Brahim Elouadi

Observation of structural transitions and Jahn-Teller distortion in LaMnO3-doped BiFeO3 thin films

Structural and functional properties of multiferroic BiFeO3 are currently under debate. We utilized Raman spectroscopy to characterize the structural properties of pure and up to 10mol% LaMnO3-doped polycrystalline BiFeO3 thin films. All films were identified as phase-pure perovskite using x-ray diffraction (XRD). Although XRD has insufficient sensitivity for detailed thin film analysis, Raman experiments showed that pure BiFeO3 structure is best described as monoclinic, and that doping with LaMnO3 causes substantial modifications, namely, Jahn–Teller distortions to the (Mn,Fe)3+O6 octahedra accompanied by structural transition toward orthorhombic. Manganese substitution is shown to be the main cause of these changes.

Crystal structures of two allotropic forms of Na2CoP2O7

Na2CoP2O7 may be prepared in two allotropie forms:I, rose,Mr=278.85, triclinic, P1,a=9.735(2),b=10.940(3),c=12.289(4) Å,α=148.78,β=121.76(1), γ=68.38(2)°,V=566.8(2) Å3,Z=4,Dmeas=3.28(5) g cm−3,Dcalc=3.267 g cm−3, λ(MoKα)=0.71069 Å,μ=37.12 cm−1, F(000)=540,T=298 K,R=5.4% for 2911 observed reflections; andII, blue,Mr=278.85, orthorhombic, P21cn,a=7.713(2) Å,b=10.271(4),c=15.378(6)°,V=1218.2(8) Å3,Z=8,Dmeas=3.06(5) g cm−3,Dcalc=3.040 g cm−1, λ(MoKα) Å,μ=34.55 cm−1, F(000)=1080,T=298 K,R=9.9% for 1450 observed reflections. Cobalt displays octahedral coordination to six oxygen atoms in the rose form whereas the metal coordination is tetrahedral in the blue form.

The crystal structure of the phosphate eulytite Ba3Bi(PO4)3

Abstract Ba 3 Bi(PO 4 ) 3 crystallizes with the eulytite structure and the cubic space group I 4 3 d . X-ray crystallography has allowed us to determine the following crystal data: a = 10.5036(4) A, V = 1158.8 A 3 , Z = 4, D cal = 5.19 g/cm 3 , D exp = 5.02 g/cm 3 . The structure was solved on a single crystal, using 343 independent reflections, with R1 = 0.0379 (wR2 = 0.0752). It could be viewed as built up from independent [PO 4 ] 3– tetrahedra, lying along rods parallel to the axis and to [001]. The rods are arranged in a manner to delimit pairs of irregular pentagonal tunnels. In addition to two oxygen sites identified in the title structure, as previously reported for other compounds, two different sites were refined for barium and bismuth contrary to what has been reported in equivalent eulytite compounds.

Correlation between structure, dielectric, and ferroelectric properties in BiFeO3–LaMnO3 solid solution thin films

Chemical solution processed BiFeO3 thin films usually show high leakage currents and poor ferroelectric properties. The optimizations of processing conditions and doping with appropriate elements have been among the most promising strategies to enhance the overall properties of BiFeO3 thin films. However, detailed studies on the effects of doping elements on the structure and through it on the properties are still lacking. In this work we investigate the interplay between structure and dielectric-electric-ferroelectric properties of BiFeO3–LaMnO3 [Bi1−xLaxFe1−xMnxO3 (0≤x≤0.1)] solid solution thin films grown on Pt-terminated silicon substrates. The BiFeO3–LaMnO3 system is shown to undergo a structural transition from monoclinic to orthorhombic-tetragonal with various degrees of orthorhombic distortion. All LMO-containing films show a Debye-like dielectric relaxation with the activation energy of 0.90 eV. Both dielectric relaxation and leakage current mechanisms were interpreted in terms of ionized oxygen ...

A new ferroelectric phosphate family

A crystal chemical study carried out on phosphate single crystals with the formula AIBIIPO4 (AI=Na, K; BII=Ca, Zn, Sr, Cd, Ba, Pb) has permitted to follow the evolution of the crystal symmetry inside these series. Second harmonic generation (SHG) tests held on powder or small single crystals and hysteresis loops displayed on ceramic samples confirmed the existence of ferroelectricity in these material series.

