The electrical and structural properties of InyGa1 ? yAs/InxAl1 ? xAs/InP quantum wells with different InAs content

In _x Al_{1 − x} As/In _y Ga_{1 − y} As/In _x Al_{1 − x} As/InP HEMT structures has been investigated with a change in the InAs molar fraction both in the quantum well and the buffer layer. The electrical parameters of the samples are measured at different temperatures. The structural parameters of the layers and the characteristics of the interfaces between them are determined by double-crystal X-ray diffraction. An increase in the Hall mobility and electron concentration, as well as in the structural quality of the samples, is observed alongside an increase in the InAs molar fraction in the quantum well. It is established that high electron mobility is retained at small (to 5%) mismatches between the buffer layer and substrate.     

Neutron diffraction investigation of the evolution of the crystal structure of oxygen-conducting solid solutions (Yb1 ? x Ca x )2Ti2O7 ( x = 0, 0.05, 0.10)

The crystal structure of pyrochlore-like solid solutions (Yb_{1 − x} Ca _x )₂Ti₂O₇ (x = 0, 0.05, 0.10) synthesized by high-temperature annealing of mechanically activated initial oxides at temperatures of 1300–1500°C is studied using neutron diffraction. It is found that the Ca²⁺ cations are located in the ytterbium sublattice, which apparently favors the splitting of one of the oxygen sublattices [O(2) (48f)] of the pyrochlore structure: the decrease in the occupancy of this sublattice is accompanied by the formation of a new sublattice O(3) (8b), whereas the other oxygen sublattice O(1) (8a) remains unchanged. This rearrangement of the anions in the oxygen subsystem due to the incorporation of an alkaline-earth cation explains the high ionic conductance (∼0.2 S/cm at 1000°C) for (Yb_0.9Ca_0.1)₂Ti₂O₇ ∼ 0.2, which is the maximum value observed to date for pyrochlores of the A ₂ B ₂O₇ type, where A = Ln and B is a Subgroup IVA element of the periodic system. The bulk and grain-boundary components of the conductivity of (Yb_0.95Ca_0.05)₂Ti₂O₇ synthesized at temperatures of 1300, 1400, and 1500°C are studied using impedance spectroscopy. It is found that the (Yb_0.95Ca_0.05)₂Ti₂O₇ sample synthesized at 1500°C has the highest total conductivity due to the increased grain-boundary component. The bulk component of the ionic conductivity of (Yb_0.95Ca_0.05)₂Ti₂O₇ is hardly affected by the synthesis temperature and depends mainly on the degree of heterovalent substitution. Original Russian Text ? A.V. Shlyakhtina, A.E. Sokolov, V.A. Ul’yanov, V.A. Trunov, M.V. Boguslavskiĭ, A.V. Levchenko, L.G. Shcherbakova, 2009, published in Kristallografiya, 2009, Vol. 54, No. 1, pp. 31–36.     

Crystal and magnetic structures of the Bi1 ? xSrxFeO3 ? δ and Bi1 ? xAxFe1 ? xMnxO3 (A = Sr, Ca) solid solutions

Bi_{1 − x} Sr _x FeO_{3 − x/2} (I), Bi_{1 − x} Sr _x Fe_{1 − x} MnxO₃ (II), and Bi_{1 − x} Ca _x Fe_{1 − x} Mn _x O₃ (III) solid solutions have been obtained. Their magnetization has been measured by X-ray and neutron diffraction and M?ssbauer spectroscopy. According to the M?ssbauer spectroscopy data, iron ions are in the trivalent state in system I. Near the concentration x ≈ 0.2, rhombohedral distortions (sp. gr. R3c) are transformed into tetragonal (P4/mmm). The symmetry of system II changes at x > 0.2 (R3c → R3c), whereas orthorhombic distortions (R3c → Pbnm) arise in system III at x > 0.2. The magnetic structure is antiferromagnetic (of G type). The samples of systems II and III exhibit weak ferromagnetism at x > 0.2 due to the Dzyaloshinski-Moriya interaction.     

