25, P = 108.21, y = 84 14'

respectively The crystals are built up from PO4, Zn04 and N H ~ O R unlts. Zn(1l) Ion is

tetrahedrally coordinated to four phosphate oxygens, whereas NH4+ ion is coord~nated

to seven phosphate oxygens and one water oxygen to form the polyhedron, which are

linked to form a three dimensional network, consist~ng two Zn(l1) ions per unit cell

(2 = 2) [21].

Direction cosines ofthe substitutional sites:

Single crystal X-ray analysis data provides the positional parameters p, q, r and

Uie unit cell dimensions a, b, c, a, p and y. For crystal system with non-orthogonal

crystal axes, the positional parameters p, q, r of the various atoms can be converted to

an orthogonal fnunework and the Cartes~an co-ordinates x, y, z could be calculated

using the relation

a b cosy ccosp

0 b siny (dsiny) ( w s a - cosp cosy )

0 0 d

where, d = [ c2 - c2 cos2 $ - ( c2/ sin2y) (cow - cosp cosy )']'"

By setting the metal atom as the origln, the coordinates of the various atoms in

the crystal surrounding the metal are calculated The normallzed Cartesian

co-ord~nates of these atoms glve the directton coslnes of the metal-ligand bond of the

co-ord~nat~on polyhedron The direct~on costnes of these metal-l~gand bonds can be

compared with the direction coslnes of the g and A- tensors, obtalned by the procedure

described in the prevlous section Sometlines, ~t IS found that the magnetlc tensor

directions winclde with some of the bond d~rect~ons, whlch may not be so in low

symmetry cases

Spin-lattice relaxations:

If a paramagnetic ion is incorporated Into a paramagnetlc host, ~t will be

interesting to study the nature and extent of d~polar lnteractlon As Co(1l) IS EPR silent

at room temperature, it is poss~ble to successfully ananlyse the data at t h ~ s temperature,

as ~f we are dealing a paramagnetlc system In a d~amagnetic host lattice. However, as

the temperature is lowered, a tremendous change In the line width has been noticed,

due to the dipolar-dipolar interaction between the paramagnetlc host and the ~mpurity.

Hence, a psnicular orientation is selected from the crystal orlentation and the variable

temperature measurements are made, from which the line widths (AB) are measured.

It is noticed that as the temperature is decreased, the line width increases with decrease

in intensity and below a certain temperature, the peaks broadened almost to a straight

line Thls type of resonance broadening is notlced even from the powder sample Thls

type of line broadening is mainly due to the d~polar interact~on of the host

paramagnetic lattice and the impurity. The I~ne-w~dth variation of the Impurity

hypertine lines in paramagnetic lattice can be understood on the b s l s of host spln-

lanice relaxation mechanism The f s t spin-latt~ce relaxation of the host ions can

randomly modulate the dipolar interaction between the paramagnetic host and the

lmpunty ions resulting In "Host sp~n-lattice relaxat~on narrowing" [22] When the

sp~n-lamce relaxation nmowlng mechanism 1s effectwe, the host spin-lattice relaxation

tlme (TI) is glven by [22,23]

TI = (3/lO)(h12ghP)(~Bmp~d2d)

~ d ' ~ = 5 l ( g h ~ n ) ~ ~ h ( ~ h + l )

where,

h = Planck's constant

p = Bohr magneton

g = the host g value

Sh = the effectwe host spin and it is taken to be %

n = the number of host spins per unit volume wh~ch can be calculated from

the crystallographic data of the crystal lanice Here, n = NA (pRvl),

where NA - Avogadro number, M - Molecular weight, p - Density

AB,= the impurity llne width.

The calculated spin-lattice relaxation times (TI) are ploned against temperature

and the graph indicates that as the temperature decreases the spin-latticc relaxatlon time

TI Increases

SirnFonia powder simulation:

The stmulatlon of the powder spectrum 1s generally carried out to venfy the

agreement between the expenmentally calculated spin Hamlltonlan parameters with

those predicted from theoretical view. The simulation of the powder spectrum 1s done

uslng the computer program SlmFonia developed and supplied by Bmcker Company.

