Structural determination of a new phase of monoclinic BaTb2Mn2O7

A new phase of monoclinic BaTb2Mn2O7 with extra superlattice lines in addition to the usual orthorhombic X-ray diffraction patterns was prepared and a Rietveld refinement was applied to analyze the crystal structure using X-ray powder diffraction data at room temperature. The refined lattice parameters were a = 0.54918(8) nm, b = 2.0149(5) nm, c = 0.54922(9) nm, and beta = 90.62degrees with the space group A2/m (no. 12). The MnO6 oxygen octahedron has distortion and tilts around a-axis by about 7degrees each other in the reverse direction.

Phase behavior of BaLn(2)Mn(2)O(7) (Ln = rare earth) with a layered perovskite structure

Many phases appear in BaLn(2)Mn(2)O(7) family (Ln = rare earth) belonging to one of the Ruddlesden-Popper type compounds, depending upon the experimental conditions such as heating conditions when prepared and composition. Some of these phases were characterized by powder X-ray diffraction method using Rietveld analysis. These phases have only a little difference in crystal structure which has fundamentally K2NiF4 type structure, although the X-ray diffraction patterns are clearly different: a little deformation or tilting of the oxygen octahedron surrounding a central manganese ion composing the main frame of this structure induce these different diffraction patterns. Phase behavior of these compounds, mainly the detailed relation between various phases in BaTb2Mn2O7, was refined including the data of high temperature X-ray diffractometry.

Phase transition of BaNd2Mn2O7

Phase transition of BaNd2Mn2O7 from orthorhombic (space group Fmmm) to tetragonal phase (I4/mmm) was studied by high temperature powder X-ray diffractometry and Rietveld analysis. The transition temperature was identified at 523 K, which is almost the same transition temperature as the compounds with other rare earth ions in this BaLn(2)Mn(2)O(7) family (Ln=Sm and Eu) with Fmmm space group. During the transition an oxygen octahedron of each phase changes a little its form, in which four oxygen atoms perpendicular to C-axis make a rectangle and a square for orthorhombic and tetragonal phases, respectively. Manganese ion is not on the center of the quadrilateral consisting of these four oxygen ions, but a little apart from the center along c-axis in both phases.

Phase behavior of rare earth manganites

Among complex oxides containing rare earth and manganese BaLn(2)Mn(2)O(7)( Ln = rare earth) with the layered perovskite type and Ln(2)(Mn, M)O-7 with pyrochlore-related structure were studied since these compounds show many kinds of phases and unique phase transitions. In BaLn(2)Mn(2)O(7) there appear many phases, depending on the synthetic conditions for each rare earth. The tetragonal phase of so-called Ruddlesden-Popper type is the fundamental structure and many kinds of deformed modification of this structure are obtained. For BaEu2Mn2O7 at least five phases have been identified from the results of X-ray diffraction analysis with the space group P4(2)/mnm, Fmmm, Immm and A2/m in addition to the fundamental tetragonal I4/mmm phase. In the pyrochlore-related type compounds, Ln(2)Mn(2-x)M(x)O(7)(M = Ta, Nb, W etc), there also appear several phases With different crystal structures. With regard to every rare earth, Ln(2)MnTaO(7) phase is stable only for excess Ta and can be obtained under high oxygen partial pressure process. This group has trigonal structure with zirkelite type ( P3(1)21 space group).

Study on the phase behavior of BaEu2Mn2O7 through heat treatment of a single crystal

A new orthorhombic phase of BaEu2Mn2O7 with the space group of Ccmm (no.63) was identified for single crystals after heat treatment and its Crystal Structure was determined by single crystal X-ray diffractometry. The volume Of the unit cell has twice the fundamental tetragonal cell and corner-shared MnO6 octahedra are slightly distorted and Mn-O-Mn angle between the neighboring octahedra tilts with an angle by around 3 degrees from b-axis. It is concluded from the results of the heat treatment of single crystals at various temperatures that this orthorhombic phase changes into a tetragonal One With superstructure (P4(2)/mnm) at 402 K and changes once more into the fundamental tetragonal phase (I4/mmm) above 552 K. The tetragonal phase with superstructure which has been expected to be an unstable one is stable between the two temperatures.

Condensed phase combustion travelling waves with sequential exothermic or endothermic reactions

The one-dimensional propagation of a combustion wave through a premixed solid fuel for two-stage kinetics is studied. We re-examine the analysis of a single reaction travelling-wave and extend it to the case of two-stage reactions. We derive an expression for the travelling wave speed in the limit of large activation energy for both reactions. The analysis shows that when both reactions are exothermic, the wave structure is similar to the single reaction case. However, when the second reaction is endothermic, the wave structure can be significantly different from single reaction case. In particular, as might be expected, a travelling wave does not necessarily exist in this case. We establish conditions in the limiting large activation energy limit for the non-existence, and for monotonicity of the temperature profile in the travelling wave.

Speaker identification using higher order spectral phase features and their effectiveness vis-a-vis Mel-Cepstral features

The effectiveness of higher-order spectral (HOS) phase features in speaker recognition is investigated by comparison with Mel Cepstral features on the same speech data. HOS phase features retain phase information from the Fourier spectrum unlikeMel–frequency Cepstral coefficients (MFCC). Gaussian mixture models are constructed from Mel– Cepstral features and HOS features, respectively, for the same data from various speakers in the Switchboard telephone Speech Corpus. Feature clusters, model parameters and classification performance are analyzed. HOS phase features on their own provide a correct identification rate of about 97% on the chosen subset of the corpus. This is the same level of accuracy as provided by MFCCs. Cluster plots and model parameters are compared to show that HOS phase features can provide complementary information to better discriminate between speakers.