Pressure effects on CrCl63− embedded in cubic Cs2NaMCl6 (M=Sc,Y) lattices: Study through periodic and cluster calculations

• Published in: The Journal of chemical physics 2008-04, Vol.128 (14)
• Main Authors: García-Lastra, J. M., Moreno, M., Barriuso, M. T.
• Format: Article
• Language: English
• ISSN: 0021-9606; 1089-7690
• DOI: 10.1063/1.2894546

The structural, elastic, vibrational, and optical properties of cubic elpasolites Cs2NaMCl6 (M=Sc,Y) containing CrCl63− complexes have been investigated by means of both periodic and cluster calculations as a function of pressure in the framework of density functional theory. Aside from calculating the host lattice bulk modulus BH and the local modulus B1 associated with the CrCl63−, complex particular attention is paid to the pressure dependence of Huang–Rhys factors, Sa and Se (related to local a1g and eg modes), and the Stokes shift associated with the first electronic excited state T2g4 (t2g2eg) of CrCl63−. The present calculations provide a big difference between BH=231kbars and B1=676kbars derived for Cs2NaScCl6:Cr3+ at zero pressure which plays a key role for a right interpretation of pressure effects on vibration frequencies and optical parameters due to CrCl63−. The significant decrease of Huang–Rhys factors, Sa and Se, due to the pressure observed experimentally is well accounted for by the present work which supports that ∂Sa∕∂P is determined by the Grüneisen constant γa of the a1g local mode (whose frequency is νa) and the dependence of 10Dq on the metal-ligand distance. At the same time, the present results point out that the Stokes shift would be little pressure dependent in the range of 0–50kbars. Accordingly the Ham effect in the T2g4 (t2g2eg) state of CrCl63− in the cubic elpasolites would also happen for a pressure up to 50kbars but the spin-orbit constant would increase with respect to that at zero pressure. From the analysis carried out in this work it is also concluded that the figures dνa∕dP=0.55cm−1∕kbar and dSa∕dP=−7.2×10−3kbar−1 extracted from the complex emission band of Cs2NaScCl6:Cr3+ are hardly compatible. This fact underlines the usefulness of ab initio calculations for helping in the analysis of complex experimental findings. Finally, as the CrCl63− unit is found to be to a good extent elastically decoupled from the rest of the elpasolite lattice, a model is shown to lead to an approximate relation between the pressure derivative of the local modulus and the Grüneisen constant γa.