Jahn-Teller Distortions, Cation Ordering and Octahedral Tilting in Perovskites

In transition metal oxides, preferential occupation of specific d orbitals on the transition metal ion can lead to the development of a long-range ordered pattern of occupied orbitals. This phenomenon, referred to as orbital ordering, is usually observed indirectly from the cooperative Jahn-Teller distortions (CJTDs) that result as a consequence of the orbital ordering. This paper examines the interplay between orbital ordering, octahedral tilting and cation ordering in perovskites. Both ternary AMX3 perovskites containing an active Jahn-Teller (J-T) ion on the octahedral site and quaternary A2MM\u27X6 perovskites containing a J-T ion on one-half of the octahedral sites have been examined. In AMX3 perovskites, the tendency is for the occupied 3d3x2-r2 and 3d3z2-r2 orbitals to order in the ac plane, as exemplified by the crystal structures of LaMnO3 and KCuF3. This arrangement maintains a favorable coordination environment for the anion sites. In AMX3 perovskites, octahedral tilting tends to enhance the magnitude of the J-T distortions. In A2MM\u27X6 perovskites, the tendency is for the occupied 3d3z2-r2 orbitals to align parallel to the c axis. This pattern maintains a favorable coordination environment about the symmetric M\u27-cation site. The orbital ordering found in rock-salt ordered A2MM\u27X6 perovskites is compatible with octahedral rotations about the c axis (Glazer tilt system a0a0c-) but appears to be incompatible with GdFeO3-type octahedral tilting (tilt system a-b+a-)

Prediction of the Crystal Structures of Perovskites Using the Software Program SPuDS

The software program SPuDS has been developed to predict the crystal structures of perovskites, including those distorted by tilting of the octahedra. The user inputs the composition and SPuDS calculates the optimal structure in ten different Glazer tilt systems. This is performed by distorting the structure to minimize the global instability index, while maintaining rigid octahedra. The location of the A-site cation is chosen so as to maximize the symmetry of its coordination environment. In its current form SPuDS can handle up to four different A-site cations in the same structure, but only one octahedral ion. Structures predicted by SPuDS are compared with a number of previously determined structures to illustrate the accuracy of this approach. SPuDS is also used to examine the prospects for synthesizing new compounds in tilt systems with multiple A-site coordination geometries (a+a+a+, a0b+b+, a0b-c+)

Structure Prediction of Ordered and Disordered Multiple Octahedral Cation Perovskites using SPuDS

The software package SPuDS has previously been shown to accurately predict crystal structures of AMX3 and A1 - xA\u27xMX3 perovskites that have undergone octahedral tilting distortions. This paper describes the extension of this technique and its accuracy for A2MM\u27X6 ordered double perovskites with the aristotype Fm3̄m cubic structure, as well as those that have undergone octahedral tilting distortions. A survey of the literature shows that roughly 70% of all ordered double perovskites undergo octahedral tilting distortions. Of the 11 distinct types of octahedral tilting that can occur in ordered perovskites, five tilt systems account for ~97% of the reported structures. SPuDS can calculate structures for the five dominant tilt systems, Fm3̄m (a0a0a0), I4/m (a0a0c-), R3̄ (a-a-a-), I2/m (a0b-b-) and P21/n (a-a-b+), as well as two additional tilt systems, Pn3̄ (a+a+a+) and P4/mnc (a0a0c+). Comparison with reported crystal structures shows that SPuDS is quite accurate at predicting distortions driven by octahedral tilting. The favored modes of octahedral tilting in ordered double perovskites are compared and contrasted with those in AMX3 perovskites. Unit-cell pseudosymmetry in Sr- and Ca-containing double perovskites is also examined. Experimentally, Sr2MM\u27O6 compounds show a much stronger tendency toward pseudosymmetry than do Ca2MM\u27O6 compounds with similar tolerance factors

Structure Determination of A2M3+TaO6 and A2M3+NbO6 Ordered Perovskites: Octahedral Tilting and Pseudosymmetry

