Rules governing Jahn-Teller order in Prussian blue analogues

TL;DR

This study investigates how Jahn-Teller distortions in Prussian blue analogues influence their structure and properties, using synchrotron X-ray diffraction and a model linking strain and crystal field effects to control these distortions.

Contribution

It introduces a simple model explaining the stability and extent of cooperative Jahn-Teller order in PBAs based on composition, strain, and crystal-field considerations.

Findings

Jahn-Teller effects in PBAs can be systematically linked to composition.

The developed model predicts the stability and extent of CJT order.

PBAs exhibit complex interplay of structural, orbital, and magnetic properties.

Abstract

Jahn-Teller distortions of transition-metal coordination environments link orbital occupancies to structure. In the solid state, such distortions can be strongly correlated through the propagation of strain and/or through orbital interactions. Cooperative Jahn-Teller (CJT) order of this kind affects the electronic, magnetic, and structural properties of the materials in which it occurs. Conventionally studied in dense ceramics, CJT order also occurs in hybrid materials, albeit the underlying phenomenology is not well established. Here we use synchrotron powder X-ray diffraction measurements to determine links between composition and Jahn-Teller effects in a series of Prussian blue analogue (PBA) families. We develop a simple model based on the dual considerations of strain and crystal-field stabilisation that can account for the stability, extent of CJT order, and crystallite strain measured experimentally. This model shows how PBA compositions might be tuned to control the emergence and nature of CJT effects. A key implication of the model is that PBAs harbour their own complex interplay between compositional, structural, orbital, and magnetic degrees of freedom related to that of strongly-correlated functional materials such as the manganite perovskites.