Characteristic constants for the crystals in formula (1)

We carried out the estimation of refractive indices with the aid of the methods of structural refractometry, which give the possibility to evaluate refractive indices, on the basis of structural data taking into account the method of the additivity of the molecular refractions of the separate fragments, on which it is possible to decompose crystal [ 10 ]. In structural refractometry the substance is characterized by the molecular refraction:

R = (n² - 1)/(n² + 2) M/d₀ = Mr, (2)

where M- molecular weight of substance, d₀ - the density of substance, r- the specific refraction. In tab. 2 the values of the molecular refractions for the investigated crystals, calculated by formula 2 for the average refractive indices n and for the wavelength 589 mmk. are given. Re- the molecular refraction, calculated from the experimental data, R_к, R_и - covalent and ionic refraction, calculated on the basis of tables [ 10 ], R_р,- the molecular refraction, calculated from the partition of crystals into the fragments [ 10 ].

The table 2.

The molecular refraction of crystals

1 - La₃Ga₅SiO₁₄, 2 - La₃Ga₅GeO₁₄, 3 - La₃Ga_5,5Nb_0,5O₁₄, 4 - Ca₃Ga₂Ge₄O₁₄, 5 - Sr₃Ga₂Ge₄O₁₄

It is evident that it is obtained a good agreement of experimental values R_э and calculated R_р, which indicates the possibility of calculating the molecular refraction according to the diagram of additivity. Thus, for this class of crystals during the calculation of the molecular refraction it is possible to use the principle of additivity and on the calculated molecular refraction it is possible to evaluate the refractive indices of new crystals of such type. Knowing the molecular refraction at different wavelengths for the separate structural fragments, it is possible to judge the dispersion of the refractive indices of complex crystals in the region of transparency. In this case the calculated and experimental values of refractive indices must coincide with an accuracy to the second sign of afterward comma (,).

Optical activity of the gallogermanates crystals

The rotation of the plane of polarization of light in the samples, oriented along the optical axis, for all crystals was measured on the spectropolarimeter with the accuracy 510^-2 grad/mm [ 8]. The values of the rotation of the plane of polarization are represented in Fig. 6. In the figure for the comparison the values of specific rotation for the quartz are given.

Fig. 6. The specific rotation of crystals

1 - La₃Ga₅SiO₁₄, 2 - La₃Ga₅GeO₁₄, 3 - La₃Ga_5,5Nb_0,5O₁₄, 4 - Ca₃Ga₂Ge₄O₁₄, 5 - Sr₃Ga₂Ge₄O₁₄, 7 - SiO₂

According to measured data it was carried out the approximation of the specific rotation of the investigated crystals of () according to the formula [ 11 ]:

= K₁²/(² - ₀²)² + K₂/(² - ₀²) (3)

In this work as the characteristic wavelength the value of wavelength, obtained from the approximation of average refractive index, is used. Values for K₁, K₂, ₀ are given in Table 3.

The table 3.

Characteristic constants for the crystals in formula (3)

Analogously we carried out the measurements of rotatory ability in the direction of optical axis for the crystals, activated by the impurities: Ca₃Ga₂Ge₄O₁₄:Cr³⁺, Sr₃Ga₂Ge₄O₁₄:Cr³⁺, La₃Ga₅SiO₁₄:Cr³⁺, Fe³⁺, Ni²⁺ and Mn^2+. Experimental results practically coincide with the results, obtained for the transparent crystals. Thus, the impurity, introduced into the matrix in the limits of 0.1 wt.%, does not influence the rotatory ability of crystals with the structure of Ca-gallogermanata. The dispersion of rotatory ability is determined by strips, which are located in the vacuum region of the spectrum. From the measurement of the specific rotation of () it is possible to define only the component of g₃₃.

As it was shown in [ 12 ], for determining the component of g₁₁ it is necessary to conduct measurements on the plate, cut out in parallel to optical axis. Since all crystals in question have weak absorption bands in the neighbor UF region and possess dichroism, during a study of optical activity in the directions, different from the direction of optical axis, this must be considered. In this case

tg2 = - 2ke^-sin и tg2 = - 2kesin, (4)

where for the plate, cut out in parallel to optical axis k = g₁₁/2n_срn - the ellipticity of natural waves in the crystal, n_ср - average refractive Д = 2рd(n)/л, = 2рd()/л.