The algorithm used in the SimFonla program for powder simulation is based on

perturbation theory, whlch IS an approximation Previously, perturbation theory has

been used in the interpretation of EPR spectra because of the speed of calculation and

the intultlveness of the results. It 1s an approximate technique for findlng the elgen

values and eigen vectors of the spin-Hamiltonian parameters The assumption made 1s

that there is a domlnant interaction, which is much larger than the other interact~ons

As the dominant Interactton becomes larger when compared to the other interactions,

the approximation becomes better. The five interactlons that are considered In the

SimFonia simulation program for the powder sample are

a) Electron Zeeman interactlon It is the interactlon ofthe magnetic moment of the

electron with externally applled magnetic field i.e , the magnetic field from the

spectrometer magnet

b) Zcro-field splitting: It occurs in electron~c systems in which the spin is greater

than 112.

c) Nuclear hyperfine mteraction. It IS an interaction between the magnetic moment

of the electron wlth the magnehc moment of the nucleus.

d) Nuclear Quadruple mteract~on. It 1s the Interaction betwmn the Quadrupole

moment of the nucleus with the local electr~c fietd grad~ents in the complex (for

system having nuclear spin greater than 112)

e) Nuclear Zceman mtemction. It IS the lnteractlon of the magnetlc moment of the

nucleus wlth the externally appl~ed magnetlc field.

The assumption made in the s~mulat~ons IS that the electron Zeeman Interaction

IS the largest followed by the zero-field spl~ttlng, hyperfine ~nteract~on, nuclear

quadrupole interact~on and the nuclear Zeeman term IS the smallest Perturbatron

theory works best when the ratio between the successwe lnteractlons is at least ten.

If the l ~ m ~ t s exceeded, perturbat~on theory still gives a good plcture of EPR spectrum,

however, ~t may not be suitable for the quantltatlve analysls And ~f the EPR spectrum

1s to be simulated w~th larger hyperfine Interactions, then second order perturbation

theory is selected to increase the accuracy of the simulation The zero-field spl~tting is

always treated to second order because they do not produce a non-zero first order term

Only allowed EPR trans~tions are s~mulated, but under some circumstances

forbidden transltlons can also appear. These corresponds to simultaneous flip of the

nucleus and flop of the electron and forb~dden EPR lines occur between the allowed

trans~t~ons or a AMF = i 2 electron transitions These forb~dden lines are not simulated

because perthation theory IS not the optimal method for calculating their positions

and intensity The SimFonia powder simulation program simulates EPR spectra for

systems having electron spin li2 to 7R. For spin greater than ID, the zero-field

splitting terms (D and E) am implemented. There are essentially no restrictions on the

spin of the nuclei. All the naturally occurring spins have been pmgrammed. The

principal axes of the electron Zeeman interaction and the zero-field splitt~ng ate

assumed to be coincident

SimFonia can simulate both types of line shapes i e , Lorentzian and Gsuss~an,

as well as combination of the two. This technique is most efficient for many

line-complicated spectra Detailed theory of the powder spectra s~mulation can be

obtained from the references [24,25]

Computer Program EPR-NMR [26].

The program sets up spin-Hamiltonian (SH) matrices and determines their

eigen values (energies) uslng "exact" diagonalization. It is a versatile program, having

many operating models tailored to a variety of appl~cations Theses modes can be

grouped Into four categories, in increasing order of complexity as follows,

a) Energy-level calculation,

b) Spectrum simulation

c) Camparison with observed data,

d) Parameter optimization

For each cetegoly, most of the operations of the lower categories remain

available, so that a good way to leam how to use the program effectively is to start at

the lowest category and work one's way up.

Wwry I: In this category, the user provides the program with SH parameters and

dmctlon and magnitudes of appllcd magnetic fields.

