minerals
Article
Luminescence Properties of Tetrahedral Coordinated Mn
2+
;
Genthelvite and Willemite Examples
Maria Czaja
1,
* , Radoslaw Lisiecki
2
, Rafal Juroszek
1
and Tomasz Krzykawski
2
Citation: Czaja, M.; Lisiecki, R.;
Juroszek, R.; Krzykawski, T.
Luminescence Properties of
Tetrahedral Coordinated Mn
2+
;
Genthelvite and Willemite Examples.
Minerals 2021, 11, 1215. https://
doi.org/10.3390/min11111215
Academic Editors: Gioacchino
Tempesta and Giovanna Agrosì
Received: 3 October 2021
Accepted: 25 October 2021
Published: 30 October 2021
Publisher’s Note: MDPI stays neutral
with regard to jurisdictional claims in
published maps and institutional affil-
iations.
Copyright: © 2021 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
1
Institute of Earth Sciences, Faculty of Natural Science, University of Silesia, B ˛ edzi ´ nska 60,
41-200 Sosnowiec, Poland; rafal.juroszek@us.edu.pl
2
Institute of Low Temperature and Structure Research, Polish Academy of Sciences, Okólna 2,
50-422 Wroclaw, Poland; r.lisiecki@intibs.pl (R.L.); tomasz.krzykawski@us.edu.pl (T.K.)
* Correspondence: maria.czaja@us.edu.pl
Abstract: The cause of the split of
4
A
4
E(
4
G) Mn
2+
excited level measured on minerals spectra
is discussed. It is our view that ΔE = |4E(4G) − 4A(4G)| should be considered an important
spectroscopic parameter. Among the possible reasons for the energy levels splitting taken under
consideration, such as the covalent bond theory, the geometric deformation of the coordination
polyhedron and the lattice site’s symmetry, the first one was found to be inappropriate. Two studied
willemite samples showed that the impurities occur in one of the two available lattice sites differently
in both crystals. Moreover, it was revealed that the calculated crystal field Dq parameter can indicate
which of the two non-equivalent lattice sites positions in the willemite crystal structure was occupied
by Mn
2+
. The above conclusions were confirmed by X-ray structure measurements. Significant
differences were also noted in the Raman spectra of these willemites.
Keywords: Mn
2+
; luminescence; energy of excited level; crystal field parameters
1. Introduction
The luminescence of synthetic materials subsidized with manganese ions, especially
(2+), is still intensively researched and has been for many years [1,2]. The emission color of
subsidized Mn
2+
compounds is usually green or orange to red. It depends on the strength of
the crystal field, i.e., the coordination number, the type of ligand and the distance between
the ligand and the manganese ion. Various materials with a halides, oxides, phosphates
and silicate matrix are synthesized in the form of glass, ceramics, and crystals, sometimes
as nanomaterials or coatings. Many synthetic materials are doped not only with Mn
2+
but also with lanthanide ions. Then, between the Mn
2+
and 4f
n
ions, the phenomenon of
energy transfer takes place [3]. This makes it possible to obtain efficient light emitters of
various colors, including white light. Moreover, some of these materials exhibit a persistent
luminescence phenomenon [4,5].
It is believed that the study of the spectroscopic properties of natural materials con-
taining Mn
2+
could be inspiring to create an optical material with the desired parameters.
Divalent manganese ion is one of the most common and well-known activators of minerals’
luminescence.
The present research has demonstrated some properties of the luminescence spectra
of Mn
2+
-bearing minerals. First, attention was paid to the splitting of the
4
E
4
A(
4
G) excited
level. The absorption/excitation bands corresponded to Mn
2+
transitions were usually
measured in the 300–580 nm range and these bands sometimes have not been single.
On these spectra, the υ
3
band of
6
S →
4
E
4
A
1
(
4
G) transition is usually distinguished due
to its intensity and sharpness. It was also often measured, but not clarified, that this
band is double. A review of available absorption/excitation spectra for Mn
2+
-bearing
minerals and synthetic materials concludes that ΔE =|
4
E(
4
G) −
4
A(
4
G)|, i.e., splitting of
these levels is an important spectroscopic parameter. In the current article, an attempt to
Minerals 2021, 11, 1215. https://doi.org/10.3390/min11111215 https://www.mdpi.com/journal/minerals