The measurements of the angles of and in the range of wavelengths from 270 to 660mmk were carried out on the plates from the crystals of La₃Ga₅SiO₁₄ [13] and Sr₃Ga₂Ge₄O₁₄:Cr³⁺ [14], cut out parallel to optical axis. The dispersion formula of the azimuth of the polarization of the passed light for these crystals is shown in Fig. 7 and 8.

Knowing values n and , from the dependences of () and () it is possible to calculate the component of g₁₁. For determining the sign the components of g₁₁ of measurement were conducted on the thin plate (d = of 0.01 mm). In this case it turned out that g₁₁ - has positive sign, and g₃₃ -negative for the right-handed crystal. The dispersion of the components of g₁₁ and g₃₃ for the crystals of La₃Ga₅SiO₁₄ and Sr₃Ga₂Ge₄O₁₄:Cr³⁺ is shown in Fig. 9 and 10.

For the analytical description of the experimental dependence of the dispersion of the components of g₁₁ and g₃₃ in the crystal of La₃Ga₅SiO₁₄ were obtained the formulas:

g₃₃ = -A(n_ср/² - ₁²) 10^-5, (4)

g₁₁ = n_ср[B/(² - ₂²) + C²/(² - ₂)²] 10^-5 (5)

where A = 0.6072?10³, B = 0.6106?10⁶, C = 0.6278?10⁶, ₁ = 198 mmk, ₂ = 156 mmk.

Spektroskopical properties of the gallogermanate of calcium of Ca₃Ga₂Ge₄O₁₄, activated by chromium ions crystal

The spectra of absorption of the crystals of Ca₃Ga₂Ge₄O₁₄, activated by chromium ions are investigated on the spectrophotometer of Specord-40 in the range 250- 900 mmk (40000- 11300 sm^-1) with 300K in the samples with a thickness of 1.9 and 0.45 mm, oriented perpendicularly to optical axis [ 15 ].

In the absorption spectra there are registered three intensive strips with frequencies in the maximums 35088 sm^-1 (285 mmk), 22650 sm^-1 (440 mmk) and 15750 sm^-1 (635 mmk), and bend in interval of 14160 is also noted. 14347 sm^-1 (695. 700 mmk) and weakly-intensive strips with 12000 sm^-1 (830 mmk), ~ 17500 sm^-1 (570 mmk) and ~2y000 sm^-1 (475 mmk). With 300K it is registered pure- electron transition with a frequency of 11374 sm^-1 (879 mmk), which relates to the strip with 12000 sm^-1 (830), which is the beginning of electron-vibrational progression with the period of 130 sm^-1 in the range 11500. 12500 sm^-1 (870. 800 mmk). For the interpretation of experimental data the calculation of the parameters of crystalline field and frequencies in the maximums of strips is executed (tab. 4).

The values of the calculated wavelengths in the ion of Cr³⁺ correlate with their experimental values. However, weak strips with 830 mmk, 570 mmk and pure- electron transition are not plotted in this diagram, apparently, these maximums correspond to the ion Of Cr⁴⁺ in the tetrahedral coordination, which coincides with the conclusions of the works [ 16 ].

Spectral measurements of circular dichroism (CD) in the crystals of Ca₃Ga₂Ge₄O₁₄ were conducted on dikhrografe in the range 250- 800 mmk, in the samples, oriented perpendicularly to optical axis, with the use of procedures, which make it possible to eliminate the possible effect of circular and linear double refraction and linear dichroism (Fig. 11).

Fig. 11. Spectra of circular dichroism of the crystals:

1 - Ca₃Ga₂Ge₄O_14:Cr³⁺ with Т =300К, 2 - Ca₃Ga₂Ge₄O_14:Cr³⁺ with Т =77К, 3 - nonactivated Ca₃Ga₂Ge₄O₁₄

It is established that in the crystals Ca₃Ga₂Ge₄O₁₄ with the impurity of the ions of Сr³⁺ CD is manifested at frequencies of all four absorption bands. Since CD appears only on the passages, for which electrodipole and magnetodipole moments simultaneously are not equal to zero, the intensities of strips in the maximums are substantially outstanding in SA and spectra CD in SA of I(b⁴Т₁): I(а⁴Т₂): I(⁴Т₂) = 6:2:1, and in the spectra CD 3:1:6. Furthermore, in the spectra CD (Fig. 11, curve 1) they are observed maximum with 335 mmk and arm in region 380-400 mmk, which according to calculation are not connected with the ions of Сr³⁺.

