INFRARED-ACTIVE PHONONS OF KMNF3 IN BOTH TETRAGONAL PHASES

The frequencies of the infrared-active phonons were measured by far-infrared methods in both tetragonal phases of KMnF3, namely at 110K (structure D4h18) and 15K (structure D 4h5). The results are discussed by comparing the frequencies of some particular phonon modes in the three structural phases of KMnF3 (Oh1, D8h18, and D4h5), also taking into consideration recent results of Raman-effect studies and neutron inelastic scattering. Standard group theoretical methods are used in this discussion. The experimental results obtained by various methods agree mostly with the group theoretical predictions. At low frequencies, however, there are some discrepancies between experimental results and group theory. For the high-frequency phonons, on the other hand, it seems that some optical data for the low-temperature tetragonal structure are missing, possibly because of a small oscillator strength or Raman cross section.