Acoustic phonon mode softening in La3S4

Publisher Website
Google Scholar
Abstract
The temperature dependences of the elastic stiffness moduli of La3S4 have been determined by pulse echo overlap measurements of ultrasonic wave velocities. While the elastic anisotropy is close to unity at room temperature, the shear modulus C'(=1/2(C11-C12)) decreases as the cubic to tetragonal phase transition temperature. T0 is approached: the phase transition is accompanied by substantial softening of the transverse (110), polarised (110) acoustic phonon mode as the Brillouin zone centre. A decrease in C11 as the temperature is reduced towards T0 shows that the longitudinal (100) mode also softens slightly. Hydrostatic pressure dependences of the elastic constants (including delta C'/ delta P) measured at room temperature are all positive: at temperatures well above T0 the application of pressure does not markedly enhance the mode softening.