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Register a new userEnhancement of SHG efficiency by urea doping in ZTS single crystals and its correlation with crystalline perfection revealed by Kurtz powder and high-resolution X-ray diffraction methods
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Enhancement of second harmonic generation (SHG) efficiency and the correlation between crystalline perfection and SHG with urea doping on tristhioureazinc(II) sulphate (ZTS) single crystals have been investigated. ZTS is a potential semiorganic nonlinear optical (NLO) material. Pure and urea doped single crystals of ZTS have been successfully grown by slow evaporation solution technique (SEST). Presence of dopants has been confirmed and analyzed by Fourier transform infrared (FTIR) spectrometer. The influence of urea doping at different concentrations on the crystalline perfection has been thoroughly assessed by high resolution X-ray diffractometry (HRXRD). HRXRD studies revealed that the crystals could accomodate urea in ZTS up to some critical concentration without any deterioration in the crystalline perfection. Above this concentration, very low angle structural grain boundaries were developed and it seems, the excess urea above the critical concentration was segregated along the grain boundaries. At very high doping concentrations, the crystals were found to contain mosaic blocks. The SHG effeiciency has been studied by using Kurtz powder technique. The relative SHG efficiency of the crystals was found to be increased substantially with the increase of urea concentration. The correlation found between the crystalline perfection and SHG efficiency was discussed.
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Effect of L-threonine (LT) doping on crystalline perfection, second harmonic generation (SHG) efficiency, optical transparency and laser damage threshold (LDT) in potassium dihydrogen phosphate (KDP) crystals grown by slow evaporation solution technique (SEST) has been investigated. The influence of doping on growth rate and morphology of the grown crystals has also been studied. Powder X-ray diffraction data confirms the crystal structure of KDP and shows a systematic variation in intensity of diffraction peaks in correlation with morphology due to varying LT concentration. No extra phase formation was observed which is further confirmed by Fourier Transform (FT) Raman studies. High-resolution X-ray diffraction curves indicate that crystalline perfection has been improved to a great extent at low concentrations with a maximum perfection at 1 mol% doping. At higher concentrations (5 to 10 mol%), it is slightly reduced due to excess incorporation of dopants at the interstitial sites of the crystalline matrix. LDT has been increased considerably with increase in doping concentration, whereas SHG efficiency was found to be maximum at 1 mol% in correlation with crystalline. The optical transparency for doped crystals has been increased as compared to that of pure KDP with a maximum value at 1 mol% doping.
Information on the lattice parameter of single crystals with known crystallographic structure allows for estimations of sample quality and composition. In many cases it is suffcient to determine one lattice parameter or the lattice spacing along a certain, high-symmetry direction, e.g. in order to determine the composition in a substitution series by taking advantage of Vegards rule. Here we present a guide to accurate measurements of single crystals with dimensions ranging from 200 $mu$m up to several millimeter using a standard powder diffractometer in Bragg-Brentano geometry. The correction of the error introduced by the sample height and the optimization of the alignment are discussed in detail. In particular for single crystals with a plate-like habit, the described procedure allows for measurement of the lattice spacings normal to the plates with high accuracy on a timescale of minutes.
Thanks to the remarkable developments of ultrafast science, one of todays challenges is to modify material state by controlling with a light pulse the coherent motions that connect two different phases. Here we show how strain waves, launched by electronic and structural precursor phenomena, determine a macroscopic transformation pathway for the semiconducting-to-metal transition with large volume change in bistable Ti$_3$O$_5$ nanocrystals. Femtosecond powder X-ray diffraction allowed us to quantify the structural deformations associated with the photoinduced phase transition on relevant time scales. We monitored the early intra-cell distortions around absorbing metal dimers, but also long range crystalline deformations dynamically governed by acoustic waves launched at the laser-exposed Ti$_3$O$_5$ surface. We rationalize these observations with a simplified elastic model, demonstrating that a macroscopic transformation occurs concomitantly with the propagating acoustic wavefront on the picosecond timescale, several decades earlier than the subsequent thermal processes governed by heat diffusion.
During past years, a number of reports have been published on synthesis of tetragonal allotrope of boron, t-B52 phase. However, no unambiguous characterization of the crystal structure has been performed to the present time, while remarkable variation of the a/c lattice-parameter ratio raises strong doubts about its uniqueness. Here the Rietveld refinement of the crystal structure of the high pressure - high temperature boron phase synthesized by a direct solid-state transformation of rhombohedral beta-B106 at 20 GPa and 2500 K has been reported for the first time. Although this boron allotrope belongs to the t-B52 type, its structure can be considered as pseudo-cubic with the a/c ratio of sqr(2).
Lithium doped sodium niobate is an ecofriendly piezoelectric material that exhibits a variety of structural phase transitions with composition and temperature. We have investigated the phase stabilities of an important composition Li0.12Na0.88NbO3 (LNN12) using a combination of powder x-ray and neutron diffraction techniques in the temperature range 300 - 1100 K. Detailed Rietveld analyses of thermo-diffractograms show a variety of structural phase transitions ranging from non-polar antiferrodistortive to ferroelectric in nature. In the temperature range of 525 K to 675 K, unambiguous experimental evidence is shown for phase coexistence of orthorhombic paraelectric O1 phase (space group Cmcm) and orthorhombic ferroelectric O2 phase (space group Pmc21). The bp primitive lattice parameter of the ferroelectric orthorhombic phase (O2 phase) decreases, while the ap and cp primitive lattice parameters show normal increase with increase in temperature. Above 675 K, in the O1 phase, all lattice parameters come close to each other and increase continuously with increase of temperature, and around 925 K, ap parameter approaches bp parameter and thus the sample undergoes an orthorhombic to tetragonal phase transition. Further as temperature increases, the cp lattice parameter decreases, and finally approaches to ap parameter, and the sample transform into the cubic phase. The continuous change in the lattice parameters reveals that the successive phase transformations from orthorhombic O1 to high temperature tetragonal phase and finally to the cubic phase are not of a strong first order type in nature. We argue that application of chemical pressure as a result of Li substitution in NaNbO3 matrix, favours the freezing of zone centre phonons over the zone boundary phonons that are known to freeze in pure NaNbO3 as function of temperature.
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