Synthesis and study of frustrated oxide and mixed anion materials

Abstract

Mixed anion systems, such as oxynitrides and oxyfluorides, are an emerging class of interesting materials. The lower stability of mixed anion systems in comparison to oxide materials has had the consequence that this area of materials research is relatively less well explored. However, the development of new synthesis techniques has resulted in the preparation of many new mixed anion systems and so a detailed understanding of their structure and how this relates to their electronic and magnetic properties is necessary. Within this Thesis, several oxide, oxynitride and oxyfluoride systems are investigated with a particular focus on the magnetic behaviour of materials based on geometrically frustrated pyrochlore and kagome lattices.

The Lu2Mo2O7 pyrochlore contains a geometrically frustrated network of vertex sharing Mo4+ (d2 S = 1) tetrahedra. Here, the solid state synthesis of Lu2Mo2O7−x is reported along with a discussion of the coexistence of two cubic pyrochlore phases that has been discovered in samples synthesised at 1600 ◦C. Powder neutron diffraction and thermogravimetric analysis have revealed that this two-phase behaviour originates from a miscibility gap between stoichiometric Lu2Mo2O7 and oxygen deficient Lu2Mo2O6.6. Magnetic susceptibility and muon spin relaxation measurements support the formation of a geometrically frustrated spin glass ground state in Lu2Mo2O7 with a spin freezing temperature Tf ∼ 16 K. Low temperature neutron diffraction has confirmed the absence of long range magnetic order and magnetic diffuse neutron scattering data have indicated the presence of competing nearest and next nearest neighbour antiferromagnetic exchange interactions in the spin glass state. The magnetic heat capacity of Lu2Mo2O7 follows a T2-dependence at the low temperatures, indicating that Lu2Mo2O7 is another rare example of an unconventional, topological spin glass, which is stable in the absence of significant chemical disorder. The magnetic properties of the oxygen deficient pyrochlore phase Lu2Mo2O6.6 are qualitatively similar to those of Lu2Mo2O7, but an increase in the spin freezing temperature Tf ∼ 20 K suggests that oxygen-vacancy disorder in Lu2Mo2O6.6 favours the onset of a glassy state at higher temperatures and enhances the degree of frustration. Oxynitride pyrochlores with the ideal composition R2Mo2O5N2 (R = rare earth) contain Mo5+ d1 S = 1 2 cations on the frustrated pyrochlore lattice and are thus ideal candidates to support exotic magnetic ground states. Here, the synthesis of oxynitride pyrochlores of the Lu2Mo2O7 system by thermal ammonolysis is discussed alongside powder neutron diffraction and susceptibility data that show no evidence for long range magnetic order and an absence of spin freezing down to at least 2 K despite the persistence of strong antiferromagnetic exchange (θ = −120 K). A comparison of the magnetic diffuse neutron scattering between the spin glass state of Lu2Mo2O7 and the oxynitride is given, which suggests that the majority of the magnetic scattering in the oxynitride system is inelastic. In addition, low temperature magnetic heat capacity shows an absence of magnetic phase transitions and a continuous density of states through a T-linear dependence down to 500 mK.

[NH4]2[C7H14N][V7O6F18], diammonium quinuclidinium vanadium(III,IV) oxyfluoride or DQVOF, is a kagome bilayer system with a geometrically frustrated two-dimensional kagome network of V4+ d1 S = 1 2 cations and V3+ d2 S = 1 cations between the kagome layers. Here, low temperature magnetisation and heat capacity data are presented, which demonstrate that the interplane V3+ d2 cations are well decoupled from the kagome layers at low temperatures such that DQVOF is a good experimental realisation of a S = 1 2 kagome antiferromagnet.

Despite significant antiferromagnetic exchange (θ = −60 K) within the kagome planes, muon spin relaxation data have confirmed the absence of spin freezing and the persistence of internal field fluctuations that are intrinsic to the kagome layers down to temperatures of 40 mK. The low temperature heat capacity of the V4+ kagome network follows T-linear behaviour down to the 300 mK, highlighting the absence of a spin gap in the low energy excitation spectrum of DQVOF. The low temperature magnetic study of DQVOF presented here thus strongly supports the formation of a gapless quantum spin liquid phase.

In the final results chapter, a discussion of the anion ordering principles in oxynitride systems is given. A high temperature, high resolution neutron diffraction study of the oxynitride perovskite SrTaO2N has revealed that the partial anion order that results in segregated Ta-N zig-zag chains is stable up to 1100 ◦C. Furthermore, these anion ordering principles are extended to the d1 perovskite oxynitrides RVO2−xN1+x (R = La, Nd, Pr) in a variable temperature neutron diffraction study, which confirms that the anion chain ordering discovered in d0 SrTaO2N is robust to electron doping. The R = La analogue also provides an interesting example of a rhombohedral oxynitride perovskite phase which coexists with an orthorhombic phase over the 4−300 K temperature range of the neutron diffraction study.