[en] A spread of compositions within the Nb^V_1-xNb^IV_xO_2-xF_1+x solid solution have been successfully synthesized in the range 0≤x≤0.48. In each case electron diffraction has revealed the presence of strong planar diffuse scattering perpendicular to each of the three <001> directions at the G±< hkq>^* regions of reciprocal space, where G represents a Bragg reflection of the ReO₃ type average structure, h and k are continuously variable and q is a fixed (at any one particular composition), composition-dependent parameter in the range from 1/3 to 1/2. Careful electron probe microanalysis indicates that the value of q is directly related to composition via the equation q=(x+1)/3 and hence acts as a 'chemical ruler' for the composition. A plausible model for the implied one-dimensional O/F ordering as well as for the observed shift in the position of the diffuse versus composition is provided via a crenel type occupational atomic modulation function

[en] A careful phase analysis study of cubic perovskite-related phases in the ternary SrO-CuO-Nb₂O₅ system has been carried out via the synthesis and compositional analysis of a range of specimens within the ternary SrO-CuO-Nb₂O₅ system. Powder XRD in conjunction with electron probe microanalysis (EPMA) has been used to determine whether the synthesized specimens are single phase or not and to determine the compositions of the various reaction products. Three quite distinct such solid solution phases have been found and their quite distinct electrical and diffraction characteristics investigated

[en] The Ni₆Se_5-xTe_x, 0< x<∼1.7, system has been carefully investigated via electron diffraction and TEM imaging. They reveal a somewhat disordered modulated superstructure phase arising from Ni ion ordering within an essentially well-defined chalcogen sub-structure. As x, and the underlying parent substructure cell dimensions increase, the incommensurate primary modulation wave-vector q characteristic of this Ni ion ordering quickly swings from close to ((1)/(2)) a^* for x=0 towards ((1)/(2))[101]^* for x≥0.5. A lock-in to ((1)/(2))[101]^* would formally transform the underlying parent Bmmb (a_p, b_p, c_p) structure into a P1a1 (a_s=2a_p, b_s=b_p, c_s=-((1)/(2))a_p+((1)/(2))c_p) superstructure phase

[en] Na₃MoO₃F₃, a member of the A₂BM^VIO₃F₃ family of elpasolite-related oxyfluorides, has been prepared by the reaction of NaF with MoO₃ at 650 deg. C. It is shown by a combined X-ray, electron and neutron diffraction study, that the true symmetry of Na₃MoO₃F₃ is not monoclinic (pseudo-orthorhombic) as previously reported but instead triclinic (metrically rhombohedral) P1. The superstructure unit cell is given by a=-a_R+b_R, b=c_R, c=(a_R+b_R+c_R), when expressed with respect to the underlying rhombohedral parent structure. Neutron diffraction refinement of the rhombohedral sub-structure shows O and F to be fully ordered with alternate (111)_R planes being occupied by O and then F. The nature of the distortion away from the high temperature Fm3-barm parent structure is consistent with a combination of phiphiphi type rotations of the octahedral framework together with ppp type displacements of the octahedral cations, which is not observed in any of the closely related A₂BM^VIO₃F₃ family members. Bond valence arguments are used to give a plausible explanation for this difference in behavior

[en] The composition range and (composite modulated) structure of compounds within the wide range non-stoichiometric LaSb₂Sn_x, 0.1=< x=<0.75, solid solution is carefully investigated via a combined electron diffraction, XRD and electron probe microanalysis study. Evidence for metastability of the LaSb₂Sn_x phase at the low x composition end of the solid solution is presented. Direct evidence is found for a reasonably (although by no means perfectly) well ordered Sn sub-structure which is, in general, mutually incommensurable with respect to a very well ordered underlying LaSb₂ sub-structure along both a and c directions. The overall (3+2)-d superspace group symmetry is given along with a discussion of the consequences as regards the arrangement of the Sn atoms. The Sn sub-structure c-axis cell dimension shows very little variation with composition x providing direct experimental evidence of the importance of Sn-Sn metallic bonding (along one-dimensional [001] Sn strings) for the stability of the phase