With a temperature decrease to 77K strips slightly are displaced into the short-wave region of the spectrum. The maximum with 390 mmk is determined at frequencies of bend, and strip with 450 mmk partially is resolved, the fixture of asymmetric (Fig. 11 curve 2). With 470 mmk the arm, which, probably, corresponds to passage ⁴А₂ ²Т₂ (table 4). (⁴А₂ was formed ²Т₂).

The table 4.

Calculated and experimental values of wavelengths in the maximums of the spectra of absorption (SA) and circular dichroism (CD) of activated by ions Cr³⁺ and nonactivated crystals Of Ca₃Ga₂Ge₄O₁₄

The calculated wavelengths _calc were calculated in the parameters: Dq = 1575, B = 761, C = 2811, a = 70; * - measurements are carried out with Т = 77К.

The spectra of magnetic circular dichroism (MCD) were measured on dikhrografe with the use of a permanent magnet, which creates the field of the strength of ~14кE. During the application of magnetic field even with 300K on the long-wave side of spectrum with 635 mmk of the ion of Сr³⁺ it is permitted purely electron transition with the maximum with 13072 sm^-1 (765 mmk) and electron-vibrational structure with a frequency of 190 sm^-1.

For the interpretation of spectra photos CD of the crystal of Ca₃Ga₂Ge₄O₁₄, activated by the ions of Сr³⁺, were investigated the spectra CD of the crystals, grown without the impurity of the ions of Сr³⁺. during a study of the spectra of absorption of unalloyed crystals with the large tension of spectra in the intensity were fixed only very weak strips with 330 and 430 mmk.

In the spectra CD were discovered the well permitted strips with 272, 330, 390 and 440 mmk (Fig. 11 curve 3). Strips with 330 and 390 mmk, as noted above were observed in the spectra CD of the crystals of Ca₃Ga₂Ge₄O₁₄, activated by the ions of Сr³⁺.Other maximums with 272 and 440 mmk overlap with spectrum bands CD with 276 and 450 mmk, which correspond to the ions of Сr³⁺. Thus from the spectra CD it is possible to conclude that the strip with 453 mmk is complex and consists of three components 440, 453 and 470 mmk.

Probably, all strips, observed in the spectra CD of unalloyed crystal can be explained by their own defects, which are formed in the process of an increase in the crystal.

Spectroscopic properties of the crystal of Sr3Ga2Ge4O14, activated by chromium ions

In this division there are represented the results of the studies, carried out on the activated by chromium of gallogermanate crystals of strontium of Sr₃Ga₂Ge₄O₁₄ (SGGO:Cr), which have different color [ 17 ]. As samples served oriented perpendicularly to the optical axis of the plate with a thickness of from 0.5 to 1.5 mm. the absorption spectra were measured in the range 250.700 mmk on the spectrophotometers of Hitachi and Spekord-40. Spectral measurement of circular dichroism was conducted in the range 250-800 mmk. The obtained results were compared with the results of the similar studies, carried out on the crystals of Ca₃Ga₂Ge₄O₁₄:Сr³⁺ [15].

The investigated crystals of Ca3Ga2Ge4O14:Сr³⁺ had green color, while the crystals of Sr3Ga2Ge4O14:Сr³⁺ had red and green color. The spectra of absorption of these crystals are represented in Fig. 12.

In all spectra there are observed wide electron-vibrational absorption bands typical for the ion of Cr³⁺ in the octahedral crystalline field: one in the ultraviolet and two in visible ranges of spectrum. The positions of the maximums of strips for all investigated samples are represented in Table 5.

According to the experimental data carried out the estimation of the parameters of crystalline field Dq and B, which will agree with the data of work [ 16 ].

During the comparison SA of the green samples of CGGO:Cr and SGGO:Cr should be noted their similarity, which attests to the fact that the replacement Ca on Sr in the gallogermanates practically did not influence on the value of crystalline field at the center of the octahedron, where the ion of Cr³⁺ is located. As can be seen from Fig. 12, for the crystals of SGGO:Cr the intensity of wide absorption band in the red region of the spectrum for the red sample is lower than the intensity of the corresponding absorption band for the green sample, which indicates the smaller concentration of the ions of Cr³⁺ in the red sample.

The table 5.

The basic spectroscopic parameters of the crystals. Sr3Ga2Ge4O14:Сr³⁺ (SGGO:Cr) and Ca3Ga2Ge4O14:Сr³⁺ (CGGO)