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Twins or multiple twins occur frequently in the orthorhombic Al20Cu2Mn3 approximant of decagonal quasi crystal (DQC). Due to the specific structural units, the twins in the Al20Cu2Mn3 approximant usually exhibit the glide-reflection characteristics. Using aberration-corrected transmission electron microscope at the atomic scale, we have observed not only glide-reflection twins but also simple mirror-reflection twins in the Al20Cu2Mn3 approximant. The two twinning modes are found to coexist in the present sample. These twins exhibit variant configurations at the twin boundaries where the tessellations of local subunits are imaged at an atomic scale. At the twin boundaries, diversified tiles such as star-like (S), bowtie-shaped and boat-shaped (B) are observed. The diversified tiles stacking with various manners allow the coexistence of the DQC and the approximant. Furthermore, the variants of B and S tiles are also found.

Besides the well-known cube-on-cube orientation relationship (OR) between M23C6 carbide and matrix, we have determined a new OR named as the twin-related OR in a long-term ageing Ni-based superalloy on the basis of the extensive and detailed electron diffraction analyses. Furthermore, by means of atomic-resolution high angle annular dark-field imaging technique which is implemented in the aberration-corrected scanning transmission electron microscope, we elucidated the interfacial characteristics between M23C6 and matrix for above two types of ORs. Taking into account of the interfacial characteristics, we propose that the twin-related OR possesses a higher total interfacial energy. Thus, its frequency of occurrence is lower than that of the cube-on-cube OR though both ORs are usually seen in the long-term ageing samples.

Plastic deformation behavior of Au/Cu multilayers with individual layer thicknesses of 25-250 nm was investigated via microindentation experiments. It was found that plastic instability of the Au/Cu multilayer exhibits strong length scale (individual layer thickness and grain size) dependence. The smaller the length scale, the easier shear bands form. In other words, plastic deformation becomes unstable with decreasing length scale. Cross-sectional observation along with plan-view indicates that the occurrence of plastic deformation instability corresponds to transformation of the deformation mechanism associated with geometrical configuration and length scale of the material. At nanometer scale, buckling-assisted interface crossing of dislocations results in local shear band, while, at submicron scale or above, local dislocation pileup-induced interface offset leads to plastic instability. Theoretical analysis is conducted to understand the length scale-dependent plastic deformation behavior of the multilayer.

Localised shear deformation of a material is usually identified as a particular feature of deformation inhomogeneity. Here, we show two different types of shear deformation-behaviour that occurred in Au-Cu multilayers subjected to microindentation load, namely, a cooperative-layer-buckling-induced shear banding in a nanoscale multilayer and a direct localised shearing across a layer interface along a shear plane in a submicron-scale multilayer. Theoretical analysis indicates that the formation of the two different types of shear deformation in the multilayers depends on a competition between the dislocation-pile-up-induced stress concentration at the layer interface and the barrier strength of the layer interface for glissile dislocation transmission.

Single crystal Tm 3+ :SrMoO 4 was grown successfully by Czochralski technique along the (0 0 1) orientation in N 2 atmosphere. The refractive index of SrMoO 4 crystal was measured at room temperature. The linear thermal expansion coefficients along c - and a -axes were measured to be 1.70 × 10 −5 °C −1 and 0.59 × 10 −5 °C −1 , respectively. Its polarized absorption spectra, fluorescence spectra and fluorescence decay curve have also been investigated at room temperature. Based on the Judd–Ofelt theory, the intensity parameters have been determined as: Ω 2 = 7.875 × 10 −20 , Ω 4 = 2.797 × 10 −20 and Ω 6 = 0.92 × 10 −20 cm 2 . The radiative probabilities, radiative branching ratios, radiative lifetimes and stimulated emission cross sections of Tm 3+ in SrMoO 4 were obtained. The quantum efficiency of the 3 H 4 level is determined to be approximately 88.56% for this material.

The polarized absorption and emission spectra have been measured for the Tm3+ doped NaY(MoO4)2 crystal and spectral parameters have been estimated from the absorption data based on the Judd–Ofelt theory. The effective intensity parameters Ω t eff (t = 2, 4, 6) are 11.67 ×10−20, 2.21 × 10−20, 1.74 × 10−20 cm2, respectively. From the intensity parameters, the radiative transition probabilities, radiative lifetimes, branching ratios and the emission cross-section have been calculated. In comparison with other Tm3+ doped laser crystals, Tm3+:NaY(MoO4)2 crystal has potential as a promising laser crystal.

