The δ-phase of SrTeO3 at 780 K1

As part of a structural investigation of strontium tellurate(IV) (STO), SrTeO3, with particular emphasis on the crystal chemistry and phase transitions, the structure of the δ-phase has been determined at 780 K using a single-crystal analysis. Both structural and non-linear optical measurements indicate that STO undergoes a γ→δ second-order ferroelectric phase transition at 633 K from the C2 (γ) to the C2/m (δ) modification. Systematic differences between the similar γ- and δ-phase structures were determined and it was found that this phase transformation can be described by a displacive mechanism

SrSeO3 from a combined X-ray and neutron powder diffraction study

Strontium trioxoselenate(IV), SrSeO3, crystallizes in the KClO3 structure type and is isotypic with BaSeO3, beta-PbSeO3 and the mineral scotlandite (PbSO3). The Sr2+ cation is nine-coordinated by O atoms. The SrO9 polyhedra are linked together by common edges to form a three-dimensional network, with channels running along the b axis where the Se4+ cations reside. They are coordinated by three O atoms to form one-sided SeO3E pyramids (E = electron lone pair), with Se - O bond lengths of 1.672 ( 6) and 1.688 ( 3) angstrom ( x 2). The SeO3E pyramids are not connected to each other; instead, they share O atoms with the SrO9 polyhedra. Except for one O atom, all other atoms (one Sr, one Se and the second O atom) are located on mirror planes.</p

A new family of layered bismuth oxohalides

A series of layered bismuth oxohalides, belonging to three new structural types, have been predicted and synthesized. In all cases, the fluorite-like [Bi2O2] layers are alternatively separated by metal-halide layers of the CsCl type and perovskite-like sheets of varied width. The structural motifs of the new compounds have been confirmed by X-ray crystallography for Bi4NdTi1.6Nb0.4Cs0.6O11Br2 (space group P4/mmm, a 3.8571(1) Angstrom, c 25.280(1) Angstrom, R-wp = 0.092, R-p = 0.063) and Bi4Nd2Ti2.4Fe0.6Cs0.6O14Cl2 (space group P4/mmm, a = 3.8424(1) Angstrom, c = 26.622(1) Angstrom, R-wp = 0.062, R-p = 0.043). These results are compared with the data on the related layered family BIPOX. The possibilities of further development of different families of layered bismuth oxohalides containing fluorite-like metal-oxygen layers are discussed.</p

SrSeO3 from a combined X-ray and neutron powder diffraction study

Strontium trioxoselenate(IV), SrSeO3, crystallizes in the KClO3 structure type and is isotypic with BaSeO3, beta-PbSeO3 and the mineral scotlandite (PbSO3). The Sr2+ cation is nine-coordinated by O atoms. The SrO9 polyhedra are linked together by common edges to form a three-dimensional network, with channels running along the b axis where the Se4+ cations reside. They are coordinated by three O atoms to form one-sided SeO3E pyramids (E = electron lone pair), with Se - O bond lengths of 1.672 ( 6) and 1.688 ( 3) angstrom ( x 2). The SeO3E pyramids are not connected to each other; instead, they share O atoms with the SrO9 polyhedra. Except for one O atom, all other atoms (one Sr, one Se and the second O atom) are located on mirror planes.</p

On the crystal structures of SrTeO3

SrTeO3 has been studied by powder neutron diffraction (PND) experiments and by second harmonic generation (SHG) method at a series of temperatures between 20 and 560 degrees C. The SrTeO3 low temperature (22 degrees C) form was found (R-wp = 1.49%, chi(2) = 1.06) to crystallize in space group C2/c with the unit cell parameters a = 28.133(9) angstrom, b = 5.9044(15) angstrom, c = 28.418(6) angstrom, beta = 114.303(17)degrees. The Sr atoms are coordinated by six, seven or eight oxygen atoms. Each Te atom has a similar 'pyramidal' geometry, coordinated by three oxygen atoms having similar Te-O. bond lengths. The Te4+ lone-pair (E) plays an active stereochemical role. The Sr-O polyhedra form an openwork framework with 2 types of channels. Inside the channels the tellurium atoms are located. The TeO3E pyramids do not connect to each other; instead they share their oxygen atoms with Sr polyhedra. The discontinuous change of the SrTeO3 lattice parameters in the region 260-310 degrees C and abrupt growth of SHG signal in this temperature region correspond to the onset of the ferroelectric phase in a first-order phase transition. However from the diffraction data acquired at 410 degrees C no evidence of lowering of the symmetry was found. The structure model of the SrTeO3 high temperature (410 degrees C) modification is proposed in the same space group C2/c. The plausible reasons of the discrepancy between the PND and SHG results are discussed. (c) 2006 Elsevier SAS. All rights reserved.</p

On the crystal structures of SrTeO3

SrTeO3 has been studied by powder neutron diffraction (PND) experiments and by second harmonic generation (SHG) method at a series of temperatures between 20 and 560 degrees C. The SrTeO3 low temperature (22 degrees C) form was found (R-wp = 1.49%, chi(2) = 1.06) to crystallize in space group C2/c with the unit cell parameters a = 28.133(9) angstrom, b = 5.9044(15) angstrom, c = 28.418(6) angstrom, beta = 114.303(17)degrees. The Sr atoms are coordinated by six, seven or eight oxygen atoms. Each Te atom has a similar 'pyramidal' geometry, coordinated by three oxygen atoms having similar Te-O. bond lengths. The Te4+ lone-pair (E) plays an active stereochemical role. The Sr-O polyhedra form an openwork framework with 2 types of channels. Inside the channels the tellurium atoms are located. The TeO3E pyramids do not connect to each other; instead they share their oxygen atoms with Sr polyhedra. The discontinuous change of the SrTeO3 lattice parameters in the region 260-310 degrees C and abrupt growth of SHG signal in this temperature region correspond to the onset of the ferroelectric phase in a first-order phase transition. However from the diffraction data acquired at 410 degrees C no evidence of lowering of the symmetry was found. The structure model of the SrTeO3 high temperature (410 degrees C) modification is proposed in the same space group C2/c. The plausible reasons of the discrepancy between the PND and SHG results are discussed. (c) 2006 Elsevier SAS. All rights reserved.</p