Elsevier

Journal of Crystal Growth

Volume 547, 1 October 2020, 125817
Journal of Crystal Growth

Growth and characterization of ErF3 doped BaF2 crystals

https://doi.org/10.1016/j.jcrysgro.2020.125817Get rights and content

Highlights

  • Various ErF3 concentrations doped BaF2 crystals were grown by Bridgman technique.

  • The optical absorption spectra and the dielectric relaxation were investigated.

  • The distribution of Er3+ ions along the crystals was studied by optical absorption.

  • The charge compensating defects were discussed using optical and dielectric spectra.

Abstract

ErF3 doped BaF2 crystals were obtained using the vertical Bridgman technique. Five crystals, with 0.05, 0.08, 0.1, 0.2 and 0.5 mol% ErF3 added to the starting material, have been investigated. The optical absorption spectra reveal the characteristic bands of the Er3+ ions. The distribution of Er3+ ions along the crystals was studied using the optical absorption spectrum of every samples (14–18 slices) cleaved from the as-grown crystals. The Er3+ ions are not uniform distributed along the BaF2 crystals. The segregation coefficient, k, was calculated using the so-called optical absorption method and the Scheil relationship between the dopant concentration and crystal growth conditions. The calculated segregation coefficient of the Er3+ ions ranges from 0.76 to 0.98, for ions with trigonal (C3v) sites, and from 0.6 to 0.86 for clusters, depending on the ErF3 concentration. In the investigated temperature range, only one type of dielectric relaxation has been observed. This relaxation, with activation energy of 0.54 eV is associated with trigonal NNN type (C3v) centers. The charge compensating defects were discussed taking into account both the optical absorption spectra and the calculated number of NNN dipoles whose relaxation were observed.

Introduction

The study of various physical properties of pure and doped fluorides (MeF2: Me = Ca, Sr, Ba) is important both of scientific and applications point of view. Pure crystals are widely used, as optical windows and lenses from IR to VUV, laser host materials, scintillators, etc. In the designing of rare-earth (RE) doped luminescent materials for various practical applications, the choice of the host is of great importance. Fluorides are considered good hosts for upconversion luminescence of trivalent rare-earth ions. BaF2 is known as the fastest scintillator for γ-rays and elementary particle detection. After adding a dopant, the radiation hardness of the BaF2 may be affected, due to the formation of color centers and of the lattice defects caused by the impurities. The use of doped crystals needs a homogeneous dopant distribution along the ingot. Joukoff et al. [1] studied the influence of various growth conditions on the chemical homogeneity of the Bridgman grown (BaF2)1-x(Y, Yb, NdF3)x crystals. The average segregation coefficient is 0.80 for Yb ions. Abe et al., [2] observed that the Tm3+ ions concentration at the edge parts of the BaF2 crystal rod is higher than that at the central part. The estimated segregation coefficient is 0.6. Stef et al., [3] investigated the Yb ions distribution along the BaF2 crystals. The segregation coefficient ranges from 0.6 to 0.84, depending on the YbF3 concentration. The study of Kuznetsov and Fedorov [4] show that the growth of homogeneous single crystals of solid solutions M1–x REx F2 + x is practically impossible in certain domains of concentration. The cell-free RE doped CaF2 crystals are easier to grow than the BaF2 ones. The trivalent RE ions dissolved in MeF2 need charge compensation in order to preserve the charge neutrality of the host. The so-called isolated centers Oh, C4v (NN), C3v (NNN) and clusters are created [5]. Studies of RE3+ ions in the BaF2 lattice using site-selective laser excitation were reported by [6] for Er3+, by [7] for Nd3+, and by [8] for Pr3+ ions. In all studies, the dominant center is of C3v symmetry. The local charge compensation creates electric dipoles whose relaxations are observed as dielectric absorption [9]. Dielectric relaxation in BaF2 crystals has been less investigated than in CaF2 and SrF2 crystals. The dielectric spectra of 0.01, 0.1 and 1 mol% ErF3 doped BaF2 were reported by [10]. Edgar and Welsh [11] studied the dielectric relaxation of the GdF3:BaF2 crystal and Nicoara and Stef [12] of the YbF3:BaF2 crystal. In all cases, only trigonal, C3v (NNN) centers were observed. Our preliminary luminescence experiments of Er doped BaF2 crystals pointed out emissions in near UV domain. It has been proved that the use of radiation in UV range is an efficient tool for medical purposes such as the treatment of tissue, improving the phototherapy methods of some skin diseases, such as psoriasis, vitiligo or lymphoma [13]. It is important to find a laser material with emission in near UV in order to design a miniature laser, easy to use by the patient itself.

