ISOSYMMETRICAL POLYMORPHIC TRANSITIONS IN ALKALI BORATES

Stanislav Filatov¹ , Rimma Bubnova² and Irina Polyakova²

¹Dept. of Crystallography, Saint Petersburg State University University Emb. 7/9, 199034, Russia, E-mail: flt@cryst.geol.pu.ru

²Institute of Silicate Chemistry, Rus. Acad. Sci., Ul. Odoevskogo, 24/2 St.Petersburg, 199155, Russia

Keywords: polymorphism, alkali borates, powder high-temperature X-ray diffraction

Unusual first-order phase transitions without change of a space group are revealed in unhydrous alkali borate systems. We will consider a-b RbB₃O₅ (space group P2₁2₁2₁), a-b KB₅O₈ (Pbca), a-b RbB₅O₈ (Pbca) and a-b NaB₃O₅ (P2₁/c) transitions.

These compounds crystallise in the glassforming region 0-40 mol. % alkali oxide where some glass properties have extrema. Within this region the addition of an alkali metal to network built up from BO₃ triangles leads to the formation of negative charged BO₄ tetrahedra rather than non-bridging oxygen atoms. Tetrahedra and triangles sharing by corners condense to various rigid arrangements which polymerize to form (some interlocking) frameworks in crystals, melts and glasses.

We finally will discuss results of powder high-temperaure X-ray diffraction experiments which demonstrate mechanism of such phase transitions. For example, a-b RbB₃O₅ transition arises from thermal continious changing of framework without breaking up boron-oxygen bonds via amorphization. During the process of this transition though the cell volume shows a rise only 3 % two of three cell parameters change 1.5 times in the opposite directions: b_a (10 A) <-> c_b (6.5 A), c_a (5.5 A) <->b_b (8.7 A), a_a (8.3 A) <-> a_b (8.5 A). Based on topological commonness of frameworks for both modifications we could suggest structural model for amorphous phase of the same composition as intermediate product of the transition. By analogy great changes in cell parameters are revealed in the course of the a-b KB₅O₈ (and a-b RbB₅O₈) transition: b_a (8.4 A) <-> c_b (14.7 A), c_a (21 A) <-> b_b (11.7 A), a_a (8.4 A) <-> a_b (7.4 A).

It may be seen that these isosymmetrical transitions are associated with (i) framework structure of both modifications and melt, (ii) remaining of twofold screw axes around of which boron-oxygen chains are formed in framework crystals, (iii) topological character of the transitions, and (iiii) hight stickiness of melts and glasses of the same chemical compositions.