Metal valences in electron-doped (Sr,La)2FeTaO6 double perovskite: A Fe Mössbauer spectroscopy study

Substitution of divalent Sr by trivalent La is found to affect the valence states of both of the two B-site cations, Fe and Ta, in the double perovskite oxide (Sr_1−xLa_x)₂FeTaO₆. Moreover, it improves the degree of order of these cations. From ⁵⁷Fe Mössbauer spectra the average Fe valence was found to decrease with increasing La substitution level, x. However, the valence of Fe decreased less than expected if the valence of Ta was assumed to remain constant. Hence, we conclude that also the valence of Ta decreases.     

Non-collinear magnetic structure of the Sr2ErRuO6 double perovskite

The crystal and magnetic structure of Sr₂ErRuO₆ has been studied by means of neutron powder diffraction as well as magnetization and susceptibility measurements. Neutron diffraction profile measured at 50 K shows that the Ru⁵⁺ and Er³⁺ are ordered in the B-sites of the perovskite-type structure, while the Sr atoms occupy the A-site. This compound crystallizes with a monoclinic unit cell, space group P2₁/n and lattice parameters are approximately √2a_p × √2a_p × 2a_p. Magnetic susceptibility measurements reveal the existence of antiferromagnetic interactions in which Ru⁵⁺ and Er³⁺ sublattices are involved. The field dependence of the magnetization indicates the presence of a weak ferromagnetic component at the transition temperature, arising from the spin canting of the antiferromagnetically ordered Ru⁵⁺ and Er³⁺ moments. Thermal evolution of the neutron diffraction patterns indicate that the Nèel temperature is 36 K and the magnetic reflections can be indexed on the basis of a propagation vector k = [0, 0, 0]. The spin arrangement is described by the AxAz magnetic modes where the Ru⁵⁺ and Er³⁺ moments are mainly aligned along the c-axis of the structure, forming an angle of 6° with the c-axis in the case of the Er³⁺ sublattice and 15° for the Ru⁵⁺ moment.     

Magnetic and Mössbauer studies of 5% Fe-doped BiMnO3

⁵⁷Fe Mössbauer spectroscopy, dc and ac magnetization, specific heat, and differential scanning calorimetry measurements were performed in a powder BiMn_0.95Fe_0.05O₃ sample prepared at 6 GPa and 1383 K. The substitution of 5% Fe for Mn increases the temperatures of the structural monoclinic-to-orthorhombic phase transition (from 768 to 779 K) and the ferromagnetic transition (from 98 to 109 K) by about 10 K in BiMn_0.95Fe_0.05O₃ compared with BiMnO₃. On the other hand, the temperature of the monoclinic-to-monoclinic phase transition associated with the orbital ordering strongly decreases in BiMn_0.95Fe_0.05O₃ (414 K) compared with that of BiMnO₃ (474 K). The saturated magnetic moment at 5 K and 5 T is also suppressed from 3.92 μ_B per formula unit in BiMnO₃ to 3.35 μ_B in BiMn_0.95Fe_0.05O₃. The large quadrupole splitting (1.18 mm/s) observed at 293 K in BiMn_0.95Fe_0.05O₃ can be explained by the strong Jahn-Teller distortion and cooperative orbital order. The quadrupole splitting reduces by two times above the orbital melting temperature.     

Structural studies of the disorder and phase transitions in the double perovskite Sr2YTaO6

The evolution of the crystal structure of the double perovskite Sr₂YTaO₆ from room temperature to 1250 °C has been studied using powder neutron and synchrotron X-ray diffraction. At room temperature Sr₂YTaO₆ crystallises in a monoclinic superstructure with the space group P2₁/n. The tilting of the octahedra evident in the room temperature structure is progressively lost on heating, resulting in a sequence of phase transitions that ultimately yields the cubic structure in space group Fm3?m. The high temperature tetragonal and cubic phases are characterised by strongly anisotropic displacements of the anions. The amount of defects in the crystal structure of Sr₂YTaO₆ is found to be sensitive to the preparative method.     

