Lfa 467 hyperflash instrument

The thermal conductivity of each sample was measured by the flash diffusivity method (ASTM E1461-13) using a Netzsch LFA 467 HyperFlash instrument (Supplementary Fig. 42). The samples were prepared in pellets of approximately 12.9 mm in diameter and 1 mm thickness. The measurements were carried out under a Helium atmosphere at 10 °C below and 15 °C above the transition temperature for the LT and HT phases, respectively, of (DA)₂MnCl₄, and at 10 °C below and above the transition temperature for the LT and HT phases, respectively, of (NA)₂CuBr₄. The measurement results were not corrected for thermal expansion due to phase changes.

To investigate the electrical
and thermal transport properties parallel and perpendicular to the
sintering direction, the sintered samples were cut and polished into
disks (10 mm diameter and ∼1.5 mm thickness, perpendicular
to the pressing direction) and cuboids of 8 × 8 × 2 mm³ parallel to the pressing direction for Hall effect and thermal
diffusivity measurements and bars of 2 × 2 × 10 mm³ (parallel and perpendicular to the pressing direction) for electrical
property measurements. The total thermal conductivity (κ_total) was calculated from the thermal diffusivity (D), heat capacity (C_p) and density
(ρ): κ_total = DC_pρ. The temperature-dependent thermal diffusivity D was measured on disk-shaped samples by a laser flash diffusivity
method using a Netzsch LFA-467 Hyperflash instrument. The temperature-dependent
heat capacity was derived using a standard sample (Pyroceram-9060).
The directions of measurement and sample shapes are illustrated in Figure 1. X-ray powder diffraction
analysis was performed with a PANalytical X’Pert PRO instrument,
using Cu Kα1 radiation (λ = 1.54059 Å) to identify
the crystal structure of each sample. Rietveld refinement was performed
using GSAS-II^{43 (link)} to obtain the lattice parameters
for all samples.