The trigonal polymorph of strontium tetraborate, β-SrB4O7

The asymmetric unit of the title compound, β-SrB4O7, contains five Sr atoms (three located on a threefold rotation axis), twelve B and 21 O atoms. The structure is made up from BO3 triangles and BO4 tetrahedra in a 1:1 ratio. Pairs of BO3 triangles are linked to BO4 tetrahedra via common corners, forming chains. These chains are further linked to adjacent chains through corner-sharing, leading to a three-dimensional framework with channels running parallel to [001]. The Sr2+ ions reside in the channels and exhibit strongly distorted polyhedra The density of the β-polymorph is considerably lower than that of α-SrB4O7, which is constructed solely from BO4 tetrahedra.

The asymmetric unit of the title compound, -SrB 4 O 7 , contains five Sr atoms (three located on a threefold rotation axis), twelve B and 21 O atoms. The structure is made up from BO 3 triangles and BO 4 tetrahedra in a 1:1 ratio. Pairs of BO 3 triangles are linked to BO 4 tetrahedra via common corners, forming chains. These chains are further linked to adjacent chains through corner-sharing, leading to a three-dimensional framework with channels running parallel to [001]. The Sr 2+ ions reside in the channels and exhibit strongly distorted polyhedra The density of the -polymorph is considerably lower than that of -SrB 4 O 7 , which is constructed solely from BO 4 tetrahedra.
SrB 4 O 7 falls in a glass-forming range within the SrO-B 2 O 3 system and can simply be obtained as a glass (Imaoka, 1959). The process of glass re-crystallization occurs through complex mechanisms with probabilistic formation of other crystalline phases, specifically of metastable phases. Such a phase was in fact observed and designated as β-SrB 4 O 7 (Polyakova & Litovchik, 2008). However, X-ray powder diffraction data of this phase and of two other new compounds described by these authors were not analysed because of impure samples. tetrahedra with shared vertices (I, II) and a framework of triangles and tetrahedra with shared vertices (V, VI) are found in these structures.
In the process of glass re-crystallization of a strontium tetraborate composition at 973-983 K during one day, we obtained β-SrB 4 O 7 crystals with dimensions of ~200-400 µm. The crystals were located on the glass surface and were optically homogeneous (i.e. crystals showed homogeneous extinction when observed under a polarizing microscope). The crystals possess strong anisotropy when abrased, and crystals with an elongated ellipsoidal shape were obtained in such a way. As it turned out, the crystals are most firm along the c-direction.
The crystal structure of (I) is built up from a three-dimensional framework of connected boron-oxygen tetrahedra. The asymmetric unit of the title structure contains five Sr (three on special positions), twelve B and 21 O atoms. Alternatively, the structural formula of the title compound can thus be written as Sr 3 B 12 O 21 . It consists of BO 3 triangles and BO 4 tetrahedra in an 1:1 ratio (3:1 for structure V and 3:4 for structure VI). They form a three-dimensional framework constructed via common vertices. The BO 3 triangles are linked to one another so that two of their vertices and the bridging O atom are located on a straight line (see O1, O3, O5; O6, O7, O10 and O11, O13, O15 in Fig.1). The plane of one triangle in such a pair is tilted relatively to the other one about the line with an angle of ~20°. The remaining two vertices are common with the same tetrahedron (e.g. see O2 and O4 in Fig.1). The BO 4 tetrahedra are connected to one another via common supplementary materials sup-2 vertices and form chains along the c-direction (Fig. 2). These chains are connected with pairs of BO 3 triangles, leading to the formation of channels in the structure. The channels are filled with strontium ions (Fig. 3). The coordination polyhedra around the strontium ions are non-regular and defined by six O atoms in the range 2.479 (3)-2.786 (3) Å when a distance < 2.8 Å is considered as relevant.
In comparison with α-SrB 4 O 7 which is constructued solely from BO 4 tetrahedra, the density of the β-polymorph is considerably lower.

Experimental
Crystals were extracted out of glass by careful dissolving of the latter in a 2% HNO 3 solution. The initial glass has been made from a mixture of SrCO 3 (99.8%) and H 3 BO 3 (99.98%) in a 1:4 ratio. The mixture was heated up to 353-363 K with addition of a small amount of water and careful mixing until CO 2 gas evolution had stopped. Then the temperature was increased slowly up to 573 K to yield a anhydrous phase. The derived mixture was then placed into a glass-carbon crucible and kept in a molten state at 1323 K during 6 h in a nitrogen atmosphere. The flux was cooled in air down to 773 K and the glass was finally annealed at 723 K during a day to remove strain. Fig. 1. View of the basic structural motif in the title compound with the atom numbering scheme. Displacement ellipsoids are drawn at the 50% probability level. Symmetry codes: (i) x, y, z-1; (ii) 1-y, 1+x-y, 1+z; (iii) -y, x-y, 1+z; (iv) 1-y, x-y, 1+z.