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By crystallization from strongly acidic aqueous solutions barium-tetrahydrogen-decaoxodiperiodate-dihydrate, BaH4I2O10·2H2O has been obtained (S.G. C2/c, No. 15) with the lattice constants a = 12.728(3), b = 7.987(2), c = 9.459(2), and β = 94.07(3). IR and Raman spectra are given and analysed with respect to the internal vibrations of the H 4 I 2 O 10 2 - ion and the hydrogen bond system. According to high temperature Raman spectra and DTA and TG measurements, the compound decomposes via unknown salts with the anion IO 4 - above 230 °C to the corresponding iodate which above 575 °C starts to disproportionate to the periodate Ba5(IO6)2.

By crystallization from strongly acidic aqueous solutions, nickel and zinc tetrahydrogen-decaoxodiperiodate hexahydrate, NiH4I2O10·6H2O and ZnH4I2O10·6H2O, have been obtained and found to be isotypic to CuH4I2O10·6H2O (P21/c, no. 14) with the lattice parameters a = 1070.3, b = 544.0, c = 1187.2 pm, β = 112.6° for the Ni compound and a = 1073.3, b = 545.3, c = 1189.5 pm and β = 112.5° for the Zn compound. The crystal structure of MgH4I2O10·6H2O has been reinvestigated and contrary to former results found to be isotypic to the Cu compound as well. IR and Raman spectra are given and analyzed with respect to the internal vibrations of the H4I2O10 2− ion and the hydrogen bonding system. According to high-temperature Raman spectra and DTA and TG measurements, the compounds decompose in the temperature range up to 300 °C via unknown salts with the anions H4IO6 2− and IO4 − to the corresponding iodates. In the case of the Ni compound the exothermic decomposition to the iodate occurs in two steps.

By solid state reaction of the binary sulfides, a new compound, Mn0.86In0.12Ga2S4, has been synthesized and its crystal structure determined. The compound crystallizes in the space group C2/m (no. 12) with a = 1232.6(2), b = 2636.9(5), c = 642.4(1) pm, β = 100.66(3)°, Z = 14. In the structure empty channels with a cross section of 320× 1082 pm2 running parallel to [001] are present.

Vibrational wavenumbers (IR and Raman) have been calculated for the diperiodate ion H4I2O102− on the basis of a DFT method and assigned to experimental wavenumbers obtained from CuH4I2O10·6H2O. To obtain vibrational wavenumbers in the range comparable to the experiment it was necessary to use the complex [(H4I2O10)7(Cu(H2O)6)6]2−. Smaller complexes lead to much too high wavenumbers for the O-H stretching vibrations and to too small wavenumbers in the range of the internal vibrations of the anion. On the basis of the results of the calculations an assignment of the Raman lines observed for CuH4I2O10· 6H2O to the vibrational modes is given. Copyright © 2008 John Wiley & Sons, Ltd.

Orange–red (CuBr)2P8Se3 was obtained from stoichiometric amounts of CuBr, P and Se by melting and subsequent annealing at 380 °C for 1 month. The crystal structure was determined from single crystal X-ray data. (CuBr)2P8Se3 crystallizes in the orthorhombic system, space group Pbcm (No. 57), with a=8.761(1) A , b=11.957(1) A , c=13.858(1) A , V=1451.8(3) A 3 , and Z=4. This compound consists of neutral P8Se3 cage molecules attached to Cu2Br2 rhombs. Vibrational spectroscopic data are reported for (CuBr)2P8Se3 and for the homologous (CuI)2P8Se3. The wavenumbers of the Cu–Br and Cu–I vibrational modes show an excellent correlation with the corresponding Cu–X bond lengths.

By crystallization from a strongly acidic aqueous solution strontium-tetrahydrogen-hexaoxoperiodate-trihydrate Sr(H 4 IO 6 ) 2 ·3H 2 O has been obtained. In the structure of this compound (S.G. P 1 ,Nr.2, Z =2, a =636.1(1) pm, b =993.9(2) pm, c =1044.5(2) pm, α =114.54(3)°, β =95.61(3)°, and γ =97.11(3)°) two crystallographically different H 4 IO 6 − anions are linked by two and three common oxygen atoms, respectively, to SrO 5 (H 2 O) 4 polyhedra which are of the form of monocapped distorted square antiprisms. The IR- and Raman-spectra of this compound are analyzed with respect to the internal vibrations of the periodate anion and the O–H vibrations of the hydrogen bonds. The dehydration and decomposition of the compound, studied by thermal analysis, high temperature Raman experiments and mass spectrometric analysis of the evolved gases, takes place via an unknown metaperiodate Sr(IO 4 ) 2 , results at 265 °C in the formation of Sr(IO 3 ) 2 in an exothermic reaction and finally Sr 5 (IO 6 ) 2 is formed in a disproportionation reaction.

