Structure of the La12GdEuB6Ge2O34borogermanate as probed by NMR and IR spectroscopy

ISSN 0036�0236, Russian Journal of Inorganic Chemistry, 2013, Vol. 58, No. 9, pp. 1090–1096. © Pleiades Publishing, Ltd., 2013. Original Russian Text © V.A. Krut’ko, V.P. Tarasov, G.A. Bandurkin, M.G. Komova, 2013, published in Zhurnal Neorganicheskoi Khimii, 2013, Vol. 58, No. 9, pp. 1217–1224. 1090 Studying the phase equilibria in the subsolidus region of the Nd2O3–B2O3–GeO2 system revealed two ternary compounds, NdBGeO5 and Nd14B6Ge2O34 [1]. It is worth noting that the LnBGeO5 compounds are known for the long series of rare earths from La to Er, whereas Ln14B6Ge2O34 compounds are known for the short series from Pr to Gd [1]. The present study focuses on one of the members of the second group of borogermanates crystallized in trigonal system (space group P31) with a noncentrosymmetric structure [2, 3], which accounts for their different, useful func� tional properties, including nonlinear optical, gyro� tropic, and other activities. The optical activity of Ln14B6Ge2O34 has been proved in studying the spec� troscopic and chiroptical characteristics of the Sm14B6Ge2O34 crystals [4]. The luminescent proper� ties of Gd14B6Ge2O34 doped with Nd 3+, Er3+, and Yb3+ ions have been studied in [5, 6]. It has been shown that the Gd14B6Ge2O34 borogermanates activated with Er 3+ and Yb3+ ions act as upconversion luminophores. The Ln14B6Ge2O34 compounds (Ln = Pr–Gd) have a framework structure composed of a set of LnOn (n = 6–10) coordination polyhedra (CPs). Such a structure prevents the synthesis of homonuclear com� pounds with lanthanum and late lanthanides (from Tb to Lu) since CN 6 is not typical of lanthanum atoms in the structure of rare earth compounds while CN 9 and 10 are not inherent to the late lanthanide atoms [7]. The substitution of two or several lanthanide cations with different coordination properties (provided that they are mutually complementary) for one cation in Ln14B6Ge2O34 enabled the synthesis of mixed�cation borogermanates with different lanthanides [7–10]. There have been synthesized compounds with La, Tb, Dy, Y, Ho, Er, and Lu, which separately do not form compounds of this composition, namely, Ln14B6Ge2O34 where Ln14 = La7Tb7, La7Dy7, La7Ho7, La7Er7, La4Tb10, etc. These compounds are isotypic with Ln14B6Ge2O34, where Ln = Pr–Gd, and contain lan� thanum capable of forming high�coordination poly� hedra (with CN 10 or higher) in mixed oxide com� pounds. Lanthanum has been suggested to stabilize the Ln14B6Ge2O34 structures since the character of melting of borogermanates changes from incongruent for compounds with one rare earth element to congruent for lanthanum�containing mixed�cation Ln14B6Ge2O34 compounds. Structure of the La12GdEuB6Ge2O34 Borogermanate as Probed by NMR and IR Spectroscopy V. A. Krut’ko, V. P. Tarasov, G. A. Bandurkin, and M. G. Komova Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Leninskii pr. 31, Moscow, 119991 Russia Received December 5, 2012 Abstract—The structure of fine crystalline borogermanate La12GdEuB6Ge2O34 has been studied by NMR and IR spectroscopy. It has been demonstrated that this compound is isostructural to the homonuclear Ln14B6Ge2O34 compounds (Ln = Pr–Gd) and crystallizes in space group P31. The rare�earth elements have been distributed over the LnOn polyhedra in La12GdEuB6Ge2O34 by analogy with the known structures. Lan� thanum can occupy positions with CN 7–10, and the symmetry of these LnOn coordination polyhedra is not higher than C2v. In the La12GdEuB6Ge2O34 structure, the LnOn coordination polyhedra are formed by oxygen atoms of oxo groups and anions, some of the oxygen atoms being shared by LnOn polyhedra. The BO3 and GeO4 groups in the structure are also bridging, i.e., are involved in