D-Type Anti-Ferromagnetic Ground State in Ca_2Mn_2O_5

We study the electronic and magnetic properties of an oxygen-deficient perovskite Ca_2 Mn_2 O_5 based on the first principle calculations. The calculations show that the ground state of Ca_2 Mn_2 O_5 is a D-type anti-ferromagnetic structure with the anti-ferromagnetic spin coupling along the c-direction. The corresponding electronic structure of the D-type state is investigated, and the results display that Ca_2 Mn_2 O_5 is an insulator with an indirect energy gap of ~2.08 eV. By the partial density-of-state analysis, the valence band maximum is mainly contributed to by the O-2 p orbitals and the conduction band minimum is contributed to by the O-2 p and Mn-3 d orbitals. Due to the Coulomb repulsion interaction between electrons, the density of state of Mn-3 d is pulled to-6--4.5 eV.     

Structural Domain Imaging and Direct Determination of Crystallographic Orientation in Noncentrosymmetric Ca_3Ru_2O_7 Using Polarized Light Reflectance

The noncentrosymmetricity of a prototypical correlated electron system Ca₃Ru₂O₇ renders extensive interest in the possible polar metallic state,along with multiple other closely competing interactions.However,the structural domain formation in this material often complicates the study of intrinsic material properties.It is crucial to fully characterize the structural domains for unrevealing underlying physics.Here,we report the domain imaging on Ca₃Ru<...     

Stability,elastic anisotropy,and electronic properties of Ca_2C_3

The systematic investigations of the mechanical, elastic, and electronic properties, and stability of the newly synthesized monoclinic C2/m-Ca_2C_3 are performed, based on the first-principles calculations. Ca_2C_3 is found to be mechanically and dynamically stable only from 0 GPa to 24 GPa. The elastic anisotropy studies show that Ca_2C_3 exhibits the elastic anisotropy increasing with the augment of pressure. Furthermore, using the HSE06 hybrid functional, the electronic properties of Ca_2C_3 under pressure are calculated. The structure can be regarded as a quasi-direct band gap semiconductor, and the pressure-induced direct-indirect band gap transition is studied in detail.     

基于La_(1.4)Ca_(1.6)Mn_2O_7结构、电输运及磁性研究(英文)

我们制备了名义成分是La1.4 Ca1.6 Mn2 O7的多晶样品,并且研究了它的结构、磁性和电性.利用Rietveld方法对X射线衍射数据进行精修的结果表明,该La1.4 Ca1.6 Mn2 O7是由钙钛矿锰氧化物La0.66 Ca0.34 MnO3和氧化钙CaO组成的复相混合物.其中,La0.66 Ca0.34 MnO3占相成分的大部分,而CaO占小部分.我们对比了该名义组分样品La1.4 Ca1.6 Mn2 O7和La0.7 Ca0.3 MnO3的铁磁居里转变温度和绝缘体-金属转变温度,结果表明该La1.4 Ca1.6 Mn2 O7的磁输运性质是由其中的钙钛矿相成分决定的.所有的这些结果表明,名义组分样品La1.4 Ca1.6 Mn2 O7实际上并没有形成所谓的Sr3Ti2O7型的双层结构,而实际形成的是以钙钛矿结构作为主相的复相混合物.     

A first-principles study of the structural and elastic properties of orthorhombic and tetragonal Ca3Mn2O7

The structural and elastic properties of multiferroic Ca3Mn2O7 with ferroelectric orthorhombic （O-phase） and paraelectric tetragonal structures （T-phase） have been studied by first-principles calculations within the generalized gradient approximation （GGA） and the GGA plus Hubbard U approaches （GGA ＋ U）. The calculated theoretical structures are in good agreement with the experimental values. The T-phase is found to be antiferromagnetic （AFM） and the AFM O-phase is more stable than the T-phase, which also agree with the experiments. On these bases, the single-crystal elastic constants （Cijs） and elastic properties of polycrystalline aggregates are investigated for the two phases. Our elasticity calculations indicate Ca3Mn2O7 is mechanically stable against volume expansions. The AFM O-phase is found to be a ductile material, while the AFM T-phase shows brittle nature and tends to be elastically isotropic. We also investigate the influence of strong correlation effects on the elastic properties, qualitatively consistent results are obtained in a reasonable range of values of U. Finally, the ionicity is discussed by Bader analysis. Our work provides useful guidance for the experimental elasticity measurements of Ca3Mn2O7, and makes the strain energy calculation in multiferroic Ca3Mn2O7 thin films possible.     

