Ultrasensitive barocaloric material for room-temperature solid-state refrigeration

The study addresses the challenge of developing efficient, low-carbon solid-state refrigeration materials that operate under small driving fields. Conventional vapor-compression refrigeration consumes 25–30% of global electricity and uses refrigerants with high global warming potential, motivating solid-state caloric alternatives. Caloric technologies (magnetocaloric, electrocaloric, elastocaloric, barocaloric) can be compact, emission-free, and potentially highly efficient. However, leading caloric materials typically require large driving fields (e.g., >2 T magnetic fields, kV–MV m−1 electric fields, ~700 MPa stress, >100–200 MPa pressure), hindering practical deployment. The purpose of this study is to identify and mechanistically understand an inorganic barocaloric material that exhibits large thermal effects near room temperature under small pressures, thereby improving barocaloric strength and practical feasibility. The hypothesis is that ammonium iodide (NH4I) exhibits a pressure-sensitive structural phase transition driven by strong coupling between molecular reorientations and lattice vibrations, enabling giant barocaloric effects at low pressures.

• Existing caloric materials often require large driving fields: magnetocalorics (>2 T), electrocalorics (kV–MV m−1), elastocalorics (~700 MPa), and typical barocalorics (>200 MPa in intermetallics; ~100 MPa in plastic crystals). These constraints limit scalability and cost-effectiveness.
• Prior barocaloric materials include intermetallics (e.g., La-Fe-Si-Co, Ni-Mn-In), Heusler alloys, oxides (BaTiO3), superionic conductors (AgI), organic-inorganic hybrids, and plastic crystals, which show large entropy changes but often at substantial pressures.
• Reports on ammonium halides have documented multiple structural phases and molecular reorientation dynamics; pressure and temperature effects on NH4I (and ND4 analogs) have been studied by diffraction, NMR, Raman, and neutron scattering, indicating order–disorder transitions and hydrogen-bond mediated dynamics. However, their barocaloric performance at room temperature and under low pressures had not been fully explored prior to this work.

Materials and sample preparation: Commercial NH4I powder (99.999% purity, Aladdin) was used.

Structural characterization: Room-temperature powder XRD on Rigaku Miniflex-600 (Cu Kα, 10°–90°, 0.02° step). Temperature-variable XRD on Rigaku Smartlab (Cu Kα): sample cooled to 175 K (held 30 min), then measured from 175–375 K in 10 K steps. Rietveld refinements performed using FullProf.

Barocaloric calorimetry: High-pressure differential scanning calorimetry (μDSC, Setaram). Constant-pressure scans: ~20 mg powder sealed in Hastelloy high-pressure vessel; empty vessel as reference. Argon gas provided hydrostatic pressures of 0.1, 10, 20, 30, 40, 50, 60, 70, 80 MPa. Temperature range 230–340 K; heating and cooling rates 1 K min−1. Variable-pressure scans at room temperature: ~42 mg powder; pressure programs of 50–90 MPa (pressurization) and 50–7.5 MPa (depressurization); heat flow vs time recorded; background determined away from phase transitions (using thermal hysteresis to avoid overlap). From heat-flow data, phase diagram (T–P), isobaric entropy change ΔSp at transition, and pressure-induced isothermal entropy change ΔSp→p were derived. Adiabatic temperature change estimated via ΔTad = TΔS/Cp using tabulated Cp.

Inelastic neutron scattering (INS) and quasi-elastic neutron scattering (QENS): Conducted on cold-neutron TOF spectrometer PELICAN (ANSTO). Instrument settings: incident wavelength 4.69 Å (Ei = 3.72 meV), energy resolution 0.135 meV (elastic). Powder sealed in annular Al can. Temperature range 100–390 K to span γ→β and β→α transitions. Measurements of empty can for background and vanadium for normalization and resolution. High-pressure INS at 0.1 and 300 MPa at 300 K using Be-Cu high-pressure cell; KBr used for pressure calibration. Data reduction with LAMP; QENS spectra analyzed in DAVE (Pan module).

Dynamics analysis: QENS spectra modeled as S(Q,ω) = f{Ae(Q)δ(ω) + ΣAqe(Q)L(Q,ω)}⊗R + linear background. Lorentzian half-widths (Γ) used to obtain relaxation times τ = 2ħ/Γ and fitted to Arrhenius behavior to extract activation energies. Elastic incoherent structure factor (EISF) computed as Ae/(Ae + Aqe) and compared to models: C2/C3 jumps, cubic tumbling, isotropic rotational diffusion. Geometric parameters used: H–H distance ~1.67 Å; N–H bond length r ≈ 1.02 Å. Configurational entropy change estimated by ΔSconf = R ln(N2/N1) based on orientational degeneracies.

Phonon DOS: Neutron-weighted DOS extracted from energy-gain side of S(Q,ω) up to 80 meV (limited by Ei), applying Bose and Debye–Waller corrections and neutron-weighting; dominated by H contributions (σ/M ratios: H ≈ 82.02, N ≈ 0.82, I ≈ 0.03 barn/amu). Temperature-dependent DOS analyzed for band softening and broadening.

Thermodynamic relations: Lattice parameters and unit-cell volumes from XRD used to compute (∂V/∂T)P and volume change ΔVt across β↔α under ambient pressure. Clausius–Clapeyron relation ΔSt = ΔVt(∂Tt/∂P)t used to cross-check calorimetric entropy change.

