Synthesis and Properties of New Oxychalcogenide Materials

Abstract

Chapter 1 gives a brief review of oxychalcogenide materials and their properties, particularly those with structures similar to systems discussed in the later chapters. Particular attention is paid to the structural and magnetic properties of La2O2Fe2OSe2-type materials, work on which is presented in chapters 3–5. These oxychalcogenides, along with ZrCuSiAs-related materials, are of interest due to their potentially interesting magnetic and conducting properties. In particular they are close relatives of the LaOFeAs superconductors.
Chapter 2 describes the synthetic and analytical techniques used in this project. The theory behind the powder diffraction techniques used throughout is described, and information on specific methodology used in the data collections and analysis of this work is given. SQUID magnetometry is also discussed.
Chapter 3 discusses the variable temperature structural and magnetic properties of La2O2Mn2OSe2 and Pr2O2Mn2OSe2, as well as those of the newly prepared material Ce2O2Mn2OSe2. These materials are observed to undergo a phase transition on cooling revealed by a change in the thermal expansion of the c cell parameter, and show evidence for the static disorder of oxide ions from the [Mn2O]2+ plane below this temperature. Pr2O2Mn2OSe2 is also shown to undergo a further transition on cooling below 36 K to an orthorhombic unit cell. Neutron diffraction data have also shown that the Mn2+ moments in these materials order with an AFM1 structure on cooling below ~180 K.
Chapter 4 describes the study of both La2O2Fe2OS2 and La2O2Fe2OSe2 by variable temperature neutron powder diffraction. These materials have been demonstrated to order with an AFM3 structure on cooling below ~100 K, which is coincident with a subtle structural change observed in the thermal expansion of the a cell parameter. The unexpected adoption of the AFM3 magnetic structure is discussed in relation to structurally similar iron-based superconductors.
Chapter 5 presents the synthesis of several new La2O2Fe2OSe2-type materials, and the magnetic structure of La2O2Co2OSe2. The stability range over which these materials form is discussed with respect to the size of the metal ions present. A comparison of the structural properties of all the A2O2M2OSe2 materials is given and the various magnetic structures they adopt discussed.
Chapter 6 reports the synthesis of a new family of transition metal oxychalcogenides (β La2O2MnSe2). Their structure has been solved from laboratory X ray diffraction data, using a combination of charge-flipping and direct methods, and confirmed by neutron diffraction. Variable temperature structural properties are discussed and show these materials undergo a phase transition on cooling associated with ordering of transition metal ions. SQUID magnetometry and neutron powder diffraction have both demonstrated these materials order antiferromagnetically at low temperatures.
Chapter 7 describes the synthesis of two new ZrCuSiAs-related transition metal containing oxychalcogenides: Ce2O2FeSe2 and La2O2ZnSe2. The structure solution of Ce2O2FeSe2 by charge-flipping is reported, alongside the determination of the magnetic structure from neutron powder diffraction data collected at 12 K. The variable temperature structural properties of La2O2ZnSe2 are also reported.