Mössbauer study of the FeV2O4---Fe3O4 system

Mössbauer spectra of the Fe_1+xV_2−xO₄ spinel solid solutions are taken to investigate the cation distribution. Room temperature spectra can be interpreted by assuming that the cation distribution is represented approximately as Fe²⁺[Fe³⁺_xV³⁺_2−x]O₄ for 0 x 0.35 and Fe³⁺[Fe²⁺Fe³⁺_x−1V³⁺_2−x]O₄ for 1 x 2 and the ionic valence arrangement changes from the 2-3-3 type (Fe²⁺[Fe³⁺_xV³⁺_2−x]O₄) to the 3-2-3 one (Fe³⁺[Fe²⁺V³⁺]O₄) in the range 0.35 x 1. Fe₂VO₄ is found to be 3-2-3 spinel, Fe³⁺[Fe²⁺V³⁺]O₄. Its paramagnetic spectrum at 473°K is, however, composed of a broad single line with isomer shift value of 0.61 mm/sec relative to stainless steel, in which the line splitting due to the ferric and ferrous ions is rendered indistinguishable.     

Dielectric relaxation in a new double perovskite oxide Ho2MgZrO6

A new double perovskite oxide holmium magnesium zirconate Ho₂MgZrO₆ (HMZ) was prepared by solid state reaction technique. The crystal structure has been determined by powder X-ray diffraction which shows monoclinic phase at room temperature with cell parameters a = 9.3028 ± 0.0030 Å, b = 5.2293 ± 0.0008 Å, c = 4.4009 ± 0.0009 Å, β = 103.3746 ± 0.0166°. An analysis of complex permittivity with frequency was carried out assuming a distribution of relaxation times. The frequency dependent electrical data are analyzed in the framework of conductivity and electric modulus formalisms. At the high temperature range, conductivity data satisfy the variable range hopping (VRH) model. In this regime, the conductivity of sample obeys Mott’s T^1/4 law, characteristic of VRH. High temperature data indicates the formation of thermally activated small polarons. The scaling behaviour of imaginary part of electric modulus suggests that the relaxation describes the same mechanism at various temperatures.     

Iron-57 Mössbauer spectroscopy of Cr2TeO6 and Fe2TeO6

Iron-57 Mössbauer spectroscopy has been used to determine the hyperfine field at a chromium site in Cr₂TeO₆ which is found to be 525 kOe. The Néel temperature for Cr₂TeO₆ containing 0.4% ⁵⁷Fe is found to be 90%K; the angle θ between V_zz and the magnetic axis is 42 ± 4°. These data are compared with those for Fe₂TeO₆ where H_eff (T = 0) = 530 kOe T_N = 203°K and θ = 90°.     

LaSrMnCoO6: a new cubic double perovskite oxide

Single-phase polycrystalline powder samples of the double perovskite oxide LaSrMnCoO₆ were synthesized by the Pechini (citrate-gel) technique. The structural, magnetic and electrical properties of the obtained powders were investigated by X-ray diffraction, electron microscopy, dc magnetization, ac susceptibility and dc resistivity measurements. The crystal structure of the new compound was found to be cubic of space group at room temperature. Below 225 K, the samples exhibit ferrimagnetic behavior with a spin-glass-like character. Resistivity measurements indicate semiconducting behavior with two different conductivity mechanisms: thermally activated behavior below 190 K and variable range hopping above 190 K.     

K+掺杂双钙钛矿Cs2AgInCl6的发光性质

采用固相球磨法制备了K⁺掺杂双钙钛矿Cs₂AgInCl₆纳米材料,该方法无需配体辅助,绿色环保。通过X射线衍射和拉曼光谱对晶体结构进行研究,通过激发光谱、发射光谱和时间分辨光谱对其发光性能进行研究。结果表明,Cs₂AgInCl₆为立方晶体,属于Fm3m空间群,由于宇称禁戒跃迁,其荧光量子产率(PLQY)低,小于0.1%。低于60%的K⁺掺杂主要取代Ag⁺的位置,引起Cs₂AgInCl₆的晶格膨胀,消除了晶格结构的反演对称性,打破了宇称禁戒跃迁,掺杂后Cs₂AgInCl₆的光致发光强度显著增强。K⁺的最佳掺杂比例为40%,Cs₂Ag_0.6K_0.4InCl₆材料发射中心波长为640 nm,半高宽为180 nm,平均荧光寿命达到29.2 ns,PLQY达到10.5%。当K⁺掺杂比例超过60%,K⁺开始取代Cs⁺的位置,产物发生相变,出现立方相的Cs_2-xK_1+x-yAg_yInCl₆和单斜相的Cs_2-xK_1+xInCl₆产物,这些产物由于强电子-声子耦合,非辐射复合占据主导地位。     

Revisiting Y2Si2O7 and Y2SiO5 polymorphic structures by Y MAS-NMR spectroscopy

This paper describes the ⁸⁹Y MAS-NMR spectra for all the established polymorphs of Y₂Si₂O₇ (y, α, β, γ and δ) and Y₂SiO₅ (X1 and X2). The combination of our spectroscopic data with the structural information published up to now from diffraction data permits the revision and correction of mistakes which appear in the literature. Finally, the influence of different structural factors, such as yttrium coordination number and Y-O distances on the ⁸⁹Y NMR isotropic chemical shift is analyzed.     