The present paper deals with the preparation of ZnIn2S4−IIIa layer-type CdIn2S4-xSex (1.75≤ x ≤ 2.75) quaternary chalcogenides and their characterization. X-ray diffraction studies of CdIn2S4-xSex (1.75 ≤ x ≤ 2.75) confirmed that all of these compounds possess the ZnIn2S4−IIIa type of structure. The variation of the composition x shows that both lattice parameters a- and c increase with a growing amount of selenium in the crystals. The calculation of microstructural parameters by an X-ray line profile analysis has shown that the intercalation of lithium in the host CdIn2S2Se2 is accompanied by a decrease in crystallite size while the dislocation density and the rms strain are increasing. The process of intercalation results in no expansion along the c-axis, probably due to no increase in the c-parameter at all in a given intercalation process; which does not have purely geometric origin but also comes from electronic effects. A radial distribution function (RDF) analysis of these quaternary chalcogenides ...

The compounds (NH4)3[Ta(O2)4], K3[Ta(O2)4], Rb3[Ta(O2)4] and Cs3[Ta(O2)4] have been prepared and investigated by X-ray powder methods as well as Raman- and IR-spectroscopy. In the case of Rb3[Ta(O2)4] the structure has been solved from single crystal data. It is shown that all these compounds are isotypic and crystallize in the K3[Cr(O2)4] type (SG I 4 2m , No. 121). The infrared- and Raman spectra (recorded on powdered samples) are discussed with respect to the internal vibrations of the peroxo-group and the dodecahedral [Ta(O2)4]3− ion. Symmetry coordinates for the [Ta(O2)4]3− ion are given from which the vibrational modes of the O–O stretching vibrations of the O22− groups, the Ta–O stretching vibrations and the Ta–O bending vibrations are deduced.

Kupfer-tetrahydrogendecaoxodiperiodat-hexahydrat CuH4I2O10·6H2O wurde durch Kristallisation aus stark saurem Milieu erhalten. In der Struktur der Verbindung ((S.G. P 21/c, Nr.14), Z = 2, a = 1060, 2(2) pm, b = 551, 1(1) pm, c = 1164, 7(2) pm, β = 111, 49(3)°) liegen centrosymmetrische [H4I2O10]2— Anionen in Form kantenverknupfter Oktaeder vor, die uber Wasserstoffbruckenbindungen zwischen den sauren, trans-standigen OH-Gruppen der Anionen zu Schichten verknupft sind. Die Ramanspektren der Verbindung werden im Bereich der inneren Schwingungen des Periodat-Anions analysiert. Die Entwasserung von CuH4I2O10·6H2O erfolgt uber CuH4I2O10·3H2O zu Cu(H2IO5)2, das sich bei 170 °C zu Cu(IO3)2 zersetzt. On Copper-tetrahydrogen-decaoxo-diperiodate-hexahydrate CuH4I2O10·6H2O: Crystal Structure, Vibrational Spectroscopy and Thermal Analysis By crystallization from a strongly acidic aqueous solution copper-tetrahydrogen-decaoxodiperiodate-hexahydrate CuH4I2O10· 6H2O has been obtained. In the structure of this compound (S.G. P 21/c, Nr.14), Z = 2, a = 1060.2(2) pm, b = 551.1(1) pm, c = 1164.7(2) pm, β = 111, 49(3)°) centrosymmetric [H4I2O10]2— anions in the form of two edge sharing octahedra form layers via hydrogen bonds originating from the acidic, trans-configurated OH groups of the anions. Raman spectra are given and analyzed with respect to the internal vibrations of the periodate anion. The dehydration of the compound takes place via CuH4I2O10·3H2O and Cu(H2IO5)2 which decomposes at 170 °C to Cu(IO3)2.

