Crystal structures of anhydrous borax -Na₂B₄O₇ and -Na₂B₄O₇ and ab initio quantum chemical calculations of structural stability on their fundamental building blocks

The crystal structures of alpha-Na2B4O7 and gamma-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. alpha-Na2B4O7 crystallized into the triclinic space group P (1) over bar; with unit cell dimensions of a = 6.5489(7) angstrom, b = 8.6261(9) angstrom, c = 10.4909(11) angstrom, alpha = 93.2540(10)degrees, beta = 94.8660(10)degrees, gamma = 90.8380(10)degrees, V= 589.45(11) angstrom 3. gamma-Na2B4O7 crystallized into the triclinic space group P (1) over bar; with unit cell dimensions of a = 6.7123(11) angstrom, b = 9.6052(17) angstrom, c = 13.270(2) angstrom, alpha = 104.183(4)degrees, beta = 91.560(4)degrees, gamma = 106.501(4)degrees, and V = 791.0(2) angstrom 3. In both alpha-Na2B4O7 and gamma-Na2B4O7, Na coordination poly-hedra with the same coordination numbers have similar coordination volumes; however, the Na polyhedra in gamma- Na2B4O7 possess more distortable environments than those in alpha-Na2B4O7. The flexibility of the Na coordina-tion environment allows these materials to adopt favorable oxygen positions, leading to an alpha-gamma phase transition. The structural stability of the FBBs in alpha-Na2B4O7 and gamma-Na2B4O7 was lowered by dehydration and recrystal-lization. Consequently, alpha-Na2B4O7 and gamma-Na2B4O7 possess FBBs with readily changeable connection geome-tries, which causes a phase transformation between alpha-Na2B4O7 and gamma-Na2B4O7 without requiring a significant amount of energy.