Ni(III) Mott–Hubbard Compounds - Material Designs and New Physical Properties in MX-and MMX-Chain Compounds - page 32

Fig. 3.2 Magnetic
susceptibilities observed in
a single crystal of
[NiBr(chxn) 2 ]Br 2 with a
magnetic field (1 T) parallel
and perpendicular to the 1D
chain. Red and blue lines
show theoretical
susceptibilities proposed by
Eggerd-Affleck-Takahashi
(EAT) with J ¼ 2,000 K

where N is the Avogadro's number, g is the Lande's factor, and k B is the
Boltzmann constant. In this equation, J is defined as H ¼ 2 J S S i · S i +1 . The

exchange interaction was estimated to be J/k B ¼ 500 K by fitting the

present data above 130 K in Eq. ( 3.1 ). The magnetic susceptibility slightly
decreased below 100 K. This suggests that some phase transition to a nonmag-
netic state, such as CDW

2,000

Ni II Ni IV ð Br Br Br Þ or spin-Peierls

states ð Br Ni III Br Ni III

Br Þ , occurs.
We measured the temperature dependence of the nuclear quadrupole resonance
(NQR) signals of the bridging Br ions because the NQR is a quite sensitive probe
for detecting subtle changes in the electron distribution around NQR nuclei.
We observed a single resonance line for 81 Br at 300 K (137.079 0.005 MHz)

and a pair of lines at 130.874 0.01MHz at 3.8 K. We assigned

these resonance signals to 81 Br nuclei on the basis of the corresponding 79 Br lines
at 164.091

0.01 and 147.786

0.01 MHz
(3.8K), which are in agreement with the reported isotope frequency ratio ( 79 Br/ 81 Br

0.005 MHz (300 K) and at 156.656 0.01 and 176.904

¼
1.1969) [ 7 ]. These resonance frequencies were assigned to bridging Br ions and not
to counter Br ions since 79 Br NQR frequencies in compounds with Ni-Br covalent
bonds have been observed in nearly the same frequency range, e.g., 126.26 MHz for
NiBr 2 [P(C 3 H 7 ) 3 ] 2 and 126.53 MHz for NiBr 2 [P(C 4 H 9 ) 3 ] 2 [ 8 ], whereas the resonance
lines for ionic Ni-Br bonds are usually at frequencies of one order of magnitude lower
than those for the present complex [ 9 ].
Figure 3.3 shows a temperature dependence of the 81 Br NQR frequencies for the
bridging Br ions in [Ni(chxn) 2 Br]Br 2 . A single 81 Br NQR peak was observed at
room temperature, which is consistent with the X-ray results, where all of the
bridging Br sites are equivalent at room temperature. The resonance frequency
gradually decreased with an increase in the temperature above 130 K due to
averaging of the electric field gradient (EFG) at Br nuclei by lattice vibrations.
The temperature dependence of the NQR frequency ( ( T )) can be described by the

harmonic oscillator model for lattice vibrations [ 10 , 11 ]:

n

ð ho nðTÞ¼n 0 1 A coth

2 kT Þ

(3.2)