# Pressure Dependence of Structural Behavior in the Polymorphs of Fe(PM–BiA)2(NCS)2

## TL;DR

This study explores how pressure affects the structure of two forms of a compound containing iron, revealing spin state transitions and superstructure formation.

## Contribution

The study identifies pressure-induced spin state transitions and superstructure formation in two polymorphs of a Fe-based compound.

## Key findings

- Both polymorphs remain stable up to 1.5 GPa, with spin state transition occurring only in PII.
- Superstructures form in both polymorphs at pressures above 2 GPa, with axis doubling observed.
- LS and HS states coexist at high pressures in both polymorphs under quasi-hydrostatic conditions.

## Abstract

The pressure dependence of structural behavior in the orthorhombic (Pccn, PI) and monoclinic (P21/c, PII) polymorphs of the compound [Fe(PM-BiA)2(NCS)2], where PM–BiA = (N–(2′–pyridylmethylene)–4-amino–bi–pheynyl), is studied with synchrotron single-crystal X-ray diffraction and vibrational spectroscopy. Both polymorphs are stable up to ∼1.5 GPa, with a spin state transition occurring only in polymorph PII under hydrostatic conditions as documented by single-crystal synchrotron diffraction. The diffraction data also provide evidence of the formation of superstructures for both PI, with a doubled c axis, and PII, with a doubled b axis, on applying pressures above 2 GPa. The LS and HS states seem to coexist at high-pressures for both polymorphs studied with synchrotron infrared spectroscopy at quasi-hydrostatic conditions. Such results indicate that the occurrence of spin-crossover transformations in [Fe(PM-BiA)2(NCS)2] might strongly depend on the stress in the sample.