By crystallisation from a strongly acidic aqueous solution calciumtetrahydrogen-hexaoxo-diperiodatetetrahydrate CaH4I2O10 ∙ 4 H2O has been obtained. In the structure of this compound (S.G. C2/c, Nr. 15), Z = 4, a = 1986,6(4), b = 564,2(1), c = 1149,4(20) pm, β = 120,62(3)°) centrosymmetric H4I2O10 2− anions in the form of edge-sharing octahedra form layers via hydrogen bonds originating from the acidic, trans-configurated OH groups of the anions. IR and Raman spectra are analysed with respect to the hydrogen bond system and the internal vibrations of the periodate anion. The dehydration of the compound takes place via Ca(IO4)2 ∙ 5 H2O and Ca(IO4)2 ∙2H2O leading at 143°C to the anhydrous metaperiodate Ca(IO4)2 which decomposes above 248°C to Ca(IO3)2.

For the compounds FeGa 2 S 4 and NiGa 2 S 4 band structure calculations have been performed by the ab initio plane wave pseudo-potential method. The valence charge density distribution points to an ionic type of chemical bonding between the transition metal atoms and the ligand atoms. Two models for the pseudo-potentials are used to calculate the band structures: (a) only s and p electrons and (b) also the d-shells of the transition metal atoms are included in the pseudo-potentials. The differences between these two cases of band structures are discussed. Energy gap formation peculiarities are analysed for both crystals. Zak's elementary energy band concept is demonstrated for the energy spectra of the considered crystals.

The magnetic properties of the new polymorphic FeIn2S2Se2 compound are presented. The system crystallizes in the aFeGa2S4 structure at low temperatures, and undergoes a transition to a MgAl2S4-type structure at T48501C. For this high-temperature phase, low-field magnetization data show a peak at T1=12.5(5) K, below which magnetic irreversibility is observed. High-temperature susceptibility fits indicate the presence of antiferromagnetic interactions with a high degree of frustration. The effective magnetic moment leff=4.54(3) lB agrees with the expected 3d 6 (S=2) configuration for Fe 2+ .

The crystal structures of MnSc2S4 and CdSc2S4 have been determined from single crystal X-ray diffraction data. MnSc2S4 crystallises in a normal spinel structure (space group Fd 3 m) while for CdSc2S4 prepared at 900°C space group F 4 3m is observed. In the structure of CdSc2S4 the tetrahedral 8a site of the spinel structure is split into two different sites one of which exhibits a site occupation factor (sof) of only 0.9. A corresponding amount of Cd is found in an additional octahedral site (sof=0.1). The structure can be looked at as a transition state between the spinel and the sodium chloride structure. CdSc2S4 prepared at 600°C adopts a normal spinel structure (space group Fd 3 m) as has been elucidated from X-ray powder diffraction data by Rietveld analysis. The structure of FeSc2S4 has been determined from X-ray powder data by Rietveld analysis. FeSc2S4 crystallises in a normal spinel structure.

The present paper deals with the microstructural parameters calculated from X-ray diffraction data, electrical and optical investigations, and scanning tunneling microscopy (STM) studies on ZnIn2S4 IIIa layer type CdIn2S4-xSex (1.75 ≤ x ≤ 2.75) quaternary chalcogenides. Microstructural parameters such as dislocation density, root-mean-square strain, stacking fault probability, crystallite size anisotropy, and layer disorder parameters of these compounds have been calculated. The temperature variation of electrical conductivity (25−400 °C) confirmed semiconducting behavior. The band gaps of all these compounds obtained from STM and optical measurements are in the range 1.57−1.77 eV and are comparable to each other irrespective of the technique used.

The system Ga 0.5 V 2 S 4 –Ga 0.5 V 2 Se 4 has been investigated by powder X-ray diffraction methods. A complete series of mixed crystals with a spinel structure with 1:1 ordering on the tetrahedral sites (space group F43m ) is formed. The lattice parameters of the cubic cell of the mixed crystals are linearly dependent on the S:Se ratio. The structures of the two border and one intermediate compound have been solved by the Rietveld method. Fractional parameters and interatomic distances are given for the different compositions. In the structure, the vanadium atoms are shifted from their ideal position to form isolated tetrahedral clusters with relatively short vanadium–vanadium distances of 288–296 pm.

