Dielectric constant of compressed solid methane at low temperature

Abstract

Measurements of the static dielectric constant and the derived Clausius−Mossotti (CM) values for the solid phases of methane at densities ranging to 20% above the triple point density are reported. The dielectric constant data for 60<T<260 K and 0.52<ρ0.60 g/cm3 may be fitted within experimental error to εs(ρ,T) = 0.7293±0.0006 − (3.2±0.3) × (10−5) T + (2.066±0.008) ρ and the CM data to CM (ρ,T) = 6.497±0.022 − (1.76±0.16) × (10−4) T + (0.061±0.04) ρ. Within experimental error, the CM function is constant across the fluid−solid transition at T = 157.2 K and is independent of density in the solid phase. These data are discussed in terms of a model based on the dielectric behavior of octupole−induced dipoles. The low temperature solid−solid P−T phase diagram is mapped using dielectric anomalies at the transitions and calculated pressures. The existence of a fourth, or δ, phase is demonstrated by a dielectric anomaly that is suggestive of ferro− or antiferroelectric behavior.