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.     

Maxwell–Wagner relaxation in the CaMn7O12 perovskite

In this work we have measured the complex dielectric permittivity of a polycrystalline material using samples of different thickness (frequency range 20–1 MHz). The results reveal that the CaMn₇O₁₂ material is electrically heterogeneous as it consists of regions of different conductivities.

In this kind of electrically heterogeneous materials the variation of the dielectric permittivity as a function of temperature can be fitted to a Maxwell–Wagner model. In the case of the CaMn₇O₁₂ sample this model fits only partially the experimental data and we need to include three regions of different conductivities to explain the obtained dielectric response.     

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.     

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.     

Excellent red phosphors of double perovskite Ca2LaMO6:Eu (M=Sb, Nb, Ta) with distorted coordination environment

Double perovskite Ca₂LaSbO₆, successfully synthesized by solid state reaction method, was identified by Rietveld refinements to crystallize in the monoclinic space group P2₁/n, which is isostructural to Ca₂LaMO₆ (M=Nb, Ta). Excellent red luminescence of Eu-doped Ca₂LaMO₆ (M=Sb, Nb, Ta) can be obtained and no luminescence quenching effect was observed when Eu-doping level reached 40%. For Ca₂La_0.6NbO₆:0.4Eu³⁺, quantum efficiencies of 20.9% and 27.7% were reached to show high light conversion and bright red emission excited at 465 nm (blue light) and 534 nm (green light), respectively, comparable to the commercial phosphors. Through systemic investigation for the series of double perovskite compounds, the excellent red emission in Ca₂LaMO₆ is attributed to highly distorted polyhedra of EuO₈ (low tolerance factor of the pervoskite), and large bond distances of La−O (low crystal field effect of the activator).     

La doping of the Sr2CoWO6 double perovskite: A structural and magnetic study

La-doped Sr₂CoWO₆ double perovskites have been prepared in air in polycrystalline form by solid-state reaction. These materials have been studied by X-ray powder diffraction (XRPD), neutron powder diffraction (NPD) and magnetic susceptibility. The structural refinement was performed from combined XRPD and NPD data (D2B instrument, λ=1.594 Å). At room temperature, the replacement of Sr²⁺ by La³⁺ induces a change of the tetragonal structure, space group I4/m of the undoped Sr₂CoWO₆ into the distorted monoclinic crystal structure, space group P2₁/n, Z=2. The structure of La-doped phases contains alternating CoO₆ and (Co/W)O₆ octahedra, almost fully ordered. On the other hand, the replacement of Sr²⁺ by La³⁺ induces a partial replacement of W⁶⁺ by Co²⁺ into the B sites, i.e. Sr_2−xLa_xCoW_1−yCo_yO₆ (y=x/4) with segregation of SrWO₄. Magnetic and neutron diffraction measurements indicate an antiferromagnetic ordering below T_N=24 K independently of the La-substitution.     

Syntheses and characterizations of complex perovskite oxynitrides LaMg1/3Ta2/3O2N, LaMg1/2Ta1/2O5/2N1/2, and BaSc0.05Ta0.95O2.1N0.9

The following complex oxynitride perovskites have been prepared: LaMg_1/3Ta_2/3O₂N, LaMg_1/2Ta_1/2O_5/2N_1/2, and BaSc_0.05Ta_0.95O_2.1N_0.9. Synchrotron X-ray powder diffraction analyses show that LaMg_1/3Ta_2/3O₂N and LaMg_1/2Ta_1/2O_5/2N_1/2 are isostructural to the oxide La₂Mg(Mg_1/3Ta_2/3)O₆ (space group P2₁/n), whereas BaSc_0.05Ta_0.95O_2.1N_0.9 has a simple cubic symmetry similarly to BaTaO₂N. The orderings of octahedral cations are markedly diminished in the above oxynitrides, as compared with the related oxides such as La₂Mg(Mg_1/3Ta_2/3)O₆ and Ba₂ScTaO₆. The optical band gaps are similar for the homologous compositions, LaMg_1/3Ta_2/3O₂N, LaMg_1/2Ta_1/2O_5/2N_1/2 and LaTaON₂ (1.9 eV), and BaSc_0.05Ta_0.95O_2.1N_0.9 and BaTaO₂N (1.8 eV), while the absorption edges become broader for the complex derivatives. As revealed from the impedance spectroscopic analysis, the oxynitrides have clearly different dielectric components from those of comparable oxides containing Ta⁵⁺. Impedance spectroscopy reveals interesting capacitor geometry in BaSc_0.05Ta_0.95O_2.1N_0.9 in which the semiconducting oxynitride grains are separated by insulating secondary phases. Most notably BaSc_0.05Ta_0.95O_2.1N_0.9 has a bulk component with a high relative permittivity (κ=7300) and the grain boundary component with an even higher κ.     