bonding of LnOn polyhedra. One of the B–O bonds in La12EuGd(BO3)6(GeO4)2O8 is elongated as compared with the B–O bond lengths in homo� nuclear compounds Pr14(BO3)6(GeO4)2O8 and Nd14(BO3)6(GeO4)2O8. In the La12GdEuB6Ge2O34 structure, germanium is located in isolated GeO4 tetrahedra with distorted Td symmetry. The local symmetry of lantha� num in fine crystalline La12GdEuB6Ge2O34 have been assessed using 139La NMR (B0 = 7.04 T, room temper� ature). For comparison, binary lanthanum compounds with a simpler structure— LaBO3, La(BO2)3, and La2GeO5—have been used. The spectra of all compounds are rather broad (ν1/2 = 180–240 kHz). The 139La NMR spectra of the LaBO3, La(BO2)3, and La12GdEu(BO3)6(GeO4)2O8 borates show a signal at (1080 ± 40) ppm, which is absent in the spectrum of La2GeO5. The shape of the 139La NMR spectra of La12GdEu(BO3)6(GeO4)2O8 and LaBO3 is characterized by the second�order quadrupole splitting with a downfield shoulder. The similarity of these spectra points to close 139La NMR chemical shifts of La12GdEu(BO3)6(GeO4)2O8 and LaBO3. No quadrupole splitting was observed in the spectra of La(BO2)3 and La2GeO5. DOI: 10.1134/S0036023613090131 PHYSICAL METHODS OF INVESTIGATION RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 58 No. 9 2013 STRUCTURE OF THE La12GdEuB6Ge2O34 BOROGERMANATE 1091 The luminescent properties of La14 – xGdxB6Ge2O34, where х = 2–12, have been studied for the Eu3+�con� taining (5, 15, and 20 at %) compounds [10]. It has been shown that these compounds are efficient multi� center luminophores in the visible region (550–700 nm) both in the fine crystalline and glass state. The La12GdEuB6Ge2O34 compound is a member of the same borogermanate series and is closest in composition to La14B6Ge2O34, which has not been synthesized. The present paper deals with studying the La12GdEuB6Ge2O34 structure by IR spectroscopy and NMR. This work is aimed at getting insight into the La12GdEuB6Ge2O34 structure, namely, into the coor� dination of rare earth elements, symmetry of ВО3 and GeO4 groups, and participation of these groups and oxygen atoms in formation of Ln coordination poly� hedra. It has been suggested to use high�sensitivity solid�state 139La NMR for assessing the local symme� try of lanthanum nuclei in La12GdEuB6Ge2O34. Binary compounds with simpler structures—LaBO3, La(BO2)3, and La2GeO5—have been used as refer� ences. EXPERIMENTAL Fine crystalline borogermanate La12GdEuB6Ge2O34, lanthanum borates LaBO3 and La(BO2)3, and the La2GeO5 orthogermanate were synthesized by the solid�phase reaction of stoichiometric amounts of Ln2O3 (99.99%), GeO2 (99.99%), and H3BO3 (reagent grade). La12GdEuB6Ge2O34 was synthesized as described in [10], and the other compounds were syn� thesized through ceramic route. Samples were placed in platinum crucibles and annealed in a silite furnace. X�ray powder diffraction patterns of fine crystalline samples were recorded at room temperature on a Rigaku powder diffractometer (CuK α radiation). Solid�state 139La NMR (42.378 MHz) spectra were recorded using a common pulse sequence at room temperature on a Bruker MSL�300 spectrometer at B0 = 7.04 T. A solution of La(ClO4)3 was used as the external reference. IR spectra were recorded as KBr pellets on a PE 1600 FTIR spectrophotometer in the range 400–4000 cm–1. RESULTS AND DISCUSSION According to X�ray diffraction data, the synthe� sized borates LaBO3 and La(BO2)3 and lanthanum orthogermanate La2O(GeO4) are single�phase. LaBO3 crystallizes in aragonite modification (JCPDF no. 12� 0762; hereinafter, the codes of the powder diffraction files of the International Center for Diffraction Data (ICDD) database are given) and La(BO2)3 and La2O(GeO4), in monoclinic modification (JCPDF no. 23�1140 and no. 40�1183, respectively). Analysis of reflection