Wir haben die Systeme CuGa5S8/CuIn5S8 und AgGa5S8/AgIn5S8 an abgeschreckten Praparaten mit Rontgenpulver- und Einkristallmethoden untersucht. Im System AgGaxIn5–xS8 wird bis x

The title systems have been investigated by powder X-ray diffraction methods on quenched samples. In the system CuIn 5 S 8-x Se x two new compounds with layered structures have been detected. Samples quenched from 800°C crystallise for 4 ≤ x ≤ 5 hexagonally with a = 395.81 and c = 3208.6 pm (x = 4) while samples quenched from 650°C for 6 ≤ x 5 S 8-x Se x no compound crystallising in a layered structure is obtained. AgIn 5 S 0.8 Se 7.2 crystallises in a Wurtzite like structure with a = 408.4 and c = 667.0 pm. The spinel type phases CuIn 5 S 8-x Se x and AgIn 5 S 8-x Se x are stable up to x

Strontium based complex perovskites are potential candidates for microwave integrated circuit applications. In the present article, we report on Raman scattering studies of cubic and noncubic structures of Sr(B0.5′Nb0.5)O3 [B′=Ga, In, Y, La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, and Yb] based complex-perovskite materials for an improved understanding of structure-property relations. The spectral results are compared to some tantalum analogues of known crystal structure. The present study reveals a higher degree of ordering for the tantalum compounds compared to those of the niobium analogues. Vibrational studies show a correlation between the tolerance factor and symmetry of these materials.

The lattice dynamics of 12 chalcopyrite type compounds CuAlS 2, CuGaS 2, CuInS 2, AgGaS 2, AgGaSe 2, AgInSe 2, ZnSiP 2, ZnGeP 2, CdSiP 2, CdGeP 2, CdSnP 2, and CdGeAs 2, have been investigated based on a 5-parameter model involving two short range stretching force constants, an interaction constant, and the effective dynamic charges of the cations. The calculated force constants, normal co-ordinates and potential energy distributions are compiled and discussed.

Structural changes accompanying thermal transformation in Na-montmorillonite samples up to a temperature of 500°C have been investigated by X-ray line profile analysis. The method of Fourier initial slope and variance analysis of X-ray line profiles have been used to calculate the different microstructural parameters like crystallite size, r.m.s. strain (2>1/2), variation of inter layer spacing (g), fraction of planes affected by such defects (γ), etc. at different stages of the dehydration process. The dehydration mechanism also has been explained based on thermogravimetric analysis (TGA).

Structures and properties of ternary and quaternary sulfides and selenides containing Gallium and /or Indium and transition elements like Sc, V, Ti, Cr, Mn, Fe, Co, Ni, Zn, Cd, and Hg crystallising in layered structures of the ZnIn2S4 type are reviewed. A survey of the different polytypic structures observed in these systems is given along with a compilation of the known compounds, their structures, their range of homogeneity and their lattice parameters. As far as available data on high pressure phase transformations, thermal stability, and band gap energies of these compounds are tabulated, too.

Application of infrared and Raman spectroscopy in solid state chemistry, as far in use at present, is recalled. Interpretation of the observed spectra with respect to structure and bonding of the respective compound is discussed. The main topics are distortion of molecular units, crystal-matrix studies, band broadening and splitting, vibrational coupling phenomena, characteristic frequencies, bond strength, etc.

The lattice dynamics of 12 chalcopyrite type compounds CuAlS 2 , CuGaS 2 , CuInS 2 , AgGaS 2 , AgGaSe 2 , AgInSe 2 , ZnSiP 2 , ZnGeP 2 , CdSiP 2 , CdGeP 2 , CdSnP 2 , and CdGeAs 2 have been investigated based on a rigid ion model. As the anisotropy of the dynamic charges is not very pronounced the calculations were done assuming isotropic charges. The calculated force constants, normal co-ordinates and potential energy distributions are compiled and discussed.

To obtain a sound basis for calculations of the lattice dynamics of chalcopyrite type compounds the literature data for the vibrational frequencies of 14 chalcogenides and 8 pnictides crystallizing in this structure are critically analysed. This is done by introducing frequency ratios of those frequencies which can be derived from zone centre or zone boundary vibrations of the sphalerite lattice presuming similar bonding and normal co-ordinates in all compounds investigated. Based on the reliable spectra vibrational frequencies have been calculated for those compounds which have not yet been investigated or for which the results given in the literature are uncertain.

The lattice dynamics of 12 chalcopyrite type compounds CuAlS 2 , CuGaS 2 ,CuInS 2 , AgGaS 2 , AgGaSe 2 , AgInSe 2 , ZnSiP 2 , ZnGeP 2 , CdSiP 2 , CdGeP 2 , CdSnP 2 , and CdGeAs 2 have been investigated based on 4 different short range force constant models. In all models a set of different solutions can be obtained which can be distinguished by their force constant patterns. For a comparison of the calculated force constants it is therefore necessary to use the data of the comparable minima. From the set of solutions one is selected by physical reasoning and complete results for a model with 8 force constants are given for this minimum for all 12 compounds. The calculated force constants, normal co-ordinates and potential energy distributions are compiled and discussed.

