NNR IN RAPIDLY ROTATED METALS By - Nottingham eTheses ...

- 83 -
change in the factor a13. Using the values of lattice parameter
a quoted by Wyckoff
(88)
(but ignoring effects due to the thermal
expansion of the lattice) the product T1a^13 as listed in Table
6.3 can be seen to be constant to within ±12%. Therefore the
range of T1 values between the cuprous halides appears simply to
reflect the strong dependence of the quadrupole interactions on
the interatomic distance.
The above finding is somewhat in contradiction to that recorded
by Guenther and Hultsch(78). They interpreted a decrease in the
ratio of their calculated to measured T1 values over the cuprous
halides as being consistent with the reported variation in the
degree of covalent bonding in these compounds: CuC1 being the most
covalent and CuI the least. An increased amount of covalency
in the lattice bond is usually associated with a paramagnetic shift
(26)
in the resonance frequency. However chemical shift measure-
ments(»'78) do not show such a correlation in the cuprous halides.
Evaluation of the absolute agreement between theory and experi-
ment depends upon the accuracy to which the individual components
of equation (6.1) are known. In calculating the spin-lattice relax-
ation time of 63Cu in CuCl at 200 K the following values were used:
3 -3 (87) (88)
Ia 3/2, d=4.14 x 10 kg ma5.406 x 10-10 m
Q- -0.16 x 10-28 m2 (84), E(T*) - 1. In the absence of any better
estimate the velocity of sound v was taken as 3.5 x 103 m s-1 as
reported for AgCl(89).
. The resulting value for T1 is then
9.46 x 103 Y2s. compared to the measured value of 12.4 m s. It
follows that a value of Y ti 103 is required to secure agreement
between experiment and theory. Similar values for y have been deter-
mined from T1 measurements on some alkali halides(22'90). This