Hyperfine interactions at 61Ni in ionic nickel compounds

d inorg, nucL Chem., 1976, Vol. 38, pp 19-21. Pergamon Press. Printed in Great Britain

HYPERFINE INTERACTIONS AT 61Ni IN IONIC NICKEL COMPOUNDS* FELIX E. OBENSHAIN Oak Ridge National Laboratory, Oak Ridge, TN 37830, U.S.A. and University of Tennessee, Knoxville. TN 37916. U.S.A. and J. C. WILLIAMS and LARRY W. HOUK Memphis State University, Memphis, TN 38152, U.S.A. (Received 2 April 1975) Abstract--Results of the analyses of ~'Ni nuclear gamma resonance (NGR) absorption spectra in Ni[P(OC.,H~hh. Ni(NO0~.6H~O, NiCh, and NiCI2.6H20 are presented. Absorber recoilless fractions, energy shifts, and magnetic hyperfine fields were obtained from theoretical curves computer fitted to the experimental data. Deduced isomer shifts correlate with ligand electronegativities and ionicities of nickel-ligand bonds. Electric quadrupole splitting of ~'Ni in NiCI:.6H20 gives a value for the ratio of the quadrupole moment of the first excited state to the ground state as Q,./Q, = -0.5 -+0.4.

NUCLEAR gamma resonance (NGR) measurements yield information about isomer shift, magnetic dipole and electric quadrupole hyperfine splitting of nuclear energy levels, which in turn reveal changes in the electronic configuration of the atom. The importance of the NGR method derives from the fact that it is a local measurement and reflects in three distinct ways the local behavior of the material under investigation. The magnitudes of these electromagnetic interactions have been measured for some nickel compounds. The effect of covalent bonding in the magnetic properties of the transition metal ion (Ni) has been studied in some cases by correlation with the observed magnetic hyperfine fields[l] at ~Ni nuclei. The present work is mainly concerned with the correlation of the isomer shifts with electronegativities of the ligand ions. M6ssbauer spectroscopy with the 67.4 keV transition in ~Ni was used to study hyperfine interactions at 6~Ni in some selected nickel compounds. The source 6'Co was produced by the reaction 64Ni(p, a)61Co with a proton energy of ~22 MeV. A nonmagnetic source-material was produced by alloying 64Ni and vanadium (64Nio.s6Vo14). All of the experiments were performed in transmission geometry with both source and absorber immersed in liquid helium (T =4.2°K). The spectrometer and other experimental details have been described elsewhere[l]. The experimental absorption spectra were computer fitted with a line-shape function[2] (transmission integral) having parameters related to physical properties of the source and absorber. From these spectra we have deduced absorber recoilless fractions, energy shifts and hyperfine fields. The computed results and the 6~Ni content of each absorber are listed in Table 1. Magnetic susceptibility studies[3] indicate that anhydrous nickel chloride is antiferromagnetic at liquid helium temperatures with a Ne61 temperature of Z,~ = 52°K. The nickel chloride hexahydrate is also antiferromagnetic in

*Research jointly sponsored by the U.S. Energy Research and Development Administration under contract with the Union Carbide Corporation and by Memphis State University. 19

Table 1. Parameters of ~'Ni M6ssbauer spectra of nickelcompound absorbers at 4.2°K

Compound Ni[P(OC2H,h], Ni(NO3)2.6H20 NiC12 NiClz.6H20 NiF2+ KNiF3NiOt

6ZNi density (mg/cm2) 0.605 3.20 7.24 6.82 4.79 2.50 2.50

fa (%) 4.6-+0.4 4.0 -+0.4 3.5_+0.3 3.4_+0-3 13.8-+1.0 14.8-+ 1.3 11.5_+ l.l

Energy shift* (tz/sec)

H,.j (kOe)

-37-+9 -+23 -+3 -+40_+3 33-+1 +43_+17 46-+ 1 +19_+2 45-+1 +9-+ 10 64::2 ~-13-+5 100::2

