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

uncertainty
- 81 -
in the values of Q. However, unlike the comparison
of relaxation times for the two copper or two bromine isotopes it
cannot be assumed that anti-shielding, covalency and other effects
are identical at the different ionic sites. Thus the factors y
will in principle differ for the two nuclei. The theory is satis-
fied if the ratio
81y/63y has the value 1.14 ± 0.30.
In the simple ionic model of the crystal lattice the magnitude
of the effective quadrupolar coupling for any nucleus is determined
by the Sternheimer anti-shielding factor y. (3).
Theoretical values
of y, depend to some extent upon the calculation procedure adopted
but for the copper and bromine ions may be taken as -15 and -123
(85
respectively .
It follows that the ratio of the corresponding
y values - given by (1 - y. ) - is in disagreement with the prev-
iously determined figure of 1.14. Experimental estimates of the
anti-shielding factors for the halide ions differ quite consider-
ably from the theoretical values. However, even when an average
experimental value of -35 is taken for Br-(86) there is still a
considerable discrepancy in the ratio
$1Y/63Y predicted by anti-
shielding effects alone and that found from measurements of Tl.
In view of the fact that the cuprous halides are covalent the dis-
agreement is perhaps hardly surprising.
A detailed comparison of Tl values between the different
cuprous halides is precluded by the absence of any reliable
values for y as well as by the lack of any reported data on the
velocity of sound in these compounds. Nevertheless by making some
simplifying assumptions an approximate calculation can be made.
The values of T1 for 63Cu in the three halides at a temperature of
200 K are listed in Table 6.3. This temperature is chosen because,