Large crystals of pure and Y b 3+ doped NaY (MoO 4 ) 2 have been grown using the Czochralski method. The major problem with these crystals is that they are black in colour, causeing total absorption in the visible range, this was resolved by annealing and the nature of the colour defect was discussed. Measurements such as DTA, thermal expansion in different crystallographic directions, and refractive index as a function of wavelength were investigated. The broad absorption band at about 980 nm and high value of the absorption cross-section are obtained for the Y b 3+ doped crystal. The results show that Y b 3+ :NaY (MoO 4 ) 2 crystal has potential as a promising laser crystal.

This paper reports crystal growth and optical characteristics of dysprosium (Dy3+) and erbium (Er3+) doped mid-infrared laser crystal temary-potassium-lead-chloride (KPb2Cl5). Two transparent crystals with good optical quality have been grown successfully by using the Bridgman technique, the largest crystal size reaches up to 010x60mm(2). Measurements of X-ray diffraction (XRD) and absorption spectra were carried out. Based on Judd-Ofelt theory, the intensity parameters Omega(t)(t=2,4,6), the experimental and theoretical oscillator strengths have been obtained. The intensity parameters Omega(t)(t=2,4,6) of Er3+ :KPb2Cl5 were calculated to be Omega(2)=5.10 x 10(-20) cm(2), Omega(4)=1.25 x 10(-20) cm(2), Omega(6)=0. 83 x 10(-20) cm(2), and the values for D Y3+ :KPb2Cl5 were calculated to be Omega(2)=6.26 x 10(-1) cm(2), Omega(4)=2.45 x 10(-20) cm(2), Omega(6)=0.04 x 10(-20) cm(2) respectively.

The Dy3+-doped NaY(MoO4)2 single crystals were grown successfully by the Czochralski technique. The main spectroscopic properties (absorption, luminescence, decay curve) of Dy3+-doped NaY(MoO4)(2) have been determined for both the sigma and pi polarizations. By using the Judd-Ofelt theory, the measured room temperature absorption spectra were applied to determine the intensity parameters, spontaneous transition probabilities, branching ratios, and radiative lifetimes of Dy3+ transitions. The results show that the Dy3+-doped Nay(MoO4)(2) crystal may realize the yellow laser operation. (C) 2006 Elsevier B.V. All rights reserved.

In this paper, we present the broadband optical amplification in bismuth-doped germanate glass, at the second telecommunication window when excited with 808 nm and 980 nm laser diodes, respectively. The amplification range is from 1272 nm to 1348 nm wavelength, which is within the O-band of silica fiber communication. This bismuth-doped glass can be used as ultra broadband amplification material for wavelength-division-multiplexing (WDM) at the second telecommunication window.

Yb3+ doped SrWO4 crystal has been grown by the Czochralski method. Polarized absorption and emission spectra together with fluorescent decay curve were measured at room temperature. The stimulated emission cross sections of the potential laser transitions were determined using the reciprocity method and the Fuchtbauer-Ladenberg formula. The potential laser gain versus wavelength for the F-2(5/2) ->-> F-2(7/2) transition was also estimated and discussed. This material is a potential candidate for a 1.0 mu m laser media. (C) 2006 Elsevier B.V. All rights reserved.

Ho3+-doped SrWO4 crystal was grown by the Czochralski method. The polarized absorption and emission spectra together with the fluorescent decay curve were measured at room temperature. The intensity parameters, radiative lifetimes and branching ratios were calculated based on the Judd-Ofelt theory. The stimulated emission cross-sections of the potential laser transitions were determined using the reciprocity method.

Tin doped NaBi(WO4)(2) of good optical quality with sizes of about 20 min x 20 mm x 40 mm was grown successfully by the Czochralski technique. The absorption spectra for both sigma- and pi-polarizations were recorded at room temperature. Based upon these data, the oscillator strengths and radiative transition rates were evaluated. The sigma- and pi-polarized emission spectra together with the fluorescence decay curve were also obtained. The emission cross-section of the potential laser transition at 1460 nm was determined to be 1.49 x 10(-19) cm(2). The results show that Tm3+:NaBi(WO4)(2) crystal is a promising laser material. (c) 2006 Elsevier B.V. All rights reserved.

