Anti-phase Tilt Research Articles

Defect dipole induced improved electrocaloric effect in modified NBT-6BT lead-free ceramics

The Rietveld refinement of the polycrystalline powders of 1% Fe and Mn-doped (Na0.5Bi0.5)0.94Ba0.06Ti0.98V0.02O3 at the Ti-site confirmed a single rhombohedral (R3c) phase. The bandgap, (Eg) was affected by the anti-phase octahedral tilt angle and the spin-orbit splitting energy of Ti4+2p3/2 and Ti4+2p1/2 states. The decrease in Bi loss and increase in the binding energy of Ba due to Fe/Mn doping has been correlated to the strengthening of Bi-O and Ba-O bonds which was revealed from the XPS studies thereby further related to the average A-O bond length from structural studies. Hence, a reduction of oxygen vacancy (VO) for the doped samples has been justified. A significant improvement of the dielectric constant, relaxation time (τ0), and the decrease in conductivity due to doping was revealed from the frequency-dependent (10 Hz-1 MHz) dielectric measurement study. The conduction and relaxation process are dominated by the short-range movement of defects. The activation energy (Ea~1 eV) revealed that there is a presence of double-ionized VOs. The ECE study showed a significant enhancement of the changes in entropy, ΔS, and the adiabatic temperature difference, ΔT, due to doping, with ΔT being highest in the Fe-doped sample. Such improvement of dielectric and ECE properties was confirmed due to the reduction of the mobility of oxygen vacancy because of the formation(MnTi′′−Vo••)/(MnTi′−Vo••)• and FeTi′′−Vo••/(FeTi′−Vo••)• defect dipoles.

Large piezoelectricity in NaNbO3-based lead-free ceramics via tuning oxygen octahedral tilt.

The development of high-performance lead-free piezoceramics for replacing Pb-based perovskites has attracted lots of attention, and comparable room-temperature piezoresponse has been realized in (Na,K)NbO3 and BaTiO3-based ceramics with local structure heterogeneity via adjusting polymorphic phase boundary. In this work, a new method of oxygen octahedron tilt design is used in NaNbO3-based lead-free ceramics, which usually shows complex oxygen octahedron tilt information inherited from NaNbO3. The substitution of Ba(Fe0.5Nb0.5)O3 and BaTiO3 with high tolerance factor into NaNbO3 leads to a gradual elimination of anti-parallel cation displacement and anti-phase tilt along the three axes; as a result, a single tetragonal phase with P4bm space group and only in-phase tilt along c axis is achieved in 0.88NaNbO3-0.04Ba(Fe0.5Nb0.5)O3-0.08BaTiO3 ceramic. Accompanying the decreased oxygen octahedral tilt degree, a drastically improved d33 up to 367 pC N-1 (over ten times that of NaNbO3 ceramic) is obtained, reaching a record high in NaNbO3-based lead-free ceramics. Together with temperature insensitive piezoresponse originating from thermally stable oxygen octahedral tilt structure, the studied NaNbO3-based materials show large potential for replacing Pb-based ceramics in some electronic devices. The oxygen octahedron tilt engineering would be used for designing more high-performance lead-free ceramics with high piezoresponse and excellent thermal stability simultaneously.

Incommensurately modulated structures in Pb(Zr1−xSnx)O3 single crystals by x-ray diffraction

Pb(Zr1−xSnx)O3 single crystals were characterized using x-ray diffraction as a function of temperature and composition. The information on the structure of two intermediate phases, situated between antiferroelectric and paraelectric phases in the composition–temperature phase diagram, has been obtained. The lower-temperature intermediate phase is characterized by incommensurate displacive modulations in the Pb sublattice. The higher temperature intermediate phase is characterized by distortions of the oxygen sublattice primarily in the form of anti-phase tilts of the oxygen octahedra. The structured diffuse scattering was also observed in the paraelectric phase along Γ–M and M–R lines pointing to the high-temperature correlated disorder of Pb ions and octahedral rotations, respectively.

