Structural reasons for the nonlinear optical properties of KTi<Subscript>0.96</Subscript>Zr<Subscript>0.04</Subscript>OPO<Subscript>4</Subscript> single crystals

This paper reports on the results of the precision X-ray structural investigations of KTi_0.96Zr_0.04OPO₄ single crystals at room temperature. It is established that the incorporation of zirconium atoms into the structure of KTiOPO₄ (KTP) crystals does not lead to substantial changes in the framework structure and results only in an insignificant decrease in the scatter of the distances in the PO₄ tetrahedra and the formation of more symmetric (TiZr)O₆ octahedra as compared to the TiO₆ octahedra. However, the incorporation of zirconium atoms into the KTP structure is accompanied by the redistribution of the electron density in the crystal as a whole, so that the electron density increases in the region of the positions occupied by the potassium atoms. This changes the nonlinear optical properties of the given series of crystals, which are estimated from the intensity of the second harmonic generation signals.     

X-Ray diffraction study of KTiOPO<Subscript>4</Subscript> single crystals doped with hafnium

Single crystals of KTi_{1 − x} Hf _x OPO₄ (x = 0.015(2), 0.035(1), and 0.128(1) are reinvestigated by precision X-ray diffraction at room temperature. It is found that the implantation of hafnium atoms in the crystal structure of KTiOPO₄ does not lead to significant changes in the framework and affects only the positions of the potassium atoms in the channel. Our studies reveal the displacements of the potassium atoms from their main and additional positions in the structure of pure KTP in all three structures studied. The largest displacements from the K1′ and K1″ additional positions are observed in the structure with x = 0.035. At this hafnium concentration, the occupancy of the main positions of potassium atoms decreases and the occupancy of the additional positions increases in relation to those in KTP. This redistribution of potassium atoms enhances the nonuniformity of distribution of the electron density in the vicinity of their positions, which is probably responsible for the increase in the nonlinear susceptibility of KTP crystals that contain 3.5% hafnium in relation to crystals of pure KTP.     

Accurate X-ray diffraction studies of KTiOPO<Subscript>4</Subscript> single crystals doped with niobium

Single crystals of potassium titanyl phosphate doped with 4% of niobium (КТР:4%Nb) and 6% of niobium (KTP:6%Nb) are studied by accurate X-ray diffraction at room temperature. The niobium atoms are localized near the Ti1 and Ti2 atomic positions, and their positions are for the first time refined independent of the titanium atomic positions. Maps of difference electron density in the vicinity of K1 and K2 atomic positions are analyzed. It is found that in the structure of crystal КТР:4%Nb, additional positions of K atoms are located farther from the main positions and from each other than in КТР and KTP:6%Nb crystals. The nonuniform distribution of electron density found in the channels of the КТР:4%Nb structure is responsible for ~20% increase in the signal of second harmonic generation.     

Crystal structure of potassium titanyl phosphate doped with zirconium

This paper reports on the results of precision X-ray structural investigations of single crystals of the compounds KTi_0.96Zr_0.04OPO₄ (at 293 K) and KTi_0.97Zr_0.03OPO₄ (at 293 and 105 K). No significant splitting of the positions occupied by potassium atoms is revealed. This result is in agreement with a considerable decrease in the electrical conductivity of potassium titanyl phosphate KTiOPO₄ (KTP) crystals doped with zirconium (KTP: Zr) as compared to crystals of undoped potassium titanyl phosphate. It is established that the difference between the Ti-O bond lengths in chains formed by titanium octahedra is not a single structural parameter responsible for the nonlinear optical properties of crystals in this series.     