In this work, we established the material and energy flow charts of a base case which contains the following units: biomass collection and transportation, pretreatment, anaerobic digestion, biogas upgrading and waste management. For this biogas system, the input feedstock are fixed to human feces and corn straw, the anaerobic digestion is performed at normal temperature 25 °C, and the biogas coming from anaerobic unit is upgraded via high pressure water scrubbing method to obtain the 98 % purity biomethane. The system material and energy flows are presented to indicate that the anaerobic digestion unit generate 90 wt % of the biogas slurry and residue, and 70 % of the energy was loss with the emission of the biogas slurry and residue. As for the system energy balance, we calculate the low heating value (LHV) based system energy efficiency considering the biomass feedstocks’ LHV. Furthermore, a system energy efficiency comparison between the base case and the modified case with heat exchange between biogas slurry and feedstock is made. The system energy efficiency results indicate that the heat exchange between the biogas slurry and the feedstock can enhance the system efficiency by about 0.3 %.

The polarized absorption, emission spectra and the fluorescence decay curve of Er3+ ions in the anisotropic NaY(MoO4)2 crystal were investigated at room temperature, and spectral parameters have been calculated from the absorption data based on the Judd–Ofelt theory. The experimental oscillator strengths for the transitions from the ground state to excited states were compared with the theoretical oscillator strengths calculated from the Judd–Ofelt theory. The emission cross-section and gain cross-section for the π- and σ-spectrum were determined for the 4H13/2 → 4H15/2 transition of special interest for laser application.

Ca3Gd2(BO3)4:Er3+ crystals of high optical quality with a large size up to ∅25 mm × 60 mm were grown by Czochralski method. The absorption spectra of the crystal in three mutually perpendicular arbitrary directions at room temperature were measured. Based on Judd–Ofelt theory and a least square fit of calculated three perpendicular directions and averaged results of the absorption line intensity (referred to as TPM method), the intensity parameters Ωt (t = 2, 4, 6), the radiative transition rates, the radiative lifetimes, the fluorescence branching ratios, the experimental and theoretical oscillator strengths were obtained. The fluorescence spectrum and decay curve under the excitation of 530 nm were presented too. The stimulated emission cross-section and radiative quantum efficiency were also diplayed.

The polarized absorption and emission spectra of Pr3+ ions in NaY(MoO4)2 single crystal were investigated. The standard and modified Judd–Ofelt theory have been applied to the measured optical absorption intensities to determine the spectroscopic parameters, including the Judd–Oflet intensity parameters Ωt (t=2,4,6), the radiative transition rates, branching ratios, and emission cross-sections for various excited levels of Pr3+ ions. In samples with Pr3+ ions concentration of 2.00×1020 cm-3, the excitation of the 1D2 manifold decays non-radiatively by the electric dipole–dipole transfer between Pr3+ neighbors. The good spectroscopic properties show the possible application of the Pr3+ doped NaY(MoO4)2 crystal as a solid-state laser.

A systematic investigation of room temperature absorption and emission spectra were presented for trivalent holmium in a crystal of NaY(MoO4)2. Oscillator strengths of the transitions to all levels up to 361 nm were calculated from the absorption spectra. Judd–Ofelt intensity parameters of Ho3+ were used to calculate the radiative transition rates, branching ratios and radiative lifetimes. The emission cross-sections for the 5S2 → 5I8 and 5I7 → 5I8 transitions of special interest for laser application were determined.

Yb3+:GdAl3(BO3)4 (hereafter Yb3+:GAB) crystals with large sizes and good optical quality have been grown by the top-seed solution growth (TSSG) method. The polarized absorption and emission spectra have been investigated at room temperature. For the σ-polarization, the intensities of both absorption and emission spectra are stronger than those for the π-polarization, the σ-absorption cross section of Yb3+ in GAB being 3.43×10-20 cm2 at 977 nm, and the σ-emission cross section being 0.98×10-20 cm2 at 1045 nm. The fluorescence lifetime of the 2 F 5/2→2 F 7/2 transition was measured to be 800 μs in the 5% doped sample used for our laser experiments, 993 μs in a 10% doped sample and 569 μs in a 0.5% doped sample. The laser parameters were estimated as: βmin=0.022, Isat=10.4 kW/cm2 and Imin=0.23 kW/cm2. About 0.4 W laseroutput at the wavelength of 1043 nm was achieved when the Yb3+:GAB crystal was pumped by a 974 nm laser diode, with 27.4% slope efficiency.