Local properties and phase transitions in Sn doped antiferroelectric PbHfO3 single crystal

Pb(Hf1−xSnx)O3 single crystals with x = 0.23 were characterized using single-crystal x-ray diffraction in the wide temperature range. The information on the structure of two intermediate phases, situated between low temperature antiferroelectric and high temperature paraelectric phases, has been obtained. The lower-temperature intermediate AFE2 phase is characterized by incommensurate displacive modulations in the Pb sublattice. The higher temperature intermediate IM phase is characterized by rotations of oxygen octahedra, primarily in the form of anti-phase tilts, which are also present in the lower-temperature AFE2 phase. For the same crystal, 119Sn Mossbauer effect in the temperature range from 300 K to 600 K has been used to study phase transitions mechanism. Two kinds of quadruple splitting have been found. It implies that two different environments of the central Sn ion exist. The observed two kinds of quadruple splitting do not disappear in the whole investigated temperature range which confirm that even far above TC the structure of paraelectric phase is locally non-centrosymmetric.

Temperature and pressure manipulation of magnetic ordering and phonon dynamics with phase transition in multiferroic GdFeO3 : Evidence from Raman scattering

We systematically investigate the detailed dynamics of the phonon and local structure of rare-earth orthoferrites ${\mathrm{GdFeO}}_{3}$ single crystal with temperature and pressure induced structural/magnetic phase transition by Raman spectroscopy. Phonon evolution related to the motion of octahedra reveals paramagnetic to antiferromagnetic ordering transition of ${\mathrm{Fe}}^{3+}$ ions at N\'eel temperature ${\phantom{\rule{4pt}{0ex}}T}_{N,Fe}$. By quantifying the polarized Raman spectra, especially the cross-polarized geometry, the lattice dynamics and distortion with local structure rearrangement during ferromagnetic transition has been also discovered. Particularly, we claim that the depolarization ratio could be quantified and used to precisely determine ferromagnetic phase transition of ${\mathrm{GdFeO}}_{3}$ and symmetry evolution simultaneously. Additionally, pressure dependence (up to 25.03 GPa) of collective phonon behavior indicates that antiphase tilt in ${\mathrm{FeO}}_{6}$ octahedra is more susceptible to the stress field than the in-phase one. The ${\mathrm{FeO}}_{6}$ octahedra presents better compressible than ${\mathrm{GdO}}_{12}$ dodecahedra in the ${\mathrm{GdFeO}}_{3}$ lattice with respect to pressure. This work has discovered the physical mechanism underlying variation of local structural symmetry, octahedra tilt, and phonon dynamics in ${\mathrm{GdFeO}}_{3}$, which can be regarded as the basic view for a series of ${\mathrm{GdFeO}}_{3}$-type perovskites and more R${\mathrm{FeO}}_{3}$ system.

Synthesis of the perovskite-type oxyfluoride AgTiO2F: an approach adopting the HSAB principle.

A synthetic approach for oxyfluorides involving the hard and soft acids and bases (HSAB) principle was examined. Using AgF composed of the Ag+ ion as a soft acid and F- as a hard base, we attempted to synthesize an oxyfluoride, AgTiO2F, that included the O2- ion as a softer base than the F- ion and the Ti4+ ion as a harder acid than the Ag+ ion. Consequently, a perovskite (Pv)-type oxyfluoride AgTiO2F was synthesized by a solid-state reaction under ambient pressure and under high pressure, and a heat treatment at 1000 °C for 30 min under a pressure of 4 GPa produced the Pv-type phase with minimal impurity phases. The lack of an optical second harmonic generation (SHG) response and the result of Rietveld refinement indicated that the compound is centro-symmetric and shows the anti-phase tilt of Ti(O, F)6 octahedra along the c-axis on the basis of the space group I4/mcm. O/F ordering on the anion site was not clarified. The phase stability of a Pv-type AgTiO2F is discussed in terms of the HSAB principle and the thermodynamics in the formation of the Pv-type phase by comparing KTiO2F and NaTiO2F. AgTiO2F has a light yellow color and a band gap energy of 2.8 eV in the visible region, which originates from the covalent bonding between the soft cation Ag+ and O2- as a softer anion than F-. We propose that the HSAB principle is useful for selecting a cation for the synthesis of oxyfluorides.

