Colossal barocaloric effects with ultralow hysteresis in two-dimensional metal-halide perovskites

Abstract

Pressure-induced thermal changes in solids-barocaloric effects-can be used to drive cooling cycles that offer a promising alternative to traditional vapor-compression technologies. Efficient barocaloric cooling requires materials that undergo reversible phase transitions with large entropy changes, high sensitivity to hydrostatic pressure, and minimal hysteresis, the combination of which has been challenging to achieve in existing barocaloric materials. Here, we report a new mechanism for achieving colossal barocaloric effects that leverages the large volume and conformational entropy changes of hydrocarbon order-disorder transitions within the organic bilayers of select two-dimensional metal-halide perovskites. Significantly, we show how the confined nature of these order-disorder phase transitions and the synthetic tunability of layered perovskites can be leveraged to reduce phase transition hysteresis through careful control over the inorganic-organic interface. The combination of ultralow hysteresis and high pressure sensitivity leads to colossal reversible isothermal entropy changes (>200 J kg^-1 K^-1) at record-low pressures (<300 bar).

Fig. 1. Barocaloric cooling with two-dimensional (2-D) metal–halide perovskites.

a Illustration of how the pressure dependence of hydrocarbon order–disorder transitions in the organic bilayers of hybrid 2-D perovskites can be leveraged to drive a barocaloric cooling cycle. Each cooling cycle begins with an adiabatic (Brayton-like cycle) or isothermal (Stirling-like cycle) increase in pressure that induces a first-order phase transition from an expanded, high-entropy phase of the 2-D perovskite to a contracted, low-entropy phase. Heat released during this exothermic transition is dissipated to a heat sink, returning the material to its original temperature but now at a lower entropy. The pressure is then adiabatically or isothermally decreased to reverse the phase transition and cool a heat source. b Comparison of phase transition entropies, ΔS_tr, and transition temperatures, T_tr, for order–disorder transitions in select 2-D perovskites of the form (C_nH_2n+1NH₃)₂MX₄ (n = 7–16; M = Mn, Cd, Cu, or Pb; X = Cl, Br, or I). Thermally induced phase transitions in 2-D perovskites are often accompanied by large changes in entropy that are sensitive to the length of the hydrocarbon chain and the identity of the metal and halide in the inorganic layer. Many of these order–disorder transitions involve one or more minor transitions that occur at lower or higher temperatures than the major transition, but (DA)₂MnCl₄ (DA = decylammonium) features a single order–disorder transition with a large entropy change near ambient temperature.

Fig. 2. Thermally induced hydrocarbon order–disorder transitions in (DA)₂MnCl₄ and (NA)₂CuBr₄ at ambient pressure.

Differential scanning calorimetry (DSC) traces for powder samples of (a) (DA)₂MnCl₄ and (b) (NA)₂CuBr₄ at 1 bar with heating (red) and cooling (blue) rates of 2 K min⁻¹. Thermal hysteresis (ΔT_hys) is indicated by the vertical gray bars. Note that ΔT_hys is calculated as the difference between heating and cooling transition onset temperatures, with ΔT_hys = T_tr,heating – T_tr,cooling. Specific volumes obtained from variable-temperature powder X-ray diffraction and He pycnometry measurements are used to determine the volume changes, ΔV, that accompany the order–disorder transition for (c) (DA)₂MnCl₄ and (d) (NA)₂CuBr₄. Variable-temperature crystal structures of the low-temperature (LT) and high-temperature (HT) phases of (e) (DA)₂MnCl₄ and (f) (NA)₂CuBr₄. Note that the LT crystal structures were both obtained at 270 K, while the HT crystal structures were obtained at 330 K and 335 K for (DA)₂MnCl₄ and (NA)₂CuBr₄, respectively. Purple, orange, green, brown, gray, and blue spheres represent Mn, Cu, Cl, Br, C, and N atoms, respectively. H atoms are omitted for clarity. Note that DA chains are disordered over a special position in both the LT and HT phases, while NA chains are modeled with two-part disorder in the LT phase and disordered over a special position in the HT phase. Geometric parameters obtained from single-crystal X-ray diffraction experiments are summarized in Supplementary Tables 14–22.