Ca3Gd2(BO3)4:Nd3+ crystals of high optical quality with a size up to 20 × 50 mm were grown by the Czochralski method. The absorption spectra of the crystal in three mutually perpendicular arbitrary directions were measured. Based on Judd–Ofelt and TPM methods, the intensity parameters Ω t (t = 2, 4, 6), the experimental and theoretical oscillator strengths, the radiative transition rates, radiative lifetime and fluorescent branching ratios of Nd3+ from 4F3/2 to 4IJ, were obtained. The fluorescence spectrum and decay curve under excitation at 808 nm were also measured. The stimulated emission cross-section and radiative quantum efficiency were also calculated.

Ca3(BO3)2 single crystals have been grown by the Czochraski technique. The absorption and emission spectra have been measured at room temperature. From the Judd–Ofelt theory analysis, the intensity parameters were determined to be Ω2 = 4.63 × 10–20 cm2, Ω4 = 2.40 × 10–20 cm2, and Ω6 = 10.4 × 10–20 cm2. The calculated radiative probabilities, radiative lifetimes, and the branching ratios have been evaluated for the 4F3/2 excited state using the calculated intensity parameters. The stimulated emission cross-section was also measured at 1060 nm. The results indicate that this crystal can be considered as a promising material for solid-state laser application. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

This paper reports the growth of Nd3+:NY(WO4)2 crystal with high optical quality and large size. Nd3+:NY(WO4)2 crystal with the dimension Φ30×75 mm2 and optical homogeneity 4×10−5 was grown by Czhchoralski method: the seed used was [1 0 0] orientation, the pulling rate and rotating rate were 0.5–1.25 mm/h and 15 rpm, respectively. The growing processes and characteristics of Nd3+:NY(WO4)2 crystal was discussed. Its unpolarized absorption spectra and emission spectra were measured. The absorption cross-section and emission cross-section were presented. Based on the Judd–Ofelt theory, we obtained the three intensity parameters: Ω 2 = 5.8 × 1 0 - 20 , Ω 4 = 5.74 × 1 0 - 20 , and Ω 6 = 4.32 × 1 0 - 20 cm 2 . The radiative probabilities, radiative lifetimes, branch ratios and quantum efficiency of Nd3+:NY(WO4)2 were calculated too.

Dy 3+ : Ca 3 (BO 3 ) 2 crystal was grown successfully by the Czochraski technique. The absorption spectrum was measured and its absorption peaks were assigned. The Judd–Ofelt intensity parameters were found to be Ω 2 =5.216×10 −20 , Ω 4 =1.858×10 −20 , Ω 6 =0.623×10 −20 cm 2 . The spectroscopic parameters of this crystal such as the oscillator strengths, radiative transition probabilities, radiative lifetime as well as the branching ratios were calculated. Also, room temperature luminescence decay curve in correspondence with the emission line 4 F 9/2 → 6 H 13/2 centered at 575 nm was measured.

Large novel birefringent pure and rare earth-doped calcium borate crystals (Ca 3 (BO 3 ) 2 ) have been grown successfully by Czochralski technique. Thermal expansion coefficients of pure Ca 3 (BO 3 ) 2 have been obtained for the first time. Trivalent rare earth ions (RE 3+ ), such as Dy 3+ - and Nd 3+ -doped calcium borate crystal, have been grown for optic application. Absorption and fluorescence spectra as well as fluorescence decay curves of Dy 3+ -, Nd 3+ -doped Ca 3 (BO 3 ) 2 crystals are measured at room temperature. Optical spectra show that there are obvious absorptions at ultraviolet to IR region. The strong emissions in visible and infrared region show that calcium borate is a suitable for rare earth-doped laser crystal.

Tm 3+ :SrWO 4 crystal of good optical quality with sizes of 15 mm×60 mm was grown successfully by Czochralski technique method. The orientation procedure of this crystal was discussed. The principal coefficients of thermal expansion along (001), (010), (100) directions were precisely measured to be 2.73×10 −5 , 1.00×10 −5 , 1.05×10 −5 °C −1 , respectively. Based on the Judd–Ofelt theory, we obtained the three intensity parameters: Ω 2 =7.41×10 −20 , Ω 4 =0.25×10 −20 , and Ω 6 =1.71×10 −20 cm 2 . The radiative probabilities, radiative life times, and branch ratios of Tm 3+ :SrWO 4 were calculated. The room temperature polarized fluorescent spectra and fluorescent decay curve were measured. The luminescent quantum efficiency of the 3 H 4 level was investigated and the quantum efficiency is determined to be approximately 69.84% for this material.