Improvement of energy storage properties with the reduction of depolarization temperature in lead-free (1 – x)Na0.5Bi0.5TiO3-xAgTaO3 ceramics

The effects of electric field and temperature on structural, dielectric, and ferroelectric properties of (1 – x)(Na0.50Bi0.50TiO3)-xAgTaO3 (x = 0, 0.03, 0.05, and 0.10) ceramics prepared via the modified sol-gel method were investigated. Rietveld refinement of synchrotron radiation x-ray diffraction data (SRPXRD) confirmed the rhombohedral (R3c) phase in all the unpoled samples. After poling, the samples remained in the rhombohedral phase for x ≤ 0.03, whereas for x ≥ 0.05, it showed a mixed rhombohedral and tetragonal (P4bm) phase. The anti-phase octahedral tilt angle was found to increase from 8.49° to 9.50° (for x = 0) and from 7.60° to 7.85° (for x = 0.10) with poling due to the long-range ordering phenomenon in the lattice system. The temperature-dependent dielectric study showed that the depolarization temperature decreases with increasing composition. Unpoled x = 0.10 composition exhibited the wide thermal stability dielectric constant in the temperature range 120-450 °C with 1795 ± 15% (tan δ < 0.041). Polarization versus electric field measurement revealed that at room temperature, the presence of anti-ferroelectric ordering increases the energy storage efficiency from 2.6% (for x = 0) to 48.2% (for x = 0.10). With increasing temperature, it increased from 48.2% (30 °C) to 85.5% (140 °C) for x = 0.10 composition. Improvement in the energy storage efficiency was correlated with structural changes probed by temperature dependent SRPXRD measurements; it confirmed the increase in antiferroelectric ordering with increasing temperature. Dielectric and ferroelectric results indicate the usefulness of this material system in the field of wide thermal stability dielectric constant and high-temperature energy storage applications.

Crystal Structure, Magnetic and Electrical Properties of Half-Doped Chromium Manganite La0.5Sr0.5Mn0.5Cr0.5O3

Polycrystalline sample La0.5Sr0.5Mn0.5Cr0.5O3 (LSMCO) was prepared by a conventional solid-state reaction method in the air, and its structure was determined by Rietveld refinement using consistent resolution time-of-flight X-ray diffraction. Different structural models have been used for analysis of X-ray diffraction data recorded at room temperature, but the best fits have been found using a mixture of rhombohedral and an orthorhombic structural phases (42(1)%/57(1)%). The rhombohedral, centro-symmetric space group R $$ \overline{3} $$ c $$ \left(\mathrm{No}.167,a\approx \sqrt{2}{a}_p,c\approx 2\sqrt{3}{a}_p,Z=6\right) $$ with octahedral tilting scheme a−a−a− leading to a three identical out-of-phase tilt angle of (Mn/Cr)-O6 octahedron along three perovskite main directions: x, y, and z-axes. The orthorhombic space group Pbnm $$ \left(\mathrm{No}.62,a\approx b\approx \sqrt{2}{a}_p,c\approx 2{a}_p,Z=4\right) $$ with octahedral tilting scheme a−a−c+, leads to an anti-phase tilts of the same magnitude about [100] and [010] directions of the elementary specimen cell and in-phase tilt of different magnitude about the [001] direction. At low temperature 10 K, Rietveld refinement of powder diffraction data revealed that the crystal structure of sample is distorted with a single orthorhombic symmetry (space group Pbnm). Magnetic measurements indicate that our investigated sample exhibits an overall anti-ferromagnetic behavior mainly attributed to the frustration induced by both competing AFM Cr3+-O-Cr3+, Mn4+-O-Mn4+ networks, and the mixed one Mn4+-O-Cr3+exchange magnetic interactions. The temperature dependence of the resistivity measured under zero field shows a semi-conducting behavior in the whole temperature range, which probably could be described by the adiabatic small polaron hoping model with relatively high polaron activation energy.