Fig. 3. Barocaloric effects in 2-D metal–halide perovskites.

DSC measurements under applied hydrostatic pressure for single-crystal samples of (a) (DA)₂MnCl₄ and (d) (NA)₂CuBr₄ with heating and cooling rates of 2 K min⁻¹. Isothermal entropy changes, ΔS_it, are calculated by the quasi-direct method for (b) (DA)₂MnCl₄ and (e) (NA)₂CuBr₄ for compression from ambient pressure and for decompression to ambient pressure. The shaded area indicates the reversible ΔS_it within this pressure range. Isobaric entropy curves are shown in Supplementary Figs. 10 and 11. Direct evaluation of pressure hysteresis, ΔP_hys, through quasi-isothermal DSC experiments for (c) (DA)₂MnCl₄ and (f) (NA)₂CuBr₄ at 311 K and 306 K, with pressure cycling from 1 to 150 bar and to 105 bar, respectively. ΔP_hys is calculated as the difference between the onset pressure for the compression-induced exotherm and the decompression-induced endotherm and is indicated by the horizontal green bar. Variable-temperature powder X-ray diffraction (PXRD) patterns for (g) (DA)₂MnCl₄ and (i) (NA)₂CuBr₄ at 360 bar and 300 bar of He, respectively, while cooling from 325 K to 280 K, with an X-ray wavelength of 0.45237 Å. The pressure dependence of the order–disorder transition temperature as determined by HP-DSC (diamonds) and PXRD (squares) is used to calculate the barocaloric coefficient, dT/dP, for (h) (DA)₂MnCl₄ and (j) (NA)₂CuBr₄. Red and blue symbols indicate the phase transition temperatures during heating and cooling, respectively. Barocaloric coefficients are summarized in Supplementary Table 8.

Fig. 4. Dependence of low-pressure reversibility on thermal contact.

The phase boundary determined from HP-DSC experiments for (a–d) (DA)₂MnCl₄ and (e–h) (NA)₂CuBr₄, using (a, b, e, f) single-crystal samples with improved thermal contact and (c, d, g, h) powder samples. Scan rates of 2 K min^–1 were used for all experiments, and He was used as the pressure-transmitting medium. DSC traces at ambient pressure are show in the left panel, with transition peak width highlighted in red and blue shades for heating and cooling, respectively. Onset temperatures are highlighted in dashed gray lines, with thermal hysteresis marked using black arrows. The pressure dependence of the onset and completion transition temperatures is shown in the right panels, and these phase boundaries illustrate the impact of the transition width on the minimum pressure required to drive a reversible isothermal entropy change (P_rev) and a reversible adiabatic temperature change (P_rev,ad). Note that the isobaric HP-DSC data for the powder samples is shown in Supplementary Fig. 16. Reversible barocaloric effects for powder and single-crystal samples are summarized in Supplementary Table 7.

Fig. 5. Isobaric DSC experiments from 300–500 bar.

DSC measurements under applied hydrostatic pressure for powder samples of (a) (DA)₂MnCl₄ and (d) (NA)₂CuBr₄ up to 500 bar, with heating and cooling rates of 0.5 K min^–1. Note that He was used as the pressure-transmitting medium. Isothermal entropy changes (ΔS_it) calculated by the quasi-direct method for (b) (DA)₂MnCl₄ and (e) (NA)₂CuBr₄. The shaded area indicates the reversible ΔS_it within this pressure range. Maximum reversible isothermal entropy change (ΔS_it,rev,max) and reversible refrigeration capacity (RC_rev) for (c) (DA)₂MnCl₄ and (f) (NA)₂CuBr₄ as a function of operating pressure, with the minimum pressures required to induce a reversible isothermal entropy change (P_rev) and a reversible adiabatic temperature change (P_rev,ad) marked using vertical lines. Note that ΔS_it,rev,max is equivalent to the peak value of each reversible isothermal entropy curve, and RC_rev is calculated as ΔS_it,rev,max × ΔT_FWHM. The dependence of RC_rev on operating pressure above P_rev,ad is highlighted using blue dashed lines. The pressure dependence of RC_rev is 4524 J kbar^–1 kg^–1 and 1830 J kbar^–1 kg^–1 for (DA)₂MnCl₄ and (NA)₂CuBr₄, respectively.

