Photoinduced phase transition in Ag3AsS3 crystals

It is shown that the illumination of samples leads to the appearance of anomalies in the temperature dependences of the propagation velocity of longitudinal ultrasonic waves along the principal crystallographic directions of proustite (Ag₃AsS₃) at T∼150 K. The features discovered are associated with a photoinduced phase transition caused by restructuring of the cation sublattice of proustite. Fiz. Tverd. Tela (St. Petersburg) 41, 702–704 (April 1999)     

Nonlinear absorption measurements in proustite (Ag3AsS3) and CdSe

Nonlinear absorption has been observed in proustite at 694 nm; however at 1.06m, in contrast to previous results, no nonlinear absorption could be detected at intensities up to the damage threshold. Measurements on CdSe at 1.06m and 1.32m show that the nonlinear absorption mechanism is two photon absorption followed by absorption by the photo-induced carriers. The implications of this for nonlinear mixing applications are discussed.     

Exciton and phonon spectra of acoustooptic Tl3AsS3 crystals

The λ-modulated exciton reflection spectra of Tl₃AsS₃ crystals are investigated at 8 and 77 K, in which the ground (n=1) and excited (n=2, 3) exciton states are revealed. Taking into account the spatial dispersion, the shapes of λ-modulated reflection spectra of the n=1 line are calculated and the basic parameters of excitons and bands are determined (the translational and reduced masses of excitons and the effective masses of electrons and light and heavy holes). The one-phonon reflection spectra are studied in the region from 50 to 500 cm^?1 in polarizations E ∥ c and E ⊥ c. The shapes of one-phonon reflection spectra are calculated and the parameters of vibrational modes E and A ₂ are determined.     

Synthetic proustite (Ag3AsS3): A summary of its properties and uses

There is at present great interest in crystals which are suitable for efficient mixing at optical frequencies. Such a crystal should be acentric, have a wide transmission band, have a large non-linear coefficient and permit phase matching. Proustite possesses such properties [1], and this paper summarises the information that has been obtained to date at R R E on these properties and their use in up-converting 10m radiation to the visible, on the available optical quality, on the electro-optic effect, and on electrical conduction in the crystal; some general observations on the type of damage caused to proustite by laser beams are given.     

Anomalous structural states of rare-earth oxides in solid-phase synthesis during continuous heating

The kinetics of solid-phase synthesis of complex rare-earth (RE) oxides from an amorphous precursor state during continuous heating is studied by X-ray diffraction. Using the synthesis of RE borates (ReBO₃) as an example, it is shown that a continuous heating of such precursors stimulates the formation of new phases which are unknown in an equilibrium state of one rare-earth element but known for other rare-earth borates. It is established that the regime of continuous heating during solid-phase synthesis of garnets (Re ₃ M ₅O₁₂) exhibiting the only crystal phase state for all rare-earth atoms results in an accelerated growth of crystallites and that the higher the heating rate the higher the growth rate.     

Investigation of structural changes in phlogopites on heating

It was shown experimentally that there are three types of curve for the temperature variation of d/d (001) for phlogopites. The relationship is linear for hard phlogopites while the soft phlogopites have negative expansion coefficients over the range of 160–250 °C. The curves for phlogopites of average hardness occupy intermediate positions. This effect is explained by the existence of layers which are capable of being dehydrated due to the presence of interstitial water.     

铂纳米晶在升温过程中结构演化与熔化特征的原子级模拟研究

采用分子动力学方法结合量子修正Sutton-Chen型多体力场,对由{100}面和{111}面构成的十四面体Pt纳米晶在升温过程中的热稳定性和熔化机制进行了计算机模拟研究,并引入统计半径和Lindemann指数来分析它的结构和形状演化过程. 结果表明：该纳米晶在1500 K时形状开始发生变化,并在1700 K时转变为球形. 铂纳米晶粒在1500 K时开始出现表面预熔,在1650 K时表面完全熔化并开始向内部传播,最终在1730 K时整体熔化为液态粒子. 表面预熔的出现对形状转变的发生是有利的. 关键词： 纳米晶 结构 熔化 分子动力学     

