[en] Reinterpretation of the neutron diffraction study of 2H-TaSe₂ by Moncton, Axe and DiSalvo (1977) reveals an ambiguity in the sense of the displacements proposed for the commensurate superlattice structure. An attempt is made to resolve this ambiguity by electrostatic and short-range energy calculations of the phase dependence of the energy of the periodic structural distortion wave. There is a fine balance between Se-Se short range repulsion, Ta-Se electrostatic and short-range repulsion and the CDW-Ta ion interaction energy terms. The analysis reveals the phase dependence of the various terms and allows the different contributions to the stability of the distortion waves to be discussed more completely than previously

[en] Re-investigation of published neutron scattering data on 2H-TaSe₂ shows that published periodic structural distortion phase angles are incorrect and thus so are the accepted atomic displacements. Furthermore an ambiguity of +-π leaves the sense of the atomic displacements undetermined. This leads to a new multi-domain model for the incommensurate superlattice involving contributions from both the preferred phase PHI and PHI+π. This +-π ambiguity is not finally resolved until the incommensurate/commensurate transition

[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 low-temperature electron diffraction study has been carried out on ThAsSe to search for evidence of structural disorder associated with the low-temperature non-magnetic Kondo effect. A highly structured and extremely complex characteristic diffuse intensity distribution has been observed at low temperature and interpreted in terms of a gradual charge density wave type phase transition upon lowering of temperature involving disordered As-As dimerization within (001) planes. Plausible models of the proposed As-As dimerization have been obtained using a group theoretical approach

[en] Tl₃MoO₃F₃, a previously unreported member of the A₂BM^VIO₃F₃ family of elpasolite-related oxyfluorides, has been prepared by the reaction of TlF with MoO₃ at 655 deg. C. DSC shows two major polymorphic phase transitions at 42 deg. C and 130 deg. C, respectively. Electron diffraction and XRD studies of the complex room temperature polymorphic form of this material indicate that the spacegroup symmetry is monoclinic P1a1 with a superstructure unit cell given by a=3a_p+c_p, b=3b_p, c=((1)/(2))(-a_p+3c_p), when expressed with respect to the underlying ideal elpasolite-type parent structure. This superstructure, while related, is not isomorphous to that recently reported for K₃MoO₃F₃. The existence of a shared subset of strong G_p±J/5[204]_p^* type satellite reflections suggests the existence of a common intermediate superstructure. A highly structured, three-dimensional continuous diffuse intensity distribution is observed in Tl₃MoO₃F₃ and Rb₂KMoO₃F₃. This suggests that a particular pattern of local O/F ordering and associated Mo ion shifts, recently shown to be responsible for the existence of this diffuse distribution in the case of K₃MoO₃F₃, may be common to the entire family of elpasolite-related A₂BM^VIO₃F₃ compounds

[en] The perovskite-based rare earth cobaltates (Ln_0.33Sr_0.67CoO_3-δ) (Ln=Y³⁺, Ho³⁺ and Dy³⁺) have been synthesized at 1100 deg. C under 1 atm oxygen. A thermogravimetric study has determined the overall oxygen content in each case while a combined electron diffraction (ED) and synchrotron X-ray diffraction study has revealed the presence of a complex, previously unreported, perovskite-related superstructure phase. ED gave a resultant C1c1 but most probably Cmcm (a=2a_p-2c_p, b=4b_p, c=2a_p+2c_p) perovskite-related superstructure, describable as a modulated I4/mmm intermediate parent structure. Synchrotron X-ray data has been used to refine the intermediate parent structures of all three compounds. Coupled Ln/Sr and O/vacancy ordering and associated structural relaxation is shown to be responsible for the observed superstructure