*Experimental shift observed relative to the ~'NiV source. +Ref. [1]. the helium region with the much lower transition temperature of T~ = 5'34°K as determined from specific heat measurements[4]. The structure determination of NiC12.6H20 by Mizuno[5] showed that the environment about each Ni ion is approximately octahedral with four water molecules forming a square in the plane bisecting the symmetry axis of the complex, which joins the two chloride ions and contains the nickel ion. The asymmetric 6'Ni NGR absorption spectrum of NiCIv6H20 (Fig. 1) indicates a quadrupole splitting not observed by Erich et al. [6]. Asymmetry is expected in the spectrum according to the model fitted; however, an anisotropic Debye-Waller factor can also cause an asymmetric quadrupole splitting (Goldanskii-Karyagin effect). In the present analysis, isotropy was assumed for the Debye-Waller factor. The electric field gradient for axial symmetry, V::, has contributions from two components: the valence electrons and the lattice. Since the ground state electronic term for NiCI2.6H20 is an orbital singlet (3A2~), an axial distortion would not alter the ground state. Thus it is expected that the lattice is the major contributor to ~:. On this basis the direction of V~z will be determined by the symmetry axis of the complex. Magnetic susceptibility measurements[7] performed on single crystals of

20

FELIX E. OBENSHAINet al. 1.0~

I

I

I

I

I

I

I

I

used and the NiV source. The isomer shift can be written as[ll]

L

LO0

&s = K (AR/R)Npe, (0)

0.99

o.ga ~ o.9z a: 0.96

0.95

0.94 -5

-4

-3

-2

0

-I

i

2

3

4

5

VELOCITY (ram/see)

Fig. 1. M6ssbauer absorption spectrum of "'Ni in NiCI~.6H~Oat 4.2°K. NiC12'6H20 showed that the preferred direction for alignment of ionic moments is along the symmetry axis. Consistent with these remarks, a model Hamiltonian having a magnetic and electric quadrupole interaction with V~=parallel to the hyperfine field was assumed. From the computer fit, we found the hyperfine field to be Hh~ = ( 4 6 - 1 ) k O e and the ratio of the quadrupole moment of the first excited state to the ground state Q,/Q~ =-0.5---0.4. This is compared with the value Q , / Q s = - l . 2 1 + - O . 1 3 obtained by G6ring[8] with a NiCr204 NGR absorption spectrum. We have calculated V°=using a point charge model and obtain V°== 5.27 x 1016V/cm 2 for the lattice contribution. Feiock and Johnson[9] have calculated the Sternheimer antishielding factor for the closed shells of nickel and they find 7==-4"33. It follows that Vr, = V°=(1-y=) = 2"81 × 10'7 V/cm 2. If we assume the quadrupole moments given in the literature, then V ~ = 16.24x 1017V/cm2. Considering the uncertainties of the calculations, the agreement here is acceptable. An experimental energy shift 6~p of the resonance absorption peak has contributions from the isomer shift 6~s and the second-order Doppler shift 6soo and is given by[10] ~exp

~(A) o,s

-

~ (NiV) ± o,~

~

a~s~o~

-

~ (NiV) II SOD

where the superscripts refer to the particular absorber

where K = l . 1 2 x l O Z * c m 4 l s e c for 6'Ni, ( A R / R ) , = -(1.2+0.5)×10 -4 is the relative change of the 6'Ni nuclear charge radius[l], and eoet(0) is the electronic charge density at the nucleus calculated from relativistic wave functions. Thus the relative electronic density at 6'Ni in various absorbers can be deduced if the relative second-order Doppler shift is known. Quantitative information concerning the lattice dynamics of a substance allows 6SOO (A~ to be accurately calculated. This has been done by Love et al. [1] for NiF2, KNiF3 and NiO. In the absence of such information, assuming a Debye model for the lattice gives ~,~D°bY"~soo -[+178/tn fa]/~/sec. For the above three compounds, this ~(A) model gives a shift about 80% of the osoo calculated with the more accurate method. Hence, we take 6~sa~= 1 ~ t,(Debye) t ~ h o •z~oso~, ,, ~ a better approximation. Table 2 lists the isomer shifts as derived from the measured experimental shifts and the computed second-order Doppler shifts. For the 64Nio.86V014source, we have taken the value ~(NiV) SOD = -96.7/x/sec appropriate for pure nickel, since the measured recoilless fraction of the source is the same for nickel. The relatively large errors associated with our derived isomer shifts arise for the most part in the uncertainty of the method for calculating 6soo. (a~ We associate an error of about 20/~/sec to the ~ ~sao~ocalculated in this way. However, the relative magnitudes of the isomer shifts reported are more certain since 6 CsAo~is about the same for each compound and changes only the absolute values. Electronegativity values (X) (measures of the effectiveness of atoms or functional groups in competition with other atoms or groups for available electrons in the molecule) have been estimated by several approaches, e.g. Pauling's scale [12] based upon thermochemical data, Mulliken's value[13] derived from the average of the ionization potential and electron affinity of the bonded atoms, Malone's approximations[14] using bond dipole moments, several empirical scales[15-18], and more recently those utilizing NQR[19] and NMR [19, 20] data. Since the M6ssbauer isomer shift is proportional to the electron charge density at the resonant nucleus which is affected by the electron withdrawing capabilities of bound atoms and groups, electronegativities should be derivable from M6ssbauer isomer shift data. Curve A in Fig. 2 illustrates a direct correlation of &s with XL for the tetrahedral Ni[P(OCzH03]4[21] and six pseudooctahedral compounds [22] studied and predicts XL values of 2.5 and