Modulated Structures in PbHfO<sub>3</sub> Crystals at High-Pressure-High-Temperature Conditions

Lead hafnate single crystals were characterized using single crystal x-ray diffraction under simultaneous application of hydrostatic pressure and high temperatures. The information on the structure of two intermediate phases, situated between antiferroelectric and paraelectric phases in the pressure-temperature phase diagram, has been obtained. The lower-temperature intermediate phase is characterized by incommensurate displacive modulations in Pb sublattice. The higher-temperature intermediate phase is characterized by oxygen framework distortion, primarily in the form of anti-phase tilts of the oxygen octahedra, which is also present in the lower-temperature intermediate phase.

Energetics of oxygen-octahedra rotations in perovskite oxides from first principles

We use first-principles methods to study oxygen-octahedra rotations in ABO3 perovskite oxides. We focus on the short-period, perfectly antiphase or in-phase, tilt patterns that characterize most compounds and control their physical (e.g., conductive, magnetic) properties. Based on an analytical form of the relevant potential energy surface, we discuss the conditions for the stability of polymorphs presenting different tilt patterns, and obtain numerical results for a collection of thirty-five representative materials. Our results reveal the mechanisms responsible for the frequent occurrence of a particular structure that combines antiphase and in-phase rotations, i.e., the orthorhombic Pbnm phase displayed by about half of all perovskite oxides and by many non-oxidic perovskites. The Pbnm phase benefits from the simultaneous occurrence of antiphase and in-phase tilt patterns that compete with each other, but not as strongly as to be mutually exclusive. We also find that secondary antipolar modes, involving the A cations, contribute to weaken the competition between different tilts and play a key role in their coexistence. Our results thus confirm and better explain previous observations for particular compounds. Interestingly, we also find that strain effects, which are known to be a major factor governing phase competition in related (e.g., ferroelectric) perovskite oxides, play no essential role as regards the relative stability of different rotational polymorphs. Further, we discuss why the Pbnm structure stops being the ground state in two opposite limits, for large and small A cations, showing that very different effects become relevant in each case. Our work thus provides a comprehensive discussion on these all-important and abundant materials, which will be useful to better understand existing compounds as well as to identify new strategies for materials engineering.

Soft antiphase tilt of oxygen octahedra in the hybrid improper multiferroic Ca3Mn1.9Ti0.1O7

We report a single crystal neutron and x-ray diffraction study of the hybrid improper multiferroic Ca3Mn1.9Ti0.1O7 (CMTO), a prototypical system where the electric polarization arises from the condensation of two lattice distortion modes.With increasing temperature (T ), the out-of-plane, antiphase tilt of MnO6 decreases in amplitude while the in-plane, in-phase rotation remains robust and experiences abrupt changes across the first-order structural transition. Application of hydrostatic pressure (P) to CMTO at room temperature shows a similar effect. The consistent behavior under both T and P reveals the softness of antiphase tilt and highlights the role of the partially occupied d orbital of the transition-metal ions in determining the stability of the octahedral distortion. Polarized neutron analysis indicates the symmetry-allowed canted ferromagnetic moment is less than the 0.04 {\mu}B/Mn site, despite a substantial out-of-plane tilt of the MnO6 octahedra.

A neutron diffuse scattering study of PbZrO3and Zr-rich PbZr1–xTixO3

A combined neutron diffuse scattering study and model analysis of the antiferroelectric crystal PbZrO3is described. Following on from earlier X-ray diffuse scattering studies, supporting evidence for disordering of oxygen octahedral tilts and Pb displacements is shown in the high-temperature cubic phase. Excess diffuse scattering intensity is found at theMandRpoints in the Brillouin zone. A shell-model molecular dynamics simulation closely reproduces the neutron diffuse scattering pattern. Both in-phase and antiphase tilts are found in the structural model, with in-phase tilts predominating. The transition between disordered and ordered structure is discussed and compared with that seen in Zr-rich PbZr1−xTixO3.