positions in the X�ray powder diffraction patterns of La12GdEuB6Ge2O34 and Pr14B6Ge2O34 (JCPDF no. 57�287) proved that they are isostructural with allowance for the shift. The pat� tern of La12GdEuB6Ge2O34 shows no extra reflections in addition to those characteristic of the Ln14B6Ge2O34 compounds with one rare earth element (Ln = Pr– Gd) and lanthanum�containing mixed cation com� pounds La7Dy7B6Ge2O34 (JCPDF no. 58�875) and La7Er7B6Ge2O34 (JCPDF no. 60�443). Hence the synthesized La12GdEuB6Ge2O34 compound is single� phase and crystallizes in trigonal system (space group Р31, Z = 3), like all borogermanates with the structural formula Ln14(BO3)6(GeO4)2O8 [2, 3]. The La12GdEuB6Ge2O34 compound contains Gd and Eu atoms and 12 lanthanum atoms. The attempts to synthesize the homonuclear La14(BO3)6(GeO4)2O8 compound (containing no other lanthanide atoms) have heretofore been unsuccessful. To get insight into the coordination of rare earth elements in the La12GdEuB6Ge2O34 structure, we have scrutinized the structural data on compounds of the same structural series (Table 1). Although the compounds presented in the table are isostructural, they are composed of differ� ent sets of LnOn coordination polyhedra (Table 1). According to X�ray crystallography data on Ln14(BO3)6(GeO4)2O8 [2, 3, 9, 11, 12], the lanthanide atoms in the unit cell occupy positions with different symmetry of the environment. This environment is rep� resented by six�, seven�, eight�, nine�, and ten�vertex polyhedra. The presence of Ln positions with CN = 11, as well as the existence of 15 and 16 coordination poly� hedra in the structure (Table 1), is due to the fact that one of the Ln atoms in the Ln14(BO3)6(GeO4)2O8 for� mula unit is disordered over two positions. In the Nd and Sm homonuclear compounds, the site occupancy factors for positions with CN = 11 and 10 are 0.37 and 0.63, respectively [2]. In the structure of the lanthanum�con� taining borogermanate La4Tb10(BO3)6(GeO4)2O8, two Tb atoms are disordered over two positions with site occupancy factors of 0.54 and 0.46, 0.48 and 0.52, respectively [9]. Disorder of some boron and oxygen atoms has also been observed [2, 9]. Ignoring the pos� sible disorder of Ln atoms in La12GdEuB6Ge2O34, let us try to assess the distribution of the Ln atoms over the coordination polyhedra. Early lanthanides have higher coordination num� bers than the middle and late lanthanides [13]. Hence, the lanthanum atoms in the structure will have a max� imum or near�to�maximum CN, while the Eu and Gd atoms will have a minimum or near�to�minimum CN. Thus, the positions with CN = 6 and 7 will be occupied by the Eu and Gd atoms with relatively small radii. The bulkier lanthanum atoms will occupy part of positions with CN = 7 and all positions with CN = 8, 9, and 10. This allows us to suggest with high probability the distri� bution of Ln positions in the La12GdEuB6Ge2O34 struc� ture (the bottom line in Table 1). As is known [13, 14], the structure of any mixed� anion rare earth compound is composed of only those structural moieties that form simpler compounds of the Ln2O3–B2O3–Ge2O3 ternary system (Fig. 1), 1092 RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 58 No. 9 2013 KRUT’KO et al. including starting oxides and intermediate binary com� pounds. Each simple compound is a “building block” as a definite Ln coordination polyhedron in the structure of a complex compound. The La12GdEuB6Ge2O34 struc� ture is composed of the coordination polyhedra of rare earth oxides, borates, and germanates. Lanthanum, europium, and gadolinium oxides provide polyhedra with CN = 6 and 7, LnBO3 orthoborates are responsi� ble for polyhedra with CN = 8 and 9, Ln2GeO5 orthogermanates give polyhedra with CN = 7 and 8, and Ln(BO2)3 metaborates supply polyhedra with