Ni(III) Mott–Hubbard Compounds - Material Designs and New Physical Properties in MX-and MMX-Chain Compounds - page 33

Fig. 3.3 Temperature
dependences of 81 Br NQR
frequencies observed in
[Ni(chxn) 2 Br]Br 2 . Dotted line
is an extrapolation to 0 K
according to the Bayer theory

150

Bayer theory

145

140

135

130

0 50 100 150 200 250 300

Temperature / K

where are the resonance frequency for the static lattice, a coefficient

that depends on modes of lattice vibrations, and the averaged vibration frequency,
respectively. The observed data were fitted with Eq. ( 3.2 ), and the extrapolated
frequency to 0 K (

n 0 , A , and o

0.2 MHz.
Upon cooling, the NQR signal disappeared around 130 K, and two lines at
130.87 and 147.78 MHz appeared again below ca. 40 K. The loss of resonance
signals between 40 and 130 K is thought to be spectrum broadening due to
fluctuation in the EFG upon phase transition. Since the averaged frequency of
these two lines is almost the same as the extrapolated frequency from the high-
temperature side, the NQR signals below 40 K are split into one of the high-
temperature signal. Two resonance lines with a large frequency separation of
16.9 MHz at low temperatures indicate the presence of two nonequivalent Br
sites, suggesting that a change in the electronic state in [Ni(chxn) 2 Br]Br 2 takes
place between 40 and 130 K.
Here we discuss the possible electronic structures of [Ni(chxn) 2 Br]Br 2 .Inan
averaged valence MH state, the environments of all Ni or bridging Br sites should
be equivalent, resulting in a single Br NQR line. In a CDW state, the bridging Br
ions are displaced, but the sites are equivalent. However, there are two nonequiva-
lent Ni sites, and hence, this state affords a single Br NQR line. In the spin-Peierls
state, which is characterized by the displacement of the Ni sites, two nonequivalent
bridging Br sites are formed in agreement with the two Br NQR lines. The splitting
of NQR signals indicates that a spin-Peierls transition occurs in [Ni(chxn) 2 Br]Br 2 in
the range of 40-130 K. This explanation is consistent with the decrease in the
magnetic susceptibility observed below 100 K. As shown in Fig. 3.2 ,

n (0)) was determined to be 139.1

w deviated

isotropically from the EAT curve below 100 K clearly. This decrease in w is due to

spin cancelation caused by the transition into a spin-Peierls state.
The charge distribution in the Ni-Br bond can be evaluated by applying the
Townes-Dailey approximation: [ 12 , 13 ]

¼ð

e 2 Qq sÞ e 2 Qq atom 1 iÞð 1 (3.3)