Refinement of the crystal structure of [Rb x (NH4)1? x ]3H(SO4)2( x = 0.11) by single-crystal X-ray and neutron diffraction: I. Phase II at 300 K

The crystal structure of a mixed [Rb_0.11(NH₄)_0.89]₃H(SO₄)₂ crystal was refined using single-crystal X-ray and neutron diffraction at 300 K. The refinement confirmed that the phase II of the crystal belongs to the monoclinic crystal system (space group C2/c). It was shown that, according to the single-crystal neutron diffraction data, ammonium ions can be represented as regular tetrahedra. The analysis of the constructed difference Fourier maps of the electron and nuclear densities of ammonium ions revealed the presence of additional peaks, which can indicate the dynamic disorder of ammonium ions in the phase II. The results obtained con-firmed that the phase transition to the orientational-glass state can occur in the mixed [Rb_0.11(NH₄)_0.89]₃H(SO₄)₂ crystal with a decrease in the temperature. Original Russian Text ? L.S. Smirnov, K. Wozniak, P. Dominiak, A. Loose, I. Natkaniec, M.V. Frontasyeva, E.V. Pomyakushina, A.I. Baranov, V.V. Dolbinina, 2008, published in Kristallografiya, 2008, Vol. 53, No. 3, pp. 450–459.     

Ionic conductivity of congruently melting Ca0.6Sr0.4F2 and Ca1 ? x ? ySryRxF2 + x (R = La, Ce, Pr, Nd) single crystals with fluorite structure

Single crystals of congruently melting compositions of the Ca_0.6Sr_0.4F₂ and Ca_{1 − x − y} Sr _y R _x F_{2 + x} (R = La, Ce, Pr, Nd; x = 0.16–0.21; y = 0.07–0.16) solid solutions with fluorite structure have been grown by the Bridgman-Stockbarger method. Their electrical properties have been investigated in the range from 473 to 823 K, and it is shown that they are ionic conductors. For Ca_0.6Sr_0.4F₂ crystals, the ionic conductivity σ = 2 × 10⁻⁶ S/cm at 673 K, and the ion transport activation energy E _a = 1.1 eV. For Ca_0.77Sr_0.07La_0.16F_2.16, Ca_0.70Sr_0.11Ce_0.19F_2.19, Ca_0.65Sr_0.15Pr_0.20F_2.20, and Ca_0.58Sr_0.21Nd_0.21F_2.21 crystals, the values of σ lie in the range from 9 × 10⁻⁷ to 2 × 10⁻⁶ S/cm at 500 K, and the activation energy E _a is 0.88–0.93 eV. The concentration and mobility of ionic charge carriers in Ca_{1 − x − y} Sr _y R _x F_{2 + x} crystals have been calculated. Original Russian Text ? N.I. Sorokin, D.N. Karimov, E.A. Krivandina, Z.I. Zhmurova, O.N. Komar’kova, 2008, published in Kristallografiya, 2008, Vol. 53, No. 2, pp. 297–303.     

Growth of Pb1 ? xSmxX (X is Se or Te and x = 0.01–0.04) films on porous silicon

Films with the composition Pb_{1 − x} Sm _x X (X is Se or Te and x = 0.01–0.04) have been prepared and their structural properties have been investigated. A method for growing a buffer porous silicon layer is described. It is established that, for all samples (except for an amorphous film on the surface of porous silicon), the real film structure consists of blocks 10–50 μm in size and is independent of the porous layer morphology.     

Defect structure and ionic conductivity of Ca1 ? xScxF2 + x (0.02 ≤ x ≤ 0.15) single crystals

Single crystals of the Ca_{1 − x} Sc _x F_{2 + x} (x = 0.106, 0.132, 0.156) solid solutions (CaF₂ structure type, space group Fm m) are investigated using X-ray diffraction. It is revealed that the crystals under investigation contain vacancies in the 8c positions and interstitial fluorine ions in the 48i positions. The coordination number of Sc³⁺ ions in the structure of the Ca_{1 −x} Sc _x F_{2 + x} solid solutions is equal to eight. The specific features of the concentration dependences of the ionic conductivity and the activation energy of ion transfer for the Ca_{1 − x} Sc _x F_{2 + x} (0.02 ≤ x ≤ 0.15) solid solutions are explained in the framework of the percolation model of conducting “defect regions.” The percolation threshold equal to 3–5 mol % ScF₃ corresponds to the model of [Ca_{14 − n} Sc _n F₆₈] octacubic clusters containing fluorine ions in the 48i positions. The ionic conductivity of the Ca_{1 − x} Sc _x F_{2 + x} solid solutions is analyzed in comparison with the change in this characteristic for the series of Ca_0.8 R _0.2F_2.2 crystals with rare-earth elements. Original Russian Text ? E.A. Sulyanova, V.N. Molchanov, N.I. Sorokin, D.N. Karimov, S.N. Sulyanov, B.P. Sobolev, 2009, published in Kristallografiya, 2009, Vol. 54, No. 4, pp. 612–622.     

