Manfred Burianek

Impurity-induced resistivity of ferroelastic domain walls in doped lead phosphate

The topology and the trace of ferroelastic domains, namely W walls, of Ca-doped lead orthophosphate (Pb1−x, Cax)3 (PO4)2 with a Ca content of 2.7% mol were studied on the monoclinic cleavage plane (100) using contact mode atomic force microscopy. Furthermore, conducting atomic force microscopy was applied using a bias voltage across the cantilever and the sample inducing a tunnelling current. As a reference a pure lead phosphate crystal was used. Only the Ca-doped lead phosphate crystals showed a considerable difference in conductivity between walls and the bulk. The conductivity in the bulk was found to be approximately 7% higher than in the domain walls. The experimental results show that ferroelastic domain walls of atomistic width can work as barriers to dielectric transport.

Crystal growth and elastic properties of orthorhombic Bi2Ga4O9

The combination of favourable oxygen conductivity at high temperatures with mechanical strength make Bi-containing compounds with mullite-type crystal structures strong candidates for use as electrolytes of solid fuel cells. Large single crystals of orthorhombic Bi2Ga4O9 with dimensions up to 20 × 20 × 10 mm3 were grown by the top-seeded solution growth technique. Their elastic constants at room temperature were determined for the first time using resonant ultrasound spectroscopy. The values given in GPa are c11 = 143.4(2), c22 = 161.7(2), c33 = 224.2(3), c44 = 69.6(1), c55 = 49.2(1), c66 = 76.5(2), c12 = 73.7(2), c13 = 62.2(3) and c23 = 70.3(3). Further, the crystal structure and the elastic properties of Bi2Ga4O9 were studied at 0 K by parameter-free ab initio calculations based on density-functional theory. On average the computed elastic constants differ from the experimental values by about 10%, indicating the reliability of the theoretical approach. Like in other mullite-type compounds the anisotropy of the longitudinal elastic stiffness is clearly controlled by the structurally dominant octahedral chains running parallel to [001]. The deviations from Cauchy relations show a significant anisotropy of the type g22>g11≈g33 which is related to the covalent character of the bonding interactions within the infinite –Bi–O–Bi–O– bond chains parallel to [010]. The mean elastic stiffness of Bi2Ga4O9 is about 40% smaller than for 2/1-mullite and sillimanite. This discrepancy can be attributed to the mechanically very soft behaviour of the Bi 6s2 lone electron pair. Its stereochemical activity is clearly evident from both the asymmetry of the bismuth coordination polyhedron and the calculated electron density maps.

Improved Single Crystal Growth of the Boron Sillenite “Bi24B2O39” and Investigation of the Crystal Structure

The boron sillenite, up to now known as the 12:1 compound Bi 24 B 2 O 39 in the system Bi 2 O 3 - B 2 O 3 and crystallizing in the space group 123, melts incongruently at 655 °C only about 25 K above the eutectic tie line and corresponding to a steep liquidus line. Single crystals with dimensions larger then 1 cm 3 have been successfully grown in [100], [110], and [111] direction by an improved Top Seeded Solution Growth (TSSG) technique equipped with crucible weighing, accelerated crystal rotation technique and air-cooled pulling rod. The structure of the boron sillenite was analyzed by X-ray diffraction method, which was possible due to the high crystalline quality achieved. A defect-free sublattice corresponding to a Bi-O framework is isostructural with all sillenites, but a 2 A environment around the origin is occupied by different cations with different population coefficients. The best calculation results in the formula Bi 24.5 BO 38.25 which is more Bi-rich than the 12:1 assumption.

Bi4B2O9: Crystal growth and some new attractive properties

For the first time, single crystals of the bismuth(2:1)borate Bi 4 B 2 O 9 have been grown by the Czochralski method. The linear optical properties of this monoclinic substance with an exceptionally high density of 8.208 g/cm 3 were investigated. Between crossed polars an extreme dispersion of the extinction angle was detected. By measuring all principal refractive indices the wavelength dependence of the extinction angle could be calculated.

Persistence of the stereochemical activity of the Bi3+ lone electron pair in Bi2Ga4O9 up to 50 GPa and crystal structure of the high-pressure phase

The crystal structure of the high-pressure phase of bismuth gallium oxide, Bi(2)Ga(4)O(9), was determined up to 30.5 (5) GPa from in situ single-crystal in-house and synchrotron X-ray diffraction. Structures were refined at ambient conditions and at pressures of 3.3 (2), 6.2 (3), 8.9 (1) and 14.9 (3) GPa for the low-pressure phase, and at 21.4 (5) and 30.5 (5) GPa for the high-pressure phase. The mode-Grüneisen parameters for the Raman modes of the low-pressure structure and the changes of the modes induced by the phase transition were obtained from Raman spectroscopic measurements. Complementary quantum-mechanical calculations based on density-functional theory were performed between 0 and 50 GPa. The phase transition is driven by a large spontaneous displacement of one O atom from a fully constrained position. The density-functional theory (DFT) model confirmed the persistence of the stereochemical activity of the lone electron pair up to at least 50 GPa in accordance with the crystal structure of the high-pressure phase. While the stereochemical activity of the lone electron pair of Bi(3+) is reduced at increasing pressure, a symmetrization of the bismuth coordination was not observed in this pressure range. This shows an unexpected stability of the localization of the lone electron pair and of its stereochemical activity at high pressure.

High-pressure phase transition of Bi2Fe4O9.

