P. S. R. Krishna

Effect of powder synthesis procedure on calcium site occupancies in barium calcium titanate: A Rietveld analysis

(Ba,Ca)TiO3 samples with the ratio of (Ba+Ca) to Ti as unity but prepared by (i) a semiwet route involving (Ba,Ca)CO3 solid solution precursors and TiO2 and (ii) the conventional dry route involving BaCO3, CaCO3, and TiO2 powders, exhibit diffuse and BaTiO3‐type sharp ferroelectric phase transitions, respectively. Rietveld analysis of neutron powder diffraction data from the two types of samples shows that, whereas in the samples prepared by the semiwet route, Ca2+ exclusively occupies the Ba2+ site, nearly half of the substituted Ca2+ occupies the Ti4+ site in the case of samples prepared by the dry route. We show that Ca replacement at the Ba site leads to diffuseness whereas Ca introduction at the Ti site leads to sharpening of the transition. To our knowledge, this is the first clear demonstration of the powder synthesis procedure affecting the substitutional sites in multicomponent ceramics and this, in turn, influencing their physical behavior drastically.

Room temperature crystal structure and relaxor ferroelectric behavior of Pb0.5Ca0.5TiO3

Superlattice reflections observed for the first time in the room temperature powder neutron diffraction patterns of (Pb0.5Ca0.5)TiO3 (PCT50) are shown to arise due to an orthorhombic distortion (space group Pbnm) resulting from tilted TiO6 octahedra and off-center location of Pb2+/Ca2+. From a study of the temperature dependence of the real and imaginary parts of the dielectric constant at various frequencies, PCT50 is found to be a relaxor ferroelectric. It is proposed that the structural frustration introduced by the tilted octahedra may be responsible for the relaxor behavior.

Nanostructures of Sr2+ doped BiFeO3 multifunctional ceramics with tunable photoluminescence and magnetic properties

Careful tuning of formation (calcination) temperature of Sr(2+) doped BiFeO(3) multiferroic ceramics results in tailorable particle morphologies ranging from spherical to pillar-like. Based on the minimization of Gibbs free energy approach, the dominant homogeneous mechanism for particle growth is suggested. The chemical substitution of a trivalent ion (Bi(3+)) by a divalent ion (Sr(2+)) causes the transformation of certain fraction of Fe(3+) to Fe(4+) and/or the appearance of oxygen vacancies. This has been respectively proved by the analysis of XPS and refinement of neutron diffraction data. Although significant modification in the particle morphology is observed, the crystal unit cell remains rhombohedral with a R3c space group but interesting variations in physical properties are achieved. O-vacancies induced strong and sharp photoluminescence activity in the IR region, similar to ZnO, is reported for the first time. This observation opens up a new application for multiferroic ceramics. SQUID M-H data confirms the straightening of the canted spin structure of BiFeO(3), which in turn results in magnetism similar to ferromagnetic materials. Findings of the magneto-dielectric effect are also discussed to claim the multiferroic nature of the sample.

Simultaneous changes of nuclear and magnetic structures across the morphotropic phase boundary in (1−x)BiFeO3−xPbTiO3

We present here the results of a powder neutron diffraction study on the (1−x)BiFeO3–xPbTiO3 (BF-xPT) system, which reveal an abrupt change in magnetic structure from a noncollinear antiferromagnetic one to a collinear G-type antiferromagnetic as the nuclear structure of the ferroelectric phase changes from monoclinic in Cc space group for x≤0.27 to tetragonal in P4mm space group for x≥0.31 across the morphotropic phase boundary (MPB). This reflects the multiferroic character of the MPB in the BF-xPT system.

Neutron diffraction study of ferroelectric Ba0.88Ca0.12TiO3

Abstract Profile analysis of neutron diffraction data on Ba 0.88 Ca 0.12 TiO 3 samples shows that Ca can enter either Ba site exclusively or both Ba and Ti sites depending on the method of preparation. Nature of structural phase transition has been examined in samples where Ca occupies Ba site only.

Neutron diffraction studies of GexSe1-x glasses

Abstract Neutron diffraction studies were performed on Ge x Se 1− x glasses for x = 0.1,0.2, 0.33 and 0.4. The structure factor S ( Q ) shows maximum intermediate-range order for x = 0.33. Analysis of the two main peaks in T ( r ) shows that these glasses have Ge(Se 1/2 ) 4 tetrahedra. Glasses with x ⩽ 0.2 consist of Se-chains cross-linked with Ge-tetrahedra while for x ⩾ 0.2 Ge-tetrahedra are present in both edge- and corner-shared configurations.

