The Earth’s lower mantle, ranging from 660 km to 2890 km depth, constitutes more than 50% of Earth’s volume and is the largest geochemical reservoir for many elements. Throughout Earth’s history, substantial amounts of material have been exchanged between the mantle and Earth’s surface and atmosphere, affecting the evolution of Earth’s atmosphere and the habitability of our planet. The lower mantle, linking the liquid outer core to the Earth’s upper mantle, is also a key component controlling mantle dynamics. Quantitative knowledge of the chemistry, mineralogy and temperature of the lower mantle is thus of key importance for interpreting the thermal evolution, geochemical properties, and dynamics of the Earth’s interior. Seismic wave velocity measurements on mantle minerals in the diamond-anvil cell. The diamond-anvils are illuminated by the probing green laser light. With the exception of the lowermost 200-300 kilometers, the lower mantle has traditionally been assumed to be chemically homogenous, a conclusion based on the absence of geophysical evidence to the contrary (although there is some debate in geochemistry). Recent evidence, from both laboratory work and geophysical measurements, suggests that chemical and/or physical properties change throughout the lower mantle (Ballmer et al., 2015, Marquardt et al. 2015, Rudolph et al. 2015, Kurnosov et al. 2017). In the mid-lower mantle, iron in the main lower mantle minerals bridgmanite and ferropericlase undergoes a change of electronic configuration, i.e. a spin crossover, that markedly affects its physical properties (Lin et al., 2013). However, at temperatures relevant to the mantle, the crossover extends over several hundreds of kilometres depths. The effects of this spin crossover on seismic wave velocities are not understood in detail (e.g. Trautner et al., 2023). Another important phase transition in the mid-lower mantle, affecting seismic wave velocities, is the tetragonal to orthorhombic distortion in SiO2 stishovite (e.g. Wang et al., 2023). This project will focus on resolving the impact of these spin and phase transitions on geophysical observables by combining traditional seismic wave velocity measurements in the laboratory and novel experimental capabilities at large-scale synchrotron research facilities (Diamond Light Source, UK and DESY, Germany), with seismological modelling. The models will be compared to the seismological record to prospect for signs of these transitions in the deep mantle and thereby enhance our understanding of the current state of the lower mantle and, ultimately, its role in the evolution of our planet.