Anomalous pressure dependence of acoustic phonons of AgGaSe₂ investigated by inelastic neutron scattering to 4.3 GPa

Abstract

The pressure dependence of the transverse acoustic (TA) phonons along [100] and [001] and the lowfrequency optical Λ 3 modes of AgGaSe 2 has been measured at pressures to 4.3 GPa by inelastic neutron scattering. We find an exceptionally strong frequency decrease of the TA modes with Grüneisen parameters as low as -10 for wave vectors close to the zone centre. The associated elastic coefficient C 44 decreases by ~50% between 0 and 2.6 GPa. The softening also affects the optical modes to a less extent. These lattice dynamical anomalies seem to be related to a continuous structural phase transition located at ~2.6 GPa. Single crystal X-ray diffraction results are consistent with a monoclinic lattice of the high pressure phase which can be regarded as a slightly distorted chalcopyrite structure, similar to AgGaS 2 .

Order-disorder transition in ZnGeP2and ZnSnP2

“…In addition, the variation of mode Grüneisen parameter over the entire Brillouin zone along the [001] direction is almost linear while it shows maxima close to the zone centre and the zone boundary along the [100] direction, for both ZnGeP 2 and ZnSnP 2 . The negative mode Gruneisen parameters of the transverse acoustic modes are related to the mechanical instability of the chalcopyrite structure (as shown later in this section), this behaviour is similar to that observed in neutron scattering experiments on AgGaSe 2 [12]. The computed variation of the elastic constant C 66 with pressure for both ZnGeP 2 and ZnSnP 2 is shown in Fig.…”

Growth and crystal structure of the layered compound TlGaSe2

“…[9]. A single crystal of TlGaSe 2 (average dimensions 0.20 mm) was selected and mounted into a glass capillary for X-ray analysis with a four circle diffractometer (Nonius Kappa CCD) using graphite monochromated MoK α radiation.…”

High‐temperature heat capacity and lattice anharmonicity of LiGaO2

“…where b 1 and b 2 are now constants and ω l is the lower cut-off frequency of optical modes having the same Grüneisen parameters. Exceptions are the optical modes with lowest frequency [34][35][36][37][38][39][40] and, in particular, the acoustic modes [43][44][45] exhibiting very small and partly negative mode Grüneisen parameters with a pronounced dependence on the wave vector q [44,45]. Therefore, the contribution of these modes to ∆C V (T) can only be calculated using relation (13) with summation over the frequency range from ω = 0 to ω = ω l .…”