Investigation of covalency and spectrum shifts in 3d transition-metal compounds

By using a correction factor of d electron effects on bond, PV theory is applied to the calculation of chemical bond;parameters of d transition-metal compounds. Racah parameters and Mossbauer isomer shifts are calculated, and the results are agreement with the experimental values.

Calculation of second-order nonlinear optical coefficients of KTiOPO4 and KTiOAsO4

The second-order nonlinear optical tensor coefficients of both KTiOPO4 (KTP) and KTiOAsO4 (KTA) are calculated from the chemical bond viewpoint. All constituent chemical bonds of both crystals are considered, and contributions of each type of bond to the total linearity and nonlinearity are determined. Calculated results agree satisfactorily with experimental data in both signs and numerical values. The calculation shows that though TiO6 groups and P(1)O-4 or As(1)O-4 groups have relatively larger linear contributions, they can only produce an advantageous environment for KOx (x = 8, 9) groups and P(2)O-4 or As(2)O-4 groups in nonlinear optical contributions. The origin of nonlinearity of KTP family crystals comes from the KOx (x = 8, 9) and P(2)O-4 groups in their crystal structures. Furthermore, the difference in optical nonlinearities of KTP type crystals is analyzed, based on the detailed calculation of nonlinearities of both KTP and KTA. (C) 1999 Academic Press.

Calculations of nonlinear optical responses of isomorphous crystals NaClO3 And NaBrO3 with natural optical activity

This work considers the isomorphous optically active crystals NaClO3 and NaBrO3. The connection between their second-order nonlinear optical (NLO) responses and chemical bond structures is established, starting from the experimental optical activities. The calculation reproduces the well-known experimental fact that crystals of NaClO3 and NaBrO3 with similar structures have different signs of optical rotation and of second harmonic generation (SHG). Unlike previous bond charge models, the method may include more than one type of bond in the calculation, and therefore may be used to study the optical activity and nonlinear optical properties of more general crystals. (C) 1998 Elsevier Science B.V. All rights reserved.

Calculation of nonlinearities of K2Ce(NO3)(5)center dot 2H(2)O and K2La(NO3)(5)center dot 2H(2)O

A quantitative investigation of structure-property relationships has been carried out in the nonlinear optical crystals K2Ce(NO3)(5) . 2H(2)O and K2La(NO3)(5) . 2H(2)O, from the chemical bond viewpoint. Chemical bond parameters and linear and nonlinear optical properties of each type of constituent chemical bond of both crystals are calculated. Theoretical results agree reasonably with experimental data, and explain quantitatively their nonlinear origins in this type of crystal. This theoretical method allows us to calculate accurately the nonlinearities of complex crystals.

Calculation of the nonlinear optical coefficient of the NdAl3(BO3)(4) crystal

For the first time, we present the calculation of the nonlinear optical coefficient of the NdAl3(BO3)(4) (NAB) crystal from a systematic and quantitative standpoint. Based on the dielectric theory of complex crystals and the Levine bond charge model, the method of calculation of the second-order nonlinear optical tensor coefficients of complex crystals has been given systematically. The chemical bond parameters and linear and nonlinear susceptibilities of the NAB crystal have been calculated in detail, and the calculated value of d(11)(NAB) is -5.81 x 10(-9) esu, which agrees with the measured value of 4.06 x 10(-9) esu.

Bond-charge calculation of nonlinear optical susceptibilities of LiXO3 type complex crystals

Second order nonlinear optical (NLO) tensor coefficients of LiXO3 (X = I, Nb, Ta) type crystals have been evaluated on the basis of the dielectric theory of complex crystals and the modified bond charge model. The current method is capable of calculating single bond contributions to the total second order NLO susceptibility. The tenser values thus calculated agree well with experimental data. By introducing the subformula equation and the concept of the effective charge of one valence electron, we are able to successfully treat such complex crystals as LiXO3 type compounds. In addition, the bond charge expression is modified to a more reasonable form for complex crystals. (C) 1998 Elsevier Science B.V.

Investigation of Chemical Bond Characteristics, Thermal Expansion Coefficients and Bulk Moduli of alpha-R2MoO6 and R2Mo2O7 (R = rare earths) by Using a Dielectric Chemical Bond Method

Theoretical researches are performed on the alpha-R2MoO6 (R = Y, Gd, Tb Dy, Ho, Er, Tm and Yb) and pyrochlore-type R2Mo2O7 (R = Y, Nd, Sm, Gd, Tb and Dy) rare earth molybdates by using chemical bond theory of dielectric description. The chemical bonding characteristics and their relationship with thermal expansion property and compressibility are explored. The calculated values of linear thermal expansion coefficient (LTEC) and bulk modulus agree well with the available experimental values. The calculations reveal that the LTECs and the bulk moduli do have linear relationship with the ionic radii of the lanthanides: the LTEC decreases from 6.80 to 6.62 10(-6)/K and the bulk modulus increases from 141 to 154 GPa when R goes in the order Gd, Tb Dy, Ho, Er, Tm, and Yb in the alpha-R2MoO6 series; while in the R2Mo2O7 series, the LTEC ranges from 6.80 to 6.61 10(-6)/K and the bulk modulus ranges from 147 to 163 GPa when R varies in the order Nd, Sm, Gd, Tb and Dy.

The influence of bond valence on bond covalency in RMn2O5 (R=La, Pr, Nd, Sm, Eu)

The relationship between bond valence and bond covalency in RMn2O5 (R = La, Pr, Nd, Sm, Eu) has been investigated by a semiempirical method. This method is the generalization of the dielectric description theory of Phillips, Van Vechten, Levine and Tanaka scheme. The results indicate that larger valences usually result in higher bond covalencies, in good agreement with the point that the excess charge in the bonding region is the origin of formation of bond covalency. Other factors, such as oxidation state of elements, only make a small contribution to bond covalency.