Fig. 6. Reversible barocaloric effects at 500 bar.

Isobaric entropy changes (ΔS_ib) during heating and cooling at ambient pressure and 500 bar are shown for (a) (DA)₂MnCl₄ and (b) (NA)₂CuBr₄. Note that the ΔS_ib curves include contributions from the heat capacity. The area between ΔS_ib(T, 1 bar)_heating and ΔS_ib(T, 500 bar)_cooling curves denotes the temperature range over which both reversible isothermal entropy changes (ΔS_it,rev) and reversible adiabatic temperature changes (ΔT_ad,rev) are accessible.

Fig. 7. Properties of representative barocaloric materials.

a Comparison of the phase-change entropy, ΔS_tr, and temperature, T_tr, for different classes of barocaloric materials. Note that ΔS_tr and T_tr are shown for endothermic transitions, and ΔS_tr represents the maximum isothermal entropy change that could be driven by the pressure-induced phase transition. b Comparison of thermal hysteresis, ΔT_hys, and barocaloric coefficient, dT/dP, for leading barocaloric materials, with dT/dP values corresponding to exothermic transitions for materials that exhibit conventional barocaloric effects and endothermic transitions for materials that exhibit inverse barocaloric effects. The minimum pressure required to achieve a reversible entropy change, P_rev, is calculated as P_rev = ΔT_hys/|dT/dP| and indicated by shading from blue (high P_rev) to white (low P_rev). A comprehensive tabulation of barocaloric properties, including reversible and irreversible ΔS_it values, is provided in Supplementary Tables 5 and 6.

Fig. 8. Variable-temperature single-crystal structures.

Conformations of the alkylammonium chains in the LT and HT phases of (a) (DA)₂MnCl₄ and (b) (NA)₂CuBr₄, with atomic displacement parameters shown at 50% probability for the C and N atoms of the alkylammonium chains. In the LT phase, decylammonium (DA) chains in (DA)₂MnCl₄ display one conformation, with a single gauche C–C bond (C2–C3), while nonylammonium (NA) chains in (NA)₂CuBr₄ adopt two conformations, alternating between chains with a gauche C1–C2 bond (chain A) and those with a gauche C2–C3 bond (chain B). Purple, orange, green, brown, gray, and blue spheres represent Mn, Cu, Cl, Br, C, and N atoms, respectively. H atoms are omitted for clarity. Note that DA chains are disordered over a special position in both the LT and HT phases, while NA chains are modeled with two-part disorder in the LT phase and disordered over a special position in the HT phase. Temperature dependence of U_equiv (equivalent isotropic displacement parameters) is shown for alkylammonium cations in (c) (DA)₂MnCl₄ and (d) (NA)₂CuBr₄, at 100 K, 270 K (LT phase), and 330/335 K (HT phase). Error bars represent standard uncertainties.

Fig. 9. Phase transition of single crystals on a substrate.

a Optical images of (DA)₂MnCl₄ single crystals directly grown on Si substrates through an anti-solvent vapor-assisted capping crystallization method. b Bright field (top) and dark field (bottom) images of a sub-micron thick single crystal. c Variable-temperature atomic force microscope (AFM) imaging experiments for a sub-micron thick (DA)₂MnCl₄ single crystal. d Height profiles of a sub-micron thick (DA)₂MnCl₄ single crystal in the low-temperature (LT, left) and high-temperature (HT, right) phases. e Thickness of a single crystal of (DA)₂MnCl₄ as a function of temperature. The heating-induced transition from the LT phase to HT phase gives rise to an 7.9% change in height, which agrees well with the 7.4% predicted from crystallographic data obtained from variable-temperature PXRD experiments at 30 °C and 45 °C. Error bars were obtained from the mean square difference between all data points in the scan region and the fitted step height. Thermal cycling AFM experiments are described in Supplementary Fig. 43.