Single crystals of the nonlinear optical (NLO) material TmAl3(BO3)4, space group R32, were grown successfully by the top seeded solution growth (TSSG) method for the first time in our laboratory. Morphology analysis reveals that the crystal is a rhombohedron with nine developed faces with the major forms (1210), (2110), and (1120) parallel to the c-axis. Grown crystals were characterized by X-ray powder diffraction (XRD), and second-harmonic generation (SHG) measurements. The two principal coefficients of thermal expansion along (001) and (100) were measured to be 5.33 × 10-6 and 2.46 × 10-6 K-1, respectively. In addition, the large difference of thermal expansion coefficients between the two crystallographic axes was discussed based on the structure of this crystal. Polarized absorption spectra were measured, and the results suggest that the optical properties of the TmAl3(BO3)4 crystal depend greatly on the polarization. This crystal is a promising candidate for NLO materials.

Pure and rare-earth (Nd3+, Tm3+ and Er3+)-doped strontium tungstates of good optical quality with sizes of about ϕ 20 mm×50 mm were grown successfully by the Czochralski technique. The (0 0 1) orientation was found to be the favorable direction for crystal growth. X-ray powder diffraction (XRD), differential thermal analysis (DTA) and differential scanning calorimeter (DSC) of pure strontium tungstate were measured. The concentrations of Nd3+, Tm3+ and Er3+ were measured and their segregation coefficients were also calculated. The absorption and emission spectra of rare-earth-doped crystals as a function of the σ and π polarizations were presented and discussed. Favorable values of the absorption cross section centered at about 800 nm suggest that Nd3+- and Tm3+-doped strontium tungstates are promising candidates for laser diode (LD) pumping.

Nd 3+ doped LaB 3 O 6 single crystal with the size up to Φ 20 × 35 mm was grown successfully by the Czochralski technique. The absorption and luminescence spectra of trivalent neodymium in LaB 3 O 6 crystal were measured at room temperature. Also, room temperature luminescence decay curve in correspondence with the emission line 4 F 3/2 → 4 I 11/2 centered at 1062 nm was measured. In the framework of the Judd–Ofelt (J–O) theory, the intensity parameters were calculated to be: Ω 2 = 0.54 × 10 −20 cm 2 , Ω 4 = 2.31 × 10 −20 cm 2 , Ω 6 = 6.94 × 10 −20 cm 2 . The calculated radiative probabilities, radiative branching ratios and radiative lifetime for the emissions from the 4 F 3/2 level of Nd 3+ ions in LaB 3 O 6 single crystal were presented. The luminescent quantum efficiency was also displayed.

The absorption and fluorescence spectra of Nd3+:Gd0.2Y0.766Al3(BO3)4 crystal have been measured. Basing on Judd–Ofelt and TPM theory, intensity parameters Ω t (t=2,4,6), radiative lifetime, experimental and theory oscillation strength, fluorescence branching ratios and stimulated emission cross-sections were obtained and related to the laser action of this crystal. A laser output of 48 mW at 1060 nm and of 78 mW at 1300 nm with the optical to optical efficiency of 20% and of 9.75% respectively were obtained under a Ti–sapphire laser pumping. Meanwhile, a laser output of 26.7 mW at 532 nm with an optical to optical efficiency of 10.68% was obtained when it was pumped by a AlGaAs diode laser.

The Ca 3 Gd 2 (BO 3 ) 4 :Yb 3+ crystal with a size up to ∅ 20 mm×50 mm was grown by the Czochralski method. The absorption and emission spectra were measured. There is a broad absorption band ranging from 850 to 1000 nm in the absorption spectrum. A broad emission spectrum from 930 to 1100 nm was exhibited from 895 nm wavelength pumping. This crystal is promising as a tunable infrared laser crystal.

The GdAl 3 (BO 3 ) 4 :Tm 3+ crystal with size up to 15 mm×15 mm×25 mm was grown from the K 2 O 3 –MoO 3 –B 2 O 3 solvent system by the top seed solution growth (TSSG) method. The absorption spectrum was measured and its absorption peaks were assigned. The emission spectrum ranging from 500 to 1800 nm was obtained from 468 nm wavelength pumping. The absorption cross section σ a at 800 nm was determined.

