Orbital structures in spinel vanadatesA 2 4 A…
Yoichi NiiHajime SagayamaT. ArimaShinobu AoyagiR. SakaiShintaro MakiEiji NishiboriHiroshi SawaKunihisa SugimotoH. OhsumiMasaki Takata
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Abstract:
Spinel FeV${}_{2}$O${}_{4}$ exhibits successive structural phase transitions, reflecting the interplay between the Fe${}^{2+}$ (3${d}^{6}$) and V${}^{3+}$ (3${d}^{2}$) ions, both of which have orbital and spin degrees of freedom. The temperature-dependent orbital shapes of Fe${}^{2+}$ and V${}^{3+}$ were investigated by means of single-crystal structure analysis, and were compared with those in MnV${}_{2}$O${}_{4}$, where only the V${}^{3+}$ ions are Jahn-Teller active. The highest-temperature transition from the cubic to the high-temperature tetragonal phase was driven by a ferroic Fe${}^{2+}$ 3${z}^{2}$-${r}^{2}$ orbital order (OO). At 110 K, where the ferrimagnetic transition takes place, the magnetic order modified the orbital shape through intratomic spin-orbit coupling, causing an orthorhombic distortion. The V${}^{3+}$ orbital order (V-OO) contributed to the lowest temperature transition from the orthorhombic to the low-temperature tetragonal phase. The V-OO in FeV${}_{2}$O${}_{4}$ was qualitatively different from that in MnV${}_{2}$O${}_{4}$. We propose that ferro-OO contains a complex orbital in FeV${}_{2}$O${}_{4}$ in contrast to the V-OO of real orbitals observed in MnV${}_{2}$O${}_{4}$.Keywords:
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We report a correlation between structural phase stability and magnetic properties of Co2FeO4 spinel oxide. We employed mechanical alloying and subsequent annealing to obtain the desired samples. The particle size of the samples changes from 25 nm to 45 nm. The structural phase separation of samples, except sample annealed at 9000C, into Co rich and Fe rich spinel phase has been examined from XRD spectrum, SEM picture, along with EDAX spectrum, and magnetic measurements. The present study indicated the ferrimagnetic character of Co2FeO4, irrespective of structural phase stability. The observation of mixed ferrimagnetic phases, associated with two Curie temperatures at TC1 and TC2 (>TC1), respectively, provides the additional support of the splitting of single cubic spinel phase in Co2FeO4 spinel oxide.
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KNbO3 (KN) nanowires were synthesized using various process conditions and their structures and morphologies were investigated. Homogeneous KN nanowires were formed in specimens synthesized at 130 °C for 24.0–48.0 h. These KN nanowires have a tetragonal structure that is known to be stable at high temperatures in the range of 225–435 °C. Tetragonal KN nanowires changed to orthorhombic KN nanoplates when the process time increased and homogeneous orthorhombic KN nanoplates existed for specimens synthesized for 144.0 h. In addition, tetragonal and orthorhombic structures coexisted in KN nanoplates synthesized at 130 °C for 72.0 h. For specimens synthesized at 100 °C, a long process time of 144.0 h was required to develop homogeneous KN nanowires that were also considered to have both tetragonal and orthorhombic structures. On the other hand, for specimens synthesized at 150 °C for 8.0 h, KN nanowires and a cube-shaped KN phase coexisted. Furthermore, a K4Nb6O17 second phase was formed in specimens synthesized for short periods of time (<8.0 h), indicating that the formation of homogeneous KN nanowires is difficult at 150 °C. Therefore, homogeneous KN nanowires with a tetragonal structure can be obtained at a low temperature of 130 °C with a short process time in the range of 24.0–48.0 h.
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Ba3.75Nd9.5Ti18O54 is known to be an orthorhombic phase built of chains of octahedra parallel to the z axis with pentagonal, tetragonal and triangular tunnels between them. It is shown here that this phase is a representative of a family of orthorhombic phases related to the tetragonal tungsten bronzes (TTB). It is assumed that some of these new phases can be ferroelectric. Crystal chemical aspects of these phases are discussed.
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The orthorhombic to tetragonal phase transition in (Ba 0.92 Ca 0.08 )(Zr 0.05 Ti 0.95 )O 3 was investigated using high-temperature X-ray diffraction between 260 and 333 K. The results established the presence of tetragonal ( P 4 mm ) and orthorhombic ( Amm 2) phase co-existence in the temperature range of 293 ≤ T ≤ 313 K. The tetragonal phase was found to increase from 27% at 293 K to 76% at 313 K. The structural refinement and line-profile analysis ruled out the presence of an intermediate monoclinic structure during P 4 mm → Amm 2 crossover. The analysis shows a pure orthorhombic ( Amm 2) structure for T < 293 K and tetragonal for T > 313 K.
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One-dimensional (1D) ferrimagnetism has recently been reported in ordered bimetallic compounds. Actually, such a behavior is not exclusive of this type of system. Ferrimagnetism can also occur in homometallic chains formed by alternating sites, or by discrete species containing an odd number of interacting metal ions. We report in this paper several examples of bimetallic or homometallic chains featuring 1D ferrimagnetism. We will also discuss the conditions for the occurrence of a long-range ferrimagnetic ordering at low temperatures.
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Complicated, alternating ferromagnetic/antiferromagnetic interactions in homometallic, one-dimensional MnII-azido systems permit an unprecedented ferrimagnetic response. One new kind of ferrimagnet with a surprising ferrimagnetic behavior in a system with ground state ST = 0 (see picture) was observed and related to the structural parameters.
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CBN crystals show a one- and a two-dimensionally modulated modification. The former is isotypic with orthorhombic Ba 4 Na 2 Nb 10 O 30 and the latter with the tetragonal tungsten bronze type of crystal structure. The orthorhombic form irreversibly transforms to the tetragonal polymorph at the ferroelectric phase transition near 603 K. Orthorhombic and tetragonal CBN24 slightly differ in the distribution of the Ba and Ca atoms over the incompletely filled Me1 and Me2 sites. The tetragonal symmetry is further broken in orthorhombic CBN24 by different amplitudes of the positional modulations of O atoms which are symmetrically equivalent in the TTB structure. A similar orthorhombic phase of CBN31 could be obtained by quenching from 1473 K.
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