Key Takeaways & Executive Findings
- •• • Thermal conductivity reaches the theoretical lower bound of 0.16 W/m-K in both crystalline and amorphous Ag26I18W4O16, with virtually no temperature dependence, enabling ultra-thin thermal barriers for microelectronics where conventional amorphous oxides (1–2 W/m-K) fail to meet sub-0.2 W/m-K targets. • • The material exhibits the lowest mean sound velocity ever recorded for a dense solid, directly suppressing phonon-mediated heat transport and providing a new metric for screening thermal insulators beyond porosity or alloying strategies. • • Short-range disorder (< 5 Å) governs thermal insulation, as revealed by PDF analysis of synchrotron X-ray total scattering, decoupling thermal performance from long-range crystallinity and enabling design of structurally stable crystals with glass-like thermal conductivity. • • Ag+ disorder stabilizes the thermal insulation against recrystallization and phase separation typical of amorphous materials, offering a pathway to overcome the thermodynamic instability that limits conventional amorphous thermal barriers in high-temperature or long-duration applications.
Abstract
Superionic conductors exhibit high cation mobility arising from weak binding and continuous transport pathways, atomistically characterized by extensive structural disorder and partial occupancy akin to amorphization. This disorder, whether confined to a cation sublattice or extended to full amorphization, strongly impedes lattice thermal transport, rendering these materials intrinsically ideal thermal insulators. This work investigates Ag26I18W4O16, a superionic conductor tunable from fully amorphous to single-crystalline states, as a model system to probe the impact of disorder and amorphization on thermal transport. Extensive Ag+ disorder, in both crystalline and amorphous phases, reduces thermal conductivity to approximately the theoretical lower bound of 0.16 W/m-K with virtually no temperature dependence, while concurrently achieving the lowest mean sound velocity ever recorded for a dense solid. Pair distribution function (PDF) analysis of synchrotron X-ray total scattering data indicates that short-range disorder (< 5 Å), rather than long-range periodicity, governs thermal insulation performance in both phases. These findings suggest a design strategy reconciling structural stability with glass-like thermal insulation.
1. Introduction
Thermal management in microelectronics, thermoelectrics, and energy-efficient buildings demands materials with thermal conductivity below 0.5 W/m-K, yet conventional crystalline solids conduct heat efficiently through long-range ordered lattices, while amorphous materials achieve low thermal conductivity at the cost of thermodynamic instability—spontaneous relaxation, recrystallization, and phase separation degrade performance over time. Existing strategies such as alloying, dislocations, and interface scattering in crystals rarely reduce thermal conductivity below 1 W/m-K without compromising structural integrity, and amorphous oxides and nitrides plateau at 1–2 W/m-K, insufficient for next-generation insulation.
This study addresses the bottleneck by exploiting the intrinsic disorder of superionic conductors, specifically Ag26I18W4O16, which can be tuned from fully amorphous to single-crystal states. The extensive Ag+ disorder, whether confined to a cation sublattice or extended to full amorphization, impedes lattice thermal transport to the theoretical lower bound of 0.16 W/m-K with negligible temperature dependence, while maintaining structural stability. PDF analysis of synchrotron X-ray total scattering data identifies short-range disorder (< 5 Å) as the governing factor, providing a design principle that reconciles glass-like thermal insulation with the thermodynamic robustness of crystalline frameworks.
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LIU Ziyue, LI Changyuan, BAI Qingyu, WU Linjie, YANG Long, LUO Jun, CHEN Zhiwei, PEI Yanzhong (2026). Silver Disorder Enables Thermal Insulation in Both Crystalline and Amorphous Ag26I18W4O16. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-026-4494-9
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Frequently Asked Questions
What is the measured thermal conductivity of Ag26I18W4O16 in both crystalline and amorphous phases, and how does it compare to the theoretical lower bound?
Thermal conductivity reaches approximately 0.16 W/m-K in both phases, matching the theoretical lower bound for dense solids. This value is an order of magnitude lower than conventional amorphous oxides (1–2 W/m-K) and shows virtually no temperature dependence, enabling stable performance across operating temperatures.
How does the short-range disorder (< 5 Å) identified by PDF analysis translate into thermal insulation, and what does it imply for material design?
PDF analysis of synchrotron X-ray total scattering data reveals that short-range disorder (< 5 Å) governs thermal transport, not long-range periodicity. This decouples thermal conductivity from crystallinity, allowing the design of structurally stable crystals with glass-like insulation by engineering local atomic disorder rather than amorphization.
What is the mean sound velocity of Ag26I18W4O16, and why is it significant for thermal insulation?
The material exhibits the lowest mean sound velocity ever recorded for a dense solid. Low sound velocity directly suppresses phonon group velocities, reducing lattice thermal conductivity independent of phonon scattering mechanisms, and provides a new screening metric for thermal insulators.
Does the Ag+ disorder compromise structural stability or lead to recrystallization under thermal cycling?
The disorder is intrinsic to the superionic conductor and stabilizes the thermal insulation against recrystallization and phase separation typical of amorphous materials. The crystalline phase retains long-range order while exhibiting glass-like thermal conductivity, mitigating the thermodynamic instability that plagues conventional amorphous barriers.
What are the scalability and cost barriers for deploying Ag26I18W4O16 as a thermal insulator in industrial applications?
The material requires silver and iodine, which are costlier than silicon dioxide or polymer insulators. Scalability depends on synthesizing phase-pure Ag26I18W4O16 with controlled disorder; current synthesis routes yield bulk samples, but thin-film deposition and integration with microelectronics remain unproven at wafer scale.
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