Mechanochemical synthesis of nonstoichiometric nanocrystals La1 ? yCayF3 ? y with a tysonite structure and nanoceramic materials from CaF2 and LaF3 crystals

The nonstoichiometric phases La_{1 − y} Ca _y F_{3 − y} (y = 0.15, 0.20) with a tysonite (LaF₃) structure have been prepared for the first time by the mechanochemical synthesis from CaF₂ and LaF₃ crystals. The average size of coherent scattering regions is approximately equal to 10–30 nm. It has been shown that the compositions of the phases prepared by the mechanochemical synthesis are inconsistent with the phase diagram of the CaF₂-LaF₃ system. The “mechanohydrolysis” of the La_{1 − y} Ca _y F_{3 − y} phase has been observed for the first time. Under these conditions, the La_{1 − y} Ca _y F_{3 − y} phase partially transforms into lanthanum calcium oxyfluoride for a milling time of 180 min with intermediate sampling. The La_{1 − y} Ca _y F_{3 − y} nanoceramic materials have been prepared from a powder of the mechanochemical synthesis product by pressing under a pressure of (2–6) × 10⁸ Pa at room temperature. The electrical conductivity of the synthesized materials at a temperature of 200°C is equal to 4.9(6) × 10⁻⁴ S/cm, and the activation energy of electrical conduction is 0.46(2) eV. These data for the nanoceramic materials coincide with those obtained for migration of fluorine vacancies in single-crystal tysonite fluoride materials. Original Russian Text ? B.P. Sobolev, I.A. Sviridov, V.I. Fadeeva, S.N. Sul’yanov, N.I. Sorokin, Z.I. Zhmurova, I.I. Khodos, A.S. Avilov, M.A. Zaporozhets, 2008, published in Kristallografiya, 2008, Vol. 53, No. 5, pp. 919–929.     

Nanostructured crystals of fluorite phases Sr1 ? xRxF2 + x (R = Y, La-Lu) and their ordering: Part III. A study of the refractive indices

The refractive indices n of Sr_{1 − x} R _x F_{2 + x} crystals (R = Y, La-Lu; 0 ≤ x ≤ 0.5) have been measured at wavelengths of 0.436, 0.546, and 0.589 μm. It is established that n increases when there is an increase in the RF₃ content x according to a weakly quadratic law for each R. For the isoconcentration series of Sr_0.9 R _0.1F_2.1 crystals, the change in n in the series of rare earth elements has a pronounced nonlinear character, which reflects the nonmonotonous change in the properties of compounds in the R series. It is shown that the method of molecular refraction additivity can be used to calculate n for Sr_{1 − x} R _x F_{2 + x} crystals. By varying the RF₃ content in them, one can obtain optical media with a gradually varied refractive index n in the range 1.44–1.55, thus filling the gap in the n values between high ones for RF₃ crystals and low ones for crystals of alkaline earth fluorides MF₂. Original Russian Text ? T.M. Glushkova, D.N. Karimov, E.A. Krivandina, Z.I. Zhmurova, B.P. Sobolev, 2009, published in Kristallografiya, 2009, Vol. 54, No. 4, pp. 642–647.     

Nanostructured crystals of fluorite phases Sr1 ? xRxF2 + x (R are rare-earth elements) and their ordering: IV. Study of the optical transmission spectra in the 2–17-μm wavelength range

Transmission spectra of two-component crystals of Sr_1−x R _x F_2+x (R = Y, La-Lu; 0 ≤ x ≤ 0.5) in the 1–17-μm wavelength range were studied. The spectral characteristics of these crystals and of single-component crystals of MF₂ (M = Ca, Sr, or Ba) and RF₃ (R = La-Nd) were compared. The transmission cutoff of Sr_1−x R _x F_2+x crystals is shifted to shorter wavelengths with increasing x. The same tendency is observed with the increasing atomic number R of rare-earth elements for two isoconcentration series of Sr_1−x R _x F_2+x (x ∼ 0.10 and 0.28). This tendency is pronounced at large x. The transmission cutoff of Sr_1−x R _x F_2+x crystals can be varied in the range of from 10.7 to 12.2 μm by changing their qualitative (R) and quantitative (x) composition. Hence, these crystals can be assigned to multicomponent fluoride optical materials with controlled optical characteristics. The Sr_1−x R _x F_2+x crystals, where R = Ce-Sm, were shown to be promising materials for the design of selective optical filters in the 2–10-μm spectral range.     