Table 2. Analysisof 61Nienergy shifts in nickel-compoundsabsorbers at 4.2°K Compound Ni[P(OC2Hs)3]4 Ni(NO3h.6HzO NiClz NiCI2.6H20 NiF* KNiF* NiO*

Nickel formal charge state

(vJsec)

0 +2 +2 +2 +2 +2 +2

-37 - 9 +23 -+3 +40 - 3 +43 -+17 +19-+2 +9-+ 10 +13-+5

6~xp

6soo(A~ 8soo+(A~97 (/~/sec) (/x/sec)

-71 -68 -65 -65 -112 -112 -103

+26 +29 +32 +32 -15 -15 -6

*Ref. [1]. tlsomer shift relative to Ni, obtained from ~%,,p.= 6,s + 6 ~A~soo-6(N~v~oo.

6,s¢ (tz/sec) -63 -+22 -6 -+20 +8 -+20 +11 -+26 +34-+2 +24-+ 12 +19-+9

Hyperfine interactions at ~'Ni in ionic nickel compounds

Acknowledgements--The authors wish to thank J. E. Tansil for his contributions during the early stages of this work and J. O. Thomson for the use of his computer program to calculate the electric field gradients. J.C.W. and L.W.H. would also like to thank the Memphis State University Research Foundation for partial support for this study.

5

4 z

3

?

~2

f .~

g -

,

5

-

0.4

i

---

~4

J

I

i 02

o 80

~60

-4-0

20

21

O

20

40

3[s ISOMER SHIFT (/z/see)

Fig. 2. Curve A is plot of 5,~ vs ligand electronegativity[24]. Curve B is a plot of 6~s vs Ni-L bond ionicity[23]. Both curves are based on data for NiF2, NiC12 and NiO with other points placed on the line.

0.4 for water and triethylphosphite, respectively. As N i - L bond ionicities are a function of the differences in XL of Ni and L [23], a similar correlation of &s with bond ionicities is expected and observed in Curve B of Fig. 2. Consistent with Pauling's calculations [24] the Ni-P bond has little ionic character in Ni[P(OC2I-Is)3]4. The main contributions to the magnetic hyperfine field of transition-metal ions in magnetically ordered compounds are the core polarization field, H,~, the orbital field, H,,, and for ions at lattice sites of lower-than-cubic symmetry, the dipolar field. An additional contact field can arise from overlap and covalent mixing of ligand and metal 4s-orbitals. The core polarization is given by H,~ = - 3 3 2 ( S ~ ) k O e , where the value -332 is from Watson and Freeman's calculation[25] for free Ni 2+ ions and (S:) is the spin expectation value in the compound. The orbital field is given by H,, = 125 x Ag x (r -3) x (S~) kOe, where Ag = g - 2.0023 and is a measure of the unquenched-orbital angular momentum. The expectation value for the spin is reduced from unity for two reasons: (1) Spin density is transferred to the ligand anions by covalent admixture. (2) Zero-point motion of an antiferromagnetically ordered spin system in its ground state reduces the net average spin on any ion. Measurements of Ag and (S:) are not available for NiCb and NiCI2.6H20 but assuming (S,.)~0.8 as it is for NiF2[I] then the values zXg = 0.33 for NiC12 and Ag = 0'28 for NiCL,.6H20[261 are obtained.

REFERENCES

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