Crystal Structure and Optical and Magnetic Properties of Pr2(MoO4)3

Praseodymium molybdate Pr2(MoO4)3 was synthesized using the standard ceramic route. The crystal structure of the material has been successfully solved in superspace group I2/b(Rβ0)00 with lattice constants a = 5.30284(4), b = 5.32699(3), c = 11.7935(1)A° , γ = 90.163(1) , and the modulation vector q = 2/3aþ0.88810(2)b. The deviation of the q vector from a rational value allows a description of the structure in terms of nanosize domains with the La2(MoO4)3-like structure separated by stacking faults. Under 450 nm excitation, (3P0 level) Pr2(MoO4)3 exhibits the characteristic red emission, with the most intense band at 649 nm corresponding to a $^3P_0\rightarrow^3F_2$ transition. Magnetic susceptibility measurements reveal Curie-Weiss paramagnetism with predominating antiferromagnetic interactions between $Pr^{3+}$ -magnetic moments and no evidence of magnetic transitions down to T = 5 K.

The commensurate composite sigma-structure of beta-tantalum

Alla Arakcheeva, Gervais Chapuis, Philip Pattison

The single-crystal investigation of the self-hosting sigma-structure of beta-tantalum (beta-Ta) at 120 K (low-temperature, LT, structure) and at 293 K (RT-I before cooling and RT-II after cooling and rewarming; RT represents room temperature) shows that this structure is indeed a specific two-component composite where the components have the same ( or an integer multiple) lattice constants but different space groups. The space groups of both host ( H) and guest ( G) components cause systematic absences, which result from their intersection. The highest symmetry of a sigma-structure can be described as [H: P4(2)/mnm; G: P4/mbm (c(G) = 0.5c(H)); composite: P4(2)/mnm]. A complete analysis of possible symmetries is presented in the Appendix. In beta-Ta, two components modify their symmetry during the thermal process 293 K (RT- I)) double right arrow 120 K (LT)) double right arrow 293 K (RT-II): [H: P (4) over bar2(1)m; G: P (4) over bar2(1)m; composite: P (4) over bar2(1)m]) double right arrow [H: P (4) over bar, G: P4/mbm (c(G) = 0.5c(H)), composite: P (4) over bar]) [H: P (4) over bar2(1)m, G: P4/mbm (c(G) = 0.5c(H)), composite: P (4) over bar2(1)m]. Thus, the phase transition is reversible with respect to H and irreversible with respect to G.

2003

Synthesis, crystal growth and physical properties of the frustrated spin ladder material BiCu2PO6

Shuang Wang

In recent years quantum antiferromagnets with an intrinsically disordered (“spin liquid”) ground state and an energy gap in the spin excitation spectrum have received a great deal of attention. In search for new experimental realizations of spin-ladder and related models, we have embarked upon a new interesting family of low-dimensional materials with the general formula BiCu2AO6 (A = P, V and As). Among them, BiCu2PO6 shows a structure where Cu spins appear to form a two-leg zigzag ladder, which is believed to be a valuable model of a quantum magnet. However, for detailed studies of quantum magnetism, single crystals are indispensible. Therefore, in this thesis, synthesis and characterization of the newly-discovered spin ladder material BiCu2PO6 (BCPO) and its derivatives in the form of powder and single crystal are presented. Polycrystalline samples of Bi(Cu1-xZnx)2PO6 (x= 0.01, 0.02, 0.03, 0.04, 0.05 and 1), Bi1-y(Ca, Sr, La )yCu2PO6 (y = 0.15, 0.5 and 1) and BiCu2(P1-zVz)O6 (z = 1) were synthesized in this work. The solid state synthesis reactions, phase purity and thermal properties of the samples, substitution solubility, and preliminary magnetic susceptibility have been studied by thermal analysis (TG-DTA), X-ray powder diffraction (XRD), and Scanning Electron Microscope (SEM) as well as DC magnetization measurements, respectively. The BiCu2PO6 structure shows a remarkable chemical flexibility and interesting substitution effects. For detailed studies of the magnetic properties, well characterized large single crystals are indispensable. Thus in this thesis, a number of single crystal growth experiments have been performed using the Travelling Solvent Floating Zone method (TSFZ). Under the elaborated crystal growth conditions (growth rate of ~1 mm/h, gas mixture of 20% O2 in Argon, total 3 pressure (6~6.5 bar), high quality cm -size single crystals of Bi(Cu1-xZnx)2PO6 (x = 0, 0.01, 0.03, 0.05) have been successfully grown for the first time. For these crystals phase purity, crystal morphology/orientation, defect density, chemical composition distribution, crystal structure (room temperature and 1.5 K) and single crystal quality were thoroughly studied based on X-ray and neutron powder diffraction, optical and scanning electron microscopy, X-ray/neutron Laue diffraction, ICP-OES/X-ray fluorescence and two-axis neutron diffraction. Moreover, the specific heat Cp (H,T), AC susceptibility χ(H,T) and DC magnetization M(H,T) measurements on well-characterized single crystal samples of the undoped BCPO and 5% Zn-doped BCPO with the field applied along main crystallographic axes were performed in order to study the unique low temperature magnetic behaviour exhibited in this system, such as anisotropy of the magnetic properties, impurity-induced magnetism by the substitution of non-2+ magnetic cations (e.g. Zn ) and frustration. The magnetic phase diagram (H-T) under low field has been constructed. Further results obtained by close collaborators, based on the measurements made on single crystal samples grown in this thesis, including field-induced quantum phase transitions, NMR, Raman studies are attached directly at the end of this thesis.

EPFL2013

Pressure dependence of the structure and electronic properties of Sr3Ir2O7

Henrik Moodysson Rønnow

We study the structural evolution of Sr3Ir2O7 as a function of pressure using x-ray diffraction. At a pressure of 54 GPa at room temperature, we observe a first-order structural phase transition, associated with a change from tetragonal to monoclinic symmetry and accompanied by a 4% volume collapse. Rietveld refinement of the high-pressure phase reveals a novel modification of the Ruddlesden-Popper structure, which adopts an altered stacking sequence of the perovskite bilayers. As the positions of the oxygen atoms could not be reliably refined from the data, we use density functional theory (local-density approximation+U+spin orbit) to optimize the crystal structure and to elucidate the electronic and magnetic properties of Sr3Ir2O7 at high pressure. In the low-pressure tetragonal phase, we find that the in-plane rotation of the IrO6 octahedra increases with pressure. The calculations further indicate that a bandwidth-driven insulator-metal transition occurs at similar to 20 GPa, along with a quenching of the magnetic moment. In the high-pressure monoclinic phase, structural optimization resulted in complex tilting and rotation of the oxygen octahedra and strongly overlapping t(2g) and e(g) bands. The t(2g) bandwidth renders both the spin-orbit coupling and electronic correlations ineffectual in opening an electronic gap, resulting in a robust metallic state for the high-pressure phase of Sr3Ir2O7.

2016

Synthesis, crystal growth and physical properties of the frustrated spin ladder material BiCu2PO6

Shuang Wang

EPFL2013