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Abstract The structure of liquid lithium pyroborate, Li₄B₂O₅(J= Li/B = 2), has been measured over a wide temperature range by high‐energy X‐ray diffraction, and compared to that of its glass and borate liquids of other compositions. The results indicate a gradual increase in tetrahedral boron fraction from 3(1)% to 6(1)% during cooling fromT= 1271(15) to 721(8) K, consistent with the largerN₄ = 10(1)% found for the glass, and literature¹¹B nuclear magnetic resonance measurements. van't Hoff analysis based on a simple boron isomerization reaction BØ₃O^2–⇌ BØO₂^2–yields ΔH= 13(1) kJ mol^–1and ΔS= 40(1) J mol^–1 K^–1for the boron coordination change from 4 to 3, which are, respectively, smaller and larger than found for singly charged isomers forJ ≤ 1. With these, we extend our model forN₄(J,T), nonbridging oxygen fractionf_nbr(J,T), configurational heat capacity , and entropyS^conf(J,T) contributions up toJ= 3. A maximum is revealed in atJ= 1, and shown semi‐quantitatively to lead to a corresponding maximum in fragility contribution, akin to that observed in the total fragilities by temperature‐modulated differential scanning calorimetry. Lithium is bound to 4.6(2) oxygen in the pyroborate liquid, with 2.7(1) bonds centered around 1.946(8) Å and 1.9(1) around 2.42(1) Å. In the glass,n_LiO= 5.4(4), the increase being due to an increase in the number of short Li–O bonds. 

Abstract The primary mechanism of optical memoristive devices relies on phase transitions between amorphous and crystalline states. The slow or energy‐hungry amorphous–crystalline transitions in optical phase‐change materials are detrimental to the scalability and performance of devices. Leveraging an integrated photonic platform, nonvolatile and reversible switching between two layered structures of indium selenide (In₂Se₃) triggered by a single nanosecond pulse is demonstrated. The high‐resolution pair distribution function reveals the detailed atomistic transition pathways between the layered structures. With interlayer “shear glide” and isosymmetric phase transition, switching between the α‐ and β‐structural states contains low re‐configurational entropy, allowing reversible switching between layered structures. Broadband refractive index contrast, optical transparency, and volumetric effect in the crystalline–crystalline phase transition are experimentally characterized in molecular‐beam‐epitaxy‐grown thin films and compared to ab initio calculations. The nonlinear resonator transmission spectra measure of incremental linear loss rate of 3.3 GHz, introduced by a 1.5 µm‐long In₂Se₃‐covered layer, resulted from the combinations of material absorption and scattering.