Investigation of acoustic phonons in CsNiF3by Brillouin scattering

Abstract

Brillouin scattering measurements were made to determine the velocities of acoustic phonons and the values of the elastic coefficients of CsNiF3. Temperature dependence of Cll and C33 was investigated between 4 and 300 K. In contradiction with the earlier measurement of Young's modulus along the c axis of the crystal, C33 was found to have a negative thermoelastic coefficient. In connection with these measurements the dispersion

Ab initiostudy of the lattice dynamics of CsNiF3

“…The magnetic moment per Ni atom equals 1.64 μ B and it stays in agreement with the FPLAPW calculations [12] which give a value of 1.63 μ B . [20,21], ultrasonic [21,22] and inelastic neutron scattering [24] In the AFA and AFB spin orderings the calculated bandgap between the highest occupied and the lowest unoccupied states is 1.04 eV for both spin channels. The FM ordering affects the positions of the majority (minority) bands, leading to a bandgap of 3.74 (1.04) eV for the spin-up (down) channels.…”

“…This set of equations represents a general form of Hooke's law and can be solved with respect to elastic constants. The calculated components of the elastic tensor in CsNiF 3 are collected in table 2 together with the experimental data determined from Brillouin scattering [20,21], ultrasonic [21,22] and inelastic neutron scattering [24] measurements. The calculated C 12 and C 44 are very close to those obtained in the Brillouin scattering experiments [20], while the components C 13 and C 33 remain inside the range determined by different experimental techniques.…”

“…It was pointed out that at higher external magnetic fields ferromagnetic ordering of Ni ions can occur as well [11,18]. Various properties of the CsNiF 3 compound have been studied experimentally [16,17,[19][20][21][22][23][24]. These investigations were performed as a function of temperature as well as external magnetic field.…”

“…These investigations were performed as a function of temperature as well as external magnetic field. Measurements of the elastic properties of this system carried out by different methods [20][21][22]24] show that CsNiF 3 is characterized by a low value of bulk modulus. This indicates CsNiF 3 to be a very soft material which could undergo a structural transformation under a pressure of a few kbar [25].…”