Ca_3NbGa_3Si_2O_(14)晶体的介电和压电特性

利用Y切和 (yxl) 30°切两种样品测量了Ca3NbGa3Si2 O1 4晶体的介电、压电和部分弹性参数 .计算了 (yxl)θ切型相关压电常数随切角的变化 .与La3Ga5SiO1 4晶体相比 ,Ca3NbGa3Si2 O1 4晶体具有更优良的压电性能 ,其压电常数d1 1=7 93× 10 - 1 2 C N ,d1 4=- 5 88× 10 - 1 2 C N .     

纳米Mn2O3的制备及其ESR研究

采用通常的化学液相均相沉淀法首次制备出了培养Ｍｎ２Ｏ３材料，Ｘ射线衍射和透射电镜观察表明纳米Ｍｎ２Ｏ３的颗粒在９－５０ｎｍ，９纳米的Ｍｎ２Ｏ３顺磁共振谱图观察到六线峰精细结构，并对此结果进行了合理的讨论。     

Thermoelectric properties of lower concentration K-doped Ca_3Co_4O_9 ceramics

The tuning of electron and phonon by ion doping is an effective method of improving the performances of thermoelectric materials. A series of lower concentration K-doped Ca_(3-x)K_xCo_4O_9(x = 0, 0.05, 0.10, 0.15) polycrystalline ceramic samples are prepared by combining citrate acid sol-gel method with cold-pressing sintering method. The single-phase compositions and plate-like grain morphologies of all samples are confirmed by x-ray diffraction and field emission scanning electron microscope. The effects of lower concentration K doping on the thermoelectric properties of the material are evaluated systematically at high temperatures(300–1026 K). Low concentration K doping causes electrical conductivity to increase up to 23% with little effect on the Seebeck coefficient. Simultaneously, the thermal conductivity of K-doped sample is lower than that of the undoped sample, and the total thermal conductivity reaches a minimum value of approximately1.30 W·m~(-1)·K~(-1), which may be suppressed mainly by the phonon thermal conduction confinement. The dimensionless figure-of-merit ZT of Ca_(2.95)K_(0.05)Co_4O_9 is close to 0.22 at 1026 K, representing an improvement of about 36% compared with that of Ca_3Co_4O_9, suggesting that lower concentration K-doped Ca_3Co_4O_9 series materials are promising thermoelectric oxides for high-temperature applications.     

双层钙钛矿锰氧化物Nd2-2xCa1+2xMn2O7(x=0.4,0.5)的磁相变特性

利用固相反应法制备了Nd2-2x Ca1+2x Mn2O7(x=0.0-0.9)多晶样品,通过FULLPROF程序对样品X射线衍射图谱进行了精修,样品的空间群为14/mmm.测量了样品x=0.4,0.5的磁性(5K相似文献   

Spin frustration and magnetic ordering in triangular lattice antiferromagnet Ca_3CoNb_2O_9

We synthesized a quasi-two-dimensional distorted triangular lattice antiferromagnet Ca3 Co Nb2O9, in which the effective spin of Co2+is 1/2 at low temperatures, whose magnetic properties were studied by dc susceptibility and magnetization techniques. The x-ray diffraction confirms the quality of our powder samples. The large Weiss constant θCW ~-55 K and the low Neel temperature TN~ 1.45 K give a frustration factor f =| θCW/TN|≈ 38, suggesting that Ca3 Co Nb2O9resides in strong frustration regime. Slightly below TN, deviation between the susceptibility data under zero-field cooling(ZFC)and field cooling(FC) is observed. A new magnetic state with 1/3 of the saturate magnetization Ms is suggested in the magnetization curve at 0.46 K. Our study indicates that Ca3 Co Nb2O9is an interesting material to investigate magnetism in triangular lattice antiferromagnets with weak anisotropy.     

