Substitution Of Ge For Si Research Articles

A family of langasite: growth and structure

A Czochralski-grown langasite family crystals of La 3 Ga 5 SiO 14 – La 3 Ga 4 ( GaSi ) O 14 , La 3 Ga 4 [ Ga ( Si , Ge ) ] O 14 , La 3 Ga 5.5 Ta 0.5 O 14 , La 3 Ga 5.5 Nb 0.5 O 14 , and Ca 3 NbGa 3 Si 2 O 14 were studied by X-ray diffraction. Our results show that the composition of crystals may vary both axially and radially. The two solid solutions isostructural with langasite, which were identified in La 3 Ga 4 ( GaSi ) O 14 crystal studied differ in the Ga and Si occupancies on the (Ga,Si) site: Ga > Si in one solid solution, and Si > Ga in the other. The effects of the melt composition (the Ga:Si ratio, the partial substitution of Ge for Si and of M=Nb, Ta for Ga), the atmosphere (the growth in Ar or Ar+O 2 ) and heat-treatment conditions (in vacuum, in air or in oxygen) on the color (orange or colorless), the crystal composition, the structural perfection and on the polarity reversal of structure were found and explained.

Magnetic characterization of intermetallicPrMn2Ge2−xSix compounds by AC susceptibility, x-ray diffraction and differential scanning calorimetrystudies

X-ray powder diffraction, AC susceptibility and differential scanningcalorimetry (DSC) studies were performed on the polycrystallinePrMn2Ge2−xSix () compounds. All compounds investigated crystallize in the body-centred tetragonalThCr2Si2-type structure withthe space group I4/mmm. Substitution of Si for Ge leads to a linear decrease of the lattice constants and the unitcell volume. The lattice constants and the unit cell volume obey Vegard’s law.Samples in this alloy system exhibit a crossover from ferromagnetic ordering forPrMn2Ge2 to antiferromagneticordering for PrMn2Si2 as a functionof Si concentration x. At low temperatures, the Pr sublattice also orders for . The samples with are ferromagnetic and have spin reorientation temperatures just below the Curie temperature. In thecases of x = 1.2 and 1.3, re-entrant ferromagnetism is observed. The samples with are antiferromagnetic below the Néel temperatureTN(Mn). By comparing our results to earlier neutron diffraction and Mössbauer studies, thex–T magnetic phase diagram has been constructed.

The crystallographic phases and magnetic properties of Fe 2MnSi 1− xGe x

Fe 2MnSi 1− x Ge x ( x=0, 0.2, 0.4, 0.5, 0.6, 0.8, 1) compounds were prepared by a mechanically activated solid-state diffusion method. Both X-ray diffraction and differential scanning calorimetry evidenced the presence of an amorphous phase after 10 h of milling. The X-ray data reveal that in the high-temperature annealing the single D0 3-type phase can be retained up to 50% substitution of Ge for Si in Fe 2MnSi. A metastable D0 3 phase is obtained after crystallization of the as-milled amorphous compounds with x>0.5. High-temperature annealing transforms the low-temperature D0 3 phase into a single D0 19 phase ( x=1) or a mixture of D0 3 and D0 19 phase ( x=0.6 and 0.8). Low-field thermomagnetic measurements show a moderately sharp ferromagnetic-paramagnetic transition, which becomes enormously broad in higher magnetic fields. The Curie temperature is significantly enhanced when going from the D0 3 phase to the D0 19 phase. Neither a magnetic-field-induced transition nor a reversible structural transition is observed throughout this compound series. The magnetocaloric effect associated with the magnetic transition is small.

Electronic structure of β-Fe(Si 2− xGe x) ternaries

The geometrical and electronic structures of β-Fe(Si 2− x Ge x ) ternaries were investigated using first principles pseudopotential calculations based on generalized gradient approximation density function theory. These compounds are indirect-bandgap semiconductors with theoretical energy band gaps ranging from 0.51 to 0.79 eV . Substitution of Ge for Si in β-FeSi 2 leads to a reduction of the gap width and an increase in the volume of the unit cell with almost the same shape of the last valence band and the first conduction band but for the Y point. The analyses of bond overlap population and Mulliken population revealed that the covalency between Fe and Si atoms are strengthened by increasing Ge content and that the electron transfer occurs from Ge to Fe or Si.

Thermoelectric properties of Ru- or Ge-doped β-FeSi 2 films prepared by electron beam deposition

Direct preparation of non-doped, and Ru- or Ge-doped β-FeSi 2 films from the gaseous phase have been studied using a ternary simultaneous electron beam deposition apparatus so as to clarify the effect of these doping elements on the thermoelectric properties and electric conductivity of β-FeSi 2. Substitution of Ru for Fe in β-FeSi 2 up to approximately 7 at.% of Ru seemed to be possible from the X-ray diffraction pattern. On the other hand, substitution of Ge for Si as small as 1.4 at.% Ge brought about the segregation. This feature was also clearly observed in scanning electron micrographs. Therefore, Ru could be successfully doped in β-FeSi 2, however, Ge could not. Though the Seebeck coefficient of β-FeSi 2 has been slightly decreased by doping of Ru, its electric conductivity has been increased by one order of magnitude.

