Four 81 Br NQR lines in 4-NH 2 C 5 H 4 NHBiBr 4 IH 2 O were observed in the temperature range between 77 and ca. 380 K; with increasing temperatures the respective sets of higher and lower two resonance lines coalesced into single lines discontinuously at 274 K, showing the occurrence of a first-order type phase transition of this crystal. The transition was confirmed with heat anomaly on a DTA curve. Each higher and lower line of high-temperature phase is assignable to the terminal Br atoms and the bridging ones of one-dimensional poly anions (BiBr 4 j ) n in the crystal structure (C2/c), which was investigated by a X-ray structure analysis at room temperature. The 1/T 1 temperature dependence of 81 Br NQR follows the usual T 2 law in the temperature range between 77 and ca. 140 K, being explained by fluctuation of the EFG at Br nucleus due to lattice vibrations. The T 1 vs. 1/T curve in the temperature range between about 160 and 190 K was describable by the exponential curves, allowing us the estimation of activation energies. These exponential behaviors of T 1 of 81 Br NQR are attributable to the fluctuations caused by the thermal motion of 4-NH 2 C 5 H 4 H + ions. Echo signals of the 81 Br NQR could not be detected above 190 K owing to poor S/N with very short T 2 .
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