CN = 10 (Table 2). (The data presented in Table 2 were obtained by X�ray crystallography analysis of rare earth oxides, borates, and germanates.) The rare earth coordination polyhedra constituting La12GdEuB6Ge2O34 have the shapes shown in Fig. 2. All polyhedra in the figure are shown in the idealized (i.e., undistorted) form. As shown in Fig. 2, the LaO7, LaO8, and LaO9 polyhedra can exist in two (CPs 13 and 14), seven (CPs 6–12), and four (CPs 2–5) forms, respectively. Table 1. Distribution of rare earth atoms over the coordination polyhedra LnOn in the Ln14(BO3)6(GeO4)2O8 structure Composiiton of the Ln14 cationic part in Ln14(BO3)6(GeO4)2O8 Number of LnOn polyhedra in the structure Number of Ln positions with CN = 6–11 CN 6 CN 7 CN 8 CN 9 CN 10 CN 11 Reference Nd14, Sm14 15* 1 3 4 5 1 1 [2] Gd14 15 1 4 4 3 2 1 [3] Nd4.3Sm4.8Eu4.9 15 2 Eu + Ln** 4 3 Eu + Ln 3 Nd + 2Ln 6 Ln – – [11] Nd3.3Sm3.71Eu3.33Gd3.6 15 1 Eu 4 2 Eu +2 Gd 3 Nd + Gd + Ln 5 Ln 2 Ln – [12] La4Tb10 16* 2 2 Tb 6 6 Tb 4 Ln 3 Ln 1 Ln – [9] La12EuGd 14 1 Eu or Gd 2 Eu, Gd, or La 4 La 5 or 6 La 2 or 1 La – Our data * The appearance of CPs 15 and 16 in the description of the Ln14(BO3)6(GeO4)2O8 structure is due to the disordering of one or two rare earth atoms over two positions.; ** Ln denotes the atoms for which positions in the Ln14(BO3)6(GeO4)2O8 structure have not been determined. Ln2O3 (LnO3)BO3 LnBO3 Ln(BO2)3 B2O3 GeO2 LnBGeO5 Ln4GeO8 Ln14(BO3)6(GeO4)2O8 La2GeO5 � Fig. 1. The Ln2O3–B2O3–GeO2 (Ln= Pr–Gd) phase dia� grams [1]. The compounds containing LnOn structural entities involved in formation of the La12GdEuB6Ge2O34 structure are shown by shaded circles. CN 10 9 8 7 6 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Fig. 2. Polyhedra constituting a polyhedral ensemble in the rare earth borate and borogermanate structure. A circled number denote the number of a polyhedron: (1) spheno� corona, (2) triangular face di� and square face mono� capped trigonal prism, (3) square face tricapped trigonal prism, (4) square face monocapped tetragonal antiprism, (5) sphenocorona with a missing vertex, (6) 1�5�2 polyhe� dron, (7) distorted square face dicapped trigonal prism, (8) cube, (9) dodecahedron, (10) square face dicapped trigo� nal prism, (11) tetragonal antiprism, (12) hexagonal bipyr� amid, (13) square face monocapped trigonal prism, (14) pentagonal bipyramid, and (15) octahedron. RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 58 No. 9 2013 STRUCTURE OF THE La12GdEuB6Ge2O34 BOROGERMANATE 1093 To assess the local symmetry of the rare earth ele� ments in La12GdEuB6Ge2O34, we use the stepwise fragmentation model suggested by Bandurkin [19]. In this model, the coordination polyhedra constituting the Ln14(BO3)6(GeO4)2O8 compounds are fragments of the maximally high�coordination ten�vertex poly� hedron, the [LnO10] sphenocorona, indentified in the structure of Nd(BO2)3 isostructural with lanthanum metaborate La(BO2)3. Since the highest symmetry of the [LnO10] sphenocorona can be (or lower) [17], symmetry of its fragments, even such as a cube (CP 8 in Fig. 2) or octahedron (CP 15), should not be higher than . 139La NMR. The local symmetry of the lanthanum nuclei in La12GdEuB6Ge2O34 was assessed by 139La NMR. Binary lanthanum compounds with simpler structures—LaBO3, La(BO2)3, and LaGeO5—were used as references. In a single solid�state 139La (I = 7/2) NMR experi� ment, only the spectrum of the central transition mI = 1/2 ↔ mI = –1/2 (mI is the nuclear magnetic spin quantum number) can be observed. The satellites resulting from the first�order quadrupole effect cover a range of several hundred kilohertzes because of the relatively large quadrupole moment of the 139La nucleus (0.21 b) and an asymmetric environment of lanthanum nuclei in fine crystalline samples [20]. In