Field-induced valence transition of EuPd 2Si 2 and related compounds

High-field magnetization has been examined for valence fluctuating Eu-based compounds. Eu(Pd 1 − x Pt x ) 2Si 2 (0 ⩽ x ⩽ 0.15) exhibits a universal relation between valence transition field H v and temperature, which suggests a single energy scale associated with the valence transition. Substitution of Ge for Si in EuPd 2Si 2 lowers H v more strongly. However, this valence transition is smeared due to a concentration fluctuation.

Phase relationships and crystallography in the pseudobinary system Gd 5Si 4Gd 5Ge 4

A study of phase relationships and crystallography in the pseudobinary system Gd 5(Si x Ge 1− x ) 4 revealed: (1) that both terminal binary compounds Gd 5Si 4 and Gd 5Ge 4 crystallize in the Sm 5Ge 4-type orthorhombic structure, and (2) the appearance of an intermediate (ternary) phase with a monoclinic crystal structure which is similar to both Gd 5Si 4 and Gd 5Ge 4. The formation of the monoclinic phase at 0.24≤ x≤0.5 [between Gd 5(Si 0.96Ge 3.03)≅Gd 5(Si 1Ge 3) and Gd 5(Si 2Ge 2)] is probably due to the large difference in bonding characteristics of Si and Ge in the Gd 5Si 4-Gd 5Ge 4 pseudobinary system which limits the ability of the mutual substitution of Si for Ge and vice versa without a change of the crystal structure. For the composition Gd 5(Si 2Ge 2) the lattice parameters of the monoclinic structure (space group P112 1/a) are a=7.580865), b=14.802(1), c=7.7799(5) A ̊ , γ=93.190(4)°. A distinct difference in the magnetic behaviors of the alloys from three different phase regions in this system follows the distinct difference in the crystal structures observed for the alloys from the three phase regions.

Investigation of magnetic properties of NdMn2(Ge1−xSix)2

The structure and magnetic properties of NdMn2(Ge1−xSix)2 compounds (x=0–1.0) have been investigated. All the compounds crystallize in ThCr2Si2-type structure. Substitution of Si for Ge leads to a linear decrease of the lattice constants and the unit-cell volume. With increasing temperature, for the compounds with x⩽0.5, both spin reorientation and ferromagnetic– paramagnetic transition are observed as well at Tr and TC(Mn), respectively. The spin reorientation temperature Tr and the Curie temperature TC(Mn) decrease linearly with Si content from 215 and 339 K at x=0–40 and 306 K at x=0.5, respectively. In compounds with 0.3<x⩽0.5 the ferromagnetic component and the antiferromagnetic component may coexit. For compounds with x⩾0.6 the ferromagnetic–antiferromagnetic and the antiferromagnetic–paramagnetic transition are observed and the Curie temperature TC(Nd) decreases monotonically while the Néel temperature TN(Mn) increases monotonically with Si content. A tendentious spin phase diagram has been obtained. The saturation magnetization at 1.5 K decreases first, goes through a minimum at x=0.5, and then increases again.

Local-symmetry effect on light emissivity from SiGe quantum wells

Photoluminescence (PL) intensity was measured from SiGe quantum wells, which were grown at different molecular beam epitaxy (MBE) temperatures (300∼800 °C) followed by subsequent annealing (900 °C). Results indicated that luminescence intensity was very low at a specific MBE temperature range (400∼650 °C), in which clustering of Ge atoms is dominant in crystal growth. Theoretical calculation shows that Ge substitution for Si causes s-like symmetry in the conduction band top, but pairing of Ge atoms changes the local symmetry of Si–Ge bonds which destroys the s-like symmetry. This result well explains the fact that intense PL is obtained only in the growth condition where Ge atoms are dispersed in Si. Additional PL experiments under [110] uniaxial stress indicated that PL intensity was very sensitive to the applied stress field. This confirms that local symmetry of Si–Ge bonds plays an important role in the light emissivity from SiGe quantum wells. © 1997 Elsevier Science S.A.