[en] Three previously reported oxidized cerium niobate phases CeNbO_4+x (x = 0.08, 0.25, and 0.33) have been synthesized and characterized by X-ray powder and electron diffraction. All three phases display structures which are modulated variants of a parent fergusonite-type Ce^IIINbO₄ structure (I2/a, a = 5.5342(2) angstrom, b = 11.4016(6) angstrom, c = 5.1583(3) angstrom, β = 94.600(5)degree). The x = 0.08 phase with parent unit cell (I2/a, a = 5.3029(8) angstrom, b = 11.483(2) angstrom, c = 5.2515(8) angstrom, β = 91.32(2)degree) is a two-dimensional, incommensurately modulated phase characterized by incommensurate primary modulation wavevectors q₁ ∼ [0.345, 0, 0.138]_p^* and q₂ ∼ [-0.069, 0, 0.172]_p^* (p for parent). The x = 0.25 phase with parent unit cell (I2/a, a = 5.3522(8) angstrom, b = 11.374(3) angstrom, c = 5.116(1) angstrom, β = 93.34(2)degree) is a commensurately modulated superstructure phase characterized by the reciprocal space unit cell a_r^* = 1/12[402]_p^*, b_r^* = 1/4[020]_p^*, and c_r^* = 1/3[101]_p^* (r for resultant). The x = 0.33 phase with parent unit cell (I1, a = 5.4374(8) angstrom, b = 11.189(2) angstrom, c = 5.1458(8) angstrom, α = 90.56(1), β = 94.37(1), γ = 88.19(1)degree) is again commensurately modulated with q = 1/3[101]_p^*. The close structural relationship between the three oxidized phases and possible interstitial oxygen sites in the Ce^IIINbO₄ structure are discussed

[en] Bond valence sum calculations are used to investigate the crystal chemistry of the elpasolite-related oxyfluoride K₃MoO₃F₃ in order to obtain insight into the type/s of structural distortion (away from an ideal, high symmetry, elpasolite type parent structure) responsible for a characteristic, highly structured, three-dimensional diffuse intensity distribution. The first required type of local structural distortion corresponds to large amplitude MoO₃F₃ octahedral rotations while the second is associated with O/F ordering and associated induced Mo ion shifts. Monte Carlo modelling is used to show how the latter when coupled with an appropriate local crystal chemical constraint can give rise to the observed structured diffuse scattering. The study is part of a wider search for diffraction evidence of oxygen/fluorine ordering in metal oxyfluoride systems

[en] A careful investigation has been carried out into the space group symmetries, structures and crystal chemistries of the 1:2 B-site ordered triple perovskites A₃Co²⁺Nb⁵⁺₂O₉ (A=Ca²⁺, Sr²⁺, Ba²⁺) using a combination of bond valence sum calculations, powder XRD and electron diffraction. A recent investigation of these compounds by Yin et al. reported a random distribution of Co²⁺ and Nb⁵⁺ ions onto the perovskite B-site positions and hence Pm3-barm (a=a_p, subscript p for parent perovskite sub-structure) space group symmetry for the A=Ba and Sr compounds and a monoclinically distorted variant thereof (although no space group was given) in the case of the A=Ca compound. An electron diffraction study, however, shows that the A=Ba compound occurs in a 1:2 B site ordered trigonal P3-barm1, a_h=b_p-c_p, b_h=c_p-a_p, c_h=a_p+b_p+c_p structure type while both the A=Ca and Sr compounds occur at room temperature in monoclinic P12₁/c1, a_m=[1-bar1-bar2]_p, b_m=[11-bar0]_p, c_m=3[110]_p, perovskite-related structure types. The latter monclinic structure type can be described as a modulated variant of the 1:2 B site ordered trigonal P3-barm1 structure type with the additional modulations associated with a b^-b^-c⁺ octahedral tilt sequence. Bond valence sum calculations are used to explain why this should be so as well as to provide a useful zeroth-order approximation to the structures of each of the compounds

[en] Reinterpretation of the neutron diffraction study of 2H-TaSe₂ by Moncton, Axe and DiSalvo (Phys. Rev. B. 16, 801 (1977)) shows ambiquity in the sense of the displacements proposed for the commensurate superlattice structure. This ambiquity is analysed using electrostatic and short-range energy calculations of the phase dependence of the energy of the periodic structural distortion wave. A new structural model for the incommensurate superlattice structure for Tsub(c) <= T <= T₀ arising from the analysis is proposed. (U.K.)