RAMAN-SCATTERING AND MICROWAVE DIELECTRIC STUDIES OF THE STRUCTURAL PHASE-TRANSITION IN THE QUASI-ONE-DIMENSIONAL FERROMAGNET (CH3)(4)NNIBR3

We report polarized Raman;spectra and 9.6 GHz dielectric constant meas urements of the quasi-one-dimensional ferromagnet (CH3)(4)NNiBr3 (TMNB ) at temperatures above and below the 134 K structural phase transitio n (T-c). The dielectric response is typical of materials for which a s tructural phase transition occurs, rising upon approach to T-c and dec reasing sharply at T-c; epsilon(1) falls gradually in the low temperat ure phase. There is a small hysteresis in the dielectric constant at T -c, indicative of the first order nature of the transition. The Raman spectra display striking changes through the structural phase transiti on as well. At room temperature, we observe the weak signature of four of the five Raman active BX(3) modes. Below T-c, we observe splitting in several degenerate vibrational modes, one of which is a BX(3) chai n mode. This suggests that the phase transition is related to a symmet ry breaking structural distortion in which the NiBr3 chains are intima tely involved. The behavior of TMNB through T-c is in contrast to that of many other hexagonal magnetic materials, most notably TMNC, where the phase transition occurs with a doubling of the unit cell and an or dering of the tetramethylammonium counterions. We suggest that this di fference is related to a combination of electrostatic and lattice expa nsion effects. Small frequency shifts of the phonons and a variation o f the slope in the dielectric constant also provide evidence for a sec ond phase transition near 95 K, which we relate to complete counterion ordering as in the case of TMCC.