Evidence for a New Non-Ferroelectric Phase Transition in (Pb1−xCax)TiO3 Ceramics for 0.60 ≤ x ≤ 0.90

Results of low temperature dielectric measurements on the mixed (Pb 1−x Ca x )TiO 3 system are presented to show that the strong dielectric anomaly observed for compositions with x < 0.60 is replaced by a very weak dielectric anomaly for x ≥ 0.60. The temperature dependence of the unit cell parameters, as obtained by Rietveld analysis of the neutron powder diffraction data, shows change of slope at the dielectric anomaly temperature. The observed dielectric peak for x ≥ 0.60 is shown to be due to a thermodynamic phase transition and not due to a relaxor or dipole glass type transition. Absence of hysteresis loop below the transition temperature reveals that this new phase transition is of non-ferroelectric type. Curie-Weiss fit to the dielectric constant gives negative Curie temperature which points towards an antiferroelectric transition.

Magnetic structure of disordered Zn0.5Co0.5Fe2−yCryO4

The disordered spinel Zn 0.5 Co 0.5 Fe 2-y Cr y O 4 (y = 0.9 and 1.1) is studied using neutron diffraction and magnetic susceptibility techniques. Comparison with earlier studies on Zn 1-x Co x FeCrO 4 shows that the effect of a slight increase in magnetic Cr-ion concentration on the B site is similar to that produced by an increase of diamagnetic Zn-ion concentration at the A site

High Pressure Phases and Amorphization of a Negative Thermal Expansion Compound TaVO5

Negative thermal expansion material TaVO5 is recently reported to have pressure induced structural phase transition and irreversible amorphization at 0.2 and above 8 GPa, respectively. Here, we have investigated the high pressure phase of TaVO5 using in situ neutron diffraction studies. The first high pressure phase is identified to be monoclinic P21/ c phase, same as the low temperature phase of TaVO5. On heating, amorphous TaVO5 transformed to a new crystalline phase, which showed signatures of higher coordination of vanadium indicating pressure induced amorphization (PIA). PIA observed in TaVO5 might be due to the kinetic hindrance of pressure induced decomposition (PID) into a compound with higher coordination of vanadium. Mechanism of PIA observed in TaVO5 is investigated by carrying out ex situ Raman, XRD, XPS, and XAS measurements. We have also proposed a pressure-temperature phase diagram of TaVO5 qualitatively delineating the phase boundaries between the ambient orthorhombic, monoclinic, and amorphous phases.

Revealing magnetic ordering and spin-phonon coupling in Y1-x Tbx MnO3 (0.1 .LEQ. x .LEQ. 0.3) compounds

The structural and magnetic properties of the Y1-x Tb x MnO3 (0.1u2009u2009⩽u2009u2009xu2009u2009⩽u2009u20090.3) compounds were investigated. Neutron diffraction patterns for all three samples, recorded at room temperature (RT), were fitted to the nuclear structure confirming the paramagnetic nature of the compounds. At 2.8u2009K, for the xu2009u2009=u2009u20090.1 sample magnetic moments of the Tb3+ ionic as well as Mn3+ ionic were ordered. At 5u2009K for the xu2009u2009=u2009u20090.2 sample only the Mn3+ ionic magnetic moments were ordered. There were six sites for Mn atoms. Three on the zu2009u2009=u2009u20090 plane and three on the zu2009u2009=u2009u20090.5 plane (where z corresponds tou2009u2009+c axis).The Mn3+ionic moments were confined to the a-b plane with a net magnitude of 2.78(3) µ B, and 2.90(3) µ B for the xu2009u2009=u2009u20090.1 and the xu2009u2009=u2009u20090.2 samples. The Tb3+ionic moments had a magnitude of 1.36(4) µ B at 2.8u2009K and were aligned antiferromagnetically along the crystallographic c-axis for the xu2009u2009=u2009u20090.1 sample. The low moment in comparison with Mn3+ free ions has been attributed to crystalline electric fields similar to that found in the parent compound YMnO3 and also in another rare earth manganite viz HoMnO3. The xu2009u2009=u2009u20090.3 sample was found to be a canonical spin glass. To investigate the role of the above spin ordering in Y1-x Tb x MnO3 in governing the phonon dynamics, temperature dependent Raman measurements were carried out. We observed the deviation of the phonon frequency near 685u2009cm-1 and its line-width from the expected anharmonic behaviour around magnetic ordering temperature for Tb substituted compounds with xu2009u2009=u2009u20090.1 and 0.2. This was attributed to the spin-phonon coupling in these systems. The anomalous behaviour of this phonon mode in the canonical spin glass compound with xu2009u2009=u2009u20090.3, indicated that the coupling sustained even in the presence of only local magnetic ordering.