[en] The average, as well as the cation and anion 'disordered', crystal structure of Li₄NbO₄F has been carefully investigated via coupled neutron and X-ray powder diffraction studies as well as via electron diffraction studies. The existence of a spectacular highly structured diffuse intensity distribution in the latter provides strong evidence for coupled Li¹⁺/Nb⁵⁺ and O^2-/F^- ordering on the Na and Cl sites of the average NaCl structure of Li₄NbO₄F. Bond valence sum calculations have been used to investigate local crystal chemistry as well as to suggest plausible local crystal chemical constraints while ab initio DFT based theoretical calculations of a 2x2x2 supercell have been carried out in order to provide additional insight into the local crystal chemistry of this compound. - Graphical abstract: An <001> zone axis EDP typical of Li₄NbO₄F.

[en] The disordered structures and low temperature dielectric relaxation properties of Bi_1.667Mg_0.70Nb_1.52O₇ (BMN) and Bi_1.67Ni_0.75Nb_1.50O₇ (BNN) misplaced-displacive cubic pyrochlores found in the Bi₂O₃-M^IIO-Nb₂O₅ (M=Mg, Ni) systems are reported. As for other recently reported Bi-pyrochlores, the metal ion vacancies are found to be confined to the pyrochlore A site. The B₂O₆ octahedral sub-structure is found to be fully occupied and well-ordered. Considerable displacive disorder, however, is found associated with the O'A₂ tetrahedral sub-structure in both cases. The A-site ions were displaced from Wyckoff position 16d (1/2 , 1/2 , 1/2 ) to 96 h (1/2 , 1/2 -ε_A, 1/2 +ε_A) while the O' oxygen was shifted from position 8b (3/8 , 3/8 , 3/8 ) to Wyckoff position 32e (3/8 +ε_o^', 3/8 +ε_o^', 3/8 +ε_o^'). The refined displacement magnitudes off the 16d and 8b sites for the A and O' sites were 0.408 A/0.423 A and 0.350 A/0.369 A for BMN/BNN, respectively. - Graphical abstract: The final displacively disordered average structure of Bi_1.667Mg_0.70Nb_1.52O₇ (BMN) projected along a close to <110> direction

[en] The binary pnictide 'SnSb' has been re-investigated using a combination of X-ray, synchrotron and electron diffraction as well as electron microprobe analysis. Its structure was found to be incommensurately modulated with an underlying rhombohedral parent structure of space group symmetry R3-bar m (No. 166), unit cell parameters a_h=b_h=4.3251(4)A, c_h=5.3376(6)A in the hexagonal setting. The incommensurate primary modulation wave vector q_h=1.3109(9)c_h^* and the superspace group symmetry is R3-bar m (0, 0, ∼1.311) (No. 166.1). The refinement of the incommensurate structure indicates that the satellite reflections arise from displacive shifts of presumably essentially pure Sn and Sb layers along the hexagonal c-axis, with increasing distance between the Sn-layers and decreasing distance between the Sb layers

[en] This paper considers stacking fault disorder in Ba₃M₁Nb₂O₉ (M=Mn²⁺ or Co²⁺) 1:2 triple perovskites and how best to model it when refining neutron powder diffraction data. The true local structures of both samples were successfully refined using a coherent stacking fault disordered model. The stacking fault disorder in Ba₃MnNb₂O₉ and Ba₃CoNb₂O₉ affects their dielectric properties

[en] A careful investigation of the previously reported single phase, pyrochlore structure type Y₂(Zr_yTi_1-y)₂O₇, 0=< y=<0.9 (YZT) solid solution has been carried out. Given the known slow rate of diffusion of cations in fluorite-related stabilized zirconia systems and the consequent difficulty in achieving equilibrium, careful attention was paid to synthesis procedures while Guinier XRD and electron diffraction were used to investigate the synthesized materials. As a consequence, a subtle but nonetheless clear two-phase region separating a pyrochlore-type solid solution field (from 0=< y=<3∼0.54) from a 'defect fluorite' type solid solution field (from ∼0.68=< y=<1) has been found. The underlying crystal chemistry of the system has been investigated using the bond valence sum approach. The dielectric properties have also been measured as a function of composition. The dielectric constant and dielectric loss of the lowest y compounds are potentially quite useful but deteriorate rapidly with increasing Zr content