Electrostrictive and relaxor ferroelectric behavior in BiAlO3-modified BaTiO3 lead-free ceramics

The crystal structure of (1−x)BaTiO3-xBiAlO3 (x = 0, 0.02, 0.05, 0.08, and 0.1) ceramics was determined using X-ray diffraction and Raman spectroscopy at room temperature, which revealed a phase transition from tetragonal to rhombohedral with increasing x. The dielectric properties were studied as a function of temperature at different frequencies, which indicated that the phase transition temperature (Tm) decreased with increasing x. The relaxor behavior was observed by frequency and temperature dependent dielectric permittivity. The Lorenz-type quadratic law was used to characterize the dielectric permittivity peaks near Tm of high-temperature slopes at 1 MHz. The temperatures Tm of dielectric permittivity peaks fit very well with the Vogel-Fulcher law in x = 0.05 and x = 0.1. The polarization hysteresis loops and electrostrictive were displayed at room temperature. The sample for x = 0.1 exhibits a slim loop with negligible hysteresis and a subtle linear feature, which is a promising transducer materia...

The crystal structure of the lacunar apatite NaPb4(PO4)3

Single crystals of NaPb4(PO4)3 were successfully grown from slowly cooled high-temperature solution. X-ray diffraction analysis has shown that the title compound crystallizes in the hexagonal system with apatite-like structure, space group P63/m, and lattice parameters a = b = 9.7249(8), c = 7.190(1) A, Z = 2. The structure was solved by the Patterson method, using 554 independent reflections. The final values of R and Rw are equal to 0.023 and 0.024, respectively. The structure is built up from regular independent [PO4]3− tetrahedra, arranged in a way to delimit cavities and hexagonal channels where sodium and lead cations are located, respectively. Two distinguished cationic positions were identified. The sixfold coordination site (6h position) is totally occupied by lead cation (Pb1), while the ninefold coordination mixed site (4f position) is statistically filled by both Pb2 (50%) and Na (50%). The mean values of the bond lengths of P–O, Pb–O and Na/Pb–O are equal to 1.545, 2.533, and 2.433 A, respectively.

Excited state absorption in Yb3+–Er3+-codoped phosphate glasses (ZnO–Al2O3–La2O3–P2O5) around the 4I13/2→4I15/2 emission spectral range

Abstract We are involved in Yb 3+ –Er 3+ -codoped phosphate glass research for eye-safe laser source near 1540 nm. Undoped and doped glasses of the composition: ZnO–Al 2 O 3 –La 2 O 3 –P 2 O 5 have been synthesised. Absorption from the 4 I 15/2 ground state, emission spectra and decay time measurements have been performed at room temperature on these materials. In addition to the usual spectroscopic characterisations, the excited state absorption 4 I 13/2 → 4 I 9/2 Er 3+ transition located at the long wavelength edge of the 4 I 13/2 → 4 I 15/2 emission transition have also been investigated at room temperature. The excited state absorption spectra were obtained with a pump and probe technique employing a double modulation scheme in the infrared wavelength region from 1300 to 1900 nm. Furthermore, the line strength of the excited state absorption transition have been calculated in order to compare it with the experimental.

NMR study of the cation distribution in the systems Li1+5xTa1-xO3 and Li1+xTa1-xTixO3

A /sup 7/Li NMR study of members of the solid-solution systems Li /sub 1+5x/ Ta /sub 1-x/ O/sub 3/ and Li /sub 1+x/ Ta /sub 1-x/ Ti /sub x/ O/sub 3/ has indicated that the excess Li/sup +/ ions occupy interstitial tetrahedral sites in the former. In the latter system, the Li/sup +/ ions appear to occupy interstitial tetrahedral sites for small values of x, but mostly octahedral sites for x > 0.1. Defect-cluster models are proposed that rationalize these findings as well as the evolution with x of the ferroelectric Curie temperature.

Variation of the ferroelectric curie temperature with the non-stoichiometry in LitaO3

Abstract Various ferroelectric solid solutions with LitaO3- related structure have been identified inside the ternary system Li2O-Ta2O5- (TiO2)2. The investigated phases are either stoichiometric or nonstoichiometric either through cationic excess or deficit with regard to LiTaO3. The greatest extension of the solid solution was obtained for Li1–xTa1–3xTi4xO3 in which the stoichiometry does not change. The ferroelectric-paraelectric transition temperature decreases when the composition deviates from LiTaO3.