Size-dependent exchange bias in single phase Mn_3O_4 nanoparticles

Glassy magnetic behavior and exchange bias phenomena are observed in single phase Mn_3O_4 nanoparticles.Dynamics scaling analysis of the ac susceptibility and the Henkel plot indicate that the observed glassy behavior at low temperature can be understood by taking into account the intrinsic behavior of the individual particles consisting of a ferrimagnetic(FIM) core and a spin-glass surface layer.Field-cooled magnetization hysteresis loops display both horizontal and vertical shifts.Dependence of the exchange bias field(H_E) on the cooling field shows an almost undamped feature up to 70 kOe,indicating the stable exchange bias state in Mn_3O_4.H_E increases as the particle size decreases due to the higher surface/volume ratio.The occurrence of the exchange bias can be attributed to the pinning effect of the frozen spin-glass surface layer upon the FIM core.     

Growth of TlBa_2Ca_2Cu_3O_9 Epitaxial Thin Films by Two-Step Method in Argon

TlBa_2 Ca_2 Cu_3 O_9(Tl-1223) films have promising applications due to their high critical temperature and strong magnetic flux pinning. Nevertheless, the preparation of pure phase Tl-1223 film is still a challenge. We successfully fabricate Tl-1223 thin films on LaAlO_3(001) substrates using dc magnetic sputtering and a post annealing two-step method in argon atmosphere. The crystallization temperature of Tl-1223 films in argon is reduced by 100℃ compared to that in oxygen. This greatly reduces the volatilization of Tl and improves the surface morphology of films. The lower annealing temperature can effectively improve the repeatability of the Tl-1223 film preparation. In addition, pure Tl-1223 phase can be obtained in a broad temperature zone,from 790℃ to 830℃. In our study, the films show homogenous and dense surface morphology using the presented method. The best critical temperature of Tl-1223 films is characterized to be 110 K, and the critical current J_c(77 K, 0 T) is up to 2.13 × 106 A/cm~2.     

Eu2+掺杂浓度对Ca2MgSi2O7∶Eu2+荧光粉发光特性的影响

采用化学共沉淀法制备了Ca_2-xMgSi₂O₇:xEu²⁺绿色荧光粉.用X射线衍射仪、荧光分光光度计及光色综合测试系统对Ca_2-xMgSi₂O₇:xEu²⁺绿色荧光粉的相结构、发光性能进行了测试.结果表明:其激发光谱分布在300–480 nm波长范围,谱峰位于389,430 nm处,可以被InGaN管芯产生的360–480 nm辐射有效激发;在波长为430 nm蓝光激发下,其发射光谱谱峰位于531 nm处.Ca_2-xMgSi₂O₇:xEu²⁺绿色荧光粉的发光强度随Eu²⁺掺杂量的增加而增强,当Eu²⁺掺杂量x为0.04时,发光强度达到最大值,而后开始降低,发生浓度猝灭.根据Dexter能量共振理论,浓度猝灭是由电偶极-电偶极相互作用引起的. 关键词： 2MgSi₂O₇∶Eu²⁺')" href="#">Ca₂MgSi₂O₇∶Eu²⁺ 绿色荧光粉 发光特性 白光发光二极管     

Orbital selection of the double [CuO_2] layer compound Ca_3Cu_2O_4Cl_2

正The discovery of high-temperature copper oxide superconductors(HTS)by Bednorz and Muller[1]in 1986 opened up a new field of superconductivity.Since then,several different families of materials have been discovered with greatly increased superconducting critical temperature(T_c)[2].Oxychloride cuprates,Ca_(n+1)Cu_nO_(2n)Cl_2,are one such type of     

