A structural study of the perovskite series Na0.75Ln0.25Ti0.5Nb0.5O3 (original) (raw)
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A Structural Study of the Perovskite Series Na1/2+xLa1/2−3xTh2xTiO3
Journal of Solid State Chemistry, 1998
An X-ray powder diffraction study of the series Na 1/ 2؉x La 1/ 2؊3x Th 2x TiO 3 is presented. The series comprises orthorhombic CaTiO 3-type perovskites (Pnma, a+c+(2 a p , b+2 a p) in the range 04x40.083 and tetragonal La 1/2 Li 1/2 TiO 3-type perovskites (P4/mmm, a ؍ a p , c+2a p) in the range 0.1004x4 0.167. The structure of the orthorhombic members is derived from the cubic aristotype by octahedral rotation. The degree of octahedral rotation is relatively small (؍ 4.1-7.6°), compared to that of other CaTiO 3-type perovskites. The tetragonal members of the series exhibit partial ordering of the cations at the A-site: the 1a and 1b sites are preferentially occupied by Na and Th, respectively. This results in alternate layers of Na and Th atoms perpendicular to the c axis of the tetragonal cell. Ti atoms are displaced from the center of the TiO 6 polyhedra due to the strong electrostatic repulsion effect of Th and La at the A-site. In contrast to previous studies, the end-member Na 2/3 Th 1/3 TiO 3 is shown not to have a cubic cell.
Cheminform, 2007
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The A-site deficient ordered perovskite Th0.25□0.75NbO3: a re-investigation
Journal of Alloys and Compounds, 2000
The crystal structures of the non-stoichiometric perovskite Th h NbO prepared by quenching from a melt and by solid-state 0.25 0.75 3 reaction are determined to be identical on the basis of single crystal X-ray diffractometry using a CCD area detector (final R52.4%) and Rietveld refinement of the XRD powder pattern (R 57.1%), respectively. The overall tetragonal symmetry of the structure [P4 /mmm; Få 53.8956(1) and 3.8994(6) A, c57.8448(2) and 7.8502(11) A, Z52; where the smaller cell dimensions correspond to the powder data], 41 is derived from the cubic perovskite aristotype by long range ordering of Th along the c-axis into 1b sites. In response to this ordering, Nb atoms are displaced 0.08 A along the c-axis towards the layers with vacant A-sites (1a). The TEM data obtained in this study indicate that long-range ordering also resulted in the formation of antiphase domains oriented parallel to [100] and [010] , and having an average p p 41 periodicity (M) of 6a. In contrast to some previous studies of this compound, neither long-range ordering of Th within the layers, nor p tilting of NbO polyhedra was observed. This study presents a statistically superior refinement of the structure of Th h NbO 6 0.25 0.75 3.
Superstructure determination of the perovskite βLa0.33NbO3
Journal of Materials Science, 2000
New interesting results in the crystal structure of the perovskite ßLa0.33NbO3 were revealed using selected area electron diffraction, powder X-ray diffraction techniques and Rietveld refinement method. Although the superstructure of ßLa0.33NbO3 could not be seen by conventional X-ray powder diffraction technique, the electron diffraction patterns revealed weak spots resulting in a superstructure array for the atoms of ßLa0.33NbO3. The crystal
New Na 0.5 Bi 0.5 TiO 3 –NaTaO 3 -Based Perovskite Ceramics
Journal of the American Ceramic Society, 2007
X-ray diffraction analyses and scanning electron microscopy revealed that Na 0.5 Bi 0.5 TiO 3 (NBT) and NaTaO 3 (NTa) form solid solutions across the whole concentration range. With increasing NTa content the symmetry of the solid solutions gradually changed from rhombohedral, on the NBT-rich side, to orthorhombic, on the NTa-rich side. No morphotropic phase boundary was found between these phases. With increasing NTa content, the perovskite lattice parameter (a p) and the sintering temperature increase, whereas the grain size decreases. In the case of pure NTa, ceramics with a secondary phase were obtained, identified as Na 2 Ta 8 O 21 , which was formed during the sintering process. A study of the dielectric properties showed that with an increasing concentration of NTa, there was a reduction and broadening of the permittivity maximum, a reduction of the temperatures of the dielectric anomalies, and a reduction of the dielectric losses.
Slicing the Perovskite Structure with Crystallographic Shear Planes: The AnBnO3n-2 Homologous Series
Slicing the Perovskite Structure with Crystallographic Shear Planes: The AnBnO3n-2 Homologous Series
A new AnBnO3n-2 homologous series of anion-deficient perovskites has been evidenced by preparation of the members with n = 5 (Pb2.9Ba2.1Fe4TiO13) and n = 6 (Pb3.8Bi0.2Ba2Fe4.2Ti1.8O16) in a single phase form. The crystal structures of these compounds were determined using a combination of transmission electron microscopy and X-ray and neutron powder diffraction (S.G. Ammm, a = 5.74313(7), b = 3.98402(4), c = 26.8378(4)A° , RI = 0.035, RP = 0.042 for Pb2.9Ba2.1Fe4TiO13 and S.G. Imma, a = 5.7199(1), b = 3.97066(7), c = 32.5245(8)A° , RI = 0.032, RP = 0.037 for Pb3.8Bi0.2Ba2Fe4.2Ti1.8O16). The crystal structures of the AnBnO3n-2 homologues are formed by slicing the perovskite structure with (101)p crystallographic shear (CS) planes. The shear planes remove a layer of oxygen atoms and displace the perovskite blocks with respect to each other by the 1/2[110]p vector. The CS planes introduce edgesharing connections of the transition metal-oxygen polyhedra at the interface between the perovskite blocks. This results in intrinsically frustrated magnetic couplings between the perovskite blocks due to a competition of the exchange interactions between the edge- and the corner-sharing metal-oxygen polyhedra. Despite the magnetic frustration, neutron powder diffraction and M€ossbauer spectroscopy reveal that Pb2.9Ba2.1Fe4TiO13 and Pb3.8Bi0.2- Ba2Fe4.2Ti1.8O16 are antiferromagnetically ordered below TN = 407 and 343 K, respectively. The Pb2.9Ba2.1Fe4TiO13 and Pb3.8Bi0.2Ba2Fe4.2Ti1.8O16 compounds are in a paraelectric state in the 5-300 K temperature range.
Crystal structure and phase transitions of sodium potassium niobate perovskites
Solid State Sciences, 2009
This paper presents the crystal structure and the phase transitions of K x Na 1Àx NbO 3 (0.4 x 0.6). X-ray diffraction measurements were used to follow the change of the unit-cell parameters and the symmetry in the temperature range 100-800 K. At room temperature all the compositions exhibited a monoclinic metric of the unit cell with a small monoclinic distortion (90.32 b 90.34 ). No major change of symmetry was evidenced in the investigated compositional range, which should be characteristic of the morphotropic phase-boundary region. With increasing temperature, the samples underwent first-order monoclinic-tetragonal and tetragonal-cubic transitions. Only the potassium-rich phases were rhombohedral at 100 K.