Compositional behavior of Raman-active phonons inPb(Zr1−xTix)O3ceramics

A systematic study of the Raman spectra of Pb(Zr1-xTix)O-3 (PZT) ceramics has been performed in a broad temperature interval (10-600 K) and a broad Ti/Zr concentration range around the morphotropic phase boundary (x = 0.25-0.70). The number of the spectral components was estimated by a standard fitting procedure with damped harmonic oscillators as well as by counting the number of peaks and shoulders with the help of a purposely designed mathematical analysis based on frequency derivatives of the Raman spectra. This last method proves to be very useful to study Raman spectra of disordered materials. For the case of PZT, the comparison with the Raman modes of PbTiO3 allows us to assign the phonon bands on both sides of the morphotropic phase boundary, and the crossover from the tetragonal to rhombohedral phase spectra is clearly visible. However, there are no indications of a systematic splitting of the E-symmetry modes into monoclinic A'-A '' doublets in the morphotropic samples. Detailed adjusting of the response function to the spectrum requires to assume additional Raman-active modes, but this holds for a much broader concentration range than that of the anticipated monoclinic phase. In particular, the lowest frequency transverse optic mode of E-symmetry (soft mode of the ferroelectric phase transition) is split into two components, a THz frequency anharmonic (central modelike) component and a resonant component (at omega similar to 80 cm(-1)), both related to the same normal coordinate. The additional Raman band appearing in this frequency range (omega similar to 65 cm(-1)) at low temperatures is rather associated with the antiphase tilt vibrations of the oxygen octahedra.

Thermoelastic and structural properties of ionically conducting cerate perovskites: (II) SrCeO3 between 1273 K and 1723 K.

The temperature dependence of the crystal structure and the thermoelastic properties of SrCeO(3) have been determined from Rietveld refinement of high resolution, neutron time-of-flight powder diffraction data collected in 5 K intervals between 1273 K and 1723 K. No evidence was found for critical behaviour in the amplitudes of the modes that soften in zone boundary phase transitions in perovskite-structured phases suggesting SrCeO(3) may remain orthorhombic, space group Pbnm from 1.2 K up to the 1 atm melting point of 2266 K. The temperature variation of the crystal structure has been determined from mode decomposition techniques and the structural evolution has been inferred from the temperature-dependences of the spontaneous shear strain and the order parameter associated with the anti-phase tilt. Thermoelastic properties have been derived from the temperature variation of the unit cell, isobaric heat capacity, and atomic displacement parameters and shows good agreement with earlier work carried out on the lightly doped system SrCe(0.95)Yb(0.05)O(ξ) (ξ∼ 3). Temperature-dependent corrections for the bond valence parameters for strontium and cerium are reported.

A high-resolution powder neutron diffraction study of the crystal structure of neighborite (NaMgF3) between 9 and 440 K

The temperature dependence of the unit-cell dimensions and the crystal structures of the fluorperovskite neighborite, NaMgF 3 (analyzed in the Pbnm setting of the space group), have been determined at 88 temperatures between 9 and 440 K from high-resolution, time-of-flight, powder neutron diffraction data. Lattice parameters exhibit saturation at low temperatures, before developing linear thermal expansion coefficients at temperatures above ~ 350 K. The temperature dependence of each axis has been analyzed, and fitted, using a two-term expression related to an Einstein internal energy function. The unit-cell parameters a and c behave in a conventional manner, however, an unexpected, and previously unobserved, region of negative linear thermal expansion has been found for the b axis in the temperature interval 20 ≤ T ≤ 90 K. Estimated, high-temperature axial thermal expansion coefficients derived from the lattice parameter fitting are in good agreement with those experimentally determined from an earlier synchrotron study, and indicate that high-temperature saturation has been achieved in neighborite by 440 K. The unit-cell volume varies smoothly and monotonically over the whole temperature interval, and the two-term Debye model of Barron has been successfully used to fit these data with characteristic temperatures of 369(2) and 1055(14) K. For the temperature interval 313.15-443.15 K, the thermodynamic Grüneisen constant has been determined using the Debye parameterization of the unit-cell volume, coupled with literature values of the isobaric molar heat capacity. The evolution of the crystal structure as a function of temperature is presented, and explained, in terms of the temperature dependence of the amplitudes of the seven symmetry-adapted basis-vectors of the aristotype phase that are consistent with the orthorhombic space group. The calculated temperature variations of the bond lengths are in excellent agreement with those experimentally determined. The primary order parameters for centrosymmetric, zone-boundary phase transitions in perovskite-structured compounds, i.e., the amplitudes of the basis vectors that transform as the irreducible representations R + 4 (anti-phase tilt) and M + 3 (in-phase tilt), have been fitted to a Landau free energy expansion that incorporates low-temperature saturation. Within the temperature range studied, the temperature dependence of the displacement corresponding to the anti-phase tilt is consistent with tricritical behavior. Experimental evidence is presented for a quadratic coupling of the in-phase tilt to the anti-phase tilt for temperatures greater than ~ 135 K, suggesting critical behavior at the orthorhombic-cubic transition is purely related to an instability at the R point of the pseudocubic Brillouin zone.