Structure of the RbTi<Subscript>0.98</Subscript>Zr<Subscript>0.02</Subscript>OPO<Subscript>4</Subscript> single crystal at temperatures of 293 and 105 K

This paper reports on the results of precision X-ray structural investigations of a RbTi_0.98Zr_0.02OPO₄ single crystal at temperatures of 293 and 105 K. It is established that the observed decrease in the temperature of the ferroelectric phase transition in RbTiOPO₄ crystals doped with zirconium is associated with the increase in the Rb-O bond lengths. The structural factors responsible for the decrease in the electrical conductivity in these crystals are revealed. An analysis is made of the structure of the helical channels which in crystals of this family are considered to be a decisive factor for the manifestation of superionic conduction. It is shown that, in structures of the KTiOPO₄ (KTP) type, the migration of ions in channels is most hindered inside the cavities.     

Growth of LiNbO<Subscript>3</Subscript>:Er Crystals and concentration dependences of their properties

A series of lithium niobate (LiNbO₃) crystals of congruent and stoichiometric compositions, doped with erbium, have been grown under non-steady-state thermal conditions. A series of LiNbO₃:Zn crystals, nominally pure LiNbO₃ crystals of congruent and stoichiometric compositions, and a LiNbO₃:B crystal have also been grown. Both growth conditions and concentration dependences of physicochemical, ferroelectric, and structural characteristics of LiNbO₃:Er crystals are investigated. The growth regular domain microstructures and periodic nanostructures in LiNbO₃:Er crystals are analyzed by optical microscopy and atomic force microscopy (AFM). A comparative study of the optical homogeneity and photorefractive properties of LiNbO₃:Er crystals of congruent and stoichiometric compositions and the Raman spectra of LiNbO₃ crystals of different compositions is performed.     

Crystal structure of the binary complex of cobalt and zinc chlorides with carbamide [Co(OCN<Subscript>2</Subscript>H<Subscript>4</Subscript>)<Subscript>5</Subscript>(H<Subscript>2</Subscript>O)][ZnCl<Subscript>4</Subscript>]

Mixed single crystals of [Co(OCN₂H₄)₅(H₂O)][ZnCl₄] were grown by the isothermal evaporation of an aqueous solution. The crystal structure of this complex was established by X-ray diffraction (R = 0.052 based on 7003 reflections). The crystals consist of [Co(OCN₂H₄)₅(H₂O)]²⁺ cations containing Co atoms in an octahedral coordination and [ZnCl₄]²⁻] anions containing Zn atoms in a tetrahedral coordination. The carbamide molecules are involved in both intramolecular and interionic hydrogen bonds. The H₂O molecule forms hydrogen bonds with the anions.     

Structure and properties of antimony-doped potassium titanyl phosphate single crystals

Antimony-doped potassium titanyl phosphate (KTP) crystals of the composition KTiOPO₄ are studied by X-ray diffraction analysis. It is shown that, with an increase of antimony content in KTP crystals, the occupancies of potassium positions change and new additional positions for potassium atoms arise. The formation of additional vacancies and splitting of the potassium-cation positions explain the considerable decrease in the temperature of the ferroelectric phase transition, enhancement of the relaxation phenomena, and an increase in electric conductivity of antimony-doped KTP crystals. Substitution of antimony atoms for titanium is accompanied by elongation of short and shortening of long Ti-O bonds in the TiO₆ octahedra, processes which result in lower intensity of second-harmonic generation in laser-irradiated KTP crystals.     

Structure and properties of niobium-doped potassium titanyl phosphate crystals

A series of potassium titanyl phosphate crystals, KTiOPO₄, with various concentrations of niobium dopant has been grown, and some of their physical properties and structural characteristics have been studied. The incorporation of a small amount of niobium results in considerable changes in the electrical conductivity of KTP: Nb crystals and the temperature of the ferroelectric phase transition. Thus, the presence of 3–4 at. % of niobium results in an increase of conductivity by more than an order of magnitude, whereas T _C decreases from 930 to 620°C. The X-ray diffraction study of the crystals has been performed at room temperature; the neutron diffraction analysis was made at temperatures of 20, 330, and 730°C. It was revealed that two crystallographically independent positions are statistically (by 90%) occupied by potassium cations, which results in the concentration of potassium atoms in the structure higher than it was expected from the condition of preservation of crystal electroneutrality. At high niobium concentrations, the monoclinic compound of the composition K₂TiNb₂P₂O₃ is formed.     

