Unlocking a Chemically Fertile Moderate-Pressure Regime in High-Pressure Borate Synthesis Using Soft-Chemistry Precursors

Borates exhibit exceptional structural diversity, yet high-pressure/high-temperature (HP/HT) synthesis has predominantly focused on increasingly extreme conditions, leaving moderate pressure–temperature regimes largely unexplored. Here, we demonstrate that this limitation is primarily kinetic rather than thermodynamic. By integrating sol–gel-derived, chemically homogeneous precursors with HP/HT treatment, we establish a precursor-controlled strategy that enables systematic access to a chemically fertile moderate-pressure regime (≤3 GPa, ≤ 800 °C). This approach allows the synthesis of known high-pressure borates (β-SmB 3 O 6, β-SnB 4 O 7, δ-BiB 3 O 6 ) under substantially milder conditions and extends the β- RE B 5 O 9 ( RE = Pr, Nd) series. Moreover, it leads to the discovery of two new families of hydrated borates, β- RE B 6 O 9 (OH) 3 ( RE = La–Nd) and RE 6 B 36 O 59 (OH) 8 ( RE = Nd, Sm, Eu), whose structures are resolved by 3D electron diffraction and powder XRD. Both families adopt polar P 6 3 symmetry and feature large 12- and 16-membered ring channel topologies within fully tetrahedral borate frameworks, an outcome that contrasts with the expected densification under compression. Temperature-dependent phase evolution further reveals that moderate HP/HT conditions can stabilize partially dehydrated, metastable frameworks inaccessible at ambient pressure. These results demonstrate that precursor-controlled kinetics can redefine accessible phase space in HP/HT synthesis, establishing moderate-pressure regimes as productive platforms for discovering structurally complex borates and beyond.