Structural Diversity of Ternary and Quaternary Alkali-Metal Pnictides

• Author / Creator

  Khatun, Mansura

• To evaluate the applicability and expand the limit of the Zintl-Klemm concept, many new alkali-metal pnictides were synthesized by high temperature solid reactions in A-M-Pn or A-Tt-Pn ternary systems and A-M-Tt-Pn quaternary systems (A = Na, K, Rb; M = Mn, Zn, Cd; Tt = Si, Ge, Sn; Pn = As, Sb). Their crystal structures were characterized by X-ray diffraction methods. Their electronic structures and bonding interactions were determined using TB-LMTO calculations. Ternary pnictides adopt their own-type structures, having unique structural patterns: corrugated chains (Rb4Zn7As7- and Rb7Mn12Sb12-type structure) and layers (KSn3As3-type structure), GeAs-type layers (NaGe6As6), and 3D-networks (K2Zn5As4). The quaternary AM1.5Tt0.5As2 pnictides prefer to crystallize either in the CaAl2Si2- or ThCr2Si2-type structure, depending on the relative sizes of atoms, as expressed through a structure map. The structure map also suggested that ACd1.5Ge0.5As2 (A = K, Rb) phases would not form; indeed, attempts to prepare them gave new triclinic phases ACdGeAs2 instead. Band structure calculations revealed electronic structures in good agreement with expectations: K2Zn5As4 (0.4 eV), KGe3As3 (0.71 eV), and KCdGeAs2 (0.80 eV) are small band-gap semiconductors; NaZn1.5Si0.5As2 (small overlap at Fermi level) is a semimetal; Rb4Zn7As7 (small gap above Fermi level), Rb7Mn12Sb12 (pseudogap above Fermi level), and NaGe6As6 (small gap below Fermi level) are metallic. Pnictides that have A-to-Pn ratio of 1:2 to 1:3 follow the Zintl concept. Rb4Zn7As7 and Rb7Mn12Sb12 are electron-deficient Zintl phases having an A-to-Pn ratio of ˂2 whereas NaGe6As6 is an electron-rich Zintl phase having an A-to-Pn ratio of ˃3. This work opens a new window to study substitutional chemistry.

• Subjects / Keywords

  • Zintl Phase
  • Pnictides
  • Arsenide
  • Alkali-Metal
  • Quaternary

• Graduation date

  Fall 2017

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3PZ5212V

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.