Time-resolved X-ray diffraction reveals the hidden mechanism of high piezoelectric activity in a uniaxial ferroelectric.

High piezoelectric activity of many ferroelectrics has been the focus of numerous recent studies. The structural origin of this activity remains poorly understood due to a lack of appropriate experimental techniques and mixing of different mechanisms related to ferroelectricity and ferroelasticity. Our work reports on the study of a uniaxial Sr_{0.5}Ba_{0.5}Nb_{2}O_{6} ferroelectric where the formation of regions with different spontaneous strains is ruled out by the symmetry and where the interrelation between piezoelectricity and ferroelectricity can be inspected in an isolated fashion. We performed x-ray diffraction experiments on a single crystalline sample under alternating electric field and observed an unknown hidden-in-the-bulk mechanism, which suggests that the highest piezoelectric activity is realized in the volumes where nucleation of small ferroelectric domains takes place. This new mechanism creates a novel roadmap for designing materials with enhanced piezoelectric properties.

Calcium barium niobate as a functional material for broadband optical frequency conversion.

We demonstrate the application of as-grown calcium barium niobate (CBN) crystal with random-sized ferroelectric domains as a broadband frequency converter. The frequency conversion process is similar to broadband harmonic generation in commonly used strontium barium niobate (SBN) crystal, but results in higher conversion efficiency reflecting a larger effective nonlinear coefficient of the CBN crystal. We also analyzed the polarization properties of the emitted radiation and determined the ratio of d32 and d33 components of the second-order susceptibility tensor of the CBN crystal.

SBN60, strontium-barium niobate at 100†K.

The title compound, Sr0.6Ba0.4Nb2O6 (strontium barium niobium oxide), belongs to the group of strontium-barium niobates with varying composition of Sr and Ba. Their general formula can be written as Sr x Ba1 - x Nb2O6. Below the Curie temperature, T c , these materials indicate ferroelectric properties. The Curie temperature for SBN60 is equal to 346±0.5 K so the structure is in the ferroelectric phase at the measurement temperature of 100 K. Characteristic for this family of compounds is the packing along the z-axis. The NbO6 corner-sharing octa-hedra surround three types of vacancy tunnels with penta-gonal, square and triangular shapes. The Sr(2+) ions partially occupy two unique sites, the first one located inside the penta-gon and the second one in the square tunnels. Consequently, they are situated on the mirror plane and the inter-section of two glide planes, respectively. The site inside the penta-gonal tunnel is additionally disordered so that the same position is shared by Ba(2+) and Sr(2+) ions whereas another part of the Ba(2+) ion occupies a different position (relative occupancies 0.43:0.41:0.16). One of the Nb(V) atoms and three of the O(2-) ions occupy general positions. The second Nb(V) atom is located on the inter-section of the mirror planes. Two remaining O(2-) ions are located on the same mirror plane. Only the Nb(V) atom and one of the O(2-) ions which is located on the mirror plane are not disordered. Each of the remaining O(2-) ions is split between two sites, with relative occupancies of 0.52:0.48 (O(2-) ions in general positions) and 0.64:0.36 (O(2-) ion on the mirror plane).

Effect of temperature on spectroscopic features relevant to laser performance of YVO4:Tm3+, GdVO4:Tm3+, and LuVO4:Tm3+ crystals.

Optical spectra and luminescence decay curves were measured for thulium-doped YVO(4), GdVO(4), and LuVO(4) single crystals as a function of temperature in the 300-670 K temperature region. In spite of structural similarity, the three systems studied display significantly different transition intensities and nonradiative relaxation rates. It was found, in particular, that the peak value of the pump band absorption intensity diminishes by about 30% for Tm:YVO(4) and Tm:GdVO(4), and the effective emission cross section for the laser transition of Tm(3+) diminishes by a factor of 2 roughly when temperature increases from 300 to 500 K. An unusually small quantum efficiency of the upper laser level in Tm:LuVO(4) has been derived from the analysis of luminescence decay curves.

Effect of temperature on spectroscopic features relevant to laser performance of YVO4:Er(3+) and GdVO4:Er(3+) crystals.

Optical spectra and luminescence decay curves were measured for erbium-doped YVO(4) and GdVO(4) single crystals as a function of temperature in the 300 K-670 K temperature region. In spite of structural similarity the two systems studied display significantly different transition intensities and nonradiative relaxation rates. It was found in particular that the intensity of parasitic upconverted emission excited at 978 nm depends weakly on temperature for YVO(4):Er(3+), whereas it decreases monotonously with increasing temperature for GdVO(4):Er(3+) and becomes negligibly small at 670 K. It was concluded that GdVO(4):Er(3+) may be a promising laser active material operating at high thermal loading conditions encountered in high-power diode-pumped IR lasers.

Influence of doping on thermal diffusivity of single crystals used in photonics: measurements based on thermal wave methods.

Three crystals used in solid-state lasers, namely, yttrium aluminum garnet (YAG), yttrium orthovanadate (YVO(4)), and gadolinium calcium oxoborate (GdCOB), were investigated to determine the influence of dopants on their thermal diffusivity. The thermal diffusivity was measured by thermal wave method with a signal detection based on mirage effect. The YAG crystals were doped with Yb or V, the YVO(4) with Nd or Ca and Tm, and the GdCOB crystals contained Nd or Yb. In all cases, the doping caused a decrease in thermal diffusivity. The analysis of complementary measurements of ultrasound velocity changes caused by dopants leads to the conclusion that impurities create phonon scattering centers. This additional scattering reduces the phonon mean free path and accordingly results in the decrease of the thermal diffusivity of the crystal. The influence of doping on lattice parameters was investigated, additionally.

High-pressure crystallography of rhombohedral PrAlO(3) perovskite.

The evolution of the crystal structure of rhombohedral PrAlO(3) perovskite with pressure has been investigated by single-crystal x-ray diffraction and Raman scattering experiments. The structural evolution as indicated by lattice strains, octahedral tilts, and the distortions of the octahedral AlO(6) and polyhedral PrO(12) groups with increasing pressure, is controlled by the relative compressibilities of the AlO(6) octahedra and the PrO(12) site. Because the AlO(6) octahedra are more compressible than the PrO(12) sites, up to 7.4 GPa the structure evolves towards the high-symmetry cubic phase like any other rhombohedral perovskite. The variation of volume of the rhombohedral phase with pressure can be represented by a third-order Birch-Murnaghan equation of state with bulk modulus K(0) = 193.0(1.2) GPa and K' = 6.6(4). Above 7.4 GPa the evolution towards a cubic phase is interrupted by a phase transition. Observations are consistent with the assignment of Imma symmetry to the high-pressure phase. Comparison with the low-temperature [Formula: see text] to Imma transition confirms that electronic interactions stabilize the Imma phase.