Nanostructured crystals of Sr1?xRxF2+x fluorite phases and their ordering: 6. Microindentation analysis of crystals

Hardness, crack resistance, brittleness, and effective fracture energy have been studied for crystals of 24 fluorite phases Sr_{1 − x} R _x F_{2 + x} (R are 14 rare earth elements (REEs); 0 < x ≤ 0.5) and SrF2 grown by the Bridgman method from a melt. These characteristics change nonlinearly with an increase in the REE content for Sr_{1 − x} R _x F_{2 + x} (0 < x ≤ 0.5) with R = La, Nd, Sm, Gd, and Lu; it is maximum in the range x < 0.1 for all REEs. The changes in a number of REEs have been traced for an isoconcentration series of Sr_0.90 R _0.10F_2.10 crystals (R = La, Nd, Sm, Gd, Ho, Er-Lu, or Y) and crystals (similar in composition) with R = Tb and Dy. The hardness of Sr_{1 − x} R _x F_{2 + x} crystals is higher by a factor of ∼2–3 than that of SrF2. The effect of decrease in microstresses in SrF₂ crystals is confirmed by the isomorphic introduction of R ³⁺ ions into this crystalline matrix.     

Synthesis and the crystal and molecular structures of (H3L · Cl)[CoCl4] and H2L[CuBr4] (L is 2,4,6-Tri(N,N-dimethylamino)methylphenol)

The complex compounds (H₃ L · Cl)[CoCl₄] (I) and H₂ L[CuBr₄] (II), where L is 2,4,6-tri(N,N-dimethylamino)methylphenol, were isolated in the crystalline state and studied by X-ray diffraction. The organic cations were found to be outer-sphere ligands. All three nitrogen atoms of the tertiary amino groups are protonated. In compound I, the H₃ L ³⁺ cation exists as the cis tautomer. In compound II, the H₂ L ²⁺ dication exists as the trans isomer. In the crystal structure, the dications are arranged in layers via hydrogen bonds.     

The X-Ray Structure and Spectroscopic Characterization of [Zn(H2O)6][{Zn(H2O)}2(ttha)]·4H2O (ttha?=?Triethylenetetraamine-N,N,N′,N″,N?,N?-hexaacetate)

Abstract  

[Zn(H₂O)₆][{Zn(H₂O)}₂(ttha)]·4H₂O (ttha = triethylenetetraamine-N,N,N′,N″,N‴,N‴-hexaacetate) has been structurally characterized (triclinic, P-1, a = 7.4014(4) ?, b = 8.5521(4) ?, c = 15.0309(7) ?, α = 73.582(1)°, β = 85.173(1)°, γ = 69.935(1)°; V = 857.15(7) ?³; Z = 1). The two Zn²⁺ ions bound by the [ttha]⁻⁶ ligand are in distorted octahedral environments. While there exists a center of symmetry in the [{Zn(H₂O)}₂(ttha)]²⁻ anion in the crystalline state, ¹³C{¹H} NMR spectroscopy demonstrates the absence of a center of symmetry in aqueous solution. Furthermore, electronic absorption spectroscopy reveals that [Zn(H₂O)₆][{Zn(H₂O)}₂(ttha)]·4H₂O reacts with vanadyl(IV) acetate in aqueous solution at room temperature to yield [(VO)₂(ttha)]²⁻. This reaction implies that Zn²⁺ can dissociate from [{Zn(H₂O)}₂(ttha)]²⁻ in aqueous solution.     

The Synthesis and Crystal Structure of (4α,8β,13β,16β)-13-Methyl-16,18-diol-17-Norkaurane: A Simultaneous Reduction Product of Isosteviol

Abstract  

(4α,8β,13β,16β)-13-methyl-16,18-diol-17-norkaurane was synthesized by esterification and reduction of isosteviol, respectively. The structure of the title compound was established by spectral analysis and X-ray diffraction studies. The compound crystallizes in the orthorhombic space group P2₁2₁2₁ with unit cell parameters: a = 7.3705 (14) ?, b = 13.508 (3) ?, c = 20.139 (4) ?, V = 2005.1 (7) ?³, Z = 4. The conformation of rings A and B is chair, whereas the conformation of ring C is unsymmetrical distorted chair. The stereochemistry of the A/B and B/C ring junctions is trans, while the five-membered ring D adopts an envelope conformation.     