Colegory 2: In category 2, the user also specifies an experiment, chosen from

field-swept or Wuency-swept electron paramagnetic resonance (EPR) or nuclear

magnetic resonance WMR), electron nucleus double resonance (ENDOR), or electron

spin echo envelope modulation (ESEEM). In addit~on, the user must ~denhfy the

transltlons of mterest. The "spectra" s~mulated conslsts of sets of transition fnquencies

or magnetlc field values, and possibly relatlve trans~tion probab~lft~es. The program

can also convolute these data with a I~ne-shape funct~on (Lorentz~an and Gaussian) to

produce a plot.

Category 3: For thls category, the user also suppl~es appropriate observed slngle crystal

data, w ~ t h transltlon labels assigned, and the program determines the degree of

consistency with data calculated from the glven SH parameters T h ~ s can include an

error analys~s on a user-selected subsets of SH parameters and/or magnet~c-field

directions.

Category 4: In the category 4, the user-selected subsets of parameters may be

optimized, so as to give better agreement between observed and calculated transltlon

frequencies. This uses a non-hear least-squares routfne, wh~ch systematically varies

the parameters so as to mlnlmize weighted difference between observed and calculated

transltlon frequencies (or fields). In thls category, user-supplied SH parameters need

only be estimates or outright guesses. T h ~ s program has been used In the calculat~on of

SH parameters for all the systems studied in thls thesis.

Referenus:

[l] D.J.E. Ingram (ed.), "Biological and Biochemlcel Applicatioos of Elecbon Spin

Resonance", Adam Hilder LTD, Londoh (1%9)

[2] C.P. Poole, "Electron spln resonance", 2* ed, Dover Publications, USA,

(1994).

[3] R S . Alger, "Electron Pamnagnetlc Resonance Techn~que and Appl~cat~ons,"

Interscience, New York, (1968).

T.H Wilrnshurst, "Electron Spin Resonance Spectrometer", Plenum, New

York, (1968).

J J. Gllman, "The art and Sc~ence of Growing crystals", John Wlley and Son

Inc., New York, (1963)

J C Bnce, "The growth of crystals from l~qu~ds", North-Holland Publ~sh~np

Company, London, (1973).

E.K. Kneubuhl, J Chem Phys , 3 3 (1960) 1074

R.H. Sands, Phys. Rev., 99 (1955) 1222

J.A Ibers, J.D Swallen, Phys Rev, 127 (1962) 1914

D.S. Schonland, Proc Phys Soc., 73 (1959) 788

J.A. Weil, H A Anderson, J Chem. Phys., 28 (1958) 864.

M H L Pryce, Proc. Phys. Soc , A63 (1950) 25

J E Geus~c, L.C. Brown, Phys Rev, 112 (1958) 64.

A Lund, T Vanngard, J Chem Phys., 42 (1965) 2979

W G. Waller, Max T. Rogers, J. Magn Res., 9 (1973) 92.

C. Sandmark, C.I. Branden, Acta. Chem. Scand ,21 (1967) 993

M.L. Mathew. W Schroeder, Acta Cryst., 835 (1979) 11.

F. Abbow R Boistelle, J. Cryst. Growth., 46 (1979) 339; and refs thereln.

E Takasaki, Uml. Intern., 30 (1975) 228

J.R. Lehr, E.H Brown, A.W Fraz~er, J.P. Smlth, R D Tasher, Tenn. Val. Auth.

Chem. Eng. Bull., (1967) No. 6

Par A. Boudjada, D. Tranquiet, J.C. Gu~tel, Acta Cryst., 836, (1960) 1176.

T. Mitsuma, J Phys Soc Jpn., I7 (1962) 128

S K Mlshra, Magn. Reson. Rev., 12 (1987) 191

A. Abragam, B Bleaney, "Electron Paramagnetic Resonance of Trans~tion

metal Ions", Clarendron Press, Oxford, (1970)

J.R Pilbrow, "Transition Ion Electron Paramagnetic Resonance", Clarendron

Press, Oxford, (1990).

EPR-NMR Program developed by F Clark, R S Dlckson, D.B. Fulton, J.

Isoya, A. Lent, D G McGavm, M J Mombourquene, R H D Nunall, P.S. Rao,

H Rinneberg, W.C. Ternant, J.A. Well, University of Saskatchewan,

Saskatwn, Canada (1996)