Crystal Structure and Physical Characterisation of [Zn<Subscript>2</Subscript>(Benzoato)<Subscript>4</Subscript>(Caffeine)<Subscript>2</Subscript>]·2 Caffeine

Abstract  

The crystal structure of [Zn₂(benzoato)₄(caffeine)₂]·(caffeine)₂ was determined by direct method and Fourier technique. The structure was refined by full-matrix least-squares method to a weighted R factor of 0.0582. The structure consists of centrosymmetric dimeric units where the two zinc(II) atoms are coordinated by four bridging benzoates in a syn–syn arrangement and two caffeine ligands at the apices of a bicapped square prism. Remaining two caffeines are bound only by hydrogen bonds. The Zn–Zn distance is 2.961(1) ?. The Zn(II) atoms are displaced by 0.365 ? from the basal plane containing four oxygen atoms towards the apical caffeine molecules. The dimeric structure of the complex is consistent with spectrum and thermal data. The structural data are compared with those found in similar [Zn₂(RCOO)₄(NL)₂] complexes.     

Structure and properties of potassium titanyl phosphate single crystals with 7 and 11 at. % Nb

This study is a continuation of research into the atomic structure and physical properties of niobium-doped potassium titanyl phosphate crystals, KTiOPO₄ (KTP: Nb). Crystals containing 7 and 11 at. % of niobium were grown and studied. With an increase in niobium content, the number of vacancies and additional potassium positions in the structure also increase. This fact accounts for an increase in both the intensities of relaxation peaks and the conductivity of KTP: Nb crystals.     

Electrical and nonlinear optical properties of KTiOPO<Subscript>4</Subscript> single crystals doped with niobium,antimony, and tantalum

Single crystals of K_{1 ? x}Ti_{1 ? x}Nb_xOPO₄ (KTP: Nb), K_{1 ? x}Ti_{1 ? x}Sb_xOPO₄ (KTP: Sb), and K_{1 ? x}Ti_{1 ? x}Ta_xOPO₄ (KTP: Ta) solid solutions are grown and their dielectric, conducting, and nonlinear optical properties are investigated. The maximum contents x of niobium, antimony, and tantalum impurities in the crystals are equal to 0.11, 0.23, and 0.25, respectively. The doping of the KTiOPO₄ crystals with niobium, antimony, and tantalum brings about the formation of additional potassium vacancies and additional potassium positions and, as a consequence, an increase in the ionic conductivity σ₃₃. An increase in the doping level leads to a smearing of the ferroelectric phase transitions and a decrease in the phase transition temperatures. The permittivity of the doped crystals exhibits a broad relaxation peak in the temperature range 200–600°C.     

A survey of research on KTP and its analogue crystals

This paper reviews the growth and properties of KTiOPOP₄ (KTP) and its analogue crystals including its isomorphous crystals and doped crystals. Based on the calculation of the refractive indices and phase matching of KTP crystals, it is shown that non-critical phase matching can be achieved theoretically. In a 4%Nb-doped KTP crystal, non-critical phase matching was obtained with a pulsed Nd:YAG laser in an efficiency of ca. 4%. Most of the isomorphous and doped KTP crystals possess similar physical, optical and nonlinear optical properties as those of KTP.RbTiOPO₄, KTiOASO₄ and RbTiOASO₄ crystals have better electrooptical figure of merit and RbTiOPO₄ crystals have a fast ion conductor character. Some doped KTP crystals have shown high second harmonic generation efficiencies and damage thresholds. But the homogeneity of KTP analogue crystals is poorer than that of KTP. On the base of damage threshold tests using a CW Argon laser, it is concluded that the best second harmonic generation crystal among all the KTP analogue crystals is still KTP itself. Special care must be taken to eliminate impurity centers to avoid the formation of gray track and photorefractive centers.     