Crystal Structures of Dipeptides Derived from the β-Amino Acids (1R,2S)-2-Aminocyclopentanecarboxylic Acid and (1S,2R,3S)-2-Amino-3-methylcyclopentanecarboxylic Acid

Abstract  

Crystals of the dimeric β-peptides 13 and 20, derived from (1R,2S)-2-aminocyclopentanecarboxylic acid and (1S,2R,3S)-2-amino-3-methylcyclopentanecarboxylic acid, respectively, were synthesised and studied by X-ray diffraction in order to establish their solid state secondary structural characteristics. Compound 13 crystallises in the monoclinic space group P 2 ₁ with cell parameters of a = 5.2682(1) ?, b = 9.1211(2) ?, c = 22.4467(6) ?, β = 91.3855(9)°, V = 1078.29(4) ?³ and Z = 2. Compound 20 crystallizes in the orthorhombic space group P 2 ₁ 2 ₁ 2 ₁ with cell parameters of a = 5.0968(1) ?, b = 11.5546(2) ?, c = 43.5414(8) ?, V = 2564.22(8) ?³ and Z = 4. In both cases adjacent molecules are linked by a series of N–H···O=C hydrogen bonds to form β-sheet like structures.     

Stereospecific Ligands and Their Complexes. III. Oxalato and Malonato-(ethylenediamine- N , N ′-di- S , S -2-propionato)-Chromate(III) Complexes. Crystal Structure of the Δ-(–)589- s - cis -K[Cr( S , S -eddp)(ox)]?·?0.5H2O

Abstract  The s-cis-[Cr(S,S-eddp)L]⁻ complexes (S,S-eddp = S,S-ethylenediamine-N,N′-di-2-propionate ion; L = oxalate or malonate ions) were prepared. The complexes were purified by ion-exchange chromatography. The geometry of the complexes has been supposed on the basis of the electronic absorption spectra, and the absolute configurations of the isolated s-cis-[Cr(S,S-eddp)L]⁻ complexes have been predicted on the basis of their circular dichroism (CD) spectra and confirmed by X-ray analysis of the crystal structure of the Δ-(–)₅₈₉-s-cis-K[Cr(S,S-eddp)(ox)] 0.5H₂O complex. Index Abstract  The title compound, s-cis-potassium-(ethylenediamine-N,N’-di-S,S-2-propionato) (oxalato)chromate(III) semihydrate was synthesized by passing the corresponding sodium salt through cation exchange-resin Merck I in potassium form and its crystal structure determined. Single crystal X-ray diffraction analysis reveals s-cis geometry of the complex cation. For Part II see Glodjović VV, Trifunović SR (2008) J Serb Chem Soc 73:541.     

X-ray investigation of the crystallization of chalcogenide glasses of the type (As2Se3)1−x:(Tl2Se)x

Amorphous and crystalline states of As₂Se₃, (As₂Se₃)₃ : Tl₂Se and As₂Se₃ : Tl₂Se have been studied using X-ray diffraction techniques. Structural changes arise during the process of annealing in the temperature range between their softening and melting points are reported and their rates investigated. The crystallization temperatures were found to be 105 ± 5 °C, 135 ± 5 °C and 180 ± 5 °C respectively. The unit cell parameters are identified for each of the three resulting crystalline phases, that for As₂Se₃ : Tl₂Se being orthorhombic while the other two are monoclinic.     

Structure and photosensitive homojunctions based on Pb1?xSnxSe epitaxial films

The growth of epitaxial films of Pb_{1 − x} Sn _x Se solid solutions of different chemical compositions (x = 0.02–0.05) on freshly cleaved BaF₂(111) faces and the structure of these films have been investigated. Photosensitive p-n homojunctions have been prepared on their basis. The homojunctions are fabricated in a unified technological cycle without breaking vacuum based on n- and p-type films of high structural quality (W _1/2 = 90–100″) that were grown using an additional selenium vapor source. The photosensitivity peak is found to shift to longer wavelengths with an increase in the Sn content in the films grown; this effect is explained by narrowing the band gap with a change in the composition.