B-site ordered perovskite LaSrMnNbO6: Synthesis, structure and antiferromagnetism

LaSrMnNbO₆ has been synthesized by high temperature solid state reaction under 1% H₂/Ar dynamic flow. The structure is determined by Rietveld refinement of the powder X-ray diffraction data. It crystallizes in the monoclinic space group P2₁/n with the unit cell parameters: a=5.69187(12), b=5.74732(10), c=8.07018(15) Å and β=90.0504(29)°, which were also confirmed by electron diffraction. The Mn²⁺ and Nb⁵⁺ ions, whose valence states are confirmed by X-ray absorption near-edge spectroscopy, are almost completely ordered over the B-site (<1% inversion) of the perovskite structure due to the large differences of both cationic size (0.19 Å) and charge. The octahedral framework displays significant tilting distortion according to Glazer’s tilt system a⁻b⁻c⁺. Upon heating, LaSrMnNbO₆ decomposes at 690 °C under O₂ flow or at 775 °C in air. The magnetic susceptibility data indicate the presence of long-range antiferromagnetic ordering at T_N=8 K; the experimentally observed effective paramagnetic moment, μ_eff=5.76 μ_B for high spin Mn²⁺ (3d⁵, S=5/2) is in good agreement with the calculated value (μ_calcd=5.92 μ_B).     

Some A6B5O18 cation-deficient perovskites in the BaO-La2O3-TiO2-Nb2O5 system

Some dielectric oxides have been synthesized and characterized in the BaO-La₂O₃-TiO₂-Nb₂O₅ system. Through Rietveld refinement of X-ray powder diffraction data, Ba₅LaTi₂Nb₃O₁₈ and Ba₄La₂Ti₃Nb₂O₁₈ are identified as the A_nB_n−1O_3n (n=6) type cation-deficient perovskites with space group and lattice constants , and for Ba₅LaTi₂Nb₃O₁₈; , and for Ba₄La₂Ti₃Nb₂O₁₈, respectively. Their ceramics exhibit high dielectric constant up to 57 and high quality factors (Qf) up to 21,273 GHz. The temperature coefficient of resonant frequency (τ_f) of these ceramics is decreased with the increase of B-site bond valence.     

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。     

Pressure and temperature-dependent structural studies of Ba2BiTaO6

The ordered double perovskite Ba₂BiTaO₆ is shown to undergo a first-order rhombohedral to monoclinic (I2/m) phase transition, which can be induced by either lowering the temperature or through the application of pressure. The structures in both phases have been refined from high resolution neutron powder diffraction data at various temperatures, while the high pressure measurements utilised synchrotron X-ray diffraction data. The rhombohedral structure is characterised by out-of-phase tilts about the [111] axis. In the monoclinic structure the tilt axis has changed to be about the [110] axis; however, the magnitude of the tilts in the two structures is remarkably similar.     

Synthesis, structures, and phase transitions of barium bismuth iridium oxide perovskites Ba2BiIrO6 and Ba3BiIr2O9

The Ba-Bi-Ir-O system is found to contain two distinct perovskite-type phases: a rock-salt ordered double perovskite Ba₂BiIrO₆; and a 6H-type hexagonal perovskite Ba₃BiIr₂O₉. Ba₂BiIrO₆ undergoes a series of symmetry-lowering phase transitions on cooling , all of which are second order except the rhombohedral→monoclinic one, which is first order. The monoclinic phase is only observed in a 2-phase rhombohedral+monoclinic regime. The transition and 2-phase region lie very close to 300 K, making the room-temperature X-ray diffraction patterns extremely complex and potentially explaining why Ba₂BiIrO₆ had not previously been identified and reported. A solid solution Ba₂Bi_1+xIr_1−xO₆, analogous to Ba₂Bi_1+xRu_1−xO₆, 0≤x≤2/3, was not observed. The 6H-type phase Ba₃BiIr₂O₉ undergoes a clean second-order phase transition P6₃/mmc→C2/c at 750 K, unlike 6H-type Ba₃LaIr₂O₉, the P6₃/mmc structure of which is highly strained below ∼750 K but fails to distort coherently to the monoclinic phase.     

