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.     

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.     

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.     

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.     

Study of the relationship between the crystal structure of nanolayers and electrical properties in Al x Ga1? x As/In y Ga1? y As pseudobinary heterostructures by double-crystal X-ray diffraction

The structural parameters of individual layers of samples of a Al _x Ga_1−x As/In _y Ga_1−y As/GaAs pseudomorphic heterostructure have been determined by double-crystal X-ray diffraction. A relationship of the technological parameters of fabrication of heterostructures with their structural and electrical properties is established. The increase in the mobility of the 2D electron gas in the samples under study, caused by the increase in the growth temperature of the Al _x Ga_1−x As spacer layer and the decrease in the time of silicon δ doping from the two sides of the quantum well, correlates well with the degree of the sample structural quality. Original Russian Text ? R.M. Imamov, I.A. Subbotin, G.B. Galiev, 2008, published in Kristallografiya, 2008, Vol. 53, No. 2, pp. 210–213.     

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.     

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.     

Investigation of Trimetallic Ligand-Pillared Oxyfluorides: Ag2Cu(pzc)2MOxF6?x (M?=?Mo, Nb, and W)

Abstract  

Three new ligand-pillared hybrid solids, Ag₂Cu(pzc)₂MO _x F_6−x (I, M = Mo, x = 2; II, M = W, x = 2; III, M = Nb, x = 1) (pzc = pyrazine-2-carboxylate) were synthesized via hydrothermal reactions at 150 °C, and their structures were determined by single-crystal X-ray diffraction (P2₁/n (No. 14), Z = 2; a = 7.2302(1), 7.2124(2), 7.2715(2) ?; b = 7.9460(1), 7.9270(2), 7.98436(3) ?; c = 13.9173(2), 13.8959(4), 13.8226(5) ?, for I, II, and III, respectively). All three are isostructural and contain unusual trimetallic (Ag₂CuMO _x F_6−x )²⁺ layers that consist of [Ag₂O₂F₂] _n and [CuMO _x F_6−x ] _n chains that alternate within the layers. Each structure also contains [MO _x F_6−x ]²⁻ octahedra with fully disordered O/F positions and with an inversion center on the M ⁿ⁺ sites, i.e., Mo⁶⁺, W⁶⁺ and Nb⁵⁺. Magnetic susceptibility measurements can be fitted to the Curie–Weiss law with a Curie constant consistent with a single non-interacting Cu(II) (S = ?) site per formula unit. Thermogravimetric analyses indicate that these hybrid compounds are stable up to ~280 °C, with each exhibiting a single weight-loss step beginning at ~300 °C that corresponds to the loss of all pyrazine-2-carboxylate ligands and additional O/F atoms via oxidation of the ligand during its removal. UV–Vis diffuse reflectance measurements show that each exhibits an optical bandgap size of ~2.8 eV, and which electronic-structure calculations show arise from excitations between the Cu(II)-based valence orbitals and the M^5+/6+-based conduction band orbitals.     

Synthesis and Crystal Structure of a New Mixed Alkali Oxalate A1?x(NH4)x(H2C2O4)(HC2O4)(H2O)2 with A?=?K, Rb

Abstract  

In the present study we report the synthesis, crystal structure, spectroscopic and thermal analysis of mixed alkali A_1−x (NH₄) _x (H₂C₂O₄)(HC₂O₄)(H₂O)₂ with A = K, Rb. Single crystal refinements showed that the two compounds Rb_0.86(NH₄)_0.14(H₂C₂O₄)(HC₂O₄)(H₂O)₂ and K_0.53(NH₄)_0.47(H₂C₂O₄)(HC₂O₄)(H₂O)₂ adopt P − 1 space group. The nine fold coordination cationic sites are randomly occupied by Rb⁺/NH₄ ⁺ and K⁺/NH₄ ⁺ respectively for rubidium and potassium compounds. The structure consists of a three-dimensional network formed by the succession along (c) axis of corrugated sheets formed by alkali polyhedra layers in the packing of four-membered rings [A₄(HC₂O₄)₂(H₂C₂O₄)₂] linked and bridged by oxalate groups that behave as bi and tetradentate ligand under three different coordination modes. The stability of the crystal lattice is ensured by interesting hydrogen bonding contacts: N–H···O and O–H···O. The thermal behavior under air reveals two anomalies at 368 and 402 K attributed respectively to a structural phase transition probably due to the reorientation of ammonium tetrahedral and to the release of crystalline water. IR spectroscopy further confirms that this material loses crystallization water gradually in the temperature range 400–460 K.     