Ag原子链的结构稳定性和磁性

采用第一性原理方法研究了Ag原子链的结构稳定性和磁学性质.在结构稳定性方面,计算了线性链、平面之字型、梯型和三条线性原子链组成的T型等链式结构.结果表明,线性原子链的结合能最小,结构最不稳定性;T型结构的结合能最大,结构最稳定.所有这些一维结构的原子间键长都小于Ag体材料的键长,表明一维下原子间的成键比体材料时更强.对Ag线性原子链的磁性计算表明,线性Ag原子链在平衡状态下并不表现出磁性,但是当原子链的原子键长被压缩了约52%时,体系可表现出铁磁性.通过Stoner判据和原子轨道相互作用图像,解释了这种磁性出现的原因. 关键词： Ag原子链 结构稳定性 磁性 从头计算     

Structure in LEED Bragg reflexes of spherical Ag crystals

LEED at room temperature is performed on spherically-shaped silver crystals grown by solidification of a drop of melt. The domains of intensity in the reciprocal lattice, which are usually rodlike, change to double cones caused by the crystal sphere. The size and the structure of Bragg reflexes were studied as a function of primary electron energy and diffraction geometry. The influence of impurities on the diffraction patterns is examined.     

Growth of metallic Ag whisker single crystals on AgI films

The growth of silver whisker nanocrystals (WNCs) on the surface of an AgI film deposited by laser ablation is studied on a Zeiss Merlin scanning electron microscope. The chemical composition of WNCs is determined by electron-probe microanalysis. The growth rate of WNCs is estimated. Possible WNC growth mechanisms under the action of an electron beam are considered.     

The influence of heating on the spectral characteristics of pure alkali halide NaCl crystals

The influence of high-temperature heating on the spectral characteristics of pure natural NaCl crystals is studied. It is shown that defects changing spectral characteristics of crystals are formed in them when heated. Temperature dependences of the luminescence intensity growth for these defects are described by the hyperbolic function. The action of X-rays on heated crystals results in the formation of new defects luminescent in regions of 435 and 570–585 nm. The efficiency of formation of these defects is obtained as a function of the temperature of heat pretreatment. Regularities in variations of the location and the intensity of luminescence of the defects obtained under the action of ultraviolet radiation and on long storage are determined.     

Initiating action of continuous heating on the formation of phases in the solid state synthesis of complex oxides of rare earth elements

The kinetics of the solid state synthesis of complex oxides of rare earth elements has been investigated by the x-ray diffraction method for various sizes of the crystallites of initial simple oxides. It has been shown that the continuous heating of the samples undergoing the solid state synthesis stimulates the formation of phases at lower temperatures and in shorter times than long-term step annealing. It has been revealed that the effectiveness of the initiating action of the continuous heating on the formation of phases increases with decreasing size of the crystallites of the initial components.     

Change in the substructure of cold worked aluminum crystals on subsequent heating

In this research, a relationship has been established between the polygonization of aluminum single crystals on annealing and the hardening curve for single crystals of fec metals. It has been shown by the Guinier-Tennevin X-ray focusing method that only high-temperature (600–640 °C) polygonization develops on annealing single crystals of pure (99.99%) aluminum cold worked by extension at –196 °C within stages I and II of the hardening curve. In crystals cold worked to the end of stage II, and in the whole of stage III, the polygonization process takes place at an appreciable rate even during low-temperature (450 °C) annealing. Subsequent high-temperature annealing of these single crystals leads to an increase in the degree of perfection of the crystal lattice of the subgrains whose formation began during low-temperature annealing.     

The mechanism of the appearance of an electromotive force on heating of SmS single crystals

We analyze the experimental variation of the concentration of conduction electrons in semiconducting SmS single crystals with increasing temperature within a shallow-impurity model. It is shown that the appearance of an electromotive force is due to accumulation of the critical concentration of free electrons, which results in screening of the Coulomb potential of Sm²⁺ impurity ions that are responsible for the creation of donor levels with an activation energy of 0.045 eV in the band gap of SmS single crystals.