Noether's theorem and low symmetry aspects concerning the crystal (ligand) field Hamiltonians and spin Hamiltonians An important theorem and a conjecture on the conserved quantities stipulated by Noether's theorem for the Hamiltonians in question formulated by us helps to elucidate the interrelationships and deeper meaning of the concepts involved. Implications of the existence of the conserved quantities for interpretation of experimental CF parameter (CFP) datasets are encapsulated in five corollaries. These considerations reveal that the feasibility of determination of CFPs from fitting experimental spectra and the reduction of the existing higher-order rotational invariants for hexagonal type II and cubic symmetry require reinterpretation. This novel approach enables adoption of better fitting strategies utilizing welldefined conserved quantities, which are invariant under CRS. The advantages of this approach are illustrated using the CFP datasets reported in literature for RE 3+ (4f N ) ions in LiYF 4 . This review deals also with the fundamental intricate aspects, hitherto not fully understood, concerning the CF Hamiltonians for the 'low symmetry' cases, including the CRS cases as well as orthorhombic ones. This includes: (1) selection of the axis systems, (2) types of CF parameters and their properties, (3) introduction of a new notion of a nominal axis system for the fitted CFP datasets, (4) implications of the Noether's theorem and the AS of CF Hamiltonians, (5) correlation properties among CFP datasets, (6) the rotational degrees of freedom and the reduction of the number of independent CFPs, and (7) extension of the multiple correlated fitting technique. Clarification of these intricate aspects enables us to provide a general framework aimed at achieving an increased compatibility and reliability of CFP datasets for transition ions at low symmetry sites in crystals.
IntroductionSpectroscopic properties of the transition ions, especially iron-group ions and lanthanides, at low symmetry sites in crystals play important role for various technologically important materials (for references, see [1][2][3][4][5]). The trivalent rare-earth (RE) 4f N ions have been most extensively studied due to applications in laser host and optoelectronic materials. The experimental techniques employed to investigate the CF/LF effects, majority of which are 'non-directional' techniques -unlike those used in the EMR (electron magnetic resonance) studies [6,7], were discussed in [3]. During our recent CF/LF studies of transition ions in crystals [1-5] we have encountered several intricate low symmetry aspects. The motivation to clarify these aspects has prompted us to embark on a comprehensive project aimed at increasing the compatibility and reliability of the crystal field parameter (CFP) datasets. This review presents a concise summary of the major findings arising from our studies concerning the intricate low symmetry properties of CF/LF Hamiltonians and spin Hamiltonians (SHs). To facilitate broad understanding, a bir...