Correlation between Polymorphism and Optical Bandwidths in AgNd(WO4)2

Abstract

The preparation and characterization of two polymorphic phases of AgNd(WO4)2 are described. The high-temperature phase of AgNd(WO4)2 is prepared as a polycrystalline powder and as a single crystal. X-ray diffraction analysis indicates that the crystal has at 300 K the tetragonal symmetry of the space group (SG) I4̄ (No. 82), with two independent crystal sites, 2b and 2d, for Nd3+ cations and structural disorder around them. The 5 K ground state optical absorption of this tetragonal crystal clearly differs from that corresponding to the monoclinic SG C2/m (No. 12) ordered phase found in polycrystalline samples prepared below 800 °C. Four times larger bandwidths and a weaker crystal field (CF), that is, lesser CF splitting for all Nd3+ 2S+1LJ manifolds, are observed for the tetragonal phase. Well-defined S4 polarization rules have been determined in the tetragonal phase, and then the observed 99 Nd3+ energy levels were labeled with the appropriate Γ7,8 or Γ5,6 irreducible representations. A detailed Hamiltonian of 26 free ion and CF parameters have been used in the simulation of the phenomenological energy levels and associated wave functions of the 4f3 configuration of Nd3+ in the tetragonal AgNd(WO4)2 single crystal, with final σ = 12.6 cm-1. The validity of the above set of CF parameters and wave functions has been established through the good reproduction of the thermal variation of the measured anisotropic paramagnetic susceptibility χ. As a result of this simulation it is shown that the larger bandwidths of the tetragonal phase contain nonresolved contributions from the two Nd3+ sites. A method to control overheating events is proposed on the basis of the nonreversibility of the tetragonal phase into the monoclinic one.