Vibrational properties of the one-dimensional, S=1/2, heisenberg antiferromagnet copper pyrazine dinitrate

We report the temperature-dependent infrared spectra of pure, deuterated, Z n-doped, and 2,6-dimethyl-substituted samples of the S = 1/2, one-dimension al Quantum Heisenberg Antiferromagnet (QHAF) copper pyrazine dinitrate (Cu( C4H4N2)(NO3)(2)) Of the more than 100 vibrational modes observed in the spe ctra, nearly one-third of them unexpectedly soften throughout the temperatu re range of investigation (300-5 K). We discuss the temperature dependence of the vibrational spectra in terms of several different models for mode so ftening. On the basis of detailed structural information and a comparison o f the infrared spectra between pure copper pyrazine dinitrate and its chemi cally modified relatives, we conclude that the unusual softening observed i n this low-dimensional molecular magnet is due to enhanced interchain hydro gen bonding with decreasing temperature.