Crystal Structure of [Me2NCH(O)]2Mg[(μ-OPri)2Al(OPri)2]2

Abstract  

Magnesium aluminum isopropoxide coordinates two solvent molecules when crystallized from dimethylformamide yielding the title compound, [Me₂NCH(O)]₂Mg[(μ-OPr ⁱ )₂Al(OPr ⁱ )₂]₂, a potential CVD and sol–gel precursor of spinel, MgAl₂O₄: monoclinic, P2₁/c, a = 18.876(3) ?, b = 12.5225(19) ?, c = 18.578(3) ?, β = 108.333(2)°, V = 4168.6(11) ?³, Z = 4, Mg[(μ-OPr ⁱ )₂Al(OPr ⁱ )₂]₂.     

具有大磁各向异性和高居里温度的二维笼目磁性材料Fe3As

最近，对二维铁磁材料的研究已成为自旋电子器件领域的热点。本文通过自旋极化密度泛函理论计算，设计出一种新型的二维材料Fe₃As,其居里温度(T_c)为300 K,可达到室温。预测的二维Fe₃As具有很强的面内Fe—Fe耦合，其大的磁各向异性能量(MAE)大约为366.7μeV,有助于材料维持长程铁磁序。这种二维Fe₃As的能带同时具有平带和狄拉克点的特征。值得注意的是，平带的位置与磁耦合的强度正相关。此外，在双轴应变的作用下，随着平带和费米面之间的距离不断减小，T_c也在逐渐升高。因此，Fe₃As单层有望成为二维室温自旋电子学器件的一种有前途的候选材料。     

锂离子电池正极材料Li(Ni1/3Co1/3Mn1/3)O2电极过程动力学研究

采用共沉淀法制备锂离子电池正极材料Li(Ni1/3Co1/3Mn1/3)O2,通过循环伏安法和电化学交流阻抗分析,探讨了锂离子在LiNi1/3Co1/3Mn1/3O2中嵌入和脱出的机制。循环伏安测试结果表明,LiNi1/3Co1/3Mn1/3O2材料结构中不存在John-teller效应,从而抑制了电极材料和电解液之间的副反应。电化学交流阻抗测试结果表明,随着电压的升高Rct(电荷转移电阻)值逐渐减小,而随着循环次数的增加Rct值逐渐增大。     

(Na1/2Bi1/2)TiO3-(Na1/2Bi1/2)(Zn1/3Nb2/3)O3无铅压电陶瓷的介电特性研究

采用固相合成法制备了(1-x)(Na1/2Bi1/2)TiO3-x(Na1/2Bi1/2)(Zn/23Nb2/3)O3(简写为(1-x)NBT-xNBZN)无铅压电陶瓷.研究了该体系陶瓷晶体结构、弥散相变特征与介电弛豫行为.X射线衍射分析表明,所研究的组成均能够形成纯钙钛矿(ABO3)型固溶体.当x≥0.5%摩尔分数时,该体系陶瓷具有三方、四方共存的晶体结构.材料的介电常数-温度曲线显示陶瓷具有两个介电反常峰Tf和Tm.修正的居里-外斯公式较好的描述了陶瓷弥散相变特征,弥散指数随x的增加而增加.x≤0.5%摩尔分数的陶瓷仅在低温介电反常峰Tf附近表现出明显的频率依赖性,随x的增加,陶瓷材料在室温和低温介电反常峰Tf之间都表现出明显的频率依赖性.根据有序-无序转变和宏畴.微畴转变理论探讨了该体系陶瓷介电弛豫特性的机理.     

乙醇共沉淀法制备纳米Li[Ni1/3CO1/3Mn1/3]O2

采用乙醇共沉淀法制备纳米Li[ Ni1/3 Co1/3 Mn1/3] O2材料.采用XRD和SEM对合成材料进行了表征.结果表明:合成材料的粒径为纳米级,平均粒径可达60 nm,此种方法合成材料具有较好的层状结构和较低的阳离子混排程度.在2.8 ～4.3 V(vs Li/Li+)条件下进行充放电测试,结果表明材料具有较好的电化学性能,尤其在高倍率下(10 C),材料的放电性能可以达到大功率用电设备的要求.     

焙烧温度对Li[Mn1/3Ni1/3CO1/3]O2结构及电化学性能影响

采用碳酸盐共沉淀法制备了Li[Mn1/3Ni1/3Co1/3]O2,研究了前驱体的焙烧温度对材料结构和电化学性能的影响.XRD测试结果表明,800℃下焙烧得到的样品具有较好的层状结构和较低的阳离子混排程度.SEM测试表明合成材料具有球状形貌,平均粒径可达5μm,组成它的一次颗粒粒径平均为200nm.在2.8～4.3V(vs.Li/Li+)0.2C条件下进行充放电测试,800℃下合成的样品的首次放电比容量最高(159.06mAh·g-1),容量损失最小,循环50次后能保持初始放电比容量的95.7;.EIS分析结果表明,800℃焙烧的样品的电化学活性最好.