Structure and magnetic properties of SmMn2(Ge1−xSix)2

The structure and magnetic properties of SmMn2(Ge1−xSix)2 compounds (x=0–1.0) have been investigated. All the compounds crystallize in ThCr2Si2-type structure. Substitution of Si for Ge leads to a linear decrease of the lattice constants and the unit-cell volume. In all compounds a transition from the ferromagnetic to the antiferromagnetic state is observed at a lower temperature T1, which first decreases, goes through a minimum at x=0.4–0.6, and then increases again with Si concentration. As temperature increases, for the compounds with x<0.3 both a antiferromagnetic–ferromagnetic transition and the ferromagnetic–paramagnetic transitions are observed as well at T2 and at the Curie temperature Tc, respectively. With increasing Si content the Tc decreases, whereas T2 increases from 140 K for x=0 to 215 K for x=0.2. For compounds with x≥0.3 the antiferromagnetic–paramagnetic transition was observed with increasing temperature and the Néel temperature increases with increasing Si content. The saturation magnetization at 1.5 K decreases first, goes through a minimum at x=0.6, and then increases again with increasing Si content. At room temperature, the saturation magnetization decreases monotonically from 3.27 μB/f.u. for x=0 to nearly zero for x=0.3.

Investigation of the transition from ferromagnetic to antiferromagnetic order in the CeMn2(GexSi1−x)2 system

Samples in the alloy system CeMn2(GexSi1−x)2 exhibit a crossover from antiferromagnetic ordering for CeMn2Si2 to ferromagnetic ordering for CeMn2Ge2 as a function of concentration. The low temperature behavior of samples in the range 0.0≤x≤1.0 using the techniques of dc magnetization, ac susceptibility, and thermal expansion were studied. From this we have determined the magnetic phase diagram which shows four concentration regions of qualitatively different low temperature behavior. Increasing substitution of Ge for Si in CeMn2Si2 shows a depression of TN and the gradual development of ferromagnetic order at T≊305 K. For 0.42≤x≤0.60 there is a clear ferromagnetic transition at T≊305 K followed by a sharp drop in magnetization at lower temperatures. For all samples ac susceptibility shows a sharp peak near 300 K indicating the onset of magnetic order at that temperature. Neither ac susceptibility nor thermal expansion measurements show clear anomalies at TN below Tc. There is evidence of mixed magnetic behavior for these samples.

Atomic origins of surface core levels on Si(111)-(7 x 7) studied by site-dependent Ge substitution.

Synchrotron-radiation photoemission has been used to examine the site-dependent Ge substitution for Si on Si(111)-(7{times}7) surfaces. The surface components of the Si 2{ital p} and Ge 3{ital d} core levels are analyzed as a function of Ge coverage and annealing temperature. The results show the lower-binding-energy component in the Si 2{ital p} core to be derived from adatom emission. Implications in regard to adatom--rest-atom charge transfer are discussed.

Raman spectroscopic investigation of the structure of silicate glasses. IV. Alkali–silico–germanate glasses

Raman spectra were measured on bulk Na2O⋅(2−x)SiO2⋅xGeO2 glasses, where x varied from 0 to 2, and on Na2O⋅SiO2⋅xGeO2 glasses, where x varied from 0 to 1. Raman spectra were measured also on Na2O⋅(1−x)SiO2⋅xGeO2 glasses crystallized at 650 °C, mostly a solid solution with the β-Na2Si2O5 structure at x between 0 and 1.5. The spectra of the glasses as well as the crystalline solid solutions can be interpreted in terms of random substitution of Ge for Si. The symmetric stretching vibrations of the nonbridging bonds Si–O−, Ge–O−, −O–Si–O−, and −O–Ge–O− are localized and exhibit two-mode behavior. On the other hand, the symmetric stretchings of the bridging bonds Si–O–Si, Ge–O–Ge, and Si–O–Ge are coupled together to give only one Raman band as the Si/Ge ratio is varied.

A Raman study of the ring defects in GeO2-SiO2 glasses

The Raman spectra of GeO2-SiO2 glasses have been studied using samples of different GeO2 contents, which are found to have the general features of the spectrum of vitreous SiO2(v-SiO2). It is demonstrated that substitution of Ge for Si in the v-SiO2 network prevents the formation of ring structures. A new vibrational band located at 570-700 cm-1 was found in the spectra of high-GeO2-content samples (above 5.0 at.%GeO2). It is also shown that the W3 vibration of the v-SiO2 network is reduced in highly GeO2 doped GeO2-SiO2 glasses.

Thermochemistry of the Amorphous System SiO2-GeO2: Comparision of Flame Hydrolysis Materials to High Temperature Fused Glasses

AbstractEnergetics and structure of germania doped silica prepared from high temperature melts and by flame hydrolysis were investigated using transposed temperature drop and solution calorimetry. Heat contents (H973-H298) are similar for first and second drops for the flame hydrolysis preforms reflecting the effect of temperature of deposition (1073 K) in creating a relaxed structure which is not easily rearranged. Heats of solution, at 973 K in molten lead borate, of fused glasses along the SiO2-GeO2 binary show ideal heats of mixing between the endmembers. This'indicates that fused glasses behave in a simple manner, with ideal substitution of Ge for Si in the tetrahedral framework. Flame hydrolysis samples (preform and annealed glasses) also show ideal heats of mixing. This suggests that the flame hydrolysis materials are energetically very similar to their bulk-melted counterparts. This insensitivity of energetics to preparation conditions is consistent with our previous studies on silica prepared at various temperatures [1].