the present work, we report the NMR spectra of the central transition for fine crystalline compounds in which lanthanum form either one coordination poly� hedron (CN of La is 9 in LaBO3 and 10 in La(BO2)3) or several coordination polyhedra. In the La2GeO5 structure, the lanthanum atoms occupy two indepen� dent positions with CN = 7 and 8; according to our data, lanthanum in La12GdEuB6Ge2O34 has several positions with CN = 7–10 (Table 1). The 139La NMR spectrum of lanthanum orthoger� manate La2GeO5 (Fig. 3) is a single symmetric line, although the lanthanum in this structure (by analogy with the isostructural Nd2GeO5) has two nonequiva� lent positions with CN = 7 and 8 (Table 2). According 2C v 2C v to X�ray crystallography data on Nd2GeO5 [18], iso� structural with La2GeO5, the [NdO8] and [NdO7] coordination polyhedra in the structure are rather dis� torted (Table 2). It is likely that this distortion is responsible for a rather broad Δν1/2 = 180 kHz) down� field�shifted 139La NMR signal (20989.8 Hz, δ = 495.3 ppm). The spectrum is presumably dominated by 139La nuclei located in a near�symmetric environment rather than by the quadrupole effect. Lanthanum metaborate La(BO2)3 gives rise to an asymmetric, significantly broadened (Δν1/2 = 240 kHz) spectrum displaying several signals (Fig. 4), although its structure contains only one type of lanthanum coordination polyhedron, a ten�vertex polyhedron (Fig. 2, CP 1). According to X�ray crystallography evi� dence [15], lanthanum in this compound has a severely distorted [LaO10] polyhedron (Table 2), and therefore, its spectrum is dominated by the second� order quadrupole effect. Indeed, the Δ value charac� terizing the scatter of La–O distances in [LaO10] in the La(BO2)3 structure is 0.445 Å, which considerably exceeds Δ = 0.23 or 0.33 Å for Nd2GeO5 (isostructural to La2GeO5). Table 2. LnOn rare earth polyhedra (n = 6–10) constituting the La12GdEuB6Ge2O34 structure Compound CN of Ln in LnOn Ln–O distances Δ**, Å References La(BO2)3 10 2.418 [2]*; 2.561 [2]; 2.579 [2]; 2.658 [2]; 2.863 [2] 0.445 [15] Nd(BO2)3 10 2.26 [2]; 2.40 [2]; 2.48 [2]; 2.59 [2]; 2.84 [2] 0.58 [16] LaBO3 9 2.40 [2]; 2.41; 2.58 [2]; 2.65 [2]; 2.74 [2] 0.34 [17], p. 160 Nd2GeO5 8 7 2.35; 2.40; 2.41; 2.46; 2.55; 2.56; 2.59; 2.68 2.33 [2]; 2.42; 2.43; 2.45; 2.56; 2.58 0.33 0.23 [18] La2O3 (A form) Eu2O3, Gd2O3 (B form) 7 6, 7 – – – – [17], p. 21 * The number of the same interatomic distances is given in the brackets; ** Δ is the scatter of the Ln–O distances in the LnOn polyhedra (n = 7–10). –450000 –350000 –250000 –150000 –50000 50000 150000 250000 350000 Hz 49 5. 3 Fig. 3. 139La NMR spectrum of fine crystalline Ln2GeO5 (room temperature, B0 = 7.04 T). 1094 RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 58 No. 9 2013 KRUT’KO et al. Noteworthy is the similarity between the spectra of LaBO3 (Fig. 5a), in which lanthanum has only the [LaO9] polyhedron (Fig. 2, CP 2–5), and the La12GdEuB6Ge2O34 borogermanate (Fig. 5b), which has a structure containing a set of lanthanum CPs with CN = 7–10 (Table 1). Both spectra show two strong peaks with close distances between them: 73710 and 76660 Hz for LaBO3 and La12GdEuB6Ge2O34, respec� tively. Both spectra have a downfield shoulder. If the lanthanum environment in LaBO3 were sym� metric, its spectrum would be a symmetric line, as for La2GeO5 (Fig. 3). However, the line shape for LaBO3 is represented by two strong peaks. Hence, the lantha� num environment is severely distorted, so that its spectrum is dominated by the second�order quadru� pole effect. In the La12GdEuB6Ge2O34 compound, the lantha� num atoms are at the centers of polyhedra with CN = 7–10, i.e., have four different environments. As has been shown, the 139La NMR chemical shift depends of the La coordination number [20]; therefore, the spec� trum can display at least four signals. At the same time, these signals can overlap each other, which can lead to a decrease in the number of signals, as is the case for La12GdEuB6Ge2O34. However, the resemblance of the two spectra (Figs. 5a and 5b) indicates that the LaBO3 and La12GdEuB6Ge2O34 compounds have close chemical shifts. In addition, their spectra display signals whose position and shape are obviously dictated by the sec� ond�order quadrupole effect. This result is quite consistent with the conclusions by Willans et al. [20] who have studied lanthanum complexes by solid�state 139La NMR and have revealed that the NMR line shape considerably depends on both the quadrupole moment and chemi� cal shift anisotropy. Lanthanum compounds in which the La atoms have a single position with CNs from 8 to 12 have been studied. It has been demonstrated that there is a linear correlation between the 139La NMR parameters and La coordination number in these compounds [20]. To this end, the NMR spectra were recorded at applied magnetic fields (B0) of 11.75 and 17.60 T. (With an increase in the applied magnetic field strength, the second�order quadrupole effect becomes weaker, while the chemical shift anisotropy becomes more pronounced.) To obtain a similar correlation for the lanthanum compounds under consideration (including the La12GdEuB6Ge2O34 borogermanate) where lanthanum has several positions, systematic NMR studies of the same compounds at a stronger applied magnetic field, as well as of other compounds at two different magnetic field values, are required. Table 3 lists the experimental chemical shifts mea� sured in this work, as well as some literature data. IR spectroscopy. Analysis of the IR spectra of fine crystalline Ln14O8(BO3)6(GeO4)2 (Ln = Pr, Nd, Sm, Eu, Gd) has been reported in [1, 23]. Figure 6 shows the change in the splitting (Δν) of the νas(B–O) anti� –450000 –350000 –250000 –150000 –50000 50000 150000 250000 350000 Hz450000 11 06 Fig. 4. 139La NMR spectrum of fine crystalline Ln(BO2)3 (room temperature, B0 = 7.04 T). –450000 –350000 –250000 –150000 –50000 50000 150000 250000 350000 Hz450000 –450000 –350000 –250000 –150000 –50000 50000 150000 250000 350000 Hz450000 44 43 4 – 32 22 7 46 38 7 – 27 34 3 (а) (b) Fig. 5. 139La NMR spectra (B0 = 7.04 T) of (a) LnBO3 and (b) La12GdEuB6Ge2O34. RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 58 No. 9 2013 STRUCTURE OF THE La12GdEuB6Ge2O34 BOROGERMANATE 1095 symmetric stretching vibration band of BO3 groups in the range 1200–1300 cm–1 as a function of the lan� thanide atomic number. In going from Nd to Sm in the Ln14O8(BO3)6(GeO4)2 structure, Δν increases, which is caused by the increase in the scatter of B–O dis� tances in the lanthanide series. In the IR spectrum of the Pr14(BO3)6(GeO4)2O8 compound, the splitting of the νas(B–O) band is minimal (Fig. 6), which is evi� dence of the minimal distortion of the BO3 group. If the and curves are extrapolated to the La coor� dinate, both curves will merge into one point. Thus, if the hypothetical La14(BO3)6(GeO4)2O8 compound existed, no splitting of the νas(B–O) band would be observed in its IR spectrum (or it would be insignificant). The lack of splitting is typical of the BO3� group with symmetry For La12EuGd(BO3)6(GeO4)2O8, the Δν of the antisymmetric stretching vibration band of BO3 groups increases, which is caused by the lowering of the symmetry of the BO3 group in going from Pr14(BO3)6(GeO4)2O8 to La12EuGd(BO3)6(GeO4)2O8. (We have completed Fig. 6 with our data obtained from the IR spectrum of the La12GdEuB6Ge2O34 com� pound (Fig. 7).) Such a character of changes is undoubtedly caused by the influence of the Eu and Gd atoms, which manifests itself in the elongation of one of the B–O bonds in La12EuGd(BO3)6(GeO4)2O8 