Pure and Nd 3+ -doped Cd 3 Y(BO 3 ) 3 crystals with sizes up to 20 mm were grown from a Li 6 B 4 O 9 solvent system by using the top seeded solution growth method. The second harmonic generation coefficients of the crystals were measured to be about two times larger than that of KH 2 PO 4 (KDP). The Absorption spectra were measured and the absorption peaks were assigned. The emission spectrum of Nd 3+ :Cd 3 Y(BO 3 ) 3 crystal was obtained under an excitation of 808 nm.

The Ca3Y2(BO3)4:Er3+ crystal with a size up to ϕ 20 mm×30 mm was grown by the Czochralski method. The absorption spectrum was measured and its absorption peaks were assigned to the corresponding transitions between the Er3+ energy levels. A broad emission spectrum from 1429.4 to 1662.8 nm was exhibited from 530 nm wavelength pumping. This crystal is promising as a tunable infrared laser crystal.

The absorption spectra of Tm3+:KGd(WO4)2 crystal in three mutually perpendicular arbitrary directions at room temperature were measured. Based on Judd–Ofelt theory and TPM method, the intensity parameters Ω t (t=2,4,6) , the radiative lifetimes, the experimental and theoretical oscillation strengths, and the fluorescence branching ratios were obtained. The broad emission spectrum from 1650 to 2000 nm was obtained under 800 nm excitation. Also, the fluorescence spectra at 455 and 665 nm were measured under 360 nm excitation. Their stimulated emission cross-sections were also calculated.

The Tm 3+ :KGd (WO 4 ) 2 crystal with the size up to 50 mm was grown from the K 2 O–WO 3 –BO 3 solvent system by using the top seeded solution growth method. The absorption spectrum was measured and its absorption peaks were confirmed. The emission spectra of the crystal at 455 and 665 nm were obtained when a light of 360 nm excited this crystal. A broad emission spectrum from 1650 to 2000 nm was obtained when a light of 800 nm excited this crystal. It is expected that this crystal may be useful as a tunable infrared laser crystal.

We report for the first time the spectroscopy and the laser operation of GdAl 3 (BO 3 ) 4 : Nd 3+ (NGAB) near 1338 nm corresponding to the 4 F 3/2 → 4 I 13/2 channel. Their features have been found favorable for self-frequency doubling in order to generate a red radiation at 669 nm and more generally for self-frequency conversions. The emission cross-section in σ-polarization was estimated to be 5.5×10 −20 cm 2 . We demonstrated self-frequency doubling despite of non-optimal experimental conditions.

Under short-pulsed longitudinal pumping at 600 nm we obtained intracavity stimulated Raman scattering frequency self-conversion of the 1352.5 nm laser line in KGd(WO4)2:Nd3+. The wavelength which was produced is eye-safe at 1539.5 nm and the conversion efficiency was 1.26% with uncoated faces of the 3 cm long crystal.

This paper reports the synthesis, structure and properties of two coordination compounds which are transparent in the visible region and show a relatively strong nonlinear optical (NLO) effect. The first crystal, Zn(2-NH 2 py) 2 CI 2 (2-aminopyridine zinc dichloride, DAZC), shows a SHG (second harmonic generation) effect 8 times as strong as that of KDP. X-ray single crystal structure analysis reveals that the molecule possesses a tetrahedron configuration around the zinc atom, and all the molecules are aligned in an almost fully parallel direction.The second compound, Zn(acac) 2 (PhTU) (phenylthiourea zinc diacetylacetate, PZDA), shows a powder SHG effect as strong as 10 times that of KDP. X-ray single crystal structure analysis shows that the molecule possesses a square pyramidal configuration around the zinc ion, and the molecules are aligned almost in a parallel arrangement in the lattice. These two crystals may represent a novel strategy for designing new transparent NLO materials.

A coordination compound with a tetrahedral molecular configuration, Zn(NH2py)2Cl2(2-NH2py=2-aminopyridine), was prepared. It is transparent in the visible region and shows second harmonic generation (SHG) effect 8.0 times as strong as that of KDP. X-ray single crystal structure analysis reveals that all Zn(NH2py)2Cl2 molecules are aligned in a fully parallel direction. The advantages and disadvantages of tetrahedral zinc coordination compounds as nonlinear optical (NLO) materials are discussed. The results may represent a novel strategy for designing a new class of transparent NLO materials.