Investigation on double perovskite Ba4Ca2Ta2O11

The structure, conductivity and water uptake of the oxygen-deficient perovskite-type compound Ba₄Ca₂Ta₂O₁₁ have been investigated. Ba₄Ca₂Ta₂O₁₁ crystallizes in the cryolite structure (cubic, Fm3m SG) with a = 8.4508(2) Å, under dry air. The compound can be partially hydrated up to a maximum water content of approximately 0.52 mol H₂O per mol Ba₄Ca₂Ta₂O₁₁. In moist air, the structure symmetry becomes monoclinic (C2/m) and the temperature dependence of total conductivity shows a different behavior because of changes in transport mechanism. Three regions can be observed as a function of temperature. For the low temperature range 200–400 °C, the protonic conduction is prevailing with an activation energy E_A = 0.85 eV. In the intermediate temperature range (400–600 °C), O²⁻ anionic and protonic conductions are mixed with an activation energy E_A = 0.45 eV and in the third region, for temperatures above 600 °C, O²⁻conduction is prevailing with an activation energy E_A = 0.85 eV.     

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。     

Electron doping effects on Sr2FeReO6

We substitute La for Sr in the Sr₂FeReO₆ double perovskite to see how electron doping affects its properties. Sr_2−xLa_xFeReO₆ compounds can be prepared up to x = 0.5. Beyond this value, a secondary phase is formed. The replacement of Sr by La leads to an increase of the unit cell volume and to a rise of the Curie temperature. However, both the saturated magnetic moment and the room-temperature magnetoresistance decrease with increasing the La content. The substituted compounds are magnetically hard and show maxima in the electrical resistivity at field values close to the coercive field. Our structural study reveals a low content of anti-site defects for all samples studied. The average Fe–O bond lengths remain almost constant upon doping whereas the Re–O distances increase as x increases. This suggests that electrons mainly go to the Re orbitals.     

Pressure-induced phase transition and octahedral tilt system change of Ba2BiSbO6

High-resolution X-ray synchrotron powder diffraction studies under high-pressure conditions are reported for the ordered double perovskite Ba₂BiSbO₆. Near 4 GPa, the oxide undergoes a pressure-induced phase transition. The symmetry of the material changes during the phase transition from space group to space group I2/m, which is consistent with a change in the octahedral tilting distortion from an a⁻a⁻a⁻ type to a⁰b⁻b⁻ type using the Glazer notation. A fit of the volume-pressure data using the Birch-Murnagaham equation of state yielded a bulk modulus of 144(8) GPa for the rhombohedral phase.     

The Jahn-Teller distortion and cation ordering in the perovskite Sr2MnSbO6

The perovskite Sr₂MnSbO₆ has been synthesized using conventional ceramic techniques and structurally characterized using high-resolution powder X-ray and neutron diffraction. The structure is tetragonal in space group I4/m. The octahedra were found to feature Jahn-Teller (JT) distortion due to the presence of Mn³⁺, and this is identified as strongly contributing to the octahedral tilting. Evidence for B-site cation ordering is presented however there is extensive anti-site disorder. The disordering of the Mn³⁺ and Sb⁵⁺ cations is believed to be a result of the similar size of these two cations and the polarizability of the Sb⁵⁺ cation. The structure was found to undergo a transition to cubic symmetry at 521 °C with removal of the octahedral tilting leading to the quenching of the JT distortion. This phase transition was found to be continuous and tricritical in nature.     

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.     

Antiferromagnetism of perovskite EuZrO3

Polycrystalline EuZrO₃ has been synthesized by the solid-state reaction between EuO and ZrO₂, and its structural and magnetic properties have been investigated. Rietveld analysis of the X-ray diffraction pattern indicates that EuZrO₃ crystallizes in an orthorhombic perovskite structure. ¹⁵¹Eu Mössbauer effect measurement reveals that almost all the europium ions are present as the divalent state and occupy distorted sites with non-axial electric field gradients, in agreement with the orthorhombic structure. In contrast to previous reports, an antiferromagnetic transition was observed around 4.1 K. The magnetic structure below the Néel temperature has been discussed.