Growth, structure, and properties of KTiOPO4 crystals doped with iron

A series of iron-doped KTiOPO₄ (KTP: Fe) single crystals in which iron substitutes for 0.1–0.3% titanium was grown. The structure of the KTP: Fe crystals was determined, and their dielectric and conducting properties were studied. An X-ray diffraction analysis failed to reveal such asmall amount of Fe⁺³ ions in titanium octahedral positions of the structure. It was found that an increase in the iron concentration results in a lowering of the symmetry of Ti(1)O₆ and Ti(2)O₆ octahedra. The splitting of the dielectric anomaly due to the ferroelectric phase transition was explained by the mechanism of incorporation of an impurity into different growth pyramids of the crystals. It was established that the aging of the KTP: Fe crystals leads to changes in the permittivity and electrical conductivity during long storage.     

The role of two coordination compounds as novel transparent nonlinear optical crystals

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-NH₂py)₂CI₂ (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.     

The defect structure of CdS crystals heavy doped with Cu,Se. and Zn atoms

In this paper the defect structure of CdS crystals doped with Cu, Se and Zn atoms has been investigated. There were used two X-ray methods: the oscillating slit and oscillating film methods. CdS crystals were obtained from the melt under pressure of Se and Zn 0.5, 0.75, 1% wt and Cu 0.25, 0.5, 0.75, 1% wt in relation to CdS crystals. It was proved that high doping damages the structure of CdS crystals and that especially Cu impurity much more disturbs the crystal lattice than Zn one. Se atoms cause rather small deformation of crystal and increase the lattice constant, this being connected with decreasing of energy gap.     

Structure of phases of the sillenite family in the Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>-V<Subscript>2</Subscript>O<Subscript>5</Subscript> system

X-ray diffraction studies of sillenite Bi₂₄V₂O₄₀ single crystals grown by the hydrothermal method are performed for a separate crystal and powdered crystals. It is found that the composition of the two specimens is described by the (Bi_{24 − x} ▭ _x )[Bi _y ³⁺V_1−y ⁵⁺]₂ O₄₀ general formula with completely populated oxygen sites but differs in the content of vacancies at the bismuth site (this was established for the first time) and the Bi: V ratio at the tetrahedral site. The structural models of all the vanadium-containing sillenites reported in the literature are considered, and the possibility that Bi atoms are located at the centers of BiO₄ tetrahedra is established.     

单掺Rb+和Cs+的KTP晶体生长及其电导率

采用高温溶液降温法在掺质浓度均为5mol;的KTP-K4溶液中分别生长了单掺Rb+和Cs+的KTP晶体,发现掺质改变了晶体生长习性,在相应生长体系中掺质Rb+和Cs+的分配系数分别为O.646和0.08,掺质KTP晶体的晶胞参数a0和b0比纯KTP晶体者略有增长.通过掺Rb+或Cs+,KTP晶体的c向电导率明显降低,但晶体在350～1100nm范围内的光透过性质未受影响.     

KTP晶体的电光研究进展

本文简述了KTP晶体的电光性能并与KD*P、LN晶体进行了比较。概括了KTP晶体电光器件研究的主要进展。对水热法生长的KTP晶体和熔剂法生长的KTP晶体在电光应用中的优缺点进行了分析。最后介绍了熔剂法生长的低电导率KTP晶体在电光领域的应用研究。     

Atomic structure and mechanism of ionic conductivity of Li<Subscript>3.31</Subscript>Ge<Subscript>0.31</Subscript>P<Subscript>0.69</Subscript>O<Subscript>4</Subscript> single crystals

The atomic structure of Li_3.31Ge_0.31P_0.69O₄ single crystals was refined based on high-precision X-ray diffraction data at 293 K. The characteristic features of the crystal structure are considered, and their influence on high ionic conductivity (Li⁺) of these crystals is discussed.