The goal of this paper is to investigate some physical properties of ErF3 doped BaF2 crystals. Two methods have been used, the optical absorption and the dielectric relaxation, to achieve these objectives. We focused on the distribution of the Er3+ ions along the crystals and on the determination of the segregation coefficient using the optical absorption method. The nature of the charge compensating defects, with dipolar configuration, was studied using the dielectric spectra, where the relaxation process of these dipoles can be observed as dielectric absorption. Only one type of charge compensating defect has been detected, namely the isolated C3v site and clusters of them. The study of these physical properties of Er3+ ions doped BaF2 crystals have not been reported previously. Knowing the physical properties of RE-doped crystals is important both from a scientific point of view and for application purposes.

Section snippets

Experimental procedure

BaF2:ErF3 crystals were grown with different ErF3 content in the starting material: 0.05, 0.08, 0.15, 0.2 and 0.5 mol%. The crystals were obtained in our Crystal Research Laboratory by vertical Bridgman technique, using a shaped graphite furnace [14]. The crystals were grown in spectral pure graphite crucible, in vacuum (~10−1 Pa). As starting material, we used crushed BaF2 optical UV–VIS windows from Crystran Ltd. (obtained using 99.99% BaF2 powder) adding to the raw material the established

Optical absorption spectra

In order to investigate the optical properties, the dopant distribution along the crystals and to determine the effective segregation coefficient, the crystals were cleaved from the bottom to the top into i = 14–18 slices. Each wafer was about 2 mm in thickness. To compare the influence of the ErF3 concentration on the optical absorption spectra of the various samples, we eliminate the different background of the samples, in this way we obtain the normalized spectrum. We compared the optical

Conclusions

BaF2 crystals doped with different ErF3 concentrations ranging from 0.05 to 0.5 mol% ErF3 were grown by using the conventional Bridgman technique. The optical and dielectric properties of the crystals were investigated. The significant results found in the present work are. (1) The optical absorption spectra of BaF2:ErF3 crystals are quite similar in band structure for CaF2: ErF3 crystals, for all samples, except the intensities of the absorption bands. These absorption bands are due to the

CRediT authorship contribution statement

Irina Nicoara: Conceptualization, Methodology, Formal analysis, Investigation, Writing - original draft, Writing - review & editing, Supervision. Marius Stef: Methodology, Formal analysis, Investigation. Gabriel Buse: Formal analysis, Investigation. Andrei Racu: Formal analysis, Investigation.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

This work was supported by the Romanian Minister of Research and Innovation, CCCDI-UEFISCDI, project number PN-III-P1-1.2-PCCDI-2017-0152/75PCCDI/2018 within the project PNCDI III.

References (25)

  • S.V. Kuznetsov et al.

    Morphological stability of solid-liquid Interface during melt crystallization of M1–x Rx F2+x solid solutions

    Inorg. Mater.

    (2008)
  • R. A. Catlow, Defect properties of anion-excess alkaline-earth fluorides. I. Low defect concentrations J. Phys. C,...
  • Cited by (8)

    • In situ quantitative yttrium and trace elements imaging analysis of Y-doped BaF<inf>2</inf> crystals by LA-ICP-MS

      2023, Talanta
      Citation Excerpt :

      In recent research, selecting doping in BaF2 crystal for slow component suppression attracts a wide research interest, such as 1 at% yttrium (Y) doping can effectively suppress the slow scintillation component in BaF2 crystal, while the fast component remains unaffected [4,5]. However, Nicoara et al. [6] showed that in as-grown crystals, the properties of multiple sites were different due to the existence of segregation effect of doped elements during crystal growth. To obtain large BaF2: Y crystals with better light response uniformity, which is vital for high energy physics applications, it is necessary to figure out the actual yttrium concentration and distribution in as-grown crystals.

    • Analysis of site symmetries of Er<sup>3+</sup> doped CaF<inf>2</inf> and BaF<inf>2</inf> crystals by high resolution photoluminescence spectroscopy

      2023, Optical Materials
      Citation Excerpt :

      The reported tendency was as follows: for CaF2, the C4v center is dominant; in the SrF2, the dominant center changes from C4v to C3v while in the BaF2, host the C3v is the predominant center. The absorption and dielectric measurements on BaF2:Er3+ crystals also confirm the presence of C3v centers as stated by authors earlier [34]. The impacts of the mentioned structural differences on optical properties are shown in Fig. 9.

    • Pulsed laser deposition and structural evolution of BaF<inf>2</inf> nanolayers in Eu-doped BaF<inf>2</inf>/Al<inf>2</inf>O<inf>3</inf> layered optical nanocomposite thin films

      2022, Thin Solid Films
      Citation Excerpt :

      A broad PL peak was found to result from trap state emission from defects or electronic centers introduced during the synthesis of BaF2 crystals [6,7]. However, after the addition of rare-earth (RE) dopants, RE-doped BaF2 materials can show various PL peaks [8,9], making this an interesting system to explore.RE-doped materials are of great interest due to their desirable PL properties and potential applications including for solar cells [10,11], bioimaging [12], and light emitting diodes (LEDs) [13]. Fluorides have served as outstanding host materials for RE ions owing to their low phonon energy and thus reduced multiphonon relaxation rates that enable a unique efficient luminescent response [14,15].

    View all citing articles on Scopus
    View full text