The system (1− x )FeIn 2 S 4 – x FeIn 2 Se 4 has been investigated by X-ray powder methods. The subsolidus phase diagram is constructed in the temperature interval 600–1000°C. The spinel type FeIn 2 S 4 exhibits a phase width up to the composition FeIn 2 S 3 Se and the layered FeIn 2 Se 4 is formed for 1≥ x ≥0.65. A new layered compound is formed for 0.55≥ x ≥0.4 which crystallizes at temperatures below 850°C in an α-FeGa 2 S 4 structure with a =363.6 pm and c =1207.1 pm ( x =0.5) for the hexagonal cell and at higher temperatures in the MgAl 2 S 4 -type with a =393.9 pm and c =3843.2 pm ( x =0.5) for the hexagonal cell. Both structures have been refined by the Rietveld-method. All phase boundaries are nearly independent from temperature.

The crystal structure of Rb3NbO8 (SG I4̅2m, Nr. 121) has been determined on single crystals to a final R = 0.041. The structure is isotypic to the K3CrO8 type. Vibrational spectra of the title compound have been recorded on powdered samples and are discussed with respect to the internal vibrations of the peroxo-group and the dodecahedral NbO8 3- ion. The stretching vibrations of the O2 2- groups are found at 838 (Raman) and 813 cm- 1(IR). A factor group analysis for the K3CrO8 type is given.

The title systems have been investigated by X-ray powder diffraction methods on samples quenched at 1300, 1100 and 900 K. A large temperature dependent area is found in the Ge-rich part of the phase diagram in which the samples form glasses. Two new crystalline compounds of composition 0.42CaS/0.15Ga 2 S 3 /0.4GeS, and 0.4SrS/0.15Ga 2 S 3 /0.45GeS 2 have been found. According to powder diffraction data they both crystallize tetragonally with a = 925.0(1), c = 602.4(1) pm for the Ca compound and a = 940.3(1), c = 618.9(1) pm for the Sr compound.

The layered chalcogenides CoGa1.7Ti0.3S4, CoGa1.7V0.3, CoGa1.6Cr0.4S4, CoGaInS4, MnGa1.5Cr0.5S4, MnGa1.4Sc0.6S4, MnGa0.8In1.2S4, MnIn2S1.6Se2.4 and CdIn2S2.1 Se1.9 have been investigated under high-pressure - high-temperature conditions (p ≤ 60 kbar, T ≤ 1600°C). MnIn2S1.6Se2.4 and CdIn2S2.1,S1.9 show a phase transition to spinel-type compounds (a = 11.022(2) and a = 11.059(1) Å, respectively). MnGa0.8In1.2S4 forms a high-pressure polymorph with an olivine-type structure (a = 12. 856(3), b = 7.500(3), c = 6.128(2)Å. All other compounds decompose under the conditions applied. The results of a Rietveld structure refinement for the two high-pressure spinels are presented

The title systems have been investigated on samples quenched at 700, 800 and 1000 °C. Five new compounds have been obtained and characterized by X-ray powder diffraction: Spinel type MgGa0.2Cr1.8S4, two compounds crystallizing in the olivine structure, MgGaCrS4 and MgGaInS4, and two layered materials, MgGa1.5Cr0.5S4 and MgGa1.5In0.5S4. Lattice para­ meters and phase widths of the compounds are given.

The mixed valence compound Ga1.93V0.97S4 has been prepared and characterized by X-ray powder methods. The compound crystallizes in an α-FeGa2S4 structure. The lattice parameters of the hexagonal cell are a = 363.6 and c = 1207.1 pm. The structure has been refined by Rietveld methods.

Single crystals of MnIn2SxSe4-x with 0.4 ≤ × ≤ 2.4 have been obtained by chemical transport reactions with iodine in the temperature gradient 860 → 800 °C. Ohmic contacts to these crystals could be obtained by treatment of the crystals with hydrogen and/or indium at 400 °C. By this treatment the conductivity of the crystals, which are very poor conductors in the untreated state, are enhanced by at least 6 orders of magnitude. The samples are photoconductors with a photo sensitivity S = (σp−σd)/σd between 1 and 10−4, depending on the treatment of the crystals.