Electrical Conductivity of the System Na<sub>1+3<i>x</i></sub>Zr<sub>2</sub>(P<sub>1-<i>x</i></sub>Ge<sub><i>x</i></sub>O<sub>4</sub>)<sub>3</sub>

The effect of substitution of Ge for Si in the system Na1+3xZr2(P1-xSixO4)3 on the phase and electrical conductivity was investigated. The compositions x=0.17 and 1.0 gave rhombohedral structure below 1100°C, with the skifts of X-ray diffraction lines to lower diffraction angles as compared with NaZr2P3O12 and Na4Zr2Si3O12, respectively. However, a difference in the diffraction patterns of Na3Zr2PGe2O12 and NASICON (Na3Zr2PSi2O12) was recognized. Arrhenius plots of σT for Na3Zr2PGe2O12 sintered at 1000°-1090°C broke in two straight lines indicating activation energies of 8.3-9.2 and 13.4-14.2koal/mol in high (500°-320°C) and low (320°-230°C) temperature regions respectively. The system Na1+3xZr2(P1-xGexO4)3 showed lower electrical conductivity and higher activation energy than the system Na1+3xZr2(P1-xSixO4)3.

Germanium‐Modified Cordierite Ceramics with Low Thermal Expansion

Magnesium cordierite ceramics modified by ionic substitution of Ge for Si were synthesized, fabricated, and characterized. The effect of the amount of ionic substitution on thermal expansion was investigated. Ionic substitution of ∼20% germanium for silicon reduces thermal expansion to zero at room temperature. A processing schedule to achieve nearly theoretical densities in germanium cordierite specimens was developed, and the microstructures of sintered samples were examined by scanning electron microscopy. Other thermal properties such as specific heat and thermal conductivity were also measured for the germanium‐modified cordierites.

A far-infrared investigation of cathodochromic sodalite

Sodalite absorption bands in the 50–350 cm −1 region were studied using Fourier transform spectroscopy. Vibrational modes were identified using chemical substitution, H 2 and vacuum-annealing, and coloration studies. A bromosodalite band at 294 cm −1 was assigned to an Si—O—Al mode based on Ge substitutions for Si. A bromosodalite band at 200 cm −1 is assigned to an Na—cage vibration based on previous H 2annealing studies and chemical substitution of Ge for Si, Ga for Al and Cl and I for Br. A 107 cm −1 band is the Na—Br related vibration, and substitution of 100% Cl for Br moved this band to 111 cm −1, H 2 and vacuum-annealing studies show a correlation between the amount of Br removed from bromosodalite during annealing and the area of the 107 cm −1 band. A band at 68 cm −1 is the Na—Br mode based on 100% substitution of Cl for Br. Coloration studies in the 50–150 cm −1 region show no changes attributable to F-center formation.

Inhomogeneous transition from the ferromagnetic state to the antiferromagnetic state in Mn5(Ge1-xSix)3alloys: an NMR study

From a thorough NMR study of the 55Mn hyperfine field distribution in Mn5(Ge1-xSix)3 alloys the authors shown that the transition from the ferromagnetic Mn5Ge3 to the antiferromagnetic Mn5Si3 occurs inhomogeneously in three steps. At the early stage of the substitution of Si for Ge (x or approximately=0.8. Then for 0.5 or=0.8 bulk antiferromagnetism occurs but a metamagnetic transition is still observed for Hext>30 kOe.

Effects of Substitution of Germanium for Silicon in Imogolite

AbstractImogolites in which a part or all of Si was substituted with Ge were synthesized from solutions containing aluminum ion and silicic and/or germanic acid. The products were similar to natural imogolite in their tubular morphology, electron diffraction patterns, and differential thermal analysis curves. However, the external diameter of the tube increased with increasing Ge substitution up to about 33 Å, and associated changes appeared in the X-ray powder diffraction patterns, suggesting that the substitution of Ge for Si caused a decrease in the curvature of the gibbsite sheet with which SiO- or GeO-tetrahedra are associated. Infrared absorption bands at 995 and 930 cm−1 in imogolite disappeared and new bands appeared at 910 and 810 cm−1 on substitution of Ge for Si, whereas those in the region between 700 and 300 cm−1 remained unchanged or changed little. The former two bands were assigned to Si-O vibrations and the latter bands to Al-O vibrations.