Transformation processes in relaxor ferroelectric PbSc0.5Ta0.5O3heavily doped with Nb and Sn

The effect of a third type of B-site cation on the temperature-and pressure-induced structural changes in ABO3-type relaxors has been studied by in-situ Raman scattering and X-ray diffraction on 0.72PbSc 0.5Ta0.5O3-0.28PbSc0.5Nb 0.5O3 (PSTN) and 0.78PbSc0.5Ta 0.5O3-0.22PbSnO3 (PSTS). The incorporation of Nb into the PST matrix is an isovalent substitution for Ta5+ on the ferroelectrically active B-site, whereas the incorporation of Sn4+ is a coupled aliovalent substitution for both types of B-cations in the host system. Both PSTN and PSTS exhibit the characteristic temperature T*, which for PSTS is shifted to slightly lower temperatures. The incorporation of Nb and Sn into the PST matrix smears the transformation processes near and below T* and suppresses the ferroelectric long-range order at low temperatures. Both PSTN and PSTS exhibit a continuous pressure-induced phase transition. The high-pressure phases have the same structural features as for pure PST: suppressed B-cation off-centre shifts, enhancement of coupled Pb-O ferroic species, and long-range order of anti-phase tilts of the BO6 octahedra. Nb-doping shifts the critical pressure pc from 1.9 GPa for pure PST to 2.5 GPa, whereas Sn-doping lowers pc to 1.3 GPa. The incorporation of both Nb and Sn decreases the degree of the overall pressure-induced structural distortion, with the effect of Sn being more pronounced than Nb. The dilution of the B-site cation system with Sn4+ results in local electric and elastic fields which reduce the coherence between nanoregions with anti-phase octahedral tilting and shifts the pressure at which long-range order of the tilts occurs to ∼ 4 GPa.

Dielectric, structural and Raman studies on(Na0.5Bi0.5TiO3)(1 − x)(BiCrO3)x ceramic

Dielectric, structural and Raman investigations were carried out on a perovskite-based solid solutionof (NBT)(1 − x)(BiCrO3)x (x = 0.00, 0.05, 0.10 and 0.15). The crystal structure is rhombohedral,R3c, for these compositions and the anti-phase(a − a − a − ) tilt angle decreaseswith increasing BiCrO3 content. The temperature and frequency dependent dielectricmeasurements show that the phase transition temperaturesTd andTR − T decrease,while Tm increases, almost linearly with an increase inBiCrO3 content. An anomalous increase in the relative dielectric permittivity is observed at higher temperature (T > Td) and higherBiCrO3 content. However, atlower temperature (T < Td) the dielectric permittivity decreases with an increase inBiCrO3 content. These effects are explained on the basis of the dynamics of oxygen defects produceddue to charge compensation. The defect related bands are observed in the Raman spectra of(NBT)(1 − x)(BiCrO3)x.

