The discovery that AgCrSe2 conducts heat like a liquid — ultrafast occupational disorder of the intercalated Ag ions selectively collapses transverse acoustic phonons above the 450 K order–disorder transition while longitudinal acoustic phonons survive, giving intrinsically ultralow lattice thermal conductivity — raised a coherent follow-up objective that remains open: converting this single-compound mechanism into a controllable, general materials-design principle. Specifically, later literature has neither experimentally demonstrated the predicted switching of intercalant interstitial-site occupation by uniaxial stress at the density-functional-theory-predicted critical stresses with the corresponding on/off switching of the liquid-like conduction regime, nor tested the mechanism beyond the Cr-based ACrX2 subfamily against the data-mined A+TM3+X2− candidate list (A = Ag, In, Tl; TM = 3d transition metals; X = S, Se), nor produced a validated quantitative design rule relating intercalant mass, gap bonding, and disorder relaxation time to transverse-phonon collapse.

Background

In a crystalline solid, heat is carried by phonons — quantized lattice vibrations — of which the heat-carrying acoustic branches are transverse (TA, atomic motion perpendicular to the propagation direction) and longitudinal (LA, motion along it); a liquid, lacking shear rigidity, supports only longitudinal waves. Superionic conductors are crystals in which one ionic sublattice becomes highly mobile, while the rest retains crystalline periodicity. AgCrSe2 is a layered superionic compound: CrSe2 octahedral layers are separated by van der Waals gaps (the weakly bonded region between layers) in which Ag ions occupy two degenerate tetrahedral interstitial sites. Around 450 K the Ag sublattice undergoes an order–disorder transition (space group R3m to R-3m), with Ag ions hopping between the interstitial sites on picosecond timescales, accompanied by a sharp rise in ionic conductivity. Li and co-workers (Nature Materials 17, 226–230, 2018) combined inelastic neutron and X-ray scattering (measurements of phonon energies and linewidths versus momentum) with density-functional-perturbation-theory calculations (DFPT, the standard first-principles method for phonon spectra) and showed that in AgCrSe2 the Ag-dominated low-energy TA phonons are damped as the transition is approached and collapse above it, because the Ag occupational fluctuations relax faster than a TA vibration period; the LA branch is strongly scattered but survives and carries heat. The result is liquid-like thermal conduction in a crystal, with an intrinsically ultralow lattice thermal conductivity κL ≈ 0.2 W/m/K around 500 K — attractive for thermoelectrics, where a low lattice thermal conductivity combined with good electronic transport raises the figure of merit ZT. The same paper made three forward-looking claims that define the present problem. First, calculations across ACrSe2 (A = Li, Ag, Cs) showed that heavy intercalants dominate the TA spectral weight, suggesting the mechanism should recur whenever a heavy, weakly bonded intercalant sits in a van der Waals gap. Second, first-principles total-energy calculations predicted that uniaxial stress along the crystallographic c axis switches the preferred Ag interstitial site between tetrahedral (R3m) and octahedral (P-3m1) coordination — implying the order–disorder physics, and hence the liquid-like conduction regime, could be mechanically switched on or off. Third, a data-mined ionic-substitution structure search (Materials Project predictor) proposed candidate chemistries of formula A+TM3+X2− (A = Ag, In, Tl; TM = 3d transition metals except Zn; X = S, Se) likely to crystallize in the R3m structure with heavy intercalants on tetrahedral sites. Later work has substantially refined the microscopic picture: hybrid propagating-and-diffusive dynamics with wavelength-dependent TA breakdown (Ding et al., PNAS 2020), large resonant four-phonon scattering (Xie et al., PRL 2020), anisotropic phonon-liquid scattering in which only in-plane zone-boundary TA modes collapse (Wang and Chen, Nano Letters 2023), and the persistence of long-wavelength heat-carrying acoustic phonons in the Cu analogue CuCrSe2 (Niedziela et al., Nature Physics 15, 73, 2019). A parallel literature measured transport across the whole ACrX2 family (Rahman et al., 2025) and family-wide atomic dynamics (Ding et al., Physical Review Materials 2025). What no later study has delivered is the control and generality that the 2018 work itself identified as decisive: experimental stress-switching of the intercalant occupation, verification in the predicted non-Cr candidates, and a quantitative predictive criterion.

Problem Statement

The overarching objective is to convert the single-compound AgCrSe2 mechanism — ultrafast intercalant occupational disorder selectively collapsing transverse acoustic phonons — into a controllable and general materials-design principle for intrinsically ultralow lattice thermal conductivity in intercalated layered crystalline solids. Within that single objective, the unresolved questions inherited from the source work are: (i) can uniaxial stress along the c axis be demonstrated experimentally to switch the intercalant interstitial-site occupation between tetrahedral (R3m) and octahedral (P-3m1) coordination in real samples at the density-functional-theory-predicted critical stresses (the R3m/P-3m1 phase boundaries of Fig. 5a of the source paper), thereby switching the liquid-like transverse-phonon-collapse regime and its ultralow lattice thermal conductivity on and off; (ii) do other predicted A+TM3+X2- compounds (A = Ag, In, Tl; TM = 3d transition metals other than Zn; X = S, Se) — not only the already-studied Cr-based ACrX2 subfamily — exhibit the same ultrafast occupational disorder, selective transverse-acoustic suppression, and ultralow lattice thermal conductivity when synthesized and measured by temperature-resolved inelastic scattering and thermal transport, the target level being candidates spanning every distinct intercalant A (Ag, In, Tl) and every distinct chalcogen X (S, Se) rather than an exhaustive screen of the entire data-mined list; and (iii) can a quantitative, validated design rule be formulated that relates the intercalant mass, the bonding strength across the van der Waals gap, the occupational relaxation time, and the transverse/longitudinal phonon spectra, predicting which intercalated compositions realize the liquid-like conduction regime. A design rule counts as validated only when it reproduces the known AgCrSe2 and ACrX2 behaviour and, without refitting, correctly predicts the outcome for at least one candidate composition not used in constructing it. These three sub-questions are jointly required components of that single objective, not independent alternatives: the objective is resolved only when all three have been addressed, and progress on one or two of them alone leaves it open.

References

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