Research Overview
The Kloß group studies novel transition metal nitrides by combining inorganic synthesis at extreme pressures and temperatures with modern characterization methods from solid-state physics to create an interdisciplinary field of research.
Synthesis Methods @ AK Kloß
Here at the Kloß group, we use very large pressures in the range from ten thousand to one million bar, ten to one thousand times higher than at the bottom of the Marianna trench, to create materials with high nitrogen content that cannot be obtained otherwise. We employ three state-of-the-art techniques of pressure generation: Hot isostatic presses for pressures up to 0.2 GPa, large volume presses for pressures up to 20 GPa, and diamond anvil cells for pressures exceeding 100 GPa.
Explore Our Research
We investigate two kinds of nitride materials: nitridometallates with 3d transition metals and nitride perovskites with heavy 4d and 5d transition metals. These compounds with general sum formula AxMyNz, where A is an electropositive element such as an alkine earth or rare earth and M is the transition metal, show a range of physical properties stemming from empty, partly filled, or filled d- and f-electron shells. To understand the structure and physics of our materials we employ characterization techniques ranging from X-ray and (magnetic) neutron diffraction to magnetisation and transport measurements, energy-dispersive X-ray spectroscopy, and UV-Vis spectroscopy.
Nitridometallates of 3d transition metals have been researched since the early 1990s. During those times, synthesis was primarily carried out at ambient pressures. While some important families of transition metal nitrides were so discovered, the materials were usually poor in nitrogen with low mean oxidation states of the transition metals. There is a discrepancy in highest oxidation states obtained between solid-state transition metal oxides and nitrides (Figure 1). The Kloß Group strives to diminish this discrepancy. Synthesis routes based on high-pressure methods proved as a key to unlocking nitrogen-rich transition metal nitrides and high oxidation states.
Figure 1: Overview of oxidation states observed in transition metal nitrides. The highest states reported in literature are indicated by a red line, our work highlighted in orange and compared to the highest observed states in transition metal oxides.
A good example is the iron(IV) nitridoferrate Ca₄FeN₄ that surpasses the previous iron(III) oxidation state limit in nitridometallates. We achieved the oxidation of iron using an azide-mediated redox reaction carried out at high-pressures provided by the large volume press. Next to structural characterization, our group also specializes in determination of physical properties such as magnetism. Here, we used a combination of magnetometry and magnetic neutron scattering (Figure 2) to probe the antiferromagnetic ground state of Ca₄FeN₄.
Figure 2: Structure of Ca₄FeN₄, as well as antiferromagnetic ordering in Ca₄FeN₄ as determined with magnetic neutron scattering and magnetometry.
The study of nitride perovskites (ABN₃) is the natural extension of oxide perovskite research, replacing O by N. The first report was already in 1995 of a compound with sum formula ThTaN3. However, ThTaN3 remained the only instance for over 25 years owed to challenging synthesis that requires either plasma-activated nitrogen for thin-film synthesis or high-temperature and high-pressure conditions.
These breakthroughs in high-pressure synthesis enabled the stabilization of ABN3 nitride perovskites as well as Ruddlesden-Popper (RP) nitrides (A₂BN₄). Our group published the second example for an ABN3 nitride perovskite LaReN3 and the first RP nitrides, Ce2TaN4, Pr2ReN4 and Nd2ReN4. Figure 1 shows the history of nitride perovskite research.
Figure 1: History of nitride perovskite research. Our contributions are marked in orange.
In our research, we explore the structure, composition and physical properties of this new type of materials. Physical properties are dependent on f- and d-electron count as well as structure, so we probe those with synchrotron and neutron diffraction as well as magnetisation measurements. Neutron diffraction is especially useful in determining the structure and composition as neutrons are sensitive to the light nitrogen atoms. Figure 2 shows structures and magnetic properties of some of our materials.
Figure 2: Structure of LaReN3, as well as structure and properties of ferromagnetic Nd2ReN4.