Date of Award
Doctor of Philosophy
Hugo Fritz Franzen
Structural changes resulting in lower symmetries can be understood in terms of electronic instabilities and Coulomb interactions. The interplay of these two interrelated factors is complicated and difficult to analyze. The RECuAs2-xP x phases, because of the variation in the chemical content (As/P substitution), allow, with the aid of band structures, Madelung energies and Landau theory, a partial unraveling of the forces important in the symmetry-breaking transitions in RECuAs2-xP x (RE = Sm, Gd, Ho and Er). Distortions of the P layers in SmCu1.15P2, GdCuP2.20 and ErCuP2 are usefully thought of as generalized Peierls distortions, i.e., they lower the electronic (and total) energy and lead to more stable structures. On the other hand, the P4/nmm → Pmmn transitions, which are observed in all studied arsenophosphide series and occur upon substitution of P for As, originate from the B1g vibrational mode and are structural adaptations to smaller P atoms. These transitions provide tighter atomic packing and better Coulomb interactions. Configurational contribution to the entropy becomes important in stabilizing the mixed occupancy in the RECuAs 2-xPx arsenophosphides. While geometric and electronic factors favor separation of the As and P atoms over two different crystallographic sites, configurational entropy stabilizes the As/P mixing on these two sites.;Progress in the research on RECuAs2- xPx was dependent upon the ability of Landau theory to predict, explain and dismiss structural models and transitions. The space group Pmmn (arising from the B 1g vibrational mode) in all mixed arsenophosphides and the existence of these mixed arsenophosphides followed from the analysis of GdCuAs 2 and GdCuP2, using Landau theory. The impossibility of obtaining the high-symmetry structure (P4/nmm) and the low symmetry structure (Pnmm) at the same temperature for the displacive continuous symmetry-breaking transition P4/ nmm → Pmmn led to the conclusion, later experimentally proven, that the tetragonal structures of the SmCu1+deltaAs 2-xPx arsenophosphides are stabilized by additional Cu atoms. On the other hand, the coexistence of the two structures with the P2/ n and Cmmm symmetries at two samples indicated that the structural change from the SmCuP2 structure (P2/ n) to the SmCu1.15P2 (Cmmm) structure is first-order and that this transition does not occur continuously upon introducing Cu atoms into the SmCuP2 structure.
Digital Repository @ Iowa State University, http://lib.dr.iastate.edu
Mozharivskyj, Yurij, "Symmetry-breaking transitions in RECuAs[subscript 2-x]P[subscript x] (RE=Sm, Gd, Ho and Er) " (2002). Retrospective Theses and Dissertations. 397.