Optical transmission enhancement of ionic crystals via superionic fluoride transfer: Growing VUV-transparent radioactive crystals
Kjeld Beeks, Tomáš Šikorský, Fabian Schaden, Martin Pressler, F. Schneider, Björn Koch, Thomas Pronebner, David Werban, Niyusha Hosseini, Georgy A. Kazakov, Jan M. Welch, Johannes H. Sterba, Florian Kraus, Thorsten Schumm
Abstract
The <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mrow><a:mn>8</a:mn><a:mtext>−</a:mtext><a:mi>eV</a:mi></a:mrow></a:math> first nuclear excited state in <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"><b:mmultiscripts><b:mi>Th</b:mi><b:mprescripts/><b:none/><b:mn>229</b:mn></b:mmultiscripts></b:math> is a candidate for implementing a nuclear clock. Doping <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:mmultiscripts><c:mi>Th</c:mi><c:mprescripts/><c:none/><c:mn>229</c:mn></c:mmultiscripts></c:math> into ionic crystals such as <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"><d:msub><d:mi>CaF</d:mi><d:mn>2</d:mn></d:msub></d:math> is expected to suppress nonradiative decay, enabling nuclear spectroscopy and the realization of a solid-state optical clock. Yet, the inherent radioactivity of <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"><e:mmultiscripts><e:mi>Th</e:mi><e:mprescripts/><e:none/><e:mn>229</e:mn></e:mmultiscripts></e:math> prohibits the growth of high-quality single crystals with high <f:math xmlns:f="http://www.w3.org/1998/Math/MathML"><f:mmultiscripts><f:mi>Th</f:mi><f:mprescripts/><f:none/><f:mn>229</f:mn></f:mmultiscripts></f:math> concentration; radiolysis causes fluoride loss, increasing absorption at <g:math xmlns:g="http://www.w3.org/1998/Math/MathML"><g:mrow><g:mn>8</g:mn><g:mspace width="0.16em"/><g:mi>eV</g:mi></g:mrow></g:math>. These radioactively doped crystals are thus a unique material for which a deeper analysis of the physical effects of radioactivity on growth, crystal structure, and electronic properties is presented. Following the analysis, we overcome the increase in absorption at <i:math xmlns:i="http://www.w3.org/1998/Math/MathML"><i:mrow><i:mn>8</i:mn><i:mspace width="0.16em"/><i:mi>eV</i:mi></i:mrow></i:math> by annealing <k:math xmlns:k="http://www.w3.org/1998/Math/MathML"><k:mmultiscripts><k:mi>Th</k:mi><k:mprescripts/><k:none/><k:mn>229</k:mn></k:mmultiscripts></k:math>-doped <l:math xmlns:l="http://www.w3.org/1998/Math/MathML"><l:msub><l:mi>CaF</l:mi><l:mn>2</l:mn></l:msub></l:math> at <m:math xmlns:m="http://www.w3.org/1998/Math/MathML"><m:mrow><m:mn>1250</m:mn><m:msup><m:mspace width="0.16em"/><m:mo>∘</m:mo></m:msup><m:mi mathvariant="normal">C</m:mi></m:mrow></m:math> in <p:math xmlns:p="http://www.w3.org/1998/Math/MathML"><p:msub><p:mi>CF</p:mi><p:mn>4</p:mn></p:msub></p:math>. This technique allows to adjust the fluoride content without crystal melting, preserving its single-crystal structure. Superionic state annealing ensures rapid fluoride distribution, creating fully transparent and radiation-hard crystals. This approach enables control over the charge state of dopants, which can be used in deep-UV optics, laser crystals, scintillators, and nuclear clocks. Published by the American Physical Society 2024