as compared with the B–O bonds of the BO3 groups in the Pr14(BO3)6(GeO4)2O8 and Nd14(BO3)6(GeO4)2O8 homonuclear compounds. The range of νas(Ge–O) vibrations of GeO4 groups shows two bands: the strong band at ~693 cm–1 and the ν I as ν II as 3 .C v 2C v weaker band at ~740 cm–1. The presence of the band at ~693 cm–1 of such a shape corresponds to the struc� ture with isolated distorted GeO4 tetrahedra with dis� torted symmetry Td. Thus, the La12GdEuB6Ge2O34 compound is isos� tructural with the Ln14(BO3)6(GeO4)2O8 boroger� manates and crystallizes in space group P31, Z = 3. According to X�ray crystallography, the unit cell of the Ln14(BO3)6(GeO4)2O8 compounds, the lanthanide atoms occupy positions of different symmetry; i.e., the formula unit, lanthanum atoms are in six�, seven�, eight�, nine�, and ten�vertex polyhedra. By analogy with known structures, the rare earth elements of the La12EuGd(BO3)6(GeO4)2O8 compound have been distributed over the coordination polyhedra (Table 1). It has been accepted that smaller lanthanide atoms (Eu and Gd) occupy positions with CN = 6 and 7, whereas the other CPs in the structure are occupied by the lanthanum atoms. In La12EuGd(BO3)6(GeO4)2O8 the LnOn coordina� tion polyhedra are formed by the oxygen atoms of oxo groups and the oxygen atoms of the anions, some of Table 3. 139La NMR parameters obtained from our experi� mental data, as well as from the literature data Compound CN of La σiso, ppm References La2O3 7 424.8 [21] La2GeO5 7, 8 495 ± 10 Our data LaBO3 9 520 ± 10 '' La(BO2)3 10 1106 ± 10 '' La12GdEuB6Ge2O34 7–10 445 ± 10 '' LaAlO3 12 375 ± 5* [22] * The value was obtained from the experimental data reported in [22]. ν 1 as, cm –1 ν II as, cm –1 1300 1290 1280 1270 1230 1220 1210 1200 La I II Ce Pr Nd Pm Sm Eu Gd Fig. 6. Change in the frequencies of the split antisymmet� ric stretching vibration band ν(B–O) of the BO3 groups in Ln14(BO3)6(GeO4)2O8 (Ln = Pr, Nd, Sm, Eu, Gd) and La12GdEuB6Ge2O34 in the range 1200–1300 cm –1 according to our data and [1, 23]. 50 48 46 44 42 40 38 36 34 400 10001500200030004000 52 54 56 58 60 62 64 66 68.3 500 32 3457.36 3542.68 2922.48 2852.71 1741.06 1652.17 1462.80 1377.77 1269.56 1196.13 1165.21 1026.08 937.19 821.25 771.01 740.09 693.71 612.56 589.37 492.75 446.37 A bs or pt io n , % ν, cm–1 Fig. 7. IR spectrum of fine crystalline La12GdEuB6Ge2O34. 1096 RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 58 No. 9 2013 KRUT’KO et al. the oxygen atoms (bridging) being shared by LnOn polyhedra. The BO3� and GeO4� groups in the struc� ture are also bridging; i.e., link LnOn polyhedra. The 139La NMR (B0 = 7.04 T, room temperature) spectra of fine crystalline La12GdEuB6Ge2O34, La2GeO5, La(BO2)3, and LaBO3, have been recorded. The spectra of all compounds are rather broad (ν1/2 = 180–240 kHz). In the 139La NMR spectra of the LaBO3, La(BO2)3, and La12GdEu(BO3)6(GeO4)2O8 borates, the strongest peak is observed at (1080 ± 40) ppm, which is absent in the spectrum of La2GeO5. The shape of the 139La NMR spectra of La12GdEu(BO3)6(GeO4)2O8 and LaBO3 is character� ized by the second�order quadrupole splitting with a downfield shoulder. No quadrupole splitting is observed in the spectra of La(BO2)3 and La2GeO5. ACKNOWLEDGMENTS We are grateful to E.N. Beresnev and I.S. Ivanova (Institute of General and Inorganic Chemistry, RAS) for their interest and valuable discussions. This study was supported by state funding (State registration no. 01.2.009 55674) and the Russian Foundation for Basic Research (project no. 11�08� 01322�a). REFERENCES 1. B. F. Dzhurinskii and G. V. Lysanova, Russ. J. Inorg. Chem. 43, 1931 (1998). 2. A. B. Ilyukhin and B. F. Dzhurinskii, Zh. Neorg. Khim. 39, 556 (1994). 3. B. F. Dzhurinskii and A. 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