A powder neutron diffraction study of the crystal structure of the fluoroperovskite NaMgF3 (neighborite) from 300 to 3.6 K

The cell dimensions and crystal structures of the fluoroperovskite NaMgF3 (neighborite), synthesized by solid state methods, have been determined by powder neutron diffraction and Rietveld refinement over the temperature range 300–3.6 K using Pt metal as an internal standard for calibration of the neutron wavelength. These data show that Pbnm NaMgF3 does not undergo any phase transitions to structures of lower symmetry with decreasing temperature. The cell dimensions and atomic coordinates together with polyhedron volumes and distortion indices are given for Pbnm NaMgF3 at 25 K intervals from 300 to 3.6 K. Decreases in the a and c cell dimensions reach a saturation point at 50 K, whereas the b dimension becomes saturated at 150 K. The distortion of the structure of Pbnm NaMgF3 from the aristotype cubic $$ Pm\ifmmode\expandafter\bar\else\expandafter\=\fi{3}m $$ structure is described in terms of the tilting of the MgF6 octahedra according to the tilt scheme a − a − c + . With decreasing temperature the antiphase tilt (a −) increases from 14.24° to 15.39°, whereas the in-phase tilt (c + ) remains effectively constant at ∼10.7°. Changes in the tilt angles are insufficient to cause changes in the coordination sphere of Na that might induce a low temperature phase transition. The structure of Pbnm NaMgF3 is also described in terms of normal mode analysis and displacements of the condensed normal modes are compared with those of Pbnm KCaF3.

First-Principles Modeling of Lithium Ordering in the LLTO (LixLa2/3-x/3TiO3) Superionic Conductor

Ab initio periodic quantum-mechanical calculations were performed on selected ordered schemes of the La−Li−□ (vacancy) distribution in the Li0.5La0.5TiO3 and Li0.3125La0.5625□0.125TiO3 phases. Perovskite superstructures were built on the basis of Z (formula units/primitive cell) = 4 and 8 in the first case (symmetries C2me to Im), and Z = 16 in the second one (Pm), with initial configurations from the experimental P4/nbm tetragonal structure of Li0.3La0.567TiO3. The B3LYP functional (hybrid DFT/Hartree−Fock) was used, with an all-electron basis set of atomic orbitals (CRYSTAL06 code). For each ordered model, maps of the electrostatic potential were computed in the ionized unrelaxed superstructure, in order to study the field acting on Li+ ions. The complete structure was then fully relaxed by energy minimization with respect to all atomic positions (78 atoms/cell in the Z = 16 case). The anti-phase octahedral tilt was reproduced for A-type cages with mixed La−Li composition in the same (001) layer, but no...

Hole doping effects in Sr2FeMo1−xWxO6 (0 ≤ x ≤ 1) double perovskites: a neutron diffraction study

The effect of the localization of electrons as Mo is substituted by W in theSr2FeMo1−xWxO6 (0≤x≤1)series has been studied by neutron powder diffraction (NPD) and SQUID magnetometry. The samplesfor x = 0.2,0.5,0.8 and 1 were prepared by soft-chemistry procedures and annealed under suitable reducing conditionsfor each member of the series. As the number of itinerant electrons drastically changes fromSr2FeMoO6 (one electron performula unit) to Sr2FeWO6 (no itinerant electrons), this series constitutes an ideal system to explore band filling effectsin the magnetic structure and the Curie temperature of half-metallic ferromagnetic doubleperovskites. The room-temperature crystal structure of the former members of the series(x≤0.5) istetragonal (I4/m) and it is characterized by a single antiphase tilt of theFeO6 and(Mo,W)O6 octahedraalong the c axis; this structure evolves to the more distorted monoclinic(P 21/n)for x = 0.8 and 1, containing three kinds of non-equivalent oxygen atoms. The drivingforce of the structural phase transition is the promotion of the voluminousFe2+ cations upon W substitution, as demonstrated by a bond valence study. Thephase transition is accompanied by a sudden decrease of the distortion of theFeO6 and (Mo,W)O6 octahedra. Our results show that the progressive localization of carriers upon Wsubstitution provides a good description of the magnetic and structural properties alongthe series. The study of the low-temperature (10 K) NPD pattern of the heavily W-dopedSr2FeMo0.2W0.8O6 suggests a lack of long-range magnetic ordering, which is consistent with the presence ofisolated ferromagnetic clusters in the insulating, antiferromagnetic matrix created byFe–O–W–O–Fe superexchange interactions.