X-Ray Study of Phase Transition in (NH₄)₂BeF₄

抄録

X-ray scattering from (NH₄)₂BeF₄ at temperatures near T_m=185 K has been measured in order to elucidate the nature of the mechanism of the phase transition. The intensity of the superlattice reflection at (1/2, 0, 6) decreases with increasing temperature and disappears at T_m indicating that the transition at T_m is of the second order. The diffuse intensity around (1/2, 0, 6) increases just above T_m. It is suggested that the transverse phonon polarized along the c-direction softens at (Remark: Graphics omitted.) in (NH₄)₂BeF₄ near T_m.