Luminescencja jonów chromu w naturalnych krzemianach

Abstract

This work presented here results of electron absorption and luminescence investigations of 14 natural silicates, which have contained chromium Cr3 + ion. The contents of it was from 0,02% and less (kunzite) to 11,2% (uvarovite). Many of minerals had also manganese, vanadium and iron admixture. Luminescence measurement as well as decay time were taken at room temperature and 77 K—4 K. The luminescence dependence on ligand crystal-field was analysed. Examples of strong and weak ligand crystal fields were demonstrated. Kammererite, pyrope, Cr-grossular, tsavorite, tanzanite, spodumene, emerald, Cr-idocrase, kotchubeite and two crystal sites in kyanite were include to the first group. Weak crystal-field was found for uwarowite, one crystal site in kyanite, fuchsite diopsyde and Cr-amphibole. The dependence of the 2E level splitting on ligand crystal field symmetry was mentioned. The smallest splitting (65 cm"1) was found for trigonal crystal field C3j in grossulars and emerald. For comparison in Cr-doped corundum the R-lines splitting is 29 cm"1. For tetragonal C4v crystal field in idocrase the splitting is 132 cm1 and the biggest are for triclinic and monoclinic symmetry in tanzanite and kyanite up to 335 cm *. The influence of the first and next-neighbour coordination on crystal field parameters was discussed for fuchsite and kotchubeite, diopside and spodumene and grossular and idocrase. The influence of other transition elements on chromium luminescence was found, too. In kyanite the chromium ions were presented in different crystal sites and among them the energy transfer was very probable. Vanadium ions were presented in tsavorite and tanzanite and the energy from chromium ions was passed on to them. It was the cause of unexpected short decay time. Because of manganese ions in spodumene the several different chromium sites were created and their decay time were short, either. For pyrope and Cr-amphibole the chromium luminescence were completely quenched for reason of iron high content. For two chromium chlorites-kammererite and kotchubeite — the clusters and Cr-Cr pairs were found. It was because of low luminescence intensity and short decay time of them. The influence of other transition ions presented in studied crystals and the high chromium content on luminescence properties was detected only qualitatively. Author had not enough examples of each kind of minerals with different contents of doped ions to discusse this problem quantitatively. The electronic and vibronic energy levels — strong and weak — were discussed. In this work for the first time the luminescence of Cr3 + ions in Cr-grossular, tsavorite, uvarovite, tanzanite, idocrase, diopside, fuchsite, kotchubeite and kammererite were presented and discussed. For the first time for chromium ions in minerals the energies of excited levels and crystal-field parameters were calculated for Cr-grossular, kyanite, tanzanite and idocrase.