Ferroelectric phase transition in PbHPO4 studied by IR-spectroscopy of internal modes

Vibrational analysis of PbHPO4 and PbDPO4 crystals

“…The weak 146 cm -1 line detected in IR [36] spectra (E || b) and suggested to be associated with impurities [26], is close to 135 cm -1 mode (A u type) calculated within the present simulation. Therefore, this line may be treated as the intrinsic lattice mode.…”

“…This value is close to the direction of spontaneous polarization (~10°) experimentally found in LHP using an etching method [50]. The frequency of this mode is nearby the frequency of a strong mode (375 cm -1 at T C ) detected in IR spectrum [36]. This phonon mode demonstrates a pronounced softening (from 400 cm -1 at 77 K to 375 cm -1 at 300 K) at increased temperature in ferroelectric phase and the sharp rise of its intensity below the phase transition.…”

“…This phonon mode demonstrates a pronounced softening (from 400 cm -1 at 77 K to 375 cm -1 at 300 K) at increased temperature in ferroelectric phase and the sharp rise of its intensity below the phase transition. The authors of paper [36] attributed this mode to the A g symmetry because the frequency of this mode is closer to ν 2 = 420 cm -1 internal mode of PO 4 group rather than to the ν 4 = 560 cm -1 mode. Moreover, according to the demands of group theory (see Table 1), ν 2 mode is allowed for observation in A g symmetry spectra, whereas it is forbidden in the B u type spectra (due to the loss of two-fold symmetry in ferroelectric phase, A g and B u type modes acquire the same A′ symmetry below T C ).…”

“…There are several reasons for this. According to IR data [36], this mode is one of the two strongest modes in the 300-550 cm -1 region in ferroelctric phase and it is clearly visible in para-phase. Since the changes of crystal structure occurring below the second order phase transition T C point are small, it is doubtful that the dipole inactive A g mode in paraelectric phase may acquire such a large dipole moment below T C to become a very strong IR mode.…”

“…A wide variety of different kinds of experimental techniques (structural [1][2][3][4][5][6][7][8], dielectric [9][10][11][12][13][14][15], resonance [16][17][18], acoustic [19], optical second harmonic generation [20], luminescence [21,22]) were applied in the investigation of LHP. The lattice dynamics of LHP and its deuterium isomorph PbDPO 4 (LDP) was intensively studied using Raman [23][24][25][26][27][28][29], hyper-Raman [30,31], Brillouin [32,33] and IR spectroscopy [34][35][36]. Comprehensive Raman study of LHP (LDP) published in the series of papers by Ohno, Loockwood and co-workers [see 24-29 and references therein] presented a scrupulous analysis of temperature evolution of Raman spectra and the eventual mechanism of phase transition.…”