Red Shift in Optical Excitations on Layered Copper Perovskites under Pressure: Role of the Orthorhombic Instability

Abstract

The red shift under pressure in optical transitions of layered compounds with CuCl64− units is explored through first‐principles calculations and the analysis of available experimental data. The results on Cu2+‐doped (C2H5NH3)2CdCl4, that is taken as a guide, show the existence of a highly anisotropic response to pressure related to a structural instability, driven by a negative force constant, that leads to an orthorhombic geometry of CuCl64− units but with a hole displaying a dominant 3z2‐r2 character (z being the direction perpendicular to the layer plane). As a result of such an instability, a pressure of only 3 GPa reduces by 0.21 Å the longest Cu2+‐Cl− distance, lying in the layer plane, while leaving unmodified the two other metal‐ligand distances. Owing to this fact, it is shown that the lowest d‐d transition would experience a red shift of 0.34 eV while the first allowed charge transfer transition is also found to be red shifted but only by 0.11 eV that reasonably concurs with the experimental value. The parallel study on Jahn‐Teller systems CdCl2:Cu2+ and NaCl:Cu2+ involving tetragonal elongated CuCl64− units shows that the reduction of the long axis by a pressure of 3 GPa is three times smaller than that for the layered (C2H5NH3)2CdCl4:Cu2+ compound. Accordingly, the optical transitions of such systems, which involve a positive force constant, are much less sensitive to pressure than in layered compounds. The origin of the red shift under pressure undergone by the lowest d‐d and charge transfer transitions of (C2H5NH3)2CdCl4:Cu2+ is discussed in detail.

Understanding Pressure Effects on Structural, Optical, and Magnetic Properties of CsMnF4 and Other 3dn Compounds

“…In other words, pressure helps the geometry of the MnF 6 3– complex to become closer to the octahedral one as R Y – R Z goes from 0.36 Å at ambient pressure to only 0.16 Å when P = 40 GPa. This behavior is very similar to that found for the hybrid layered perovskite (C 2 H 5 NH 3 ) 2 CdCl 4 doped with Cu 2+ and plays a key role for explaining the shifts undergone by d–d transitions under pressure, 71 a question analyzed in the next subsection.…”

Chemical Bonding, 10Dq Parameter and Superexchange in the Model Compound KNiF3

“…Based on both assumptions and Eq. ( 7), then J(AFM) would be related to 10Dq of a single NiF 6 4À unit as follows JðAFMÞ / ð10DqÞ 2 (13) Although this expression suggests a strong dependence of J(AFM) upon R in KNiF 3 the calculated values in Figure 3 show that q σ is practically equal to q π and thus neither the assumption in Eq. ( 12) nor the conclusion of Eq.…”

“…As regards the CrF 6 3− complex in Cr 3+ ‐doped KZnF 3 , the first excited state is 4 T 2 at ambient pressure but becomes 2 E for a pressure of about 8 GPa [11] a situation also observed [12] for K 2 NaGaF 6 : Cr 3+ . Along this line, an applied pressure on copper layered perovskites leads to a surprising red shift of some optical transitions [13] …”

“…Along this line, an applied pressure on copper layered perovskites leads to a surprising red shift of some optical transitions. [13] The present work deals with pressure effects on model insulating compounds displaying cubic symmetry and involving octahedral TM complexes whose d-d transitions are described in the approximate Tanabe-Sugano framework through the three B, C and 10Dq quantities. [6,7] Interestingly, the two Racah parameters, B and C, of the MX 6 unit are systematically smaller than B 0 and C 0 corresponding to the free TM cation.…”

Understanding the Local Structure, Magnetism, and Optical Properties in Layered Compounds with d9Ions: Insight into Silver Fluorides and K2CuF4

“…In the realm of insulating layered perovskites, , much research is currently focused on those containing the 3d 9 ion Cu 2+ due to their attractive optical and magnetic properties. − Inorganic compounds − like K 2 CuF 4 or Rb 2 CuCl 4 as well as organic–inorganic hybrid perovskites of the (C n H 2 n +1 NH 3 ) 2 CuCl 4 or [NH 3 (CH 2 ) n NH 3 ]­CuX 4 (X = Cl and Br) families − belong to these kinds of materials. In all these insulating transition-metal compounds, properties are greatly dependent on the equilibrium geometry and the related electronic structure of the CuX 6 4– complex (X = F, Cl, and Br) formed in the crystal.…”