Precision adiabatic scanning calorimetry of a nematic – ferroelectric nematic phase transition

ABSTRACT

In high-resolution adiabatic scanning calorimetry (ASC) experiments, data for the temperature dependence of the specific enthalpy, h(T), and of the specific heat capacity, c_p(T), are simultaneously obtained, from which the order of the phase transition and critical behaviour can be evaluated. ASC was applied to study the nematic to ferroelectric nematic phase transition (N–N_F) in the liquid crystal molecule 4-[(4-nitrophenoxy)carbonyl]phenyl 2,4-dimethoxybenzoate (RM734). The N–N_F was found to be very weakly first order with a latent heat Δh = 0.115 ± 0.005 J/g. The pretransitional specific heat capacity behaviour is substantially larger in the high-temperature N phase than in the low-temperature N_F phase. In both phases the power-law analysis of c_p(T) resulted in a critical exponent α = 0.50 ± 0.05 and amplitude ratio A_NF/A_N = 0.42 ± 0.03. The very small latent heat and the value of α indicate that the N–N_F transition is close to a tricritical point. This is confirmed by a value of the order parameter exponent β ≈ 0.25, recently obtained from electric polarisation measurements. Invoking two-scale-factor universality, it follows from the low value of A_NF/A_N ratio that the size of the critical fluctuations is much larger in the N_F phase than in the N phase.

Graphical Abstract

KEYWORDS:

• Heat capacity
• enthalpy
• ferroelectric nematic
• tricritical point

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported by NSF Condensed Matter Physics Grants DMR 1710711 and DMR 2005170, and by Materials Research Science and Engineering Center (MRSEC) Grant DMR 1420736.