Crystal structures of anhydrous borax α-Na<sub>2</sub>B<sub>4</sub>O<sub>7</sub> and γ-Na<sub>2</sub>B<sub>4</sub>O<sub>7</sub> and ab initio quantum chemical calculations of structural stability on their fundamental building blocks

Abstract

The crystal structures of α-Na2B4O7 and γ-Na2B4O7 formed during the heating of borax were investigated by single-crystal X-ray diffraction. In addition, the structural stability of the fundamental building blocks (FBBs) was examined using ab initio quantum chemical calculations. α-Na2B4O7 crystallized into the triclinic space group P1 with unit cell dimensions of a = 6.5489(7) Å, b = 8.6261(9) Å, c = 10.4909(11) Å, α = 93.2540(10)°, β = 94.8660(10)°, γ = 90.8380(10)°, V = 589.45(11) Å3. γ-Na2B4O7 crystallized into the triclinic space group P1 with unit cell dimensions of a = 6.7123(11) Å, b = 9.6052(17) Å, c = 13.270(2) Å, α = 104.183(4)°, β = 91.560(4)°, γ = 106.501(4)°, and V = 791.0(2) Å3. In both α-Na2B4O7 and γ-Na2B4O7, Na coordination polyhedra with the same coordination numbers have similar coordination volumes; however, the Na polyhedra in γ-Na2B4O7 possess more distortable environments than those in α-Na2B4O7. The flexibility of the Na coordination environment allows these materials to adopt favorable oxygen positions, leading to an α-γ phase transition. The structural stability of the FBBs in α-Na2B4O7 and γ-Na2B4O7 was lowered by dehydration and recrystallization. Consequently, α-Na2B4O7 and γ-Na2B4O7 possess FBBs with readily changeable connection geometries, which causes a phase transformation between α-Na2B4O7 and γ-Na2B4O7 without requiring a significant amount of energy.