The high-pressure behaviour of Bi2Fe4O9 was analysed by in situ powder and single-crystal x-ray diffraction and Raman spectroscopy. Pressures up to 34.3(8) GPa were generated using the diamond anvil cell technique. A reversible phase transition is observed at approximately 6.89(6) GPa and the high-pressure structure is stable up to 26.3(1) GPa. At higher pressures the onset of amorphization is observed. The crystal structures were refined from single-crystal data at ambient pressure and pressures of 4.49(2), 6.46(2), 7.26(2) and 9.4(1) GPa. The high-pressure structure is isotypic to the high-pressure structure of Bi2Ga4O9. The lower phase transition pressure of Bi2Fe4O9 with respect to that of Bi2Ga4O9 (16 GPa) confirms the previously proposed strong influence of cation substitution on the high-pressure stability and the misfit of Ga3+ and Fe3+ in tetrahedral coordination at high pressure. A fit of a second-order Birch–Murnaghan equation of state to the p–V data results in K0 = 74(3) GPa for the low-pressure phase and K0 = 79(2) GPa for the high-pressure phase. The mode Grüneisen parameters were obtained from Raman-spectroscopic measurements.

Thermal expansion and elastic properties of mullite-type Bi2Ga4O9 and Bi2Fe4O9 single crystals

Abstract Resonant ultrasound spectroscopy was used to characterize the elastic properties of single crystal orthorhombic Bi2Ga4O9 and Bi2Fe4O9 between room temperature and about 1200 K. Additionally, the coefficients of thermal expansion were studied in the range 100 K to 1280 K using high-resolution dilatometry and X-ray powder diffraction. The elastic constants at 295 K are in GPa c11 = 143.4(1), c22 = 161.9(1), c33 = 224.5(1), c44 = 68.4(1), c55 = 49.3(1), c66 = 76.6(1), c12 = 74.2(1), c13 = 62.2(1), c23 = 70.5(1) for Bi2Ga4O9, and c11 = 106.7(1), c22 = 141.2(1), c33 = 183.7(2), c44 = 53.7(1), c55 = 41.9(1), c66 = 63.8(1), c12 = 63.5(1), c13 = 59.8(1), c23 = 63.4(2) for Bi2Fe4O9. In both mullite-type compounds the strong bond chains built up by edge-sharing coordination octahedra extending parallel to [001] dominate the anisotropy of their elastic and thermoelastic properties. Smaller variations of elastic anisotropy within the (001) plane can be attributed to the specific type of cross-linking of the octahedral chains. The temperature evolution of the cij shows no hint on any structural instability or glass-like transition that might be related to the suspected ion conductivity at high temperatures. However, in both crystal species characteristic anelastic relaxation phenomena occur in the ultrasonic frequency regime close to room temperature. The smallest thermal expansion is observed in the plane perpendicular to the stiffest octahedral chains. A model is discussed to explain the apparent discrepancy in terms of cross-correlations within the three-dimensional framework of edge- and corner-linked coordination polyhedra.

Incommensurate modulation of calcium barium niobate (CBN28 and Ce:CBN28)

The incommensurately modulated crystal structures of Ca(0.28)Ba(0.72)Nb(2)O(6) (CBN28) and Ce(0.02)Ca(0.25)Ba(0.72)Nb(2)O(6) (Ce:CBN28) were refined in the supercentred setting X4bm(AA0,-AA0) of the 3 + 2-dimensional superspace group P4bm(aa½,-aa½). Both compounds are isostructural with a tetragonal tungsten bronze-type structure. The modulation of CBN28 consists of a wavy distribution of Ba and Ca atoms as well as vacancies on the incompletely occupied Me2 site with 15-fold oxygen coordination. The occupational modulation is coupled with a modulation of the atomic displacement parameters and a very weak modulation of the positional parameters of Me2. The surrounding O atoms show strong displacive modulations with amplitudes up to ca 0.2 Å owing to the cooperative tilting of the rigid NbO(6) octahedra. The Me1 site with 12-fold coordination and Nb atoms are hardly affected by the modulations. Only first-order satellites were observed and the modulations are described by first-order harmonics. In Ce:CBN28 cerium appears to be located on both the Me2 and Me1 sites. Wavevectors and structural modulations are only weakly modified upon substitutional incorporation of 0.02 cerium per formula unit of calcium.

Relaxor behavior of ferroelectric Ca0.22Sr0.12Ba0.66Nb2O6

The relaxor behavior of tetragonal tungsten bronze uniaxial relaxor ferroelectric calcium strontium barium niobate (Ca0.22Sr0.12Ba0.66Nb2O6 or CSBN-22) single crystal was studied by measuring elastic constants and thermal expansion with the aid of resonant ultrasound spectroscopy and dilatometry, respectively, in the temperature range 300 K–1503 K. Thermal expansion yields evidence of the Burns temperature TB and the intermediate characteristic temperature T*, which was also supported by the temperature evolutions of the elastic constants cij. CSBN-22 was found to be ∼2%–3% elastically stiffer than CBN-28. The presented results open the perspective to understand the relaxor behavior of CSBN.

Anomalous elastic behavior of relaxor ferroelectric Ca0.28Ba0.72Nb2O6:Ce studied by resonant ultrasound spectroscopy

Elastic behavior of tetragonal tungsten bronze uniaxial relaxor ferroelectric cerium doped Ca0.28Ba0.72Nb2O6 single crystal was investigated employing resonant ultrasound spectroscopy in the temperature range from room temperature up to 1323 K. Doping of cerium lowers the phase transition temperature Tc and Burns temperature Tb significantly, however, intermediate characteristic temperature T* (between the Burns temperature Tb and the temperature of maximum dielectric permittivity Tm) remains same as for pure Ca0.28Ba0.72Nb2O6. All independent elastic constants evolved differently with temperature, reflecting their coupling to different types of the reorientational motion of the polar nanoregions through their interaction with the acoustic waves.