Browsing by Author "Shoemaker, DP"

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    Crystal structure analysis of tungsten bronzes beta-SrTa(2)O(6) and beta '-SrTa(2)O(6) by synchrotron X-ray and neutron powder diffraction
    (Academic Press Inc Elsevier Science, 2012-07-01) Lee, E; Park, CH; Shoemaker, DP; Avdeev, M; Kim, YI
    Strontium ditantalum oxide SrTa(2)O(6) exists in alpha-, beta-, and beta'-polymorphs. Herein the crystal structures of the latter two were studied using synchrotron X-ray and constant-wavelength neutron powder diffraction. While beta'-SrTa(2)O(6) [space group P4/mbm, a=12.47099(1) angstrom, c=3.898210(5) angstrom, V=606.271(2) angstrom(3), Z=5] belongs to the regular tetragonal tungsten bronze (TTB) family, it contains locally disordered strontium atoms within the pentagonal channel. beta-SrTa(2)O(6) [space group Pnam, a = 12.36603(2) angstrom, b=12.43467(2) angstrom, c=7.72403(1) angstrom, V=1187.705(4) angstrom(3), Z=10] can be described as an orthorhombic modification of the TTB, where the octahedral tilting distortion effectively alleviates the bonding strains around TaO(6) and SrO(13) polyhedra. For comparison, rynersonite type alpha-SrTa(2)O(6) [space group Prima, a=11.00610(6) angstrom, b=7.63397(3) angstrom, c=5.62634(3) angstrom, V=472.727(5) angstrom(3), and Z=4] is built from edge-shared dimer units of TaO(6) octahedra. As measured by diffuse-reflection absorption spectroscopy, alpha-, beta- and beta'-SrTa(2)O(6) have indirect band gap energies of 4.4, 4.0, and 3.8 eV, respectively. © 2012, Elsevier Ltd.
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    Two-step magnetic ordering into a canted state in ferrimagnetic monoclinic Mn3As2
    (Elsevier, 2021-02-01) Karigerasi, MH; Lam, BH; Avdeev, M; Shoemaker, DP
    We report the magnetic structure of monoclinic Mn3As2 at 3 ​K and 250 ​K using neutron powder diffraction measurements. From magnetometry data, the Curie temperature of Mn3As2 was confirmed to be around 270 ​K. Calorimetry analysis showed the presence of another transition at 225 ​K. At 270 ​K, Mn3As2 undergoes a k=0 ferrimagnetic ordering in the magnetic space group C2/m (#12.58) with Mn moments pointing along b. Below 225 ​K, there is a canting of Mn moments in the ac plane which produces a multi-k non-collinear magnetic structure in space group C2/c (#15.85). The components of Mn moments along b follow k=0 ordering and the components along a and c have k=[0012] propagation vector. The change in the magnetic ground state with temperature provides a deeper insight into the factors that govern magnetic ordering in Mn–As compounds. © 2020 Elsevier Inc.