Bond covalencies in R2BaCuO5 (R = Sm, Gd, Dy, Ho, Y, Er, Tm, Yb, Lu)

Bond covalencies in R2BaCuO5 (R = Sm, Gd, Dy, Ho, Y, Er, Tm, Yb, Lu) were calculated by means of a semiempirical method. This method is the generalization of the dielectric description theory of Phillips-Van Vechten-Levine-Tanaka scheme. The present paper presents the formula concerning the decomposing of complex crystals which are usually anisotropic systems into the sum of binary crystals which are isotropic systems. It can be seen that although the bond covalency is related to many physical quantities, it is mainly influenced by bond valence or bond charge, and a higher bond valence will produce higher bond covalency.

Semiempirical method for the evaluation of bond covalency in complex crystals

We report a semiempirical method for the evaluation of bond covalency in complex crystals. This method is the extension of the dielectric description theory delivered by Phillips, Van Vechten, Levine, and Tanaka (PVLT) which is mainly suitable for binary crystals. Our method offers the advantage of applicability to a broad class of complex materials. The simplicity of the approach allows a broader class of researchers to access the method easily and to calculate not only the bond covalency but also other useful. properties such as bulk modulus. For a series study, a useful trend can be illustrated and often the prediction of the properties of the-missing one(s) among the series can be possible. Finally, examples are given to show how the method is applied and the procedure is transferable to other complex crystals.

Calculation of chemical bond parameters in La1-xCaxCrO3 (0.0 <= x <= 0.3)

The chemical bond parameters, that is, bond covalency, bond susceptibility, and macroscopic linear susceptibility of La1-xCaxCrO3 (x = 0.0, 0.1, 0.2, 0.3) has been calculated using a semiempirical method. This method is the generalization of the dielectric description theory proposed by Phillips, Van Vechten, Levine, and Tanaka (PVLT). In the calculation of bond valence, two schemes were adopted. One is the bond valence sums (BVS) scheme, and the other is the equal-valence scheme. Both schemes suggest that for the title compounds bond covalency and bond susceptibility are mainly influenced by bond valence and are insensitive to the Ca doping level or structural change. Larger bond valences usually result in higher bond covalency and bond susceptibility. The macroscopic linear susceptibility increases (only slightly for BVS scheme) with the increasing Ca doping level. (C) 1999 John Wiley & Sons, Inc.

Influence of bond valence on bond covalency in calcium doped lanthanum chromite

The influence of bond valence on bond covalency in La1-xCaxCrO3(x =0.0, 0.1, 0.2, 0.3) has been studied by using semiempirical method. This method is the extension of the dielectric description theory proposed by Phillips, Van Vechten, levine and Tanaka (PVLT). In the calculation of bond valence, two schemes were adopted. The first is the equal-valence scheme, and the second is Bond Valence Sums (BVS) scheme. Both schemes suggest that for the title compound bond covalency be mainly influenced by bond valence, and insensitive to the Ca doping level. Generally speaking, larger bond valences usually result in higher bond covalencies.

Evaluation of bond covalency and bond valence in Sr-doped perovskite-type compounds

Formulas for decomposing of complex crystals to a sum of binary crystals are described and applied to the study of bond covalency in La1-xSrxFeO3 (0.0 less than or equal to x less than or equal to 0.9) and Ca1-xSrxMnO3 (0.0 less than or equal to x less than or equal to 0.5). The bond valence is treated by bond-valence sums scheme. The results indicate that, for both compounds, with the increasing doping level, the bond covalency and bond valence show the same trend, namely, larger bond covalency corresponds to higher bond valence. For La1-xSrxFeO3, with the increase of doping level, the bond covalency of La-O, Ca-O decreases in the orthorhombic (0.0 less than or equal to x less than or equal to 0.2) and rhombohedral (0.4 less than or equal to x less than or equal to 0.7) systems, then increases slightly for the cubic (0.8 less than or equal to x less than or equal to 0.9) system, but that of Fe-O increases for all crystal systems. A sharp decrease in bond covalency was observed where the crystal changes from orthorhombic to rhombohedral, while a smooth trend was seen for the rhombohedral-to-cubic transition. On the other hand, for orthorhombic Ca1-xSrxMnO3, the bond covalency of Ca-O, Sr-O, and Mn-O (4-coordinate site) decreases with the increasing doping level, that of Mn-O (2-coordinate site) increases.

Chemical bond analysis of the correlation between crystal structure and nonlinear optical properties of complex crystals

Second order nonlinear optical (NLO) properties of single crystals with complex structures are studied, from the chemical bond viewpoint. Contributions of each type of constituent chemical bond to the total linearity and nonlinearity are calculated from the actual crystal structure, using the chemical bond theory of complex crystals and the modified bond charge model. We have quantitatively proposed certain relationships between the crystal structure and its NLO properties. Several relations have been established from the calculation. Our method makes it possible for us to identify, predict and modify new NLO materials according to our needs. (C) 1999 Elsevier Science B.V. All rights reserved.

Study on the bond covalency and bond susceptibility in NdCr1-xMxO3 (M=Mn, Fe, Co)

Bond covalency, bond susceptibility and macroscopic linear susceptibility in NdCr1-xMxO3 (M=Mn, Fe, Co, 0.0 less than or equal to x less than or equal to 1.0) are investigated by complex chemical bond theory. The results indicate the bond covalencies are insensitive to the doping level. With the increasing doping level, the macroscopic linear susceptibilities increase for M=Mn, Fe, decrease for M=Co. The valence state of Cr can be strongly influenced by the properties of the doping ions.