Lattice dynamical calculations for thiogallate-type compounds A Ga 2 X 4 with A = Cd or Hg, and X = S or Se have been performed on the basis of a short-range force constant model. Discrepancies in the experimental frequencies and crystal structures reported for these compounds in the literature by different groups are discussed with respect to the short-range force constants and potential energy distributions resulting from our calculations.

The title system has been investigated by X-ray powder diffraction methods on samples tempered at 1300, 1100 and 900 K. A large temperature-dependent area is found in which the samples form glasses. A new crystalline compound of composition Ba2.7Ga5.4Ge3.6S18 has been found, crystallizing trigonally in space group R3 with a = 9.600 A and c = 8.684 A . The crystal structure of this compound has been determined from single-crystal X-ray diffraction data and refined to R = 0.101 (Rw = 0.13). Ba and S together form a distorted c.c.p. array in which Ga and Ge occupy tetrahedral sites.

The system (1 - x)NiIn2S4 - xNiIn2Se4 has been investigated by X-ray powder methods. The subsolidus phase diagram is constructed in the temperature interval 600 -1000 °C. The spinel type NiIn2S4 exhibits a phase width up to the composition NiIn2S2Se2 at 900 °C and NiIn2S2.8Se1.2 at 600 °C. A new layered compound is formed for 0.7 ≥ x ≥ 0.5 at 600 °C and for x = 0.6 at 900 °C which crystallizes in the MgAl2S4-type with a = 392.4 and c = 3739.6 pm (x = 0.6) for the hexagonal cell.

In the quasibinary system MnIn 2 S 4 -MnIn 2 Se 4 a series of solid solutions of general formula MnIn 2 S x Se 4- x exists for 2.4 ≥ x ≥ 0.2 with the layered MgAl 2 S 4 structure. The samples have been characterized by X-ray powder diffraction and investigated by electrical measurements and diffuse reflectance spectroscopy. The compounds are semiconductors with optical band gaps varying between 1.6 and 1.2 eV depending on x . From Arrhenius plots of the conductivity data at least two different activation energies E a can be obtained: in the temperature interval from 50 to 200°C we find E a = 0.5 eV, independent of the composition of the samples, while in the high temperature region from 200 to 400°C, E a varies with x from 1.35 to 0.56 eV.

The crystal structure of (Mn0.6Ti0.4)(Ga1.6Mn0.4)S4 has been solved from X-ray single crystal data and refined to R = 0.036 (Rw = 0.040). The compound crystallizes in the FeGa2S4 type structure of space group P[unk]m1 [a = 370.7(1), c = 1215.9(2) pm, V = 0.1447 nm3, Z = 1, Dx = 3.598 Mg · m−3].

Optical transmittance and reflectance spectra of MnIn2S2Se2 single crystals are measured in the wavelength range from 0.5 to 30 μm. The interference method is used to determine the refractive index of the compound for wavelengths between 0.8 and 12 μm. From an analysis of the absorption spectrum it follows that the fundamental edge is due to forbidden indirect transitions between parabolic bands for the polarization E ⊥ c with a gap energy of 1.50 eV at room temperature. Optische Transmissions- und Reflexionsspektren von MnIn2S2Se2-Einkristallen wurden im Wellenlangenbereich von 0,5 bis 30 μm gemessen. Mit Hilfe der Interferenzmethode wird der Brechungsindex der Verbindung fur Wellenlangen zwischen 0.8 und 12 μm bestimmt. Aus einer Analyse des Absorptionsspektrums folgt, das die fundamentale Absorptionskante fur die Polarisation E ⊥ c auf verbotene indirekte Ubergange zwischen parabolischen Bandern zuruckzufuhren ist und das die Breite der verbotenen Zone bei Raumtemperatur 1,50 eV betragt.

The title system has been investigated by X-ray powder diffraction methods on samples tempered at 1300, 1100 and 900 K. A large temperature-dependent area is found in which the samples form glasses. A new crystalline compound of composition Ba2.7Ga5.4Ge3.6S18 has been found, crystallizing trigonally in space group R3 with a = 9.600 A and c = 8.684 A . The crystal structure of this compound has been determined from single-crystal X-ray diffraction data and refined to R = 0.101 (Rw = 0.13). Ba and S together form a distorted c.c.p. array in which Ga and Ge occupy tetrahedral sites.