PHYSICAL REVIEW B 105, 014105 (2022) Polyamorphism and liquid-liquid phase transition in B 2 O 3 Giovanni Carini Jr., 1 Antonino Bartolotta, 2 Gaetano Di Marco, 2 Barbara Fazio , 2 Mauro Federico, 1 Valentino Romano, 1 Giuseppe Carini, 1, 2, * and Giovanna D’Angelo 1, 2 1 Dipartimento MIFT, Universita’ di Messina, Viale F. Stagno d’Alcontres 31, I-98166 Messina, Italy 2 IPCF del CNR, Sezione di Messina, Viale F. Stagno d’Alcontres 37, I-98158 Messina, Italy (Received 24 May 2021; revised 21 December 2021; accepted 3 January 2022; published 18 January 2022) Pressure of 4 GPa applied on liquid B 2 O 3 leads to the formation of fourfold coordinated boron atoms and the resulting pressure-quenched glasses reflect the morphology of a “two-species” liquid mainly formed from triangular BO 3 and tetrahedral BO 4 groups. Raman spectra of compacted glasses show that pressure quenching of the liquid preserves the two species, also favoring the formation of two superstructural units: boroxol rings (B 3 O 6 ) involving only BO 3 units and pentaborate groups (two boroxol rings linked by a fourfold coordinated boron atom). Calorimetric analysis up to the liquid state shows that these polyamorphic glasses are single-phase systems characterized by a single glass transition with a much higher T g and a lower thermodynamic fragility than those of normal v-B 2 O 3 . Above T g , a sharp endothermic process due to the inverse liquid-liquid phase transition converting the coordination of boron atoms from 4 to 3 is also revealed. It leads to recovering the classical structure (at ambient pressure) of the “single-species” liquid B 2 O 3 . DOI: 10.1103/PhysRevB.105.014105 I. INTRODUCTION Polymorphs are stable forms of a crystalline solid having the same stoichiometry but different structural topology which must, however, preserve the translational periodicity. Classical examples are the eight polymorphs of SiO 2 [1] and the two polymorphs of GeO 2 [1] and B 2 O 3 [2]. While transitions be- tween different stable phases at ambient pressure are regulated by the temperature with reconstructive topological changes, some high-pressure phases of SiO 2 (stishovite, rutile-type structure), GeO 2 (rutile-type structure, tetragonal), and B 2 O 3 (B 2 O 3 − II, tetrahedral) imply changes of the short-range or- der with variations of the cation coordination from 4 to 6 for both SiO 2 and GeO 2 and from 3 to 4 for B 2 O 3 [1–3]. These latter examples emphasize the effect of high pressures on crystalline solids and are of special interest with regard to the possibility for their liquid state of the coexistence of different states having different local densities between which dynamic transitions mainly driven by GPa pressures can oc- cur [4–6]. A first-order phase transition between two different liquid forms of phosphorous (from a molecular to a polymeric structure) has been clearly observed by in situ x-ray diffrac- tion over a narrow pressure interval (0.02 GPa) [4]. Quite recently [7], very refined in situ measurements of density, x-ray diffraction, and Raman scattering over wide pressure (0–3 GPa) and temperature (300–1100 K) ranges evidenced a first-order liquid-liquid transition in sulfur, with the further important observation of a liquid-liquid critical point. Liquid- liquid phase transitions (LLT) revealed on different inorganic systems under high pressure by experiments and computer simulations [6,8,9] raise the question about their real nature: * carini@unime.it sharp first-order phase transitions as in crystals or smooth transitions? We explore the correlation between polymorphism in liq- uids and the modifications of the short- and medium-range orders by an investigation of heat capacity and Raman scat- tering on pressure-quenched glasses of B 2 O 3 . Even if a glass cannot be considered as an ergodic system, its structure re- flects that of the supercooled liquid, so evidence of polyamor- phism in glasses gives an unambiguous indication of the existence of distinct states in the corresponding liquid [10]. B 2 O 3 was chosen as the model system because the pressure dependence of viscosity η along the melting curve revealed a sudden decrease, above 3.5 GPa, towards values smaller than 1 Pa s [11]. This viscosity drop has been ascribed [11,12] to the gradual conversion of threefold to fourfold coordinated borons (change of BO 3 in BO 4 ). BO 3 and BO 4 groups are reminiscent of the basic blocks building up the two crys- talline polymorphs of B 2 O 3 ,B 2 O 3 -I [13] and B 2 O 3 -II [14], respectively. Here, we prove that pressure-quenched B 2 O 3 glasses, where two polymorphic structures coexist (the “normal” one based on threefold coordinated boron atoms and the more “packed” one formed by clusters of triangular BO 3 units connected by fourfold coordinated atoms), occur as single-phase systems characterized by a single glass tran- sition with a much higher T g than that of normal v-B 2 O 3 . Quite above T g , i.e., in the liquid phase, they also experi- ence a phase transformation (a liquid-liquid transition) which converts the fourfold coordinated borons into tricoordinated atoms. These findings give clear evidence for the existence of polyamorphism in B 2 O 3 glasses and in the liquids from which glasses are obtained by pressure quenching, also clari- fying the nature of the pressure-driven LLT between the two states. 2469-9950/2022/105(1)/014105(7) 014105-1 ©2022 American Physical Society