Integrated experimental-theoretical study of optical absorption in Na3Cr2(PO4)3: Neuhauser-model analysis of spin-orbit effects.

The compound Na3Cr2(PO4)3 is commonly synthesized via a high-temperature solid-state reaction. Its morphological characteristics were examined by scanning electron microscopy (SEM) and the homogeneity of its chemical composition was confirmed by energy dispersive X-ray (EDX). Analysis of the optical absorption spectrum indicates a direct band gap of 3.1 eV and an Urbach energy of 0.59 eV. Intriguingly, an interference dip appears in the lowest-energy 4T2g(4F) band, attributed to spin-orbit coupling between the states 2Eg(2G) and 4T2g(4F). This spectral feature was modeled using Neuhauser's approach, based on coupled potential energy surfaces. For the electronic study, we consider the zero-phonon lines ZPL of the d-d transitions using the autocorrelation function. From this analysis, the electronic structure of Cr3+ (3d3) ions in Na3Cr2(PO4)3 was elucidated, enabling accurate determination of Racah and crystal-field parameters. The experimentally observed and theoretically predicted energy levels showed strong agreement. Overall, the study effectively highlights the influence of spin-orbit interactions as interpreted through the coupled potential energy surface framework.