The temperature dependence of I-127 NQR and DSC as well as the crystal stru
cture at room temperature of the title compound were determined. This compo
und shows a first-order phase transition of an order-disorder type at 245 K
. Eight I-127(nu(1): m = +/-1/2 <-> +/-3/2) NQR lines of 79.57, 81.86, 82.5
6, 83.36, 84.68, 87.72, 88.34, and 88.86 MHz, and corresponding eight I-127
(nu(2): M = +/- 3/2 <-> 5/2) NQR lines were observed at liquid nitrogen tem
perature. Three I-127(nu(1)) NQR lines with an intensity ratio of 1:1:2 in
the order of decreasing frequency were observed just above the transition p
oint and two NQR lines except for the middle-frequency line disappeared aro
und room temperature. This temperature behavior of NQR lines is very simila
r to that observed in [N(CH3)(4)](2)HgI4. Another first-order phase transit
ion takes place at 527 K. The structure of the room-temperature phase was r
edetermined: orthorhombic, Pnma, Z = 4, a = 1342.8(3), b = 975.7(2), c = 16
96.5(3) pm. The NQR result of three lines with an intensity ratio of 1:1:2
is in agreement with this structure. The thermal displacement parameters of
atoms in both cations and anions are large.