β‐RE1‐xBixB3O6 (RE: Sm, Eu, Gd, Tb, Dy, Ho, Er, Y): Bi3+ Substitution Induced Formation of Metastable Rare Earth Borates at Ambient Pressure.

There emerge great interests in the syntheses of metastable polyborates; however, most are involved with the high-pressure technique. A facile method to synthesize metastable rare earth borates at ambient pressure is eagerly required for the large-scale production and property investigation. Here we demonstrate the critical role of Bi3+ substitutions in the stabilization of metastable β-REB3O6 (RE = Sm, Eu, Gd, Tb, Dy, Ho, Er, and Y) at ambient pressure, where the Bi3+-to-RE3+ substitutions would efficiently reduce the synthetic temperatures to 735–820 °C, well below the upper limit of thermodynamically stable window (840–980 °C). Partial solid solutions of β-RE1–xBixB3O6 were prepared, and the ranges of the solution were also studied experimentally. The thermal behaviors of β-RE0.8Bi0.2B3O6 were investigated by differential thermal analyses and powder X-ray diffraction, and they were divided into two categories; that is, β-RE0.8Bi0.2B3O6 (RE = Sm, Eu, Gd) transfers to α-RE0.8Bi0.2B3O6 with further increa...