Crystallographic and magnetic properties of CaLaMnMoO6 double perovskite

Powder neutron diffraction studies show that CaLaMnMoO₆ double perovskite crystallizes in monoclinic P2₁/n, with a=5.56961(9), b=5.71514(9), and β=90.043(1)°. Mn and Mo occupy the 2c and 2d positions, respectively, with 6.0(4)% Mn/Mo anti-site mixing. Temperature-dependent magnetic susceptibility measurements reveal that CaLaMnMoO₆ is ferrimagnetic, with T_N=92(3) K, below which large magnetic frustration is detected. The zero-field magnetic moment measured at 5 K is about 1.2 μ_B, comparable to that of ALaMnMoO₆ (A=Ba and Sr), but much lower than expected for antiparallel ordering of formally Mn²⁺ (d⁵) and Mo⁵⁺ (d¹). Moreover, no long-range magnetic ordering is observed in neutron diffraction data down to 4 K. The magnetic frustration is discussed in the framework of nearest-neighbors next-nearest-neighbors magnetic frustration.     

On the space group of the double perovskite Ba2PrIrO6

The structure of the double perovskite Ba₂PrIrO₆ has been re-examined by profile analysis of X-ray diffraction data. It has been shown that neither the monoclinic P2₁/n nor the tetragonal P4/mnc space group correctly describes its structure. Ba₂PrIrO₆ has cubic symmetry, space group , cell parameter . In agreement with earlier observations, Ba₂PrIrO₆ is an ordered perovskite containing the valence pair Pr(IV)/Ir(IV).     

Crystal structure, magnetic susceptibility and Mössbauer spectroscopy of the mixed-valence iron phosphate Na1/2Cu4/3Fe2(PO4)3

Single crystals of a new mixed-valent iron phosphate Na_1/2Cu_4/3Fe₂(PO₄)₃ have been synthesized by a flux method and structurally characterized from X-ray diffraction data. Crystal data: space group ; ; ; ; α=105.881(1)°; β=107.202(1)°; γ=101.467(1)°; Z=2; R₁=0.03; wR₂=0.093. The three-dimensional structure was found to be closely related to that of the well known Howardevansite structural type. It results from infinite chains of CuO₅ and FeO₆ polyhedra, joined together by (Cu,□)O₆ octahedra and PO₄ tetrahedra by corner-sharing. The large cavities in framework are occupied by Na⁺ ions. The magnetic susceptibility study revealed an antiferromagnetic behavior with Neel temperature of approximately 40 K. The Mössbauer spectroscopy confirmed the presence of iron in both +2 and +3 oxidation states.     

Observation by mössbauer spectroscopy of a crystal phase transition in FeCl2, 2py and αCoCl2, 2py

A crystal phase transition occurs at 235 and 150°K for FeCl₂. 2py and α-⁵⁷CoCl₂, 2py respectively, as deduced from the anomalous temperature dependence of the quadrupole interaction at the ⁵⁷Fe site. FeCl₂, 2py orders magnetically at 10.5°K.     

Crystal structures and phase transitions of Sr2CrSbO6

Sr₂CrSbO₆ was synthesized by the conventional solid-state reaction process. X-ray powder diffraction (XRPD) and neutron powder diffraction (NPD) has been used to reinvestigate the structure at room temperature and to study the phase transitions at high- and low-temperature. Rietveld analysis revealed that Sr₂CrSbO₆ crystallizes at room temperature in a monoclinic system having a space group I2/m, with a=5.5574(1) Å; b=5.5782(1) Å; c=7.8506(2) Å and β=90.06(2), no P2₁/n space group as was previously reported. The high-temperature study (300-870 K) has shown that the compound presents the following temperature induced phase-transition sequence: I2/m-I4/m-Fm-3m. The low-temperature study (100-300 K) demonstrated that the room-temperature I2/m monoclinic symmetry seems to be stable down to 100 K.     