The room-temperature crystal structures of six A2M3+M5+O6 ordered perovskites have been determined from neutron and X-ray powder diffraction data. Ba2YNbO6 adopts the aristotype high-symmetry cubic structure (space group Fm3̄m, Z = 4). The symmetries of the remaining five compounds were lowered by octahedral tilting distortions. Out-of-phase rotations of the octahedra about the c axis were observed in Sr2CrTaO6 and Sr2GaTaO6, which lowers the symmetry to tetragonal (space group = I4/m, Z = 2, Glazer tilt system = a0a0c-). Octahedral tilting analogous to that seen in GdFeO3 occurs in Sr2ScNbO6, Ca2AlNbO6 and Ca2CrTaO6, which lowers the symmetry to monoclinic (space group P21/n, Z = 2, Glazer tilt system = a-a-c+). The Sr2MTaO6 (M = Cr, Ga, Sc) compounds have unit-cell dimensions that are highly pseudo-cubic. Ca2AlNbO6 and Ca2CrTaO6 have unit-cell dimensions that are strongly pseudo-orthorhombic. This high degree of pseudosymmetry complicates the space-group assignment and structure determination. The space-group symmetries, unit-cell dimensions and cation ordering characteristics of an additional 13 compositions, as determined from X-ray powder diffraction data, are also reported. An analysis of the crystal structures of 32 A2MTaO6 and A2MNbO6 perovskites shows that in general the octahedral tilt system strongly correlates with the tolerance factor

Spin-phonon coupling in Gd(Co1/2Mn1/2)O3 perovskite

We have investigated the temperature-dependent Raman-active phonons and the magnetic properties of Gd(Co1/2Mn1/2)O3 perovskite ceramics in the temperature range from 40 K to 300 K. The samples crystallized in an orthorhombic distorted simple perovskite, whose symmetry belongs to the Pnma space group. The data reveals spin-phonon coupling near the ferromagnetic transition occurring at around 120 K. The correlation of the Raman and magnetization data suggests that the structural order influences the magnitude of the spin-phonon coupling.Comment: 3 Figures, suplementary materia

Study of the volume and spin collapse in orthoferrite LuFeO_3 using LDA+U

Rare earth (R) orthoferrites RFeO_3 exhibit large volume transitions associated with a spin collapse. We present here ab initio calculations on LuFeO_3. We show that taking into account the strong correlation among the Fe-3d electrons is necessary. Indeed, with the LDA+U method in the Projector Augmented Wave (PAW), we are able to describe the isostructural phase transition at 50 GPa, as well as a volume discontinuity of 6.0% at the transition and the considerable reduction of the magnetic moment on the Fe ions. We further investigate the effect of the variation of U and J and find a linear dependence of the transition pressure on these parameters. We give an interpretation for the non-intuitive effect of J. This emphasizes the need for a correct determination of these parameters especially when the LDA+U is applied to systems (e.g in geophysical investigations) where the transition pressure is a priori unknown

Reactions of Oxygen Atoms with Van der Waals Complexes: The Effect of Complex Formation on the Internal Energy Distribution in the Products

Reactions of atomic oxygen with complexes containing HCl are investigated and the OH product state distributions are compared to those observed for the corresponding reactions of HCl monomers. In previous studies of reactions of O(3P) with HCl and hydrocarbon complexes, rotationally colder OH product state distributions were observed, when compared to the corresponding reactions of monomers. In contrast, we find that reactions of O(1D) with HCl clusters yield OH rotational distributions that are unaffected by the incorporation of HCl into a van der Waals complex. Quasiclassical trajectories are run on collisions of oxygen with HCl and Ar⋯HCl at 1 eV collision energies to investigate the differences in the dynamics of the O(1D) and O(3P) reactions. It is found that when the van der Waals complex is longer lived than the collision complex, rotational and vibrational cooling are observed. In contrast, when the dissociation of the van der Waals complex is prompt, compared to the collision complex lifetime, the effects of complex formation on the internal energy of the OH product become negligible

1,12-Diferrocenyldodecane at 100 K

1,12-Diferrocenyldodecane, [Fe2(C5H5)2(C22H32)], was synthesized from ferrocene and 1,12-dodecanedioyl chloride, followed by Clemmensen reduction. The single-crystal structure was determined at 100 K by X-ray diffraction and the spectroscopic and cyclic voltammetric data of 1,12-diferrocenyldodecane and its precursor are reported