• Giant barocaloric effect in NH4I near room temperature with maximum reversible isothermal entropy change ΔSp→p,max ≈ 71 J K−1 kg−1 under small driving pressure (~40 MPa saturation on heating).
• Very large pressure sensitivity of the transition temperature: dTt/dP ≈ 0.79 K MPa−1 (heating) and 0.81 K MPa−1 (cooling), exceeding most leading barocaloric materials. This yields a broad reversible working window of ~41 K under 80 MPa.
• Giant barocaloric strength: ΔSmax/ΔP ≈ 1.78 J K−1 kg−1 MPa−1, among the highest reported, especially for inorganic materials. Volumetric ΔS ≈ 0.21 J K−1 cm−3.
• Estimated adiabatic temperature change ΔTad ≈ 34 K from ΔS and Cp, among the largest for barocaloric materials.
• Direct variable-pressure calorimetry at 298 K shows pressure-induced entropy changes of ~62.7 J K−1 kg−1 (pressurization 50–90 MPa) and ~65.6 J K−1 kg−1 (depressurization 50–7.5 MPa), consistent with constant-pressure calorimetry.
• Phase behavior: NH4I exhibits γ (P4/nmm) → β (Pm3m) → α (Fm3m) transitions. XRD confirms first-order β→α on heating. Unit-cell thermal expansivities yield marginal lattice-entropy contributions (<0.4 J K−1 kg−1 per phase). Volume change across β↔α: ΔVt ≈ 5.87 × 10−5 m3 kg−1 (~16.95%). Clausius–Clapeyron gives ΔSt ≈ 74.3 J K−1 kg−1, agreeing with calorimetry.
• Reorientation dynamics: QENS shows hydrogen dynamics activated above ~190 K (β and α phases). Relaxation is localized (Γ nearly Q-independent). Activation energies: Ea ≈ 126(5) meV in β-phase and 25(1) meV in α-phase; reorientation at 300 K is ~25× faster than at 260 K. EISF fits consistent with cubic tumbling or isotropic rotational diffusion; orientational degeneracy change (β: Td-like two DOF; α: sixfold single-approach model) yields ΔSconfig ≈ 63 J K−1 kg−1 across β→α, close to measured ΔS.
• Lattice dynamics: Phonon DOS shows pronounced softening of optical bands (On, Oxy, Ozy) and broadening near β→α (~275 K), indicating strong anharmonicity and shallow potentials.
• Pressure response: At 300 K, applying 300 MPa converts dynamics from α-like to β-like: fast reorientation mode is suppressed (Γ decreases), and optical phonon features (∼19 and ∼34 meV) re-emerge, evidencing strong coupling of pressure to both reorientation and lattice vibrations.

The findings demonstrate that NH4I achieves large reversible entropy changes near room temperature under unusually small pressures due to a steep dTt/dP. This directly addresses the key practical barrier of large driving fields in caloric materials, enabling potentially compact, low-cost barocaloric systems. Mechanistically, the giant ΔS arises primarily from configurational entropy associated with the increased orientational degrees of freedom of [NH4]+ tetrahedra across β→α, while the exceptional pressure sensitivity of Tt is rooted in strong coupling between molecular reorientations and lattice vibrations mediated by N–H···I hydrogen bonds. Temperature softens optical phonons (weakening H-bonds) and accelerates reorientation, stabilizing α-phase; pressure hardens phonons and strengthens/reorganizes H-bonds, suppressing reorientation and stabilizing β-phase. The consistency among calorimetry, XRD-derived volumes, Clausius–Clapeyron estimates, QENS activation energies, and INS phonon trends corroborates the proposed coupling-driven transition mechanism. These results position NH4I as a benchmark inorganic barocaloric with high strength, large ΔTad, and broad operating span under accessible pressures, advancing the feasibility of room-temperature solid-state refrigeration.

This work identifies NH4I as an ultrasensitive inorganic barocaloric material operating around room temperature with a giant entropy change (~71 J K−1 kg−1), very high barocaloric strength (~1.78 J K−1 kg−1 MPa−1), low saturation pressure (~40 MPa), and wide reversible temperature span (~41 K at 80 MPa). Neutron scattering and XRD reveal that strong coupling between [NH4]+ reorientations and lattice vibrations, via N–H···I hydrogen bonds, underpins the steep dTt/dP and the large configurational entropy change across β↔α. These insights suggest design rules for future barocaloric materials: enhance reorientation–vibration coupling and orientational degeneracy changes while maintaining low hysteresis and accessible transition temperatures. Future research could focus on direct ΔTad measurements under cycling, engineering microstructures to reduce hysteresis and improve cyclability, exploring alloying or compositional tuning among ammonium halides, and device-level demonstrations leveraging the low-pressure operation.

• The distinction between reorientation models (cubic tumbling vs isotropic rotational diffusion) could not be resolved due to the limited Q range of the QENS data.
• The γ→β transition is subtle in diffraction/QENS and was inferred via MSD and spectral broadening; detailed structural resolution of this transition is limited.
• The adiabatic temperature change (ΔTad ≈ 34 K) is an estimate based on Cp and ΔS, not a direct measurement.
• Direct pressure-induced calorimetry was performed at room temperature; comprehensive mapping of ΔS over temperature under direct pressure cycling is limited.
• Practical cycling stability, mechanical integrity under repeated pressurization, and thermal hysteresis impacts on device performance were not extensively evaluated.