Crystal growth, structures, and optical properties of the cubic double perovskites Ba2MgWO6 and Ba2ZnWO6

Single crystals of the tungstates Ba₂MgWO₆ and Ba₂ZnWO₆ have been grown for the first time. The crystals were prepared with molten potassium carbonate acting as a flux. According to the single-crystal X-ray diffraction structure determination, the compounds crystallize in space group Fm3¯m of the cubic system with a double perovskite structure, A₂BB′O₆. These structural findings were confirmed with neutron diffraction on polycrystalline samples synthesized by a high-temperature solid-state route. Both sets of diffraction data reveal that the M²⁺ and W⁶⁺ cations are fully ordered on the B and B′ sites. Ba₂MgWO₆ and Ba₂ZnWO₆ exhibit room-temperature luminescence with green and yellow emissions, respectively.     

Phase analysis of surface layers by Mössbauer spectroscopy

Mössbauer spectroscopy—now 25 years old—is described as a very useful tool to perform phase analyses. The experimental technique and the use of the spectral parameters to derive qualitative and quantitative informations are briefly discussed. Special scattering techniques (gamma scattering and conversion electron detection) are now applied to investigate surface layers. Some examples, where the product composition as well as the kinetics were investigated, demonstrate the successful application in corrosion research: Corrosion of the inner surface of hot water pipes, the effect of corrosion inhibitors, the influence of aggressive components, and the nondestructive analysis of sublayer rusting beneath a coating. The general advantages of Mössbauer spectroscopy in surface studies are summarized.     

X-ray diffraction and Mössbauer spectroscopy studies of LiFe0.5Ti1.5O4 – A new primitive cubic ordered spinel

LiFe_0.5Ti_1.5O₄ was synthesized by solid-state reaction carried out at 900 °C in flowing argon atmosphere, followed by rapid quenching of the reaction product to room temperature. The compound has been characterized by X-ray powder diffraction (XRD) and ⁵⁷Fe Mössbauer effect spectroscopy (MES). It crystallizes in the space group P4₃32, a = 8.4048(1) Å. Results from Rietveld structural refinement indicated 1:3 cation ordering on the octahedral sites: Li occupies the octahedral (4b) sites, Ti occupies the octahedral (12d) sites, while the tetrahedral (8c) sites have mixed (Fe/Li) occupancy. A small, about 5%, inversion of Fe on the (4b) sites has been detected. The MES data is consistent with cation distribution and oxidation state of Fe, determined from the structural data.The title compound is thermally unstable in air atmosphere. At 800 °C it transforms to a mixture of two Fe³⁺ containing phases – a face centred cubic spinel Li_(1+y)/2Fe_(5−3y)/2Ti_yO₄ and a Li_(z−1)/2Fe_(7−3z)/2Ti_zO₅ – pseudobrookite. The major product of thermal treatment at 1000 °C is a ramsdellite type lithium titanium iron(III) oxide, accompanied by traces of rutile and pseudobrookite.     

K+掺杂双钙钛矿Cs2AgInCl6的发光性质

采用固相球磨法制备了K⁺掺杂双钙钛矿Cs₂AgInCl₆纳米材料,该方法无需配体辅助,绿色环保。通过X射线衍射谱和拉曼光谱对晶体结构进行研究,通过激发光谱、发射光谱和时间分辨光谱对其发光性能进行研究。结果表明,Cs₂AgInCl₆为立方晶体,属于Fm3m空间群,由于宇称禁戒跃迁,其荧光量子产率(PLQY)低,小于0.1%。低于60%的K⁺掺杂主要取代Ag⁺的位置,引起Cs₂AgInCl₆的晶格膨胀,消除了晶格结构的反演对称性,打破了宇称禁戒跃迁,掺杂后Cs₂AgInCl₆的光致发光强度显著增强。K⁺的最佳掺杂比例为40%,Cs₂Ag_0.6K_0.4InCl₆发出中心波长为640 nm,半高宽为180 nm,平均荧光寿命达到29.2 ns,PLQY达到10.5%。当K⁺掺杂比例超过60%,K⁺开始取代Cs⁺的位置,产物发生相变,出现立方相的Cs_2-xK_1+x-yAg_yInCl₆和单斜相的Cs_2-xK_1+xInCl₆产物,这些产物由于强电子-声子耦合,非辐射复合占据主导地位。