Interplay of lattice and charge on the CDW-order emergence in KCP

An interesting Charge density wave (CDW) compound is the Krogmann’s salt, K₂Pt(CN)₄Br_0.3 · xH₂O [1], which was thoroughly studied in the ‘70. The CDW phase arises from a sub-structure made of columnar stacks of [Pt(CN₄)]^1.7- ions, with an inter distance between Pt of 2.9 Å, comparable to the metallic distance. In contrast, the distance between Pt-chains is ~10 Å, leading to highly anisotropy behaviour associated with a partially-filled d_z² band. The phase transition occurs in two steps: an initial CDW formation at ~600K and a final 3D structural ordering at ~77K [2]. The phase transition can be characterized as “sluggish” and a useful model for its description is the 2D XY model, where the phase of the CDW is represented as a ‘pseudo-spin’. A total neutron scattering technique was chosen to detect and differentiate the inelastic and elastic contributions, allowing to track the evolution of the diffuse scattering signal throughout the phase transition.

[1] Krogmann K. & Hausen, H. D. (1968), Z. Anorg. Allg. Chem. 358, 67

[2] Pouget, J.-P. (2016), C. R. Physique 17, 332