Excellent ethanol sensing properties based on Er_2O_3-Fe_2O_3 nanotubes

In this work, pure α-Fe2O3 and Er2O3-Fe2O3 nanotubes were synthesized by a simple single-capillary electrospinning technology followed by calcination treatment. The morphologies and crystal structures of the as-prepared samples were characterized by scanning electron microscopy and x-ray diffraction, respectively. The gas-sensing properties of the as-prepared samples have been researched, and the result shows that the Er2O3-Fe2O3 nanotubes exhibit much better sensitivity to ethanol. The response value of Er2O3-Fe2O3 nanotubes to 10 ppm ethanol is 21 at the operating temperature240?, which is 14 times larger than that of pure α-Fe2O3 nanotubes(response value is 1.5). The ethanol sensing properties of α-Fe2O3 nanotubes are remarkably enhanced by doping Er, and the lowest detection limit of Er2O3-Fe2O3 nanotubes is300 ppb, to which the response value is about 2. The response and recovery times are about 4 s and 70 s to 10 ppm ethanol,respectively. In addition, the Er2O3-Fe2O3 nanotubes possess good selectivity and long-term stability.     

Pressure dependence of elastic constants of orthorhombic Li2Ge7O15 above and below the ferroelectric transition

The pressure derivatives of the elastic constants c_ij of orthorhombic Li₂Ge₇O₁₅ have been determined at 293 K by the method of pressure-induced shifts of resonance frequencies of thick plates at ca. 15 MHz in the range between 0 and 1500 bar. Approaching the transition at ca. 630 bar, all P_ij = dc_ij/dp (i, j = 1, 2, 3; p pressure) develop strongly negative values. At higher pressures a similar behaviour with reversed sign is observed. The pressure derivatives of the pure “shear resistances” c₄₄, c₅₅, and c₆₆ depend only slightly on pressure even in the vicinity of the transition. The main interactions driving the transition are of the totally symmetric type. The values dK^?1/dp (K volume compressibility) deviate strongly from the quasi-invariant value of ca. 5 observed in almost all stable crystals (dK^?1/dp = ? 1750 at 620 bar and 1380 at 700 bar). The anomalous piezoelastic behaviour reflects the anomalous thermoelastic behaviour: negative P_ij in the low pressure (high temperature) phase correspond to positive T_ij = d log c_ij/dT (T temperature) and vice versa.     

Investigation of electronic,elastic, and optical properties of topological electride Ca_3Pb via first-principles calculations

Electrides are unique materials with the anionic electrons confined to the interstitial sites, expecting important applications in various areas. In this work, the electronic structure and detailed physical properties of topological electride Ca_3Pb are studied theoretically. By comparing the crystal structures and band structures of Ca_3Pb and Ca_3Pb O, we find that after removing O_2-ions from Ca_3Pb O, the remaining electrons are confined in the vacancies of the Ca_6 octahedra centers,playing the role as anions and forming an additional energy band compared with that of Ca_3Pb. These interstitial electrons partially result in the low work function of Ca_3Pb. Moreover, the calculated mechanic properties imply that Ca_3Pb has a strong brittleness. In addition, the dielectric functions and optical properties of Ca_3Pb are also analyzed.     

Effect of pressure on phonon spectra and elastic properties of orthorhombic GeSe

The pressure dependences of the phonon frequencies of the Brillouin zone center and also the elastic constants of the GeSe compound have been calculated by the density functional theory method using the ABINIT program package. The results have been compared with the available data of theoretical calculations and measurements of the pressure dependences of the Raman frequency. The calculations have demonstrated that the compound undergoes a continuous phase transition from the simple orthorhombic to body-centered orthorhombic lattice at a pressure about 29 GPa.     

An electron spin resonance study of Eu doping effect in La_(4/3)Sr_(5/3)Mn_2O_7 single crystal

The effect of Eu3+ion doping in the La sites of single-crystal La4/3Sr5/3Mn2O7was investigated. Electron spin resonance(ESR) was applied to La4/3Sr5/3Mn2O7and(La0.8Eu0.2)4/3Sr5/3Mn2O7single crystals. A phase separation and phase transitions were observed from the ESR spectra data. Between 350 K and 300 K, both paramagnetic resonance(PMR)and anisotropic ferromagnetic resonance(FMR) lines were observed in the ab plane and the c axis direction, suggesting a coexistence of the paramagnetic(PM) phase and the ferromagnetic(FM) phase. The magnetization measurement reveals a spin-glass-like behavior in single-crystal(La0.8Eu0.2)4/3Sr5/3Mn2O7below the temperature of spin freezing Tf(～ 29.5 K).